gcc
GNU project C and C++ compiler
see also :
cpp - gcov - as - ld - gdb
Synopsis
gcc
[-c|-S|-E]
[-std=standard]
[-g] [-pg]
[-Olevel]
[-Wwarn...] [-pedantic]
[-Idir...]
[-Ldir...]
[-Dmacro[=defn]...]
[-Umacro]
[-foption...]
[-mmachine-option...]
[-o outfile] [@file]
infile...
Only the most
useful options are listed here; see below for the remainder.
g++ accepts mostly the same options as
gcc.
add an example, a script, a trick and tips
examples
source
make CC=gcc CCLD=gcc bits
make
source
CC=gcc
GCC="gcc -E" ./configure
--enable-mods-shared=all
source
gcc and g++ in Mac
You need to install the XCode tools from here.
source
Premade Virtualbox images
If I understand your question correctly, you are wanting a VBox
image that is a pre-made linux distro that you will run, but ssh
into to communicate, to do some non-gui development? But you do
not have a lot of RAM?
I guess my first concern is that even with just running a CLI
interface, won't VBox take up your RAM anyways? Do you have
enough to run vbox in the first place (which I believe is the
amount needed for you host system + the amount needed for your
virtual instance)?
Second, though no pre-made distros come to mind (bearing, I have
done no research), it seems given your unique circumstances it
might be to your benefit to download something like Debian-minimal and install the core system and the
utilities you need on top of that. It will require a one-time
setup, but once it's done, you can just reuse your own custom
image.
At this point in linux's development, I am all for
package-managed systems, even when space is a concern.
I know this doesn't directly answer the question at hand, but I
hope it helps a little.
Cheers!
source
Upgrading GCC on CentOS
You could try Remi's repository. It usually has more up to date
packages than the normal CentOS repository.
source
How to benchmark kernel (-Os vs -O2)
If your -Os compiled kernel "seems excellent" I think you
shouldn't care benchmarking it, here is why:
The problem with benchmarks will always be to choose what types
of loads you test.
Even if Phoronix Test Suite is fine to highlight
differences between different kernel revisions you can't use it
yourself to prove your kernel performs better for your own load
because you don't ask it the same operations on a day to day
basis.
In case you still want to try this:
Maybe you could try benchmarking the applications you're using
most of the time or those who takes a long time to complete (3D
rendering/compilations/OLAP-style queries aka cubes/rainbow
tables generation...) to see if you can find a gain.
I personally highly doubt you'll see any real (measurable,
repeatable) speedup with -Os on a modern desktop CPU (embedded
CPU can gain some performances though). The little more
aggressive optimizations with -O2 (source article) might be more interesting than the
smallest size of the -Os kernel.
If you need more infos/wants to talk about gcc optimizations more
thoroughly you can go on the #gentoo channel on freenode IRC or
on the gentoo forums, but remember: just don't mention
the term "ricer" ^^
source
Installing Network Simulator (NS-2, allinone) on Linux Mandriva
From your log, its looks like you are running the display on the
localhost only. My suggestion is to use the following export
command:
% export DISPLAY=:0.0
and try running it again. I use this same command on my Ubuntu
12.10 system and it works.
source
gcc running out of memory in Ubuntu 12.10
Wow, that's a project!
Looks like you'll have to use a cross-compiler running on 64-bit
host system to build this project. Firefox is built in such way
IIRC.
GCC probably have exhausted all of its address space due to
virtual memory fragmentation, and also 900 meg you see in stats
is probably a commited physical memory, which is usually lower
than reserved virtual memory.
Also, every 32-bit process can access no more than 2Gb of memory,
no matter which size of physical memory and swap file you have.
source
how to install a old version of gcc compiler from software manger and use it with plain "gcc" command?
Just install the package gcc-4.4
along with the
other version of gcc. The next step would be to change the
default gcc
to this specific version. Most elegant
and easy way is using update-alternatives
. This is
all explained in this
Stackoverflow.com answer.
source
How can I install gcj on Linux?
You probably need to install the gcj package. If you're using Red
Hat, fire up Terminal and something like this should work:
yum update
yum install gcj (or java-gcj-compat)
source
Error installing GCC on Fedora
Try to update the package audit
first.
yum update audit
source
How to install an old version of gcc using yum on fedora 18?
If you're prepared to settle for gcc version 3.4 instead, try:
yum install compat-gcc-34
if not, then you'll have to build your own from a source tarball
on the gnu website:
http://www.gnu.org/software/gcc/
The build process will take quite some time, but it's simple
enough.
source
How to break out of a program in an infinite loop?
source
Error while loading shared libraries - libwebsock
Try running ldconfig
manually (as root) in case it
wasn't run during installation of the libs. The shared linker
uses a cache to look up shared libraries, and
ldconfig
is needed to update it.
source
I've installed gcc 4.6.1 on lion, do I overwrite default gcc?
I hacked around this using a previous method (ran into the EXACT
same problem) and unfortunately I could not come up with a non
hacky solution.. however:
Using which gcc
will tell you where the default
installed gcc is. Now if I were you, I would move the old gcc to
a new directory and create a symbolic link to the new gcc
directory in the old directory.
Try gcc -v
again after that.
Sorry for the illegitimate method. I have yet to come up with a
clean solution.
+1 to anyone who does if I see it.
Good luck
description
When you invoke
GCC , it normally does preprocessing,
compilation, assembly and linking. The "overall
options" allow you to stop this process at an
intermediate stage. For example, the -c option
says not to run the linker. Then the output consists of
object files output by the assembler.
Other options
are passed on to one stage of processing. Some options
control the preprocessor and others the compiler itself. Yet
other options control the assembler and linker; most of
these are not documented here, since you rarely need to use
any of them.
Most of the
command-line options that you can use with
GCC are useful for C programs; when an option
is only useful with another language (usually C
++ ), the explanation says so explicitly. If
the description for a particular option does not mention a
source language, you can use that option with all supported
languages.
The gcc
program accepts options and file names as operands. Many
options have multi-letter names; therefore multiple
single-letter options may not be grouped:
-dv is very different from
-d -v.
You can mix
options and other arguments. For the most part, the order
you use doesn’t matter. Order does matter when you use
several options of the same kind; for example, if you
specify -L more than once, the directories are
searched in the order specified. Also, the placement of the
-l option is significant.
Many options
have long names starting with -f or with
-W---for example,
-fmove-loop-invariants,
-Wformat and so on. Most of these have both
positive and negative forms; the negative form of
-ffoo would be -fno-foo.
This manual documents only one of these two forms, whichever
one is not the default.
options
Option
Summary
Here is a summary of all the options, grouped by type.
Explanations are in the following sections.
Overall Options
-c -S -E
-o file
-no-canonical-prefixes -pipe
-pass-exit-codes -x
language -v -###
--help[=class[,...]]
--target-help --version
-wrapper @file
-fplugin=file
-fplugin-arg-name=arg
-fdump-ada-spec[-slim]
-fdump-go-spec=file
C Language Options
-ansi
-std=standard -fgnu89-inline
-aux-info filename
-fallow-parameterless-variadic-functions
-fno-asm -fno-builtin
-fno-builtin-function
-fhosted -ffreestanding -fopenmp
-fms-extensions -fplan9-extensions
-trigraphs -no-integrated-cpp
-traditional -traditional-cpp
-fallow-single-precision
-fcond-mismatch
-flax-vector-conversions
-fsigned-bitfields -fsigned-char
-funsigned-bitfields
-funsigned-char
C ++ Language
Options
-fabi-version=n
-fno-access-control
-fcheck-new -fconserve-space
-fconstexpr-depth=n
-ffriend-injection
-fno-elide-constructors
-fno-enforce-eh-specs
-ffor-scope -fno-for-scope
-fno-gnu-keywords
-fno-implicit-templates
-fno-implicit-inline-templates
-fno-implement-inlines
-fms-extensions
-fno-nonansi-builtins
-fnothrow-opt
-fno-operator-names
-fno-optional-diags -fpermissive
-fno-pretty-templates -frepo
-fno-rtti -fstats
-ftemplate-depth=n
-fno-threadsafe-statics
-fuse-cxa-atexit -fno-weak
-nostdinc++ -fno-default-inline
-fvisibility-inlines-hidden
-fvisibility-ms-compat -Wabi
-Wconversion-null
-Wctor-dtor-privacy
-Wdelete-non-virtual-dtor
-Wnarrowing -Wnoexcept
-Wnon-virtual-dtor -Wreorder
-Weffc++ -Wstrict-null-sentinel
-Wno-non-template-friend
-Wold-style-cast
-Woverloaded-virtual
-Wno-pmf-conversions
-Wsign-promo
Objective-C and
Objective-C ++ Language Options
-fconstant-string-class=class-name
-fgnu-runtime -fnext-runtime
-fno-nil-receivers
-fobjc-abi-version=n
-fobjc-call-cxx-cdtors
-fobjc-direct-dispatch
-fobjc-exceptions -fobjc-gc
-fobjc-nilcheck -fobjc-std=objc1
-freplace-objc-classes
-fzero-link -gen-decls
-Wassign-intercept -Wno-protocol
-Wselector -Wstrict-selector-match
-Wundeclared-selector
Language Independent
Options
-fmessage-length=n
-fdiagnostics-show-location=[once|every-line]
-fno-diagnostics-show-option
Warning Options
-fsyntax-only
-fmax-errors=n -pedantic
-pedantic-errors -w -Wextra
-Wall -Waddress -Waggregate-return
-Warray-bounds -Wno-attributes
-Wno-builtin-macro-redefined
-Wc++-compat -Wc++11-compat
-Wcast-align -Wcast-qual
-Wchar-subscripts -Wclobbered
-Wcomment -Wconversion
-Wcoverage-mismatch -Wno-cpp
-Wno-deprecated
-Wno-deprecated-declarations
-Wdisabled-optimization
-Wno-div-by-zero
-Wdouble-promotion -Wempty-body
-Wenum-compare
-Wno-endif-labels -Werror
-Werror=* -Wfatal-errors
-Wfloat-equal -Wformat -Wformat=2
-Wno-format-contains-nul
-Wno-format-extra-args
-Wformat-nonliteral
-Wformat-security -Wformat-y2k
-Wframe-larger-than=len
-Wno-free-nonheap-object
-Wjump-misses-init
-Wignored-qualifiers -Wimplicit
-Wimplicit-function-declaration
-Wimplicit-int -Winit-self
-Winline -Wmaybe-uninitialized
-Wno-int-to-pointer-cast
-Wno-invalid-offsetof
-Winvalid-pch
-Wlarger-than=len
-Wunsafe-loop-optimizations
-Wlogical-op -Wlong-long
-Wmain -Wmaybe-uninitialized
-Wmissing-braces
-Wmissing-field-initializers
-Wmissing-format-attribute
-Wmissing-include-dirs
-Wno-mudflap -Wno-multichar
-Wnonnull -Wno-overflow
-Woverlength-strings -Wpacked
-Wpacked-bitfield-compat -Wpadded
-Wparentheses -Wpedantic-ms-format
-Wno-pedantic-ms-format
-Wpointer-arith
-Wno-pointer-to-int-cast
-Wredundant-decls -Wreturn-type
-Wsequence-point -Wshadow
-Wsign-compare -Wsign-conversion
-Wstack-protector
-Wstack-usage=len
-Wstrict-aliasing
-Wstrict-aliasing=n
-Wstrict-overflow
-Wstrict-overflow=n
-Wsuggest-attribute=[pure|const|noreturn]
-Wswitch -Wswitch-default
-Wswitch-enum -Wsync-nand
-Wsystem-headers -Wtrampolines
-Wtrigraphs -Wtype-limits -Wundef
-Wuninitialized -Wunknown-pragmas
-Wno-pragmas
-Wunsuffixed-float-constants
-Wunused -Wunused-function
-Wunused-label
-Wunused-local-typedefs
-Wunused-parameter
-Wno-unused-result
-Wunused-value -Wunused-variable
-Wunused-but-set-parameter
-Wunused-but-set-variable
-Wvariadic-macros
-Wvector-operation-performance -Wvla
-Wvolatile-register-var
-Wwrite-strings
-Wzero-as-null-pointer-constant
C and Objective-C-only
Warning Options
-Wbad-function-cast
-Wmissing-declarations
-Wmissing-parameter-type
-Wmissing-prototypes
-Wnested-externs
-Wold-style-declaration
-Wold-style-definition
-Wstrict-prototypes -Wtraditional
-Wtraditional-conversion
-Wdeclaration-after-statement
-Wpointer-sign
Debugging Options
-dletters
-dumpspecs -dumpmachine -dumpversion
-fdbg-cnt-list
-fdbg-cnt=counter-value-list
-fdisable-ipa-pass_name
-fdisable-rtl-pass_name
-fdisable-rtl-pass-name=range-list
-fdisable-tree-pass_name
-fdisable-tree-pass-name=range-list
-fdump-noaddr -fdump-unnumbered
-fdump-unnumbered-links
-fdump-translation-unit[-n]
-fdump-class-hierarchy[-n]
-fdump-ipa-all
-fdump-ipa-cgraph
-fdump-ipa-inline
-fdump-passes -fdump-statistics
-fdump-tree-all
-fdump-tree-original[-n]
-fdump-tree-optimized[-n]
-fdump-tree-cfg
-fdump-tree-vcg
-fdump-tree-alias
-fdump-tree-ch
-fdump-tree-ssa[-n]
-fdump-tree-pre[-n]
-fdump-tree-ccp[-n]
-fdump-tree-dce[-n]
-fdump-tree-gimple[-raw]
-fdump-tree-mudflap[-n]
-fdump-tree-dom[-n]
-fdump-tree-dse[-n]
-fdump-tree-phiprop[-n]
-fdump-tree-phiopt[-n]
-fdump-tree-forwprop[-n]
-fdump-tree-copyrename[-n]
-fdump-tree-nrv
-fdump-tree-vect
-fdump-tree-sink
-fdump-tree-sra[-n]
-fdump-tree-forwprop[-n]
-fdump-tree-fre[-n]
-fdump-tree-vrp[-n]
-ftree-vectorizer-verbose=n
-fdump-tree-storeccp[-n]
-fdump-final-insns=file
-fcompare-debug[=opts]
-fcompare-debug-second
-feliminate-dwarf2-dups
-feliminate-unused-debug-types
-feliminate-unused-debug-symbols
-femit-class-debug-always
-fenable-kind-pass
-fenable-kind-pass=range-list
-fdebug-types-section
-fmem-report
-fpre-ipa-mem-report
-fpost-ipa-mem-report
-fprofile-arcs
-frandom-seed=string
-fsched-verbose=n
-fsel-sched-verbose
-fsel-sched-dump-cfg
-fsel-sched-pipelining-verbose
-fstack-usage -ftest-coverage
-ftime-report -fvar-tracking
-fvar-tracking-assignments
-fvar-tracking-assignments-toggle
-g -glevel -gtoggle
-gcoff -gdwarf-version
-ggdb -grecord-gcc-switches
-gno-record-gcc-switches
-gstabs -gstabs+ -gstrict-dwarf
-gno-strict-dwarf -gvms
-gxcoff -gxcoff+
-fno-merge-debug-strings
-fno-dwarf2-cfi-asm
-fdebug-prefix-map=old=new
-femit-struct-debug-baseonly
-femit-struct-debug-reduced
-femit-struct-debug-detailed[=spec-list]
-p -pg
-print-file-name=library
-print-libgcc-file-name
-print-multi-directory
-print-multi-lib
-print-multi-os-directory
-print-prog-name=program
-print-search-dirs -Q
-print-sysroot
-print-sysroot-headers-suffix
-save-temps -save-temps=cwd
-save-temps=obj
-time[=file]
Optimization Options
-falign-functions[=n]
-falign-jumps[=n]
-falign-labels[=n]
-falign-loops[=n]
-fassociative-math
-fauto-inc-dec
-fbranch-probabilities
-fbranch-target-load-optimize
-fbranch-target-load-optimize2
-fbtr-bb-exclusive
-fcaller-saves
-fcheck-data-deps
-fcombine-stack-adjustments
-fconserve-stack -fcompare-elim
-fcprop-registers -fcrossjumping
-fcse-follow-jumps
-fcse-skip-blocks
-fcx-fortran-rules
-fcx-limited-range
-fdata-sections -fdce
-fdelayed-branch
-fdelete-null-pointer-checks
-fdevirtualize -fdse
-fearly-inlining -fipa-sra
-fexpensive-optimizations
-ffat-lto-objects -ffast-math
-ffinite-math-only
-ffloat-store
-fexcess-precision=style
-fforward-propagate
-ffp-contract=style
-ffunction-sections -fgcse
-fgcse-after-reload -fgcse-las
-fgcse-lm -fgraphite-identity
-fgcse-sm -fif-conversion
-fif-conversion2 -findirect-inlining
-finline-functions
-finline-functions-called-once
-finline-limit=n
-finline-small-functions
-fipa-cp -fipa-cp-clone
-fipa-matrix-reorg -fipa-pta
-fipa-profile -fipa-pure-const
-fipa-reference
-fira-algorithm=algorithm
-fira-region=region
-fira-loop-pressure
-fno-ira-share-save-slots
-fno-ira-share-spill-slots
-fira-verbose=n -fivopts
-fkeep-inline-functions
-fkeep-static-consts
-floop-block -floop-flatten
-floop-interchange
-floop-strip-mine
-floop-parallelize-all -flto
-flto-compression-level
-flto-partition=alg
-flto-report
-fmerge-all-constants
-fmerge-constants -fmodulo-sched
-fmodulo-sched-allow-regmoves
-fmove-loop-invariants fmudflap
-fmudflapir -fmudflapth
-fno-branch-count-reg
-fno-default-inline
-fno-defer-pop
-fno-function-cse
-fno-guess-branch-probability
-fno-inline -fno-math-errno
-fno-peephole -fno-peephole2
-fno-sched-interblock
-fno-sched-spec
-fno-signed-zeros
-fno-toplevel-reorder
-fno-trapping-math
-fno-zero-initialized-in-bss
-fomit-frame-pointer
-foptimize-register-move
-foptimize-sibling-calls
-fpartial-inlining -fpeel-loops
-fpredictive-commoning
-fprefetch-loop-arrays
-fprofile-correction
-fprofile-dir=path
-fprofile-generate
-fprofile-generate=path
-fprofile-use
-fprofile-use=path
-fprofile-values
-freciprocal-math -free -fregmove
-frename-registers -freorder-blocks
-freorder-blocks-and-partition
-freorder-functions
-frerun-cse-after-loop
-freschedule-modulo-scheduled-loops
-frounding-math
-fsched2-use-superblocks
-fsched-pressure
-fsched-spec-load
-fsched-spec-load-dangerous
-fsched-stalled-insns-dep[=n]
-fsched-stalled-insns[=n]
-fsched-group-heuristic
-fsched-critical-path-heuristic
-fsched-spec-insn-heuristic
-fsched-rank-heuristic
-fsched-last-insn-heuristic
-fsched-dep-count-heuristic
-fschedule-insns -fschedule-insns2
-fsection-anchors
-fselective-scheduling
-fselective-scheduling2
-fsel-sched-pipelining
-fsel-sched-pipelining-outer-loops
-fshrink-wrap -fsignaling-nans
-fsingle-precision-constant
-fsplit-ivs-in-unroller
-fsplit-wide-types
-fstack-protector
-fstack-protector-all
-fstrict-aliasing -fstrict-overflow
-fthread-jumps -ftracer
-ftree-bit-ccp
-ftree-builtin-call-dce
-ftree-ccp -ftree-ch
-ftree-coalesce-inline-vars
-ftree-coalesce-vars
-ftree-copy-prop
-ftree-copyrename -ftree-dce
-ftree-dominator-opts
-ftree-dse -ftree-forwprop
-ftree-fre
-ftree-loop-if-convert
-ftree-loop-if-convert-stores
-ftree-loop-im -ftree-phiprop
-ftree-loop-distribution
-ftree-loop-distribute-patterns
-ftree-loop-ivcanon
-ftree-loop-linear
-ftree-loop-optimize
-ftree-parallelize-loops=n
-ftree-pre
-ftree-partial-pre -ftree-pta
-ftree-reassoc -ftree-sink
-ftree-sra
-ftree-switch-conversion
-ftree-tail-merge -ftree-ter
-ftree-vect-loop-version
-ftree-vectorize -ftree-vrp
-funit-at-a-time
-funroll-all-loops
-funroll-loops
-funsafe-loop-optimizations
-funsafe-math-optimizations
-funswitch-loops
-fvariable-expansion-in-unroller
-fvect-cost-model -fvpt -fweb
-fwhole-program -fwpa
-fuse-ld=linker
-fuse-linker-plugin
--param name=value
-O -O0 -O1 -O2 -O3
-Os -Ofast
Preprocessor Options
-Aquestion=answer
-A-question[=answer]
-C -dD -dI -dM -dN
-Dmacro[=defn] -E
-H -idirafter dir
-include file -imacros
file -iprefix file
-iwithprefix dir
-iwithprefixbefore dir
-isystem dir -imultilib
dir -isysroot dir -M
-MM -MF -MG -MP -MQ -MT
-nostdinc -P -fdebug-cpp
-ftrack-macro-expansion
-fworking-directory -remap
-trigraphs -undef -Umacro
-Wp,option -Xpreprocessor
option
Assembler Option
-Wa,option
-Xassembler option
Linker Options
object-file-name
-llibrary -nostartfiles
-nodefaultlibs -nostdlib -pie
-rdynamic -s -static
-static-libgcc -static-libstdc++
-shared -shared-libgcc -symbolic
-T script -Wl,option
-Xlinker option -u
symbol
Directory Options
-Bprefix
-Idir
-iplugindir=dir
-iquotedir -Ldir
-specs=file -I-
--sysroot=dir
Machine Dependent
Options
AArch64 Options
-mbig-endian -mlittle-endian
-mgeneral-regs-only -mcmodel=tiny
-mcmodel=small -mcmodel=large
-mstrict-align
-momit-leaf-frame-pointer
-mno-omit-leaf-frame-pointer
-mtls-dialect=desc
-mtls-dialect=traditional
-march=name -mcpu=name
-mtune=name
Adapteva
Epiphany Options -mhalf-reg-file
-mprefer-short-insn-regs
-mbranch-cost=num -mcmove
-mnops=num -msoft-cmpsf
-msplit-lohi -mpost-inc
-mpost-modify
-mstack-offset=num
-mround-nearest -mlong-calls
-mshort-calls -msmall16
-mfp-mode=mode
-mvect-double
-max-vect-align=num
-msplit-vecmove-early
-m1reg-reg
ARM
Options -mapcs-frame
-mno-apcs-frame
-mabi=name
-mapcs-stack-check
-mno-apcs-stack-check
-mapcs-float -mno-apcs-float
-mapcs-reentrant
-mno-apcs-reentrant
-msched-prolog
-mno-sched-prolog
-mlittle-endian -mbig-endian
-mwords-little-endian
-mfloat-abi=name -mfpe
-mfp16-format=name
-mthumb-interwork
-mno-thumb-interwork
-mcpu=name -march=name
-mfpu=name
-mstructure-size-boundary=n
-mabort-on-noreturn
-mlong-calls -mno-long-calls
-msingle-pic-base
-mno-single-pic-base
-mpic-register=reg
-mnop-fun-dllimport
-mcirrus-fix-invalid-insns
-mno-cirrus-fix-invalid-insns
-mpoke-function-name -mthumb
-marm -mtpcs-frame
-mtpcs-leaf-frame
-mcaller-super-interworking
-mcallee-super-interworking
-mtp=name
-mtls-dialect=dialect
-mword-relocations
-mfix-cortex-m3-ldrd
-munaligned-access
-mneon-for-64bits
AVR
Options -mmcu=mcu
-maccumulate-args
-mbranch-cost=cost
-mcall-prologues -mint8
-mno-interrupts -mrelax
-mshort-calls -mstrict-X
-mtiny-stack
Blackfin
Options
-mcpu=cpu[-sirevision]
-msim
-momit-leaf-frame-pointer
-mno-omit-leaf-frame-pointer
-mspecld-anomaly
-mno-specld-anomaly
-mcsync-anomaly
-mno-csync-anomaly -mlow-64k
-mno-low64k -mstack-check-l1
-mid-shared-library
-mno-id-shared-library
-mshared-library-id=n
-mleaf-id-shared-library
-mno-leaf-id-shared-library
-msep-data -mno-sep-data
-mlong-calls -mno-long-calls
-mfast-fp -minline-plt
-mmulticore -mcorea -mcoreb -msdram
-micplb
C6X
Options -mbig-endian
-mlittle-endian -march=cpu
-msim -msdata=sdata-type
CRIS
Options -mcpu=cpu
-march=cpu
-mtune=cpu
-mmax-stack-frame=n
-melinux-stacksize=n
-metrax4 -metrax100 -mpdebug
-mcc-init -mno-side-effects
-mstack-align -mdata-align
-mconst-align -m32-bit
-m16-bit -m8-bit
-mno-prologue-epilogue
-mno-gotplt -melf -maout
-melinux -mlinux -sim -sim2
-mmul-bug-workaround
-mno-mul-bug-workaround
CR16
Options -mmac -mcr16cplus -mcr16c
-msim -mint32 -mbit-ops
-mdata-model=model
Darwin
Options -all_load -allowable_client
-arch -arch_errors_fatal -arch_only
-bind_at_load -bundle -bundle_loader
-client_name -compatibility_version
-current_version -dead_strip
-dependency-file -dylib_file
-dylinker_install_name -dynamic
-dynamiclib -exported_symbols_list
-filelist -flat_namespace
-force_cpusubtype_ALL -force_flat_namespace
-headerpad_max_install_names -iframework
-image_base -init -install_name
-keep_private_externs -multi_module
-multiply_defined -multiply_defined_unused
-noall_load -no_dead_strip_inits_and_terms
-nofixprebinding -nomultidefs -noprebind
-noseglinkedit -pagezero_size -prebind
-prebind_all_twolevel_modules -private_bundle
-read_only_relocs -sectalign
-sectobjectsymbols -whyload -seg1addr
-sectcreate -sectobjectsymbols -sectorder
-segaddr -segs_read_only_addr
-segs_read_write_addr -seg_addr_table
-seg_addr_table_filename -seglinkedit
-segprot -segs_read_only_addr
-segs_read_write_addr -single_module
-static -sub_library -sub_umbrella
-twolevel_namespace -umbrella -undefined
-unexported_symbols_list
-weak_reference_mismatches -whatsloaded -F
-gused -gfull
-mmacosx-version-min=version
-mkernel -mone-byte-bool
DEC
Alpha Options -mno-fp-regs
-msoft-float -malpha-as -mgas
-mieee -mieee-with-inexact
-mieee-conformant
-mfp-trap-mode=mode
-mfp-rounding-mode=mode
-mtrap-precision=mode
-mbuild-constants
-mcpu=cpu-type
-mtune=cpu-type -mbwx
-mmax -mfix -mcix -mfloat-vax
-mfloat-ieee -mexplicit-relocs
-msmall-data -mlarge-data
-msmall-text -mlarge-text
-mmemory-latency=time
DEC
Alpha/VMS Options
-mvms-return-codes
-mdebug-main=prefix
-mmalloc64
FR30
Options -msmall-model
-mno-lsim
FRV
Options -mgpr-32 -mgpr-64
-mfpr-32 -mfpr-64
-mhard-float -msoft-float
-malloc-cc -mfixed-cc -mdword
-mno-dword -mdouble
-mno-double -mmedia -mno-media
-mmuladd -mno-muladd -mfdpic
-minline-plt -mgprel-ro
-multilib-library-pic
-mlinked-fp -mlong-calls
-malign-labels -mlibrary-pic
-macc-4 -macc-8 -mpack
-mno-pack -mno-eflags
-mcond-move -mno-cond-move
-moptimize-membar
-mno-optimize-membar -mscc
-mno-scc -mcond-exec
-mno-cond-exec -mvliw-branch
-mno-vliw-branch
-mmulti-cond-exec
-mno-multi-cond-exec
-mnested-cond-exec
-mno-nested-cond-exec
-mtomcat-stats -mTLS -mtls
-mcpu=cpu
GNU/Linux
Options -mglibc -muclibc -mbionic
-mandroid -tno-android-cc
-tno-android-ld
H8/300
Options -mrelax -mh -ms -mn
-mint32 -malign-300
HPPA
Options -march=architecture-type
-mbig-switch -mdisable-fpregs
-mdisable-indexing
-mfast-indirect-calls -mgas
-mgnu-ld -mhp-ld
-mfixed-range=register-range
-mjump-in-delay
-mlinker-opt -mlong-calls
-mlong-load-store
-mno-big-switch
-mno-disable-fpregs
-mno-disable-indexing
-mno-fast-indirect-calls
-mno-gas
-mno-jump-in-delay
-mno-long-load-store
-mno-portable-runtime
-mno-soft-float
-mno-space-regs -msoft-float
-mpa-risc-1-0
-mpa-risc-1-1
-mpa-risc-2-0
-mportable-runtime
-mschedule=cpu-type
-mspace-regs -msio -mwsio
-munix=unix-std -nolibdld
-static -threads
i386 and
x86-64 Options -mtune=cpu-type
-march=cpu-type
-mfpmath=unit
-masm=dialect
-mno-fancy-math-387
-mno-fp-ret-in-387
-msoft-float
-mno-wide-multiply -mrtd
-malign-double
-mpreferred-stack-boundary=num
-mincoming-stack-boundary=num
-mcld -mcx16 -msahf -mmovbe
-mcrc32 -mrecip -mrecip=opt
-mvzeroupper -mprefer-avx128
-mmmx -msse -msse2 -msse3
-mssse3 -msse4.1 -msse4.2 -msse4
-mavx -mavx2 -maes -mpclmul
-mfsgsbase -mrdrnd -mf16c -mfma
-msse4a -m3dnow -mpopcnt -mabm
-mbmi -mtbm -mfma4 -mxop
-mlzcnt -mbmi2 -mlwp -mthreads
-mno-align-stringops
-minline-all-stringops
-minline-stringops-dynamically
-mstringop-strategy=alg
-mpush-args
-maccumulate-outgoing-args
-m128bit-long-double
-m96bit-long-double
-mregparm=num -msseregparm
-mveclibabi=type
-mvect8-ret-in-mem -mpc32
-mpc64 -mpc80 -mstackrealign
-momit-leaf-frame-pointer
-mno-red-zone
-mno-tls-direct-seg-refs
-mcmodel=code-model
-mabi=name
-maddress-mode=mode -m32
-m64 -mx32
-mlarge-data-threshold=num
-msse2avx -mfentry -m8bit-idiv
-mavx256-split-unaligned-load
-mavx256-split-unaligned-store
i386 and
x86-64 Windows Options -mconsole
-mcygwin -mno-cygwin -mdll
-mnop-fun-dllimport -mthread
-municode -mwin32 -mwindows
-fno-set-stack-executable
IA-64
Options -mbig-endian
-mlittle-endian -mgnu-as
-mgnu-ld -mno-pic
-mvolatile-asm-stop
-mregister-names -msdata
-mno-sdata -mconstant-gp
-mauto-pic -mfused-madd
-minline-float-divide-min-latency
-minline-float-divide-max-throughput
-mno-inline-float-divide
-minline-int-divide-min-latency
-minline-int-divide-max-throughput
-mno-inline-int-divide
-minline-sqrt-min-latency
-minline-sqrt-max-throughput
-mno-inline-sqrt -mdwarf2-asm
-mearly-stop-bits
-mfixed-range=register-range
-mtls-size=tls-size
-mtune=cpu-type -milp32
-mlp64 -msched-br-data-spec
-msched-ar-data-spec
-msched-control-spec
-msched-br-in-data-spec
-msched-ar-in-data-spec
-msched-in-control-spec
-msched-spec-ldc
-msched-spec-control-ldc
-msched-prefer-non-data-spec-insns
-msched-prefer-non-control-spec-insns
-msched-stop-bits-after-every-cycle
-msched-count-spec-in-critical-path
-msel-sched-dont-check-control-spec
-msched-fp-mem-deps-zero-cost
-msched-max-memory-insns-hard-limit
-msched-max-memory-insns=max-insns
IA-64/VMS
Options -mvms-return-codes
-mdebug-main=prefix
-mmalloc64
LM32
Options -mbarrel-shift-enabled
-mdivide-enabled -mmultiply-enabled
-msign-extend-enabled
-muser-enabled
M32R/D
Options -m32r2 -m32rx -m32r
-mdebug -malign-loops
-mno-align-loops
-missue-rate=number
-mbranch-cost=number
-mmodel=code-size-model-type
-msdata=sdata-type
-mno-flush-func
-mflush-func=name
-mno-flush-trap
-mflush-trap=number -G
num
M32C
Options -mcpu=cpu -msim
-memregs=number
M680x0
Options -march=arch
-mcpu=cpu
-mtune=tune -m68000
-m68020 -m68020-40 -m68020-60
-m68030 -m68040 -m68060 -mcpu32
-m5200 -m5206e -m528x -m5307
-m5407 -mcfv4e -mbitfield
-mno-bitfield -mc68000 -mc68020
-mnobitfield -mrtd -mno-rtd
-mdiv -mno-div -mshort
-mno-short -mhard-float
-m68881 -msoft-float -mpcrel
-malign-int -mstrict-align
-msep-data -mno-sep-data
-mshared-library-id=n
-mid-shared-library
-mno-id-shared-library -mxgot
-mno-xgot
MCore
Options -mhardlit -mno-hardlit
-mdiv -mno-div
-mrelax-immediates
-mno-relax-immediates
-mwide-bitfields
-mno-wide-bitfields
-m4byte-functions
-mno-4byte-functions
-mcallgraph-data
-mno-callgraph-data
-mslow-bytes -mno-slow-bytes
-mno-lsim -mlittle-endian
-mbig-endian -m210 -m340
-mstack-increment
MeP
Options -mabsdiff -mall-opts
-maverage -mbased=n -mbitops
-mc=n -mclip
-mconfig=name -mcop -mcop32
-mcop64 -mivc2 -mdc -mdiv -meb
-mel -mio-volatile -ml -mleadz
-mm -mminmax -mmult -mno-opts
-mrepeat -ms -msatur -msdram
-msim -msimnovec -mtf
-mtiny=n
MicroBlaze
Options -msoft-float
-mhard-float -msmall-divides
-mcpu=cpu -mmemcpy
-mxl-soft-mul
-mxl-soft-div
-mxl-barrel-shift
-mxl-pattern-compare
-mxl-stack-check
-mxl-gp-opt -mno-clearbss
-mxl-multiply-high
-mxl-float-convert
-mxl-float-sqrt
-mxl-mode-app-model
MIPS
Options -EL -EB
-march=arch -mtune=arch
-mips1 -mips2 -mips3 -mips4
-mips32 -mips32r2 -mips64 -mips64r2
-mips16 -mno-mips16
-mflip-mips16 -minterlink-mips16
-mno-interlink-mips16
-mabi=abi -mabicalls
-mno-abicalls -mshared
-mno-shared -mplt -mno-plt
-mxgot -mno-xgot -mgp32 -mgp64
-mfp32 -mfp64 -mhard-float
-msoft-float -msingle-float
-mdouble-float -mdsp -mno-dsp
-mdspr2 -mno-dspr2
-mfpu=fpu-type -msmartmips
-mno-smartmips -mpaired-single
-mno-paired-single -mdmx
-mno-mdmx -mips3d -mno-mips3d
-mmt -mno-mt -mllsc
-mno-llsc -mlong64 -mlong32
-msym32 -mno-sym32 -Gnum
-mlocal-sdata
-mno-local-sdata
-mextern-sdata
-mno-extern-sdata -mgpopt
-mno-gopt -membedded-data
-mno-embedded-data
-muninit-const-in-rodata
-mno-uninit-const-in-rodata
-mcode-readable=setting
-msplit-addresses
-mno-split-addresses
-mexplicit-relocs
-mno-explicit-relocs
-mcheck-zero-division
-mno-check-zero-division
-mdivide-traps -mdivide-breaks
-mmemcpy -mno-memcpy
-mlong-calls -mno-long-calls
-mmad -mno-mad -mfused-madd
-mno-fused-madd -nocpp
-mfix-24k -mno-fix-24k
-mfix-r4000 -mno-fix-r4000
-mfix-r4400 -mno-fix-r4400
-mfix-r10000 -mno-fix-r10000
-mfix-vr4120 -mno-fix-vr4120
-mfix-vr4130 -mno-fix-vr4130
-mfix-sb1 -mno-fix-sb1
-mflush-func=func
-mno-flush-func
-mbranch-cost=num
-mbranch-likely
-mno-branch-likely
-mfp-exceptions
-mno-fp-exceptions
-mvr4130-align
-mno-vr4130-align -msynci
-mno-synci -mrelax-pic-calls
-mno-relax-pic-calls
-mmcount-ra-address
MMIX
Options -mlibfuncs -mno-libfuncs
-mepsilon -mno-epsilon -mabi=gnu
-mabi=mmixware -mzero-extend
-mknuthdiv -mtoplevel-symbols -melf
-mbranch-predict
-mno-branch-predict
-mbase-addresses
-mno-base-addresses
-msingle-exit
-mno-single-exit
MN10300
Options -mmult-bug
-mno-mult-bug -mno-am33
-mam33 -mam33-2 -mam34
-mtune=cpu-type
-mreturn-pointer-on-d0
-mno-crt0 -mrelax -mliw
-msetlb
PDP-11
Options -mfpu -msoft-float
-mac0 -mno-ac0 -m40 -m45
-m10 -mbcopy -mbcopy-builtin
-mint32 -mno-int16 -mint16
-mno-int32 -mfloat32
-mno-float64 -mfloat64
-mno-float32 -mabshi
-mno-abshi -mbranch-expensive
-mbranch-cheap -munix-asm
-mdec-asm
picoChip
Options -mae=ae_type
-mvliw-lookahead=N
-msymbol-as-address
-mno-inefficient-warnings
PowerPC
Options See RS/6000 and PowerPC
Options.
RL78
Options -msim -mmul=none -mmul=g13
-mmul=rl78
RS/6000
and PowerPC Options -mcpu=cpu-type
-mtune=cpu-type
-mcmodel=code-model -mpower
-mno-power -mpower2
-mno-power2 -mpowerpc -mpowerpc64
-mno-powerpc -maltivec
-mno-altivec -mpowerpc-gpopt
-mno-powerpc-gpopt
-mpowerpc-gfxopt
-mno-powerpc-gfxopt -mmfcrf
-mno-mfcrf -mpopcntb
-mno-popcntb -mpopcntd
-mno-popcntd -mfprnd
-mno-fprnd -mcmpb -mno-cmpb
-mmfpgpr -mno-mfpgpr
-mhard-dfp -mno-hard-dfp
-mnew-mnemonics -mold-mnemonics
-mfull-toc -mminimal-toc
-mno-fp-in-toc
-mno-sum-in-toc -m64
-m32 -mxl-compat
-mno-xl-compat -mpe
-malign-power -malign-natural
-msoft-float -mhard-float
-mmultiple -mno-multiple
-msingle-float -mdouble-float
-msimple-fpu -mstring
-mno-string -mupdate
-mno-update
-mavoid-indexed-addresses
-mno-avoid-indexed-addresses
-mfused-madd -mno-fused-madd
-mbit-align -mno-bit-align
-mstrict-align
-mno-strict-align -mrelocatable
-mno-relocatable -mrelocatable-lib
-mno-relocatable-lib -mtoc
-mno-toc -mlittle
-mlittle-endian -mbig
-mbig-endian -mdynamic-no-pic
-maltivec -mswdiv
-msingle-pic-base
-mprioritize-restricted-insns=priority
-msched-costly-dep=dependence_type
-minsert-sched-nops=scheme
-mcall-sysv -mcall-netbsd
-maix-struct-return
-msvr4-struct-return
-mabi=abi-type -msecure-plt
-mbss-plt
-mblock-move-inline-limit=num
-misel -mno-isel -misel=yes
-misel=no -mspe -mno-spe
-mspe=yes -mspe=no -mpaired
-mgen-cell-microcode
-mwarn-cell-microcode -mvrsave
-mno-vrsave -mmulhw -mno-mulhw
-mdlmzb -mno-dlmzb
-mfloat-gprs=yes -mfloat-gprs=no
-mfloat-gprs=single
-mfloat-gprs=double -mprototype
-mno-prototype -msim -mmvme
-mads -myellowknife -memb -msdata
-msdata=opt -mvxworks -G
num -pthread -mrecip
-mrecip=opt -mno-recip
-mrecip-precision
-mno-recip-precision
-mveclibabi=type -mfriz
-mno-friz
-mpointers-to-nested-functions
-mno-pointers-to-nested-functions
-msave-toc-indirect
-mno-save-toc-indirect
RX
Options -m64bit-doubles
-m32bit-doubles -fpu -nofpu
-mcpu= -mbig-endian-data
-mlittle-endian-data
-msmall-data -msim -mno-sim
-mas100-syntax
-mno-as100-syntax -mrelax
-mmax-constant-size=
-mint-register= -mpid
-msave-acc-in-interrupts
S/390 and
zSeries Options -mtune=cpu-type
-march=cpu-type
-mhard-float -msoft-float
-mhard-dfp -mno-hard-dfp
-mlong-double-64
-mlong-double-128 -mbackchain
-mno-backchain -mpacked-stack
-mno-packed-stack -msmall-exec
-mno-small-exec -mmvcle
-mno-mvcle -m64 -m31 -mdebug
-mno-debug -mesa -mzarch
-mtpf-trace -mno-tpf-trace
-mfused-madd -mno-fused-madd
-mwarn-framesize -mwarn-dynamicstack
-mstack-size -mstack-guard
Score
Options -meb -mel -mnhwloop
-muls -mmac -mscore5 -mscore5u
-mscore7 -mscore7d
SH
Options -m1 -m2 -m2e
-m2a-nofpu -m2a-single-only
-m2a-single -m2a -m3 -m3e
-m4-nofpu -m4-single-only
-m4-single -m4 -m4a-nofpu
-m4a-single-only -m4a-single
-m4a -m4al -m5-64media
-m5-64media-nofpu -m5-32media
-m5-32media-nofpu -m5-compact
-m5-compact-nofpu -mb -ml
-mdalign -mrelax -mbigtable -mfmovd
-mhitachi -mrenesas -mno-renesas
-mnomacsave -mieee -mno-ieee
-mbitops -misize
-minline-ic_invalidate -mpadstruct
-mspace -mprefergot -musermode
-multcost=number
-mdiv=strategy
-mdivsi3_libfunc=name
-mfixed-range=register-range
-madjust-unroll
-mindexed-addressing
-mgettrcost=number
-mpt-fixed
-maccumulate-outgoing-args
-minvalid-symbols -msoft-atomic
-mbranch-cost=num
-mcbranchdi -mcmpeqdi
-mfused-madd -mpretend-cmove
Solaris 2
Options -mimpure-text
-mno-impure-text -pthreads
-pthread
SPARC
Options -mcpu=cpu-type
-mtune=cpu-type
-mcmodel=code-model
-mmemory-model=mem-model
-m32 -m64 -mapp-regs
-mno-app-regs -mfaster-structs
-mno-faster-structs -mflat
-mno-flat -mfpu -mno-fpu
-mhard-float -msoft-float
-mhard-quad-float
-msoft-quad-float -mstack-bias
-mno-stack-bias
-munaligned-doubles
-mno-unaligned-doubles -mv8plus
-mno-v8plus -mvis -mno-vis
-mvis2 -mno-vis2 -mvis3
-mno-vis3 -mfmaf -mno-fmaf
-mpopc -mno-popc
-mfix-at697f
SPU
Options -mwarn-reloc
-merror-reloc -msafe-dma
-munsafe-dma -mbranch-hints
-msmall-mem -mlarge-mem
-mstdmain
-mfixed-range=register-range
-mea32 -mea64
-maddress-space-conversion
-mno-address-space-conversion
-mcache-size=cache-size
-matomic-updates
-mno-atomic-updates
System V
Options -Qy -Qn
-YP,paths -Ym,dir
TILE-Gx
Options -mcpu=cpu -m32
-m64
TILEPro
Options -mcpu=cpu
-m32
V850
Options -mlong-calls
-mno-long-calls -mep
-mno-ep -mprolog-function
-mno-prolog-function -mspace
-mtda=n -msda=n
-mzda=n -mapp-regs
-mno-app-regs -mdisable-callt
-mno-disable-callt -mv850e2v3
-mv850e2 -mv850e1 -mv850es -mv850e
-mv850 -mbig-switch
VAX
Options -mg -mgnu -munix
VxWorks
Options -mrtp -non-static
-Bstatic -Bdynamic -Xbind-lazy
-Xbind-now
x86-64
Options See i386 and x86-64 Options.
Xstormy16
Options -msim
Xtensa
Options -mconst16 -mno-const16
-mfused-madd -mno-fused-madd
-mforce-no-pic
-mserialize-volatile
-mno-serialize-volatile
-mtext-section-literals
-mno-text-section-literals
-mtarget-align
-mno-target-align -mlongcalls
-mno-longcalls
zSeries
Options See S/390 and zSeries Options.
Code Generation
Options
-fcall-saved-reg
-fcall-used-reg
-ffixed-reg -fexceptions
-fnon-call-exceptions
-funwind-tables
-fasynchronous-unwind-tables
-finhibit-size-directive
-finstrument-functions
-finstrument-functions-exclude-function-list=sym,sym,...
-finstrument-functions-exclude-file-list=file,file,...
-fno-common -fno-ident
-fpcc-struct-return -fpic
-fPIC -fpie -fPIE
-fno-jump-tables
-frecord-gcc-switches
-freg-struct-return
-fshort-enums -fshort-double
-fshort-wchar -fverbose-asm
-fpack-struct[=n]
-fstack-check
-fstack-limit-register=reg
-fstack-limit-symbol=sym
-fno-stack-limit
-fsplit-stack -fleading-underscore
-ftls-model=model -ftrapv
-fwrapv -fbounds-check -fvisibility
-fstrict-volatile-bitfields
Options
Controlling the Kind of Output
Compilation can involve up to four stages: preprocessing,
compilation proper, assembly and linking, always in that
order. GCC is capable of preprocessing and
compiling several files either into several assembler input
files, or into one assembler input file; then each assembler
input file produces an object file, and linking combines all
the object files (those newly compiled, and those specified
as input) into an executable file.
For any given
input file, the file name suffix determines what kind of
compilation is done:
file.c
C source code that must be
preprocessed.
file.i
C source code that should not
be preprocessed.
file.ii
C ++ source code
that should not be preprocessed.
file.m
Objective-C source code. Note
that you must link with the libobjc library to make
an Objective-C program work.
file.mi
Objective-C source code that
should not be preprocessed.
file.mm
file.M
Objective-C
++ source code. Note that you must link with
the libobjc library to make an Objective-C
++ program work. Note that .M refers
to a literal capital M.
file.mii
Objective-C
++ source code that should not be
preprocessed.
file.h
C, C ++ ,
Objective-C or Objective-C ++ header
file to be turned into a precompiled header (default), or C,
C ++ header file to be turned into an Ada
spec (via the -fdump-ada-spec
switch).
file.cc
file.cp
file.cxx
file.cpp
file.CPP
file.c++
file.C
C ++ source code
that must be preprocessed. Note that in .cxx, the
last two letters must both be literally x. Likewise,
.C refers to a literal capital C.
file.mm
file.M
Objective-C
++ source code that must be preprocessed.
file.mii
Objective-C
++ source code that should not be
preprocessed.
file.hh
file.H
file.hp
file.hxx
file.hpp
file.HPP
file.h++
file.tcc
C ++ header file
to be turned into a precompiled header or Ada spec.
file.f
file.for
file.ftn
Fixed form Fortran source code
that should not be preprocessed.
file.F
file.FOR
file.fpp
file.FPP
file.FTN
Fixed form Fortran source code
that must be preprocessed (with the traditional
preprocessor).
file.f90
file.f95
file.f03
file.f08
Free form Fortran source code
that should not be preprocessed.
file.F90
file.F95
file.F03
file.F08
Free form Fortran source code
that must be preprocessed (with the traditional
preprocessor).
file.go
Go source code.
file.ads
Ada source code file that
contains a library unit declaration (a declaration of a
package, subprogram, or generic, or a generic
instantiation), or a library unit renaming declaration (a
package, generic, or subprogram renaming declaration). Such
files are also called specs.
file.adb
Ada source code file containing
a library unit body (a subprogram or package body). Such
files are also called bodies.
file.s
Assembler code.
file.S
file.sx
Assembler code that must be
preprocessed.
other
An object file to be fed
straight into linking. Any file name with no recognized
suffix is treated this way.
You can specify
the input language explicitly with the -x
option:
-x language
Specify explicitly the
language for the following input files (rather than
letting the compiler choose a default based on the file name
suffix). This option applies to all following input files
until the next -x option. Possible values for
language are:
c c-header cpp-output
c++ c++-header c++-cpp-output
objective-c objective-c-header objective-c-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
assembler assembler-with-cpp
ada
f77 f77-cpp-input f95 f95-cpp-input
go
java
-x none
Turn off any specification of a
language, so that subsequent files are handled according to
their file name suffixes (as they are if -x has
not been used at all).
-pass-exit-codes
Normally the gcc program
will exit with the code of 1 if any phase of the compiler
returns a non-success return code. If you specify
-pass-exit-codes, the gcc
program will instead return with numerically highest error
produced by any phase that returned an error indication. The
C, C ++ , and Fortran frontends return 4, if
an internal compiler error is encountered.
If you only
want some of the stages of compilation, you can use
-x (or filename suffixes) to tell gcc
where to start, and one of the options -c,
-S, or -E to say where gcc
is to stop. Note that some combinations (for example,
-x cpp-output -E) instruct gcc to
do nothing at all.
-c
Compile or assemble the source files, but do not link.
The linking stage simply is not done. The ultimate output is
in the form of an object file for each source file.
By default, the
object file name for a source file is made by replacing the
suffix .c, .i, .s, etc., with
.o.
Unrecognized
input files, not requiring compilation or assembly, are
ignored.
-S
Stop after the stage of
compilation proper; do not assemble. The output is in the
form of an assembler code file for each non-assembler input
file specified.
By default, the
assembler file name for a source file is made by replacing
the suffix .c, .i, etc., with .s.
Input files
that don’t require compilation are ignored.
-E
Stop after the preprocessing
stage; do not run the compiler proper. The output is in the
form of preprocessed source code, which is sent to the
standard output.
Input files
that don’t require preprocessing are ignored.
-o file
Place output in file
file. This applies regardless to whatever sort of
output is being produced, whether it be an executable file,
an object file, an assembler file or preprocessed C
code.
If
-o is not specified, the default is to put an
executable file in a.out, the object file for
source.suffix in source.o, its assembler file
in source.s, a precompiled header file in
source.suffix.gch, and all preprocessed C source on
standard output.
-v
Print (on standard error output) the commands executed
to run the stages of compilation. Also print the version
number of the compiler driver program and of the
preprocessor and the compiler proper.
-###
Like -v except the
commands are not executed and arguments are quoted unless
they contain only alphanumeric characters or
"./-_". This is useful for shell
scripts to capture the driver-generated command lines.
-pipe
Use pipes rather than temporary
files for communication between the various stages of
compilation. This fails to work on some systems where the
assembler is unable to read from a pipe; but the
GNU assembler has no trouble.
--help
Print (on the standard output)
a description of the command-line options understood by
gcc. If the -v option is also specified
then --help will also be passed on to the
various processes invoked by gcc, so that they can
display the command-line options they accept. If the
-Wextra option has also been specified (prior
to the --help option), then command-line
options that have no documentation associated with them will
also be displayed.
--target-help
Print (on the standard output)
a description of target-specific command-line options for
each tool. For some targets extra target-specific
information may also be printed.
--help={class|[^]qualifier}[,...]
Print (on the standard output)
a description of the command-line options understood by the
compiler that fit into all specified classes and qualifiers.
These are the supported classes:
optimizers
This will display all of the
optimization options supported by the compiler.
warnings
This will display all of the
options controlling warning messages produced by the
compiler.
target
This will display
target-specific options. Unlike the
--target-help option however,
target-specific options of the linker and assembler will not
be displayed. This is because those tools do not currently
support the extended --help= syntax.
params
This will display the values
recognized by the --param option.
language
This will display the options
supported for language, where language is the
name of one of the languages supported in this version of
GCC .
common
This will display the options
that are common to all languages.
These are the
supported qualifiers:
undocumented
Display only those options that
are undocumented.
joined
Display options taking an
argument that appears after an equal sign in the same
continuous piece of text, such as:
--help=target.
separate
Display options taking an
argument that appears as a separate word following the
original option, such as: -o output-file.
Thus for
example to display all the undocumented target-specific
switches supported by the compiler the following can be
used:
--help=target,undocumented
The sense of a
qualifier can be inverted by prefixing it with the ^
character, so for example to display all binary warning
options (i.e., ones that are either on or off and that do
not take an argument) that have a description, use:
--help=warnings,^joined,^undocumented
The argument to
--help= should not consist solely of
inverted qualifiers.
Combining
several classes is possible, although this usually restricts
the output by so much that there is nothing to display. One
case where it does work however is when one of the classes
is target. So for example to display all the
target-specific optimization options the following can be
used:
--help=target,optimizers
The
--help= option can be repeated on the
command line. Each successive use will display its requested
class of options, skipping those that have already been
displayed.
If the
-Q option appears on the command line before
the --help= option, then the descriptive
text displayed by --help= is changed.
Instead of describing the displayed options, an indication
is given as to whether the option is enabled, disabled or
set to a specific value (assuming that the compiler knows
this at the point where the --help=
option is used).
Here is a
truncated example from the ARM port of
gcc:
% gcc -Q -mabi=2 --help=target -c
The following options are target specific:
-mabi= 2
-mabort-on-noreturn [disabled]
-mapcs [disabled]
The output is
sensitive to the effects of previous command-line options,
so for example it is possible to find out which
optimizations are enabled at -O2 by using:
-Q -O2 --help=optimizers
Alternatively
you can discover which binary optimizations are enabled by
-O3 by using:
gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled
-no-canonical-prefixes
Do not expand any symbolic
links, resolve references to /../ or /./, or
make the path absolute when generating a relative
prefix.
--version
Display the version number and
copyrights of the invoked GCC .
-wrapper
Invoke all subcommands under a
wrapper program. The name of the wrapper program and its
parameters are passed as a comma separated list.
gcc -c t.c -wrapper gdb,--args
This will
invoke all subprograms of gcc under gdb
--args, thus the invocation of cc1
will be gdb --args cc1 ....
-fplugin=name.so
Load the plugin code in file
name.so, assumed to be a shared object to be
dlopen’d by the compiler. The base name of the shared
object file is used to identify the plugin for the purposes
of argument parsing (See
-fplugin-arg-name-key=value
below). Each plugin should define the callback functions
specified in the Plugins API .
-fplugin-arg-name-key=value
Define an argument called
key with a value of value for the plugin
called name.
-fdump-ada-spec[-slim]
For C and C ++
source and include files, generate corresponding Ada
specs.
-fdump-go-spec=file
For input files in any
language, generate corresponding Go declarations in
file. This generates Go "const",
"type", "var", and
"func" declarations which may be a useful
way to start writing a Go interface to code written in some
other language.
@file
Read command-line options from
file. The options read are inserted in place of the
original @file option. If file does not exist,
or cannot be read, then the option will be treated
literally, and not removed.
Options in
file are separated by whitespace. A whitespace
character may be included in an option by surrounding the
entire option in either single or double quotes. Any
character (including a backslash) may be included by
prefixing the character to be included with a backslash. The
file may itself contain additional @file
options; any such options will be processed recursively.
Compiling C
++ Programs
C ++ source files conventionally use one of
the suffixes .C, .cc, .cpp,
.CPP, .c++, .cp, or .cxx; C
++ header files often use .hh,
.hpp, .H, or (for shared template code)
.tcc; and preprocessed C ++ files use
the suffix .ii. GCC recognizes files
with these names and compiles them as C ++
programs even if you call the compiler the same way as for
compiling C programs (usually with the name gcc).
However, the
use of gcc does not add the C ++
library. g++ is a program that calls
GCC and treats .c, .h and
.i files as C ++ source files instead
of C source files unless -x is used, and
automatically specifies linking against the C
++ library. This program is also useful when
precompiling a C header file with a .h extension for
use in C ++ compilations. On many systems,
g++ is also installed with the name c++.
When you
compile C ++ programs, you may specify many
of the same command-line options that you use for compiling
programs in any language; or command-line options meaningful
for C and related languages; or options that are meaningful
only for C ++ programs.
Options
Controlling C Dialect
The following options control the dialect of C (or languages
derived from C, such as C ++ , Objective-C
and Objective-C ++ ) that the compiler
accepts:
-ansi
In C mode, this is equivalent
to -std=c90. In C ++ mode, it is
equivalent to -std=c++98.
This turns off
certain features of GCC that are incompatible
with ISO C90 (when compiling C code), or of
standard C ++ (when compiling C
++ code), such as the
"asm" and "typeof"
keywords, and predefined macros such as
"unix" and "vax" that
identify the type of system you are using. It also enables
the undesirable and rarely used ISO trigraph
feature. For the C compiler, it disables recognition of C
++ style // comments as well as the
"inline" keyword.
The alternate
keywords "__asm__",
"__extension__",
"__inline__" and
"__typeof__" continue to work despite
-ansi. You would not want to use them in an
ISO C program, of course, but it is useful to
put them in header files that might be included in
compilations done with -ansi. Alternate
predefined macros such as "__unix__" and
"__vax__" are also available, with or
without -ansi.
The
-ansi option does not cause non-ISO programs to
be rejected gratuitously. For that, -pedantic
is required in addition to -ansi.
The macro
"__STRICT_ANSI__" is predefined when the
-ansi option is used. Some header files may
notice this macro and refrain from declaring certain
functions or defining certain macros that the
ISO standard doesn’t call for; this is
to avoid interfering with any programs that might use these
names for other things.
Functions that
would normally be built in but do not have semantics defined
by ISO C (such as "alloca"
and "ffs") are not built-in functions
when -ansi is used.
-std=
Determine the language
standard. This option is currently only supported when
compiling C or C ++ .
The compiler
can accept several base standards, such as c90 or
c++98, and GNU dialects of those
standards, such as gnu90 or gnu++98. By
specifying a base standard, the compiler will accept all
programs following that standard and those using
GNU extensions that do not contradict it. For
example, -std=c90 turns off certain features of
GCC that are incompatible with
ISO C90, such as the "asm"
and "typeof" keywords, but not other
GNU extensions that do not have a meaning in
ISO C90, such as omitting the middle term of
a "?:" expression. On the other hand, by
specifying a GNU dialect of a standard, all
features the compiler support are enabled, even when those
features change the meaning of the base standard and some
strict-conforming programs may be rejected. The particular
standard is used by -pedantic to identify which
features are GNU extensions given that
version of the standard. For example -std=gnu90
-pedantic would warn about C ++
style // comments, while -std=gnu99
-pedantic would not.
A value for
this option must be provided; possible values are
iso9899:1990
Support all ISO
C90 programs (certain GNU extensions that
conflict with ISO C90 are disabled). Same as
-ansi for C code.
iso9899:199409
ISO C90 as
modified in amendment 1.
iso9899:1999
iso9899:199x
ISO C99. Note
that this standard is not yet fully supported; see
<http://gcc.gnu.org/gcc-4.7/c99status.html>
for more information. The names c9x and
iso9899:199x are deprecated.
iso9899:2011
ISO C11, the
2011 revision of the ISO C standard. Support
is incomplete and experimental. The name c1x is
deprecated.
gnu90
gnu89
GNU dialect of
ISO C90 (including some C99 features). This
is the default for C code.
gnu99
gnu9x
GNU dialect of
ISO C99. When ISO C99 is fully
implemented in GCC , this will become the
default. The name gnu9x is deprecated.
gnu11
gnu1x
GNU dialect of
ISO C11. Support is incomplete and
experimental. The name gnu1x is deprecated.
c++98
The 1998 ISO C
++ standard plus amendments. Same as
-ansi for C ++ code.
gnu++98
GNU dialect of
-std=c++98. This is the default for C
++ code.
c++11
The 2011 ISO C
++ standard plus amendments. Support for C
++ 11 is still experimental, and may change
in incompatible ways in future releases.
gnu++11
GNU dialect of
-std=c++11. Support for C ++ 11
is still experimental, and may change in incompatible ways
in future releases.
-fgnu89-inline
The option
-fgnu89-inline tells GCC
to use the traditional GNU semantics for
"inline" functions when in C99 mode.
This option is accepted and ignored by GCC
versions 4.1.3 up to but not including 4.3. In
GCC versions 4.3 and later it changes the
behavior of GCC in C99 mode. Using this
option is roughly equivalent to adding the
"gnu_inline" function attribute to all
inline functions.
The option
-fno-gnu89-inline explicitly tells
GCC to use the C99 semantics for
"inline" when in C99 or gnu99 mode (i.e.,
it specifies the default behavior). This option was first
supported in GCC 4.3. This option is not
supported in -std=c90 or
-std=gnu90 mode.
The
preprocessor macros "__GNUC_GNU_INLINE__"
and "__GNUC_STDC_INLINE__" may be used to
check which semantics are in effect for
"inline" functions.
-aux-info
filename
Output to the given filename
prototyped declarations for all functions declared and/or
defined in a translation unit, including those in header
files. This option is silently ignored in any language other
than C.
Besides
declarations, the file indicates, in comments, the origin of
each declaration (source file and line), whether the
declaration was implicit, prototyped or unprototyped
(I, N for new or O for old,
respectively, in the first character after the line number
and the colon), and whether it came from a declaration or a
definition (C or F, respectively, in the
following character). In the case of function definitions, a
K&R-style list of arguments followed by their
declarations is also provided, inside comments, after the
declaration.
-fallow-parameterless-variadic-functions
Accept variadic functions
without named parameters.
Although it is
possible to define such a function, this is not very useful
as it is not possible to read the arguments. This is only
supported for C as this construct is allowed by C
++ .
-fno-asm
Do not recognize
"asm", "inline" or
"typeof" as a keyword, so that code can
use these words as identifiers. You can use the keywords
"__asm__",
"__inline__" and
"__typeof__" instead. -ansi
implies -fno-asm.
In C
++ , this switch only affects the
"typeof" keyword, since
"asm" and "inline" are
standard keywords. You may want to use the
-fno-gnu-keywords flag instead,
which has the same effect. In C99 mode
(-std=c99 or -std=gnu99), this
switch only affects the "asm" and
"typeof" keywords, since
"inline" is a standard keyword in
ISO C99.
-fno-builtin
-fno-builtin-function
Don’t recognize built-in
functions that do not begin with __builtin_ as
prefix.
GCC
normally generates special code to handle certain built-in
functions more efficiently; for instance, calls to
"alloca" may become single instructions
which adjust the stack directly, and calls to
"memcpy" may become inline copy loops.
The resulting code is often both smaller and faster, but
since the function calls no longer appear as such, you
cannot set a breakpoint on those calls, nor can you change
the behavior of the functions by linking with a different
library. In addition, when a function is recognized as a
built-in function, GCC may use information
about that function to warn about problems with calls to
that function, or to generate more efficient code, even if
the resulting code still contains calls to that function.
For example, warnings are given with -Wformat
for bad calls to "printf", when
"printf" is built in, and
"strlen" is known not to modify global
memory.
With the
-fno-builtin-function option
only the built-in function function is disabled.
function must not begin with __builtin_. If a
function is named that is not built-in in this version of
GCC , this option is ignored. There is no
corresponding -fbuiltin-function
option; if you wish to enable built-in functions selectively
when using -fno-builtin or
-ffreestanding, you may define macros such
as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
-fhosted
Assert that compilation takes
place in a hosted environment. This implies
-fbuiltin. A hosted environment is one in which
the entire standard library is available, and in which
"main" has a return type of
"int". Examples are nearly everything
except a kernel. This is equivalent to
-fno-freestanding.
-ffreestanding
Assert that compilation takes
place in a freestanding environment. This implies
-fno-builtin. A freestanding environment
is one in which the standard library may not exist, and
program startup may not necessarily be at
"main". The most obvious example is an
OS kernel. This is equivalent to
-fno-hosted.
-fopenmp
Enable handling of OpenMP
directives "#pragma omp" in C/C
++ and "!$omp" in Fortran.
When -fopenmp is specified, the compiler
generates parallel code according to the OpenMP Application
Program Interface v3.0
<http://www.openmp.org/>. This option implies
-pthread, and thus is only supported on targets
that have support for -pthread.
-fgnu-tm
When the option
-fgnu-tm is specified, the compiler will
generate code for the Linux variant of Intel’s current
Transactional Memory ABI specification
document (Revision 1.1, May 6 2009). This is an experimental
feature whose interface may change in future versions of
GCC , as the official specification changes.
Please note that not all architectures are supported for
this feature.
For more
information on GCC ’s support for
transactional memory,
Note that the
transactional memory feature is not supported with non-call
exceptions
(-fnon-call-exceptions).
-fms-extensions
Accept some non-standard
constructs used in Microsoft header files.
In C
++ code, this allows member names in
structures to be similar to previous types declarations.
typedef int UOW;
struct ABC {
UOW UOW;
};
Some cases of
unnamed fields in structures and unions are only accepted
with this option.
-fplan9-extensions
Accept some non-standard
constructs used in Plan 9 code.
This enables
-fms-extensions, permits passing pointers
to structures with anonymous fields to functions that expect
pointers to elements of the type of the field, and permits
referring to anonymous fields declared using a typedef. This
is only supported for C, not C ++ .
-trigraphs
Support ISO C
trigraphs. The -ansi option (and
-std options for strict ISO C
conformance) implies -trigraphs.
-no-integrated-cpp
Performs a compilation in two
passes: preprocessing and compiling. This option allows a
user supplied "cc1", "cc1plus", or
"cc1obj" via the -B option. The user
supplied compilation step can then add in an additional
preprocessing step after normal preprocessing but before
compiling. The default is to use the integrated cpp
(internal cpp)
The semantics
of this option will change if "cc1",
"cc1plus", and "cc1obj" are merged.
-traditional
-traditional-cpp
Formerly, these options caused
GCC to attempt to emulate a pre-standard C
compiler. They are now only supported with the
-E switch. The preprocessor continues to
support a pre-standard mode. See the GNU CPP
manual for details.
-fcond-mismatch
Allow conditional expressions
with mismatched types in the second and third arguments. The
value of such an expression is void. This option is not
supported for C ++ .
-flax-vector-conversions
Allow implicit conversions
between vectors with differing numbers of elements and/or
incompatible element types. This option should not be used
for new code.
-funsigned-char
Let the type
"char" be unsigned, like
"unsigned char".
Each kind of
machine has a default for what "char"
should be. It is either like "unsigned
char" by default or like "signed
char" by default.
Ideally, a
portable program should always use "signed
char" or "unsigned char" when
it depends on the signedness of an object. But many programs
have been written to use plain "char" and
expect it to be signed, or expect it to be unsigned,
depending on the machines they were written for. This
option, and its inverse, let you make such a program work
with the opposite default.
The type
"char" is always a distinct type from
each of "signed char" or
"unsigned char", even though its behavior
is always just like one of those two.
-fsigned-char
Let the type
"char" be signed, like "signed
char".
Note that this
is equivalent to
-fno-unsigned-char, which is the
negative form of -funsigned-char.
Likewise, the option
-fno-signed-char is equivalent to
-funsigned-char.
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
These options control whether a
bit-field is signed or unsigned, when the declaration does
not use either "signed" or
"unsigned". By default, such a bit-field
is signed, because this is consistent: the basic integer
types such as "int" are signed types.
Options
Controlling C ++ Dialect
This section describes the command-line options that are
only meaningful for C ++ programs; but you
can also use most of the GNU compiler options
regardless of what language your program is in. For example,
you might compile a file "firstClass.C"
like this:
g++ -g -frepo -O -c firstClass.C
In this
example, only -frepo is an option meant only
for C ++ programs; you can use the other
options with any language supported by GCC
.
Here is a list
of options that are only for compiling C
++ programs:
-fabi-version=n
Use version n of the C
++ ABI . Version 2 is the version of the
C ++ ABI that first appeared in G++ 3.4.
Version 1 is the version of the C ++ ABI
that first appeared in G++ 3.2. Version 0 will always
be the version that conforms most closely to the C ++
ABI specification. Therefore, the ABI
obtained using version 0 will change as ABI
bugs are fixed.
The
default is version 2.
Version
3 corrects an error in mangling a constant address as a
template argument.
Version
4, which first appeared in G++ 4.5, implements a standard
mangling for vector types.
Version
5, which first appeared in G++ 4.6, corrects the mangling of
attribute const/volatile on function pointer types, decltype
of a plain decl, and use of a function parameter in the
declaration of another
parameter.
Version
6, which first appeared in G++ 4.7, corrects the promotion
behavior of C ++ 11 scoped enums and the mangling
of template argument packs, const/static_cast, prefix ++ and
--, and a class scope function used as a
template argument.
See
also -Wabi.
-fno-access-control
Turn off
all access checking. This switch is mainly useful for
working around bugs in the access control
code.
-fcheck-new
Check that
the pointer returned by "operator new" is
non-null before attempting to modify the storage allocated.
This check is normally unnecessary because the C ++
standard specifies that "operator
new" will only return 0 if it is declared
throw(), in which case the compiler will
always check the return value even without this option. In
all other cases, when "operator new" has
a non-empty exception specification, memory exhaustion is
signalled by throwing "std::bad_alloc".
See also new (nothrow).
-fconserve-space
Put
uninitialized or run-time-initialized global variables into
the common segment, as C does. This saves space in the
executable at the cost of not diagnosing duplicate
definitions. If you compile with this flag and your program
mysteriously crashes after "main()" has
completed, you may have an object that is being destroyed
twice because two definitions were
merged.
This
option is no longer useful on most targets, now that support
has been added for putting variables into BSS
without making them common.
-fconstexpr-depth=n
Set the
maximum nested evaluation depth for C ++ 11
constexpr functions to n. A limit is needed to detect
endless recursion during constant expression evaluation. The
minimum specified by the standard is
512.
-fdeduce-init-list
Enable
deduction of a template type parameter as
std::initializer_list from a brace-enclosed initializer
list, i.e.
template <class T> auto forward(T t) -> decltype (realfn (t))
return realfn (t);
void f()
forward({1,2}); // call forward<std::initializer_list<int>>
}
This
deduction was implemented as a possible extension to the
originally proposed semantics for the C ++ 11
standard, but was not part of the final standard, so it is
disabled by default. This option is deprecated, and may be
removed in a future version of
G++.
-ffriend-injection
Inject
friend functions into the enclosing namespace, so that they
are visible outside the scope of the class in which they are
declared. Friend functions were documented to work this way
in the old Annotated C ++ Reference Manual, and
versions of G++ before 4.1 always worked that way. However,
in ISO C ++ a friend function that is
not declared in an enclosing scope can only be found using
argument dependent lookup. This option causes friends to be
injected as they were in earlier
releases.
This
option is for compatibility, and may be removed in a future
release of G++.
-fno-elide-constructors
The C
++ standard allows an implementation to omit creating a
temporary that is only used to initialize another object of
the same type. Specifying this option disables that
optimization, and forces G++ to call the copy constructor in
all cases.
-fno-enforce-eh-specs
Don’t
generate code to check for violation of exception
specifications at run time. This option violates the C
++ standard, but may be useful for reducing code size
in production builds, much like defining NDEBUG
. This does not give user code permission to throw
exceptions in violation of the exception specifications; the
compiler will still optimize based on the specifications, so
throwing an unexpected exception will result in undefined
behavior.
-ffor-scope
-fno-for-scope
If
-ffor-scope is specified, the scope of
variables declared in a for-init-statement is limited
to the for loop itself, as specified by the C
++ standard. If -fno-for-scope
is specified, the scope of variables declared in a
for-init-statement extends to the end of the
enclosing scope, as was the case in old versions of G++, and
other (traditional) implementations of C ++
.
The
default if neither flag is given to follow the standard, but
to allow and give a warning for old-style code that would
otherwise be invalid, or have different
behavior.
-fno-gnu-keywords
Do not
recognize "typeof" as a keyword, so that
code can use this word as an identifier. You can use the
keyword "__typeof__" instead.
-ansi implies
-fno-gnu-keywords.
-fno-implicit-templates
Never emit
code for non-inline templates that are instantiated
implicitly (i.e. by use); only emit code for explicit
instantiations.
-fno-implicit-inline-templates
Don’t
emit code for implicit instantiations of inline templates,
either. The default is to handle inlines differently so that
compiles with and without optimization will need the same
set of explicit instantiations.
-fno-implement-inlines
To save
space, do not emit out-of-line copies of inline functions
controlled by #pragma implementation. This will cause
linker errors if these functions are not inlined everywhere
they are called.
-fms-extensions
Disable
pedantic warnings about constructs used in MFC ,
such as implicit int and getting a pointer to member
function via non-standard
syntax.
-fno-nonansi-builtins
Disable
built-in declarations of functions that are not mandated
by ANSI/ISO C. These include
"ffs", "alloca",
"_exit", "index",
"bzero", "conjf", and
other related functions.
-fnothrow-opt
Treat a
"throw()" exception specification as
though it were a "noexcept" specification
to reduce or eliminate the text size overhead relative to a
function with no exception specification. If the function
has local variables of types with non-trivial destructors,
the exception specification will actually make the function
smaller because the EH cleanups for those
variables can be optimized away. The semantic effect is that
an exception thrown out of a function with such an exception
specification will result in a call to
"terminate" rather than
"unexpected".
-fno-operator-names
Do not
treat the operator name keywords "and",
"bitand", "bitor",
"compl", "not",
"or" and "xor" as
synonyms as keywords.
-fno-optional-diags
Disable
diagnostics that the standard says a compiler does not need
to issue. Currently, the only such diagnostic issued by G++
is the one for a name having multiple meanings within a
class.
-fpermissive
Downgrade
some diagnostics about nonconformant code from errors to
warnings. Thus, using -fpermissive will allow
some nonconforming code to
compile.
-fno-pretty-templates
When an
error message refers to a specialization of a function
template, the compiler will normally print the signature of
the template followed by the template arguments and any
typedefs or typenames in the signature (e.g. "void
f(T) [with T = int]" rather than "void
f(int)") so that it’s clear which template
is involved. When an error message refers to a
specialization of a class template, the compiler will omit
any template arguments that match the default template
arguments for that template. If either of these behaviors
make it harder to understand the error message rather than
easier, using -fno-pretty-templates
will disable them.
-frepo
Enable
automatic template instantiation at link time. This option
also implies
-fno-implicit-templates.
-fno-rtti
Disable
generation of information about every class with virtual
functions for use by the C ++ run-time type
identification features (dynamic_cast and
typeid). If you don’t use those parts of the
language, you can save some space by using this flag. Note
that exception handling uses the same information, but it
will generate it as needed. The dynamic_cast operator
can still be used for casts that do not require run-time
type information, i.e. casts to "void *"
or to unambiguous base classes.
-fstats
Emit
statistics about front-end processing at the end of the
compilation. This information is generally only useful to
the G++ development team.
-fstrict-enums
Allow the
compiler to optimize using the assumption that a value of
enumerated type can only be one of the values of the
enumeration (as defined in the C ++ standard;
basically, a value that can be represented in the minimum
number of bits needed to represent all the enumerators).
This assumption may not be valid if the program uses a cast
to convert an arbitrary integer value to the enumerated
type.
-ftemplate-depth=n
Set the
maximum instantiation depth for template classes to
n. A limit on the template instantiation depth is
needed to detect endless recursions during template class
instantiation. ANSI/ISO C ++
conforming programs must not rely on a maximum depth
greater than 17 (changed to 1024 in C ++ 11). The
default value is 900, as the compiler can run out of stack
space before hitting 1024 in some
situations.
-fno-threadsafe-statics
Do not emit
the extra code to use the routines specified in the C
++ ABI for thread-safe initialization of local
statics. You can use this option to reduce code size
slightly in code that doesn’t need to be
thread-safe.
-fuse-cxa-atexit
Register
destructors for objects with static storage duration with
the "__cxa_atexit" function rather than
the "atexit" function. This option is
required for fully standards-compliant handling of static
destructors, but will only work if your C library supports
"__cxa_atexit".
-fno-use-cxa-get-exception-ptr
Don’t
use the "__cxa_get_exception_ptr" runtime
routine. This will cause
"std::uncaught_exception" to be
incorrect, but is necessary if the runtime routine is not
available.
-fvisibility-inlines-hidden
This
switch declares that the user does not attempt to compare
pointers to inline functions or methods where the addresses
of the two functions were taken in different shared
objects.
The
effect of this is that GCC may, effectively, mark
inline methods with "__attribute__ ((visibility
("hidden")))" so that they do not appear
in the export table of a DSO and do not require
a PLT indirection when used within the DSO
. Enabling this option can have a dramatic effect on
load and link times of a DSO as it massively
reduces the size of the dynamic export table when the
library makes heavy use of
templates.
The
behavior of this switch is not quite the same as marking the
methods as hidden directly, because it does not affect
static variables local to the function or cause the compiler
to deduce that the function is defined in only one shared
object.
You
may mark a method as having a visibility explicitly to
negate the effect of the switch for that method. For
example, if you do want to compare pointers to a particular
inline method, you might mark it as having default
visibility. Marking the enclosing class with explicit
visibility will have no
effect.
Explicitly
instantiated inline methods are unaffected by this option as
their linkage might otherwise cross a shared library
boundary.
-fvisibility-ms-compat
This
flag attempts to use visibility settings to make GCC
’s C ++ linkage model compatible with
that of Microsoft Visual
Studio.
The
flag makes these changes to GCC ’s linkage
model:
1.
It sets the default visibility
to "hidden", like
-fvisibility=hidden.
2.
Types, but not their members,
are not hidden by default.
3.
The One Definition Rule is
relaxed for types without explicit visibility specifications
that are defined in more than one different shared object:
those declarations are permitted if they would have been
permitted when this option was not
used.
In
new code it is better to use
-fvisibility=hidden and export those classes
that are intended to be externally visible. Unfortunately it
is possible for code to rely, perhaps accidentally, on the
Visual Studio behavior.
Among
the consequences of these changes are that static data
members of the same type with the same name but defined in
different shared objects will be different, so changing one
will not change the other; and that pointers to function
members defined in different shared objects may not compare
equal. When this flag is given, it is a violation of
the ODR to define types with the same name
differently.
-fno-weak
Do not
use weak symbol support, even if it is provided by the
linker. By default, G++ will use weak symbols if they are
available. This option exists only for testing, and should
not be used by end-users; it will result in inferior code
and has no benefits. This option may be removed in a future
release of G++.
-nostdinc++
Do not
search for header files in the standard directories specific
to C ++ , but do still search the other standard
directories. (This option is used when building the C
++ library.)
In
addition, these optimization, warning, and code generation
options have meanings only for C ++ programs:
-fno-default-inline
Do not
assume inline for functions defined inside a class
scope.
Note that these functions will have linkage like inline
functions; they just won’t be inlined by
default.
-Wabi
(C, Objective-C, C ++ and Objective-C
++ only)
Warn
when G++ generates code that is probably not compatible with
the vendor-neutral C ++ ABI . Although an
effort has been made to warn about all such cases, there are
probably some cases that are not warned about, even though
G++ is generating incompatible code. There may also be cases
where warnings are emitted even though the code that is
generated will be
compatible.
You
should rewrite your code to avoid these warnings if you are
concerned about the fact that code generated by G++ may not
be binary compatible with code generated by other
compilers.
The
known incompatibilities in
-fabi-version=2 (the default)
include:
•
A template with a non-type
template parameter of reference type is mangled
incorrectly:
extern int N;
template <int &> struct S {};
void n (S<N>) {2}
This
is fixed in
-fabi-version=3.
•
SIMD
vector types declared using "__attribute
((vector_size))" are mangled in a non-standard way
that does not allow for overloading of functions taking
vectors of different
sizes.
The
mangling is changed in
-fabi-version=4.
The
known incompatibilities in
-fabi-version=1
include:
•
Incorrect
handling of tail-padding for bit-fields. G++ may attempt to
pack data into the same byte as a base class. For
example:
struct A { virtual void f(); int f1 : 1; };
struct B : public A { int f2 : 1; };
In
this case, G++ will place "B::f2" into
the same byte as"A::f1"; other compilers
will not. You can avoid this problem by explicitly padding
"A" so that its size is a multiple of the
byte size on your platform; that will cause G++ and other
compilers to layout "B"
identically.
•
Incorrect
handling of tail-padding for virtual bases. G++ does not use
tail padding when laying out virtual bases. For
example:
struct A { virtual void f(); char c1; };
struct B { B(); char c2; };
struct C : public A, public virtual B {};
In
this case, G++ will not place "B" into
the tail-padding for "A"; other compilers
will. You can avoid this problem by explicitly padding
"A" so that its size is a multiple of its
alignment (ignoring virtual base classes); that will cause
G++ and other compilers to layout "C"
identically.
•
Incorrect
handling of bit-fields with declared widths greater than
that of their underlying types, when the bit-fields appear
in a union. For
example:
union U { int i : 4096; };
Assuming
that an "int" does not have 4096 bits,
G++ will make the union too small by the number of bits in
an
"int".
•
Empty
classes can be placed at incorrect offsets. For
example:
struct A {};
struct B {
A a;
virtual void f ();
struct C : public B, public A {};
G++
will place the "A" base class of
"C" at a nonzero offset; it should be
placed at offset zero. G++ mistakenly believes that the
"A" data member of "B"
is already at offset
zero.
•
Names
of template functions whose types involve
"typename" or template template
parameters can be mangled
incorrectly.
template <typename Q>
void f(typename Q::X) {}
template <template <typename> class Q>
void f(typename Q<int>::X) {}
Instantiations
of these templates may be mangled
incorrectly.
It
also warns psABI related changes. The known psABI changes at
this point include:
•
For
SYSV/x86-64, when passing union with long double, it
is changed to pass in memory as specified in psABI. For
example:
union U {
long double ld;
int i;
};
"union
U" will always be passed in
memory.
-Wctor-dtor-privacy
(C ++ and Objective-C ++
only)
Warn
when a class seems unusable because all the constructors or
destructors in that class are private, and it has neither
friends nor public static member
functions.
-Wdelete-non-virtual-dtor
(C ++ and Objective-C ++
only)
Warn
when delete is used to destroy an instance of a class
that has virtual functions and non-virtual destructor. It is
unsafe to delete an instance of a derived class through a
pointer to a base class if the base class does not have a
virtual destructor. This warning is enabled by
-Wall.
-Wnarrowing
(C ++ and Objective-C ++
only)
Warn
when a narrowing conversion prohibited by C ++ 11
occurs within { },
e.g.
int i = { 2.2 }; // error: narrowing from double to int
This
flag is included in -Wall and
-Wc++11-compat.
With
-std=c++11, -Wno-narrowing
suppresses the diagnostic required by the standard. Note
that this does not affect the meaning of well-formed code;
narrowing conversions are still considered ill-formed
in SFINAE
context.
-Wnoexcept
(C ++ and Objective-C ++
only)
Warn
when a noexcept-expression evaluates to false because of a
call to a function that does not have a non-throwing
exception specification (i.e. throw() or
noexcept) but is known by the compiler to never throw
an exception.
-Wnon-virtual-dtor
(C ++ and Objective-C ++
only)
Warn
when a class has virtual functions and accessible
non-virtual destructor, in which case it would be possible
but unsafe to delete an instance of a derived class through
a pointer to the base class. This warning is also enabled if
-Weffc++ is
specified.
-Wreorder
(C ++ and Objective-C ++
only)
Warn
when the order of member initializers given in the code does
not match the order in which they must be executed. For
instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
The
compiler will rearrange the member initializers for i
and j to match the declaration order of the members,
emitting a warning to that effect. This warning is enabled
by
-Wall.
The
following -W... options are not affected by
-Wall.
-Weffc++ (C ++ and
Objective-C ++
only)
Warn
about violations of the following style guidelines from
Scott Meyers’ Effective C ++ ,
Second Edition
book:
•
Item 11: Define a copy
constructor and an assignment operator for classes with
dynamically allocated
memory.
•
Item 12: Prefer
initialization to assignment in
constructors.
•
Item 14: Make destructors
virtual in base
classes.
•
Item 15: Have
"operator=" return a reference to
*this.
•
Item 23: Don’t try
to return a reference when you must return an
object.
Also
warn about violations of the following style guidelines from
Scott Meyers’ More Effective C ++
book:
•
Item
6: Distinguish between prefix and postfix forms of increment
and decrement
operators.
•
Item 7: Never overload
"&&", "||", or
",".
When
selecting this option, be aware that the standard library
headers do not obey all of these guidelines; use grep
-v to filter out those
warnings.
-Wstrict-null-sentinel
(C ++ and Objective-C ++
only)
Warn
also about the use of an uncasted "NULL"
as sentinel. When compiling only with GCC this is
a valid sentinel, as "NULL" is defined to
"__null". Although it is a null pointer
constant not a null pointer, it is guaranteed to be of the
same size as a pointer. But this use is not portable across
different compilers.
-Wno-non-template-friend
(C ++ and Objective-C ++
only)
Disable
warnings when non-templatized friend functions are declared
within a template. Since the advent of explicit template
specification support in G++, if the name of the friend is
an unqualified-id (i.e., friend foo(int)), the
C ++ language specification demands that the
friend declare or define an ordinary, nontemplate function.
(Section 14.5.3). Before G++ implemented explicit
specification, unqualified-ids could be interpreted as a
particular specialization of a templatized function. Because
this non-conforming behavior is no longer the default
behavior for G++,
-Wnon-template-friend allows the
compiler to check existing code for potential trouble spots
and is on by default. This new compiler behavior can be
turned off with
-Wno-non-template-friend,
which keeps the conformant compiler code but disables the
helpful warning.
-Wold-style-cast
(C ++ and Objective-C ++
only)
Warn
if an old-style (C-style) cast to a non-void type is
used within a C ++ program. The new-style casts
(dynamic_cast, static_cast,
reinterpret_cast, and const_cast) are less
vulnerable to unintended effects and much easier to search
for.
-Woverloaded-virtual
(C ++ and Objective-C ++
only)
Warn
when a function declaration hides virtual functions from a
base class. For example,
in:
struct A {
virtual void f();
struct B: public A {
void f(int);
};
the
"A" class version of
"f" is hidden in "B",
and code like:
B* b;
b->f();
will
fail to compile.
-Wno-pmf-conversions
(C ++ and Objective-C ++
only)
Disable
the diagnostic for converting a bound pointer to member
function to a plain
pointer.
-Wsign-promo
(C ++ and Objective-C ++
only)
Warn
when overload resolution chooses a promotion from unsigned
or enumerated type to a signed type, over a conversion to an
unsigned type of the same size. Previous versions of G++
would try to preserve unsignedness, but the standard
mandates the current
behavior.
struct A {
operator int ();
A& operator = (int);
main ()
A a,b;
a = b;
}
In
this example, G++ will synthesize a default A&
operator = (const A&);, while cfront will use the
user-defined operator
=.
Options
Controlling Objective-C and Objective-C
++ Dialects
( NOTE: This manual does not describe the
Objective-C and Objective-C ++ languages
themselves.
This
section describes the command-line options that are only
meaningful for Objective-C and Objective-C ++
programs, but you can also use most of the
language-independent GNU compiler options. For
example, you might compile a file
"some_class.m" like
this:
gcc -g -fgnu-runtime -O -c some_class.m
In
this example, -fgnu-runtime is an option
meant only for Objective-C and Objective-C ++
programs; you can use the other options with any
language supported by GCC
.
Note
that since Objective-C is an extension of the C language,
Objective-C compilations may also use options specific to
the C front-end (e.g., -Wtraditional).
Similarly, Objective-C ++ compilations may
use C ++ -specific options (e.g.,
-Wabi).
Here
is a list of options that are only for compiling
Objective-C and Objective-C ++ programs:
-fconstant-string-class=class-name
Use
class-name as the name of the class to instantiate
for each literal string specified with the syntax
"@"..."". The default class
name is "NXConstantString" if the
GNU runtime is being used, and
"NSConstantString" if the NeXT runtime is
being used (see below). The
-fconstant-cfstrings option, if also
present, will override the
-fconstant-string-class setting and
cause "@"..."" literals to be
laid out as constant CoreFoundation
strings.
-fgnu-runtime
Generate
object code compatible with the standard GNU
Objective-C runtime. This is the default for most types
of systems.
-fnext-runtime
Generate
output compatible with the NeXT runtime. This is the default
for NeXT-based systems, including Darwin and Mac OS
X. The macro "__NEXT_RUNTIME__" is
predefined if (and only if) this option is
used.
-fno-nil-receivers
Assume
that all Objective-C message dispatches ("[receiver
message:arg]") in this translation unit ensure
that the receiver is not "nil". This
allows for more efficient entry points in the runtime to be
used. This option is only available in conjunction with the
NeXT runtime and ABI version 0 or
1.
-fobjc-abi-version=n
Use
version n of the Objective-C ABI for the
selected runtime. This option is currently supported only
for the NeXT runtime. In that case, Version 0 is the
traditional (32-bit) ABI without support
for properties and other Objective-C 2.0 additions. Version
1 is the traditional (32-bit) ABI with
support for properties and other Objective-C 2.0 additions.
Version 2 is the modern (64-bit) ABI . If
nothing is specified, the default is Version 0 on
32-bit target machines, and Version 2 on 64-bit
target machines.
-fobjc-call-cxx-cdtors
For
each Objective-C class, check if any of its instance
variables is a C ++ object with a non-trivial
default constructor. If so, synthesize a special
"- (id) .cxx_construct" instance
method which will run non-trivial default constructors on
any such instance variables, in order, and then return
"self". Similarly, check if any instance
variable is a C ++ object with a non-trivial
destructor, and if so, synthesize a special
"- (void) .cxx_destruct" method
which will run all such default destructors, in reverse
order.
The
"- (id) .cxx_construct" and
"- (void) .cxx_destruct" methods
thusly generated will only operate on instance variables
declared in the current Objective-C class, and not those
inherited from superclasses. It is the responsibility of the
Objective-C runtime to invoke all such methods in an
object’s inheritance hierarchy. The "-
(id) .cxx_construct" methods will be invoked by
the runtime immediately after a new object instance is
allocated; the "- (void)
.cxx_destruct" methods will be invoked immediately
before the runtime deallocates an object
instance.
As
of this writing, only the NeXT runtime on Mac OS
X 10.4 and later has support for invoking the
"- (id) .cxx_construct" and
"- (void) .cxx_destruct"
methods.
-fobjc-direct-dispatch
Allow
fast jumps to the message dispatcher. On Darwin this is
accomplished via the comm
page.
-fobjc-exceptions
Enable
syntactic support for structured exception handling in
Objective-C, similar to what is offered by C ++
and Java. This option is required to use the
Objective-C keywords @try, @throw,
@catch, @finally and
@synchronized. This option is available with both
the GNU runtime and the NeXT runtime (but not
available in conjunction with the NeXT runtime on Mac
OS X 10.2 and
earlier).
-fobjc-gc
Enable
garbage collection ( GC ) in Objective-C and
Objective-C ++ programs. This option is
only available with the NeXT runtime; the GNU
runtime has a different garbage collection
implementation that does not require special compiler
flags.
-fobjc-nilcheck
For
the NeXT runtime with version 2 of the ABI ,
check for a nil receiver in method invocations before doing
the actual method call. This is the default and can be
disabled using -fno-objc-nilcheck.
Class methods and super calls are never checked for nil in
this way no matter what this flag is set to. Currently this
flag does nothing when the GNU runtime, or an
older version of the NeXT runtime ABI , is
used.
-fobjc-std=objc1
Conform
to the language syntax of Objective-C 1.0, the language
recognized by GCC 4.0. This only affects the
Objective-C additions to the C/C ++ language; it
does not affect conformance to C/C ++ standards,
which is controlled by the separate C/C ++
dialect option flags. When this option is used with the
Objective-C or Objective-C ++ compiler, any
Objective-C syntax that is not recognized by GCC
4.0 is rejected. This is useful if you need to make
sure that your Objective-C code can be compiled with older
versions of GCC
.
-freplace-objc-classes
Emit
a special marker instructing ld(1) not
to statically link in the resulting object file, and allow
dyld(1) to load it in at run time
instead. This is used in conjunction with the
Fix-and-Continue debugging mode, where the object file in
question may be recompiled and dynamically reloaded in the
course of program execution, without the need to restart the
program itself. Currently, Fix-and-Continue functionality is
only available in conjunction with the NeXT runtime on
Mac OS X 10.3 and
later.
-fzero-link
When
compiling for the NeXT runtime, the compiler ordinarily
replaces calls to
"objc_getClass("...")" (when
the name of the class is known at compile time) with static
class references that get initialized at load time, which
improves run-time performance. Specifying the
-fzero-link flag suppresses this behavior
and causes calls to
"objc_getClass("...")" to be
retained. This is useful in Zero-Link debugging mode, since
it allows for individual class implementations to be
modified during program execution. The GNU
runtime currently always retains calls to
"objc_get_class("...")"
regardless of command-line
options.
-gen-decls
Dump
interface declarations for all classes seen in the source
file to a file named
sourcename.decl.
-Wassign-intercept
(Objective-C and Objective-C ++
only)
Warn
whenever an Objective-C assignment is being intercepted by
the garbage
collector.
-Wno-protocol
(Objective-C and Objective-C ++
only)
If
a class is declared to implement a protocol, a warning is
issued for every method in the protocol that is not
implemented by the class. The default behavior is to issue a
warning for every method not explicitly implemented in the
class, even if a method implementation is inherited from the
superclass. If you use the -Wno-protocol
option, then methods inherited from the superclass are
considered to be implemented, and no warning is issued for
them.
-Wselector
(Objective-C and Objective-C ++
only)
Warn
if multiple methods of different types for the same selector
are found during compilation. The check is performed on the
list of methods in the final stage of compilation.
Additionally, a check is performed for each selector
appearing in a "@selector(...)"
expression, and a corresponding method for that selector has
been found during compilation. Because these checks scan the
method table only at the end of compilation, these warnings
are not produced if the final stage of compilation is not
reached, for example because an error is found during
compilation, or because the -fsyntax-only
option is being
used.
-Wstrict-selector-match
(Objective-C and Objective-C ++
only)
Warn
if multiple methods with differing argument and/or return
types are found for a given selector when attempting to send
a message using this selector to a receiver of type
"id" or "Class". When
this flag is off (which is the default behavior), the
compiler will omit such warnings if any differences found
are confined to types that share the same size and
alignment.
-Wundeclared-selector
(Objective-C and Objective-C ++
only)
Warn
if a "@selector(...)" expression
referring to an undeclared selector is found. A selector is
considered undeclared if no method with that name has been
declared before the "@selector(...)"
expression, either explicitly in an @interface or
@protocol declaration, or implicitly in an
@implementation section. This option always
performs its checks as soon as a
"@selector(...)" expression is found,
while -Wselector only performs its checks in
the final stage of compilation. This also enforces the
coding style convention that methods and selectors must be
declared before being
used.
-print-objc-runtime-info
Generate
C header describing the largest structure that is passed by
value, if any.
Options
to Control Diagnostic Messages Formatting
Traditionally, diagnostic messages have been formatted
irrespective of the output device’s aspect (e.g. its
width, ...). The options described below can be used to
control the diagnostic messages formatting algorithm, e.g.
how many characters per line, how often source location
information should be reported. Right now, only the C
++ front end can honor these options. However it is
expected, in the near future, that the remaining front ends
would be able to digest them correctly.
-fmessage-length=n
Try
to format error messages so that they fit on lines of about
n characters. The default is 72 characters for
g++ and 0 for the rest of the front ends supported
by GCC . If n is zero, then no
line-wrapping will be done; each error message will appear
on a single line.
-fdiagnostics-show-location=once
Only
meaningful in line-wrapping mode. Instructs the diagnostic
messages reporter to emit once source location
information; that is, in case the message is too long to fit
on a single physical line and has to be wrapped, the source
location won’t be emitted (as prefix) again, over and
over, in subsequent continuation lines. This is the default
behavior.
-fdiagnostics-show-location=every-line
Only
meaningful in line-wrapping mode. Instructs the diagnostic
messages reporter to emit the same source location
information (as prefix) for physical lines that result from
the process of breaking a message which is too long to fit
on a single line.
-fno-diagnostics-show-option
By
default, each diagnostic emitted includes text indicating
the command-line option that directly controls the
diagnostic (if such an option is known to the diagnostic
machinery). Specifying the
-fno-diagnostics-show-option
flag suppresses that
behavior.
Options
to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions
that are not inherently erroneous but that are risky or
suggest there may have been an
error.
The
following language-independent options do not enable
specific warnings but control the kinds of diagnostics
produced by GCC .
-fsyntax-only
Check
the code for syntax errors, but don’t do anything
beyond that.
-fmax-errors=n
Limits
the maximum number of error messages to n, at which
point GCC bails out rather than attempting to
continue processing the source code. If n is 0 (the
default), there is no limit on the number of error messages
produced. If -Wfatal-errors is also
specified, then -Wfatal-errors takes
precedence over this
option.
-w
Inhibit all warning
messages.
-Werror
Make
all warnings into
errors.
-Werror=
Make
the specified warning into an error. The specifier for a
warning is appended, for example -Werror=switch
turns the warnings controlled by -Wswitch into
errors. This switch takes a negative form, to be used to
negate -Werror for specific warnings, for
example -Wno-error=switch makes
-Wswitch warnings not be errors, even when
-Werror is in
effect.
The
warning message for each controllable warning includes the
option that controls the warning. That option can then be
used with -Werror= and
-Wno-error= as described above. (Printing
of the option in the warning message can be disabled using
the
-fno-diagnostics-show-option
flag.)
Note
that specifying -Werror=foo
automatically implies -Wfoo. However,
-Wno-error=foo does not imply
anything.
-Wfatal-errors
This
option causes the compiler to abort compilation on the first
error occurred rather than trying to keep going and printing
further error
messages.
You
can request many specific warnings with options beginning
-W, for example -Wimplicit to
request warnings on implicit declarations. Each of these
specific warning options also has a negative form beginning
-Wno- to turn off warnings; for example,
-Wno-implicit. This manual lists only one
of the two forms, whichever is not the default. For further,
language-specific options also refer to C ++
Dialect Options and Objective-C and
Objective-C ++ Dialect
Options.
When
an unrecognized warning option is requested (e.g.,
-Wunknown-warning), GCC will
emit a diagnostic stating that the option is not recognized.
However, if the -Wno- form is used, the
behavior is slightly different: No diagnostic will be
produced for -Wno-unknown-warning
unless other diagnostics are being produced. This allows the
use of new -Wno- options with old
compilers, but if something goes wrong, the compiler will
warn that an unrecognized option was used.
-pedantic
Issue
all the warnings demanded by strict ISO C
and ISO C ++ ; reject all programs
that use forbidden extensions, and some other programs that
do not follow ISO C and ISO C ++
. For ISO C, follows the version of
the ISO C standard specified by any
-std option
used.
Valid
ISO C and ISO C ++ programs
should compile properly with or without this option (though
a rare few will require -ansi or a
-std option specifying the required version
of ISO C). However, without this option,
certain GNU extensions and traditional C and
C ++ features are supported as well. With this
option, they are
rejected.
-pedantic
does not cause warning messages for use of the alternate
keywords whose names begin and end with __. Pedantic
warnings are also disabled in the expression that follows
"__extension__". However, only system
header files should use these escape routes; application
programs should avoid
them.
Some
users try to use -pedantic to check programs
for strict ISO C conformance. They soon find that
it does not do quite what they want: it finds some non-ISO
practices, but not all---only those for
which ISO C requires a diagnostic, and
some others for which diagnostics have been
added.
A
feature to report any failure to conform to ISO C
might be useful in some instances, but would require
considerable additional work and would be quite different
from -pedantic. We don’t have plans to
support such a feature in the near
future.
Where
the standard specified with -std represents
a GNU extended dialect of C, such as gnu90
or gnu99, there is a corresponding base
standard, the version of ISO C on which
the GNU extended dialect is based. Warnings from
-pedantic are given where they are required by
the base standard. (It would not make sense for such
warnings to be given only for features not in the
specified GNU C dialect, since by definition
the GNU dialects of C include all features the
compiler supports with the given option, and there would be
nothing to warn
about.)
-pedantic-errors
Like
-pedantic, except that errors are produced
rather than
warnings.
-Wall
This
enables all the warnings about constructions that some users
consider questionable, and that are easy to avoid (or modify
to prevent the warning), even in conjunction with macros.
This also enables some language-specific warnings described
in C ++ Dialect Options
and Objective-C and Objective-C
++ Dialect
Options.
-Wall
turns on the following warning
flags:
-Waddress
-Warray-bounds (only with -O2)
-Wc++11-compat -Wchar-subscripts
-Wenum-compare (in C/Objc; this is on by
default in C ++ )
-Wimplicit-int (C and Objective-C only)
-Wimplicit-function-declaration (C
and Objective-C only) -Wcomment -Wformat
-Wmain (only for C/ObjC and unless
-ffreestanding)
-Wmaybe-uninitialized
-Wmissing-braces -Wnonnull
-Wparentheses -Wpointer-sign
-Wreorder -Wreturn-type
-Wsequence-point -Wsign-compare
(only in C ++ ) -Wstrict-aliasing
-Wstrict-overflow=1 -Wswitch
-Wtrigraphs -Wuninitialized
-Wunknown-pragmas -Wunused-function
-Wunused-label -Wunused-value
-Wunused-variable
-Wvolatile-register-var
Note
that some warning flags are not implied by
-Wall. Some of them warn about constructions
that users generally do not consider questionable, but which
occasionally you might wish to check for; others warn about
constructions that are necessary or hard to avoid in some
cases, and there is no simple way to modify the code to
suppress the warning. Some of them are enabled by
-Wextra but many of them must be enabled
individually.
-Wextra
This
enables some extra warning flags that are not enabled by
-Wall. (This option used to be called
-W. The older name is still supported, but the
newer name is more
descriptive.)
-Wclobbered
-Wempty-body -Wignored-qualifiers
-Wmissing-field-initializers
-Wmissing-parameter-type (C only)
-Wold-style-declaration (C only)
-Woverride-init -Wsign-compare
-Wtype-limits -Wuninitialized
-Wunused-parameter (only with
-Wunused or -Wall)
-Wunused-but-set-parameter
(only with -Wunused or
-Wall)
The
option -Wextra also prints warning messages for
the following cases:
•
A pointer is compared
against integer zero with <, <=,
>, or
>=.
•
(C ++ only) An
enumerator and a non-enumerator both appear in a conditional
expression.
•
(C ++ only)
Ambiguous virtual
bases.
•
(C ++ only)
Subscripting an array that has been declared
register.
•
(C ++ only)
Taking the address of a variable that has been declared
register.
•
(C ++ only) A
base class is not initialized in a derived class’ copy
constructor.
-Wchar-subscripts
Warn
if an array subscript has type "char".
This is a common cause of error, as programmers often forget
that this type is signed on some machines. This warning is
enabled by
-Wall.
-Wcomment
Warn
whenever a comment-start sequence /* appears in a
/* comment, or whenever a Backslash-Newline appears
in a // comment. This warning is enabled by
-Wall.
-Wno-coverage-mismatch
Warn
if feedback profiles do not match when using the
-fprofile-use option. If a source file
was changed between -fprofile-gen and
-fprofile-use, the files with the profile
feedback can fail to match the source file and GCC
cannot use the profile feedback information. By
default, this warning is enabled and is treated as an error.
-Wno-coverage-mismatch can be used
to disable the warning or
-Wno-error=coverage-mismatch can be
used to disable the error. Disabling the error for this
warning can result in poorly optimized code and is useful
only in the case of very minor changes such as bug fixes to
an existing code-base. Completely disabling the warning is
not recommended.
-Wno-cpp
(C,
Objective-C, C ++ , Objective-C ++
and Fortran
only)
Suppress
warning messages emitted by "#warning"
directives.
-Wdouble-promotion
(C, C ++ , Objective-C and
Objective-C ++
only)
Give
a warning when a value of type "float" is
implicitly promoted to "double". CPUs
with a 32-bit "single-precision"
floating-point unit implement "float" in
hardware, but emulate "double" in
software. On such a machine, doing computations using
"double" values is much more expensive
because of the overhead required for software
emulation.
It
is easy to accidentally do computations with
"double" because floating-point literals
are implicitly of type "double". For
example, in:
float area(float radius)
return 3.14159 * radius * radius;
}
the
compiler will perform the entire computation with
"double" because the floating-point
literal is a
"double".
-Wformat
Check
calls to "printf" and
"scanf", etc., to make sure that the
arguments supplied have types appropriate to the format
string specified, and that the conversions specified in the
format string make sense. This includes standard functions,
and others specified by format attributes, in the
"printf", "scanf",
"strftime" and
"strfmon" (an X/Open extension, not in
the C standard) families (or other target-specific
families). Which functions are checked without format
attributes having been specified depends on the standard
version selected, and such checks of functions without the
attribute specified are disabled by
-ffreestanding or
-fno-builtin.
The
formats are checked against the format features supported
by GNU libc version 2.2. These include all
ISO C90 and C99 features, as well as features from the
Single Unix Specification and some BSD and
GNU extensions. Other library implementations may not
support all these features; GCC does not support
warning about features that go beyond a particular
library’s limitations. However, if
-pedantic is used with -Wformat,
warnings will be given about format features not in the
selected standard version (but not for
"strfmon" formats, since those are not in
any version of the C
standard).
Since
-Wformat also checks for null format arguments
for several functions, -Wformat also implies
-Wnonnull.
-Wformat
is included in -Wall. For more control over
some aspects of format checking, the options
-Wformat-y2k,
-Wno-format-extra-args,
-Wno-format-zero-length,
-Wformat-nonliteral,
-Wformat-security, and
-Wformat=2 are available, but are not included
in
-Wall.
NOTE:
In Ubuntu 8.10 and later versions this option is
enabled by default for C, C ++ , ObjC, ObjC++. To
disable, use
-Wformat=0.
-Wformat-y2k
If
-Wformat is specified, also warn about
"strftime" formats that may yield only a
two-digit year.
-Wno-format-contains-nul
If
-Wformat is specified, do not warn about format
strings that contain NUL
bytes.
-Wno-format-extra-args
If
-Wformat is specified, do not warn about excess
arguments to a "printf" or
"scanf" format function. The C standard
specifies that such arguments are
ignored.
Where
the unused arguments lie between used arguments that are
specified with $ operand number specifications,
normally warnings are still given, since the implementation
could not know what type to pass to
"va_arg" to skip the unused arguments.
However, in the case of "scanf" formats,
this option will suppress the warning if the unused
arguments are all pointers, since the Single Unix
Specification says that such unused arguments are
allowed.
-Wno-format-zero-length
If
-Wformat is specified, do not warn about
zero-length formats. The C standard specifies that
zero-length formats are
allowed.
-Wformat-nonliteral
If
-Wformat is specified, also warn if the format
string is not a string literal and so cannot be checked,
unless the format function takes its format arguments as a
"va_list".
-Wformat-security
If
-Wformat is specified, also warn about uses of
format functions that represent possible security problems.
At present, this warns about calls to
"printf" and "scanf"
functions where the format string is not a string literal
and there are no format arguments, as in "printf
(foo);". This may be a security hole if the format
string came from untrusted input and contains %n.
(This is currently a subset of what
-Wformat-nonliteral warns about, but in
future warnings may be added to
-Wformat-security that are not included
in
-Wformat-nonliteral.)
NOTE:
In Ubuntu 8.10 and later versions this option is
enabled by default for C, C ++ , ObjC, ObjC++. To
disable, use -Wno-format-security,
or disable all format warnings with -Wformat=0.
To make format security warnings fatal, specify
-Werror=format-security.
-Wformat=2
Enable
-Wformat plus format checks not included in
-Wformat. Currently equivalent to
-Wformat -Wformat-nonliteral
-Wformat-security
-Wformat-y2k.
-Wnonnull
Warn
about passing a null pointer for arguments marked as
requiring a non-null value by the
"nonnull" function
attribute.
-Wnonnull
is included in -Wall and -Wformat.
It can be disabled with the -Wno-nonnull
option.
-Winit-self
(C, C ++ , Objective-C and
Objective-C ++
only)
Warn
about uninitialized variables that are initialized with
themselves. Note this option can only be used with the
-Wuninitialized
option.
For
example, GCC will warn about
"i" being uninitialized in the following
snippet only when -Winit-self has been
specified:
int f()
int i = i;
return i;
}
-Wimplicit-int
(C and Objective-C
only)
Warn
when a declaration does not specify a type. This warning is
enabled by
-Wall.
-Wimplicit-function-declaration
(C and Objective-C
only)
Give
a warning whenever a function is used before being declared.
In C99 mode (-std=c99 or
-std=gnu99), this warning is enabled by default
and it is made into an error by
-pedantic-errors. This warning is also
enabled by
-Wall.
-Wimplicit
(C and Objective-C
only)
Same
as -Wimplicit-int and
-Wimplicit-function-declaration.
This warning is enabled by
-Wall.
-Wignored-qualifiers
(C and C ++
only)
Warn
if the return type of a function has a type qualifier such
as "const". For ISO C such a
type qualifier has no effect, since the value returned by a
function is not an lvalue. For C ++ , the warning
is only emitted for scalar types or
"void". ISO C prohibits
qualified "void" return types on function
definitions, so such return types always receive a warning
even without this
option.
This
warning is also enabled by
-Wextra.
-Wmain
Warn
if the type of main is suspicious. main should
be a function with external linkage, returning int, taking
either zero arguments, two, or three arguments of
appropriate types. This warning is enabled by default in
C ++ and is enabled by either -Wall
or
-pedantic.
-Wmissing-braces
Warn
if an aggregate or union initializer is not fully bracketed.
In the following example, the initializer for a is
not fully bracketed, but that for b is fully
bracketed.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
This
warning is enabled by
-Wall.
-Wmissing-include-dirs
(C, C ++ , Objective-C and
Objective-C ++
only)
Warn
if a user-supplied include directory does not
exist.
-Wparentheses
Warn
if parentheses are omitted in certain contexts, such as when
there is an assignment in a context where a truth value is
expected, or when operators are nested whose precedence
people often get confused
about.
Also
warn if a comparison like x<=y<=z appears; this
is equivalent to (x<=y ? 1 : 0) <= z, which is
a different interpretation from that of ordinary
mathematical
notation.
Also
warn about constructions where there may be confusion to
which "if" statement an
"else" branch belongs. Here is an example
of such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}
In
C/C ++ , every "else" branch
belongs to the innermost possible "if"
statement, which in this example is "if
(b)". This is often not what the programmer
expected, as illustrated in the above example by indentation
the programmer chose. When there is the potential for this
confusion, GCC will issue a warning when this
flag is specified. To eliminate the warning, add explicit
braces around the innermost "if"
statement so there is no way the "else"
could belong to the enclosing "if". The
resulting code would look like
this:
{
if (a)
if (b)
foo ();
else
bar ();
}
Also
warn for dangerous uses of the ?: with omitted middle
operand GNU extension. When the condition in the
?: operator is a boolean expression the omitted value will
be always 1. Often the user expects it to be a value
computed inside the conditional expression
instead.
This
warning is enabled by
-Wall.
-Wsequence-point
Warn
about code that may have undefined semantics because of
violations of sequence point rules in the C and C ++
standards.
The
C and C ++ standards defines the order in which
expressions in a C/C ++ program are evaluated in
terms of sequence points, which represent a partial
ordering between the execution of parts of the program:
those executed before the sequence point, and those executed
after it. These occur after the evaluation of a full
expression (one which is not part of a larger expression),
after the evaluation of the first operand of a
"&&", "||",
"? :" or "," (comma)
operator, before a function is called (but after the
evaluation of its arguments and the expression denoting the
called function), and in certain other places. Other than as
expressed by the sequence point rules, the order of
evaluation of subexpressions of an expression is not
specified. All these rules describe only a partial order
rather than a total order, since, for example, if two
functions are called within one expression with no sequence
point between them, the order in which the functions are
called is not specified. However, the standards committee
have ruled that function calls do not
overlap.
It
is not specified when between sequence points modifications
to the values of objects take effect. Programs whose
behavior depends on this have undefined behavior; the C and
C ++ standards specify that "Between the
previous and next sequence point an object shall have its
stored value modified at most once by the evaluation of an
expression. Furthermore, the prior value shall be read only
to determine the value to be stored.". If a program
breaks these rules, the results on any particular
implementation are entirely
unpredictable.
Examples
of code with undefined behavior are "a =
a++;", "a[n] = b[n++]" and
"a[i++] = i;". Some more complicated
cases are not diagnosed by this option, and it may give an
occasional false positive result, but in general it has been
found fairly effective at detecting this sort of problem in
programs.
The
standard is worded confusingly, therefore there is some
debate over the precise meaning of the sequence point rules
in subtle cases. Links to discussions of the problem,
including proposed formal definitions, may be found on
the GCC readings page, at
<http://gcc.gnu.org/readings.html>.
This
warning is enabled by -Wall for C and C
++ .
-Wreturn-type
Warn
whenever a function is defined with a return-type that
defaults to "int". Also warn about any
"return" statement with no return-value
in a function whose return-type is not
"void" (falling off the end of the
function body is considered returning without a value), and
about a "return" statement with an
expression in a function whose return-type is
"void".
For
C ++ , a function without return type always
produces a diagnostic message, even when
-Wno-return-type is specified. The
only exceptions are main and functions defined in
system headers.
This
warning is enabled by
-Wall.
-Wswitch
Warn
whenever a "switch" statement has an
index of enumerated type and lacks a
"case" for one or more of the named codes
of that enumeration. (The presence of a
"default" label prevents this warning.)
"case" labels outside the enumeration
range also provoke warnings when this option is used (even
if there is a "default" label). This
warning is enabled by
-Wall.
-Wswitch-default
Warn
whenever a "switch" statement does not
have a "default"
case.
-Wswitch-enum
Warn
whenever a "switch" statement has an
index of enumerated type and lacks a
"case" for one or more of the named codes
of that enumeration. "case" labels
outside the enumeration range also provoke warnings when
this option is used. The only difference between
-Wswitch and this option is that this option
gives a warning about an omitted enumeration code even if
there is a "default"
label.
-Wsync-nand
(C and C ++
only)
Warn
when "__sync_fetch_and_nand" and
"__sync_nand_and_fetch" built-in
functions are used. These functions changed semantics
in GCC
4.4.
-Wtrigraphs
Warn
if any trigraphs are encountered that might change the
meaning of the program (trigraphs within comments are not
warned about). This warning is enabled by
-Wall.
-Wunused-but-set-parameter
Warn
whenever a function parameter is assigned to, but otherwise
unused (aside from its
declaration).
To
suppress this warning use the unused
attribute.
This
warning is also enabled by -Wunused together
with
-Wextra.
-Wunused-but-set-variable
Warn
whenever a local variable is assigned to, but otherwise
unused (aside from its declaration). This warning is enabled
by
-Wall.
To
suppress this warning use the unused
attribute.
This
warning is also enabled by -Wunused, which is
enabled by
-Wall.
-Wunused-function
Warn
whenever a static function is declared but not defined or a
non-inline static function is unused. This warning is
enabled by
-Wall.
-Wunused-label
Warn
whenever a label is declared but not used. This warning is
enabled by
-Wall.
To
suppress this warning use the unused
attribute.
-Wunused-local-typedefs
(C, Objective-C, C ++ and Objective-C
++ only)
Warn
when a typedef locally defined in a function is not
used.
-Wunused-parameter
Warn
whenever a function parameter is unused aside from its
declaration.
To
suppress this warning use the unused
attribute.
-Wno-unused-result
Do
not warn if a caller of a function marked with attribute
"warn_unused_result" does not use its
return value. The default is
-Wunused-result.
-Wunused-variable
Warn
whenever a local variable or non-constant static variable is
unused aside from its declaration. This warning is enabled
by
-Wall.
To
suppress this warning use the unused
attribute.
-Wunused-value
Warn
whenever a statement computes a result that is explicitly
not used. To suppress this warning cast the unused
expression to void. This includes an
expression-statement or the left-hand side of a comma
expression that contains no side effects. For example, an
expression such as x[i,j] will cause a warning, while
x[(void)i,j] will
not.
This
warning is enabled by
-Wall.
-Wunused
All
the above -Wunused options
combined.
In
order to get a warning about an unused function parameter,
you must either specify -Wextra -Wunused
(note that -Wall implies
-Wunused), or separately specify
-Wunused-parameter.
-Wuninitialized
Warn
if an automatic variable is used without first being
initialized or if a variable may be clobbered by a
"setjmp" call. In C ++ , warn
if a non-static reference or non-static const member
appears in a class without
constructors.
If
you want to warn about code that uses the uninitialized
value of the variable in its own initializer, use the
-Winit-self
option.
These
warnings occur for individual uninitialized or clobbered
elements of structure, union or array variables as well as
for variables that are uninitialized or clobbered as a
whole. They do not occur for variables or elements declared
"volatile". Because these warnings depend
on optimization, the exact variables or elements for which
there are warnings will depend on the precise optimization
options and version of GCC
used.
Note
that there may be no warning about a variable that is used
only to compute a value that itself is never used, because
such computations may be deleted by data flow analysis
before the warnings are
printed.
-Wmaybe-uninitialized
For
an automatic variable, if there exists a path from the
function entry to a use of the variable that is initialized,
but there exist some other paths the variable is not
initialized, the compiler will emit a warning if it can not
prove the uninitialized paths do not happen at run time.
These warnings are made optional because GCC is
not smart enough to see all the reasons why the code might
be correct despite appearing to have an error. Here is one
example of how this can
happen:
{
int x;
switch (y)
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
foo (x);
}
If
the value of "y" is always 1, 2 or 3,
then "x" is always initialized, but
GCC doesn’t know this. To suppress the warning,
the user needs to provide a default case with
assert(0) or similar
code.
This
option also warns when a non-volatile automatic variable
might be changed by a call to "longjmp".
These warnings as well are possible only in optimizing
compilation.
The
compiler sees only the calls to "setjmp".
It cannot know where "longjmp" will be
called; in fact, a signal handler could call it at any point
in the code. As a result, you may get a warning even when
there is in fact no problem because
"longjmp" cannot in fact be called at the
place that would cause a
problem.
Some
spurious warnings can be avoided if you declare all the
functions you use that never return as
"noreturn".
This
warning is enabled by -Wall or
-Wextra.
-Wunknown-pragmas
Warn
when a "#pragma" directive is encountered
that is not understood by GCC . If this
command-line option is used, warnings will even be issued
for unknown pragmas in system header files. This is not the
case if the warnings were only enabled by the
-Wall command-line
option.
-Wno-pragmas
Do
not warn about misuses of pragmas, such as incorrect
parameters, invalid syntax, or conflicts between pragmas.
See also
-Wunknown-pragmas.
-Wstrict-aliasing
This
option is only active when
-fstrict-aliasing is active. It warns
about code that might break the strict aliasing rules that
the compiler is using for optimization. The warning does not
catch all cases, but does attempt to catch the more common
pitfalls. It is included in -Wall. It is
equivalent to
-Wstrict-aliasing=3
-Wstrict-aliasing=n
This
option is only active when
-fstrict-aliasing is active. It warns
about code that might break the strict aliasing rules that
the compiler is using for optimization. Higher levels
correspond to higher accuracy (fewer false positives).
Higher levels also correspond to more effort, similar to the
way -O works. -Wstrict-aliasing is
equivalent to -Wstrict-aliasing=n, with
n=3.
Level
1: Most aggressive, quick, least accurate. Possibly useful
when higher levels do not warn but
-fstrict-aliasing still breaks the code, as it
has very few false negatives. However, it has many false
positives. Warns for all pointer conversions between
possibly incompatible types, even if never dereferenced.
Runs in the front end
only.
Level
2: Aggressive, quick, not too precise. May still have many
false positives (not as many as level 1 though), and few
false negatives (but possibly more than level 1). Unlike
level 1, it only warns when an address is taken. Warns about
incomplete types. Runs in the front end
only.
Level
3 (default for -Wstrict-aliasing): Should
have very few false positives and few false negatives.
Slightly slower than levels 1 or 2 when optimization is
enabled. Takes care of the common pun+dereference pattern in
the front end: "*(int*)&some_float".
If optimization is enabled, it also runs in the back end,
where it deals with multiple statement cases using
flow-sensitive points-to information. Only warns when the
converted pointer is dereferenced. Does not warn about
incomplete types.
-Wstrict-overflow
-Wstrict-overflow=n
This
option is only active when
-fstrict-overflow is active. It warns
about cases where the compiler optimizes based on the
assumption that signed overflow does not occur. Note that it
does not warn about all cases where the code might overflow:
it only warns about cases where the compiler implements some
optimization. Thus this warning depends on the optimization
level.
An
optimization that assumes that signed overflow does not
occur is perfectly safe if the values of the variables
involved are such that overflow never does, in fact, occur.
Therefore this warning can easily give a false positive: a
warning about code that is not actually a problem. To help
focus on important issues, several warning levels are
defined. No warnings are issued for the use of undefined
signed overflow when estimating how many iterations a loop
will require, in particular when determining whether a loop
will be executed at all.
-Wstrict-overflow=1
Warn
about cases that are both questionable and easy to avoid.
For example: "x + 1 > x"; with
-fstrict-overflow, the compiler will
simplify this to 1. This level of
-Wstrict-overflow is enabled by
-Wall; higher levels are not, and must be
explicitly
requested.
-Wstrict-overflow=2
Also
warn about other cases where a comparison is simplified to a
constant. For example: "abs (x) >= 0".
This can only be simplified when
-fstrict-overflow is in effect, because
"abs (INT_MIN)" overflows to
"INT_MIN", which is less than zero.
-Wstrict-overflow (with no level) is the
same as
-Wstrict-overflow=2.
-Wstrict-overflow=3
Also
warn about other cases where a comparison is simplified. For
example: "x + 1 > 1" will be
simplified to "x >
0".
-Wstrict-overflow=4
Also
warn about other simplifications not covered by the above
cases. For example: "(x * 10) / 5" will
be simplified to "x *
2".
-Wstrict-overflow=5
Also
warn about cases where the compiler reduces the magnitude of
a constant involved in a comparison. For example:
"x + 2 > y" will be simplified to
"x + 1 >= y". This is reported only at
the highest warning level because this simplification
applies to many comparisons, so this warning level will give
a very large number of false
positives.
-Wsuggest-attribute=[pure|const|noreturn]
Warn
for cases where adding an attribute may be beneficial. The
attributes currently supported are listed below.
-Wsuggest-attribute=pure
-Wsuggest-attribute=const
-Wsuggest-attribute=noreturn
Warn
about functions that might be candidates for attributes
"pure", "const" or
"noreturn". The compiler only warns for
functions visible in other compilation units or (in the case
of "pure" and "const")
if it cannot prove that the function returns normally. A
function returns normally if it doesn’t contain an
infinite loop nor returns abnormally by throwing, calling
"abort()" or trapping. This analysis
requires option -fipa-pure-const,
which is enabled by default at -O and higher.
Higher optimization levels improve the accuracy of the
analysis.
-Warray-bounds
This
option is only active when -ftree-vrp is
active (default for -O2 and above). It warns
about subscripts to arrays that are always out of bounds.
This warning is enabled by
-Wall.
-Wno-div-by-zero
Do
not warn about compile-time integer division by zero.
Floating-point division by zero is not warned about, as it
can be a legitimate way of obtaining infinities and
NaNs.
-Wsystem-headers
Print
warning messages for constructs found in system header
files. Warnings from system headers are normally suppressed,
on the assumption that they usually do not indicate real
problems and would only make the compiler output harder to
read. Using this command-line option tells GCC to
emit warnings from system headers as if they occurred in
user code. However, note that using -Wall in
conjunction with this option will not warn about
unknown pragmas in system headers---for
that, -Wunknown-pragmas must also be
used.
-Wtrampolines
Warn about trampolines generated for pointers to nested functions.
A trampoline is a small piece of data or code that is created at run
time on the stack when the address of a nested function is taken, and
is used to call the nested function indirectly. For some targets, it
is made up of data only and thus requires no special treatment. But,
for most targets, it is made up of code and thus requires the stack
to be made executable in order for the program to work properly.
-Wfloat-equal
Warn
if floating-point values are used in equality
comparisons.
The
idea behind this is that sometimes it is convenient (for the
programmer) to consider floating-point values as
approximations to infinitely precise real numbers. If you
are doing this, then you need to compute (by analyzing the
code, or in some other way) the maximum or likely maximum
error that the computation introduces, and allow for it when
performing comparisons (and when producing output, but
that’s a different problem). In particular, instead of
testing for equality, you would check to see whether the two
values have ranges that overlap; and this is done with the
relational operators, so equality comparisons are probably
mistaken.
-Wtraditional
(C and Objective-C
only)
Warn
about certain constructs that behave differently in
traditional and ISO C. Also warn about ISO
C constructs that have no traditional C equivalent,
and/or problematic constructs that should be
avoided.
•
Macro parameters that
appear within string literals in the macro body. In
traditional C macro replacement takes place within string
literals, but does not in ISO
C.
•
In traditional C, some
preprocessor directives did not exist. Traditional
preprocessors would only consider a line to be a directive
if the # appeared in column 1 on the line. Therefore
-Wtraditional warns about directives that
traditional C understands but would ignore because the
# does not appear as the first character on the line.
It also suggests you hide directives like #pragma not
understood by traditional C by indenting them. Some
traditional implementations would not recognize
#elif, so it suggests avoiding it
altogether.
•
A function-like macro that
appears without
arguments.
•
The unary plus
operator.
•
The U integer
constant suffix, or the F or L floating-point
constant suffixes. (Traditional C does support the L
suffix on integer constants.) Note, these suffixes appear in
macros defined in the system headers of most modern systems,
e.g. the _MIN/_MAX macros in
"<limits.h>". Use of these macros
in user code might normally lead to spurious warnings,
however GCC ’s integrated preprocessor has
enough context to avoid warning in these
cases.
•
A function declared
external in one block and then used after the end of the
block.
•
A
"switch" statement has an operand of type
"long".
•
A
non-"static" function declaration
follows a "static" one. This construct is
not accepted by some traditional C
compilers.
•
The ISO type of
an integer constant has a different width or signedness from
its traditional type. This warning is only issued if the
base of the constant is ten. I.e. hexadecimal or octal
values, which typically represent bit patterns, are not
warned about.
•
Usage of ISO
string concatenation is
detected.
•
Initialization of
automatic
aggregates.
•
Identifier conflicts with
labels. Traditional C lacks a separate namespace for
labels.
•
Initialization of unions.
If the initializer is zero, the warning is omitted. This is
done under the assumption that the zero initializer in user
code appears conditioned on e.g.
"__STDC__" to avoid missing initializer
warnings and relies on default initialization to zero in the
traditional C case.
•
Conversions by prototypes
between fixed/floating-point values and vice versa.
The absence of these prototypes when compiling with
traditional C would cause serious problems. This is a subset
of the possible conversion warnings, for the full set use
-Wtraditional-conversion.
•
Use of ISO C
style function definitions. This warning intentionally is
not issued for prototype declarations or variadic
functions because these ISO C features will
appear in your code when using libiberty’s traditional
C compatibility macros, "PARAMS" and
"VPARAMS". This warning is also bypassed
for nested functions because that feature is already a
GCC extension and thus not relevant to traditional C
compatibility.
-Wtraditional-conversion
(C and Objective-C
only)
Warn
if a prototype causes a type conversion that is different
from what would happen to the same argument in the absence
of a prototype. This includes conversions of fixed point to
floating and vice versa, and conversions changing the width
or signedness of a fixed-point argument except when the same
as the default
promotion.
-Wdeclaration-after-statement
(C and Objective-C
only)
Warn
when a declaration is found after a statement in a block.
This construct, known from C ++ , was introduced
with ISO C99 and is by default allowed in
GCC . It is not supported by ISO C90 and was
not supported by GCC versions before GCC
3.0.
-Wundef
Warn
if an undefined identifier is evaluated in an #if
directive.
-Wno-endif-labels
Do
not warn whenever an #else or an #endif are
followed by text.
-Wshadow
Warn
whenever a local variable or type declaration shadows
another variable, parameter, type, or class member (in
C ++ ), or whenever a built-in function is
shadowed. Note that in C ++ , the compiler will
not warn if a local variable shadows a struct/class/enum,
but will warn if it shadows an explicit
typedef.
-Wlarger-than=len
Warn
whenever an object of larger than len bytes is
defined.
-Wframe-larger-than=len
Warn
if the size of a function frame is larger than len
bytes. The computation done to determine the stack frame
size is approximate and not conservative. The actual
requirements may be somewhat greater than len even if
you do not get a warning. In addition, any space allocated
via "alloca", variable-length arrays, or
related constructs is not included by the compiler when
determining whether or not to issue a
warning.
-Wno-free-nonheap-object
Do
not warn when attempting to free an object that was not
allocated on the
heap.
-Wstack-usage=len
Warn
if the stack usage of a function might be larger than
len bytes. The computation done to determine the
stack usage is conservative. Any space allocated via
"alloca", variable-length arrays, or
related constructs is included by the compiler when
determining whether or not to issue a
warning.
The
message is in keeping with the output of
-fstack-usage.
•
If the stack usage is
fully static but exceeds the specified amount,
it’s:
warning: stack usage is 1120 bytes
•
If
the stack usage is (partly) dynamic but bounded,
it’s:
warning: stack usage might be 1648 bytes
•
If
the stack usage is (partly) dynamic and not bounded,
it’s:
warning: stack usage might be unbounded
-Wunsafe-loop-optimizations
Warn
if the loop cannot be optimized because the compiler could
not assume anything on the bounds of the loop indices. With
-funsafe-loop-optimizations warn if
the compiler made such
assumptions.
-Wno-pedantic-ms-format
(MinGW targets only)
Disables
the warnings about non-ISO "printf" /
"scanf" format width specifiers
"I32", "I64", and
"I" used on Windows targets depending on
the MS runtime, when you are using the options
-Wformat and -pedantic without
gnu-extensions.
-Wpointer-arith
Warn
about anything that depends on the "size of" a
function type or of "void". GNU
C assigns these types a size of 1, for convenience in
calculations with "void *" pointers and
pointers to functions. In C ++ , warn also when
an arithmetic operation involves "NULL".
This warning is also enabled by
-pedantic.
-Wtype-limits
Warn
if a comparison is always true or always false due to the
limited range of the data type, but do not warn for constant
expressions. For example, warn if an unsigned variable is
compared against zero with < or >=. This
warning is also enabled by
-Wextra.
-Wbad-function-cast
(C and Objective-C
only)
Warn
whenever a function call is cast to a non-matching type. For
example, warn if "int malloc()" is cast
to "anything
*".
-Wc++-compat
(C and Objective-C
only)
Warn
about ISO C constructs that are outside of the
common subset of ISO C and ISO C
++ , e.g. request for implicit conversion from
"void *" to a pointer to
non-"void"
type.
-Wc++11-compat
(C ++ and Objective-C ++
only)
Warn
about C ++ constructs whose meaning differs
between ISO C ++ 1998 and ISO
C ++ 2011, e.g., identifiers in ISO
C ++ 1998 that are keywords in ISO
C ++ 2011. This warning turns on
-Wnarrowing and is enabled by
-Wall.
-Wcast-qual
Warn
whenever a pointer is cast so as to remove a type qualifier
from the target type. For example, warn if a "const
char *" is cast to an ordinary "char
*".
Also
warn when making a cast that introduces a type qualifier in
an unsafe way. For example, casting "char
**" to "const char **" is
unsafe, as in this
example:
/* p is char ** value. */
const char **q = (const char **) p;
/* Assignment of readonly string to const char * is OK. */
*q = "string";
/* Now char** pointer points to read-only memory. */
**p = 'b';
-Wcast-align
Warn
whenever a pointer is cast such that the required alignment
of the target is increased. For example, warn if a
"char *" is cast to an "int
*" on machines where integers can only be accessed
at two- or four-byte
boundaries.
-Wwrite-strings
When
compiling C, give string constants the type "const
char[length]" so that copying the
address of one into a non-"const"
"char *" pointer will get a warning. These
warnings will help you find at compile time code that can
try to write into a string constant, but only if you have
been very careful about using "const" in
declarations and prototypes. Otherwise, it will just be a
nuisance. This is why we did not make -Wall
request these
warnings.
When
compiling C ++ , warn about the deprecated
conversion from string literals to "char
*". This warning is enabled by default for C
++ programs.
-Wclobbered
Warn
for variables that might be changed by longjmp or
vfork. This warning is also enabled by
-Wextra.
-Wconversion
Warn
for implicit conversions that may alter a value. This
includes conversions between real and integer, like
"abs (x)" when "x" is
"double"; conversions between signed and
unsigned, like "unsigned ui = -1";
and conversions to smaller types, like "sqrtf
(M_PI)". Do not warn for explicit casts like
"abs ((int) x)" and "ui =
(unsigned) -1", or if the value is not
changed by the conversion like in "abs
(2.0)". Warnings about conversions between signed
and unsigned integers can be disabled by using
-Wno-sign-conversion.
For
C ++ , also warn for confusing overload
resolution for user-defined conversions; and conversions
that will never use a type conversion operator: conversions
to "void", the same type, a base class or
a reference to them. Warnings about conversions between
signed and unsigned integers are disabled by default in
C ++ unless -Wsign-conversion
is explicitly
enabled.
-Wno-conversion-null
(C ++ and Objective-C ++
only)
Do
not warn for conversions between "NULL"
and non-pointer types. -Wconversion-null
is enabled by
default.
-Wzero-as-null-pointer-constant
(C ++ and Objective-C ++
only)
Warn
when a literal ’0’ is used as null pointer
constant. This can be useful to facilitate the conversion to
"nullptr" in C ++
11.
-Wempty-body
Warn
if an empty body occurs in an if, else or
do while statement. This warning is also enabled by
-Wextra.
-Wenum-compare
Warn
about a comparison between values of different enumerated
types. In C ++ enumeral mismatches in conditional
expressions are also diagnosed and the warning is enabled by
default. In C this warning is enabled by
-Wall.
-Wjump-misses-init
(C, Objective-C
only)
Warn
if a "goto" statement or a
"switch" statement jumps forward across
the initialization of a variable, or jumps backward to a
label after the variable has been initialized. This only
warns about variables that are initialized when they are
declared. This warning is only supported for C and
Objective-C; in C ++ this sort of branch is an
error in any case.
-Wjump-misses-init
is included in -Wc++-compat. It can be
disabled with the
-Wno-jump-misses-init
option.
-Wsign-compare
Warn
when a comparison between signed and unsigned values could
produce an incorrect result when the signed value is
converted to unsigned. This warning is also enabled by
-Wextra; to get the other warnings of
-Wextra without this warning, use
-Wextra
-Wno-sign-compare.
-Wsign-conversion
Warn
for implicit conversions that may change the sign of an
integer value, like assigning a signed integer expression to
an unsigned integer variable. An explicit cast silences the
warning. In C, this option is enabled also by
-Wconversion.
-Waddress
Warn
about suspicious uses of memory addresses. These include
using the address of a function in a conditional expression,
such as "void func(void); if (func)", and
comparisons against the memory address of a string literal,
such as "if (x == "abc")". Such
uses typically indicate a programmer error: the address of a
function always evaluates to true, so their use in a
conditional usually indicate that the programmer forgot the
parentheses in a function call; and comparisons against
string literals result in unspecified behavior and are not
portable in C, so they usually indicate that the programmer
intended to use "strcmp". This warning is
enabled by
-Wall.
-Wlogical-op
Warn
about suspicious uses of logical operators in expressions.
This includes using logical operators in contexts where a
bit-wise operator is likely to be
expected.
-Waggregate-return
Warn
if any functions that return structures or unions are
defined or called. (In languages where you can return an
array, this also elicits a
warning.)
-Wno-attributes
Do
not warn if an unexpected "__attribute__"
is used, such as unrecognized attributes, function
attributes applied to variables, etc. This will not stop
errors for incorrect use of supported
attributes.
-Wno-builtin-macro-redefined
Do
not warn if certain built-in macros are redefined. This
suppresses warnings for redefinition of
"__TIMESTAMP__",
"__TIME__",
"__DATE__",
"__FILE__", and
"__BASE_FILE__".
-Wstrict-prototypes
(C and Objective-C
only)
Warn
if a function is declared or defined without specifying the
argument types. (An old-style function definition is
permitted without a warning if preceded by a declaration
that specifies the argument
types.)
-Wold-style-declaration
(C and Objective-C
only)
Warn
for obsolescent usages, according to the C Standard, in a
declaration. For example, warn if storage-class specifiers
like "static" are not the first things in
a declaration. This warning is also enabled by
-Wextra.
-Wold-style-definition
(C and Objective-C
only)
Warn
if an old-style function definition is used. A warning is
given even if there is a previous
prototype.
-Wmissing-parameter-type
(C and Objective-C
only)
A
function parameter is declared without a type specifier in
K&R-style
functions:
void foo(bar) { }
This
warning is also enabled by
-Wextra.
-Wmissing-prototypes
(C and Objective-C
only)
Warn
if a global function is defined without a previous prototype
declaration. This warning is issued even if the definition
itself provides a prototype. The aim is to detect global
functions that are not declared in header
files.
-Wmissing-declarations
Warn
if a global function is defined without a previous
declaration. Do so even if the definition itself provides a
prototype. Use this option to detect global functions that
are not declared in header files. In C ++ , no
warnings are issued for function templates, or for inline
functions, or for functions in anonymous
namespaces.
-Wmissing-field-initializers
Warn
if a structure’s initializer has some fields missing.
For example, the following code would cause such a warning,
because "x.h" is implicitly
zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
This
option does not warn about designated initializers, so the
following modification would not trigger a
warning:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };
This
warning is included in -Wextra. To get other
-Wextra warnings without this one, use
-Wextra
-Wno-missing-field-initializers.
-Wmissing-format-attribute
Warn
about function pointers that might be candidates for
"format" attributes. Note these are only
possible candidates, not absolute ones. GCC will
guess that function pointers with
"format" attributes that are used in
assignment, initialization, parameter passing or return
statements should have a corresponding
"format" attribute in the resulting type.
I.e. the left-hand side of the assignment or initialization,
the type of the parameter variable, or the return type of
the containing function respectively should also have a
"format" attribute to avoid the
warning.
GCC
will also warn about function definitions that might be
candidates for "format" attributes.
Again, these are only possible candidates. GCC
will guess that "format" attributes
might be appropriate for any function that calls a function
like "vprintf" or
"vscanf", but this might not always be
the case, and some functions for which
"format" attributes are appropriate may
not be detected.
-Wno-multichar
Do
not warn if a multicharacter constant (’
FOOF ’) is used. Usually they
indicate a typo in the user’s code, as they have
implementation-defined values, and should not be used in
portable code.
-Wnormalized=<none|id|nfc|nfkc>
In
ISO C and ISO C ++ , two
identifiers are different if they are different sequences of
characters. However, sometimes when characters outside the
basic ASCII character set are used, you can have
two different character sequences that look the same. To
avoid confusion, the ISO 10646 standard sets out
some normalization rules which when applied ensure
that two sequences that look the same are turned into the
same sequence. GCC can warn you if you are using
identifiers that have not been normalized; this option
controls that
warning.
There
are four levels of warning supported by GCC . The
default is -Wnormalized=nfc, which warns about
any identifier that is not in the ISO 10646
"C" normalized form, NFC .
NFC is the recommended form for most
uses.
Unfortunately,
there are some characters allowed in identifiers by
ISO C and ISO C ++ that, when
turned into NFC , are not allowed in identifiers.
That is, there’s no way to use these symbols in
portable ISO C or C ++ and have all
your identifiers in NFC .
-Wnormalized=id suppresses the warning for
these characters. It is hoped that future versions of the
standards involved will correct this, which is why this
option is not the
default.
You
can switch the warning off for all characters by writing
-Wnormalized=none. You would only want to do
this if you were using some other normalization scheme (like
"D"), because otherwise you can easily create bugs
that are literally impossible to
see.
Some
characters in ISO 10646 have distinct meanings
but look identical in some fonts or display methodologies,
especially once formatting has been applied. For instance
"\u207F", " SUPERSCRIPT LATIN
SMALL LETTER N", will display just like a regular
"n" that has been placed in a
superscript. ISO 10646 defines the
NFKC normalization scheme to convert all these
into a standard form as well, and GCC will warn
if your code is not in NFKC if you use
-Wnormalized=nfkc. This warning is comparable
to warning about every identifier that contains the letter O
because it might be confused with the digit 0, and so is not
the default, but may be useful as a local coding convention
if the programming environment is unable to be fixed to
display these characters
distinctly.
-Wno-deprecated
Do
not warn about usage of deprecated
features.
-Wno-deprecated-declarations
Do
not warn about uses of functions, variables, and types
marked as deprecated by using the
"deprecated"
attribute.
-Wno-overflow
Do
not warn about compile-time overflow in constant
expressions.
-Woverride-init
(C and Objective-C
only)
Warn
if an initialized field without side effects is overridden
when using designated
initializers.
This
warning is included in -Wextra. To get other
-Wextra warnings without this one, use
-Wextra
-Wno-override-init.
-Wpacked
Warn
if a structure is given the packed attribute, but the packed
attribute has no effect on the layout or size of the
structure. Such structures may be mis-aligned for little
benefit. For instance, in this code, the variable
"f.x" in "struct bar"
will be misaligned even though "struct
bar" does not itself have the packed
attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
-Wpacked-bitfield-compat
The
4.1, 4.2 and 4.3 series of GCC ignore the
"packed" attribute on bit-fields of type
"char". This has been fixed in GCC
4.4 but the change can lead to differences in the
structure layout. GCC informs you when the offset
of such a field has changed in GCC 4.4. For
example there is no longer a 4-bit padding between
field "a" and "b" in
this structure:
struct foo
char a:4;
char b:8;
} __attribute__ ((packed));
This
warning is enabled by default. Use
-Wno-packed-bitfield-compat
to disable this
warning.
-Wpadded
Warn
if padding is included in a structure, either to align an
element of the structure or to align the whole structure.
Sometimes when this happens it is possible to rearrange the
fields of the structure to reduce the padding and so make
the structure
smaller.
-Wredundant-decls
Warn
if anything is declared more than once in the same scope,
even in cases where multiple declaration is valid and
changes nothing.
-Wnested-externs
(C and Objective-C
only)
Warn
if an "extern" declaration is encountered
within a function.
-Winline
Warn
if a function can not be inlined and it was declared as
inline. Even with this option, the compiler will not warn
about failures to inline functions declared in system
headers.
The
compiler uses a variety of heuristics to determine whether
or not to inline a function. For example, the compiler takes
into account the size of the function being inlined and the
amount of inlining that has already been done in the current
function. Therefore, seemingly insignificant changes in the
source program can cause the warnings produced by
-Winline to appear or
disappear.
-Wno-invalid-offsetof
(C ++ and Objective-C ++
only)
Suppress
warnings from applying the offsetof macro to a
non-POD type. According to the 1998 ISO C
++ standard, applying offsetof to a non-POD type
is undefined. In existing C ++ implementations,
however, offsetof typically gives meaningful results
even when applied to certain kinds of non-POD types. (Such
as a simple struct that fails to be a POD
type only by virtue of having a constructor.) This flag
is for users who are aware that they are writing nonportable
code and who have deliberately chosen to ignore the warning
about it.
The
restrictions on offsetof may be relaxed in a future
version of the C ++
standard.
-Wno-int-to-pointer-cast
Suppress
warnings from casts to pointer type of an integer of a
different size. In C ++ , casting to a pointer
type of smaller size is an error.
Wint-to-pointer-cast is enabled by
default.
-Wno-pointer-to-int-cast
(C and Objective-C
only)
Suppress
warnings from casts from a pointer to an integer type of a
different size.
-Winvalid-pch
Warn
if a precompiled header is found in the search path but
can’t be used.
-Wlong-long
Warn
if long long type is used. This is enabled by either
-pedantic or -Wtraditional
in ISO C90 and C ++ 98 modes. To
inhibit the warning messages, use
-Wno-long-long.
-Wvariadic-macros
Warn
if variadic macros are used in pedantic ISO C90
mode, or the GNU alternate syntax when in
pedantic ISO C99 mode. This is default. To
inhibit the warning messages, use
-Wno-variadic-macros.
-Wvector-operation-performance
Warn
if vector operation is not implemented via SIMD
capabilities of the architecture. Mainly useful for the
performance tuning. Vector operation can be implemented
"piecewise", which means that the scalar
operation is performed on every vector element; "in
parallel", which means that the vector operation
is implemented using scalars of wider type, which normally
is more performance efficient; and "as a single
scalar", which means that vector fits into a
scalar type.
-Wvla
Warn
if variable length array is used in the code.
-Wno-vla will prevent the
-pedantic warning of the variable length
array.
-Wvolatile-register-var
Warn
if a register variable is declared volatile. The volatile
modifier does not inhibit all optimizations that may
eliminate reads and/or writes to register variables. This
warning is enabled by
-Wall.
-Wdisabled-optimization
Warn
if a requested optimization pass is disabled. This warning
does not generally indicate that there is anything wrong
with your code; it merely indicates that GCC
’s optimizers were unable to handle the code
effectively. Often, the problem is that your code is too big
or too complex; GCC will refuse to optimize
programs when the optimization itself is likely to take
inordinate amounts of
time.
-Wpointer-sign
(C and Objective-C
only)
Warn
for pointer argument passing or assignment with different
signedness. This option is only supported for C and
Objective-C. It is implied by -Wall and by
-pedantic, which can be disabled with
-Wno-pointer-sign.
-Wstack-protector
This
option is only active when
-fstack-protector is active. It warns
about functions that will not be protected against stack
smashing.
-Wno-mudflap
Suppress
warnings about constructs that cannot be instrumented by
-fmudflap.
-Woverlength-strings
Warn
about string constants that are longer than the
"minimum maximum" length specified in the C
standard. Modern compilers generally allow string constants
that are much longer than the standard’s minimum
limit, but very portable programs should avoid using longer
strings.
The
limit applies after string constant concatenation,
and does not count the trailing NUL . In C90, the
limit was 509 characters; in C99, it was raised to 4095.
C ++ 98 does not specify a normative minimum
maximum, so we do not diagnose overlength strings in C
++ .
This
option is implied by -pedantic, and can be
disabled with
-Wno-overlength-strings.
-Wunsuffixed-float-constants
(C and Objective-C
only)
GCC
will issue a warning for any floating constant that
does not have a suffix. When used together with
-Wsystem-headers it will warn about such
constants in system header files. This can be useful when
preparing code to use with the
"FLOAT_CONST_DECIMAL64" pragma from the
decimal floating-point extension to
C99.
Options
for Debugging Your Program or GCC
GCC has various special options that are used for
debugging either your program or
GCC:
-g
Produce debugging
information in the operating system’s native format
(stabs, COFF , XCOFF , or DWARF
2). GDB can work with this debugging
information.
On
most systems that use stabs format, -g enables
use of extra debugging information that only GDB
can use; this extra information makes debugging work
better in GDB but will probably make other
debuggers crash or refuse to read the program. If you want
to control for certain whether to generate the extra
information, use -gstabs+,
-gstabs, -gxcoff+,
-gxcoff, or -gvms (see
below).
GCC
allows you to use -g with -O.
The shortcuts taken by optimized code may occasionally
produce surprising results: some variables you declared may
not exist at all; flow of control may briefly move where you
did not expect it; some statements may not be executed
because they compute constant results or their values were
already at hand; some statements may execute in different
places because they were moved out of
loops.
Nevertheless
it proves possible to debug optimized output. This makes it
reasonable to use the optimizer for programs that might have
bugs.
The
following options are useful when GCC is
generated with the capability for more than one debugging
format.
-ggdb
Produce
debugging information for use by GDB . This means
to use the most expressive format available ( DWARF
2, stabs, or the native format if neither of those are
supported), including GDB extensions if at all
possible.
-gstabs
Produce
debugging information in stabs format (if that is
supported), without GDB extensions. This is the
format used by DBX on most BSD
systems. On MIPS , Alpha and System V
Release 4 systems this option produces stabs debugging
output that is not understood by DBX or SDB
. On System V Release 4 systems this option requires
the GNU
assembler.
-feliminate-unused-debug-symbols
Produce
debugging information in stabs format (if that is
supported), for only symbols that are actually
used.
-femit-class-debug-always
Instead
of emitting debugging information for a C ++
class in only one object file, emit it in all object
files using the class. This option should be used only with
debuggers that are unable to handle the way GCC
normally emits debugging information for classes
because using this option will increase the size of
debugging information by as much as a factor of
two.
-fno-debug-types-section
By
default when using DWARF v4 or higher type DIEs
will be put into their own .debug_types section instead of
making them part of the .debug_info section. It is more
efficient to put them in a separate comdat sections since
the linker will then be able to remove duplicates. But not
all DWARF consumers support .debug_types sections
yet.
-gstabs+
Produce
debugging information in stabs format (if that is
supported), using GNU extensions understood only
by the GNU debugger ( GDB ). The use
of these extensions is likely to make other debuggers crash
or refuse to read the
program.
-gcoff
Produce
debugging information in COFF format (if that is
supported). This is the format used by SDB on
most System V systems prior to System V Release
4.
-gxcoff
Produce
debugging information in XCOFF format (if that is
supported). This is the format used by the DBX
debugger on IBM RS/6000
systems.
-gxcoff+
Produce
debugging information in XCOFF format (if that is
supported), using GNU extensions understood only
by the GNU debugger ( GDB ). The use
of these extensions is likely to make other debuggers crash
or refuse to read the program, and may cause assemblers
other than the GNU assembler ( GAS )
to fail with an
error.
-gdwarf-version
Produce
debugging information in DWARF format (if that is
supported). This is the format used by DBX
on IRIX 6. The value of version may
be either 2, 3 or 4; the default version is
2.
Note
that with DWARF version 2 some ports require, and
will always use, some non-conflicting DWARF 3
extensions in the unwind
tables.
Version
4 may require GDB 7.0 and
-fvar-tracking-assignments for
maximum benefit.
-grecord-gcc-switches
This
switch causes the command-line options used to invoke the
compiler that may affect code generation to be appended to
the DW_AT_producer attribute in DWARF debugging
information. The options are concatenated with spaces
separating them from each other and from the compiler
version. See also
-frecord-gcc-switches for another
way of storing compiler options into the object
file.
-gno-record-gcc-switches
Disallow
appending command-line options to the DW_AT_producer
attribute in DWARF debugging information. This is
the default.
-gstrict-dwarf
Disallow
using extensions of later DWARF standard version
than selected with
-gdwarf-version. On most targets
using non-conflicting DWARF extensions from later
standard versions is
allowed.
-gno-strict-dwarf
Allow
using extensions of later DWARF standard version
than selected with
-gdwarf-version.
-gvms
Produce
debugging information in VMS debug format (if
that is supported). This is the format used by DEBUG
on VMS
systems.
-glevel
-ggdblevel
-gstabslevel
-gcofflevel
-gxcofflevel
-gvmslevel
Request
debugging information and also use level to specify
how much information. The default level is
2.
Level
0 produces no debug information at all. Thus,
-g0 negates
-g.
Level
1 produces minimal information, enough for making backtraces
in parts of the program that you don’t plan to debug.
This includes descriptions of functions and external
variables, but no information about local variables and no
line numbers.
Level
3 includes extra information, such as all the macro
definitions present in the program. Some debuggers support
macro expansion when you use
-g3.
-gdwarf-2
does not accept a concatenated debug level, because
GCC used to support an option -gdwarf that
meant to generate debug information in version 1 of
the DWARF format (which is very different from
version 2), and it would have been too confusing. That debug
format is long obsolete, but the option cannot be changed
now. Instead use an additional -glevel
option to change the debug level for DWARF
.
-gtoggle
Turn
off generation of debug info, if leaving out this option
would have generated it, or turn it on at level 2 otherwise.
The position of this argument in the command line does not
matter, it takes effect after all other options are
processed, and it does so only once, no matter how many
times it is given. This is mainly intended to be used with
-fcompare-debug.
-fdump-final-insns[=file]
Dump
the final internal representation ( RTL ) to
file. If the optional argument is omitted (or if
file is "."), the name of the dump
file will be determined by appending
".gkd" to the compilation output file
name.
-fcompare-debug[=opts]
If
no error occurs during compilation, run the compiler a
second time, adding opts and
-fcompare-debug-second to the
arguments passed to the second compilation. Dump the final
internal representation in both compilations, and print an
error if they
differ.
If
the equal sign is omitted, the default -gtoggle
is used.
The
environment variable GCC_COMPARE_DEBUG ,
if defined, non-empty and nonzero, implicitly enables
-fcompare-debug. If
GCC_COMPARE_DEBUG is defined to a string
starting with a dash, then it is used for opts,
otherwise the default -gtoggle is
used.
-fcompare-debug=,
with the equal sign but without opts, is equivalent
to -fno-compare-debug, which
disables the dumping of the final representation and the
second compilation, preventing even
GCC_COMPARE_DEBUG from taking
effect.
To
verify full coverage during
-fcompare-debug testing, set
GCC_COMPARE_DEBUG to say
-fcompare-debug-not-overridden,
which GCC will reject as an invalid option in any
actual compilation (rather than preprocessing, assembly or
linking). To get just a warning, setting
GCC_COMPARE_DEBUG to
-w%n-fcompare-debug not overridden
will do.
-fcompare-debug-second
This
option is implicitly passed to the compiler for the second
compilation requested by -fcompare-debug,
along with options to silence warnings, and omitting other
options that would cause side-effect compiler outputs to
files or to the standard output. Dump files and preserved
temporary files are renamed so as to contain the
".gk" additional extension during the
second compilation, to avoid overwriting those generated by
the first.
When
this option is passed to the compiler driver, it causes the
first compilation to be skipped, which makes it
useful for little other than debugging the compiler
proper.
-feliminate-dwarf2-dups
Compress
DWARF2 debugging information by eliminating duplicated
information about each symbol. This option only makes sense
when generating DWARF2 debugging information with
-gdwarf-2.
-femit-struct-debug-baseonly
Emit
debug information for struct-like types only when the base
name of the compilation source file matches the base name of
file in which the struct was
defined.
This
option substantially reduces the size of debugging
information, but at significant potential loss in type
information to the debugger. See
-femit-struct-debug-reduced
for a less aggressive option. See
-femit-struct-debug-detailed
for more detailed
control.
This
option works only with DWARF
2.
-femit-struct-debug-reduced
Emit
debug information for struct-like types only when the base
name of the compilation source file matches the base name of
file in which the type was defined, unless the struct is a
template or defined in a system
header.
This
option significantly reduces the size of debugging
information, with some potential loss in type information to
the debugger. See
-femit-struct-debug-baseonly
for a more aggressive option. See
-femit-struct-debug-detailed
for more detailed
control.
This
option works only with DWARF
2.
-femit-struct-debug-detailed[=spec-list]
Specify
the struct-like types for which the compiler will generate
debug information. The intent is to reduce duplicate struct
debug information between different object files within the
same program.
This
option is a detailed version of
-femit-struct-debug-reduced
and
-femit-struct-debug-baseonly,
which will serve for most
needs.
A
specification has the
syntax[dir:|ind:][ord:|gen:](any|sys|base|none)
The
optional first word limits the specification to structs that
are used directly (dir:) or used indirectly
(ind:). A struct type is used directly when it is the
type of a variable, member. Indirect uses arise through
pointers to structs. That is, when use of an incomplete
struct would be legal, the use is indirect. An example is
struct one direct; struct two *
indirect;.
The
optional second word limits the specification to ordinary
structs (ord:) or generic structs (gen:).
Generic structs are a bit complicated to explain. For
C ++ , these are non-explicit specializations of
template classes, or non-template classes within the above.
Other programming languages have generics, but
-femit-struct-debug-detailed
does not yet implement
them.
The
third word specifies the source files for those structs for
which the compiler will emit debug information. The values
none and any have the normal meaning. The
value base means that the base of name of the file in
which the type declaration appears must match the base of
the name of the main compilation file. In practice, this
means that types declared in foo.c and foo.h
will have debug information, but types declared in other
header will not. The value sys means those types
satisfying base or declared in system or compiler
headers.
You
may need to experiment to determine the best settings for
your application.
The
default is
-femit-struct-debug-detailed=all.
This
option works only with DWARF
2.
-fno-merge-debug-strings
Direct
the linker to not merge together strings in the debugging
information that are identical in different object files.
Merging is not supported by all assemblers or linkers.
Merging decreases the size of the debug information in the
output file at the cost of increasing link processing time.
Merging is enabled by
default.
-fdebug-prefix-map=old=new
When
compiling files in directory old, record debugging
information describing them as in new
instead.
-fno-dwarf2-cfi-asm
Emit
DWARF 2 unwind info as compiler generated
".eh_frame" section instead of
using GAS ".cfi_*"
directives.
-p
Generate extra code to
write profile information suitable for the analysis program
prof. You must use this option when compiling the
source files you want data about, and you must also use it
when linking.
-pg
Generate extra code to
write profile information suitable for the analysis program
gprof. You must use this option when compiling the
source files you want data about, and you must also use it
when linking.
-Q
Makes the compiler print
out each function name as it is compiled, and print some
statistics about each pass when it
finishes.
-ftime-report
Makes
the compiler print some statistics about the time consumed
by each pass when it
finishes.
-fmem-report
Makes
the compiler print some statistics about permanent memory
allocation when it
finishes.
-fpre-ipa-mem-report
-fpost-ipa-mem-report
Makes
the compiler print some statistics about permanent memory
allocation before or after interprocedural
optimization.
-fstack-usage
Makes
the compiler output stack usage information for the program,
on a per-function basis. The filename for the dump is made
by appending .su to the auxname.
auxname is generated from the name of the output
file, if explicitly specified and it is not an executable,
otherwise it is the basename of the source file. An entry is
made up of three
fields:
•
The name of the
function.
•
A number of
bytes.
•
One or more qualifiers:
"static", "dynamic",
"bounded".
The
qualifier "static" means that the
function manipulates the stack statically: a fixed number of
bytes are allocated for the frame on function entry and
released on function exit; no stack adjustments are
otherwise made in the function. The second field is this
fixed number of
bytes.
The
qualifier "dynamic" means that the
function manipulates the stack dynamically: in addition to
the static allocation described above, stack adjustments are
made in the body of the function, for example to push/pop
arguments around function calls. If the qualifier
"bounded" is also present, the amount of
these adjustments is bounded at compile time and the second
field is an upper bound of the total amount of stack used by
the function. If it is not present, the amount of these
adjustments is not bounded at compile time and the second
field only represents the bounded
part.
-fprofile-arcs
Add
code so that program flow arcs are instrumented.
During execution the program records how many times each
branch and call is executed and how many times it is taken
or returns. When the compiled program exits it saves this
data to a file called auxname.gcda for each source
file. The data may be used for profile-directed
optimizations (-fbranch-probabilities),
or for test coverage analysis
(-ftest-coverage). Each object
file’s auxname is generated from the name of
the output file, if explicitly specified and it is not the
final executable, otherwise it is the basename of the source
file. In both cases any suffix is removed (e.g.
foo.gcda for input file dir/foo.c, or
dir/foo.gcda for output file specified as -o
dir/foo.o).
--coverage
This
option is used to compile and link code instrumented for
coverage analysis. The option is a synonym for
-fprofile-arcs
-ftest-coverage (when compiling) and
-lgcov (when linking). See the documentation
for those options for more
details.
•
Compile the source files
with -fprofile-arcs plus optimization and
code generation options. For test coverage analysis, use the
additional -ftest-coverage option. You do
not need to profile every source file in a
program.
•
Link your object files
with -lgcov or
-fprofile-arcs (the latter implies the
former).
•
Run the program on a
representative workload to generate the arc profile
information. This may be repeated any number of times. You
can run concurrent instances of your program, and provided
that the file system supports locking, the data files will
be correctly updated. Also "fork" calls
are detected and correctly handled (double counting will not
happen).
•
For profile-directed
optimizations, compile the source files again with the same
optimization and code generation options plus
-fbranch-probabilities.
•
For test coverage
analysis, use gcov to produce human readable
information from the .gcno and .gcda files.
Refer to the gcov documentation for further
information.
With
-fprofile-arcs, for each function of your
program GCC creates a program flow graph, then
finds a spanning tree for the graph. Only arcs that are not
on the spanning tree have to be instrumented: the compiler
adds code to count the number of times that these arcs are
executed. When an arc is the only exit or only entrance to a
block, the instrumentation code can be added to the block;
otherwise, a new basic block must be created to hold the
instrumentation
code.
-ftest-coverage
Produce
a notes file that the gcov code-coverage utility can
use to show program coverage. Each source file’s note
file is called auxname.gcno. Refer to the
-fprofile-arcs option above for a
description of auxname and instructions on how to
generate test coverage data. Coverage data will match the
source files more closely, if you do not
optimize.
-fdbg-cnt-list
Print
the name and the counter upper bound for all debug
counters.
-fdbg-cnt=counter-value-list
Set
the internal debug counter upper bound.
counter-value-list is a comma-separated list of
name:value pairs which sets the upper bound of
each debug counter name to value. All debug
counters have the initial upper bound of
UINT_MAX , thus dbg_cnt() returns true
always unless the upper bound is set by this option. e.g.
With -fdbg-cnt=dce:10,tail_call:0 dbg_cnt(dce)
will return true only for first 10
invocations
-fenable-kind-pass
-fdisable-kind-pass=range-list
This
is a set of debugging options that are used to explicitly
disable/enable optimization passes. For compiler users,
regular options for enabling/disabling passes should be used
instead.
*<-fdisable-ipa-pass>
Disable
ipa pass pass. pass is the pass name. If the
same pass is statically invoked in the compiler multiple
times, the pass name should be appended with a sequential
number starting from
1.
*<-fdisable-rtl-pass>
*<-fdisable-rtl-pass=range-list>
Disable
rtl pass pass. pass is the pass name. If the
same pass is statically invoked in the compiler multiple
times, the pass name should be appended with a sequential
number starting from 1. range-list is a comma
seperated list of function ranges or assembler names. Each
range is a number pair seperated by a colon. The range is
inclusive in both ends. If the range is trivial, the number
pair can be simplified as a single number. If the
function’s cgraph node’s uid is falling
within one of the specified ranges, the pass is
disabled for that function. The uid is shown in the
function header of a dump file, and the pass names can be
dumped by using option
-fdump-passes.
*<-fdisable-tree-pass>
*<-fdisable-tree-pass=range-list>
Disable
tree pass pass. See -fdisable-rtl
for the description of option
arguments.
*<-fenable-ipa-pass>
Enable
ipa pass pass. pass is the pass name. If the
same pass is statically invoked in the compiler multiple
times, the pass name should be appended with a sequential
number starting from
1.
*<-fenable-rtl-pass>
*<-fenable-rtl-pass=range-list>
Enable
rtl pass pass. See -fdisable-rtl
for option argument description and
examples.
*<-fenable-tree-pass>
*<-fenable-tree-pass=range-list>
Enable
tree pass pass. See -fdisable-rtl
for the description of option
arguments.
# disable ccp1 for all functions
-fdisable-tree-ccp1
# disable complete unroll for function whose cgraph node uid is 1
-fenable-tree-cunroll=1
# disable gcse2 for functions at the following ranges [1,1],
# [300,400], and [400,1000]
# disable gcse2 for functions foo and foo2
-fdisable-rtl-gcse2=foo,foo2
# disable early inlining
-fdisable-tree-einline
# disable ipa inlining
-fdisable-ipa-inline
# enable tree full unroll
-fenable-tree-unroll
-dletters
-fdump-rtl-pass
Says
to make debugging dumps during compilation at times
specified by letters. This is used for debugging the
RTL-based passes of the compiler. The file names for most of
the dumps are made by appending a pass number and a word to
the dumpname, and the files are created in the
directory of the output file. Note that the pass number is
computed statically as passes get registered into the pass
manager. Thus the numbering is not related to the dynamic
order of execution of passes. In particular, a pass
installed by a plugin could have a number over 200 even if
it executed quite early. dumpname is generated from
the name of the output file, if explicitly specified and it
is not an executable, otherwise it is the basename of the
source file. These switches may have different effects when
-E is used for
preprocessing.
Debug
dumps can be enabled with a -fdump-rtl
switch or some -d option letters. Here
are the possible letters for use in pass and
letters, and their meanings:
-fdump-rtl-alignments
Dump
after branch alignments have been
computed.
-fdump-rtl-asmcons
Dump
after fixing rtl statements that have unsatisfied in/out
constraints.
-fdump-rtl-auto_inc_dec
Dump
after auto-inc-dec discovery. This pass is only run on
architectures that have auto inc or auto dec
instructions.
-fdump-rtl-barriers
Dump
after cleaning up the barrier
instructions.
-fdump-rtl-bbpart
Dump
after partitioning hot and cold basic
blocks.
-fdump-rtl-bbro
Dump
after block
reordering.
-fdump-rtl-btl1
-fdump-rtl-btl2
-fdump-rtl-btl1
and -fdump-rtl-btl2 enable dumping
after the two branch target load optimization
passes.
-fdump-rtl-bypass
Dump
after jump bypassing and control flow
optimizations.
-fdump-rtl-combine
Dump
after the RTL instruction combination
pass.
-fdump-rtl-compgotos
Dump
after duplicating the computed
gotos.
-fdump-rtl-ce1
-fdump-rtl-ce2
-fdump-rtl-ce3
-fdump-rtl-ce1,
-fdump-rtl-ce2, and
-fdump-rtl-ce3 enable dumping after
the three if conversion
passes.
-fdump-rtl-cprop_hardreg
Dump
after hard register copy
propagation.
-fdump-rtl-csa
Dump
after combining stack
adjustments.
-fdump-rtl-cse1
-fdump-rtl-cse2
-fdump-rtl-cse1
and -fdump-rtl-cse2 enable dumping
after the two common sub-expression elimination
passes.
-fdump-rtl-dce
Dump
after the standalone dead code elimination
passes.
-fdump-rtl-dbr
Dump
after delayed branch
scheduling.
-fdump-rtl-dce1
-fdump-rtl-dce2
-fdump-rtl-dce1
and -fdump-rtl-dce2 enable dumping
after the two dead store elimination
passes.
-fdump-rtl-eh
Dump
after finalization of EH handling
code.
-fdump-rtl-eh_ranges
Dump
after conversion of EH handling range
regions.
-fdump-rtl-expand
Dump
after RTL
generation.
-fdump-rtl-fwprop1
-fdump-rtl-fwprop2
-fdump-rtl-fwprop1
and -fdump-rtl-fwprop2 enable
dumping after the two forward propagation
passes.
-fdump-rtl-gcse1
-fdump-rtl-gcse2
-fdump-rtl-gcse1
and -fdump-rtl-gcse2 enable dumping
after global common subexpression
elimination.
-fdump-rtl-init-regs
Dump
after the initialization of the
registers.
-fdump-rtl-initvals
Dump
after the computation of the initial value
sets.
-fdump-rtl-into_cfglayout
Dump
after converting to cfglayout
mode.
-fdump-rtl-ira
Dump
after iterated register
allocation.
-fdump-rtl-jump
Dump
after the second jump
optimization.
-fdump-rtl-loop2
-fdump-rtl-loop2
enables dumping after the rtl loop optimization
passes.
-fdump-rtl-mach
Dump
after performing the machine dependent reorganization pass,
if that pass exists.
-fdump-rtl-mode_sw
Dump
after removing redundant mode
switches.
-fdump-rtl-rnreg
Dump
after register
renumbering.
-fdump-rtl-outof_cfglayout
Dump
after converting from cfglayout
mode.
-fdump-rtl-peephole2
Dump
after the peephole
pass.
-fdump-rtl-postreload
Dump
after post-reload
optimizations.
-fdump-rtl-pro_and_epilogue
Dump
after generating the function prologues and
epilogues.
-fdump-rtl-regmove
Dump
after the register move
pass.
-fdump-rtl-sched1
-fdump-rtl-sched2
-fdump-rtl-sched1
and -fdump-rtl-sched2 enable
dumping after the basic block scheduling
passes.
-fdump-rtl-see
Dump
after sign extension
elimination.
-fdump-rtl-seqabstr
Dump
after common sequence
discovery.
-fdump-rtl-shorten
Dump
after shortening
branches.
-fdump-rtl-sibling
Dump
after sibling call
optimizations.
-fdump-rtl-split1
-fdump-rtl-split2
-fdump-rtl-split3
-fdump-rtl-split4
-fdump-rtl-split5
-fdump-rtl-split1,
-fdump-rtl-split2,
-fdump-rtl-split3,
-fdump-rtl-split4 and
-fdump-rtl-split5 enable dumping
after five rounds of instruction
splitting.
-fdump-rtl-sms
Dump
after modulo scheduling. This pass is only run on some
architectures.
-fdump-rtl-stack
Dump
after conversion from GCC ’s "flat
register file" registers to the x87’s stack-like
registers. This pass is only run on x86
variants.
-fdump-rtl-subreg1
-fdump-rtl-subreg2
-fdump-rtl-subreg1
and -fdump-rtl-subreg2 enable
dumping after the two subreg expansion
passes.
-fdump-rtl-unshare
Dump
after all rtl has been
unshared.
-fdump-rtl-vartrack
Dump
after variable
tracking.
-fdump-rtl-vregs
Dump
after converting virtual registers to hard
registers.
-fdump-rtl-web
Dump
after live range
splitting.
-fdump-rtl-regclass
-fdump-rtl-subregs_of_mode_init
-fdump-rtl-subregs_of_mode_finish
-fdump-rtl-dfinit
-fdump-rtl-dfinish
These
dumps are defined but always produce empty
files.
-fdump-rtl-all
Produce
all the dumps listed
above.
-dA
Annotate the assembler
output with miscellaneous debugging
information.
-dD
Dump all macro
definitions, at the end of preprocessing, in addition to
normal output.
-dH
Produce a core dump
whenever an error
occurs.
-dp
Annotate the assembler
output with a comment indicating which pattern and
alternative was used. The length of each instruction is also
printed.
-dP
Dump the RTL in
the assembler output as a comment before each instruction.
Also turns on -dp
annotation.
-dv
For each of the other
indicated dump files
(-fdump-rtl-pass), dump a
representation of the control flow graph suitable for
viewing with VCG to
file.pass.vcg.
-dx
Just generate RTL
for a function instead of compiling it. Usually used
with
-fdump-rtl-expand.
-fdump-noaddr
When
doing debugging dumps, suppress address output. This makes
it more feasible to use diff on debugging dumps for compiler
invocations with different compiler binaries and/or
different text / bss / data / heap / stack / dso start
locations.
-fdump-unnumbered
When
doing debugging dumps, suppress instruction numbers and
address output. This makes it more feasible to use diff on
debugging dumps for compiler invocations with different
options, in particular with and without
-g.
-fdump-unnumbered-links
When
doing debugging dumps (see -d option above),
suppress instruction numbers for the links to the previous
and next instructions in a
sequence.
-fdump-translation-unit
(C ++ only)
-fdump-translation-unit-options
(C ++
only)
Dump
a representation of the tree structure for the entire
translation unit to a file. The file name is made by
appending .tu to the source file name, and the file
is created in the same directory as the output file. If the
-options form is used, options
controls the details of the dump as described for the
-fdump-tree
options.
-fdump-class-hierarchy
(C ++ only)
-fdump-class-hierarchy-options
(C ++
only)
Dump
a representation of each class’s hierarchy and virtual
function table layout to a file. The file name is made by
appending .class to the source file name, and the
file is created in the same directory as the output file. If
the -options form is used,
options controls the details of the dump as described
for the -fdump-tree
options.
-fdump-ipa-switch
Control
the dumping at various stages of inter-procedural analysis
language tree to a file. The file name is generated by
appending a switch specific suffix to the source file name,
and the file is created in the same directory as the output
file. The following dumps are
possible:
all
Enables all
inter-procedural analysis
dumps.
cgraph
Dumps
information about call-graph optimization, unused function
removal, and inlining
decisions.
inline
Dump
after function
inlining.
-fdump-passes
Dump
the list of optimization passes that are turned on and off
by the current command-line
options.
-fdump-statistics-option
Enable
and control dumping of pass statistics in a separate file.
The file name is generated by appending a suffix ending in
.statistics to the source file name, and the file is
created in the same directory as the output file. If the
-option form is used,
-stats will cause counters to be summed over
the whole compilation unit while -details will
dump every event as the passes generate them. The default
with no option is to sum counters for each function
compiled.
-fdump-tree-switch
-fdump-tree-switch-options
Control
the dumping at various stages of processing the intermediate
language tree to a file. The file name is generated by
appending a switch specific suffix to the source file name,
and the file is created in the same directory as the output
file. If the -options form is used,
options is a list of - separated options
which control the details of the dump. Not all options are
applicable to all dumps; those that are not meaningful will
be ignored. The following options are available
address
Print
the address of each node. Usually this is not meaningful as
it changes according to the environment and source file. Its
primary use is for tying up a dump file with a debug
environment.
asmname
If
"DECL_ASSEMBLER_NAME" has been set for a
given decl, use that in the dump instead of
"DECL_NAME". Its primary use is ease of
use working backward from mangled names in the assembly
file.
slim
Inhibit
dumping of members of a scope or body of a function merely
because that scope has been reached. Only dump such items
when they are directly reachable by some other path. When
dumping pretty-printed trees, this option inhibits dumping
the bodies of control
structures.
raw
Print a raw representation
of the tree. By default, trees are pretty-printed into a
C-like
representation.
details
Enable
more detailed dumps (not honored by every dump
option).
stats
Enable
dumping various statistics about the pass (not honored by
every dump option).
blocks
Enable
showing basic block boundaries (disabled in raw
dumps).
vops
Enable
showing virtual operands for every
statement.
lineno
Enable
showing line numbers for
statements.
uid
Enable showing the
unique ID ("DECL_UID") for
each variable.
verbose
Enable
showing the tree dump for each
statement.
eh
Enable showing the
EH region number holding each
statement.
scev
Enable
showing scalar evolution analysis
details.
all
Turn on all options,
except raw, slim, verbose and
lineno.
The
following tree dumps are possible:
original
Dump
before any tree based optimization, to
file.original.
optimized
Dump
after all tree based optimization, to
file.optimized.
gimple
Dump
each function before and after the gimplification pass to a
file. The file name is made by appending .gimple to
the source file
name.
cfg
Dump the control flow
graph of each function to a file. The file name is made by
appending .cfg to the source file
name.
vcg
Dump the control flow
graph of each function to a file in VCG format.
The file name is made by appending .vcg to the source
file name. Note that if the file contains more than one
function, the generated file cannot be used directly
by VCG . You will need to cut and paste each
function’s graph into its own separate file
first.
ch
Dump each function after
copying loop headers. The file name is made by appending
.ch to the source file
name.
ssa
Dump SSA
related information to a file. The file name is made by
appending .ssa to the source file
name.
alias
Dump
aliasing information for each function. The file name is
made by appending .alias to the source file
name.
ccp
Dump each function
after CCP . The file name is made by appending
.ccp to the source file
name.
storeccp
Dump
each function after STORE-CCP. The file name is made by
appending .storeccp to the source file
name.
pre
Dump trees after partial
redundancy elimination. The file name is made by appending
.pre to the source file
name.
fre
Dump trees after full
redundancy elimination. The file name is made by appending
.fre to the source file
name.
copyprop
Dump
trees after copy propagation. The file name is made by
appending .copyprop to the source file
name.
store_copyprop
Dump
trees after store copy-propagation. The file name is made by
appending .store_copyprop to the source file
name.
dce
Dump each function after
dead code elimination. The file name is made by appending
.dce to the source file
name.
mudflap
Dump
each function after adding mudflap instrumentation. The file
name is made by appending .mudflap to the source file
name.
sra
Dump each function after
performing scalar replacement of aggregates. The file name
is made by appending .sra to the source file
name.
sink
Dump
each function after performing code sinking. The file name
is made by appending .sink to the source file
name.
dom
Dump each function after
applying dominator tree optimizations. The file name is made
by appending .dom to the source file
name.
dse
Dump each function after
applying dead store elimination. The file name is made by
appending .dse to the source file
name.
phiopt
Dump
each function after optimizing PHI nodes into
straightline code. The file name is made by appending
.phiopt to the source file
name.
forwprop
Dump
each function after forward propagating single use
variables. The file name is made by appending
.forwprop to the source file
name.
copyrename
Dump
each function after applying the copy rename optimization.
The file name is made by appending .copyrename to the
source file name.
nrv
Dump each function after
applying the named return value optimization on generic
trees. The file name is made by appending .nrv to the
source file name.
vect
Dump
each function after applying vectorization of loops. The
file name is made by appending .vect to the source
file name.
slp
Dump each function after
applying vectorization of basic blocks. The file name is
made by appending .slp to the source file
name.
vrp
Dump each function after
Value Range Propagation ( VRP ). The file name is
made by appending .vrp to the source file
name.
all
Enable all the available
tree dumps with the flags provided in this
option.
-ftree-vectorizer-verbose=n
This
option controls the amount of debugging output the
vectorizer prints. This information is written to standard
error, unless -fdump-tree-all or
-fdump-tree-vect is specified, in
which case it is output to the usual dump listing file,
.vect. For n=0 no diagnostic information is
reported. If n=1 the vectorizer reports each loop
that got vectorized, and the total number of loops that got
vectorized. If n=2 the vectorizer also reports
non-vectorized loops that passed the first analysis phase
(vect_analyze_loop_form) - i.e. countable, inner-most,
single-bb, single-entry/exit loops. This is the same
verbosity level that
-fdump-tree-vect-stats uses.
Higher verbosity levels mean either more information dumped
for each reported loop, or same amount of information
reported for more loops: if n=3, vectorizer cost
model information is reported. If n=4, alignment
related information is added to the reports. If n=5,
data-references related information (e.g. memory
dependences, memory access-patterns) is added to the
reports. If n=6, the vectorizer reports also
non-vectorized inner-most loops that did not pass the first
analysis phase (i.e., may not be countable, or may have
complicated control-flow). If n=7, the vectorizer
reports also non-vectorized nested loops. If
n=8, SLP related information is added to
the reports. For n=9, all the information the
vectorizer generates during its analysis and transformation
is reported. This is the same verbosity level that
-fdump-tree-vect-details
uses.
-frandom-seed=string
This
option provides a seed that GCC uses when it
would otherwise use random numbers. It is used to generate
certain symbol names that have to be different in every
compiled file. It is also used to place unique stamps in
coverage data files and the object files that produce them.
You can use the -frandom-seed option to
produce reproducibly identical object
files.
The
string should be different for every file you
compile.
-fsched-verbose=n
On
targets that use instruction scheduling, this option
controls the amount of debugging output the scheduler
prints. This information is written to standard error,
unless -fdump-rtl-sched1 or
-fdump-rtl-sched2 is specified, in
which case it is output to the usual dump listing file,
.sched1 or .sched2 respectively. However for
n greater than nine, the output is always printed to
standard error.
For
n greater than zero,
-fsched-verbose outputs the same
information as -fdump-rtl-sched1
and -fdump-rtl-sched2. For n
greater than one, it also output basic block probabilities,
detailed ready list information and unit/insn info. For
n greater than two, it includes RTL at
abort point, control-flow and regions info. And for n
over four, -fsched-verbose also includes
dependence info.
-save-temps
-save-temps=cwd
Store
the usual "temporary" intermediate files
permanently; place them in the current directory and name
them based on the source file. Thus, compiling foo.c
with -c -save-temps would produce
files foo.i and foo.s, as well as
foo.o. This creates a preprocessed foo.i
output file even though the compiler now normally uses an
integrated
preprocessor.
When
used in combination with the -x command-line
option, -save-temps is sensible enough to
avoid over writing an input source file with the same
extension as an intermediate file. The corresponding
intermediate file may be obtained by renaming the source
file before using
-save-temps.
If
you invoke GCC in parallel, compiling several
different source files that share a common base name in
different subdirectories or the same source file compiled
for multiple output destinations, it is likely that the
different parallel compilers will interfere with each other,
and overwrite the temporary files. For
instance:
gcc -save-temps -o outdir1/foo.o indir1/foo.c&
gcc -save-temps -o outdir2/foo.o indir2/foo.c&
may
result in foo.i and foo.o being written to
simultaneously by both
compilers.
-save-temps=obj
Store
the usual "temporary" intermediate files
permanently. If the -o option is used, the
temporary files are based on the object file. If the
-o option is not used, the
-save-temps=obj switch behaves like
-save-temps.
For
example:
gcc -save-temps=obj -c foo.c
gcc -save-temps=obj -c bar.c -o dir/xbar.o
gcc -save-temps=obj foobar.c -o dir2/yfoobar
would
create foo.i, foo.s, dir/xbar.i,
dir/xbar.s, dir2/yfoobar.i,
dir2/yfoobar.s, and
dir2/yfoobar.o.
-time[=file]
Report
the CPU time taken by each subprocess in the
compilation sequence. For C source files, this is the
compiler proper and assembler (plus the linker if linking is
done).
Without
the specification of an output file, the output looks like
this:
# cc1 0.12 0.01
# as 0.00 0.01
The
first number on each line is the "user time", that
is time spent executing the program itself. The second
number is "system time", time spent executing
operating system routines on behalf of the program. Both
numbers are in
seconds.
With
the specification of an output file, the output is appended
to the named file, and it looks like
this:
0.12 0.01 cc1 <options>
0.00 0.01 as <options>
The
"user time" and the "system time" are
moved before the program name, and the options passed to the
program are displayed, so that one can later tell what file
was being compiled, and with which
options.
-fvar-tracking
Run
variable tracking pass. It computes where variables are
stored at each position in code. Better debugging
information is then generated (if the debugging information
format supports this
information).
It
is enabled by default when compiling with optimization
(-Os, -O, -O2, ...),
debugging information (-g) and the debug info
format supports it.
-fvar-tracking-assignments
Annotate
assignments to user variables early in the compilation and
attempt to carry the annotations over throughout the
compilation all the way to the end, in an attempt to improve
debug information while optimizing. Use of
-gdwarf-4 is recommended along with
it.
It
can be enabled even if var-tracking is disabled, in which
case annotations will be created and maintained, but
discarded at the
end.
-fvar-tracking-assignments-toggle
Toggle
-fvar-tracking-assignments, in the
same way that -gtoggle toggles
-g.
-print-file-name=library
Print
the full absolute name of the library file library
that would be used when linking---and
don’t do anything else. With this option, GCC
does not compile or link anything; it just prints the
file name.
-print-multi-directory
Print
the directory name corresponding to the multilib selected by
any other switches present in the command line. This
directory is supposed to exist in
GCC_EXEC_PREFIX
.
-print-multi-lib
Print
the mapping from multilib directory names to compiler
switches that enable them. The directory name is separated
from the switches by ;, and each switch starts with
an @ instead of the -, without spaces
between multiple switches. This is supposed to ease
shell-processing.
-print-multi-os-directory
Print
the path to OS libraries for the selected
multilib, relative to some lib subdirectory. If
OS libraries are present in the lib subdirectory
and no multilibs are used, this is usually just .,
if OS libraries are present in libsuffix
sibling directories this prints e.g. ../lib64,
../lib or ../lib32, or if OS
libraries are present in lib/subdir
subdirectories it prints e.g. amd64, sparcv9
or ev6.
-print-multiarch
Print
the path to OS libraries for the selected
multiarch, relative to some lib
subdirectory.
-print-prog-name=program
Like
-print-file-name, but searches for
a program such as
cpp.
-print-libgcc-file-name
Same
as
-print-file-name=libgcc.a.
This
is useful when you use -nostdlib or
-nodefaultlibs but you do want to link with
libgcc.a. You can
do
gcc -nostdlib <files>... `gcc -print-libgcc-file-name`
-print-search-dirs
Print
the name of the configured installation directory and a list
of program and library directories gcc will
search---and don’t do anything
else.
This
is useful when gcc prints the error message
installation problem, cannot exec cpp0: No such file or
directory. To resolve this you either need to put
cpp0 and the other compiler components where
gcc expects to find them, or you can set the
environment variable GCC_EXEC_PREFIX to
the directory where you installed them. Don’t forget
the trailing
/.
-print-sysroot
Print
the target sysroot directory that will be used during
compilation. This is the target sysroot specified either at
configure time or using the --sysroot
option, possibly with an extra suffix that depends on
compilation options. If no target sysroot is specified, the
option prints
nothing.
-print-sysroot-headers-suffix
Print
the suffix added to the target sysroot when searching for
headers, or give an error if the compiler is not configured
with such a suffix---and don’t do
anything else.
-dumpmachine
Print
the compiler’s target machine (for example,
i686-pc-linux-gnu)---and
don’t do anything
else.
-dumpversion
Print
the compiler version (for example,
3.0)---and don’t do anything
else.
-dumpspecs
Print
the compiler’s built-in specs---and
don’t do anything else. (This is used when GCC
itself is being
built.)
-feliminate-unused-debug-types
Normally,
when producing DWARF2 output, GCC will
emit debugging information for all types declared in a
compilation unit, regardless of whether or not they are
actually used in that compilation unit. Sometimes this is
useful, such as if, in the debugger, you want to cast a
value to a type that is not actually used in your program
(but is declared). More often, however, this results in a
significant amount of wasted space. With this option,
GCC will avoid producing debug symbol output for types
that are nowhere used in the source file being
compiled.
Options
That Control Optimization
These options control various sorts of
optimizations.
Without
any optimization option, the compiler’s goal is to
reduce the cost of compilation and to make debugging produce
the expected results. Statements are independent: if you
stop the program with a breakpoint between statements, you
can then assign a new value to any variable or change the
program counter to any other statement in the function and
get exactly the results you would expect from the source
code.
Turning
on optimization flags makes the compiler attempt to improve
the performance and/or code size at the expense of
compilation time and possibly the ability to debug the
program.
The
compiler performs optimization based on the knowledge it has
of the program. Compiling multiple files at once to a single
output file mode allows the compiler to use information
gained from all of the files when compiling each of
them.
Not
all optimizations are controlled directly by a flag. Only
optimizations that have a flag are listed in this
section.
Most
optimizations are only enabled if an -O level
is set on the command line. Otherwise they are disabled,
even if individual optimization flags are
specified.
Depending
on the target and how GCC was configured, a
slightly different set of optimizations may be enabled at
each -O level than those listed here. You can
invoke GCC with -Q
--help=optimizers to find out the exact set
of optimizations that are enabled at each
level.
-O
-O1
Optimize. Optimizing
compilation takes somewhat more time, and a lot more memory
for a large
function.
With
-O, the compiler tries to reduce code size and
execution time, without performing any optimizations that
take a great deal of compilation
time.
-O
turns on the following optimization
flags:
-fauto-inc-dec
-fcompare-elim -fcprop-registers
-fdce -fdefer-pop
-fdelayed-branch -fdse
-fguess-branch-probability
-fif-conversion2 -fif-conversion
-fipa-pure-const -fipa-profile
-fipa-reference -fmerge-constants
-fsplit-wide-types
-ftree-bit-ccp
-ftree-builtin-call-dce
-ftree-ccp -ftree-ch
-ftree-copyrename -ftree-dce
-ftree-dominator-opts
-ftree-dse -ftree-forwprop
-ftree-fre -ftree-phiprop
-ftree-sra -ftree-pta
-ftree-ter
-funit-at-a-time
-O
also turns on -fomit-frame-pointer
on machines where doing so does not interfere with
debugging.
-O2
Optimize
even more. GCC performs nearly all supported
optimizations that do not involve a space-speed tradeoff. As
compared to -O, this option increases both
compilation time and the performance of the generated
code.
-O2
turns on all optimization flags specified by
-O. It also turns on the following optimization
flags: -fthread-jumps
-falign-functions -falign-jumps
-falign-loops -falign-labels
-fcaller-saves -fcrossjumping
-fcse-follow-jumps
-fcse-skip-blocks
-fdelete-null-pointer-checks
-fdevirtualize -fexpensive-optimizations
-fgcse -fgcse-lm
-finline-small-functions
-findirect-inlining -fipa-sra
-foptimize-sibling-calls
-fpartial-inlining -fpeephole2
-fregmove -freorder-blocks
-freorder-functions
-frerun-cse-after-loop
-fsched-interblock -fsched-spec
-fschedule-insns -fschedule-insns2
-fstrict-aliasing -fstrict-overflow
-ftree-switch-conversion
-ftree-tail-merge -ftree-pre
-ftree-vrp
Please
note the warning under -fgcse about invoking
-O2 on programs that use computed
gotos.
NOTE:
In Ubuntu 8.10 and later versions,
-D_FORTIFY_SOURCE=2 is set by default, and is
activated when -O is set to 2 or higher. This
enables additional compile-time and run-time checks for
several libc functions. To disable, specify either
-U_FORTIFY_SOURCE or
-D_FORTIFY_SOURCE=0.
-O3
Optimize
yet more. -O3 turns on all optimizations
specified by -O2 and also turns on the
-finline-functions,
-funswitch-loops,
-fpredictive-commoning,
-fgcse-after-reload,
-ftree-vectorize,
-ftree-partial-pre and
-fipa-cp-clone
options.
-O0
Reduce compilation time
and make debugging produce the expected results. This is the
default.
-Os
Optimize for size.
-Os enables all -O2 optimizations
that do not typically increase code size. It also performs
further optimizations designed to reduce code
size.
-Os
disables the following optimization flags:
-falign-functions -falign-jumps
-falign-loops -falign-labels
-freorder-blocks
-freorder-blocks-and-partition
-fprefetch-loop-arrays
-ftree-vect-loop-version
-Ofast
Disregard
strict standards compliance. -Ofast enables all
-O3 optimizations. It also enables
optimizations that are not valid for all standard compliant
programs. It turns on -ffast-math and the
Fortran-specific
-fno-protect-parens and
-fstack-arrays.
If
you use multiple -O options, with or without
level numbers, the last such option is the one that is
effective.
Options
of the form -fflag specify
machine-independent flags. Most flags have both positive and
negative forms; the negative form of -ffoo
would be -fno-foo. In the table below,
only one of the forms is listed---the one
you typically will use. You can figure out the other form by
either removing no- or adding
it.
The
following options control specific optimizations. They are
either activated by -O options or are related
to ones that are. You can use the following flags in the
rare cases when "fine-tuning" of optimizations to
be performed is desired.
-fno-default-inline
Do
not make member functions inline by default merely because
they are defined inside the class scope (C ++
only). Otherwise, when you specify -O,
member functions defined inside class scope are compiled
inline by default; i.e., you don’t need to add
inline in front of the member function
name.
-fno-defer-pop
Always
pop the arguments to each function call as soon as that
function returns. For machines that must pop arguments after
a function call, the compiler normally lets arguments
accumulate on the stack for several function calls and pops
them all at once.
Disabled
at levels -O, -O2,
-O3,
-Os.
-fforward-propagate
Perform
a forward propagation pass on RTL . The pass
tries to combine two instructions and checks if the result
can be simplified. If loop unrolling is active, two passes
are performed and the second is scheduled after loop
unrolling.
This
option is enabled by default at optimization levels
-O, -O2, -O3,
-Os.
-ffp-contract=style
-ffp-contract=off
disables floating-point expression contraction.
-ffp-contract=fast enables floating-point
expression contraction such as forming of fused multiply-add
operations if the target has native support for them.
-ffp-contract=on enables floating-point
expression contraction if allowed by the language standard.
This is currently not implemented and treated equal to
-ffp-contract=off.
The
default is
-ffp-contract=fast.
-fomit-frame-pointer
Don’t
keep the frame pointer in a register for functions that
don’t need one. This avoids the instructions to save,
set up and restore frame pointers; it also makes an extra
register available in many functions. It also makes
debugging impossible on some
machines.
On
some machines, such as the VAX , this flag has no
effect, because the standard calling sequence automatically
handles the frame pointer and nothing is saved by pretending
it doesn’t exist. The machine-description macro
"FRAME_POINTER_REQUIRED" controls whether
a target machine supports this
flag.
Starting
with GCC version 4.6, the default setting (when
not optimizing for size) for 32-bit Linux x86 and
32-bit Darwin x86 targets has been changed to
-fomit-frame-pointer. The default
can be reverted to
-fno-omit-frame-pointer by
configuring GCC with the
--enable-frame-pointer
configure option.
Enabled
at levels -O, -O2,
-O3,
-Os.
-foptimize-sibling-calls
Optimize
sibling and tail recursive
calls.
Enabled
at levels -O2, -O3,
-Os.
-fno-inline
Do
not expand any functions inline apart from those marked with
the "always_inline" attribute. This is
the default when not
optimizing.
Single
functions can be exempted from inlining by marking them with
the "noinline"
attribute.
-finline-small-functions
Integrate
functions into their callers when their body is smaller than
expected function call code (so overall size of program gets
smaller). The compiler heuristically decides which functions
are simple enough to be worth integrating in this way. This
inlining applies to all functions, even those not declared
inline.
Enabled
at level
-O2.
-findirect-inlining
Inline
also indirect calls that are discovered to be known at
compile time thanks to previous inlining. This option has
any effect only when inlining itself is turned on by the
-finline-functions or
-finline-small-functions
options.
Enabled
at level
-O2.
-finline-functions
Consider
all functions for inlining, even if they are not declared
inline. The compiler heuristically decides which functions
are worth integrating in this
way.
If
all calls to a given function are integrated, and the
function is declared "static", then the
function is normally not output as assembler code in its own
right.
Enabled
at level
-O3.
-finline-functions-called-once
Consider
all "static" functions called once for
inlining into their caller even if they are not marked
"inline". If a call to a given function
is integrated, then the function is not output as assembler
code in its own
right.
Enabled
at levels -O1, -O2,
-O3 and
-Os.
-fearly-inlining
Inline
functions marked by "always_inline" and
functions whose body seems smaller than the function call
overhead early before doing
-fprofile-generate instrumentation and
real inlining pass. Doing so makes profiling significantly
cheaper and usually inlining faster on programs having large
chains of nested wrapper
functions.
Enabled
by default.
-fipa-sra
Perform
interprocedural scalar replacement of aggregates, removal of
unused parameters and replacement of parameters passed by
reference by parameters passed by
value.
Enabled
at levels -O2, -O3 and
-Os.
-finline-limit=n
By
default, GCC limits the size of functions that
can be inlined. This flag allows coarse control of this
limit. n is the size of functions that can be inlined
in number of pseudo
instructions.
Inlining
is actually controlled by a number of parameters, which may
be specified individually by using
--param name=value.
The -finline-limit=n option sets
some of these parameters as follows:
max-inline-insns-single
is
set to n/2.
max-inline-insns-auto
is
set to n/2.
See
below for a documentation of the individual parameters
controlling inlining and for the defaults of these
parameters.
Note:
there may be no value to -finline-limit
that results in default
behavior.
Note:
pseudo instruction represents, in this particular context,
an abstract measurement of function’s size. In no way
does it represent a count of assembly instructions and as
such its exact meaning might change from one release to an
another.
-fno-keep-inline-dllexport
This
is a more fine-grained version of
-fkeep-inline-functions, which
applies only to functions that are declared using the
"dllexport" attribute or
declspec
-fkeep-inline-functions
In
C, emit "static" functions that are
declared "inline" into the object file,
even if the function has been inlined into all of its
callers. This switch does not affect functions using the
"extern inline" extension in GNU
C90. In C ++ , emit any and all inline
functions into the object
file.
-fkeep-static-consts
Emit
variables declared "static const" when
optimization isn’t turned on, even if the variables
aren’t
referenced.
GCC
enables this option by default. If you want to force
the compiler to check if the variable was referenced,
regardless of whether or not optimization is turned on, use
the -fno-keep-static-consts
option.
-fmerge-constants
Attempt
to merge identical constants (string constants and
floating-point constants) across compilation
units.
This
option is the default for optimized compilation if the
assembler and linker support it. Use
-fno-merge-constants to inhibit
this behavior.
Enabled
at levels -O, -O2,
-O3,
-Os.
-fmerge-all-constants
Attempt
to merge identical constants and identical
variables.
This
option implies -fmerge-constants. In
addition to -fmerge-constants this
considers e.g. even constant initialized arrays or
initialized constant variables with integral or
floating-point types. Languages like C or C ++
require each variable, including multiple instances of
the same variable in recursive calls, to have distinct
locations, so using this option will result in
non-conforming
behavior.
-fmodulo-sched
Perform
swing modulo scheduling immediately before the first
scheduling pass. This pass looks at innermost loops and
reorders their instructions by overlapping different
iterations.
-fmodulo-sched-allow-regmoves
Perform
more aggressive SMS based modulo scheduling with
register moves allowed. By setting this flag certain
anti-dependences edges will be deleted which will trigger
the generation of reg-moves based on the life-range
analysis. This option is effective only with
-fmodulo-sched
enabled.
-fno-branch-count-reg
Do
not use "decrement and branch" instructions on a
count register, but instead generate a sequence of
instructions that decrement a register, compare it against
zero, then branch based upon the result. This option is only
meaningful on architectures that support such instructions,
which include x86, PowerPC, IA-64 and
S/390.
The
default is
-fbranch-count-reg.
-fno-function-cse
Do
not put function addresses in registers; make each
instruction that calls a constant function contain the
function’s address
explicitly.
This
option results in less efficient code, but some strange
hacks that alter the assembler output may be confused by the
optimizations performed when this option is not
used.
The
default is
-ffunction-cse
-fno-zero-initialized-in-bss
If
the target supports a BSS section, GCC
by default puts variables that are initialized to zero
into BSS . This can save space in the resulting
code.
This
option turns off this behavior because some programs
explicitly rely on variables going to the data section.
E.g., so that the resulting executable can find the
beginning of that section and/or make assumptions based on
that.
The
default is
-fzero-initialized-in-bss.
-fmudflap
-fmudflapth
-fmudflapir
For
front-ends that support it (C and C ++ ),
instrument all risky pointer/array dereferencing operations,
some standard library string/heap functions, and some other
associated constructs with range/validity tests. Modules so
instrumented should be immune to buffer overflows, invalid
heap use, and some other classes of C/C ++
programming errors. The instrumentation relies on a
separate runtime library (libmudflap), which will be
linked into a program if -fmudflap is given at
link time. Run-time behavior of the instrumented program is
controlled by the MUDFLAP_OPTIONS
environment variable. See "env
MUDFLAP_OPTIONS=-help a.out" for its
options.
Use
-fmudflapth instead of -fmudflap
to compile and to link if your program is multi-threaded.
Use -fmudflapir, in addition to
-fmudflap or -fmudflapth, if
instrumentation should ignore pointer reads. This produces
less instrumentation (and therefore faster execution) and
still provides some protection against outright memory
corrupting writes, but allows erroneously read data to
propagate within a
program.
-fthread-jumps
Perform
optimizations where we check to see if a jump branches to a
location where another comparison subsumed by the first is
found. If so, the first branch is redirected to either the
destination of the second branch or a point immediately
following it, depending on whether the condition is known to
be true or false.
Enabled
at levels -O2, -O3,
-Os.
-fsplit-wide-types
When
using a type that occupies multiple registers, such as
"long long" on a 32-bit system,
split the registers apart and allocate them independently.
This normally generates better code for those types, but may
make debugging more
difficult.
Enabled
at levels -O, -O2,
-O3,
-Os.
-fcse-follow-jumps
In
common subexpression elimination ( CSE ), scan
through jump instructions when the target of the jump is not
reached by any other path. For example, when CSE
encounters an "if" statement with an
"else" clause, CSE will follow
the jump when the condition tested is
false.
Enabled
at levels -O2, -O3,
-Os.
-fcse-skip-blocks
This
is similar to -fcse-follow-jumps,
but causes CSE to follow jumps that conditionally
skip over blocks. When CSE encounters a simple
"if" statement with no else clause,
-fcse-skip-blocks causes CSE
to follow the jump around the body of the
"if".
Enabled
at levels -O2, -O3,
-Os.
-frerun-cse-after-loop
Re-run
common subexpression elimination after loop optimizations
has been performed.
Enabled
at levels -O2, -O3,
-Os.
-fgcse
Perform
a global common subexpression elimination pass. This pass
also performs global constant and copy
propagation.
Note:
When compiling a program using computed gotos, a GCC
extension, you may get better run-time performance if
you disable the global common subexpression elimination pass
by adding -fno-gcse to the command
line.
Enabled
at levels -O2, -O3,
-Os.
-fgcse-lm
When
-fgcse-lm is enabled, global common
subexpression elimination will attempt to move loads that
are only killed by stores into themselves. This allows a
loop containing a load/store sequence to be changed to a
load outside the loop, and a copy/store within the
loop.
Enabled
by default when gcse is
enabled.
-fgcse-sm
When
-fgcse-sm is enabled, a store motion pass
is run after global common subexpression elimination. This
pass will attempt to move stores out of loops. When used in
conjunction with -fgcse-lm, loops
containing a load/store sequence can be changed to a load
before the loop and a store after the
loop.
Not
enabled at any optimization
level.
-fgcse-las
When
-fgcse-las is enabled, the global common
subexpression elimination pass eliminates redundant loads
that come after stores to the same memory location (both
partial and full
redundancies).
Not
enabled at any optimization
level.
-fgcse-after-reload
When
-fgcse-after-reload is enabled, a
redundant load elimination pass is performed after reload.
The purpose of this pass is to cleanup redundant
spilling.
-funsafe-loop-optimizations
If
given, the loop optimizer will assume that loop indices do
not overflow, and that the loops with nontrivial exit
condition are not infinite. This enables a wider range of
loop optimizations even if the loop optimizer itself cannot
prove that these assumptions are valid. Using
-Wunsafe-loop-optimizations, the
compiler will warn you if it finds this kind of
loop.
-fcrossjumping
Perform
cross-jumping transformation. This transformation unifies
equivalent code and save code size. The resulting code may
or may not perform better than without
cross-jumping.
Enabled
at levels -O2, -O3,
-Os.
-fauto-inc-dec
Combine
increments or decrements of addresses with memory accesses.
This pass is always skipped on architectures that do not
have instructions to support this. Enabled by default at
-O and higher on architectures that support
this.
-fdce
Perform
dead code elimination ( DCE ) on RTL .
Enabled by default at -O and
higher.
-fdse
Perform
dead store elimination ( DSE ) on RTL
. Enabled by default at -O and
higher.
-fif-conversion
Attempt
to transform conditional jumps into branch-less equivalents.
This include use of conditional moves, min, max, set flags
and abs instructions, and some tricks doable by standard
arithmetics. The use of conditional execution on chips where
it is available is controlled by
"if-conversion2".
Enabled
at levels -O, -O2,
-O3,
-Os.
-fif-conversion2
Use
conditional execution (where available) to transform
conditional jumps into branch-less
equivalents.
Enabled
at levels -O, -O2,
-O3,
-Os.
-fdelete-null-pointer-checks
Assume
that programs cannot safely dereference null pointers, and
that no code or data element resides there. This enables
simple constant folding optimizations at all optimization
levels. In addition, other optimization passes in GCC
use this flag to control global dataflow analyses that
eliminate useless checks for null pointers; these assume
that if a pointer is checked after it has already been
dereferenced, it cannot be
null.
Note
however that in some environments this assumption is not
true. Use
-fno-delete-null-pointer-checks
to disable this optimization for programs that depend on
that behavior.
Some
targets, especially embedded ones, disable this option at
all levels. Otherwise it is enabled at all levels:
-O0, -O1, -O2,
-O3, -Os. Passes that use the
information are enabled independently at different
optimization levels.
-fdevirtualize
Attempt
to convert calls to virtual functions to direct calls. This
is done both within a procedure and interprocedurally as
part of indirect inlining
("-findirect-inlining") and
interprocedural constant propagation
(-fipa-cp). Enabled at levels
-O2, -O3,
-Os.
-fexpensive-optimizations
Perform
a number of minor optimizations that are relatively
expensive.
Enabled
at levels -O2, -O3,
-Os.
-free
Attempt
to remove redundant extension instructions. This is
especially helpful for the x86-64 architecture which
implicitly zero-extends in 64-bit registers after
writing to their lower 32-bit
half.
Enabled
for x86 at levels -O2,
-O3.
-foptimize-register-move
-fregmove
Attempt
to reassign register numbers in move instructions and as
operands of other simple instructions in order to maximize
the amount of register tying. This is especially helpful on
machines with two-operand
instructions.
Note
-fregmove and
-foptimize-register-move are the
same optimization.
Enabled
at levels -O2, -O3,
-Os.
-fira-algorithm=algorithm
Use
the specified coloring algorithm for the integrated register
allocator. The algorithm argument can be
priority, which specifies Chow’s priority
coloring, or CB , which specifies
Chaitin-Briggs coloring. Chaitin-Briggs coloring is not
implemented for all architectures, but for those targets
that do support it, it is the default because it generates
better code.
-fira-region=region
Use
specified regions for the integrated register allocator. The
region argument should be one of the
following:
all
Use all loops as register
allocation regions. This can give the best results for
machines with a small and/or irregular register
set.
mixed
Use
all loops except for loops with small register pressure as
the regions. This value usually gives the best results in
most cases and for most architectures, and is enabled by
default when compiling with optimization for speed
(-O, -O2,
...).
one
Use all functions as a
single region. This typically results in the smallest code
size, and is enabled by default for -Os or
-O0.
-fira-loop-pressure
Use
IRA to evaluate register pressure in loops for
decisions to move loop invariants. This option usually
results in generation of faster and smaller code on machines
with large register files (>= 32 registers), but it can
slow the compiler
down.
This
option is enabled at level -O3 for some
targets.
-fno-ira-share-save-slots
Disable
sharing of stack slots used for saving call-used hard
registers living through a call. Each hard register gets a
separate stack slot, and as a result function stack frames
are larger.
-fno-ira-share-spill-slots
Disable
sharing of stack slots allocated for pseudo-registers. Each
pseudo-register that does not get a hard register gets a
separate stack slot, and as a result function stack frames
are larger.
-fira-verbose=n
Control
the verbosity of the dump file for the integrated register
allocator. The default value is 5. If the value n is
greater or equal to 10, the dump output is sent to stderr
using the same format as n minus
10.
-fdelayed-branch
If
supported for the target machine, attempt to reorder
instructions to exploit instruction slots available after
delayed branch
instructions.
Enabled
at levels -O, -O2,
-O3,
-Os.
-fschedule-insns
If
supported for the target machine, attempt to reorder
instructions to eliminate execution stalls due to required
data being unavailable. This helps machines that have slow
floating point or memory load instructions by allowing other
instructions to be issued until the result of the load or
floating-point instruction is
required.
Enabled
at levels -O2,
-O3.
-fschedule-insns2
Similar
to -fschedule-insns, but requests an
additional pass of instruction scheduling after register
allocation has been done. This is especially useful on
machines with a relatively small number of registers and
where memory load instructions take more than one
cycle.
Enabled
at levels -O2, -O3,
-Os.
-fno-sched-interblock
Don’t
schedule instructions across basic blocks. This is normally
enabled by default when scheduling before register
allocation, i.e. with -fschedule-insns or
at -O2 or
higher.
-fno-sched-spec
Don’t
allow speculative motion of non-load instructions. This is
normally enabled by default when scheduling before register
allocation, i.e. with -fschedule-insns or
at -O2 or
higher.
-fsched-pressure
Enable
register pressure sensitive insn scheduling before the
register allocation. This only makes sense when scheduling
before register allocation is enabled, i.e. with
-fschedule-insns or at -O2
or higher. Usage of this option can improve the generated
code and decrease its size by preventing register pressure
increase above the number of available hard registers and as
a consequence register spills in the register
allocation.
-fsched-spec-load
Allow
speculative motion of some load instructions. This only
makes sense when scheduling before register allocation, i.e.
with -fschedule-insns or at
-O2 or
higher.
-fsched-spec-load-dangerous
Allow
speculative motion of more load instructions. This only
makes sense when scheduling before register allocation, i.e.
with -fschedule-insns or at
-O2 or
higher.
-fsched-stalled-insns
-fsched-stalled-insns=n
Define
how many insns (if any) can be moved prematurely from the
queue of stalled insns into the ready list, during the
second scheduling pass.
-fno-sched-stalled-insns
means that no insns will be moved prematurely,
-fsched-stalled-insns=0 means there
is no limit on how many queued insns can be moved
prematurely. -fsched-stalled-insns
without a value is equivalent to
-fsched-stalled-insns=1.
-fsched-stalled-insns-dep
-fsched-stalled-insns-dep=n
Define
how many insn groups (cycles) will be examined for a
dependency on a stalled insn that is candidate for premature
removal from the queue of stalled insns. This has an effect
only during the second scheduling pass, and only if
-fsched-stalled-insns is used.
-fno-sched-stalled-insns-dep
is equivalent to
-fsched-stalled-insns-dep=0.
-fsched-stalled-insns-dep
without a value is equivalent to
-fsched-stalled-insns-dep=1.
-fsched2-use-superblocks
When
scheduling after register allocation, do use superblock
scheduling algorithm. Superblock scheduling allows motion
across basic block boundaries resulting on faster schedules.
This option is experimental, as not all machine descriptions
used by GCC model the CPU closely
enough to avoid unreliable results from the
algorithm.
This
only makes sense when scheduling after register allocation,
i.e. with -fschedule-insns2 or at
-O2 or
higher.
-fsched-group-heuristic
Enable
the group heuristic in the scheduler. This heuristic favors
the instruction that belongs to a schedule group. This is
enabled by default when scheduling is enabled, i.e. with
-fschedule-insns or
-fschedule-insns2 or at -O2
or higher.
-fsched-critical-path-heuristic
Enable
the critical-path heuristic in the scheduler. This heuristic
favors instructions on the critical path. This is enabled by
default when scheduling is enabled, i.e. with
-fschedule-insns or
-fschedule-insns2 or at -O2
or higher.
-fsched-spec-insn-heuristic
Enable
the speculative instruction heuristic in the scheduler. This
heuristic favors speculative instructions with greater
dependency weakness. This is enabled by default when
scheduling is enabled, i.e. with
-fschedule-insns or
-fschedule-insns2 or at -O2
or higher.
-fsched-rank-heuristic
Enable
the rank heuristic in the scheduler. This heuristic favors
the instruction belonging to a basic block with greater size
or frequency. This is enabled by default when scheduling is
enabled, i.e. with -fschedule-insns or
-fschedule-insns2 or at -O2
or higher.
-fsched-last-insn-heuristic
Enable
the last-instruction heuristic in the scheduler. This
heuristic favors the instruction that is less dependent on
the last instruction scheduled. This is enabled by default
when scheduling is enabled, i.e. with
-fschedule-insns or
-fschedule-insns2 or at -O2
or higher.
-fsched-dep-count-heuristic
Enable
the dependent-count heuristic in the scheduler. This
heuristic favors the instruction that has more instructions
depending on it. This is enabled by default when scheduling
is enabled, i.e. with -fschedule-insns or
-fschedule-insns2 or at -O2
or higher.
-freschedule-modulo-scheduled-loops
The
modulo scheduling comes before the traditional scheduling,
if a loop was modulo scheduled we may want to prevent the
later scheduling passes from changing its schedule, we use
this option to control
that.
-fselective-scheduling
Schedule
instructions using selective scheduling algorithm. Selective
scheduling runs instead of the first scheduler
pass.
-fselective-scheduling2
Schedule
instructions using selective scheduling algorithm. Selective
scheduling runs instead of the second scheduler
pass.
-fsel-sched-pipelining
Enable
software pipelining of innermost loops during selective
scheduling. This option has no effect until one of
-fselective-scheduling or
-fselective-scheduling2 is turned
on.
-fsel-sched-pipelining-outer-loops
When
pipelining loops during selective scheduling, also pipeline
outer loops. This option has no effect until
-fsel-sched-pipelining is turned
on.
-fshrink-wrap
Emit
function prologues only before parts of the function that
need it, rather than at the top of the function. This flag
is enabled by default at -O and
higher.
-fcaller-saves
Enable
values to be allocated in registers that will be clobbered
by function calls, by emitting extra instructions to save
and restore the registers around such calls. Such allocation
is done only when it seems to result in better code than
would otherwise be
produced.
This
option is always enabled by default on certain machines,
usually those which have no call-preserved registers to use
instead.
Enabled
at levels -O2, -O3,
-Os.
-fcombine-stack-adjustments
Tracks
stack adjustments (pushes and pops) and stack memory
references and then tries to find ways to combine
them.
Enabled
by default at -O1 and
higher.
-fconserve-stack
Attempt
to minimize stack usage. The compiler will attempt to use
less stack space, even if that makes the program slower.
This option implies setting the large-stack-frame
parameter to 100 and the large-stack-frame-growth
parameter to 400.
-ftree-reassoc
Perform
reassociation on trees. This flag is enabled by default at
-O and
higher.
-ftree-pre
Perform
partial redundancy elimination ( PRE ) on trees.
This flag is enabled by default at -O2 and
-O3.
-ftree-partial-pre
Make
partial redundancy elimination ( PRE ) more
aggressive. This flag is enabled by default at
-O3.
-ftree-forwprop
Perform
forward propagation on trees. This flag is enabled by
default at -O and
higher.
-ftree-fre
Perform
full redundancy elimination ( FRE ) on trees. The
difference between FRE and PRE is
that FRE only considers expressions that are
computed on all paths leading to the redundant computation.
This analysis is faster than PRE , though it
exposes fewer redundancies. This flag is enabled by default
at -O and
higher.
-ftree-phiprop
Perform
hoisting of loads from conditional pointers on trees. This
pass is enabled by default at -O and
higher.
-ftree-copy-prop
Perform
copy propagation on trees. This pass eliminates unnecessary
copy operations. This flag is enabled by default at
-O and
higher.
-fipa-pure-const
Discover
which functions are pure or constant. Enabled by default at
-O and
higher.
-fipa-reference
Discover
which static variables do not escape cannot escape the
compilation unit. Enabled by default at -O and
higher.
-fipa-pta
Perform
interprocedural pointer analysis and interprocedural
modification and reference analysis. This option can cause
excessive memory and compile-time usage on large compilation
units. It is not enabled by default at any optimization
level.
-fipa-profile
Perform
interprocedural profile propagation. The functions called
only from cold functions are marked as cold. Also functions
executed once (such as "cold",
"noreturn", static constructors or
destructors) are identified. Cold functions and loop less
parts of functions executed once are then optimized for
size. Enabled by default at -O and
higher.
-fipa-cp
Perform
interprocedural constant propagation. This optimization
analyzes the program to determine when values passed to
functions are constants and then optimizes accordingly. This
optimization can substantially increase performance if the
application has constants passed to functions. This flag is
enabled by default at -O2, -Os and
-O3.
-fipa-cp-clone
Perform
function cloning to make interprocedural constant
propagation stronger. When enabled, interprocedural constant
propagation will perform function cloning when externally
visible function can be called with constant arguments.
Because this optimization can create multiple copies of
functions, it may significantly increase code size (see
--param
ipcp-unit-growth=value). This flag is
enabled by default at
-O3.
-fipa-matrix-reorg
Perform
matrix flattening and transposing. Matrix flattening tries
to replace an m-dimensional matrix with its equivalent
n-dimensional matrix, where n < m. This reduces the
level of indirection needed for accessing the elements of
the matrix. The second optimization is matrix transposing,
which attempts to change the order of the matrix’s
dimensions in order to improve cache locality. Both
optimizations need the -fwhole-program
flag. Transposing is enabled only if profiling information
is available.
-ftree-sink
Perform
forward store motion on trees. This flag is enabled by
default at -O and
higher.
-ftree-bit-ccp
Perform
sparse conditional bit constant propagation on trees and
propagate pointer alignment information. This pass only
operates on local scalar variables and is enabled by default
at -O and higher. It requires that
-ftree-ccp is
enabled.
-ftree-ccp
Perform
sparse conditional constant propagation ( CCP )
on trees. This pass only operates on local scalar variables
and is enabled by default at -O and
higher.
-ftree-switch-conversion
Perform
conversion of simple initializations in a switch to
initializations from a scalar array. This flag is enabled by
default at -O2 and
higher.
-ftree-tail-merge
Look
for identical code sequences. When found, replace one with a
jump to the other. This optimization is known as tail
merging or cross jumping. This flag is enabled by default at
-O2 and higher. The compilation time in this
pass can be limited using max-tail-merge-comparisons
parameter and max-tail-merge-iterations
parameter.
-ftree-dce
Perform
dead code elimination ( DCE ) on trees. This flag
is enabled by default at -O and
higher.
-ftree-builtin-call-dce
Perform
conditional dead code elimination ( DCE ) for
calls to builtin functions that may set
"errno" but are otherwise side-effect
free. This flag is enabled by default at -O2
and higher if -Os is not also
specified.
-ftree-dominator-opts
Perform
a variety of simple scalar cleanups (constant/copy
propagation, redundancy elimination, range propagation and
expression simplification) based on a dominator tree
traversal. This also performs jump threading (to reduce
jumps to jumps). This flag is enabled by default at
-O and
higher.
-ftree-dse
Perform
dead store elimination ( DSE ) on trees. A dead
store is a store into a memory location that is later
overwritten by another store without any intervening loads.
In this case the earlier store can be deleted. This flag is
enabled by default at -O and
higher.
-ftree-ch
Perform
loop header copying on trees. This is beneficial since it
increases effectiveness of code motion optimizations. It
also saves one jump. This flag is enabled by default at
-O and higher. It is not enabled for
-Os, since it usually increases code
size.
-ftree-loop-optimize
Perform
loop optimizations on trees. This flag is enabled by default
at -O and
higher.
-ftree-loop-linear
Perform
loop interchange transformations on tree. Same as
-floop-interchange. To use this code
transformation, GCC has to be configured with
--with-ppl and
--with-cloog to enable the Graphite
loop transformation
infrastructure.
-floop-interchange
Perform
loop interchange transformations on loops. Interchanging two
nested loops switches the inner and outer loops. For
example, given a loop
like:
DO J = 1, M
DO I = 1, N
A(J, I) = A(J, I) * C
ENDDO
ENDDO
loop
interchange will transform the loop as if the user had
written:
DO I = 1, N
DO J = 1, M
A(J, I) = A(J, I) * C
ENDDO
ENDDO
which
can be beneficial when "N" is larger than
the caches, because in Fortran, the elements of an array are
stored in memory contiguously by column, and the original
loop iterates over rows, potentially creating at each access
a cache miss. This optimization applies to all the languages
supported by GCC and is not limited to Fortran.
To use this code transformation, GCC has to be
configured with --with-ppl and
--with-cloog to enable the Graphite
loop transformation
infrastructure.
-floop-strip-mine
Perform
loop strip mining transformations on loops. Strip mining
splits a loop into two nested loops. The outer loop has
strides equal to the strip size and the inner loop has
strides of the original loop within a strip. The strip
length can be changed using the loop-block-tile-size
parameter. For example, given a loop
like:
DO I = 1, N
A(I) = A(I) + C
ENDDO
loop
strip mining will transform the loop as if the user had
written:
DO II = 1, N, 51
DO I = II, min (II + 50, N)
A(I) = A(I) + C
ENDDO
ENDDO
This
optimization applies to all the languages supported by
GCC and is not limited to Fortran. To use this code
transformation, GCC has to be configured with
--with-ppl and
--with-cloog to enable the Graphite
loop transformation
infrastructure.
-floop-block
Perform
loop blocking transformations on loops. Blocking strip mines
each loop in the loop nest such that the memory accesses of
the element loops fit inside caches. The strip length can be
changed using the loop-block-tile-size parameter. For
example, given a loop
like:
DO I = 1, N
DO J = 1, M
A(J, I) = B(I) + C(J)
ENDDO
ENDDO
loop
blocking will transform the loop as if the user had
written:
DO II = 1, N, 51
DO JJ = 1, M, 51
DO I = II, min (II + 50, N)
DO J = JJ, min (JJ + 50, M)
A(J, I) = B(I) + C(J)
ENDDO
ENDDO
ENDDO
ENDDO
which
can be beneficial when "M" is larger than
the caches, because the innermost loop will iterate over a
smaller amount of data which can be kept in the caches. This
optimization applies to all the languages supported by
GCC and is not limited to Fortran. To use this code
transformation, GCC has to be configured with
--with-ppl and
--with-cloog to enable the Graphite
loop transformation
infrastructure.
-fgraphite-identity
Enable
the identity transformation for graphite. For every SCoP we
generate the polyhedral representation and transform it back
to gimple. Using -fgraphite-identity we
can check the costs or benefits of the GIMPLE
-> GRAPHITE ->
GIMPLE transformation. Some minimal optimizations are
also performed by the code generator CLooG, like index
splitting and dead code elimination in
loops.
-floop-flatten
Removes
the loop nesting structure: transforms the loop nest into a
single loop. This transformation can be useful as an
enablement transform for vectorization and parallelization.
This feature is experimental. To use this code
transformation, GCC has to be configured with
--with-ppl and
--with-cloog to enable the Graphite
loop transformation
infrastructure.
-floop-parallelize-all
Use
the Graphite data dependence analysis to identify loops that
can be parallelized. Parallelize all the loops that can be
analyzed to not contain loop carried dependences without
checking that it is profitable to parallelize the
loops.
-fcheck-data-deps
Compare
the results of several data dependence analyzers. This
option is used for debugging the data dependence
analyzers.
-ftree-loop-if-convert
Attempt
to transform conditional jumps in the innermost loops to
branch-less equivalents. The intent is to remove
control-flow from the innermost loops in order to improve
the ability of the vectorization pass to handle these loops.
This is enabled by default if vectorization is
enabled.
-ftree-loop-if-convert-stores
Attempt
to also if-convert conditional jumps containing memory
writes. This transformation can be unsafe for multi-threaded
programs as it transforms conditional memory writes into
unconditional memory writes. For
example,
for (i = 0; i < N; i++)
if (cond)
A[i] = expr;
would
be transformed to
for (i = 0; i < N; i++)
A[i] = cond ? expr : A[i];
potentially
producing data
races.
-ftree-loop-distribution
Perform
loop distribution. This flag can improve cache performance
on big loop bodies and allow further loop optimizations,
like parallelization or vectorization, to take place. For
example, the loop
DO I = 1, N
A(I) = B(I) + C
D(I) = E(I) * F
ENDDO
is
transformed to
DO I = 1, N
A(I) = B(I) + C
ENDDO
DO I = 1, N
D(I) = E(I) * F
ENDDO
-ftree-loop-distribute-patterns
Perform
loop distribution of patterns that can be code generated
with calls to a library. This flag is enabled by default at
-O3.
This
pass distributes the initialization loops and generates a
call to memset zero. For example, the
loop
DO I = 1, N
A(I) = 0
B(I) = A(I) + I
ENDDO
is
transformed to
DO I = 1, N
A(I) = 0
ENDDO
DO I = 1, N
B(I) = A(I) + I
ENDDO
and
the initialization loop is transformed into a call to memset
zero.
-ftree-loop-im
Perform
loop invariant motion on trees. This pass moves only
invariants that would be hard to handle at RTL
level (function calls, operations that expand to
nontrivial sequences of insns). With
-funswitch-loops it also moves operands
of conditions that are invariant out of the loop, so that we
can use just trivial invariantness analysis in loop
unswitching. The pass also includes store
motion.
-ftree-loop-ivcanon
Create
a canonical counter for number of iterations in loops for
which determining number of iterations requires complicated
analysis. Later optimizations then may determine the number
easily. Useful especially in connection with
unrolling.
-fivopts
Perform
induction variable optimizations (strength reduction,
induction variable merging and induction variable
elimination) on
trees.
-ftree-parallelize-loops=n
Parallelize
loops, i.e., split their iteration space to run in n
threads. This is only possible for loops whose iterations
are independent and can be arbitrarily reordered. The
optimization is only profitable on multiprocessor machines,
for loops that are CPU-intensive, rather than constrained
e.g. by memory bandwidth. This option implies
-pthread, and thus is only supported on targets
that have support for
-pthread.
-ftree-pta
Perform
function-local points-to analysis on trees. This flag is
enabled by default at -O and
higher.
-ftree-sra
Perform
scalar replacement of aggregates. This pass replaces
structure references with scalars to prevent committing
structures to memory too early. This flag is enabled by
default at -O and
higher.
-ftree-copyrename
Perform
copy renaming on trees. This pass attempts to rename
compiler temporaries to other variables at copy locations,
usually resulting in variable names which more closely
resemble the original variables. This flag is enabled by
default at -O and
higher.
-ftree-coalesce-inlined-vars
Tell
the copyrename pass (see
-ftree-copyrename) to attempt to combine
small user-defined variables too, but only if they were
inlined from other functions. It is a more limited form of
-ftree-coalesce-vars. This may harm
debug information of such inlined variables, but it will
keep variables of the inlined-into function apart from each
other, such that they are more likely to contain the
expected values in a debugging session. This was the default
in GCC versions older than
4.7.
-ftree-coalesce-vars
Tell
the copyrename pass (see
-ftree-copyrename) to attempt to combine
small user-defined variables too, instead of just compiler
temporaries. This may severely limit the ability to debug an
optimized program compiled with
-fno-var-tracking-assignments.
In the negated form, this flag prevents SSA
coalescing of user variables, including inlined ones.
This option is enabled by
default.
-ftree-ter
Perform
temporary expression replacement during the SSA-
>normal phase. Single use/single def temporaries are
replaced at their use location with their defining
expression. This results in non-GIMPLE code, but gives the
expanders much more complex trees to work on resulting in
better RTL generation. This is enabled by default
at -O and
higher.
-ftree-vectorize
Perform
loop vectorization on trees. This flag is enabled by default
at -O3.
-ftree-slp-vectorize
Perform
basic block vectorization on trees. This flag is enabled by
default at -O3 and when
-ftree-vectorize is
enabled.
-ftree-vect-loop-version
Perform
loop versioning when doing loop vectorization on trees. When
a loop appears to be vectorizable except that data alignment
or data dependence cannot be determined at compile time,
then vectorized and non-vectorized versions of the loop are
generated along with run-time checks for alignment or
dependence to control which version is executed. This option
is enabled by default except at level -Os where
it is disabled.
-fvect-cost-model
Enable
cost model for
vectorization.
-ftree-vrp
Perform
Value Range Propagation on trees. This is similar to the
constant propagation pass, but instead of values, ranges of
values are propagated. This allows the optimizers to remove
unnecessary range checks like array bound checks and null
pointer checks. This is enabled by default at
-O2 and higher. Null pointer check elimination
is only done if
-fdelete-null-pointer-checks
is enabled.
-ftracer
Perform
tail duplication to enlarge superblock size. This
transformation simplifies the control flow of the function
allowing other optimizations to do better
job.
-funroll-loops
Unroll
loops whose number of iterations can be determined at
compile time or upon entry to the loop.
-funroll-loops implies
-frerun-cse-after-loop. This
option makes code larger, and may or may not make it run
faster.
-funroll-all-loops
Unroll
all loops, even if their number of iterations is uncertain
when the loop is entered. This usually makes programs run
more slowly. -funroll-all-loops
implies the same options as
-funroll-loops,
-fsplit-ivs-in-unroller
Enables
expressing of values of induction variables in later
iterations of the unrolled loop using the value in the first
iteration. This breaks long dependency chains, thus
improving efficiency of the scheduling
passes.
Combination
of -fweb and CSE is often sufficient
to obtain the same effect. However in cases the loop body is
more complicated than a single basic block, this is not
reliable. It also does not work at all on some of the
architectures due to restrictions in the CSE
pass.
This
optimization is enabled by
default.
-fvariable-expansion-in-unroller
With
this option, the compiler will create multiple copies of
some local variables when unrolling a loop which can result
in superior code.
-fpartial-inlining
Inline
parts of functions. This option has any effect only when
inlining itself is turned on by the
-finline-functions or
-finline-small-functions
options.
Enabled
at level
-O2.
-fpredictive-commoning
Perform
predictive commoning optimization, i.e., reusing
computations (especially memory loads and stores) performed
in previous iterations of
loops.
This
option is enabled at level
-O3.
-fprefetch-loop-arrays
If
supported by the target machine, generate instructions to
prefetch memory to improve the performance of loops that
access large arrays.
This
option may generate better or worse code; results are highly
dependent on the structure of loops within the source
code.
Disabled
at level
-Os.
-fno-peephole
-fno-peephole2
Disable
any machine-specific peephole optimizations. The difference
between -fno-peephole and
-fno-peephole2 is in how they are
implemented in the compiler; some targets use one, some use
the other, a few use
both.
-fpeephole
is enabled by default. -fpeephole2 enabled at
levels -O2, -O3,
-Os.
-fno-guess-branch-probability
Do
not guess branch probabilities using
heuristics.
GCC
will use heuristics to guess branch probabilities if
they are not provided by profiling feedback
(-fprofile-arcs). These heuristics are
based on the control flow graph. If some branch
probabilities are specified by __builtin_expect, then
the heuristics will be used to guess branch probabilities
for the rest of the control flow graph, taking the
__builtin_expect info into account. The interactions
between the heuristics and __builtin_expect can be
complex, and in some cases, it may be useful to disable the
heuristics so that the effects of __builtin_expect
are easier to
understand.
The
default is
-fguess-branch-probability at
levels -O, -O2, -O3,
-Os.
-freorder-blocks
Reorder
basic blocks in the compiled function in order to reduce
number of taken branches and improve code
locality.
Enabled
at levels -O2,
-O3.
-freorder-blocks-and-partition
In
addition to reordering basic blocks in the compiled
function, in order to reduce number of taken branches,
partitions hot and cold basic blocks into separate sections
of the assembly and .o files, to improve paging and cache
locality
performance.
This
optimization is automatically turned off in the presence of
exception handling, for linkonce sections, for functions
with a user-defined section attribute and on any
architecture that does not support named
sections.
-freorder-functions
Reorder
functions in the object file in order to improve code
locality. This is implemented by using special subsections
".text.hot" for most frequently executed
functions and ".text.unlikely" for
unlikely executed functions. Reordering is done by the
linker so object file format must support named sections and
linker must place them in a reasonable
way.
Also
profile feedback must be available in to make this option
effective. See -fprofile-arcs for
details.
Enabled
at levels -O2, -O3,
-Os.
-fstrict-aliasing
Allow
the compiler to assume the strictest aliasing rules
applicable to the language being compiled. For C (and
C ++ ), this activates optimizations based on the
type of expressions. In particular, an object of one type is
assumed never to reside at the same address as an object of
a different type, unless the types are almost the same. For
example, an "unsigned int" can alias an
"int", but not a
"void*" or a "double".
A character type may alias any other
type.
Pay
special attention to code like
this:
union a_union {
int i;
double d;
int f() {
union a_union t;
t.d = 3.0;
return t.i;
}
The
practice of reading from a different union member than the
one most recently written to (called
"type-punning") is common. Even with
-fstrict-aliasing, type-punning is
allowed, provided the memory is accessed through the union
type. So, the code above will work as expected. However,
this code might not:
int f() {
union a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
Similarly,
access by taking the address, casting the resulting pointer
and dereferencing the result has undefined behavior, even if
the cast uses a union type,
e.g.:
int f() {
double d = 3.0;
return ((union a_union *) &d)->i;
}
The
-fstrict-aliasing option is enabled at
levels -O2, -O3,
-Os.
-fstrict-overflow
Allow
the compiler to assume strict signed overflow rules,
depending on the language being compiled. For C (and C
++ ) this means that overflow when doing arithmetic
with signed numbers is undefined, which means that the
compiler may assume that it will not happen. This permits
various optimizations. For example, the compiler will assume
that an expression like "i + 10 > i"
will always be true for signed "i". This
assumption is only valid if signed overflow is undefined, as
the expression is false if "i + 10"
overflows when using twos complement arithmetic. When this
option is in effect any attempt to determine whether an
operation on signed numbers will overflow must be written
carefully to not actually involve
overflow.
This
option also allows the compiler to assume strict pointer
semantics: given a pointer to an object, if adding an offset
to that pointer does not produce a pointer to the same
object, the addition is undefined. This permits the compiler
to conclude that "p + u > p" is always
true for a pointer "p" and unsigned
integer "u". This assumption is only
valid because pointer wraparound is undefined, as the
expression is false if "p + u" overflows
using twos complement
arithmetic.
See
also the -fwrapv option. Using
-fwrapv means that integer signed overflow is
fully defined: it wraps. When -fwrapv is used,
there is no difference between
-fstrict-overflow and
-fno-strict-overflow for integers.
With -fwrapv certain types of overflow are
permitted. For example, if the compiler gets an overflow
when doing arithmetic on constants, the overflowed value can
still be used with -fwrapv, but not
otherwise.
The
-fstrict-overflow option is enabled at
levels -O2, -O3,
-Os.
-falign-functions
-falign-functions=n
Align
the start of functions to the next power-of-two greater than
n, skipping up to n bytes. For instance,
-falign-functions=32 aligns functions to
the next 32-byte boundary, but
-falign-functions=24 would align to the
next 32-byte boundary only if this can be done by
skipping 23 bytes or
less.
-fno-align-functions
and -falign-functions=1 are equivalent
and mean that functions will not be
aligned.
Some
assemblers only support this flag when n is a power
of two; in that case, it is rounded
up.
If
n is not specified or is zero, use a
machine-dependent
default.
Enabled
at levels -O2,
-O3.
-falign-labels
-falign-labels=n
Align
all branch targets to a power-of-two boundary, skipping up
to n bytes like -falign-functions.
This option can easily make code slower, because it must
insert dummy operations for when the branch target is
reached in the usual flow of the
code.
-fno-align-labels
and -falign-labels=1 are equivalent and
mean that labels will not be
aligned.
If
-falign-loops or
-falign-jumps are applicable and are
greater than this value, then their values are used
instead.
If
n is not specified or is zero, use a
machine-dependent default which is very likely to be
1, meaning no
alignment.
Enabled
at levels -O2,
-O3.
-falign-loops
-falign-loops=n
Align
loops to a power-of-two boundary, skipping up to n
bytes like -falign-functions. The hope is
that the loop will be executed many times, which will make
up for any execution of the dummy
operations.
-fno-align-loops
and -falign-loops=1 are equivalent and
mean that loops will not be
aligned.
If
n is not specified or is zero, use a
machine-dependent
default.
Enabled
at levels -O2,
-O3.
-falign-jumps
-falign-jumps=n
Align
branch targets to a power-of-two boundary, for branch
targets where the targets can only be reached by jumping,
skipping up to n bytes like
-falign-functions. In this case, no dummy
operations need be
executed.
-fno-align-jumps
and -falign-jumps=1 are equivalent and
mean that loops will not be
aligned.
If
n is not specified or is zero, use a
machine-dependent
default.
Enabled
at levels -O2,
-O3.
-funit-at-a-time
This
option is left for compatibility reasons.
-funit-at-a-time has no
effect, while
-fno-unit-at-a-time
implies -fno-toplevel-reorder and
-fno-section-anchors.
Enabled
by default.
-fno-toplevel-reorder
Do
not reorder top-level functions, variables, and
"asm" statements. Output them in the same
order that they appear in the input file. When this option
is used, unreferenced static variables will not be removed.
This option is intended to support existing code that relies
on a particular ordering. For new code, it is better to use
attributes.
Enabled
at level -O0. When disabled explicitly, it also
implies -fno-section-anchors, which
is otherwise enabled at -O0 on some
targets.
-fweb
Constructs
webs as commonly used for register allocation purposes and
assign each web individual pseudo register. This allows the
register allocation pass to operate on pseudos directly, but
also strengthens several other optimization passes, such
as CSE , loop optimizer and trivial dead code
remover. It can, however, make debugging impossible, since
variables will no longer stay in a "home
register".
Enabled
by default with
-funroll-loops.
-fwhole-program
Assume
that the current compilation unit represents the whole
program being compiled. All public functions and variables
with the exception of "main" and those
merged by attribute "externally_visible"
become static functions and in effect are optimized more
aggressively by interprocedural optimizers. If gold
is used as the linker plugin,
"externally_visible" attributes are
automatically added to functions (not variable yet due to a
current gold issue) that are accessed outside
of LTO objects according to resolution file
produced by gold. For other linkers that cannot
generate resolution file, explicit
"externally_visible" attributes are still
necessary. While this option is equivalent to proper use of
the "static" keyword for programs
consisting of a single file, in combination with option
-flto this flag can be used to compile many
smaller scale programs since the functions and variables
become local for the whole combined compilation unit, not
for the single source file
itself.
This
option implies -fwhole-file for Fortran
programs.
-flto[=n]
This
option runs the standard link-time optimizer. When invoked
with source code, it generates GIMPLE (one
of GCC ’s internal representations) and
writes it to special ELF sections in the object
file. When the object files are linked together, all the
function bodies are read from these ELF sections
and instantiated as if they had been part of the same
translation unit.
To
use the link-time optimizer, -flto needs to be
specified at compile time and during the final link. For
example:
gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.o
The
first two invocations to GCC save a bytecode
representation of GIMPLE into special ELF
sections inside foo.o and bar.o. The
final invocation reads the GIMPLE bytecode from
foo.o and bar.o, merges the two files into a
single internal image, and compiles the result as usual.
Since both foo.o and bar.o are merged into a
single image, this causes all the interprocedural analyses
and optimizations in GCC to work across the two
files as if they were a single one. This means, for example,
that the inliner is able to inline functions in bar.o
into functions in foo.o and
vice-versa.
Another
(simpler) way to enable link-time optimization
is:
gcc -o myprog -flto -O2 foo.c bar.c
The
above generates bytecode for foo.c and bar.c,
merges them together into a single GIMPLE
representation and optimizes them as usual to produce
myprog.
The
only important thing to keep in mind is that to enable
link-time optimizations the -flto flag needs to
be passed to both the compile and the link
commands.
To
make whole program optimization effective, it is necessary
to make certain whole program assumptions. The compiler
needs to know what functions and variables can be accessed
by libraries and runtime outside of the link-time optimized
unit. When supported by the linker, the linker plugin (see
-fuse-linker-plugin) passes
information to the compiler about used and externally
visible symbols. When the linker plugin is not available,
-fwhole-program should be used to allow
the compiler to make these assumptions, which leads to more
aggressive optimization
decisions.
Note
that when a file is compiled with -flto, the
generated object file is larger than a regular object file
because it contains GIMPLE bytecodes and the
usual final code. This means that object files with
LTO information can be linked as normal object files;
if -flto is not passed to the linker, no
interprocedural optimizations are
applied.
Additionally,
the optimization flags used to compile individual files are
not necessarily related to those used at link time. For
instance,
gcc -c -O0 -flto foo.c
gcc -c -O0 -flto bar.c
gcc -o myprog -flto -O3 foo.o bar.o
This
produces individual object files with unoptimized assembler
code, but the resulting binary myprog is optimized at
-O3. If, instead, the final binary is generated
without -flto, then myprog is not
optimized.
When
producing the final binary with -flto,
GCC only applies link-time optimizations to those files
that contain bytecode. Therefore, you can mix and match
object files and libraries with GIMPLE bytecodes
and final object code. GCC automatically selects
which files to optimize in LTO mode and which
files to link without further
processing.
There
are some code generation flags preserved by GCC
when generating bytecodes, as they need to be used
during the final link stage. Currently, the following
options are saved into the GIMPLE bytecode files:
-fPIC, -fcommon and all the
-m target
flags.
At
link time, these options are read in and reapplied. Note
that the current implementation makes no attempt to
recognize conflicting values for these options. If different
files have conflicting option values (e.g., one file is
compiled with -fPIC and another isn’t),
the compiler simply uses the last value read from the
bytecode files. It is recommended, then, that you compile
all the files participating in the same link with the same
options.
If
LTO encounters objects with C linkage declared with
incompatible types in separate translation units to be
linked together (undefined behavior according to ISO
C99 6.2.7), a non-fatal diagnostic may be issued. The
behavior is still undefined at run
time.
Another
feature of LTO is that it is possible to apply
interprocedural optimizations on files written in different
languages. This requires support in the language front end.
Currently, the C, C ++ and Fortran front ends are
capable of emitting GIMPLE bytecodes, so
something like this should
work:
gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran
Notice
that the final link is done with g++ to get the
C ++ runtime libraries and
-lgfortran is added to get the Fortran runtime
libraries. In general, when mixing languages in LTO
mode, you should use the same link command options as
when mixing languages in a regular (non-LTO) compilation;
all you need to add is -flto to all the compile
and link commands.
If
object files containing GIMPLE bytecode are
stored in a library archive, say libfoo.a, it is
possible to extract and use them in an LTO link
if you are using a linker with plugin support. To enable
this feature, use the flag
-fuse-linker-plugin at link
time:
gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo
With
the linker plugin enabled, the linker extracts the
needed GIMPLE files from libfoo.a and
passes them on to the running GCC to make them
part of the aggregated GIMPLE image to be
optimized.
If
you are not using a linker with plugin support and/or do not
enable the linker plugin, then the objects inside
libfoo.a are extracted and linked as usual, but they
do not participate in the LTO optimization
process.
Link-time
optimizations do not require the presence of the whole
program to operate. If the program does not require any
symbols to be exported, it is possible to combine
-flto and -fwhole-program to
allow the interprocedural optimizers to use more aggressive
assumptions which may lead to improved optimization
opportunities. Use of -fwhole-program is
not needed when linker plugin is active (see
-fuse-linker-plugin).
The
current implementation of LTO makes no attempt to
generate bytecode that is portable between different types
of hosts. The bytecode files are versioned and there is a
strict version check, so bytecode files generated in one
version of GCC will not work with an older/newer
version of GCC
.
Link-time
optimization does not work well with generation of debugging
information. Combining -flto with
-g is currently experimental and expected to
produce wrong
results.
If
you specify the optional n, the optimization and code
generation done at link time is executed in parallel using
n parallel jobs by utilizing an installed make
program. The environment variable MAKE may
be used to override the program used. The default value for
n is 1.
You
can also specify -flto=jobserver to use
GNU make’s job server mode to determine the
number of parallel jobs. This is useful when the Makefile
calling GCC is already executing in parallel. You
must prepend a + to the command recipe in the parent
Makefile for this to work. This option likely only works
if MAKE is GNU
make.
This
option is disabled by
default
-flto-partition=alg
Specify
the partitioning algorithm used by the link-time optimizer.
The value is either "1to1" to specify a
partitioning mirroring the original source files or
"balanced" to specify partitioning into
equally sized chunks (whenever possible). Specifying
"none" as an algorithm disables
partitioning and streaming completely. The default value is
"balanced".
-flto-compression-level=n
This
option specifies the level of compression used for
intermediate language written to LTO object
files, and is only meaningful in conjunction with LTO
mode (-flto). Valid values are 0 (no
compression) to 9 (maximum compression). Values outside this
range are clamped to either 0 or 9. If the option is not
given, a default balanced compression setting is
used.
-flto-report
Prints
a report with internal details on the workings of the
link-time optimizer. The contents of this report vary from
version to version. It is meant to be useful to GCC
developers when processing object files in LTO
mode (via
-flto).
Disabled
by default.
-fuse-linker-plugin
Enables
the use of a linker plugin during link-time optimization.
This option relies on plugin support in the linker, which is
available in gold or in GNU ld 2.21 or
newer.
This
option enables the extraction of object files with
GIMPLE bytecode out of library archives. This improves
the quality of optimization by exposing more code to the
link-time optimizer. This information specifies what symbols
can be accessed externally (by non-LTO object or during
dynamic linking). Resulting code quality improvements on
binaries (and shared libraries that use hidden visibility)
are similar to
"-fwhole-program". See
-flto for a description of the effect of this
flag and how to use
it.
This
option is enabled by default when LTO support
in GCC is enabled and GCC was
configured for use with a linker supporting plugins (
GNU ld 2.21 or newer or
gold).
-ffat-lto-objects
Fat
LTO objects are object files that contain both the
intermediate language and the object code. This makes them
usable for both LTO linking and normal linking.
This option is effective only when compiling with
-flto and is ignored at link
time.
-fno-fat-lto-objects
improves compilation time over plain LTO , but
requires the complete toolchain to be aware of LTO
. It requires a linker with linker plugin support for
basic functionality. Additionally, nm, ar and ranlib need to
support linker plugins to allow a full-featured build
environment (capable of building static libraries
etc).
The
default is -ffat-lto-objects but
this default is intended to change in future releases when
linker plugin enabled environments become more
common.
-fcompare-elim
After
register allocation and post-register allocation instruction
splitting, identify arithmetic instructions that compute
processor flags similar to a comparison operation based on
that arithmetic. If possible, eliminate the explicit
comparison
operation.
This
pass only applies to certain targets that cannot explicitly
represent the comparison operation before register
allocation is
complete.
Enabled
at levels -O, -O2,
-O3,
-Os.
-fuse-ld=gold
Use
the gold linker instead of the default
linker.
-fuse-ld=bfd
Use
the ld.bfd linker instead of the default
linker.
-fcprop-registers
After
register allocation and post-register allocation instruction
splitting, we perform a copy-propagation pass to try to
reduce scheduling dependencies and occasionally eliminate
the copy.
Enabled
at levels -O, -O2,
-O3,
-Os.
-fprofile-correction
Profiles
collected using an instrumented binary for multi-threaded
programs may be inconsistent due to missed counter updates.
When this option is specified, GCC will use
heuristics to correct or smooth out such inconsistencies. By
default, GCC will emit an error message when an
inconsistent profile is
detected.
-fprofile-dir=path
Set
the directory to search for the profile data files in to
path. This option affects only the profile data
generated by -fprofile-generate,
-ftest-coverage,
-fprofile-arcs and used by
-fprofile-use and
-fbranch-probabilities and its related
options. Both absolute and relative paths can be used. By
default, GCC will use the current directory as
path, thus the profile data file will appear in the
same directory as the object
file.
-fprofile-generate
-fprofile-generate=path
Enable
options usually used for instrumenting application to
produce profile useful for later recompilation with profile
feedback based optimization. You must use
-fprofile-generate both when compiling
and when linking your
program.
The
following options are enabled:
"-fprofile-arcs",
"-fprofile-values",
"-fvpt".
If
path is specified, GCC will look at the
path to find the profile feedback data files. See
-fprofile-dir.
-fprofile-use
-fprofile-use=path
Enable
profile feedback directed optimizations, and optimizations
generally profitable only with profile feedback
available.
The
following options are enabled:
"-fbranch-probabilities",
"-fvpt",
"-funroll-loops",
"-fpeel-loops",
"-ftracer"
By
default, GCC emits an error message if the
feedback profiles do not match the source code. This error
can be turned into a warning by using
-Wcoverage-mismatch. Note this may result
in poorly optimized
code.
If
path is specified, GCC will look at the
path to find the profile feedback data files. See
-fprofile-dir.
The
following options control compiler behavior regarding
floating-point arithmetic. These options trade off between
speed and correctness. All must be specifically enabled.
-ffloat-store
Do
not store floating-point variables in registers, and inhibit
other options that might change whether a floating-point
value is taken from a register or
memory.
This
option prevents undesirable excess precision on machines
such as the 68000 where the floating registers (of the
68881) keep more precision than a
"double" is supposed to have. Similarly
for the x86 architecture. For most programs, the excess
precision does only good, but a few programs rely on the
precise definition of IEEE floating point. Use
-ffloat-store for such programs, after
modifying them to store all pertinent intermediate
computations into
variables.
-fexcess-precision=style
This
option allows further control over excess precision on
machines where floating-point registers have more precision
than the IEEE "float"
and "double" types and the processor
does not support operations rounding to those types. By
default, -fexcess-precision=fast is in
effect; this means that operations are carried out in the
precision of the registers and that it is unpredictable when
rounding to the types specified in the source code takes
place. When compiling C, if
-fexcess-precision=standard is specified
then excess precision will follow the rules specified
in ISO C99; in particular, both casts and
assignments cause values to be rounded to their semantic
types (whereas -ffloat-store only affects
assignments). This option is enabled by default for C if a
strict conformance option such as -std=c99 is
used.
-fexcess-precision=standard
is not implemented for languages other than C, and has no
effect if
-funsafe-math-optimizations or
-ffast-math is specified. On the x86, it
also has no effect if -mfpmath=sse or
-mfpmath=sse+387 is specified; in the former
case, IEEE semantics apply without excess
precision, and in the latter, rounding is
unpredictable.
-ffast-math
Sets
-fno-math-errno,
-funsafe-math-optimizations,
-ffinite-math-only,
-fno-rounding-math,
-fno-signaling-nans and
-fcx-limited-range.
This
option causes the preprocessor macro
"__FAST_MATH__" to be
defined.
This
option is not turned on by any -O option
besides -Ofast since it can result in incorrect
output for programs that depend on an exact implementation
of IEEE or ISO rules/specifications
for math functions. It may, however, yield faster code for
programs that do not require the guarantees of these
specifications.
-fno-math-errno
Do
not set ERRNO after calling math functions that
are executed with a single instruction, e.g., sqrt. A
program that relies on IEEE exceptions for math
error handling may want to use this flag for speed while
maintaining IEEE arithmetic
compatibility.
This
option is not turned on by any -O option since
it can result in incorrect output for programs that depend
on an exact implementation of IEEE or ISO
rules/specifications for math functions. It may,
however, yield faster code for programs that do not require
the guarantees of these
specifications.
The
default is
-fmath-errno.
On
Darwin systems, the math library never sets
"errno". There is therefore no reason for
the compiler to consider the possibility that it might, and
-fno-math-errno is the
default.
-funsafe-math-optimizations
Allow
optimizations for floating-point arithmetic that (a) assume
that arguments and results are valid and (b) may
violate IEEE or ANSI standards. When
used at link-time, it may include libraries or startup files
that change the default FPU control word or other
similar
optimizations.
This
option is not turned on by any -O option since
it can result in incorrect output for programs that depend
on an exact implementation of IEEE or ISO
rules/specifications for math functions. It may,
however, yield faster code for programs that do not require
the guarantees of these specifications. Enables
-fno-signed-zeros,
-fno-trapping-math,
-fassociative-math and
-freciprocal-math.
The
default is
-fno-unsafe-math-optimizations.
-fassociative-math
Allow
re-association of operands in series of floating-point
operations. This violates the ISO C and C
++ language standard by possibly changing computation
result. NOTE: re-ordering may change the sign of
zero as well as ignore NaNs and inhibit or create underflow
or overflow (and thus cannot be used on code that relies on
rounding behavior like "(x + 2**52) -
2**52". May also reorder floating-point
comparisons and thus may not be used when ordered
comparisons are required. This option requires that both
-fno-signed-zeros and
-fno-trapping-math be in effect.
Moreover, it doesn’t make much sense with
-frounding-math. For Fortran the option
is automatically enabled when both
-fno-signed-zeros and
-fno-trapping-math are in
effect.
The
default is
-fno-associative-math.
-freciprocal-math
Allow
the reciprocal of a value to be used instead of dividing by
the value if this enables optimizations. For example
"x / y" can be replaced with "x
* (1/y)", which is useful if
"(1/y)" is subject to common
subexpression elimination. Note that this loses precision
and increases the number of flops operating on the
value.
The
default is
-fno-reciprocal-math.
-ffinite-math-only
Allow
optimizations for floating-point arithmetic that assume that
arguments and results are not NaNs or
+-Infs.
This
option is not turned on by any -O option since
it can result in incorrect output for programs that depend
on an exact implementation of IEEE or ISO
rules/specifications for math functions. It may,
however, yield faster code for programs that do not require
the guarantees of these
specifications.
The
default is
-fno-finite-math-only.
-fno-signed-zeros
Allow
optimizations for floating-point arithmetic that ignore the
signedness of zero. IEEE arithmetic specifies the
behavior of distinct +0.0 and -0.0 values, which then
prohibits simplification of expressions such as x+0.0 or
0.0*x (even with
-ffinite-math-only). This option
implies that the sign of a zero result isn’t
significant.
The
default is
-fsigned-zeros.
-fno-trapping-math
Compile
code assuming that floating-point operations cannot generate
user-visible traps. These traps include division by zero,
overflow, underflow, inexact result and invalid operation.
This option requires that
-fno-signaling-nans be in effect.
Setting this option may allow faster code if one relies on
"non-stop" IEEE arithmetic, for
example.
This
option should never be turned on by any -O
option since it can result in incorrect output for programs
that depend on an exact implementation of IEEE
or ISO rules/specifications for math
functions.
The
default is
-ftrapping-math.
-frounding-math
Disable
transformations and optimizations that assume default
floating-point rounding behavior. This is round-to-zero for
all floating point to integer conversions, and
round-to-nearest for all other arithmetic truncations. This
option should be specified for programs that change
the FP rounding mode dynamically, or that may be
executed with a non-default rounding mode. This option
disables constant folding of floating-point expressions at
compile time (which may be affected by rounding mode) and
arithmetic transformations that are unsafe in the presence
of sign-dependent rounding
modes.
The
default is
-fno-rounding-math.
This
option is experimental and does not currently guarantee to
disable all GCC optimizations that are affected
by rounding mode. Future versions of GCC may
provide finer control of this setting using C99’s
"FENV_ACCESS" pragma. This command-line
option will be used to specify the default state for
"FENV_ACCESS".
-fsignaling-nans
Compile
code assuming that IEEE signaling NaNs may
generate user-visible traps during floating-point
operations. Setting this option disables optimizations that
may change the number of exceptions visible with signaling
NaNs. This option implies
-ftrapping-math.
This
option causes the preprocessor macro
"__SUPPORT_SNAN__" to be
defined.
The
default is
-fno-signaling-nans.
This
option is experimental and does not currently guarantee to
disable all GCC optimizations that affect
signaling NaN
behavior.
-fsingle-precision-constant
Treat
floating-point constants as single precision instead of
implicitly converting them to double-precision
constants.
-fcx-limited-range
When
enabled, this option states that a range reduction step is
not needed when performing complex division. Also, there is
no checking whether the result of a complex multiplication
or division is "NaN + I*NaN", with an
attempt to rescue the situation in that case. The default is
-fno-cx-limited-range, but is
enabled by
-ffast-math.
This
option controls the default setting of the ISO
C99 "CX_LIMITED_RANGE" pragma.
Nevertheless, the option applies to all
languages.
-fcx-fortran-rules
Complex
multiplication and division follow Fortran rules. Range
reduction is done as part of complex division, but there is
no checking whether the result of a complex multiplication
or division is "NaN + I*NaN", with an
attempt to rescue the situation in that
case.
The
default is
-fno-cx-fortran-rules.
The
following options control optimizations that may improve
performance, but are not enabled by any -O
options. This section includes experimental options that may
produce broken code.
-fbranch-probabilities
After
running a program compiled with
-fprofile-arcs, you can compile it a
second time using -fbranch-probabilities,
to improve optimizations based on the number of times each
branch was taken. When the program compiled with
-fprofile-arcs exits it saves arc
execution counts to a file called sourcename.gcda for
each source file. The information in this data file is very
dependent on the structure of the generated code, so you
must use the same source code and the same optimization
options for both
compilations.
With
-fbranch-probabilities, GCC
puts a REG_BR_PROB note on each
JUMP_INSN and CALL_INSN .
These can be used to improve optimization. Currently, they
are only used in one place: in reorg.c, instead of
guessing which path a branch is most likely to take,
the REG_BR_PROB values are used to exactly
determine which path is taken more
often.
-fprofile-values
If
combined with -fprofile-arcs, it adds
code so that some data about values of expressions in the
program is gathered.
With
-fbranch-probabilities, it reads back the
data gathered from profiling values of expressions for usage
in optimizations.
Enabled
with -fprofile-generate and
-fprofile-use.
-fvpt
If
combined with -fprofile-arcs, it
instructs the compiler to add a code to gather information
about values of
expressions.
With
-fbranch-probabilities, it reads back the
data gathered and actually performs the optimizations based
on them. Currently the optimizations include specialization
of division operation using the knowledge about the value of
the denominator.
-frename-registers
Attempt
to avoid false dependencies in scheduled code by making use
of registers left over after register allocation. This
optimization will most benefit processors with lots of
registers. Depending on the debug information format adopted
by the target, however, it can make debugging impossible,
since variables will no longer stay in a "home
register".
Enabled
by default with -funroll-loops and
-fpeel-loops.
-ftracer
Perform
tail duplication to enlarge superblock size. This
transformation simplifies the control flow of the function
allowing other optimizations to do better
job.
Enabled
with
-fprofile-use.
-funroll-loops
Unroll
loops whose number of iterations can be determined at
compile time or upon entry to the loop.
-funroll-loops implies
-frerun-cse-after-loop,
-fweb and
-frename-registers. It also turns on
complete loop peeling (i.e. complete removal of loops with
small constant number of iterations). This option makes code
larger, and may or may not make it run
faster.
Enabled
with
-fprofile-use.
-funroll-all-loops
Unroll
all loops, even if their number of iterations is uncertain
when the loop is entered. This usually makes programs run
more slowly. -funroll-all-loops
implies the same options as
-funroll-loops.
-fpeel-loops
Peels
loops for which there is enough information that they do not
roll much (from profile feedback). It also turns on complete
loop peeling (i.e. complete removal of loops with small
constant number of
iterations).
Enabled
with
-fprofile-use.
-fmove-loop-invariants
Enables
the loop invariant motion pass in the RTL loop
optimizer. Enabled at level
-O1
-funswitch-loops
Move
branches with loop invariant conditions out of the loop,
with duplicates of the loop on both branches (modified
according to result of the
condition).
-ffunction-sections
-fdata-sections
Place
each function or data item into its own section in the
output file if the target supports arbitrary sections. The
name of the function or the name of the data item determines
the section’s name in the output
file.
Use
these options on systems where the linker can perform
optimizations to improve locality of reference in the
instruction space. Most systems using the ELF
object format and SPARC processors running
Solaris 2 have linkers with such optimizations. AIX
may have these optimizations in the
future.
Only
use these options when there are significant benefits from
doing so. When you specify these options, the assembler and
linker will create larger object and executable files and
will also be slower. You will not be able to use
"gprof" on all systems if you specify
this option and you may have problems with debugging if you
specify both this option and
-g.
-fbranch-target-load-optimize
Perform
branch target register load optimization before prologue /
epilogue threading. The use of target registers can
typically be exposed only during reload, thus hoisting loads
out of loops and doing inter-block scheduling needs a
separate optimization
pass.
-fbranch-target-load-optimize2
Perform
branch target register load optimization after prologue /
epilogue threading.
-fbtr-bb-exclusive
When
performing branch target register load optimization,
don’t reuse branch target registers in within any
basic block.
-fstack-protector
Emit
extra code to check for buffer overflows, such as stack
smashing attacks. This is done by adding a guard variable to
functions with vulnerable objects. This includes functions
that call alloca, and functions with buffers larger than 8
bytes. The guards are initialized when a function is entered
and then checked when the function exits. If a guard check
fails, an error message is printed and the program
exits.
NOTE:
In Ubuntu 6.10 and later versions this option is
enabled by default for C, C ++ , ObjC, ObjC++, if
none of -fno-stack-protector,
-nostdlib, nor -ffreestanding are
found.
-fstack-protector-all
Like
-fstack-protector except that all
functions are
protected.
-fsection-anchors
Try
to reduce the number of symbolic address calculations by
using shared "anchor" symbols to address nearby
objects. This transformation can help to reduce the number
of GOT entries and GOT accesses on
some targets.
For
example, the implementation of the following function
"foo":
static int a, b, c;
int foo (void) { return a + b + c; }
would
usually calculate the addresses of all three variables, but
if you compile it with -fsection-anchors,
it will access the variables from a common anchor point
instead. The effect is similar to the following pseudocode
(which isn’t valid
C):
int foo (void)
register int *xr = &x;
return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}
Not
all targets support this
option.
--param
name=value
In
some places, GCC uses various constants to
control the amount of optimization that is done. For
example, GCC will not inline functions that
contain more than a certain number of instructions. You can
control some of these constants on the command line using
the --param
option.
The
names of specific parameters, and the meaning of the values,
are tied to the internals of the compiler, and are subject
to change without notice in future
releases.
In
each case, the value is an integer. The allowable
choices for name are given in the following table:
predictable-branch-outcome
When
branch is predicted to be taken with probability lower than
this threshold (in percent), then it is considered well
predictable. The default is
10.
max-crossjump-edges
The
maximum number of incoming edges to consider for
crossjumping. The algorithm used by
-fcrossjumping is O(N^2) in the number of edges
incoming to each block. Increasing values mean more
aggressive optimization, making the compilation time
increase with probably small improvement in executable
size.
min-crossjump-insns
The
minimum number of instructions that must be matched at the
end of two blocks before crossjumping will be performed on
them. This value is ignored in the case where all
instructions in the block being crossjumped from are
matched. The default value is
5.
max-grow-copy-bb-insns
The
maximum code size expansion factor when copying basic blocks
instead of jumping. The expansion is relative to a jump
instruction. The default value is
8.
max-goto-duplication-insns
The
maximum number of instructions to duplicate to a block that
jumps to a computed goto. To avoid O(N^2) behavior in a
number of passes, GCC factors computed gotos
early in the compilation process, and unfactors them as late
as possible. Only computed jumps at the end of a basic
blocks with no more than max-goto-duplication-insns are
unfactored. The default value is
8.
max-delay-slot-insn-search
The
maximum number of instructions to consider when looking for
an instruction to fill a delay slot. If more than this
arbitrary number of instructions is searched, the time
savings from filling the delay slot will be minimal so stop
searching. Increasing values mean more aggressive
optimization, making the compilation time increase with
probably small improvement in execution
time.
max-delay-slot-live-search
When
trying to fill delay slots, the maximum number of
instructions to consider when searching for a block with
valid live register information. Increasing this arbitrarily
chosen value means more aggressive optimization, increasing
the compilation time. This parameter should be removed when
the delay slot code is rewritten to maintain the
control-flow graph.
max-gcse-memory
The
approximate maximum amount of memory that will be allocated
in order to perform the global common subexpression
elimination optimization. If more memory than specified is
required, the optimization will not be
done.
max-gcse-insertion-ratio
If
the ratio of expression insertions to deletions is larger
than this value for any expression, then RTL PRE
will insert or remove the expression and thus leave
partially redundant computations in the instruction stream.
The default value is
20.
max-pending-list-length
The
maximum number of pending dependencies scheduling will allow
before flushing the current state and starting over. Large
functions with few branches or calls can create excessively
large lists which needlessly consume memory and
resources.
max-modulo-backtrack-attempts
The
maximum number of backtrack attempts the scheduler should
make when modulo scheduling a loop. Larger values can
exponentially increase compilation
time.
max-inline-insns-single
Several
parameters control the tree inliner used in gcc. This number
sets the maximum number of instructions (counted in
GCC ’s internal representation) in a single
function that the tree inliner will consider for inlining.
This only affects functions declared inline and methods
implemented in a class declaration (C ++ ). The
default value is
400.
max-inline-insns-auto
When
you use -finline-functions (included in
-O3), a lot of functions that would otherwise
not be considered for inlining by the compiler will be
investigated. To those functions, a different (more
restrictive) limit compared to functions declared inline can
be applied. The default value is
40.
large-function-insns
The
limit specifying really large functions. For functions
larger than this limit after inlining, inlining is
constrained by --param
large-function-growth. This parameter is useful
primarily to avoid extreme compilation time caused by
non-linear algorithms used by the back end. The default
value is 2700.
large-function-growth
Specifies
maximal growth of large function caused by inlining in
percents. The default value is 100 which limits large
function growth to 2.0 times the original
size.
large-unit-insns
The
limit specifying large translation unit. Growth caused by
inlining of units larger than this limit is limited by
--param inline-unit-growth. For small
units this might be too tight (consider unit consisting of
function A that is inline and B that just calls A three
time. If B is small relative to A, the growth of unit is
300\% and yet such inlining is very sane. For very large
units consisting of small inlineable functions however the
overall unit growth limit is needed to avoid exponential
explosion of code size. Thus for smaller units, the size is
increased to --param large-unit-insns
before applying --param
inline-unit-growth. The default is
10000
inline-unit-growth
Specifies
maximal overall growth of the compilation unit caused by
inlining. The default value is 30 which limits unit growth
to 1.3 times the original
size.
ipcp-unit-growth
Specifies
maximal overall growth of the compilation unit caused by
interprocedural constant propagation. The default value is
10 which limits unit growth to 1.1 times the original
size.
large-stack-frame
The
limit specifying large stack frames. While inlining the
algorithm is trying to not grow past this limit too much.
Default value is 256
bytes.
large-stack-frame-growth
Specifies
maximal growth of large stack frames caused by inlining in
percents. The default value is 1000 which limits large stack
frame growth to 11 times the original
size.
max-inline-insns-recursive
max-inline-insns-recursive-auto
Specifies
maximum number of instructions out-of-line copy of self
recursive inline function can grow into by performing
recursive inlining.
For
functions declared inline --param
max-inline-insns-recursive is taken into account. For
function not declared inline, recursive inlining happens
only when -finline-functions (included in
-O3) is enabled and --param
max-inline-insns-recursive-auto is used. The default
value is 450.
max-inline-recursive-depth
max-inline-recursive-depth-auto
Specifies
maximum recursion depth used by the recursive
inlining.
For
functions declared inline --param
max-inline-recursive-depth is taken into account. For
function not declared inline, recursive inlining happens
only when -finline-functions (included in
-O3) is enabled and --param
max-inline-recursive-depth-auto is used. The default
value is 8.
min-inline-recursive-probability
Recursive
inlining is profitable only for function having deep
recursion in average and can hurt for function having little
recursion depth by increasing the prologue size or
complexity of function body to other
optimizers.
When
profile feedback is available (see
-fprofile-generate) the actual recursion
depth can be guessed from probability that function will
recurse via given call expression. This parameter limits
inlining only to call expression whose probability exceeds
given threshold (in percents). The default value is
10.
early-inlining-insns
Specify
growth that early inliner can make. In effect it increases
amount of inlining for code having large abstraction
penalty. The default value is
10.
max-early-inliner-iterations
max-early-inliner-iterations
Limit
of iterations of early inliner. This basically bounds number
of nested indirect calls early inliner can resolve. Deeper
chains are still handled by late
inlining.
comdat-sharing-probability
comdat-sharing-probability
Probability
(in percent) that C ++ inline function with
comdat visibility will be shared across multiple compilation
units. The default value is
20.
min-vect-loop-bound
The
minimum number of iterations under which a loop will not get
vectorized when -ftree-vectorize is used.
The number of iterations after vectorization needs to be
greater than the value specified by this option to allow
vectorization. The default value is
0.
gcse-cost-distance-ratio
Scaling
factor in calculation of maximum distance an expression can
be moved by GCSE optimizations. This is currently
supported only in the code hoisting pass. The bigger the
ratio, the more aggressive code hoisting will be with simple
expressions, i.e., the expressions that have cost less than
gcse-unrestricted-cost. Specifying 0 will disable
hoisting of simple expressions. The default value is
10.
gcse-unrestricted-cost
Cost,
roughly measured as the cost of a single typical machine
instruction, at which GCSE optimizations will not
constrain the distance an expression can travel. This is
currently supported only in the code hoisting pass. The
lesser the cost, the more aggressive code hoisting will be.
Specifying 0 will allow all expressions to travel
unrestricted distances. The default value is
3.
max-hoist-depth
The
depth of search in the dominator tree for expressions to
hoist. This is used to avoid quadratic behavior in hoisting
algorithm. The value of 0 will avoid limiting the search,
but may slow down compilation of huge functions. The default
value is 30.
max-tail-merge-comparisons
The
maximum amount of similar bbs to compare a bb with. This is
used to avoid quadratic behavior in tree tail merging. The
default value is 10.
max-tail-merge-iterations
The
maximum amount of iterations of the pass over the function.
This is used to limit compilation time in tree tail merging.
The default value is
2.
max-unrolled-insns
The
maximum number of instructions that a loop should have if
that loop is unrolled, and if the loop is unrolled, it
determines how many times the loop code is
unrolled.
max-average-unrolled-insns
The
maximum number of instructions biased by probabilities of
their execution that a loop should have if that loop is
unrolled, and if the loop is unrolled, it determines how
many times the loop code is
unrolled.
max-unroll-times
The
maximum number of unrollings of a single
loop.
max-peeled-insns
The
maximum number of instructions that a loop should have if
that loop is peeled, and if the loop is peeled, it
determines how many times the loop code is
peeled.
max-peel-times
The
maximum number of peelings of a single
loop.
max-completely-peeled-insns
The
maximum number of insns of a completely peeled
loop.
max-completely-peel-times
The
maximum number of iterations of a loop to be suitable for
complete peeling.
max-completely-peel-loop-nest-depth
The
maximum depth of a loop nest suitable for complete
peeling.
max-unswitch-insns
The
maximum number of insns of an unswitched
loop.
max-unswitch-level
The
maximum number of branches unswitched in a single
loop.
lim-expensive
The
minimum cost of an expensive expression in the loop
invariant motion.
iv-consider-all-candidates-bound
Bound
on number of candidates for induction variables below that
all candidates are considered for each use in induction
variable optimizations. Only the most relevant candidates
are considered if there are more candidates, to avoid
quadratic time
complexity.
iv-max-considered-uses
The
induction variable optimizations give up on loops that
contain more induction variable
uses.
iv-always-prune-cand-set-bound
If
number of candidates in the set is smaller than this value,
we always try to remove unnecessary ivs from the set during
its optimization when a new iv is added to the
set.
scev-max-expr-size
Bound
on size of expressions used in the scalar evolutions
analyzer. Large expressions slow the
analyzer.
scev-max-expr-complexity
Bound
on the complexity of the expressions in the scalar
evolutions analyzer. Complex expressions slow the
analyzer.
omega-max-vars
The
maximum number of variables in an Omega constraint system.
The default value is
128.
omega-max-geqs
The
maximum number of inequalities in an Omega constraint
system. The default value is
256.
omega-max-eqs
The
maximum number of equalities in an Omega constraint system.
The default value is
128.
omega-max-wild-cards
The
maximum number of wildcard variables that the Omega solver
will be able to insert. The default value is
18.
omega-hash-table-size
The
size of the hash table in the Omega solver. The default
value is 550.
omega-max-keys
The
maximal number of keys used by the Omega solver. The default
value is 500.
omega-eliminate-redundant-constraints
When
set to 1, use expensive methods to eliminate all redundant
constraints. The default value is
0.
vect-max-version-for-alignment-checks
The
maximum number of run-time checks that can be performed when
doing loop versioning for alignment in the vectorizer. See
option ftree-vect-loop-version for more
information.
vect-max-version-for-alias-checks
The
maximum number of run-time checks that can be performed when
doing loop versioning for alias in the vectorizer. See
option ftree-vect-loop-version for more
information.
max-iterations-to-track
The
maximum number of iterations of a loop the brute force
algorithm for analysis of # of iterations of the loop tries
to evaluate.
hot-bb-count-fraction
Select
fraction of the maximal count of repetitions of basic block
in program given basic block needs to have to be considered
hot.
hot-bb-frequency-fraction
Select
fraction of the entry block frequency of executions of basic
block in function given basic block needs to have to be
considered hot.
max-predicted-iterations
The
maximum number of loop iterations we predict statically.
This is useful in cases where function contain single loop
with known bound and other loop with unknown. We predict the
known number of iterations correctly, while the unknown
number of iterations average to roughly 10. This means that
the loop without bounds would appear artificially cold
relative to the other
one.
align-threshold
Select
fraction of the maximal frequency of executions of basic
block in function given basic block will get
aligned.
align-loop-iterations
A
loop expected to iterate at lest the selected number of
iterations will get
aligned.
tracer-dynamic-coverage
tracer-dynamic-coverage-feedback
This
value is used to limit superblock formation once the given
percentage of executed instructions is covered. This limits
unnecessary code size
expansion.
The
tracer-dynamic-coverage-feedback is used only when
profile feedback is available. The real profiles (as opposed
to statically estimated ones) are much less balanced
allowing the threshold to be larger
value.
tracer-max-code-growth
Stop
tail duplication once code growth has reached given
percentage. This is rather hokey argument, as most of the
duplicates will be eliminated later in cross jumping, so it
may be set to much higher values than is the desired code
growth.
tracer-min-branch-ratio
Stop
reverse growth when the reverse probability of best edge is
less than this threshold (in
percent).
tracer-min-branch-ratio
tracer-min-branch-ratio-feedback
Stop
forward growth if the best edge do have probability lower
than this threshold.
Similarly
to tracer-dynamic-coverage two values are present,
one for compilation for profile feedback and one for
compilation without. The value for compilation with profile
feedback needs to be more conservative (higher) in order to
make tracer
effective.
max-cse-path-length
Maximum
number of basic blocks on path that cse considers. The
default is 10.
max-cse-insns
The
maximum instructions CSE process before flushing.
The default is 1000.
ggc-min-expand
GCC
uses a garbage collector to manage its own memory
allocation. This parameter specifies the minimum percentage
by which the garbage collector’s heap should be
allowed to expand between collections. Tuning this may
improve compilation speed; it has no effect on code
generation.
The
default is 30% + 70% * ( RAM/1GB ) with an upper
bound of 100% when RAM >= 1GB. If
"getrlimit" is available, the notion of
" RAM " is the smallest of actual
RAM and "RLIMIT_DATA" or
"RLIMIT_AS". If GCC is not
able to calculate RAM on a particular platform,
the lower bound of 30% is used. Setting this parameter and
ggc-min-heapsize to zero causes a full collection to
occur at every opportunity. This is extremely slow, but can
be useful for
debugging.
ggc-min-heapsize
Minimum
size of the garbage collector’s heap before it begins
bothering to collect garbage. The first collection occurs
after the heap expands by ggc-min-expand% beyond
ggc-min-heapsize. Again, tuning this may improve
compilation speed, and has no effect on code
generation.
The
default is the smaller of RAM/8 ,
RLIMIT_RSS , or a limit that tries to ensure that
RLIMIT_DATA or RLIMIT_AS are not exceeded,
but with a lower bound of 4096 (four megabytes) and an upper
bound of 131072 (128 megabytes). If GCC is not
able to calculate RAM on a particular platform,
the lower bound is used. Setting this parameter very large
effectively disables garbage collection. Setting this
parameter and ggc-min-expand to zero causes a full
collection to occur at every
opportunity.
max-reload-search-insns
The
maximum number of instruction reload should look backward
for equivalent register. Increasing values mean more
aggressive optimization, making the compilation time
increase with probably slightly better performance. The
default value is
100.
max-cselib-memory-locations
The
maximum number of memory locations cselib should take into
account. Increasing values mean more aggressive
optimization, making the compilation time increase with
probably slightly better performance. The default value is
500.
reorder-blocks-duplicate
reorder-blocks-duplicate-feedback
Used
by basic block reordering pass to decide whether to use
unconditional branch or duplicate the code on its
destination. Code is duplicated when its estimated size is
smaller than this value multiplied by the estimated size of
unconditional jump in the hot spots of the
program.
The
reorder-block-duplicate-feedback is used only when
profile feedback is available and may be set to higher
values than reorder-block-duplicate since information
about the hot spots is more
accurate.
max-sched-ready-insns
The
maximum number of instructions ready to be issued the
scheduler should consider at any given time during the first
scheduling pass. Increasing values mean more thorough
searches, making the compilation time increase with probably
little benefit. The default value is
100.
max-sched-region-blocks
The
maximum number of blocks in a region to be considered for
interblock scheduling. The default value is
10.
max-pipeline-region-blocks
The
maximum number of blocks in a region to be considered for
pipelining in the selective scheduler. The default value is
15.
max-sched-region-insns
The
maximum number of insns in a region to be considered for
interblock scheduling. The default value is
100.
max-pipeline-region-insns
The
maximum number of insns in a region to be considered for
pipelining in the selective scheduler. The default value is
200.
min-spec-prob
The
minimum probability (in percents) of reaching a source block
for interblock speculative scheduling. The default value is
40.
max-sched-extend-regions-iters
The
maximum number of iterations through CFG to
extend regions. 0 - disable region extension, N
- do at most N iterations. The default value is
0.
max-sched-insn-conflict-delay
The
maximum conflict delay for an insn to be considered for
speculative motion. The default value is
3.
sched-spec-prob-cutoff
The
minimal probability of speculation success (in percents), so
that speculative insn will be scheduled. The default value
is 40.
sched-mem-true-dep-cost
Minimal
distance (in CPU cycles) between store and load
targeting same memory locations. The default value is
1.
selsched-max-lookahead
The
maximum size of the lookahead window of selective
scheduling. It is a depth of search for available
instructions. The default value is
50.
selsched-max-sched-times
The
maximum number of times that an instruction will be
scheduled during selective scheduling. This is the limit on
the number of iterations through which the instruction may
be pipelined. The default value is
2.
selsched-max-insns-to-rename
The
maximum number of best instructions in the ready list that
are considered for renaming in the selective scheduler. The
default value is 2.
sms-min-sc
The
minimum value of stage count that swing modulo scheduler
will generate. The default value is
2.
max-last-value-rtl
The
maximum size measured as number of RTLs that can be recorded
in an expression in combiner for a pseudo register as last
known value of that register. The default is
10000.
integer-share-limit
Small
integer constants can use a shared data structure, reducing
the compiler’s memory usage and increasing its speed.
This sets the maximum value of a shared integer constant.
The default value is
256.
min-virtual-mappings
Specifies
the minimum number of virtual mappings in the
incremental SSA updater that should be registered
to trigger the virtual mappings heuristic defined by
virtual-mappings-ratio. The default value is
100.
virtual-mappings-ratio
If
the number of virtual mappings is virtual-mappings-ratio
bigger than the number of virtual symbols to be updated,
then the incremental SSA updater switches to a
full update for those symbols. The default ratio is
3.
ssp-buffer-size
The
minimum size of buffers (i.e. arrays) that will receive
stack smashing protection when
-fstack-protection is
used.
This
default before Ubuntu 10.10 was "8". Currently it
is "4", to increase the number of functions
protected by the stack
protector.
max-jump-thread-duplication-stmts
Maximum
number of statements allowed in a block that needs to be
duplicated when threading
jumps.
max-fields-for-field-sensitive
Maximum
number of fields in a structure we will treat in a field
sensitive manner during pointer analysis. The default is
zero for -O0, and -O1 and 100 for -Os,
-O2, and
-O3.
prefetch-latency
Estimate
on average number of instructions that are executed before
prefetch finishes. The distance we prefetch ahead is
proportional to this constant. Increasing this number may
also lead to less streams being prefetched (see
simultaneous-prefetches).
simultaneous-prefetches
Maximum
number of prefetches that can run at the same
time.
l1-cache-line-size
The
size of cache line in L1 cache, in
bytes.
l1-cache-size
The
size of L1 cache, in
kilobytes.
l2-cache-size
The
size of L2 cache, in
kilobytes.
min-insn-to-prefetch-ratio
The
minimum ratio between the number of instructions and the
number of prefetches to enable prefetching in a
loop.
prefetch-min-insn-to-mem-ratio
The
minimum ratio between the number of instructions and the
number of memory references to enable prefetching in a
loop.
use-canonical-types
Whether
the compiler should use the "canonical" type
system. By default, this should always be 1, which uses a
more efficient internal mechanism for comparing types in
C ++ and Objective-C ++ .
However, if bugs in the canonical type system are causing
compilation failures, set this value to 0 to disable
canonical types.
switch-conversion-max-branch-ratio
Switch
initialization conversion will refuse to create arrays that
are bigger than switch-conversion-max-branch-ratio
times the number of branches in the
switch.
max-partial-antic-length
Maximum
length of the partial antic set computed during the tree
partial redundancy elimination optimization
(-ftree-pre) when optimizing at
-O3 and above. For some sorts of source code
the enhanced partial redundancy elimination optimization can
run away, consuming all of the memory available on the host
machine. This parameter sets a limit on the length of the
sets that are computed, which prevents the runaway behavior.
Setting a value of 0 for this parameter will allow an
unlimited set
length.
sccvn-max-scc-size
Maximum
size of a strongly connected component ( SCC )
during SCCVN processing. If this limit is
hit, SCCVN processing for the whole function will
not be done and optimizations depending on it will be
disabled. The default maximum SCC size is
10000.
ira-max-loops-num
IRA
uses regional register allocation by default. If a
function contains more loops than the number given by this
parameter, only at most the given number of the most
frequently-executed loops form regions for regional register
allocation. The default value of the parameter is
100.
ira-max-conflict-table-size
Although
IRA uses a sophisticated algorithm to compress the
conflict table, the table can still require excessive
amounts of memory for huge functions. If the conflict table
for a function could be more than the size in MB
given by this parameter, the register allocator instead
uses a faster, simpler, and lower-quality algorithm that
does not require building a pseudo-register conflict table.
The default value of the parameter is
2000.
ira-loop-reserved-regs
IRA
can be used to evaluate more accurate register pressure
in loops for decisions to move loop invariants (see
-O3). The number of available registers
reserved for some other purposes is given by this parameter.
The default value of the parameter is 2, which is the
minimal number of registers needed by typical instructions.
This value is the best found from numerous
experiments.
loop-invariant-max-bbs-in-loop
Loop
invariant motion can be very expensive, both in compilation
time and in amount of needed compile-time memory, with very
large loops. Loops with more basic blocks than this
parameter won’t have loop invariant motion
optimization performed on them. The default value of the
parameter is 1000 for -O1 and 10000 for -O2 and
above.
loop-max-datarefs-for-datadeps
Building
data dapendencies is expensive for very large loops. This
parameter limits the number of data references in loops that
are considered for data dependence analysis. These large
loops will not be handled then by the optimizations using
loop data dependencies. The default value is
1000.
max-vartrack-size
Sets
a maximum number of hash table slots to use during variable
tracking dataflow analysis of any function. If this limit is
exceeded with variable tracking at assignments enabled,
analysis for that function is retried without it, after
removing all debug insns from the function. If the limit is
exceeded even without debug insns, var tracking analysis is
completely disabled for the function. Setting the parameter
to zero makes it
unlimited.
max-vartrack-expr-depth
Sets
a maximum number of recursion levels when attempting to map
variable names or debug temporaries to value expressions.
This trades compilation time for more complete debug
information. If this is set too low, value expressions that
are available and could be represented in debug information
may end up not being used; setting this higher may enable
the compiler to find more complex debug expressions, but
compile time and memory use may grow. The default is
12.
min-nondebug-insn-uid
Use
uids starting at this parameter for nondebug insns. The
range below the parameter is reserved exclusively for debug
insns created by
-fvar-tracking-assignments, but
debug insns may get (non-overlapping) uids above it if the
reserved range is
exhausted.
ipa-sra-ptr-growth-factor
IPA-SRA
will replace a pointer to an aggregate with one or more new
parameters only when their cumulative size is less or equal
to ipa-sra-ptr-growth-factor times the size of the
original pointer
parameter.
tm-max-aggregate-size
When
making copies of thread-local variables in a transaction,
this parameter specifies the size in bytes after which
variables will be saved with the logging functions as
opposed to save/restore code sequence pairs. This option
only applies when using
-fgnu-tm.
graphite-max-nb-scop-params
To
avoid exponential effects in the Graphite loop transforms,
the number of parameters in a Static Control Part (SCoP) is
bounded. The default value is 10 parameters. A variable
whose value is unknown at compilation time and defined
outside a SCoP is a parameter of the
SCoP.
graphite-max-bbs-per-function
To
avoid exponential effects in the detection of SCoPs, the
size of the functions analyzed by Graphite is bounded. The
default value is 100 basic
blocks.
loop-block-tile-size
Loop
blocking or strip mining transforms, enabled with
-floop-block or
-floop-strip-mine, strip mine each
loop in the loop nest by a given number of iterations. The
strip length can be changed using the
loop-block-tile-size parameter. The default value is
51 iterations.
ipa-cp-value-list-size
IPA-CP
attempts to track all possible values and types passed to a
function’s parameter in order to propagate them and
perform devirtualization. ipa-cp-value-list-size is
the maximum number of values and types it stores per one
formal parameter of a
function.
lto-partitions
Specify
desired number of partitions produced during WHOPR
compilation. The number of partitions should exceed the
number of CPUs used for compilation. The default value is
32.
lto-minpartition
Size
of minimal partition for WHOPR (in estimated
instructions). This prevents expenses of splitting very
small programs into too many
partitions.
cxx-max-namespaces-for-diagnostic-help
The
maximum number of namespaces to consult for suggestions when
C ++ name lookup fails for an identifier. The
default is 1000.
sink-frequency-threshold
The
maximum relative execution frequency (in percents) of the
target block relative to a statement’s original block
to allow statement sinking of a statement. Larger numbers
result in more aggressive statement sinking. The default
value is 75. A small positive adjustment is applied for
statements with memory operands as those are even more
profitable so sink.
max-stores-to-sink
The
maximum number of conditional stores paires that can be
sunk. Set to 0 if either vectorization
(-ftree-vectorize) or if-conversion
(-ftree-loop-if-convert) is
disabled. The default is
2.
allow-load-data-races
Allow
optimizers to introduce new data races on loads. Set to 1 to
allow, otherwise to 0. This option is enabled by default
unless implicitly set by the
-fmemory-model=
option.
allow-store-data-races
Allow
optimizers to introduce new data races on stores. Set to 1
to allow, otherwise to 0. This option is enabled by default
unless implicitly set by the
-fmemory-model=
option.
allow-packed-load-data-races
Allow
optimizers to introduce new data races on packed data loads.
Set to 1 to allow, otherwise to 0. This option is enabled by
default unless implicitly set by the
-fmemory-model=
option.
allow-packed-store-data-races
Allow
optimizers to introduce new data races on packed data
stores. Set to 1 to allow, otherwise to 0. This option is
enabled by default unless implicitly set by the
-fmemory-model=
option.
case-values-threshold
The
smallest number of different values for which it is best to
use a jump-table instead of a tree of conditional branches.
If the value is 0, use the default for the machine. The
default is 0.
tree-reassoc-width
Set
the maximum number of instructions executed in parallel in
reassociated tree. This parameter overrides target dependent
heuristics used by default if has non zero
value.
Options
Controlling the Preprocessor
These options control the C preprocessor, which is run on
each C source file before actual
compilation.
If
you use the -E option, nothing is done except
preprocessing. Some of these options make sense only
together with -E because they cause the
preprocessor output to be unsuitable for actual compilation.
-Wp,option
You
can use -Wp,option to bypass the
compiler driver and pass option directly through to
the preprocessor. If option contains commas, it is
split into multiple options at the commas. However, many
options are modified, translated or interpreted by the
compiler driver before being passed to the preprocessor, and
-Wp forcibly bypasses this phase. The
preprocessor’s direct interface is undocumented and
subject to change, so whenever possible you should avoid
using -Wp and let the driver handle the options
instead.
-Xpreprocessor
option
Pass
option as an option to the preprocessor. You can use
this to supply system-specific preprocessor options
that GCC does not know how to
recognize.
If
you want to pass an option that takes an argument, you must
use -Xpreprocessor twice, once for the option
and once for the
argument.
-D
name
Predefine
name as a macro, with definition
1.
-D
name=definition
The
contents of definition are tokenized and processed as
if they appeared during translation phase three in a
#define directive. In particular, the definition will
be truncated by embedded newline
characters.
If
you are invoking the preprocessor from a shell or shell-like
program you may need to use the shell’s quoting syntax
to protect characters such as spaces that have a meaning in
the shell syntax.
If
you wish to define a function-like macro on the command
line, write its argument list with surrounding parentheses
before the equals sign (if any). Parentheses are meaningful
to most shells, so you will need to quote the option. With
sh and csh,
-D’name(args...)=definition’
works.
-D
and -U options are processed in the order they
are given on the command line. All -imacros
file and -include file options
are processed after all -D and -U
options.
-U
name
Cancel
any previous definition of name, either built in or
provided with a -D
option.
-undef
Do
not predefine any system-specific or GCC-specific macros.
The standard predefined macros remain
defined.
-I
dir
Add
the directory dir to the list of directories to be
searched for header files. Directories named by
-I are searched before the standard system
include directories. If the directory dir is a
standard system include directory, the option is ignored to
ensure that the default search order for system directories
and the special treatment of system headers are not defeated
. If dir begins with "=", then the
"=" will be replaced by the sysroot
prefix; see --sysroot and
-isysroot.
-o
file
Write
output to file. This is the same as specifying
file as the second non-option argument to cpp.
gcc has a different interpretation of a second
non-option argument, so you must use -o to
specify the output
file.
-Wall
Turns
on all optional warnings which are desirable for normal
code. At present this is -Wcomment,
-Wtrigraphs, -Wmultichar and a
warning about integer promotion causing a change of sign in
"#if" expressions. Note that many of the
preprocessor’s warnings are on by default and have no
options to control
them.
-Wcomment
-Wcomments
Warn
whenever a comment-start sequence /* appears in a
/* comment, or whenever a backslash-newline appears
in a // comment. (Both forms have the same
effect.)
-Wtrigraphs
Most
trigraphs in comments cannot affect the meaning of the
program. However, a trigraph that would form an escaped
newline (??/ at the end of a line) can, by changing
where the comment begins or ends. Therefore, only trigraphs
that would form escaped newlines produce warnings inside a
comment.
This
option is implied by -Wall. If
-Wall is not given, this option is still
enabled unless trigraphs are enabled. To get trigraph
conversion without warnings, but get the other
-Wall warnings, use -trigraphs
-Wall
-Wno-trigraphs.
-Wtraditional
Warn
about certain constructs that behave differently in
traditional and ISO C. Also warn about ISO
C constructs that have no traditional C equivalent, and
problematic constructs which should be
avoided.
-Wundef
Warn
whenever an identifier which is not a macro is encountered
in an #if directive, outside of defined. Such
identifiers are replaced with
zero.
-Wunused-macros
Warn
about macros defined in the main file that are unused. A
macro is used if it is expanded or tested for
existence at least once. The preprocessor will also warn if
the macro has not been used at the time it is redefined or
undefined.
Built-in
macros, macros defined on the command line, and macros
defined in include files are not warned
about.
Note:
If a macro is actually used, but only used in skipped
conditional blocks, then CPP will report it as
unused. To avoid the warning in such a case, you might
improve the scope of the macro’s definition by, for
example, moving it into the first skipped block.
Alternatively, you could provide a dummy use with something
like:
#if defined the_macro_causing_the_warning
#endif
-Wendif-labels
Warn
whenever an #else or an #endif are followed by
text. This usually happens in code of the
form
#if FOO
#else FOO
#endif FOO
The
second and third "FOO" should be in
comments, but often are not in older programs. This warning
is on by default.
-Werror
Make
all warnings into hard errors. Source code which triggers
warnings will be
rejected.
-Wsystem-headers
Issue
warnings for code in system headers. These are normally
unhelpful in finding bugs in your own code, therefore
suppressed. If you are responsible for the system library,
you may want to see
them.
-w
Suppress all warnings,
including those which GNU CPP issues by
default.
-pedantic
Issue
all the mandatory diagnostics listed in the C standard. Some
of them are left out by default, since they trigger
frequently on harmless
code.
-pedantic-errors
Issue
all the mandatory diagnostics, and make all mandatory
diagnostics into errors. This includes mandatory diagnostics
that GCC issues without -pedantic
but treats as
warnings.
-M
Instead of outputting the
result of preprocessing, output a rule suitable for
make describing the dependencies of the main source
file. The preprocessor outputs one make rule
containing the object file name for that source file, a
colon, and the names of all the included files, including
those coming from -include or
-imacros command line
options.
Unless
specified explicitly (with -MT or
-MQ), the object file name consists of the name
of the source file with any suffix replaced with object file
suffix and with any leading directory parts removed. If
there are many included files then the rule is split into
several lines using \-newline. The rule has no
commands.
This
option does not suppress the preprocessor’s debug
output, such as -dM. To avoid mixing such debug
output with the dependency rules you should explicitly
specify the dependency output file with -MF, or
use an environment variable like
DEPENDENCIES_OUTPUT . Debug output will still be
sent to the regular output stream as
normal.
Passing
-M to the driver implies -E, and
suppresses warnings with an implicit
-w.
-MM
Like
-M but do not mention header files that are
found in system header directories, nor header files that
are included, directly or indirectly, from such a
header.
This
implies that the choice of angle brackets or double quotes
in an #include directive does not in itself determine
whether that header will appear in -MM
dependency output. This is a slight change in semantics
from GCC versions 3.0 and
earlier.
-MF
file
When
used with -M or -MM, specifies a
file to write the dependencies to. If no -MF
switch is given the preprocessor sends the rules to the same
place it would have sent preprocessed
output.
When
used with the driver options -MD or
-MMD, -MF overrides the default
dependency output
file.
-MG
In conjunction with an
option such as -M requesting dependency
generation, -MG assumes missing header files
are generated files and adds them to the dependency list
without raising an error. The dependency filename is taken
directly from the "#include" directive
without prepending any path. -MG also
suppresses preprocessed output, as a missing header file
renders this
useless.
This
feature is used in automatic updating of
makefiles.
-MP
This
option instructs CPP to add a phony target for
each dependency other than the main file, causing each to
depend on nothing. These dummy rules work around errors
make gives if you remove header files without
updating the Makefile to
match.
This
is typical output:
test.o: test.c test.h
test.h:
-MT
target
Change
the target of the rule emitted by dependency generation. By
default CPP takes the name of the main input
file, deletes any directory components and any file suffix
such as .c, and appends the platform’s usual
object suffix. The result is the
target.
An
-MT option will set the target to be exactly
the string you specify. If you want multiple targets, you
can specify them as a single argument to -MT,
or use multiple -MT
options.
For
example, -MT ’$(objpfx)foo.o’
might give
$(objpfx)foo.o: foo.c
-MQ
target
Same
as -MT, but it quotes any characters which are
special to Make.
-MQ ’$(objpfx)foo.o’
gives
$$(objpfx)foo.o: foo.c
The
default target is automatically quoted, as if it were given
with
-MQ.
-MD
-MD is
equivalent to -M -MF file, except
that -E is not implied. The driver determines
file based on whether an -o option is
given. If it is, the driver uses its argument but with a
suffix of .d, otherwise it takes the name of the
input file, removes any directory components and suffix, and
applies a .d
suffix.
If
-MD is used in conjunction with
-E, any -o switch is understood to
specify the dependency output file, but if used without
-E, each -o is understood to
specify a target object
file.
Since
-E is not implied, -MD can be used
to generate a dependency output file as a side-effect of the
compilation process.
-MMD
Like
-MD except mention only user header files, not
system header files.
-fpch-deps
When
using precompiled headers, this flag will cause the
dependency-output flags to also list the files from the
precompiled header’s dependencies. If not specified
only the precompiled header would be listed and not the
files that were used to create it because those files are
not consulted when a precompiled header is
used.
-fpch-preprocess
This
option allows use of a precompiled header together with
-E. It inserts a special
"#pragma", "#pragma GCC
pch_preprocess
"filename"" in the
output to mark the place where the precompiled header was
found, and its filename. When
-fpreprocessed is in use, GCC
recognizes this "#pragma" and loads
the PCH .
This
option is off by default, because the resulting preprocessed
output is only really suitable as input to GCC .
It is switched on by
-save-temps.
You
should not write this "#pragma" in your
own code, but it is safe to edit the filename if the
PCH file is available in a different location. The
filename may be absolute or it may be relative to GCC
’s current
directory.
-x
c
-x c++
-x objective-c
-x
assembler-with-cpp
Specify
the source language: C, C ++ , Objective-C, or
assembly. This has nothing to do with standards conformance
or extensions; it merely selects which base syntax to
expect. If you give none of these options, cpp will deduce
the language from the extension of the source file:
.c, .cc, .m, or .S. Some other
common extensions for C ++ and assembly are also
recognized. If cpp does not recognize the extension, it will
treat the file as C; this is the most generic
mode.
Note:
Previous versions of cpp accepted a -lang
option which selected both the language and the standards
conformance level. This option has been removed, because it
conflicts with the -l
option.
-std=standard
-ansi
Specify
the standard to which the code should conform.
Currently CPP knows about C and C ++
standards; others may be added in the
future.
standard
may be one of:
"c90"
"c89"
"iso9899:1990"
The
ISO C standard from 1990. c90 is the customary
shorthand for this version of the
standard.
The
-ansi option is equivalent to
-std=c90.
"iso9899:199409"
The
1990 C standard, as amended in
1994.
"iso9899:1999"
"c99"
"iso9899:199x"
"c9x"
The
revised ISO C standard, published in December
1999. Before publication, this was known as
C9X.
"iso9899:2011"
"c11"
"c1x"
The
revised ISO C standard, published in December
2011. Before publication, this was known as
C1X.
"gnu90"
"gnu89"
The
1990 C standard plus GNU extensions. This is the
default.
"gnu99"
"gnu9x"
The
1999 C standard plus GNU
extensions.
"gnu11"
"gnu1x"
The
2011 C standard plus GNU
extensions.
"c++98"
The
1998 ISO C ++ standard plus
amendments.
"gnu++98"
The
same as -std=c++98 plus GNU
extensions. This is the default for C ++
code.
-I-
Split the include path.
Any directories specified with -I options
before -I- are searched only for headers
requested with
"#include "file"";
they are not searched for
"#include <file>".
If additional directories are specified with -I
options after the -I-, those directories
are searched for all #include
directives.
In
addition, -I- inhibits the use of the
directory of the current file directory as the first search
directory for
"#include "file"".
This option has been
deprecated.
-nostdinc
Do
not search the standard system directories for header files.
Only the directories you have specified with -I
options (and the directory of the current file, if
appropriate) are
searched.
-nostdinc++
Do
not search for header files in the C ++
-specific standard directories, but do still
search the other standard directories. (This option is used
when building the C ++
library.)
-include
file
Process
file as if "#include
"file"" appeared as the first line of
the primary source file. However, the first directory
searched for file is the preprocessor’s working
directory instead of the directory containing the
main source file. If not found there, it is searched for in
the remainder of the "#include
"..."" search chain as
normal.
If
multiple -include options are given, the files
are included in the order they appear on the command
line.
-imacros
file
Exactly
like -include, except that any output produced
by scanning file is thrown away. Macros it defines
remain defined. This allows you to acquire all the macros
from a header without also processing its
declarations.
All
files specified by -imacros are processed
before all files specified by
-include.
-idirafter
dir
Search
dir for header files, but do it after all
directories specified with -I and the standard
system directories have been exhausted. dir is
treated as a system include directory. If dir begins
with "=", then the "="
will be replaced by the sysroot prefix; see
--sysroot and
-isysroot.
-iprefix
prefix
Specify
prefix as the prefix for subsequent
-iwithprefix options. If the prefix represents
a directory, you should include the final
/.
-iwithprefix
dir
-iwithprefixbefore
dir
Append
dir to the prefix specified previously with
-iprefix, and add the resulting directory to
the include search path. -iwithprefixbefore
puts it in the same place -I would;
-iwithprefix puts it where
-idirafter
would.
-isysroot
dir
This
option is like the --sysroot option, but
applies only to header files (except for Darwin targets,
where it applies to both header files and libraries). See
the --sysroot option for more
information.
-imultilib
dir
Use
dir as a subdirectory of the directory containing
target-specific C ++
headers.
-isystem
dir
Search
dir for header files, after all directories specified
by -I but before the standard system
directories. Mark it as a system directory, so that it gets
the same special treatment as is applied to the standard
system directories. If dir begins with
"=", then the "=" will
be replaced by the sysroot prefix; see
--sysroot and
-isysroot.
-iquote
dir
Search
dir only for header files requested with
"#include "file"";
they are not searched for
"#include <file>",
before all directories specified by -I and
before the standard system directories. If dir begins
with "=", then the "="
will be replaced by the sysroot prefix; see
--sysroot and
-isysroot.
-fdirectives-only
When
preprocessing, handle directives, but do not expand
macros.
The
option’s behavior depends on the -E and
-fpreprocessed
options.
With
-E, preprocessing is limited to the handling of
directives such as "#define",
"#ifdef", and
"#error". Other preprocessor operations,
such as macro expansion and trigraph conversion are not
performed. In addition, the -dD option is
implicitly enabled.
With
-fpreprocessed, predefinition of command line
and most builtin macros is disabled. Macros such as
"__LINE__", which are contextually
dependent, are handled normally. This enables compilation of
files previously preprocessed with "-E
-fdirectives-only".
With
both -E and -fpreprocessed, the
rules for -fpreprocessed take precedence. This
enables full preprocessing of files previously preprocessed
with "-E
-fdirectives-only".
-fdollars-in-identifiers
Accept
$ in
identifiers.
-fextended-identifiers
Accept
universal character names in identifiers. This option is
experimental; in a future version of GCC , it
will be enabled by default for C99 and C ++
.
-fpreprocessed
Indicate
to the preprocessor that the input file has already been
preprocessed. This suppresses things like macro expansion,
trigraph conversion, escaped newline splicing, and
processing of most directives. The preprocessor still
recognizes and removes comments, so that you can pass a file
preprocessed with -C to the compiler without
problems. In this mode the integrated preprocessor is little
more than a tokenizer for the front
ends.
-fpreprocessed
is implicit if the input file has one of the extensions
.i, .ii or .mi. These are the
extensions that GCC uses for preprocessed files
created by
-save-temps.
-ftabstop=width
Set
the distance between tab stops. This helps the preprocessor
report correct column numbers in warnings or errors, even if
tabs appear on the line. If the value is less than 1 or
greater than 100, the option is ignored. The default is
8.
-fdebug-cpp
This
option is only useful for debugging GCC . When
used with -E, dumps debugging information about
location maps. Every token in the output is preceded by the
dump of the map its location belongs to. The dump of the map
holding the location of a token would
be:
{"P":F</file/path>;"F":F</includer/path>;"L":<line_num>;"C":<col_num>;"S":<system_header_p>;"M":<map_address>;"E":<macro_expansion_p>,"loc":<location>}
When
used without -E, this option has no
effect.
-ftrack-macro-expansion[=level]
Track
locations of tokens across macro expansions. This allows the
compiler to emit diagnostic about the current macro
expansion stack when a compilation error occurs in a macro
expansion. Using this option makes the preprocessor and the
compiler consume more memory. The level parameter can
be used to choose the level of precision of token location
tracking thus decreasing the memory consumption if
necessary. Value 0 of level de-activates this
option just as if no
-ftrack-macro-expansion was present
on the command line. Value 1 tracks tokens locations
in a degraded mode for the sake of minimal memory overhead.
In this mode all tokens resulting from the expansion of an
argument of a function-like macro have the same location.
Value 2 tracks tokens locations completely. This
value is the most memory hungry. When this option is given
no argument, the default parameter value is
2.
-fexec-charset=charset
Set
the execution character set, used for string and character
constants. The default is UTF-8 .
charset can be any encoding supported by the
system’s "iconv" library
routine.
-fwide-exec-charset=charset
Set
the wide execution character set, used for wide string and
character constants. The default is UTF-32
or UTF-16 , whichever corresponds to
the width of "wchar_t". As with
-fexec-charset, charset can be any
encoding supported by the system’s
"iconv" library routine; however, you
will have problems with encodings that do not fit exactly in
"wchar_t".
-finput-charset=charset
Set
the input character set, used for translation from the
character set of the input file to the source character set
used by GCC . If the locale does not specify,
or GCC cannot get this information from the
locale, the default is UTF-8 . This can be
overridden by either the locale or this command line option.
Currently the command line option takes precedence if
there’s a conflict. charset can be any encoding
supported by the system’s "iconv"
library routine.
-fworking-directory
Enable
generation of linemarkers in the preprocessor output that
will let the compiler know the current working directory at
the time of preprocessing. When this option is enabled, the
preprocessor will emit, after the initial linemarker, a
second linemarker with the current working directory
followed by two slashes. GCC will use this
directory, when it’s present in the preprocessed
input, as the directory emitted as the current working
directory in some debugging information formats. This option
is implicitly enabled if debugging information is enabled,
but this can be inhibited with the negated form
-fno-working-directory. If the
-P flag is present in the command line, this
option has no effect, since no "#line"
directives are emitted
whatsoever.
-fno-show-column
Do
not print column numbers in diagnostics. This may be
necessary if diagnostics are being scanned by a program that
does not understand the column numbers, such as
dejagnu.
-A
predicate=answer
Make
an assertion with the predicate predicate and answer
answer. This form is preferred to the older form
-A
predicate(answer), which is
still supported, because it does not use shell special
characters.
-A
-predicate=answer
Cancel
an assertion with the predicate predicate and answer
answer.
-dCHARS
CHARS
is a sequence of one or more of the following
characters, and must not be preceded by a space. Other
characters are interpreted by the compiler proper, or
reserved for future versions of GCC , and so are
silently ignored. If you specify characters whose behavior
conflicts, the result is
undefined.
M
Instead of the normal
output, generate a list of #define directives for all
the macros defined during the execution of the preprocessor,
including predefined macros. This gives you a way of finding
out what is predefined in your version of the preprocessor.
Assuming you have no file foo.h, the
command
touch foo.h; cpp -dM foo.h
will
show all the predefined
macros.
If
you use -dM without the -E option,
-dM is interpreted as a synonym for
-fdump-rtl-mach.
D
Like
M except in two respects: it does not include
the predefined macros, and it outputs both the
#define directives and the result of preprocessing.
Both kinds of output go to the standard output
file.
N
Like D, but emit
only the macro names, not their
expansions.
I
Output #include
directives in addition to the result of
preprocessing.
U
Like D except that
only macros that are expanded, or whose definedness is
tested in preprocessor directives, are output; the output is
delayed until the use or test of the macro; and
#undef directives are also output for macros tested
but undefined at the
time.
-P
Inhibit
generation of linemarkers in the output from the
preprocessor. This might be useful when running the
preprocessor on something that is not C code, and will be
sent to a program which might be confused by the
linemarkers.
-C
Do
not discard comments. All comments are passed through to the
output file, except for comments in processed directives,
which are deleted along with the
directive.
You
should be prepared for side effects when using
-C; it causes the preprocessor to treat
comments as tokens in their own right. For example, comments
appearing at the start of what would be a directive line
have the effect of turning that line into an ordinary source
line, since the first token on the line is no longer a
#.
-CC
Do
not discard comments, including during macro expansion. This
is like -C, except that comments contained
within macros are also passed through to the output file
where the macro is
expanded.
In
addition to the side-effects of the -C option,
the -CC option causes all C ++
-style comments inside a macro to be converted to
C-style comments. This is to prevent later use of that
macro from inadvertently commenting out the remainder of the
source line.
The
-CC option is generally used to support lint
comments.
-traditional-cpp
Try
to imitate the behavior of old-fashioned C preprocessors, as
opposed to ISO C
preprocessors.
-trigraphs
Process
trigraph sequences. These are three-character sequences, all
starting with ??, that are defined by ISO
C to stand for single characters. For example,
??/ stands for \, so ’??/n’
is a character constant for a newline. By default, GCC
ignores trigraphs, but in standard-conforming modes it
converts them. See the -std and
-ansi
options.
The
nine trigraphs and their replacements
are
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~
-remap
Enable
special code to work around file systems which only permit
very short file names, such as
MS-DOS.
--help
--target-help
Print
text describing all the command line options instead of
preprocessing
anything.
-v
Verbose mode. Print
out GNU CPP ’s version number at the
beginning of execution, and report the final form of the
include path.
-H
Print the name of each
header file used, in addition to other normal activities.
Each name is indented to show how deep in the
#include stack it is. Precompiled header files are
also printed, even if they are found to be invalid; an
invalid precompiled header file is printed with ...x
and a valid one with ...!
.
-version
--version
Print
out GNU CPP ’s version number. With one
dash, proceed to preprocess as normal. With two dashes, exit
immediately.
Passing
Options to the Assembler
You can pass options to the assembler.
-Wa,option
Pass
option as an option to the assembler. If
option contains commas, it is split into multiple
options at the
commas.
-Xassembler
option
Pass
option as an option to the assembler. You can use
this to supply system-specific assembler options that
GCC does not know how to
recognize.
If
you want to pass an option that takes an argument, you must
use -Xassembler twice, once for the option and
once for the
argument.
Options
for Linking
These options come into play when the compiler links object
files into an executable output file. They are meaningless
if the compiler is not doing a link step.
object-file-name
A
file name that does not end in a special recognized suffix
is considered to name an object file or library. (Object
files are distinguished from libraries by the linker
according to the file contents.) If linking is done, these
object files are used as input to the
linker.
-c
-S
-E
If any of these options is
used, then the linker is not run, and object file names
should not be used as
arguments.
-llibrary
-l
library
Search
the library named library when linking. (The second
alternative with the library as a separate argument is only
for POSIX compliance and is not
recommended.)
It
makes a difference where in the command you write this
option; the linker searches and processes libraries and
object files in the order they are specified. Thus, foo.o
-lz bar.o searches library z after file
foo.o but before bar.o. If bar.o refers
to functions in z, those functions may not be
loaded.
The
linker searches a standard list of directories for the
library, which is actually a file named liblibrary.a.
The linker then uses this file as if it had been specified
precisely by name.
The
directories searched include several standard system
directories plus any that you specify with
-L.
Normally
the files found this way are library
files---archive files whose members are
object files. The linker handles an archive file by scanning
through it for members which define symbols that have so far
been referenced but not defined. But if the file that is
found is an ordinary object file, it is linked in the usual
fashion. The only difference between using an
-l option and specifying a file name is that
-l surrounds library with lib and
.a and searches several
directories.
-lobjc
You
need this special case of the -l option in
order to link an Objective-C or Objective-C ++
program.
-nostartfiles
Do
not use the standard system startup files when linking. The
standard system libraries are used normally, unless
-nostdlib or -nodefaultlibs is
used.
-nodefaultlibs
Do
not use the standard system libraries when linking. Only the
libraries you specify will be passed to the linker, options
specifying linkage of the system libraries, such as
"-static-libgcc" or
"-shared-libgcc", will be
ignored. The standard startup files are used normally,
unless -nostartfiles is used. The compiler may
generate calls to "memcmp",
"memset", "memcpy" and
"memmove". These entries are usually
resolved by entries in libc. These entry points should be
supplied through some other mechanism when this option is
specified.
-nostdlib
Do
not use the standard system startup files or libraries when
linking. No startup files and only the libraries you specify
will be passed to the linker, options specifying linkage of
the system libraries, such as
"-static-libgcc" or
"-shared-libgcc", will be
ignored. The compiler may generate calls to
"memcmp", "memset",
"memcpy" and
"memmove". These entries are usually
resolved by entries in libc. These entry points should be
supplied through some other mechanism when this option is
specified.
One
of the standard libraries bypassed by -nostdlib
and -nodefaultlibs is libgcc.a, a
library of internal subroutines which GCC uses to
overcome shortcomings of particular machines, or special
needs for some
languages.
In
most cases, you need libgcc.a even when you want to
avoid other standard libraries. In other words, when you
specify -nostdlib or
-nodefaultlibs you should usually specify
-lgcc as well. This ensures that you have no
unresolved references to internal GCC library
subroutines. (For example, __main, used to ensure
C ++ constructors will be
called.)
-pie
Produce
a position independent executable on targets that support
it. For predictable results, you must also specify the same
set of options that were used to generate code
(-fpie, -fPIE, or model
suboptions) when you specify this
option.
-rdynamic
Pass
the flag -export-dynamic to the ELF
linker, on targets that support it. This instructs the
linker to add all symbols, not only used ones, to the
dynamic symbol table. This option is needed for some uses of
"dlopen" or to allow obtaining backtraces
from within a
program.
-s
Remove all symbol table
and relocation information from the
executable.
-static
On
systems that support dynamic linking, this prevents linking
with the shared libraries. On other systems, this option has
no effect.
-shared
Produce
a shared object which can then be linked with other objects
to form an executable. Not all systems support this option.
For predictable results, you must also specify the same set
of options that were used to generate code
(-fpic, -fPIC, or model
suboptions) when you specify this
option.[1]
-shared-libgcc
-static-libgcc
On
systems that provide libgcc as a shared library,
these options force the use of either the shared or static
version respectively. If no shared version of libgcc
was built when the compiler was configured, these options
have no effect.
There
are several situations in which an application should use
the shared libgcc instead of the static version. The
most common of these is when the application wishes to throw
and catch exceptions across different shared libraries. In
that case, each of the libraries as well as the application
itself should use the shared
libgcc.
Therefore,
the G++ and GCJ drivers automatically add
-shared-libgcc whenever you build a
shared library or a main executable, because C ++
and Java programs typically use exceptions, so this is
the right thing to
do.
If,
instead, you use the GCC driver to create shared
libraries, you may find that they will not always be linked
with the shared libgcc. If GCC finds, at
its configuration time, that you have a non-GNU linker or
a GNU linker that does not support option
--eh-frame-hdr, it will link
the shared version of libgcc into shared libraries by
default. Otherwise, it will take advantage of the linker and
optimize away the linking with the shared version of
libgcc, linking with the static version of libgcc by
default. This allows exceptions to propagate through such
shared libraries, without incurring relocation costs at
library load time.
However,
if a library or main executable is supposed to throw or
catch exceptions, you must link it using the G++ or
GCJ driver, as appropriate for the languages used in
the program, or using the option
-shared-libgcc, such that it is linked
with the shared
libgcc.
-static-libstdc++
When
the g++ program is used to link a C ++
program, it will normally automatically link against
libstdc++. If libstdc++ is available as a
shared library, and the -static option is not
used, then this will link against the shared version of
libstdc++. That is normally fine. However, it is
sometimes useful to freeze the version of libstdc++
used by the program without going all the way to a fully
static link. The -static-libstdc++ option
directs the g++ driver to link libstdc++
statically, without necessarily linking other libraries
statically.
-symbolic
Bind
references to global symbols when building a shared object.
Warn about any unresolved references (unless overridden by
the link editor option -Xlinker -z
-Xlinker defs). Only a few systems support this
option.
-T
script
Use
script as the linker script. This option is supported
by most systems using the GNU linker. On some
targets, such as bare-board targets without an operating
system, the -T option may be required when
linking to avoid references to undefined
symbols.
-Xlinker
option
Pass
option as an option to the linker. You can use this
to supply system-specific linker options that GCC
does not
recognize.
If
you want to pass an option that takes a separate argument,
you must use -Xlinker twice, once for the
option and once for the argument. For example, to pass
-assert definitions, you must write
-Xlinker -assert -Xlinker
definitions. It does not work to write -Xlinker
"-assert definitions", because this
passes the entire string as a single argument, which is not
what the linker
expects.
When
using the GNU linker, it is usually more
convenient to pass arguments to linker options using the
option=value syntax than as separate
arguments. For example, you can specify -Xlinker
-Map=output.map rather than -Xlinker
-Map -Xlinker output.map. Other linkers may
not support this syntax for command-line
options.
-Wl,option
Pass
option as an option to the linker. If option
contains commas, it is split into multiple options at the
commas. You can use this syntax to pass an argument to the
option. For example, -Wl,-Map,output.map
passes -Map output.map to the linker. When
using the GNU linker, you can also get the same
effect with
-Wl,-Map=output.map.
NOTE:
In Ubuntu 8.10 and later versions, for LDFLAGS
, the option -Wl,-z,relro is used.
To disable, use
-Wl,-z,norelro.
-u
symbol
Pretend
the symbol symbol is undefined, to force linking of
library modules to define it. You can use -u
multiple times with different symbols to force loading of
additional library
modules.
Options
for Directory Search
These options specify directories to search for header
files, for libraries and for parts of the compiler:
-Idir
Add
the directory dir to the head of the list of
directories to be searched for header files. This can be
used to override a system header file, substituting your own
version, since these directories are searched before the
system header file directories. However, you should not use
this option to add directories that contain vendor-supplied
system header files (use -isystem for that). If
you use more than one -I option, the
directories are scanned in left-to-right order; the standard
system directories come
after.
If
a standard system include directory, or a directory
specified with -isystem, is also specified with
-I, the -I option will be ignored.
The directory will still be searched but as a system
directory at its normal position in the system include
chain. This is to ensure that GCC ’s
procedure to fix buggy system headers and the ordering for
the include_next directive are not inadvertently changed. If
you really need to change the search order for system
directories, use the -nostdinc and/or
-isystem
options.
-iplugindir=dir
Set
the directory to search for plugins that are passed by
-fplugin=name instead of
-fplugin=path/name.so.
This option is not meant to be used by the user, but only
passed by the
driver.
-iquotedir
Add
the directory dir to the head of the list of
directories to be searched for header files only for the
case of #include "file"; they
are not searched for #include
<file>, otherwise just like
-I.
-Ldir
Add
directory dir to the list of directories to be
searched for
-l.
-Bprefix
This
option specifies where to find the executables, libraries,
include files, and data files of the compiler
itself.
The
compiler driver program runs one or more of the subprograms
cpp, cc1, as and ld. It tries
prefix as a prefix for each program it tries to run,
both with and without
machine/version/.
For
each subprogram to be run, the compiler driver first tries
the -B prefix, if any. If that name is not
found, or if -B was not specified, the driver
tries two standard prefixes, /usr/lib/gcc/ and
/usr/local/lib/gcc/. If neither of those results in a
file name that is found, the unmodified program name is
searched for using the directories specified in your
PATH environment
variable.
The
compiler will check to see if the path provided by the
-B refers to a directory, and if necessary it
will add a directory separator character at the end of the
path.
-B
prefixes that effectively specify directory names also apply
to libraries in the linker, because the compiler translates
these options into -L options for the linker.
They also apply to includes files in the preprocessor,
because the compiler translates these options into
-isystem options for the preprocessor. In this
case, the compiler appends include to the
prefix.
The
runtime support file libgcc.a can also be searched
for using the -B prefix, if needed. If it is
not found there, the two standard prefixes above are tried,
and that is all. The file is left out of the link if it is
not found by those
means.
Another
way to specify a prefix much like the -B prefix
is to use the environment variable
GCC_EXEC_PREFIX
.
As
a special kludge, if the path provided by -B is
[dir/]stageN/, where N is a number in the
range 0 to 9, then it will be replaced by
[dir/]include. This is to help with boot-strapping
the compiler.
-specs=file
Process
file after the compiler reads in the standard
specs file, in order to override the defaults which
the gcc driver program uses when determining what
switches to pass to cc1, cc1plus, as,
ld, etc. More than one
-specs=file can be specified on the
command line, and they are processed in order, from left to
right.
--sysroot=dir
Use
dir as the logical root directory for headers and
libraries. For example, if the compiler would normally
search for headers in /usr/include and libraries in
/usr/lib, it will instead search
dir/usr/include and
dir/usr/lib.
If
you use both this option and the -isysroot
option, then the --sysroot option will
apply to libraries, but the -isysroot option
will apply to header
files.
The
GNU linker (beginning with version 2.16) has the
necessary support for this option. If your linker does not
support this option, the header file aspect of
--sysroot will still work, but the
library aspect will
not.
-I-
This option has been
deprecated. Please use -iquote instead for
-I directories before the
-I- and remove the
-I-. Any directories you specify with
-I options before the -I-
option are searched only for the case of #include
"file"; they are not searched
for #include
<file>.
If
additional directories are specified with -I
options after the -I-, these directories
are searched for all #include directives. (Ordinarily
all -I directories are used this
way.)
In
addition, the -I- option inhibits the use
of the current directory (where the current input file came
from) as the first search directory for #include
"file". There is no way to
override this effect of -I-. With
-I. you can specify searching the directory
that was current when the compiler was invoked. That is not
exactly the same as what the preprocessor does by default,
but it is often
satisfactory.
-I-
does not inhibit the use of the standard system directories
for header files. Thus, -I- and
-nostdinc are
independent.
Specifying
Target Machine and Compiler Version
The usual way to run GCC is to run the executable
called gcc, or machine-gcc when
cross-compiling, or
machine-gcc-version to run
a version other than the one that was installed
last.
Hardware
Models and Configurations
Each target machine types can have its own special options,
starting with -m, to choose among various
hardware models or configurations---for
example, 68010 vs 68020, floating coprocessor or none. A
single installed version of the compiler can compile for any
model or configuration, according to the options
specified.
Some
configurations of the compiler also support additional
special options, usually for compatibility with other
compilers on the same
platform.
Adapteva
Epiphany Options
These
-m options are defined for Adapteva Epiphany:
-mhalf-reg-file
Don’t
allocate any register in the range
"r32"..."r63". That
allows code to run on hardware variants that lack these
registers.
-mprefer-short-insn-regs
Preferrentially
allocate registers that allow short instruction generation.
This can result in increasesd instruction count, so if this
reduces or increases code size might vary from case to
case.
-mbranch-cost=num
Set
the cost of branches to roughly num
"simple" instructions. This cost is only a
heuristic and is not guaranteed to produce consistent
results across
releases.
-mcmove
Enable
the generation of conditional
moves.
-mnops=num
Emit
num nops before every other generated
instruction.
-mno-soft-cmpsf
For
single-precision floating-point comparisons, emit an fsub
instruction and test the flags. This is faster than a
software comparison, but can get incorrect results in the
presence of NaNs, or when two different small numbers are
compared such that their difference is calculated as zero.
The default is -msoft-cmpsf, which uses
slower, but IEEE-compliant, software
comparisons.
-mstack-offset=num
Set
the offset between the top of the stack and the stack
pointer. E.g., a value of 8 means that the eight bytes in
the range sp+0...sp+7 can be used by leaf functions without
stack allocation. Values other than 8 or 16
are untested and unlikely to work. Note also that this
option changes the ABI , compiling a program with
a different stack offset than the libraries have been
compiled with will generally not work. This option can be
useful if you want to evaluate if a different stack offset
would give you better code, but to actually use a different
stack offset to build working programs, it is recommended to
configure the toolchain with the appropriate
--with-stack-offset=num
option.
-mno-round-nearest
Make
the scheduler assume that the rounding mode has been set to
truncating. The default is
-mround-nearest.
-mlong-calls
If
not otherwise specified by an attribute, assume all calls
might be beyond the offset range of the b / bl instructions,
and therefore load the function address into a register
before performing a (otherwise direct) call. This is the
default.
-mshort-calls
If
not otherwise specified by an attribute, assume all direct
calls are in the range of the b / bl instructions, so use
these instructions for direct calls. The default is
-mlong-calls.
-msmall16
Assume
addresses can be loaded as 16-bit unsigned values.
This does not apply to function addresses for which
-mlong-calls semantics are in
effect.
-mfp-mode=mode
Set
the prevailing mode of the floating-point unit. This
determines the floating-point mode that is provided and
expected at function call and return time. Making this mode
match the mode you predominantly need at function start can
make your programs smaller and faster by avoiding
unnecessary mode
switches.
mode
can be set to one the following values:
caller
Any
mode at function entry is valid, and retained or restored
when the function returns, and when it calls other
functions. This mode is useful for compiling libraries or
other compilation units you might want to incorporate into
different programs with different prevailing FPU
modes, and the convenience of being able to use a
single object file outweighs the size and speed overhead for
any extra mode switching that might be needed, compared with
what would be needed with a more specific choice of
prevailing FPU
mode.
truncate
This
is the mode used for floating-point calculations with
truncating (i.e. round towards zero) rounding mode. That
includes conversion from floating point to
integer.
round-nearest
This
is the mode used for floating-point calculations with
round-to-nearest-or-even rounding
mode.
int
This is the mode used to
perform integer calculations in the FPU , e.g.
integer multiply, or integer
multiply-and-accumulate.
The
default is
-mfp-mode=caller
-mnosplit-lohi
-mno-postinc
-mno-postmodify
Code
generation tweaks that disable, respectively, splitting of
32-bit loads, generation of post-increment addresses,
and generation of post-modify addresses. The defaults are
msplit-lohi, -mpost-inc, and
-mpost-modify.
-mnovect-double
Change
the preferred SIMD mode to SImode. The default is
-mvect-double, which uses DImode as
preferred SIMD
mode.
-max-vect-align=num
The
maximum alignment for SIMD vector mode types.
num may be 4 or 8. The default is 8. Note that this
is an ABI change, even though many library
function interfaces will be unaffected, if they don’t
use SIMD vector modes in places where they affect
size and/or alignment of relevant
types.
-msplit-vecmove-early
Split
vector moves into single word moves before reload. In theory
this could give better register allocation, but so far the
reverse seems to be generally the
case.
-m1reg-reg
Specify
a register to hold the constant -1, which makes
loading small negative constants and certain bitmasks
faster. Allowable values for reg are r43 and r63, which
specify to use that register as a fixed register, and none,
which means that no register is used for this purpose. The
default is
-m1reg-none.
AArch64
Options
These
options are defined for AArch64 implementations:
-mbig-endian
Generate
big-endian code. This is the default when GCC is
configured for an aarch64_be-*-*
target.
-mgeneral-regs-only
Generate
code which uses only the general
registers.
-mlittle-endian
Generate
little-endian code. This is the default when GCC
is configured for an aarch64-*-* but
not an aarch64_be-*-*
target.
-mcmodel=tiny
Generate
code for the tiny code model. The program and its statically
defined symbols must be within 1GB of each other. Pointers
are 64 bits. Programs can be statically or dynamically
linked. This model is not fully implemented and mostly
treated as
"small".
-mcmodel=small
Generate
code for the small code model. The program and its
statically defined symbols must be within 4GB of each other.
Pointers are 64 bits. Programs can be statically or
dynamically linked. This is the default code
model.
-mcmodel=large
Generate
code for the large code model. This makes no assumptions
about addresses and sizes of sections. Pointers are 64 bits.
Programs can be statically linked
only.
-mstrict-align
Do
not assume that unaligned memory references will be handled
by the system.
-momit-leaf-frame-pointer
-mno-omit-leaf-frame-pointer
Omit
or keep the frame pointer in leaf functions. The former
behaviour is the
default.
-mtls-dialect=desc
Use
TLS descriptors as the thread-local storage mechanism
for dynamic accesses of TLS variables. This is
the default.
-mtls-dialect=traditional
Use
traditional TLS as the thread-local storage
mechanism for dynamic accesses of TLS
variables.
-march=name
Specify
the name of the target architecture, optionally suffixed by
one or more feature modifiers. This option has the form
-march=arch{+[no]feature}*,
where the only value for arch is
armv8-a. The possible values for feature
are documented in the sub-section
below.
Where
conflicting feature modifiers are specified, the right-most
feature is used.
GCC
uses this name to determine what kind of instructions
it can emit when generating assembly code. This option can
be used in conjunction with or instead of the
-mcpu=
option.
-mcpu=name
Specify
the name of the target processor, optionally suffixed by one
or more feature modifiers. This option has the form
-mcpu=cpu{+[no]feature}*,
where the possible values for cpu are generic,
large. The possible values for feature are
documented in the sub-section
below.
Where
conflicting feature modifiers are specified, the right-most
feature is used.
GCC
uses this name to determine what kind of instructions
it can emit when generating assembly
code.
-mtune=name
Specify
the name of the processor to tune the performance for. The
code will be tuned as if the target processor were of the
type specified in this option, but still using instructions
compatible with the target processor specified by a
-mcpu= option. This option cannot be suffixed
by feature
modifiers.
-march
and -mcpu feature
modifiers
Feature
modifiers used with -march and
-mcpu can be one the following:
crypto
Enable
Crypto extension. This implies Advanced SIMD is
enabled.
fp
Enable floating-point
instructions.
simd
Enable
Advanced SIMD instructions. This implies
floating-point instructions are enabled. This is the default
for all current possible values for options
-march and
-mcpu=.
ARM
Options
These
-m options are defined for Advanced RISC
Machines ( ARM ) architectures:
-mabi=name
Generate
code for the specified ABI . Permissible values
are: apcs-gnu, atpcs, aapcs,
aapcs-linux and
iwmmxt.
-mapcs-frame
Generate
a stack frame that is compliant with the ARM
Procedure Call Standard for all functions, even if this
is not strictly necessary for correct execution of the code.
Specifying -fomit-frame-pointer
with this option will cause the stack frames not to be
generated for leaf functions. The default is
-mno-apcs-frame.
-mapcs
This
is a synonym for
-mapcs-frame.
-mthumb-interwork
Generate
code that supports calling between the ARM and
Thumb instruction sets. Without this option, on pre-v5
architectures, the two instruction sets cannot be reliably
used inside one program. The default is
-mno-thumb-interwork, since
slightly larger code is generated when
-mthumb-interwork is specified. In
AAPCS configurations this option is
meaningless.
-mno-sched-prolog
Prevent
the reordering of instructions in the function prologue, or
the merging of those instruction with the instructions in
the function’s body. This means that all functions
will start with a recognizable set of instructions (or in
fact one of a choice from a small set of different function
prologues), and this information can be used to locate the
start if functions inside an executable piece of code. The
default is
-msched-prolog.
-mfloat-abi=name
Specifies
which floating-point ABI to use. Permissible
values are: soft, softfp and
hard.
Specifying
soft causes GCC to generate output
containing library calls for floating-point operations.
softfp allows the generation of code using hardware
floating-point instructions, but still uses the soft-float
calling conventions. hard allows generation of
floating-point instructions and uses FPU-specific calling
conventions.
The
default depends on the specific target configuration. Note
that the hard-float and soft-float ABIs are not
link-compatible; you must compile your entire program with
the same ABI , and link with a compatible set of
libraries.
-mlittle-endian
Generate
code for a processor running in little-endian mode. This is
the default for all standard
configurations.
-mbig-endian
Generate
code for a processor running in big-endian mode; the default
is to compile code for a little-endian
processor.
-mwords-little-endian
This
option only applies when generating code for big-endian
processors. Generate code for a little-endian word order but
a big-endian byte order. That is, a byte order of the form
32107654. Note: this option should only be used if
you require compatibility with code for big-endian ARM
processors generated by versions of the compiler prior
to 2.8. This option is now
deprecated.
-mcpu=name
This
specifies the name of the target ARM
processor. GCC uses this name to determine
what kind of instructions it can emit when generating
assembly code. Permissible names are: arm2,
arm250, arm3, arm6, arm60,
arm600, arm610, arm620, arm7,
arm7m, arm7d, arm7dm, arm7di,
arm7dmi, arm70, arm700, arm700i,
arm710, arm710c, arm7100,
arm720, arm7500, arm7500fe,
arm7tdmi, arm7tdmi-s, arm710t,
arm720t, arm740t, strongarm,
strongarm110, strongarm1100,
strongarm1110, arm8, arm810,
arm9, arm9e, arm920, arm920t,
arm922t, arm946e-s,
arm966e-s, arm968e-s,
arm926ej-s, arm940t, arm9tdmi,
arm10tdmi, arm1020t, arm1026ej-s,
arm10e, arm1020e, arm1022e,
arm1136j-s, arm1136jf-s,
mpcore, mpcorenovfp, arm1156t2-s,
arm1156t2f-s, arm1176jz-s,
arm1176jzf-s, cortex-a5,
cortex-a7, cortex-a8,
cortex-a9, cortex-a15,
cortex-r4, cortex-r4f,
cortex-r5, cortex-m4,
cortex-m3, cortex-m1,
cortex-m0, xscale, iwmmxt,
iwmmxt2, ep9312, fa526, fa626,
fa606te, fa626te, fmp626,
fa726te.
-mcpu=generic-arch
is also permissible, and is equivalent to
-march=arch
-mtune=generic-arch. See
-mtune for more
information.
-mcpu=native
causes the compiler to auto-detect the CPU of the
build computer. At present, this feature is only supported
on Linux, and not all architectures are recognized. If the
auto-detect is unsuccessful the option has no
effect.
-mtune=name
This
option is very similar to the -mcpu= option,
except that instead of specifying the actual target
processor type, and hence restricting which instructions can
be used, it specifies that GCC should tune the
performance of the code as if the target were of the type
specified in this option, but still choosing the
instructions that it will generate based on the CPU
specified by a -mcpu= option. For
some ARM implementations better performance can
be obtained by using this
option.
-mtune=generic-arch
specifies that GCC should tune the performance
for a blend of processors within architecture arch.
The aim is to generate code that run well on the current
most popular processors, balancing between optimizations
that benefit some CPUs in the range, and avoiding
performance pitfalls of other CPUs. The effects of this
option may change in future GCC versions as
CPU models come and
go.
-mtune=native
causes the compiler to auto-detect the CPU of the
build computer. At present, this feature is only supported
on Linux, and not all architectures are recognized. If the
auto-detect is unsuccessful the option has no
effect.
-march=name
This
specifies the name of the target ARM
architecture. GCC uses this name to
determine what kind of instructions it can emit when
generating assembly code. This option can be used in
conjunction with or instead of the -mcpu=
option. Permissible names are: armv2, armv2a,
armv3, armv3m, armv4, armv4t,
armv5, armv5t, armv5e, armv5te,
armv6, armv6j, armv6t2, armv6z,
armv6zk, armv6-m, armv7,
armv7-a, armv7-r,
armv7-m, iwmmxt, iwmmxt2,
ep9312.
-march=native
causes the compiler to auto-detect the architecture of the
build computer. At present, this feature is only supported
on Linux, and not all architectures are recognized. If the
auto-detect is unsuccessful the option has no
effect.
-mfpu=name
-mfpe=number
-mfp=number
This
specifies what floating-point hardware (or hardware
emulation) is available on the target. Permissible names
are: fpa, fpe2, fpe3, maverick,
vfp, vfpv3, vfpv3-fp16,
vfpv3-d16, vfpv3-d16-fp16,
vfpv3xd, vfpv3xd-fp16, neon,
neon-fp16, vfpv4,
vfpv4-d16, fpv4-sp-d16 and
neon-vfpv4. -mfp and
-mfpe are synonyms for
-mfpu=fpenumber, for
compatibility with older versions of GCC
.
If
-msoft-float is specified this specifies
the format of floating-point
values.
If
the selected floating-point hardware includes the NEON
extension (e.g. -mfpu=neon), note
that floating-point operations will not be used by GCC
’s auto-vectorization pass unless
-funsafe-math-optimizations is also
specified. This is because NEON hardware does not
fully implement the IEEE 754 standard for
floating-point arithmetic (in particular denormal values are
treated as zero), so the use of NEON instructions
may lead to a loss of
precision.
-mfp16-format=name
Specify
the format of the "__fp16" half-precision
floating-point type. Permissible names are none,
ieee, and alternative; the default is
none, in which case the "__fp16"
type is not defined.
-mstructure-size-boundary=n
The
size of all structures and unions will be rounded up to a
multiple of the number of bits set by this option.
Permissible values are 8, 32 and 64. The default value
varies for different toolchains. For the COFF
targeted toolchain the default value is 8. A value of
64 is only allowed if the underlying ABI supports
it.
Specifying
the larger number can produce faster, more efficient code,
but can also increase the size of the program. Different
values are potentially incompatible. Code compiled with one
value cannot necessarily expect to work with code or
libraries compiled with another value, if they exchange
information using structures or
unions.
-mabort-on-noreturn
Generate
a call to the function "abort" at the end
of a "noreturn" function. It will be
executed if the function tries to
return.
-mlong-calls
-mno-long-calls
Tells
the compiler to perform function calls by first loading the
address of the function into a register and then performing
a subroutine call on this register. This switch is needed if
the target function will lie outside of the 64 megabyte
addressing range of the offset based version of subroutine
call instruction.
Even
if this switch is enabled, not all function calls will be
turned into long calls. The heuristic is that static
functions, functions that have the short-call
attribute, functions that are inside the scope of a
#pragma no_long_calls directive and functions whose
definitions have already been compiled within the current
compilation unit, will not be turned into long calls. The
exception to this rule is that weak function definitions,
functions with the long-call attribute or the
section attribute, and functions that are within the
scope of a #pragma long_calls directive, will always
be turned into long
calls.
This
feature is not enabled by default. Specifying
-mno-long-calls will restore the
default behavior, as will placing the function calls within
the scope of a #pragma long_calls_off directive. Note
these switches have no effect on how the compiler generates
code to handle function calls via function
pointers.
-msingle-pic-base
Treat
the register used for PIC addressing as
read-only, rather than loading it in the prologue for each
function. The runtime system is responsible for initializing
this register with an appropriate value before execution
begins.
-mpic-register=reg
Specify
the register to be used for PIC addressing. The
default is R10 unless stack-checking is enabled, when R9 is
used.
-mcirrus-fix-invalid-insns
Insert
NOPs into the instruction stream to in order to work around
problems with invalid Maverick instruction combinations.
This option is only valid if the -mcpu=ep9312
option has been used to enable generation of instructions
for the Cirrus Maverick floating-point co-processor. This
option is not enabled by default, since the problem is only
present in older Maverick implementations. The default can
be re-enabled by use of the
-mno-cirrus-fix-invalid-insns
switch.
-mpoke-function-name
Write
the name of each function into the text section, directly
preceding the function prologue. The generated code is
similar to this:
t0
.ascii "arm_poke_function_name", 0
.align
t1
.word 0xff000000 + (t1 - t0)
arm_poke_function_name
mov ip, sp
stmfd sp!, {fp, ip, lr, pc}
sub fp, ip, #4
When
performing a stack backtrace, code can inspect the value of
"pc" stored at "fp +
0". If the trace function then looks at location
"pc - 12" and the top 8 bits are
set, then we know that there is a function name embedded
immediately preceding this location and has length
"((pc[-3]) &
0xff000000)".
-mthumb
-marm
Select
between generating code that executes in ARM and
Thumb states. The default for most configurations is to
generate code that executes in ARM state, but the
default can be changed by configuring GCC with
the --with-mode=state
configure option.
-mtpcs-frame
Generate
a stack frame that is compliant with the Thumb Procedure
Call Standard for all non-leaf functions. (A leaf function
is one that does not call any other functions.) The default
is
-mno-tpcs-frame.
-mtpcs-leaf-frame
Generate
a stack frame that is compliant with the Thumb Procedure
Call Standard for all leaf functions. (A leaf function is
one that does not call any other functions.) The default is
-mno-apcs-leaf-frame.
-mcallee-super-interworking
Gives
all externally visible functions in the file being compiled
an ARM instruction set header which switches to
Thumb mode before executing the rest of the function. This
allows these functions to be called from non-interworking
code. This option is not valid in AAPCS
configurations because interworking is enabled by
default.
-mcaller-super-interworking
Allows
calls via function pointers (including virtual functions) to
execute correctly regardless of whether the target code has
been compiled for interworking or not. There is a small
overhead in the cost of executing a function pointer if this
option is enabled. This option is not valid in AAPCS
configurations because interworking is enabled by
default.
-mtp=name
Specify
the access model for the thread local storage pointer. The
valid models are soft, which generates calls to
"__aeabi_read_tp", cp15, which
fetches the thread pointer from "cp15"
directly (supported in the arm6k architecture), and
auto, which uses the best available method for the
selected processor. The default setting is
auto.
-mtls-dialect=dialect
Specify
the dialect to use for accessing thread local storage. Two
dialects are supported --- gnu and
gnu2. The gnu dialect selects the
original GNU scheme for supporting local and
global dynamic TLS models. The gnu2
dialect selects the GNU descriptor scheme, which
provides better performance for shared libraries. The
GNU descriptor scheme is compatible with the original
scheme, but does require new assembler, linker and library
support. Initial and local exec TLS models are
unaffected by this option and always use the original
scheme.
-mword-relocations
Only
generate absolute relocations on word-sized values (i.e.
R_ARM_ABS32). This is enabled by default on targets
(uClinux, SymbianOS) where the runtime loader imposes this
restriction, and when -fpic or
-fPIC is
specified.
-mfix-cortex-m3-ldrd
Some
Cortex-M3 cores can cause data corruption when
"ldrd" instructions with overlapping
destination and base registers are used. This option avoids
generating these instructions. This option is enabled by
default when -mcpu=cortex-m3 is
specified.
-munaligned-access
-mno-unaligned-access
Enables
(or disables) reading and writing of 16- and 32-
bit values from addresses that are not 16- or
32- bit aligned. By default unaligned access is
disabled for all pre-ARMv6 and all ARMv6-M
architectures, and enabled for all other architectures. If
unaligned access is not enabled then words in packed data
structures will be accessed a byte at a
time.
The
ARM attribute
"Tag_CPU_unaligned_access" will be set in
the generated object file to either true or false, depending
upon the setting of this option. If unaligned access is
enabled then the preprocessor symbol
"__ARM_FEATURE_UNALIGNED" will also be
defined.
-mneon-for-64bits
Enables
using Neon to handle scalar 64-bits operations. This
is disabled by default since the cost of moving data from
core registers to Neon is
high.
AVR
Options
-mmcu=mcu
Specify
Atmel AVR instruction set architectures (
ISA ) or MCU
type.
The
default for this option
is@tie{}"avr2".
GCC
supports the following AVR devices and ISAs:
"avr2"
"Classic"
devices with up to 8@tie{}KiB of program memory.
mcu@tie{}= "attiny22",
"attiny26",
"at90c8534",
"at90s2313",
"at90s2323",
"at90s2333",
"at90s2343",
"at90s4414",
"at90s4433",
"at90s4434",
"at90s8515",
"at90s8535".
"avr25"
"Classic"
devices with up to 8@tie{}KiB of program memory and with the
"MOVW" instruction.
mcu@tie{}= "ata6289",
"attiny13",
"attiny13a",
"attiny2313",
"attiny2313a",
"attiny24",
"attiny24a",
"attiny25",
"attiny261",
"attiny261a",
"attiny43u",
"attiny4313",
"attiny44",
"attiny44a",
"attiny45",
"attiny461",
"attiny461a",
"attiny48",
"attiny84",
"attiny84a",
"attiny85",
"attiny861",
"attiny861a",
"attiny87",
"attiny88",
"at86rf401".
"avr3"
"Classic"
devices with 16@tie{}KiB up to 64@tie{}KiB of program
memory. mcu@tie{}=
"at43usb355",
"at76c711".
"avr31"
"Classic"
devices with 128@tie{}KiB of program memory.
mcu@tie{}= "atmega103",
"at43usb320".
"avr35"
"Classic"
devices with 16@tie{}KiB up to 64@tie{}KiB of program memory
and with the "MOVW" instruction.
mcu@tie{}= "atmega16u2",
"atmega32u2",
"atmega8u2",
"attiny167",
"at90usb162",
"at90usb82".
"avr4"
"Enhanced"
devices with up to 8@tie{}KiB of program memory.
mcu@tie{}= "atmega48",
"atmega48a",
"atmega48p",
"atmega8",
"atmega8hva",
"atmega8515",
"atmega8535",
"atmega88",
"atmega88a",
"atmega88p",
"atmega88pa",
"at90pwm1",
"at90pwm2",
"at90pwm2b",
"at90pwm3",
"at90pwm3b",
"at90pwm81".
"avr5"
"Enhanced"
devices with 16@tie{}KiB up to 64@tie{}KiB of program
memory. mcu@tie{}=
"atmega16",
"atmega16a",
"atmega16hva",
"atmega16hva2",
"atmega16hvb",
"atmega16m1",
"atmega16u4",
"atmega161",
"atmega162",
"atmega163",
"atmega164a",
"atmega164p",
"atmega165",
"atmega165a",
"atmega165p",
"atmega168",
"atmega168a",
"atmega168p",
"atmega169",
"atmega169a",
"atmega169p",
"atmega169pa",
"atmega32",
"atmega32c1",
"atmega32hvb",
"atmega32m1",
"atmega32u4",
"atmega32u6",
"atmega323",
"atmega324a",
"atmega324p",
"atmega324pa",
"atmega325",
"atmega325a",
"atmega325p",
"atmega3250",
"atmega3250a",
"atmega3250p",
"atmega328",
"atmega328p",
"atmega329",
"atmega329a",
"atmega329p",
"atmega329pa",
"atmega3290",
"atmega3290a",
"atmega3290p",
"atmega406",
"atmega64",
"atmega64c1",
"atmega64hve",
"atmega64m1",
"atmega640",
"atmega644",
"atmega644a",
"atmega644p",
"atmega644pa",
"atmega645",
"atmega645a",
"atmega645p",
"atmega6450",
"atmega6450a",
"atmega6450p",
"atmega649",
"atmega649a",
"atmega649p",
"atmega6490",
"at90can32",
"at90can64",
"at90pwm216",
"at90pwm316",
"at90scr100",
"at90usb646",
"at90usb647", "at94k",
"m3000".
"avr51"
"Enhanced"
devices with 128@tie{}KiB of program memory.
mcu@tie{}= "atmega128",
"atmega128rfa1",
"atmega1280",
"atmega1281",
"atmega1284p",
"at90can128",
"at90usb1286",
"at90usb1287".
"avr6"
"Enhanced"
devices with 3-byte PC , i.e. with more
than 128@tie{}KiB of program memory.
mcu@tie{}= "atmega2560",
"atmega2561".
"avrxmega2"
"
XMEGA " devices with more than 8@tie{}KiB and up
to 64@tie{}KiB of program memory. mcu@tie{}=
"atxmega16a4",
"atxmega16d4",
"atxmega16x1",
"atxmega32a4",
"atxmega32d4",
"atxmega32x1".
"avrxmega4"
"
XMEGA " devices with more than 64@tie{}KiB and up
to 128@tie{}KiB of program memory.
mcu@tie{}= "atxmega64a3",
"atxmega64d3".
"avrxmega5"
"
XMEGA " devices with more than 64@tie{}KiB and up
to 128@tie{}KiB of program memory and more than 64@tie{}KiB
of RAM . mcu@tie{}=
"atxmega64a1",
"atxmega64a1u".
"avrxmega6"
"
XMEGA " devices with more than 128@tie{}KiB of
program memory. mcu@tie{}=
"atxmega128a3",
"atxmega128d3",
"atxmega192a3",
"atxmega192d3",
"atxmega256a3",
"atxmega256a3b",
"atxmega256a3bu",
"atxmega256d3".
"avrxmega7"
"
XMEGA " devices with more than 128@tie{}KiB of
program memory and more than 64@tie{}KiB of RAM .
mcu@tie{}=
"atxmega128a1",
"atxmega128a1u".
"avr1"
This
ISA is implemented by the minimal AVR core
and supported for assembler only. mcu@tie{}=
"attiny11",
"attiny12",
"attiny15",
"attiny28",
"at90s1200".
-maccumulate-args
Accumulate
outgoing function arguments and acquire/release the needed
stack space for outgoing function arguments once in function
prologue/epilogue. Without this option, outgoing arguments
are pushed before calling a function and popped
afterwards.
Popping
the arguments after the function call can be expensive
on AVR so that accumulating the stack space might
lead to smaller executables because arguments need not to be
removed from the stack after such a function
call.
This
option can lead to reduced code size for functions that
perform several calls to functions that get their arguments
on the stack like calls to printf-like
functions.
-mbranch-cost=cost
Set
the branch costs for conditional branch instructions to
cost. Reasonable values for cost are small,
non-negative integers. The default branch cost is
0.
-mcall-prologues
Functions
prologues/epilogues are expanded as calls to appropriate
subroutines. Code size is
smaller.
-mint8
Assume
"int" to be 8-bit integer. This
affects the sizes of all types: a "char"
is 1 byte, an "int" is 1 byte, a
"long" is 2 bytes, and "long
long" is 4 bytes. Please note that this option
does not conform to the C standards, but it results in
smaller code size.
-mno-interrupts
Generated
code is not compatible with hardware interrupts. Code size
is smaller.
-mrelax
Try
to replace "CALL" resp.
"JMP" instruction by the shorter
"RCALL" resp. "RJMP"
instruction if applicable. Setting
"-mrelax" just adds the
"--relax" option to the
linker command line when the linker is
called.
Jump
relaxing is performed by the linker because jump offsets are
not known before code is located. Therefore, the assembler
code generated by the compiler is the same, but the
instructions in the executable may differ from instructions
in the assembler
code.
Relaxing
must be turned on if linker stubs are needed, see the
section on "EIND" and linker stubs
below.
-mshort-calls
This
option has been deprecated and will be removed in GCC
4.8. See "-mrelax" for a
replacement.
Use
"RCALL"/"RJMP"
instructions even on devices with 16@tie{}KiB or more of
program memory, i.e. on devices that have the
"CALL" and "JMP"
instructions.
-msp8
Treat
the stack pointer register as an 8-bit register, i.e.
assume the high byte of the stack pointer is zero. In
general, you don’t need to set this option by
hand.
This
option is used internally by the compiler to select and
build multilibs for architectures "avr2"
and "avr25". These architectures mix
devices with and without "SPH". For any
setting other than "-mmcu=avr2" or
"-mmcu=avr25" the compiler driver
will add or remove this option from the compiler
proper’s command line, because the compiler then knows
if the device or architecture has an 8-bit stack
pointer and thus no "SPH" register or
not.
-mstrict-X
Use
address register "X" in a way proposed by
the hardware. This means that "X" is only
used in indirect, post-increment or pre-decrement
addressing.
Without
this option, the "X" register may be used
in the same way as "Y" or
"Z" which then is emulated by additional
instructions. For example, loading a value with
"X+const" addressing with a small
non-negative "const < 64" to a
register Rn is performed
as
adiw r26, const ; X += const
ld <Rn>, X ; <Rn> = *X
sbiw r26, const ; X -= const
-mtiny-stack
Only
change the lower 8@tie{}bits of the stack
pointer.
"EIND"
and Devices with more than 128 Ki Bytes of
Flash
Pointers
in the implementation are 16@tie{}bits wide. The address of
a function or label is represented as word address so that
indirect jumps and calls can target any code address in the
range of 64@tie{}Ki
words.
In
order to facilitate indirect jump on devices with more than
128@tie{}Ki bytes of program memory space, there is a
special function register called "EIND"
that serves as most significant part of the target address
when "EICALL" or
"EIJMP" instructions are
used.
Indirect
jumps and calls on these devices are handled as follows by
the compiler and are subject to some
limitations:
•
The compiler never sets
"EIND".
•
The compiler uses
"EIND" implicitely in
"EICALL"/"EIJMP"
instructions or might read "EIND"
directly in order to emulate an indirect call/jump by means
of a "RET"
instruction.
•
The compiler assumes that
"EIND" never changes during the startup
code or during the application. In particular,
"EIND" is not saved/restored in function
or interrupt service routine
prologue/epilogue.
•
For indirect calls to
functions and computed goto, the linker generates
stubs. Stubs are jump pads sometimes also called
trampolines. Thus, the indirect call/jump jumps to
such a stub. The stub contains a direct jump to the desired
address.
•
Linker relaxation must be
turned on so that the linker will generate the stubs
correctly an all situaltion. See the compiler option
"-mrelax" and the linler option
"--relax". There are corner
cases where the linker is supposed to generate stubs but
aborts without relaxation and without a helpful error
message.
•
The default linker script
is arranged for code with "EIND = 0". If
code is supposed to work for a setup with "EIND !=
0", a custom linker script has to be used in order
to place the sections whose name start with
".trampolines" into the segment where
"EIND" points
to.
•
The startup code from
libgcc never sets "EIND". Notice that
startup code is a blend of code from libgcc and AVR-LibC.
For the impact of AVR-LibC on "EIND", see
the AVR-LibC user manual
("http://nongnu.org/avr-libc/user-manual/").
•
It is legitimate for
user-specific startup code to set up
"EIND" early, for example by means of
initialization code located in section
".init3". Such code runs prior to general
startup code that initializes RAM and calls
constructors, but after the bit of startup code from
AVR-LibC that sets "EIND" to the segment
where the vector table is
located.
#include <avr/io.h>
static void
__attribute__((section(".init3"),naked,used,no_instrument_function))
init3_set_eind (void)
__asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
"out %i0,r24" :: "n" (&EIND) : "r24","memory");
}
The
"__trampolines_start" symbol is defined
in the linker
script.
•
Stubs
are generated automatically by the linker if the following
two conditions are
met:
-<The
address of a label is taken by means of the "gs"
modifier>
(short
for generate stubs) like
so:
LDI r24, lo8(gs(<func>))
LDI r25, hi8(gs(<func>))
-<The
final location of that label is in a code
segment>
outside
the segment where the stubs are
located.
•
The compiler emits such
"gs" modifiers for code labels in the
following
situations:
-<Taking
address of a function or code label.>
-<Computed goto.>
-<If prologue-save function is used, see
-mcall-prologues>
command-line
option.
-<Switch/case
dispatch tables. If you do not want such
dispatch>
tables
you can specify the
-fno-jump-tables command-line
option.
-<C
and C ++ constructors/destructors called during
startup/shutdown.>
-<If the tools hit a "gs()" modifier
explained above.>
•
Jumping to non-symbolic
addresses like so is not
supported:
int main (void)
/* Call function at word address 0x2 */
return ((int(*)(void)) 0x2)();
}
Instead,
a stub has to be set up, i.e. the function has to be called
through a symbol ("func_4" in the
example):
int main (void)
extern int func_4 (void);
/* Call function at byte address 0x4 */
return func_4();
}
and
the application be linked with
"-Wl,--defsym,func_4=0x4".
Alternatively, "func_4" can be defined in
the linker script.
Handling
of the "RAMPD",
"RAMPX", "RAMPY" and
"RAMPZ" Special Function
Registers
Some
AVR devices support memories larger than the
64@tie{}KiB range that can be accessed with 16-bit
pointers. To access memory locations outside this
64@tie{}KiB range, the contentent of a
"RAMP" register is used as high part of
the address: The "X",
"Y", "Z" address
register is concatenated with the
"RAMPX", "RAMPY",
"RAMPZ" special function register,
respectively, to get a wide address. Similarly,
"RAMPD" is used together with direct
addressing.
•
The
startup code initializes the "RAMP"
special function registers with
zero.
•
If a AVR
Named Address Spaces,named address space
other than generic or "__flash" is
used, then "RAMPZ" is set as needed
before the
operation.
•
If the device
supports RAM larger than 64@tie{KiB} and the
compiler needs to change "RAMPZ" to
accomplish an operation, "RAMPZ" is reset
to zero after the
operation.
•
If the device comes with a
specific "RAMP" register, the ISR
prologue/epilogue saves/restores that SFR
and initializes it with zero in case the ISR
code might (implicitly) use
it.
•
RAM larger than
64@tie{KiB} is not supported by GCC for AVR
targets. If you use inline assembler to read from
locations outside the 16-bit address range and change
one of the "RAMP" registers, you must
reset it to zero after the
access.
AVR
Built-in Macros
GCC
defines several built-in macros so that the user code
can test for the presence or absence of features. Almost any
of the following built-in macros are deduced from device
capabilities and thus triggered by the
"-mmcu=" command-line
option.
For
even more AVR-specific built-in macros see AVR
Named Address Spaces and AVR
Built-in Functions.
"__AVR_ARCH__"
Build-in
macro that resolves to a decimal number that identifies the
architecture and depends on the
"-mmcu=mcu" option.
Possible values are:
2,
25, 3, 31, 35,
4, 5, 51, 6,
102, 104, 105, 106,
107
for
mcu="avr2",
"avr25", "avr3",
"avr31", "avr35",
"avr4", "avr5",
"avr51", "avr6",
"avrxmega2",
"avrxmega4",
"avrxmega5",
"avrxmega6",
"avrxmega7", respectively. If mcu
specifies a device, this built-in macro is set accordingly.
For example, with "-mmcu=atmega8"
the macro will be defined to
4.
"__AVR_Device__"
Setting
"-mmcu=device"
defines this built-in macro which reflects the
device’s name. For example,
"-mmcu=atmega8" defines the
built-in macro "__AVR_ATmega8__",
"-mmcu=attiny261a" defines
"__AVR_ATtiny261A__",
etc.
The
built-in macros’ names follow the scheme
"__AVR_Device__" where
Device is the device name as from the AVR
user manual. The difference between Device in
the built-in macro and device in
"-mmcu=device" is
that the latter is always
lowercase.
If
device is not a device but only a core architecture
like "avr51", this macro will not be
defined.
"__AVR_XMEGA__"
The
device/architecture belongs to the XMEGA family
of devices.
"__AVR_HAVE_ELPM__"
The
device has the the "ELPM"
instruction.
"__AVR_HAVE_ELPMX__"
The
device has the "ELPM
Rn,Z" and "ELPM
Rn,Z+"
instructions.
"__AVR_HAVE_MOVW__"
The
device has the "MOVW" instruction to
perform 16-bit register-register
moves.
"__AVR_HAVE_LPMX__"
The
device has the "LPM Rn,Z"
and "LPM Rn,Z+"
instructions.
"__AVR_HAVE_MUL__"
The
device has a hardware
multiplier.
"__AVR_HAVE_JMP_CALL__"
The
device has the "JMP" and
"CALL" instructions. This is the case for
devices with at least 16@tie{}KiB of program memory and if
"-mshort-calls" is not
set.
"__AVR_HAVE_EIJMP_EICALL__"
"__AVR_3_BYTE_PC__"
The
device has the "EIJMP" and
"EICALL" instructions. This is the case
for devices with more than 128@tie{}KiB of program memory.
This also means that the program counter ( PC )
is 3@tie{}bytes
wide.
"__AVR_2_BYTE_PC__"
The
program counter ( PC ) is 2@tie{}bytes wide. This
is the case for devices with up to 128@tie{}KiB of program
memory.
"__AVR_HAVE_8BIT_SP__"
"__AVR_HAVE_16BIT_SP__"
The
stack pointer ( SP ) register is treated as
8-bit respectively 16-bit register by the
compiler. The definition of these macros is affected by
"-mtiny-stack".
"__AVR_HAVE_SPH__"
"__AVR_SP8__"
The
device has the SPH (high part of stack pointer)
special function register or has an 8-bit stack
pointer, respectively. The definition of these macros is
affected by "-mmcu=" and in the
cases of "-mmcu=avr2" and
"-mmcu=avr25" also by
"-msp8".
"__AVR_HAVE_RAMPD__"
"__AVR_HAVE_RAMPX__"
"__AVR_HAVE_RAMPY__"
"__AVR_HAVE_RAMPZ__"
The
device has the "RAMPD",
"RAMPX", "RAMPY",
"RAMPZ" special function register,
respectively.
"__NO_INTERRUPTS__"
This
macro reflects the
"-mno-interrupts" command
line option.
"__AVR_ERRATA_SKIP__"
"__AVR_ERRATA_SKIP_JMP_CALL__"
Some
AVR devices ( AT90S8515 , ATmega103) must
not skip 32-bit instructions because of a hardware
erratum. Skip instructions are "SBRS",
"SBRC", "SBIS",
"SBIC" and "CPSE". The
second macro is only defined if
"__AVR_HAVE_JMP_CALL__" is also
set.
"__AVR_SFR_OFFSET__=offset"
Instructions
that can address I/O special function registers directly
like "IN", "OUT",
"SBI", etc. may use a different address
as if addressed by an instruction to access RAM
like "LD" or
"STS". This offset depends on the device
architecture and has to be subtracted from the RAM
address in order to get the respective
I/O@tie{}address.
"__WITH_AVRLIBC__"
The
compiler is configured to be used together with AVR-Libc.
See the
"--with-avrlibc"
configure option.
Blackfin
Options
-mcpu=cpu[-sirevision]
Specifies
the name of the target Blackfin processor. Currently,
cpu can be one of bf512, bf514,
bf516, bf518, bf522, bf523,
bf524, bf525, bf526, bf527,
bf531, bf532, bf533, bf534,
bf536, bf537, bf538, bf539,
bf542, bf544, bf547, bf548,
bf549, bf542m, bf544m, bf547m,
bf548m, bf549m, bf561, bf592.
The optional sirevision specifies the silicon
revision of the target Blackfin processor. Any workarounds
available for the targeted silicon revision will be enabled.
If sirevision is none, no workarounds are
enabled. If sirevision is any, all workarounds
for the targeted processor will be enabled. The
"__SILICON_REVISION__" macro is defined
to two hexadecimal digits representing the major and minor
numbers in the silicon revision. If sirevision is
none, the "__SILICON_REVISION__"
is not defined. If sirevision is any, the
"__SILICON_REVISION__" is defined to be
0xffff. If this optional sirevision is not
used, GCC assumes the latest known silicon
revision of the targeted Blackfin
processor.
Support
for bf561 is incomplete. For bf561, Only the
processor macro is defined. Without this option,
bf532 is used as the processor by default. The
corresponding predefined processor macros for cpu is
to be defined. And for bfin-elf toolchain, this
causes the hardware BSP provided by libgloss to
be linked in if -msim is not
given.
-msim
Specifies
that the program will be run on the simulator. This causes
the simulator BSP provided by libgloss to be
linked in. This option has effect only for bfin-elf
toolchain. Certain other options, such as
-mid-shared-library and
-mfdpic, imply
-msim.
-momit-leaf-frame-pointer
Don’t
keep the frame pointer in a register for leaf functions.
This avoids the instructions to save, set up and restore
frame pointers and makes an extra register available in leaf
functions. The option
-fomit-frame-pointer removes the
frame pointer for all functions, which might make debugging
harder.
-mspecld-anomaly
When
enabled, the compiler will ensure that the generated code
does not contain speculative loads after jump instructions.
If this option is used,
"__WORKAROUND_SPECULATIVE_LOADS" is
defined.
-mno-specld-anomaly
Don’t
generate extra code to prevent speculative loads from
occurring.
-mcsync-anomaly
When
enabled, the compiler will ensure that the generated code
does not contain CSYNC or SSYNC
instructions too soon after conditional branches. If
this option is used,
"__WORKAROUND_SPECULATIVE_SYNCS" is
defined.
-mno-csync-anomaly
Don’t
generate extra code to prevent CSYNC or
SSYNC instructions from occurring too soon after a
conditional branch.
-mlow-64k
When
enabled, the compiler is free to take advantage of the
knowledge that the entire program fits into the low 64k of
memory.
-mno-low-64k
Assume
that the program is arbitrarily large. This is the
default.
-mstack-check-l1
Do
stack checking using information placed into L1 scratchpad
memory by the uClinux
kernel.
-mid-shared-library
Generate
code that supports shared libraries via the library ID
method. This allows for execute in place and shared
libraries in an environment without virtual memory
management. This option implies -fPIC. With a
bfin-elf target, this option implies
-msim.
-mno-id-shared-library
Generate
code that doesn’t assume ID based shared
libraries are being used. This is the
default.
-mleaf-id-shared-library
Generate
code that supports shared libraries via the library ID
method, but assumes that this library or executable
won’t link against any other ID shared
libraries. That allows the compiler to use faster code for
jumps and calls.
-mno-leaf-id-shared-library
Do
not assume that the code being compiled won’t link
against any ID shared libraries. Slower code will
be generated for jump and call
insns.
-mshared-library-id=n
Specified
the identification number of the ID based shared
library being compiled. Specifying a value of 0 will
generate more compact code, specifying other values will
force the allocation of that number to the current library
but is no more space or time efficient than omitting this
option.
-msep-data
Generate
code that allows the data segment to be located in a
different area of memory from the text segment. This allows
for execute in place in an environment without virtual
memory management by eliminating relocations against the
text section.
-mno-sep-data
Generate
code that assumes that the data segment follows the text
segment. This is the
default.
-mlong-calls
-mno-long-calls
Tells
the compiler to perform function calls by first loading the
address of the function into a register and then performing
a subroutine call on this register. This switch is needed if
the target function lies outside of the 24-bit
addressing range of the offset-based version of subroutine
call instruction.
This
feature is not enabled by default. Specifying
-mno-long-calls will restore the
default behavior. Note these switches have no effect on how
the compiler generates code to handle function calls via
function pointers.
-mfast-fp
Link
with the fast floating-point library. This library relaxes
some of the IEEE floating-point standard’s
rules for checking inputs against Not-a-Number ( NAN
), in the interest of
performance.
-minline-plt
Enable
inlining of PLT entries in function calls to
functions that are not known to bind locally. It has no
effect without
-mfdpic.
-mmulticore
Build
standalone application for multicore Blackfin processor.
Proper start files and link scripts will be used to support
multicore. This option defines
"__BFIN_MULTICORE". It can only be used
with
-mcpu=bf561[-sirevision].
It can be used with -mcorea or
-mcoreb. If it’s used without
-mcorea or -mcoreb, single
application/dual core programming model is used. In this
model, the main function of Core B should be named as
coreb_main. If it’s used with -mcorea or
-mcoreb, one application per core programming
model is used. If this option is not used, single core
application programming model is
used.
-mcorea
Build
standalone application for Core A of BF561 when
using one application per core programming model. Proper
start files and link scripts will be used to support Core A.
This option defines "__BFIN_COREA". It
must be used with
-mmulticore.
-mcoreb
Build
standalone application for Core B of BF561 when
using one application per core programming model. Proper
start files and link scripts will be used to support Core B.
This option defines "__BFIN_COREB". When
this option is used, coreb_main should be used instead of
main. It must be used with
-mmulticore.
-msdram
Build
standalone application for SDRAM . Proper start
files and link scripts will be used to put the application
into SDRAM . Loader should initialize SDRAM
before loading the application into SDRAM .
This option defines
"__BFIN_SDRAM".
-micplb
Assume
that ICPLBs are enabled at run time. This has an effect on
certain anomaly workarounds. For Linux targets, the default
is to assume ICPLBs are enabled; for standalone applications
the default is off.
C6X
Options
-march=name
This
specifies the name of the target architecture. GCC
uses this name to determine what kind of instructions
it can emit when generating assembly code. Permissible names
are: c62x, c64x, c64x+, c67x,
c67x+,
c674x.
-mbig-endian
Generate
code for a big-endian
target.
-mlittle-endian
Generate
code for a little-endian target. This is the
default.
-msim
Choose
startup files and linker script suitable for the
simulator.
-msdata=default
Put
small global and static data in the .neardata
section, which is pointed to by register
"B14". Put small uninitialized global and
static data in the .bss section, which is adjacent to
the .neardata section. Put small read-only data into
the .rodata section. The corresponding sections used
for large pieces of data are .fardata, .far
and .const.
-msdata=all
Put
all data, not just small objets, into the sections reserved
for small data, and use addressing relative to the
"B14" register to access
them.
-msdata=none
Make
no use of the sections reserved for small data, and use
absolute addresses to access all data. Put all initialized
global and static data in the .fardata section, and
all uninitialized data in the .far section. Put all
constant data into the .const
section.
CRIS
Options
These
options are defined specifically for the CRIS
ports.
-march=architecture-type
-mcpu=architecture-type
Generate
code for the specified architecture. The choices for
architecture-type are v3, v8 and
v10 for respectively ETRAX 4,
ETRAX 100, and ETRAX
100 LX . Default is v0 except
for cris-axis-linux-gnu, where the default is
v10.
-mtune=architecture-type
Tune
to architecture-type everything applicable about the
generated code, except for the ABI and the set of
available instructions. The choices for
architecture-type are the same as for
-march=architecture-type.
-mmax-stack-frame=n
Warn
when the stack frame of a function exceeds n
bytes.
-metrax4
-metrax100
The
options -metrax4 and -metrax100
are synonyms for -march=v3 and
-march=v8
respectively.
-mmul-bug-workaround
-mno-mul-bug-workaround
Work
around a bug in the "muls" and
"mulu" instructions for CPU
models where it applies. This option is active by
default.
-mpdebug
Enable
CRIS-specific verbose debug-related information in the
assembly code. This option also has the effect to turn off
the #NO_APP formatted-code indicator to the assembler
at the beginning of the assembly
file.
-mcc-init
Do
not use condition-code results from previous instruction;
always emit compare and test instructions before use of
condition codes.
-mno-side-effects
Do
not emit instructions with side-effects in addressing modes
other than
post-increment.
-mstack-align
-mno-stack-align
-mdata-align
-mno-data-align
-mconst-align
-mno-const-align
These
options (no-options) arranges (eliminate arrangements) for
the stack-frame, individual data and constants to be aligned
for the maximum single data access size for the chosen
CPU model. The default is to arrange for 32-bit
alignment. ABI details such as structure layout
are not affected by these
options.
-m32-bit
-m16-bit
-m8-bit
Similar
to the stack- data- and const-align options
above, these options arrange for stack-frame, writable data
and constants to all be 32-bit, 16-bit or
8-bit aligned. The default is 32-bit
alignment.
-mno-prologue-epilogue
-mprologue-epilogue
With
-mno-prologue-epilogue, the normal
function prologue and epilogue which set up the stack frame
are omitted and no return instructions or return sequences
are generated in the code. Use this option only together
with visual inspection of the compiled code: no warnings or
errors are generated when call-saved registers must be
saved, or storage for local variable needs to be
allocated.
-mno-gotplt
-mgotplt
With
-fpic and -fPIC, don’t
generate (do generate) instruction sequences that load
addresses for functions from the PLT part of
the GOT rather than (traditional on other
architectures) calls to the PLT . The default is
-mgotplt.
-melf
Legacy
no-op option only recognized with the cris-axis-elf and
cris-axis-linux-gnu
targets.
-mlinux
Legacy
no-op option only recognized with the cris-axis-linux-gnu
target.
-sim
This
option, recognized for the cris-axis-elf arranges to link
with input-output functions from a simulator library. Code,
initialized data and zero-initialized data are allocated
consecutively.
-sim2
Like
-sim, but pass linker options to locate
initialized data at 0x40000000 and zero-initialized data at
0x80000000.
CR16
Options
These
options are defined specifically for the CR16
ports.
-mmac
Enable
the use of multiply-accumulate instructions. Disabled by
default.
-mcr16cplus
-mcr16c
Generate
code for CR16C or CR16C+
architecture. CR16C+ architecture is
default.
-msim
Links
the library libsim.a which is in compatible with simulator.
Applicable to elf compiler
only.
-mint32
Choose
integer type as 32-bit
wide.
-mbit-ops
Generates
sbit/cbit instructions for bit
manipulations.
-mdata-model=model
Choose
a data model. The choices for model are near,
far or medium. medium is default.
However, far is not valid when -mcr16c option
is chosen as CR16C architecture does not support
far data model.
Darwin
Options
These
options are defined for all architectures running the Darwin
operating system.
FSF
GCC on Darwin does not create "fat" object
files; it will create an object file for the single
architecture that it was built to target.
Apple’s GCC on Darwin does create
"fat" files if multiple -arch options
are used; it does so by running the compiler or linker
multiple times and joining the results together with
lipo.
The
subtype of the file created (like ppc7400 or
ppc970 or i686) is determined by the flags
that specify the ISA that GCC is
targetting, like -mcpu or -march.
The -force_cpusubtype_ALL option can be used to
override this.
The
Darwin tools vary in their behavior when presented with
an ISA mismatch. The assembler, as, will
only permit instructions to be used that are valid for the
subtype of the file it is generating, so you cannot put
64-bit instructions in a ppc750 object file.
The linker for shared libraries, /usr/bin/libtool,
will fail and print an error if asked to create a shared
library with a less restrictive subtype than its input files
(for instance, trying to put a ppc970 object file in
a ppc7400 library). The linker for executables,
ld, will quietly give the executable the most
restrictive subtype of any of its input files.
-Fdir
Add
the framework directory dir to the head of the list
of directories to be searched for header files. These
directories are interleaved with those specified by
-I options and are scanned in a left-to-right
order.
A
framework directory is a directory with frameworks in it. A
framework is a directory with a "Headers"
and/or "PrivateHeaders" directory contained
directly in it that ends in ".framework".
The name of a framework is the name of this directory
excluding the ".framework". Headers
associated with the framework are found in one of those two
directories, with "Headers" being searched
first. A subframework is a framework directory that is in a
framework’s "Frameworks" directory.
Includes of subframework headers can only appear in a header
of a framework that contains the subframework, or in a
sibling subframework header. Two subframeworks are siblings
if they occur in the same framework. A subframework should
not have the same name as a framework, a warning will be
issued if this is violated. Currently a subframework cannot
have subframeworks, in the future, the mechanism may be
extended to support this. The standard frameworks can be
found in "/System/Library/Frameworks" and
"/Library/Frameworks". An example include
looks like "#include
<Framework/header.h>", where
Framework denotes the name of the framework and
header.h is found in the "PrivateHeaders"
or "Headers"
directory.
-iframeworkdir
Like
-F except the directory is a treated as a
system directory. The main difference between this
-iframework and -F is that with
-iframework the compiler does not warn about
constructs contained within header files found via
dir. This option is valid only for the C family of
languages.
-gused
Emit
debugging information for symbols that are used. For
STABS debugging format, this enables
-feliminate-unused-debug-symbols.
This is by default ON
.
-gfull
Emit
debugging information for all symbols and
types.
-mmacosx-version-min=version
The
earliest version of MacOS X that this executable will run on
is version. Typical values of version include
10.1, 10.2, and
10.3.9.
If
the compiler was built to use the system’s headers by
default, then the default for this option is the system
version on which the compiler is running, otherwise the
default is to make choices that are compatible with as many
systems and code bases as
possible.
-mkernel
Enable
kernel development mode. The -mkernel option
sets -static, -fno-common,
-fno-cxa-atexit,
-fno-exceptions,
-fno-non-call-exceptions,
-fapple-kext,
-fno-weak and
-fno-rtti where applicable. This mode
also sets -mno-altivec,
-msoft-float,
-fno-builtin and
-mlong-branch for PowerPC
targets.
-mone-byte-bool
Override
the defaults for bool so that sizeof(bool)==1.
By default sizeof(bool) is 4 when compiling
for Darwin/PowerPC and 1 when compiling for
Darwin/x86, so this option has no effect on
x86.
Warning:
The -mone-byte-bool switch
causes GCC to generate code that is not binary
compatible with code generated without that switch. Using
this switch may require recompiling all other modules in a
program, including system libraries. Use this switch to
conform to a non-default data
model.
-mfix-and-continue
-ffix-and-continue
-findirect-data
Generate
code suitable for fast turn around development. Needed to
enable gdb to dynamically load ".o" files
into already running programs.
-findirect-data and
-ffix-and-continue are provided for
backwards
compatibility.
-all_load
Loads
all members of static archive libraries. See man
ld(1) for more
information.
-arch_errors_fatal
Cause
the errors having to do with files that have the wrong
architecture to be
fatal.
-bind_at_load
Causes
the output file to be marked such that the dynamic linker
will bind all undefined references when the file is loaded
or launched.
-bundle
Produce
a Mach-o bundle format file. See man ld(1) for more
information.
-bundle_loader
executable
This
option specifies the executable that will be loading
the build output file being linked. See man ld(1) for
more information.
-dynamiclib
When
passed this option, GCC will produce a dynamic
library instead of an executable when linking, using the
Darwin libtool
command.
-force_cpusubtype_ALL
This
causes GCC ’s output file to have the
ALL subtype, instead of one controlled by the
-mcpu or -march
option.
-allowable_client
client_name
-client_name
-compatibility_version
-current_version
-dead_strip
-dependency-file
-dylib_file
-dylinker_install_name
-dynamic
-exported_symbols_list
-filelist
-flat_namespace
-force_flat_namespace
-headerpad_max_install_names
-image_base
-init
-install_name
-keep_private_externs
-multi_module
-multiply_defined
-multiply_defined_unused
-noall_load
-no_dead_strip_inits_and_terms
-nofixprebinding
-nomultidefs
-noprebind
-noseglinkedit
-pagezero_size
-prebind
-prebind_all_twolevel_modules
-private_bundle
-read_only_relocs
-sectalign
-sectobjectsymbols
-whyload
-seg1addr
-sectcreate
-sectobjectsymbols
-sectorder
-segaddr
-segs_read_only_addr
-segs_read_write_addr
-seg_addr_table
-seg_addr_table_filename
-seglinkedit
-segprot
-segs_read_only_addr
-segs_read_write_addr
-single_module
-static
-sub_library
-sub_umbrella
-twolevel_namespace
-umbrella
-undefined
-unexported_symbols_list
-weak_reference_mismatches
-whatsloaded
These
options are passed to the Darwin linker. The Darwin linker
man page describes them in
detail.
DEC
Alpha
Options
These
-m options are defined for the DEC
Alpha implementations:
-mno-soft-float
-msoft-float
Use
(do not use) the hardware floating-point instructions for
floating-point operations. When
-msoft-float is specified, functions in
libgcc.a will be used to perform floating-point
operations. Unless they are replaced by routines that
emulate the floating-point operations, or compiled in such a
way as to call such emulations routines, these routines will
issue floating-point operations. If you are compiling for an
Alpha without floating-point operations, you must ensure
that the library is built so as not to call
them.
Note
that Alpha implementations without floating-point operations
are required to have floating-point
registers.
-mfp-reg
-mno-fp-regs
Generate
code that uses (does not use) the floating-point register
set. -mno-fp-regs implies
-msoft-float. If the floating-point
register set is not used, floating-point operands are passed
in integer registers as if they were integers and
floating-point results are passed in $0 instead of
$f0. This is a non-standard calling sequence, so
any function with a floating-point argument or return value
called by code compiled with
-mno-fp-regs must also be compiled
with that option.
A
typical use of this option is building a kernel that does
not use, and hence need not save and restore, any
floating-point
registers.
-mieee
The
Alpha architecture implements floating-point hardware
optimized for maximum performance. It is mostly compliant
with the IEEE floating-point standard. However,
for full compliance, software assistance is required. This
option generates code fully IEEE-compliant code
except that the inexact-flag is not maintained
(see below). If this option is turned on, the preprocessor
macro "_IEEE_FP" is defined during
compilation. The resulting code is less efficient but is
able to correctly support denormalized numbers and
exceptional IEEE values such as not-a-number and
plus/minus infinity. Other Alpha compilers call this option
-ieee_with_no_inexact.
-mieee-with-inexact
This
is like -mieee except the generated code also
maintains the IEEE
inexact-flag. Turning on this option
causes the generated code to implement fully-compliant
IEEE math. In addition to
"_IEEE_FP",
"_IEEE_FP_EXACT" is defined as a
preprocessor macro. On some Alpha implementations the
resulting code may execute significantly slower than the
code generated by default. Since there is very little code
that depends on the inexact-flag, you should normally
not specify this option. Other Alpha compilers call this
option
-ieee_with_inexact.
-mfp-trap-mode=trap-mode
This
option controls what floating-point related traps are
enabled. Other Alpha compilers call this option
-fptm trap-mode. The trap mode can be
set to one of four
values:
n
This is the default
(normal) setting. The only traps that are enabled are the
ones that cannot be disabled in software (e.g., division by
zero trap).
u
In addition to the traps
enabled by n, underflow traps are enabled as
well.
su
Like u, but the
instructions are marked to be safe for software completion
(see Alpha architecture manual for
details).
sui
Like su, but
inexact traps are enabled as
well.
-mfp-rounding-mode=rounding-mode
Selects
the IEEE rounding mode. Other Alpha compilers
call this option -fprm rounding-mode.
The rounding-mode can be one
of:
n
Normal IEEE
rounding mode. Floating-point numbers are rounded
towards the nearest machine number or towards the even
machine number in case of a
tie.
m
Round towards minus
infinity.
c
Chopped rounding mode.
Floating-point numbers are rounded towards
zero.
d
Dynamic rounding mode. A
field in the floating-point control register (fpcr,
see Alpha architecture reference manual) controls the
rounding mode in effect. The C library initializes this
register for rounding towards plus infinity. Thus, unless
your program modifies the fpcr, d corresponds
to round towards plus
infinity.
-mtrap-precision=trap-precision
In
the Alpha architecture, floating-point traps are imprecise.
This means without software assistance it is impossible to
recover from a floating trap and program execution normally
needs to be terminated. GCC can generate code
that can assist operating system trap handlers in
determining the exact location that caused a floating-point
trap. Depending on the requirements of an application,
different levels of precisions can be
selected:
p
Program precision. This
option is the default and means a trap handler can only
identify which program caused a floating-point
exception.
f
Function precision. The
trap handler can determine the function that caused a
floating-point
exception.
i
Instruction precision. The
trap handler can determine the exact instruction that caused
a floating-point
exception.
Other
Alpha compilers provide the equivalent options called
-scope_safe and
-resumption_safe.
-mieee-conformant
This
option marks the generated code as IEEE
conformant. You must not use this option unless you
also specify -mtrap-precision=i and
either -mfp-trap-mode=su or
-mfp-trap-mode=sui. Its only effect
is to emit the line .eflag 48 in the function
prologue of the generated assembly file. Under DEC
Unix, this has the effect that IEEE-conformant math
library routines will be linked
in.
-mbuild-constants
Normally
GCC examines a 32- or 64-bit integer
constant to see if it can construct it from smaller
constants in two or three instructions. If it cannot, it
will output the constant as a literal and generate code to
load it from the data segment at run
time.
Use
this option to require GCC to construct
all integer constants using code, even if it takes
more instructions (the maximum is
six).
You
would typically use this option to build a shared library
dynamic loader. Itself a shared library, it must relocate
itself in memory before it can find the variables and
constants in its own data
segment.
-malpha-as
-mgas
Select
whether to generate code to be assembled by the
vendor-supplied assembler (-malpha-as) or
by the GNU assembler
-mgas.
-mbwx
-mno-bwx
-mcix
-mno-cix
-mfix
-mno-fix
-mmax
-mno-max
Indicate
whether GCC should generate code to use the
optional BWX , CIX , FIX
and MAX instruction sets. The default is to
use the instruction sets supported by the CPU
type specified via -mcpu= option or that
of the CPU on which GCC was built if
none was specified.
-mfloat-vax
-mfloat-ieee
Generate
code that uses (does not use) VAX F and G
floating-point arithmetic instead of IEEE single
and double
precision.
-mexplicit-relocs
-mno-explicit-relocs
Older
Alpha assemblers provided no way to generate symbol
relocations except via assembler macros. Use of these macros
does not allow optimal instruction scheduling. GNU
binutils as of version 2.12 supports a new syntax that
allows the compiler to explicitly mark which relocations
should apply to which instructions. This option is mostly
useful for debugging, as GCC detects the
capabilities of the assembler when it is built and sets the
default accordingly.
-msmall-data
-mlarge-data
When
-mexplicit-relocs is in effect, static
data is accessed via gp-relative relocations. When
-msmall-data is used, objects 8 bytes
long or smaller are placed in a small data area (the
".sdata" and ".sbss"
sections) and are accessed via 16-bit relocations off
of the $gp register. This limits the size of the
small data area to 64KB, but allows the variables to be
directly accessed via a single
instruction.
The
default is -mlarge-data. With this option
the data area is limited to just below 2GB. Programs that
require more than 2GB of data must use
"malloc" or "mmap" to
allocate the data in the heap instead of in the
program’s data
segment.
When
generating code for shared libraries, -fpic
implies -msmall-data and
-fPIC implies
-mlarge-data.
-msmall-text
-mlarge-text
When
-msmall-text is used, the compiler
assumes that the code of the entire program (or shared
library) fits in 4MB, and is thus reachable with a branch
instruction. When -msmall-data is used,
the compiler can assume that all local symbols share the
same $gp value, and thus reduce the number of
instructions required for a function call from 4 to
1.
The
default is
-mlarge-text.
-mcpu=cpu_type
Set
the instruction set and instruction scheduling parameters
for machine type cpu_type. You can specify either
the EV style name or the corresponding
chip number. GCC supports scheduling parameters
for the EV4 , EV5 and EV6
family of processors and will choose the default values
for the instruction set from the processor you specify. If
you do not specify a processor type, GCC will
default to the processor on which the compiler was
built.
Supported
values for cpu_type
are
ev45
21064
Schedules
as an EV4 and has no instruction set
extensions.
21164
Schedules
as an EV5 and has no instruction set
extensions.
ev56
21164a
Schedules
as an EV5 and supports the BWX
extension.
pca56
21164pc
21164PC
Schedules
as an EV5 and supports the BWX
and MAX
extensions.
21264
Schedules
as an EV6 and supports the BWX ,
FIX , and MAX
extensions.
ev67
21264a
Schedules
as an EV6 and supports the BWX ,
CIX , FIX , and MAX
extensions.
Native
toolchains also support the value native, which
selects the best architecture option for the host processor.
-mcpu=native has no effect if GCC
does not recognize the
processor.
-mtune=cpu_type
Set
only the instruction scheduling parameters for machine type
cpu_type. The instruction set is not
changed.
Native
toolchains also support the value native, which
selects the best architecture option for the host processor.
-mtune=native has no effect if GCC
does not recognize the
processor.
-mmemory-latency=time
Sets
the latency the scheduler should assume for typical memory
references as seen by the application. This number is highly
dependent on the memory access patterns used by the
application and the size of the external cache on the
machine.
Valid
options for time are
number
A
decimal number representing clock
cycles.
main
The
compiler contains estimates of the number of clock cycles
for "typical" EV4 & EV5
hardware for the Level 1, 2 & 3 caches (also called
Dcache, Scache, and Bcache), as well as to main memory. Note
that L3 is only valid for EV5
.
DEC
Alpha/VMS
Options
These
-m options are defined for the DEC
Alpha/VMS implementations:
-mvms-return-codes
Return
VMS condition codes from main. The default is to
return POSIX style condition (e.g. error)
codes.
-mdebug-main=prefix
Flag
the first routine whose name starts with prefix as
the main routine for the
debugger.
-mmalloc64
Default
to 64-bit memory allocation
routines.
FR30
Options
These
options are defined specifically for the FR30
port.
-msmall-model
Use
the small address space model. This can produce smaller
code, but it does assume that all symbolic values and
addresses will fit into a 20-bit
range.
-mno-lsim
Assume
that runtime support has been provided and so there is no
need to include the simulator library (libsim.a) on
the linker command
line.
FRV
Options
-mgpr-32
Only
use the first 32 general-purpose
registers.
-mgpr-64
Use
all 64 general-purpose
registers.
-mfpr-32
Use
only the first 32 floating-point
registers.
-mfpr-64
Use
all 64 floating-point
registers.
-mhard-float
Use
hardware instructions for floating-point
operations.
-msoft-float
Use
library routines for floating-point
operations.
-malloc-cc
Dynamically
allocate condition code
registers.
-mfixed-cc
Do
not try to dynamically allocate condition code registers,
only use "icc0" and
"fcc0".
-mdword
Change
ABI to use double word
insns.
-mno-dword
Do
not use double word
instructions.
-mdouble
Use
floating-point double
instructions.
-mno-double
Do
not use floating-point double
instructions.
-mmedia
Use
media instructions.
-mno-media
Do
not use media
instructions.
-mmuladd
Use
multiply and add/subtract
instructions.
-mno-muladd
Do
not use multiply and add/subtract
instructions.
-mfdpic
Select
the FDPIC ABI , which uses function descriptors
to represent pointers to functions. Without any
PIC/PIE-related options, it implies
-fPIE. With -fpic or
-fpie, it assumes GOT entries and
small data are within a 12-bit range from the
GOT base address; with -fPIC or
-fPIE, GOT offsets are computed with
32 bits. With a bfin-elf target, this option implies
-msim.
-minline-plt
Enable
inlining of PLT entries in function calls to
functions that are not known to bind locally. It has no
effect without -mfdpic. It’s enabled by
default if optimizing for speed and compiling for shared
libraries (i.e., -fPIC or -fpic),
or when an optimization option such as -O3 or
above is present in the command
line.
-mTLS
Assume
a large TLS segment when generating thread-local
code.
-mtls
Do
not assume a large TLS segment when generating
thread-local code.
-mgprel-ro
Enable
the use of "GPREL" relocations in
the FDPIC ABI for data that is known to be in
read-only sections. It’s enabled by default, except
for -fpic or -fpie: even though it
may help make the global offset table smaller, it trades 1
instruction for 4. With -fPIC or
-fPIE, it trades 3 instructions for 4, one of
which may be shared by multiple symbols, and it avoids the
need for a GOT entry for the referenced symbol,
so it’s more likely to be a win. If it is not,
-mno-gprel-ro can be used to
disable it.
-multilib-library-pic
Link
with the (library, not FD ) pic libraries.
It’s implied by -mlibrary-pic, as
well as by -fPIC and -fpic without
-mfdpic. You should never have to use it
explicitly.
-mlinked-fp
Follow
the EABI requirement of always creating a frame
pointer whenever a stack frame is allocated. This option is
enabled by default and can be disabled with
-mno-linked-fp.
-mlong-calls
Use
indirect addressing to call functions outside the current
compilation unit. This allows the functions to be placed
anywhere within the 32-bit address
space.
-malign-labels
Try
to align labels to an 8-byte boundary by inserting
nops into the previous packet. This option only has an
effect when VLIW packing is enabled. It
doesn’t create new packets; it merely adds nops to
existing ones.
-mlibrary-pic
Generate
position-independent EABI
code.
-macc-4
Use
only the first four media accumulator
registers.
-macc-8
Use
all eight media accumulator
registers.
-mpack
Pack
VLIW
instructions.
-mno-pack
Do
not pack VLIW
instructions.
-mno-eflags
Do
not mark ABI switches in
e_flags.
-mcond-move
Enable
the use of conditional-move instructions
(default).
This
switch is mainly for debugging the compiler and will likely
be removed in a future
version.
-mno-cond-move
Disable
the use of conditional-move
instructions.
This
switch is mainly for debugging the compiler and will likely
be removed in a future
version.
-mscc
Enable
the use of conditional set instructions
(default).
This
switch is mainly for debugging the compiler and will likely
be removed in a future
version.
-mno-scc
Disable
the use of conditional set
instructions.
This
switch is mainly for debugging the compiler and will likely
be removed in a future
version.
-mcond-exec
Enable
the use of conditional execution
(default).
This
switch is mainly for debugging the compiler and will likely
be removed in a future
version.
-mno-cond-exec
Disable
the use of conditional
execution.
This
switch is mainly for debugging the compiler and will likely
be removed in a future
version.
-mvliw-branch
Run
a pass to pack branches into VLIW instructions
(default).
This
switch is mainly for debugging the compiler and will likely
be removed in a future
version.
-mno-vliw-branch
Do
not run a pass to pack branches into VLIW
instructions.
This
switch is mainly for debugging the compiler and will likely
be removed in a future
version.
-mmulti-cond-exec
Enable
optimization of "&&" and
"||" in conditional execution
(default).
This
switch is mainly for debugging the compiler and will likely
be removed in a future
version.
-mno-multi-cond-exec
Disable
optimization of "&&" and
"||" in conditional
execution.
This
switch is mainly for debugging the compiler and will likely
be removed in a future
version.
-mnested-cond-exec
Enable
nested conditional execution optimizations
(default).
This
switch is mainly for debugging the compiler and will likely
be removed in a future
version.
-mno-nested-cond-exec
Disable
nested conditional execution
optimizations.
This
switch is mainly for debugging the compiler and will likely
be removed in a future
version.
-moptimize-membar
This
switch removes redundant "membar"
instructions from the compiler generated code. It is enabled
by default.
-mno-optimize-membar
This
switch disables the automatic removal of redundant
"membar" instructions from the generated
code.
-mtomcat-stats
Cause
gas to print out tomcat
statistics.
-mcpu=cpu
Select
the processor type for which to generate code. Possible
values are frv, fr550, tomcat,
fr500, fr450, fr405, fr400,
fr300 and
simple.
GNU/Linux
Options
These
-m options are defined for GNU/Linux targets:
-mglibc
Use
the GNU C library. This is the default except on
*-*-linux-*uclibc* and
*-*-linux-*android*
targets.
-muclibc
Use
uClibc C library. This is the default on
*-*-linux-*uclibc*
targets.
-mbionic
Use
Bionic C library. This is the default on
*-*-linux-*android*
targets.
-mandroid
Compile
code compatible with Android platform. This is the default
on *-*-linux-*android*
targets.
When
compiling, this option enables -mbionic,
-fPIC, -fno-exceptions and
-fno-rtti by default. When linking, this
option makes the GCC driver pass Android-specific
options to the linker. Finally, this option causes the
preprocessor macro "__ANDROID__" to be
defined.
-tno-android-cc
Disable
compilation effects of -mandroid, i.e., do not
enable -mbionic, -fPIC,
-fno-exceptions and
-fno-rtti by
default.
-tno-android-ld
Disable
linking effects of -mandroid, i.e., pass
standard Linux linking options to the
linker.
H8/300
Options
These
-m options are defined for the H8/300
implementations:
-mrelax
Shorten
some address references at link time, when possible; uses
the linker option
-relax.
-mh
Generate code for the
H8/300H.
-ms
Generate code for the
H8S.
-mn
Generate code for the H8S
and H8/300H in the normal mode. This switch must be used
either with -mh or
-ms.
-ms2600
Generate
code for the H8S/2600. This switch must be used with
-ms.
-mint32
Make
"int" data 32 bits by
default.
-malign-300
On
the H8/300H and H8S, use the same alignment rules as for the
H8/300. The default for the H8/300H and H8S is to align
longs and floats on 4-byte boundaries.
-malign-300 causes them to be aligned on
2-byte boundaries. This option has no effect on the
H8/300.
HPPA
Options
These
-m options are defined for the HPPA
family of computers:
-march=architecture-type
Generate
code for the specified architecture. The choices for
architecture-type are 1.0 for PA
1.0, 1.1 for PA 1.1, and 2.0
for PA 2.0 processors. Refer to
/usr/lib/sched.models on an HP-UX system to determine
the proper architecture option for your machine. Code
compiled for lower numbered architectures will run on higher
numbered architectures, but not the other way
around.
-mpa-risc-1-0
-mpa-risc-1-1
-mpa-risc-2-0
Synonyms
for -march=1.0, -march=1.1, and
-march=2.0
respectively.
-mbig-switch
Generate
code suitable for big switch tables. Use this option only if
the assembler/linker complain about out of range branches
within a switch
table.
-mjump-in-delay
Fill
delay slots of function calls with unconditional jump
instructions by modifying the return pointer for the
function call to be the target of the conditional
jump.
-mdisable-fpregs
Prevent
floating-point registers from being used in any manner. This
is necessary for compiling kernels that perform lazy context
switching of floating-point registers. If you use this
option and attempt to perform floating-point operations, the
compiler aborts.
-mdisable-indexing
Prevent
the compiler from using indexing address modes. This avoids
some rather obscure problems when compiling MIG
generated code under MACH
.
-mno-space-regs
Generate
code that assumes the target has no space registers. This
allows GCC to generate faster indirect calls and
use unscaled index address
modes.
Such
code is suitable for level 0 PA systems and
kernels.
-mfast-indirect-calls
Generate
code that assumes calls never cross space boundaries. This
allows GCC to emit code that performs faster
indirect calls.
This
option will not work in the presence of shared libraries or
nested functions.
-mfixed-range=register-range
Generate
code treating the given register range as fixed registers. A
fixed register is one that the register allocator can not
use. This is useful when compiling kernel code. A register
range is specified as two registers separated by a dash.
Multiple register ranges can be specified separated by a
comma.
-mlong-load-store
Generate
3-instruction load and store sequences as sometimes
required by the HP-UX 10 linker. This is equivalent to the
+k option to the HP
compilers.
-mportable-runtime
Use
the portable calling conventions proposed by HP
for ELF
systems.
-mgas
Enable
the use of assembler directives only GAS
understands.
-mschedule=cpu-type
Schedule
code according to the constraints for the machine type
cpu-type. The choices for cpu-type are 700
7100, 7100LC, 7200, 7300 and
8000. Refer to /usr/lib/sched.models on an
HP-UX system to determine the proper scheduling option for
your machine. The default scheduling is
8000.
-mlinker-opt
Enable
the optimization pass in the HP-UX linker. Note this makes
symbolic debugging impossible. It also triggers a bug in the
HP-UX 8 and HP-UX 9 linkers in which they give bogus error
messages when linking some
programs.
-msoft-float
Generate
output containing library calls for floating point.
Warning: the requisite libraries are not available
for all HPPA targets. Normally the facilities of
the machine’s usual C compiler are used, but this
cannot be done directly in cross-compilation. You must make
your own arrangements to provide suitable library functions
for
cross-compilation.
-msoft-float
changes the calling convention in the output file;
therefore, it is only useful if you compile all of a
program with this option. In particular, you need to compile
libgcc.a, the library that comes with GCC
, with -msoft-float in order for
this to work.
-msio
Generate
the predefine, "_SIO", for server
IO . The default is -mwsio. This generates
the predefines, "__hp9000s700",
"__hp9000s700__" and
"_WSIO", for workstation IO .
These options are available under HP-UX and
HI-UX.
-mgnu-ld
Use
GNU ld specific options. This passes
-shared to ld when building a shared library.
It is the default when GCC is configured,
explicitly or implicitly, with the GNU linker.
This option does not have any affect on which ld is called,
it only changes what parameters are passed to that ld. The
ld that is called is determined by the
--with-ld configure option,
GCC ’s program search path, and finally by the
user’s PATH . The linker used
by GCC can be printed using which ’gcc
-print-prog-name=ld’. This
option is only available on the 64-bit HP-UX GCC
, i.e. configured with
hppa*64*-*-hpux*.
-mhp-ld
Use
HP ld specific options. This passes -b to
ld when building a shared library and passes +Accept
TypeMismatch to ld on all links. It is the default
when GCC is configured, explicitly or implicitly,
with the HP linker. This option does not have any
affect on which ld is called, it only changes what
parameters are passed to that ld. The ld that is called is
determined by the --with-ld
configure option, GCC ’s program search
path, and finally by the user’s PATH
. The linker used by GCC can be printed
using which ’gcc
-print-prog-name=ld’. This
option is only available on the 64-bit HP-UX GCC
, i.e. configured with
hppa*64*-*-hpux*.
-mlong-calls
Generate
code that uses long call sequences. This ensures that a call
is always able to reach linker generated stubs. The default
is to generate long calls only when the distance from the
call site to the beginning of the function or translation
unit, as the case may be, exceeds a predefined limit set by
the branch type being used. The limits for normal calls are
7,600,000 and 240,000 bytes, respectively for the PA
2.0 and PA 1.X architectures. Sibcalls are
always limited at 240,000
bytes.
Distances
are measured from the beginning of functions when using the
-ffunction-sections option, or when using
the -mgas and
-mno-portable-runtime options
together under HP-UX with the SOM
linker.
It
is normally not desirable to use this option as it will
degrade performance. However, it may be useful in large
applications, particularly when partial linking is used to
build the
application.
The
types of long calls used depends on the capabilities of the
assembler and linker, and the type of code being generated.
The impact on systems that support long absolute calls, and
long pic symbol-difference or pc-relative calls should be
relatively small. However, an indirect call is used on
32-bit ELF systems in pic code and it is
quite long.
-munix=unix-std
Generate
compiler predefines and select a startfile for the
specified UNIX standard. The choices for
unix-std are 93, 95 and 98.
93 is supported on all HP-UX versions. 95 is
available on HP-UX 10.10 and later. 98 is available
on HP-UX 11.11 and later. The default values are 93
for HP-UX 10.00, 95 for HP-UX 10.10 though to 11.00,
and 98 for HP-UX 11.11 and
later.
-munix=93
provides the same predefines as GCC 3.3 and 3.4.
-munix=95 provides additional predefines for
"XOPEN_UNIX" and
"_XOPEN_SOURCE_EXTENDED", and the
startfile unix95.o. -munix=98 provides
additional predefines for "_XOPEN_UNIX",
"_XOPEN_SOURCE_EXTENDED",
"_INCLUDE__STDC_A1_SOURCE" and
"_INCLUDE_XOPEN_SOURCE_500", and the
startfile
unix98.o.
It
is important to note that this option changes the
interfaces for various library routines. It also affects the
operational behavior of the C library. Thus, extreme
care is needed in using this
option.
Library
code that is intended to operate with more than one
UNIX standard must test, set and restore the variable
__xpg4_extended_mask as appropriate. Most GNU
software doesn’t provide this
capability.
-nolibdld
Suppress
the generation of link options to search libdld.sl when the
-static option is specified on HP-UX 10 and
later.
-static
The
HP-UX implementation of setlocale in libc has a dependency
on libdld.sl. There isn’t an archive version of
libdld.sl. Thus, when the -static option is
specified, special link options are needed to resolve this
dependency.
On
HP-UX 10 and later, the GCC driver adds the
necessary options to link with libdld.sl when the
-static option is specified. This causes the
resulting binary to be dynamic. On the 64-bit port,
the linkers generate dynamic binaries by default in any
case. The -nolibdld option can be used to
prevent the GCC driver from adding these link
options.
-threads
Add
support for multithreading with the dce thread
library under HP-UX. This option sets flags for both the
preprocessor and
linker.
Intel
386 and AMD x86-64
Options
These
-m options are defined for the i386 and
x86-64 family of computers:
-mtune=cpu-type
Tune
to cpu-type everything applicable about the generated
code, except for the ABI and the set of available
instructions. The choices for cpu-type are:
generic
Produce
code optimized for the most common IA32/AMD64/EM64T
processors. If you know the CPU on which
your code will run, then you should use the corresponding
-mtune option instead of
-mtune=generic. But, if you do not know exactly
what CPU users of your application will have,
then you should use this
option.
As
new processors are deployed in the marketplace, the behavior
of this option will change. Therefore, if you upgrade to a
newer version of GCC , the code generated option
will change to reflect the processors that were most common
when that version of GCC was
released.
There
is no -march=generic option because
-march indicates the instruction set the
compiler can use, and there is no generic instruction set
applicable to all processors. In contrast,
-mtune indicates the processor (or, in this
case, collection of processors) for which the code is
optimized.
native
This
selects the CPU to tune for at compilation time
by determining the processor type of the compiling machine.
Using -mtune=native will produce code optimized
for the local machine under the constraints of the selected
instruction set. Using -march=native will
enable all instruction subsets supported by the local
machine (hence the result might not run on different
machines).
i386
Original
Intel’s i386 CPU
.
i486
Intel’s
i486 CPU . (No scheduling is implemented for this
chip.)
i586,
pentium
Intel
Pentium CPU with no MMX
support.
pentium-mmx
Intel
PentiumMMX CPU based on Pentium core with
MMX instruction set
support.
pentiumpro
Intel
PentiumPro CPU
.
i686
Same
as "generic", but when used as
"march" option, PentiumPro instruction
set will be used, so the code will run on all i686 family
chips.
pentium2
Intel
Pentium2 CPU based on PentiumPro core with
MMX instruction set
support.
pentium3,
pentium3m
Intel
Pentium3 CPU based on PentiumPro core with
MMX and SSE instruction set
support.
pentium-m
Low
power version of Intel Pentium3 CPU with
MMX , SSE and SSE2 instruction
set support. Used by Centrino
notebooks.
pentium4,
pentium4m
Intel
Pentium4 CPU with MMX , SSE
and SSE2 instruction set
support.
prescott
Improved
version of Intel Pentium4 CPU with MMX
, SSE , SSE2 and SSE3
instruction set
support.
nocona
Improved
version of Intel Pentium4 CPU with 64-bit
extensions, MMX , SSE , SSE2
and SSE3 instruction set
support.
core2
Intel
Core2 CPU with 64-bit extensions, MMX
, SSE , SSE2 , SSE3
and SSSE3 instruction set
support.
corei7
Intel
Core i7 CPU with 64-bit extensions,
MMX , SSE , SSE2 , SSE3
, SSSE3 , SSE4 .1 and SSE4
.2 instruction set
support.
corei7-avx
Intel
Core i7 CPU with 64-bit extensions,
MMX , SSE , SSE2 , SSE3
, SSSE3 , SSE4 .1, SSE4
.2, AVX , AES and PCLMUL
instruction set
support.
core-avx-i
Intel
Core CPU with 64-bit extensions, MMX
, SSE , SSE2 , SSE3
, SSSE3 , SSE4 .1, SSE4
.2, AVX , AES , PCLMUL
, FSGSBASE , RDRND and F16C
instruction set
support.
atom
Intel
Atom CPU with 64-bit extensions, MMX
, SSE , SSE2 , SSE3
and SSSE3 instruction set
support.
k6
AMD K6 CPU
with MMX instruction set
support.
k6-2,
k6-3
Improved
versions of AMD K6 CPU with MMX
and 3DNow! instruction set
support.
athlon,
athlon-tbird
AMD
Athlon CPU with MMX , 3dNOW!,
enhanced 3DNow! and SSE prefetch instructions
support.
athlon-4,
athlon-xp,
athlon-mp
Improved
AMD Athlon CPU with MMX , 3DNow!,
enhanced 3DNow! and full SSE instruction set
support.
k8,
opteron, athlon64,
athlon-fx
AMD
K8 core based CPUs with x86-64 instruction set
support. (This supersets MMX , SSE
, SSE2 , 3DNow!, enhanced 3DNow! and
64-bit instruction set
extensions.)
k8-sse3,
opteron-sse3,
athlon64-sse3
Improved
versions of k8, opteron and athlon64 with SSE3
instruction set
support.
amdfam10,
barcelona
AMD
Family 10h core based CPUs with x86-64
instruction set support. (This supersets MMX
, SSE , SSE2 , SSE3
, SSE4A , 3DNow!, enhanced 3DNow!, ABM
and 64-bit instruction set
extensions.)
bdver1
AMD
Family 15h core based CPUs with x86-64
instruction set support. (This supersets FMA4
, AVX , XOP , LWP
, AES , PCL_MUL , CX16
, MMX , SSE , SSE2
, SSE3 , SSE4A , SSSE3
, SSE4 .1, SSE4 .2, ABM
and 64-bit instruction set
extensions.)
bdver2
AMD
Family 15h core based CPUs with x86-64
instruction set support. (This supersets BMI
, TBM , F16C, FMA , AVX
, XOP , LWP , AES
, PCL_MUL , CX16 , MMX
, SSE , SSE2 , SSE3
, SSE4A , SSSE3 , SSE4
.1, SSE4 .2, ABM and 64-bit
instruction set
extensions.)
btver1
AMD
Family 14h core based CPUs with x86-64
instruction set support. (This supersets MMX
, SSE , SSE2 , SSE3
, SSSE3 , SSE4A , CX16
, ABM and 64-bit instruction set
extensions.)
winchip-c6
IDT
Winchip C6 CPU , dealt in same way as i486
with additional MMX instruction set
support.
winchip2
IDT
Winchip2 CPU , dealt in same way as i486
with additional MMX and 3DNow! instruction set
support.
c3
Via C3 CPU
with MMX and 3DNow! instruction set support.
(No scheduling is implemented for this
chip.)
c3-2
Via
C3-2 CPU with MMX and SSE
instruction set support. (No scheduling is implemented
for this chip.)
geode
Embedded
AMD CPU with MMX and 3DNow! instruction set
support.
While
picking a specific cpu-type will schedule things
appropriately for that particular chip, the compiler will
not generate any code that does not run on the default
machine type without the -march=cpu-type
option being used. For example, if GCC is
configured for i686-pc-linux-gnu then
-mtune=pentium4 will generate code that is
tuned for Pentium4 but will still run on i686
machines.
-march=cpu-type
Generate
instructions for the machine type cpu-type. The
choices for cpu-type are the same as for
-mtune. Moreover, specifying
-march=cpu-type implies
-mtune=cpu-type.
-mcpu=cpu-type
A
deprecated synonym for
-mtune.
-mfpmath=unit
Generate
floating-point arithmetic for selected unit unit. The
choices for unit
are:
387
Use the standard 387
floating-point coprocessor present on the majority of chips
and emulated otherwise. Code compiled with this option runs
almost everywhere. The temporary results are computed in
80-bit precision instead of the precision specified by
the type, resulting in slightly different results compared
to most of other chips. See -ffloat-store
for more detailed
description.
This
is the default choice for i386
compiler.
sse
Use
scalar floating-point instructions present in the SSE
instruction set. This instruction set is supported by
Pentium3 and newer chips, in the AMD line by
Athlon-4, Athlon-xp and Athlon-mp chips. The earlier
version of SSE instruction set supports only
single-precision arithmetic, thus the double and
extended-precision arithmetic are still done using 387. A
later version, present only in Pentium4 and the future
AMD x86-64 chips, supports double-precision
arithmetic too.
For
the i386 compiler, you need to use
-march=cpu-type, -msse or
-msse2 switches to enable SSE
extensions and make this option effective. For the
x86-64 compiler, these extensions are enabled by
default.
The
resulting code should be considerably faster in the majority
of cases and avoid the numerical instability problems of 387
code, but may break some existing code that expects
temporaries to be 80
bits.
This
is the default choice for the x86-64
compiler.
sse,387
sse+387
both
Attempt
to utilize both instruction sets at once. This effectively
double the amount of available registers and on chips with
separate execution units for 387 and SSE the
execution resources too. Use this option with care, as it is
still experimental, because the GCC register
allocator does not model separate functional units well
resulting in instable
performance.
-masm=dialect
Output
asm instructions using selected dialect. Supported
choices are intel or att (the default one).
Darwin does not support
intel.
-mieee-fp
-mno-ieee-fp
Control
whether or not the compiler uses IEEE
floating-point comparisons. These handle correctly the
case where the result of a comparison is
unordered.
-msoft-float
Generate
output containing library calls for floating point.
Warning: the requisite libraries are not part
of GCC . Normally the facilities of the
machine’s usual C compiler are used, but this
can’t be done directly in cross-compilation. You must
make your own arrangements to provide suitable library
functions for
cross-compilation.
On
machines where a function returns floating-point results in
the 80387 register stack, some floating-point opcodes may be
emitted even if -msoft-float is
used.
-mno-fp-ret-in-387
Do
not use the FPU registers for return values of
functions.
The
usual calling convention has functions return values of
types "float" and
"double" in an FPU register,
even if there is no FPU . The idea is that the
operating system should emulate an FPU
.
The
option
-mno-fp-ret-in-387
causes such values to be returned in ordinary CPU
registers
instead.
-mno-fancy-math-387
Some
387 emulators do not support the "sin",
"cos" and "sqrt"
instructions for the 387. Specify this option to avoid
generating those instructions. This option is the default on
FreeBSD, OpenBSD and NetBSD. This option is overridden when
-march indicates that the target CPU
will always have an FPU and so the
instruction will not need emulation. As of revision 2.6.1,
these instructions are not generated unless you also use the
-funsafe-math-optimizations
switch.
-malign-double
-mno-align-double
Control
whether GCC aligns "double",
"long double", and "long
long" variables on a two-word boundary or a
one-word boundary. Aligning "double"
variables on a two-word boundary produces code that runs
somewhat faster on a Pentium at the expense of more
memory.
On
x86-64, -malign-double is enabled
by default.
Warning:
if you use the -malign-double switch,
structures containing the above types will be aligned
differently than the published application binary interface
specifications for the 386 and will not be binary compatible
with structures in code compiled without that
switch.
-m96bit-long-double
-m128bit-long-double
These
switches control the size of "long
double" type. The i386 application binary
interface specifies the size to be 96 bits, so
-m96bit-long-double is the default
in 32-bit
mode.
Modern
architectures (Pentium and newer) prefer "long
double" to be aligned to an 8- or
16-byte boundary. In arrays or structures conforming
to the ABI , this is not possible. So specifying
-m128bit-long-double aligns
"long double" to a 16-byte boundary
by padding the "long double" with an
additional 32-bit
zero.
In
the x86-64 compiler,
-m128bit-long-double is the default
choice as its ABI specifies that "long
double" is to be aligned on 16-byte
boundary.
Notice
that neither of these options enable any extra precision
over the x87 standard of 80 bits for a "long
double".
Warning:
if you override the default value for your target ABI
, the structures and arrays containing "long
double" variables will change their size as well
as function calling convention for function taking
"long double" will be modified. Hence
they will not be binary compatible with arrays or structures
in code compiled without that
switch.
-mlarge-data-threshold=number
When
-mcmodel=medium is specified, the data greater
than threshold are placed in large data section. This
value must be the same across all object linked into the
binary and defaults to
65535.
-mrtd
Use
a different function-calling convention, in which functions
that take a fixed number of arguments return with the
"ret" num instruction, which pops
their arguments while returning. This saves one instruction
in the caller since there is no need to pop the arguments
there.
You
can specify that an individual function is called with this
calling sequence with the function attribute stdcall.
You can also override the -mrtd option by using
the function attribute
cdecl.
Warning:
this calling convention is incompatible with the one
normally used on Unix, so you cannot use it if you need to
call libraries compiled with the Unix
compiler.
Also,
you must provide function prototypes for all functions that
take variable numbers of arguments (including
"printf"); otherwise incorrect code will
be generated for calls to those
functions.
In
addition, seriously incorrect code will result if you call a
function with too many arguments. (Normally, extra arguments
are harmlessly
ignored.)
-mregparm=num
Control
how many registers are used to pass integer arguments. By
default, no registers are used to pass arguments, and at
most 3 registers can be used. You can control this behavior
for a specific function by using the function attribute
regparm.
Warning:
if you use this switch, and num is nonzero, then you
must build all modules with the same value, including any
libraries. This includes the system libraries and startup
modules.
-msseregparm
Use
SSE register passing conventions for float and double
arguments and return values. You can control this behavior
for a specific function by using the function attribute
sseregparm.
Warning:
if you use this switch then you must build all modules with
the same value, including any libraries. This includes the
system libraries and startup
modules.
-mvect8-ret-in-mem
Return
8-byte vectors in memory instead of MMX
registers. This is the default on Solaris@tie{}8 and 9
and VxWorks to match the ABI of the Sun Studio
compilers until version 12. Later compiler versions
(starting with Studio 12 Update@tie{}1) follow the ABI
used by other x86 targets, which is the default on
Solaris@tie{}10 and later. Only use this option if
you need to remain compatible with existing code produced by
those previous compiler versions or older versions of
GCC .
-mpc32
-mpc64
-mpc80
Set
80387 floating-point precision to 32, 64 or 80 bits. When
-mpc32 is specified, the significands of
results of floating-point operations are rounded to 24 bits
(single precision); -mpc64 rounds the
significands of results of floating-point operations to 53
bits (double precision) and -mpc80 rounds the
significands of results of floating-point operations to 64
bits (extended double precision), which is the default. When
this option is used, floating-point operations in higher
precisions are not available to the programmer without
setting the FPU control word
explicitly.
Setting
the rounding of floating-point operations to less than the
default 80 bits can speed some programs by 2% or more. Note
that some mathematical libraries assume that
extended-precision (80-bit) floating-point operations
are enabled by default; routines in such libraries could
suffer significant loss of accuracy, typically through
so-called "catastrophic cancellation", when this
option is used to set the precision to less than extended
precision.
-mstackrealign
Realign
the stack at entry. On the Intel x86, the
-mstackrealign option will generate an
alternate prologue and epilogue that realigns the run-time
stack if necessary. This supports mixing legacy codes that
keep a 4-byte aligned stack with modern codes that
keep a 16-byte stack for SSE compatibility.
See also the attribute
"force_align_arg_pointer", applicable to
individual
functions.
-mpreferred-stack-boundary=num
Attempt
to keep the stack boundary aligned to a 2 raised to
num byte boundary. If
-mpreferred-stack-boundary is not
specified, the default is 4 (16 bytes or 128
bits).
Warning:
When generating code for the x86-64 architecture
with SSE extensions disabled,
-mpreferred-stack-boundary=3 can be
used to keep the stack boundary aligned to 8 byte boundary.
You must build all modules with
-mpreferred-stack-boundary=3,
including any libraries. This includes the system libraries
and startup modules.
-mincoming-stack-boundary=num
Assume
the incoming stack is aligned to a 2 raised to num
byte boundary. If
-mincoming-stack-boundary is not
specified, the one specified by
-mpreferred-stack-boundary will be
used.
On
Pentium and PentiumPro, "double" and
"long double" values should be aligned to
an 8-byte boundary (see
-malign-double) or suffer significant run
time performance penalties. On Pentium III , the
Streaming SIMD Extension ( SSE ) data
type "__m128" may not work properly if it
is not 16-byte
aligned.
To
ensure proper alignment of this values on the stack, the
stack boundary must be as aligned as that required by any
value stored on the stack. Further, every function must be
generated such that it keeps the stack aligned. Thus calling
a function compiled with a higher preferred stack boundary
from a function compiled with a lower preferred stack
boundary will most likely misalign the stack. It is
recommended that libraries that use callbacks always use the
default setting.
This
extra alignment does consume extra stack space, and
generally increases code size. Code that is sensitive to
stack space usage, such as embedded systems and operating
system kernels, may want to reduce the preferred alignment
to
-mpreferred-stack-boundary=2.
-mmmx
-mno-mmx
-msse
-mno-sse
-msse2
-mno-sse2
-msse3
-mno-sse3
-mssse3
-mno-ssse3
-msse4.1
-mno-sse4.1
-msse4.2
-mno-sse4.2
-msse4
-mno-sse4
-mavx
-mno-avx
-mavx2
-mno-avx2
-maes
-mno-aes
-mpclmul
-mno-pclmul
-mfsgsbase
-mno-fsgsbase
-mrdrnd
-mno-rdrnd
-mf16c
-mno-f16c
-mfma
-mno-fma
-msse4a
-mno-sse4a
-mfma4
-mno-fma4
-mxop
-mno-xop
-mlwp
-mno-lwp
-m3dnow
-mno-3dnow
-mpopcnt
-mno-popcnt
-mabm
-mno-abm
-mbmi
-mbmi2
-mno-bmi
-mno-bmi2
-mlzcnt
-mno-lzcnt
-mtbm
-mno-tbm
These
switches enable or disable the use of instructions in
the MMX , SSE , SSE2
, SSE3 , SSSE3 , SSE4
.1, AVX , AVX2 , AES
, PCLMUL , FSGSBASE , RDRND
, F16C, FMA , SSE4A , FMA4
, XOP , LWP , ABM
, BMI , BMI2 , LZCNT
or 3DNow!
extended instruction sets. These extensions are also
available as built-in functions: see X86 Built-in
Functions, for details of the functions enabled and
disabled by these
switches.
To
have SSE/SSE2 instructions generated
automatically from floating-point code (as opposed to 387
instructions), see
-mfpmath=sse.
GCC
depresses SSEx instructions when -mavx is
used. Instead, it generates new AVX instructions
or AVX equivalence for all SSEx instructions when
needed.
These
options will enable GCC to use these extended
instructions in generated code, even without
-mfpmath=sse. Applications that perform
run-time CPU detection must compile separate
files for each supported architecture, using the appropriate
flags. In particular, the file containing the CPU
detection code should be compiled without these
options.
-mcld
This
option instructs GCC to emit a
"cld" instruction in the prologue of
functions that use string instructions. String instructions
depend on the DF flag to select between
autoincrement or autodecrement mode. While the ABI
specifies the DF flag to be cleared on
function entry, some operating systems violate this
specification by not clearing the DF flag in
their exception dispatchers. The exception handler can be
invoked with the DF flag set, which leads to
wrong direction mode when string instructions are used. This
option can be enabled by default on 32-bit x86 targets
by configuring GCC with the
--enable-cld configure option.
Generation of "cld" instructions can be
suppressed with the -mno-cld compiler
option in this case.
-mvzeroupper
This
option instructs GCC to emit a
"vzeroupper" instruction before a
transfer of control flow out of the function to
minimize AVX to SSE transition penalty
as well as remove unnecessary zeroupper
intrinsics.
-mprefer-avx128
This
option instructs GCC to use 128-bit
AVX instructions instead of 256-bit AVX
instructions in the
auto-vectorizer.
-mcx16
This
option will enable GCC to use CMPXCHG16B
instruction in generated code. CMPXCHG16B
allows for atomic operations on 128-bit double
quadword (or oword) data types. This is useful for high
resolution counters that could be updated by multiple
processors (or cores). This instruction is generated as part
of atomic built-in functions: see __sync Builtins or
__atomic Builtins for
details.
-msahf
This
option will enable GCC to use SAHF
instruction in generated 64-bit code. Early Intel
CPUs with Intel 64 lacked LAHF and SAHF
instructions supported by AMD64 until
introduction of Pentium 4 G1 step in December 2005.
LAHF and SAHF are load and store
instructions, respectively, for certain status flags. In
64-bit mode, SAHF instruction is used to
optimize "fmod",
"drem" or "remainder"
built-in functions: see Other Builtins for
details.
-mmovbe
This
option will enable GCC to use movbe instruction
to implement "__builtin_bswap32" and
"__builtin_bswap64".
-mcrc32
This
option will enable built-in functions,
"__builtin_ia32_crc32qi",
"__builtin_ia32_crc32hi".
"__builtin_ia32_crc32si" and
"__builtin_ia32_crc32di" to generate the
crc32 machine
instruction.
-mrecip
This
option will enable GCC to use RCPSS
and RSQRTSS instructions (and their
vectorized variants RCPPS and RSQRTPS
) with an additional Newton-Raphson step to increase
precision instead of DIVSS and SQRTSS
(and their vectorized variants) for single-precision
floating-point arguments. These instructions are generated
only when
-funsafe-math-optimizations is
enabled together with
-finite-math-only and
-fno-trapping-math. Note that while
the throughput of the sequence is higher than the throughput
of the non-reciprocal instruction, the precision of the
sequence can be decreased by up to 2 ulp (i.e. the inverse
of 1.0 equals
0.99999994).
Note
that GCC implements
"1.0f/sqrtf(x)" in terms
of RSQRTSS (or RSQRTPS ) already with
-ffast-math (or the above option
combination), and doesn’t need
-mrecip.
Also
note that GCC emits the above sequence with
additional Newton-Raphson step for vectorized single-float
division and vectorized
"sqrtf(x)" already with
-ffast-math (or the above option
combination), and doesn’t need
-mrecip.
-mrecip=opt
This
option allows to control which reciprocal estimate
instructions may be used. opt is a comma separated
list of options, which may be preceded by a
"!" to invert the option:
"all": enable all estimate instructions,
"default": enable the default
instructions, equivalent to -mrecip,
"none": disable all estimate
instructions, equivalent to -mno-recip,
"div": enable the approximation for
scalar division, "vec-div": enable
the approximation for vectorized division,
"sqrt": enable the approximation for
scalar square root, "vec-sqrt":
enable the approximation for vectorized square
root.
So
for example, -mrecip=all,!sqrt would enable all
of the reciprocal approximations, except for square
root.
-mveclibabi=type
Specifies
the ABI type to use for vectorizing intrinsics
using an external library. Supported types are
"svml" for the Intel short vector math
library and "acml" for the AMD
math core library style of interfacing. GCC
will currently emit calls to
"vmldExp2", "vmldLn2",
"vmldLog102",
"vmldLog102",
"vmldPow2",
"vmldTanh2",
"vmldTan2",
"vmldAtan2",
"vmldAtanh2",
"vmldCbrt2",
"vmldSinh2",
"vmldSin2",
"vmldAsinh2",
"vmldAsin2",
"vmldCosh2",
"vmldCos2",
"vmldAcosh2",
"vmldAcos2",
"vmlsExp4", "vmlsLn4",
"vmlsLog104",
"vmlsLog104",
"vmlsPow4",
"vmlsTanh4",
"vmlsTan4",
"vmlsAtan4",
"vmlsAtanh4",
"vmlsCbrt4",
"vmlsSinh4",
"vmlsSin4",
"vmlsAsinh4",
"vmlsAsin4",
"vmlsCosh4",
"vmlsCos4",
"vmlsAcosh4" and
"vmlsAcos4" for corresponding function
type when -mveclibabi=svml is used and
"__vrd2_sin",
"__vrd2_cos",
"__vrd2_exp",
"__vrd2_log",
"__vrd2_log2",
"__vrd2_log10",
"__vrs4_sinf",
"__vrs4_cosf",
"__vrs4_expf",
"__vrs4_logf",
"__vrs4_log2f",
"__vrs4_log10f" and
"__vrs4_powf" for corresponding function
type when -mveclibabi=acml is used. Both
-ftree-vectorize and
-funsafe-math-optimizations have to
be enabled. A SVML or ACML ABI
compatible library will have to be specified at link
time.
-mabi=name
Generate
code for the specified calling convention. Permissible
values are: sysv for the ABI used on
GNU/Linux and other systems and ms for the
Microsoft ABI . The default is to use the
Microsoft ABI when targeting Windows. On all
other systems, the default is the SYSV ABI . You
can control this behavior for a specific function by using
the function attribute
ms_abi/sysv_abi.
-mtls-dialect=type
Generate
code to access thread-local storage using the gnu or
gnu2 conventions. gnu is the conservative
default; gnu2 is more efficient, but it may add
compile- and run-time requirements that cannot be
satisfied on all
systems.
-mpush-args
-mno-push-args
Use
PUSH operations to store outgoing parameters. This
method is shorter and usually equally fast as method
using SUB/MOV operations and is enabled by
default. In some cases disabling it may improve performance
because of improved scheduling and reduced
dependencies.
-maccumulate-outgoing-args
If
enabled, the maximum amount of space required for outgoing
arguments will be computed in the function prologue. This is
faster on most modern CPUs because of reduced dependencies,
improved scheduling and reduced stack usage when preferred
stack boundary is not equal to 2. The drawback is a notable
increase in code size. This switch implies
-mno-push-args.
-mthreads
Support
thread-safe exception handling on Mingw32. Code that
relies on thread-safe exception handling must compile and
link all code with the -mthreads option. When
compiling, -mthreads defines
-D_MT; when linking, it links in a special
thread helper library -lmingwthrd which cleans
up per thread exception handling
data.
-mno-align-stringops
Do
not align destination of inlined string operations. This
switch reduces code size and improves performance in case
the destination is already aligned, but GCC
doesn’t know about
it.
-minline-all-stringops
By
default GCC inlines string operations only when
the destination is known to be aligned to least a
4-byte boundary. This enables more inlining, increase
code size, but may improve performance of code that depends
on fast memcpy, strlen and memset for short
lengths.
-minline-stringops-dynamically
For
string operations of unknown size, use run-time checks with
inline code for small blocks and a library call for large
blocks.
-mstringop-strategy=alg
Overwrite
internal decision heuristic about particular algorithm to
inline string operation with. The allowed values are
"rep_byte",
"rep_4byte",
"rep_8byte" for expanding using i386
"rep" prefix of specified size,
"byte_loop", "loop",
"unrolled_loop" for expanding inline
loop, "libcall" for always expanding
library call.
-momit-leaf-frame-pointer
Don’t
keep the frame pointer in a register for leaf functions.
This avoids the instructions to save, set up and restore
frame pointers and makes an extra register available in leaf
functions. The option
-fomit-frame-pointer removes the
frame pointer for all functions, which might make debugging
harder.
-mtls-direct-seg-refs
-mno-tls-direct-seg-refs
Controls
whether TLS variables may be accessed with
offsets from the TLS segment register
(%gs for 32-bit, %fs for
64-bit), or whether the thread base pointer must be
added. Whether or not this is legal depends on the operating
system, and whether it maps the segment to cover the
entire TLS
area.
For
systems that use GNU libc, the default is
on.
-msse2avx
-mno-sse2avx
Specify
that the assembler should encode SSE instructions
with VEX prefix. The option -mavx
turns this on by
default.
-mfentry
-mno-fentry
If
profiling is active -pg put the profiling
counter call before prologue. Note: On x86 architectures the
attribute "ms_hook_prologue" isn’t
possible at the moment for -mfentry and
-pg.
-m8bit-idiv
-mno-8bit-idiv
On
some processors, like Intel Atom, 8-bit unsigned
integer divide is much faster than 32-bit/64-bit
integer divide. This option generates a run-time check. If
both dividend and divisor are within range of 0 to 255,
8-bit unsigned integer divide is used instead of
32-bit/64-bit integer
divide.
-mavx256-split-unaligned-load
-mavx256-split-unaligned-store
Split
32-byte AVX unaligned load and
store.
These
-m switches are supported in addition to the
above on AMD x86-64 processors in
64-bit environments.
-m32
-m64
-mx32
Generate
code for a 32-bit or 64-bit environment. The
-m32 option sets int, long and pointer to 32
bits and generates code that runs on any i386 system. The
-m64 option sets int to 32 bits and long and
pointer to 64 bits and generates code for AMD
’s x86-64 architecture. The
-mx32 option sets int, long and pointer to 32
bits and generates code for AMD ’s
x86-64 architecture. For darwin only the
-m64 option turns off the
-fno-pic and
-mdynamic-no-pic
options.
-mno-red-zone
Do
not use a so called red zone for x86-64 code. The red
zone is mandated by the x86-64 ABI , it is
a 128-byte area beyond the location of the stack
pointer that will not be modified by signal or interrupt
handlers and therefore can be used for temporary data
without adjusting the stack pointer. The flag
-mno-red-zone disables this red
zone.
-mcmodel=small
Generate
code for the small code model: the program and its symbols
must be linked in the lower 2 GB of the address
space. Pointers are 64 bits. Programs can be statically or
dynamically linked. This is the default code
model.
-mcmodel=kernel
Generate
code for the kernel code model. The kernel runs in the
negative 2 GB of the address space. This model
has to be used for Linux kernel
code.
-mcmodel=medium
Generate
code for the medium model: The program is linked in the
lower 2 GB of the address space. Small symbols
are also placed there. Symbols with sizes larger than
-mlarge-data-threshold are put into
large data or bss sections and can be located above 2GB.
Programs can be statically or dynamically
linked.
-mcmodel=large
Generate
code for the large model: This model makes no assumptions
about addresses and sizes of
sections.
-maddress-mode=long
Generate
code for long address mode. This is only supported for
64-bit and x32 environments. It is the default address
mode for 64-bit
environments.
-maddress-mode=short
Generate
code for short address mode. This is only supported for
32-bit and x32 environments. It is the default address
mode for 32-bit and x32
environments.
i386
and x86-64 Windows
Options
These
additional options are available for Windows targets:
-mconsole
This
option is available for Cygwin and MinGW targets. It
specifies that a console application is to be generated, by
instructing the linker to set the PE header
subsystem type required for console applications. This is
the default behavior for Cygwin and MinGW
targets.
-mdll
This
option is available for Cygwin and MinGW targets. It
specifies that a DLL - a dynamic link
library - is to be generated, enabling the selection
of the required runtime startup object and entry
point.
-mnop-fun-dllimport
This
option is available for Cygwin and MinGW targets. It
specifies that the dllimport attribute should be
ignored.
-mthread
This
option is available for MinGW targets. It specifies that
MinGW-specific thread support is to be
used.
-municode
This
option is available for mingw-w64 targets. It
specifies that the UNICODE macro is getting
pre-defined and that the unicode capable runtime startup
code is chosen.
-mwin32
This
option is available for Cygwin and MinGW targets. It
specifies that the typical Windows pre-defined macros are to
be set in the pre-processor, but does not influence the
choice of runtime library/startup
code.
-mwindows
This
option is available for Cygwin and MinGW targets. It
specifies that a GUI application is to be
generated by instructing the linker to set the PE
header subsystem type
appropriately.
-fno-set-stack-executable
This
option is available for MinGW targets. It specifies that the
executable flag for stack used by nested functions
isn’t set. This is necessary for binaries running in
kernel mode of Windows, as there the user32 API ,
which is used to set executable privileges, isn’t
available.
-mpe-aligned-commons
This
option is available for Cygwin and MinGW targets. It
specifies that the GNU extension to the PE
file format that permits the correct alignment of
COMMON variables should be used when generating code.
It will be enabled by default if GCC detects that
the target assembler found during configuration supports the
feature.
See
also under i386 and x86-64 Options for standard
options.
IA-64
Options
These
are the -m options defined for the Intel
IA-64 architecture.
-mbig-endian
Generate
code for a big-endian target. This is the default for
HP-UX.
-mlittle-endian
Generate
code for a little-endian target. This is the default
for AIX5 and
GNU/Linux.
-mgnu-as
-mno-gnu-as
Generate
(or don’t) code for the GNU assembler. This
is the default.
-mgnu-ld
-mno-gnu-ld
Generate
(or don’t) code for the GNU linker. This is
the default.
-mno-pic
Generate
code that does not use a global pointer register. The result
is not position independent code, and violates the
IA-64 ABI
.
-mvolatile-asm-stop
-mno-volatile-asm-stop
Generate
(or don’t) a stop bit immediately before and after
volatile asm
statements.
-mregister-names
-mno-register-names
Generate
(or don’t) in, loc, and out
register names for the stacked registers. This may make
assembler output more
readable.
-mno-sdata
-msdata
Disable
(or enable) optimizations that use the small data section.
This may be useful for working around optimizer
bugs.
-mconstant-gp
Generate
code that uses a single constant global pointer value. This
is useful when compiling kernel
code.
-mauto-pic
Generate
code that is self-relocatable. This implies
-mconstant-gp. This is useful when
compiling firmware
code.
-minline-float-divide-min-latency
Generate
code for inline divides of floating-point values using the
minimum latency
algorithm.
-minline-float-divide-max-throughput
Generate
code for inline divides of floating-point values using the
maximum throughput
algorithm.
-mno-inline-float-divide
Do
not generate inline code for divides of floating-point
values.
-minline-int-divide-min-latency
Generate
code for inline divides of integer values using the minimum
latency algorithm.
-minline-int-divide-max-throughput
Generate
code for inline divides of integer values using the maximum
throughput
algorithm.
-mno-inline-int-divide
Do
not generate inline code for divides of integer
values.
-minline-sqrt-min-latency
Generate
code for inline square roots using the minimum latency
algorithm.
-minline-sqrt-max-throughput
Generate
code for inline square roots using the maximum throughput
algorithm.
-mno-inline-sqrt
Do
not generate inline code for
sqrt.
-mfused-madd
-mno-fused-madd
Do
(don’t) generate code that uses the fused multiply/add
or multiply/subtract instructions. The default is to use
these instructions.
-mno-dwarf2-asm
-mdwarf2-asm
Don’t
(or do) generate assembler code for the DWARF2
line number debugging info. This may be useful when not
using the GNU
assembler.
-mearly-stop-bits
-mno-early-stop-bits
Allow
stop bits to be placed earlier than immediately preceding
the instruction that triggered the stop bit. This can
improve instruction scheduling, but does not always do
so.
-mfixed-range=register-range
Generate
code treating the given register range as fixed registers. A
fixed register is one that the register allocator can not
use. This is useful when compiling kernel code. A register
range is specified as two registers separated by a dash.
Multiple register ranges can be specified separated by a
comma.
-mtls-size=tls-size
Specify
bit size of immediate TLS offsets. Valid values
are 14, 22, and 64.
-mtune=cpu-type
Tune
the instruction scheduling for a particular CPU ,
Valid values are itanium, itanium1, merced, itanium2, and
mckinley.
-milp32
-mlp64
Generate
code for a 32-bit or 64-bit environment. The
32-bit environment sets int, long and pointer to 32
bits. The 64-bit environment sets int to 32 bits and
long and pointer to 64 bits. These are HP-UX specific
flags.
-mno-sched-br-data-spec
-msched-br-data-spec
(Dis/En)able
data speculative scheduling before reload. This will result
in generation of the ld.a instructions and the corresponding
check instructions (ld.c / chk.a). The default is
’disable’.
-msched-ar-data-spec
-mno-sched-ar-data-spec
(En/Dis)able
data speculative scheduling after reload. This will result
in generation of the ld.a instructions and the corresponding
check instructions (ld.c / chk.a). The default is
’enable’.
-mno-sched-control-spec
-msched-control-spec
(Dis/En)able
control speculative scheduling. This feature is available
only during region scheduling (i.e. before reload). This
will result in generation of the ld.s instructions and the
corresponding check instructions chk.s . The default is
’disable’.
-msched-br-in-data-spec
-mno-sched-br-in-data-spec
(En/Dis)able
speculative scheduling of the instructions that are
dependent on the data speculative loads before reload. This
is effective only with
-msched-br-data-spec enabled.
The default is
’enable’.
-msched-ar-in-data-spec
-mno-sched-ar-in-data-spec
(En/Dis)able
speculative scheduling of the instructions that are
dependent on the data speculative loads after reload. This
is effective only with
-msched-ar-data-spec enabled.
The default is
’enable’.
-msched-in-control-spec
-mno-sched-in-control-spec
(En/Dis)able
speculative scheduling of the instructions that are
dependent on the control speculative loads. This is
effective only with
-msched-control-spec enabled. The
default is
’enable’.
-mno-sched-prefer-non-data-spec-insns
-msched-prefer-non-data-spec-insns
If
enabled, data speculative instructions will be chosen for
schedule only if there are no other choices at the moment.
This will make the use of the data speculation much more
conservative. The default is
’disable’.
-mno-sched-prefer-non-control-spec-insns
-msched-prefer-non-control-spec-insns
If
enabled, control speculative instructions will be chosen for
schedule only if there are no other choices at the moment.
This will make the use of the control speculation much more
conservative. The default is
’disable’.
-mno-sched-count-spec-in-critical-path
-msched-count-spec-in-critical-path
If
enabled, speculative dependencies will be considered during
computation of the instructions priorities. This will make
the use of the speculation a bit more conservative. The
default is
’disable’.
-msched-spec-ldc
Use
a simple data speculation check. This option is on by
default.
-msched-control-spec-ldc
Use
a simple check for control speculation. This option is on by
default.
-msched-stop-bits-after-every-cycle
Place
a stop bit after every cycle when scheduling. This option is
on by default.
-msched-fp-mem-deps-zero-cost
Assume
that floating-point stores and loads are not likely to cause
a conflict when placed into the same instruction group. This
option is disabled by
default.
-msel-sched-dont-check-control-spec
Generate
checks for control speculation in selective scheduling. This
flag is disabled by
default.
-msched-max-memory-insns=max-insns
Limit
on the number of memory insns per instruction group, giving
lower priority to subsequent memory insns attempting to
schedule in the same instruction group. Frequently useful to
prevent cache bank conflicts. The default value is
1.
-msched-max-memory-insns-hard-limit
Disallow
more than
’msched-max-memory-insns’ in
instruction group. Otherwise, limit is ’soft’
meaning that we would prefer non-memory operations when
limit is reached but may still schedule memory
operations.
IA-64/VMS
Options
These
-m options are defined for the
IA-64/VMS implementations:
-mvms-return-codes
Return
VMS condition codes from main. The default is to
return POSIX style condition (e.g. error)
codes.
-mdebug-main=prefix
Flag
the first routine whose name starts with prefix as
the main routine for the
debugger.
-mmalloc64
Default
to 64-bit memory allocation
routines.
LM32
Options
These
-m options are defined for the Lattice Mico32
architecture:
-mbarrel-shift-enabled
Enable
barrel-shift
instructions.
-mdivide-enabled
Enable
divide and modulus
instructions.
-mmultiply-enabled
Enable
multiply
instructions.
-msign-extend-enabled
Enable
sign extend
instructions.
-muser-enabled
Enable
user-defined
instructions.
M32C
Options
-mcpu=name
Select
the CPU for which code is generated. name
may be one of r8c for the R8C/Tiny series,
m16c for the M16C (up to /60) series, m32cm
for the M16C/80 series, or m32c for the M32C/80
series.
-msim
Specifies
that the program will be run on the simulator. This causes
an alternate runtime library to be linked in which supports,
for example, file I/O. You must not use this option when
generating programs that will run on real hardware; you must
provide your own runtime library for whatever I/O functions
are needed.
-memregs=number
Specifies
the number of memory-based pseudo-registers GCC
will use during code generation. These pseudo-registers
will be used like real registers, so there is a tradeoff
between GCC ’s ability to fit the code into
available registers, and the performance penalty of using
memory instead of registers. Note that all modules in a
program must be compiled with the same value for this
option. Because of that, you must not use this option with
the default runtime libraries gcc
builds.
M32R/D
Options
These
-m options are defined for Renesas M32R/D
architectures:
-m32r2
Generate
code for the M32R/2.
-m32rx
Generate
code for the M32R/X.
-m32r
Generate
code for the M32R. This is the
default.
-mmodel=small
Assume
all objects live in the lower 16MB of memory (so that their
addresses can be loaded with the "ld24"
instruction), and assume all subroutines are reachable with
the "bl" instruction. This is the
default.
The
addressability of a particular object can be set with the
"model"
attribute.
-mmodel=medium
Assume
objects may be anywhere in the 32-bit address space
(the compiler will generate "seth/add3"
instructions to load their addresses), and assume all
subroutines are reachable with the "bl"
instruction.
-mmodel=large
Assume
objects may be anywhere in the 32-bit address space
(the compiler will generate "seth/add3"
instructions to load their addresses), and assume
subroutines may not be reachable with the
"bl" instruction (the compiler will
generate the much slower "seth/add3/jl"
instruction
sequence).
-msdata=none
Disable
use of the small data area. Variables will be put into one
of .data, bss, or .rodata (unless the
"section" attribute has been specified).
This is the default.
The
small data area consists of sections .sdata and
.sbss. Objects may be explicitly put in the small
data area with the "section" attribute
using one of these
sections.
-msdata=sdata
Put
small global and static data in the small data area, but do
not generate special code to reference
them.
-msdata=use
Put
small global and static data in the small data area, and
generate special instructions to reference
them.
-G
num
Put
global and static objects less than or equal to num
bytes into the small data or bss sections instead of the
normal data or bss sections. The default value of num
is 8. The -msdata option must be set to one of
sdata or use for this option to have any
effect.
All
modules should be compiled with the same -G
num value. Compiling with different values of
num may or may not work; if it doesn’t the
linker will give an error
message---incorrect code will not be
generated.
-mdebug
Makes
the M32R specific code in the compiler display some
statistics that might help in debugging
programs.
-malign-loops
Align
all loops to a 32-byte
boundary.
-mno-align-loops
Do
not enforce a 32-byte alignment for loops. This is the
default.
-missue-rate=number
Issue
number instructions per cycle. number can only
be 1 or 2.
-mbranch-cost=number
number
can only be 1 or 2. If it is 1 then branches will be
preferred over conditional code, if it is 2, then the
opposite will apply.
-mflush-trap=number
Specifies
the trap number to use to flush the cache. The default is
12. Valid numbers are between 0 and 15
inclusive.
-mno-flush-trap
Specifies
that the cache cannot be flushed by using a
trap.
-mflush-func=name
Specifies
the name of the operating system function to call to flush
the cache. The default is _flush_cache, but a
function call will only be used if a trap is not
available.
-mno-flush-func
Indicates
that there is no OS function for flushing the
cache.
M680x0
Options
These
are the -m options defined for M680x0 and
ColdFire processors. The default settings depend on which
architecture was selected when the compiler was configured;
the defaults for the most common choices are given below.
-march=arch
Generate
code for a specific M680x0 or ColdFire instruction set
architecture. Permissible values of arch for M680x0
architectures are: 68000, 68010, 68020,
68030, 68040, 68060 and cpu32.
ColdFire architectures are selected according to
Freescale’s ISA classification and the
permissible values are: isaa, isaaplus,
isab and
isac.
gcc
defines a macro __mcfarch__ whenever it
is generating code for a ColdFire target. The arch in
this macro is one of the -march arguments given
above.
When
used together, -march and -mtune
select code that runs on a family of similar processors but
that is optimized for a particular
microarchitecture.
-mcpu=cpu
Generate
code for a specific M680x0 or ColdFire processor. The M680x0
cpus are: 68000, 68010, 68020,
68030, 68040, 68060, 68302,
68332 and cpu32. The ColdFire cpus are
given by the table below, which also classifies the CPUs
into families:
Family : -mcpu arguments
51 : 51 51ac 51cn 51em 51qe
5206 : 5202 5204 5206
5206e : 5206e
5208 : 5207 5208
5211a : 5210a 5211a
5213 : 5211 5212 5213
5216 : 5214 5216
52235 : 52230 52231 52232 52233 52234 52235
5225 : 5224 5225
52259 : 52252 52254 52255 52256 52258 52259
5235 : 5232 5233 5234 5235 523x
5249 : 5249
5250 : 5250
5271 : 5270 5271
5272 : 5272
5275 : 5274 5275
5282 : 5280 5281 5282 528x
53017 : 53011 53012 53013 53014 53015 53016 53017
5307 : 5307
5329 : 5327 5328 5329 532x
5373 : 5372 5373 537x
5407 : 5407
5475 : 5470 5471 5472 5473 5474 5475 547x 5480 5481
5482 5483 5484
5485
-mcpu=cpu
overrides -march=arch if arch is
compatible with cpu. Other combinations of
-mcpu and -march are
rejected.
gcc
defines the macro __mcf_cpu_cpu when ColdFire
target cpu is selected. It also defines
__mcf_family_family, where the value of
family is given by the table
above.
-mtune=tune
Tune
the code for a particular microarchitecture, within the
constraints set by -march and
-mcpu. The M680x0 microarchitectures are:
68000, 68010, 68020, 68030,
68040, 68060 and cpu32. The ColdFire
microarchitectures are: cfv1, cfv2,
cfv3, cfv4 and
cfv4e.
You
can also use -mtune=68020-40 for code
that needs to run relatively well on 68020, 68030 and 68040
targets. -mtune=68020-60 is similar but
includes 68060 targets as well. These two options select the
same tuning decisions as -m68020-40 and
-m68020-60
respectively.
gcc
defines the macros __mcarch and
__mcarch__ when tuning for 680x0
architecture arch. It also defines
mcarch unless either -ansi or a
non-GNU -std option is used. If gcc is tuning
for a range of architectures, as selected by
-mtune=68020-40 or
-mtune=68020-60, it defines the macros
for every architecture in the
range.
gcc
also defines the macro __muarch__ when
tuning for ColdFire microarchitecture uarch, where
uarch is one of the arguments given
above.
-m68000
-mc68000
Generate
output for a 68000. This is the default when the compiler is
configured for 68000-based systems. It is equivalent
to
-march=68000.
Use
this option for microcontrollers with a 68000 or EC000
core, including the 68008, 68302, 68306, 68307, 68322,
68328 and 68356.
-m68010
Generate
output for a 68010. This is the default when the compiler is
configured for 68010-based systems. It is equivalent
to
-march=68010.
-m68020
-mc68020
Generate
output for a 68020. This is the default when the compiler is
configured for 68020-based systems. It is equivalent
to
-march=68020.
-m68030
Generate
output for a 68030. This is the default when the compiler is
configured for 68030-based systems. It is equivalent
to
-march=68030.
-m68040
Generate
output for a 68040. This is the default when the compiler is
configured for 68040-based systems. It is equivalent
to
-march=68040.
This
option inhibits the use of 68881/68882 instructions that
have to be emulated by software on the 68040. Use this
option if your 68040 does not have code to emulate those
instructions.
-m68060
Generate
output for a 68060. This is the default when the compiler is
configured for 68060-based systems. It is equivalent
to
-march=68060.
This
option inhibits the use of 68020 and 68881/68882
instructions that have to be emulated by software on the
68060. Use this option if your 68060 does not have code to
emulate those
instructions.
-mcpu32
Generate
output for a CPU32 . This is the default when the
compiler is configured for CPU32-based systems. It is
equivalent to
-march=cpu32.
Use
this option for microcontrollers with a CPU32
or CPU32+ core, including the 68330, 68331,
68332, 68333, 68334, 68336, 68340, 68341, 68349 and
68360.
-m5200
Generate
output for a 520X ColdFire CPU . This is the
default when the compiler is configured for 520X-based
systems. It is equivalent to -mcpu=5206, and is
now deprecated in favor of that
option.
Use
this option for microcontroller with a 5200 core, including
the MCF5202 , MCF5203 , MCF5204
and MCF5206
.
-m5206e
Generate
output for a 5206e ColdFire CPU . The option is
now deprecated in favor of the equivalent
-mcpu=5206e.
-m528x
Generate
output for a member of the ColdFire 528X family. The option
is now deprecated in favor of the equivalent
-mcpu=528x.
-m5307
Generate
output for a ColdFire 5307 CPU . The option is
now deprecated in favor of the equivalent
-mcpu=5307.
-m5407
Generate
output for a ColdFire 5407 CPU . The option is
now deprecated in favor of the equivalent
-mcpu=5407.
-mcfv4e
Generate
output for a ColdFire V4e family CPU (e.g.
547x/548x). This includes use of hardware floating-point
instructions. The option is equivalent to
-mcpu=547x, and is now deprecated in favor of
that option.
-m68020-40
Generate
output for a 68040, without using any of the new
instructions. This results in code that can run relatively
efficiently on either a 68020/68881 or a 68030 or a 68040.
The generated code does use the 68881 instructions that are
emulated on the
68040.
The
option is equivalent to -march=68020
-mtune=68020-40.
-m68020-60
Generate
output for a 68060, without using any of the new
instructions. This results in code that can run relatively
efficiently on either a 68020/68881 or a 68030 or a 68040.
The generated code does use the 68881 instructions that are
emulated on the
68060.
The
option is equivalent to -march=68020
-mtune=68020-60.
-mhard-float
-m68881
Generate
floating-point instructions. This is the default for 68020
and above, and for ColdFire devices that have an FPU
. It defines the macro __HAVE_68881__ on M680x0
targets and __mcffpu__ on ColdFire
targets.
-msoft-float
Do
not generate floating-point instructions; use library calls
instead. This is the default for 68000, 68010, and 68832
targets. It is also the default for ColdFire devices that
have no FPU
.
-mdiv
-mno-div
Generate
(do not generate) ColdFire hardware divide and remainder
instructions. If -march is used without
-mcpu, the default is "on" for
ColdFire architectures and "off" for M680x0
architectures. Otherwise, the default is taken from the
target CPU (either the default CPU ,
or the one specified by -mcpu). For example,
the default is "off" for -mcpu=5206
and "on" for
-mcpu=5206e.
gcc
defines the macro __mcfhwdiv__ when this option is
enabled.
-mshort
Consider
type "int" to be 16 bits wide, like
"short int". Additionally, parameters
passed on the stack are also aligned to a 16-bit
boundary even on targets whose API mandates
promotion to
32-bit.
-mno-short
Do
not consider type "int" to be 16 bits
wide. This is the
default.
-mnobitfield
-mno-bitfield
Do
not use the bit-field instructions. The
-m68000, -mcpu32 and
-m5200 options imply
-mnobitfield.
-mbitfield
Do
use the bit-field instructions. The -m68020
option implies -mbitfield. This is the default
if you use a configuration designed for a
68020.
-mrtd
Use
a different function-calling convention, in which functions
that take a fixed number of arguments return with the
"rtd" instruction, which pops their
arguments while returning. This saves one instruction in the
caller since there is no need to pop the arguments
there.
This
calling convention is incompatible with the one normally
used on Unix, so you cannot use it if you need to call
libraries compiled with the Unix
compiler.
Also,
you must provide function prototypes for all functions that
take variable numbers of arguments (including
"printf"); otherwise incorrect code will
be generated for calls to those
functions.
In
addition, seriously incorrect code will result if you call a
function with too many arguments. (Normally, extra arguments
are harmlessly
ignored.)
The
"rtd" instruction is supported by the
68010, 68020, 68030, 68040, 68060 and CPU32
processors, but not by the 68000 or
5200.
-mno-rtd
Do
not use the calling conventions selected by
-mrtd. This is the
default.
-malign-int
-mno-align-int
Control
whether GCC aligns "int",
"long", "long long",
"float", "double", and
"long double" variables on a 32-bit
boundary (-malign-int) or a 16-bit
boundary (-mno-align-int). Aligning
variables on 32-bit boundaries produces code that runs
somewhat faster on processors with 32-bit busses at
the expense of more
memory.
Warning:
if you use the -malign-int switch,
GCC will align structures containing the above types
differently than most published application binary interface
specifications for the
m68k.
-mpcrel
Use
the pc-relative addressing mode of the 68000 directly,
instead of using a global offset table. At present, this
option implies -fpic, allowing at most a
16-bit offset for pc-relative addressing.
-fPIC is not presently supported with
-mpcrel, though this could be supported for
68020 and higher
processors.
-mno-strict-align
-mstrict-align
Do
not (do) assume that unaligned memory references will be
handled by the
system.
-msep-data
Generate
code that allows the data segment to be located in a
different area of memory from the text segment. This allows
for execute in place in an environment without virtual
memory management. This option implies
-fPIC.
-mno-sep-data
Generate
code that assumes that the data segment follows the text
segment. This is the
default.
-mid-shared-library
Generate
code that supports shared libraries via the library ID
method. This allows for execute in place and shared
libraries in an environment without virtual memory
management. This option implies
-fPIC.
-mno-id-shared-library
Generate
code that doesn’t assume ID based shared
libraries are being used. This is the
default.
-mshared-library-id=n
Specified
the identification number of the ID based shared
library being compiled. Specifying a value of 0 will
generate more compact code, specifying other values will
force the allocation of that number to the current library
but is no more space or time efficient than omitting this
option.
-mxgot
-mno-xgot
When
generating position-independent code for ColdFire, generate
code that works if the GOT has more than 8192
entries. This code is larger and slower than code generated
without this option. On M680x0 processors, this option is
not needed; -fPIC
suffices.
GCC
normally uses a single instruction to load values from
the GOT . While this is relatively efficient, it
only works if the GOT is smaller than about 64k.
Anything larger causes the linker to report an error such
as:
relocation truncated to fit: R_68K_GOT16O foobar
If
this happens, you should recompile your code with
-mxgot. It should then work with very large
GOTs. However, code generated with -mxgot is
less efficient, since it takes 4 instructions to fetch the
value of a global
symbol.
Note
that some linkers, including newer versions of the GNU
linker, can create multiple GOTs and sort GOT
entries. If you have such a linker, you should only
need to use -mxgot when compiling a single
object file that accesses more than 8192 GOT
entries. Very few
do.
These
options have no effect unless GCC is generating
position-independent
code.
MCore
Options
These
are the -m options defined for the Motorola
M*Core processors.
-mhardlit
-mno-hardlit
Inline
constants into the code stream if it can be done in two
instructions or
less.
-mdiv
-mno-div
Use
the divide instruction. (Enabled by
default).
-mrelax-immediate
-mno-relax-immediate
Allow
arbitrary sized immediates in bit
operations.
-mwide-bitfields
-mno-wide-bitfields
Always
treat bit-fields as
int-sized.
-m4byte-functions
-mno-4byte-functions
Force
all functions to be aligned to a 4-byte
boundary.
-mcallgraph-data
-mno-callgraph-data
Emit
callgraph
information.
-mslow-bytes
-mno-slow-bytes
Prefer
word access when reading byte
quantities.
-mlittle-endian
-mbig-endian
Generate
code for a little-endian
target.
-m210
-m340
Generate
code for the 210
processor.
-mno-lsim
Assume
that runtime support has been provided and so omit the
simulator library (libsim.a) from the linker command
line.
-mstack-increment=size
Set
the maximum amount for a single stack increment operation.
Large values can increase the speed of programs that contain
functions that need a large amount of stack space, but they
can also trigger a segmentation fault if the stack is
extended too much. The default value is
0x1000.
MeP
Options
-mabsdiff
Enables
the "abs" instruction, which is the
absolute difference between two
registers.
-mall-opts
Enables
all the optional instructions - average, multiply,
divide, bit operations, leading zero, absolute difference,
min/max, clip, and
saturation.
-maverage
Enables
the "ave" instruction, which computes the
average of two
registers.
-mbased=n
Variables
of size n bytes or smaller will be placed in the
".based" section by default. Based
variables use the $tp register as a base register,
and there is a 128-byte limit to the
".based"
section.
-mbitops
Enables
the bit operation instructions - bit test
("btstm"), set
("bsetm"), clear
("bclrm"), invert
("bnotm"), and test-and-set
("tas").
-mc=name
Selects
which section constant data will be placed in. name
may be "tiny", "near",
or
"far".
-mclip
Enables
the "clip" instruction. Note that
"-mclip" is not useful unless you
also provide
"-mminmax".
-mconfig=name
Selects
one of the build-in core configurations. Each MeP chip has
one or more modules in it; each module has a core CPU
and a variety of coprocessors, optional instructions,
and peripherals. The
"MeP-Integrator" tool, not part
of GCC , provides these configurations through
this option; using this option is the same as using all the
corresponding command-line options. The default
configuration is
"default".
-mcop
Enables
the coprocessor instructions. By default, this is a
32-bit coprocessor. Note that the coprocessor is
normally enabled via the
"-mconfig="
option.
-mcop32
Enables
the 32-bit coprocessor’s
instructions.
-mcop64
Enables
the 64-bit coprocessor’s
instructions.
-mivc2
Enables
IVC2 scheduling. IVC2 is a
64-bit VLIW
coprocessor.
-mdc
Causes
constant variables to be placed in the
".near"
section.
-mdiv
Enables
the "div" and "divu"
instructions.
-meb
Generate
big-endian code.
-mel
Generate
little-endian code.
-mio-volatile
Tells
the compiler that any variable marked with the
"io" attribute is to be considered
volatile.
-ml
Causes variables to be
assigned to the ".far" section by
default.
-mleadz
Enables
the "leadz" (leading zero)
instruction.
-mm
Causes variables to be
assigned to the ".near" section by
default.
-mminmax
Enables
the "min" and "max"
instructions.
-mmult
Enables
the multiplication and multiply-accumulate
instructions.
-mno-opts
Disables
all the optional instructions enabled by
"-mall-opts".
-mrepeat
Enables
the "repeat" and
"erepeat" instructions, used for
low-overhead
looping.
-ms
Causes all variables to
default to the ".tiny" section. Note that
there is a 65536-byte limit to this section. Accesses
to these variables use the %gp base
register.
-msatur
Enables
the saturation instructions. Note that the compiler does not
currently generate these itself, but this option is included
for compatibility with other tools, like
"as".
-msdram
Link
the SDRAM-based runtime instead of the default ROM-based
runtime.
-msim
Link
the simulator runtime
libraries.
-msimnovec
Link
the simulator runtime libraries, excluding built-in support
for reset and exception vectors and
tables.
-mtf
Causes
all functions to default to the ".far"
section. Without this option, functions default to the
".near"
section.
-mtiny=n
Variables
that are n bytes or smaller will be allocated to the
".tiny" section. These variables use the
$gp base register. The default for this option is
4, but note that there’s a 65536-byte limit to
the ".tiny"
section.
MicroBlaze
Options
-msoft-float
Use
software emulation for floating point
(default).
-mhard-float
Use
hardware floating-point
instructions.
-mmemcpy
Do
not optimize block moves, use
"memcpy".
-mno-clearbss
This
option is deprecated. Use
-fno-zero-initialized-in-bss
instead.
-mcpu=cpu-type
Use
features of and schedule code for given CPU .
Supported values are in the format
vX. YY
.Z, where X is a major
version, YY is the minor version, and
Z is compatibility code. Example values are
v3.00.a, v4.00.b, v5.00.a,
v5.00.b, v5.00.b,
v6.00.a.
-mxl-soft-mul
Use
software multiply emulation
(default).
-mxl-soft-div
Use
software emulation for divides
(default).
-mxl-barrel-shift
Use
the hardware barrel
shifter.
-mxl-pattern-compare
Use
pattern compare
instructions.
-msmall-divides
Use
table lookup optimization for small signed integer
divisions.
-mxl-stack-check
This
option is deprecated. Use -fstack-check
instead.
-mxl-gp-opt
Use
GP relative sdata/sbss
sections.
-mxl-multiply-high
Use
multiply high instructions for high part of 32x32
multiply.
-mxl-float-convert
Use
hardware floating-point conversion
instructions.
-mxl-float-sqrt
Use
hardware floating-point square root
instruction.
-mxl-mode-app-model
Select
application model app-model. Valid models are
executable
normal
executable (default), uses startup code
crt0.o.
xmdstub
for
use with Xilinx Microprocessor Debugger ( XMD )
based software intrusive debug agent called xmdstub. This
uses startup file crt1.o and sets the start address
of the program to be
0x800.
bootstrap
for
applications that are loaded using a bootloader. This model
uses startup file crt2.o which does not contain a
processor reset vector handler. This is suitable for
transferring control on a processor reset to the bootloader
rather than the
application.
novectors
for
applications that do not require any of the MicroBlaze
vectors. This option may be useful for applications running
within a monitoring application. This model uses
crt3.o as a startup
file.
Option
-xl-mode-app-model is a
deprecated alias for
-mxl-mode-app-model.
MIPS
Options
-EB
Generate big-endian
code.
-EL
Generate little-endian
code. This is the default for mips*el-*-*
configurations.
-march=arch
Generate
code that will run on arch, which can be the name of
a generic MIPS ISA , or the name of a particular
processor. The ISA names are: mips1,
mips2, mips3, mips4, mips32,
mips32r2, mips64 and mips64r2. The
processor names are: 4kc, 4km, 4kp,
4ksc, 4kec, 4kem, 4kep,
4ksd, 5kc, 5kf, 20kc,
24kc, 24kf2_1, 24kf1_1, 24kec,
24kef2_1, 24kef1_1, 34kc,
34kf2_1, 34kf1_1, 74kc, 74kf2_1,
74kf1_1, 74kf3_2, 1004kc,
1004kf2_1, 1004kf1_1, loongson2e,
loongson2f, loongson3a, m4k,
octeon, octeon+, octeon2, orion,
r2000, r3000, r3900, r4000,
r4400, r4600, r4650, r6000,
r8000, rm7000, rm9000, r10000,
r12000, r14000, r16000, sb1,
sr71000, vr4100, vr4111, vr4120,
vr4130, vr4300, vr5000, vr5400,
vr5500 and xlr. The special value
from-abi selects the most compatible architecture for
the selected ABI (that is, mips1 for
32-bit ABIs and mips3 for 64-bit
ABIs).
Native
Linux/GNU and IRIX toolchains also support the
value native, which selects the best architecture
option for the host processor. -march=native
has no effect if GCC does not recognize the
processor.
In
processor names, a final 000 can be abbreviated as
k (for example, -march=r2k). Prefixes
are optional, and vr may be written
r.
Names
of the form nf2_1 refer to processors with
FPUs clocked at half the rate of the core, names of the form
nf1_1 refer to processors with FPUs clocked at
the same rate as the core, and names of the form
nf3_2 refer to processors with FPUs clocked a
ratio of 3:2 with respect to the core. For compatibility
reasons, nf is accepted as a synonym for
nf2_1 while nx and
bfx are accepted as synonyms for
nf1_1.
GCC
defines two macros based on the value of this option.
The first is _MIPS_ARCH, which gives the name of
target architecture, as a string. The second has the form
_MIPS_ARCH_foo, where foo is the
capitalized value of _MIPS_ARCH. For example,
-march=r2000 will set _MIPS_ARCH to
"r2000" and define the macro
_MIPS_ARCH_R2000.
Note
that the _MIPS_ARCH macro uses the processor names
given above. In other words, it will have the full prefix
and will not abbreviate 000 as k. In the case
of from-abi, the macro names the resolved
architecture (either "mips1" or
"mips3"). It names the default architecture
when no -march option is
given.
-mtune=arch
Optimize
for arch. Among other things, this option controls
the way instructions are scheduled, and the perceived cost
of arithmetic operations. The list of arch values is
the same as for
-march.
When
this option is not used, GCC will optimize for
the processor specified by -march. By using
-march and -mtune together, it is
possible to generate code that will run on a family of
processors, but optimize the code for one particular member
of that family.
-mtune
defines the macros _MIPS_TUNE and
_MIPS_TUNE_foo, which work in the same way as
the -march ones described
above.
-mips1
Equivalent
to
-march=mips1.
-mips2
Equivalent
to
-march=mips2.
-mips3
Equivalent
to
-march=mips3.
-mips4
Equivalent
to
-march=mips4.
-mips32
Equivalent
to
-march=mips32.
-mips32r2
Equivalent
to
-march=mips32r2.
-mips64
Equivalent
to
-march=mips64.
-mips64r2
Equivalent
to
-march=mips64r2.
-mips16
-mno-mips16
Generate
(do not generate) MIPS16 code. If GCC
is targetting a MIPS32 or MIPS64
architecture, it will make use of the MIPS16e ASE
.
MIPS16
code generation can also be controlled on a
per-function basis by means of "mips16"
and "nomips16"
attributes.
-mflip-mips16
Generate
MIPS16 code on alternating functions. This option is
provided for regression testing of mixed
MIPS16/non-MIPS16 code generation, and is not intended
for ordinary use in compiling user
code.
-minterlink-mips16
-mno-interlink-mips16
Require
(do not require) that non-MIPS16 code be
link-compatible with MIPS16
code.
For
example, non-MIPS16 code cannot jump directly to
MIPS16 code; it must either use a call or an indirect
jump. -minterlink-mips16 therefore
disables direct jumps unless GCC knows that the
target of the jump is not MIPS16
.
-mabi=32
-mabi=o64
-mabi=n32
-mabi=64
-mabi=eabi
Generate
code for the given ABI
.
Note
that the EABI has a 32-bit and a
64-bit variant. GCC normally generates
64-bit code when you select a 64-bit
architecture, but you can use -mgp32 to get
32-bit code
instead.
For
information about the O64 ABI , see
<http://gcc.gnu.org/projects/mipso64-abi.html>.
GCC
supports a variant of the o32 ABI in which
floating-point registers are 64 rather than 32 bits wide.
You can select this combination with -mabi=32
-mfp64. This ABI relies on the
mthc1 and mfhc1 instructions and is therefore
only supported for MIPS32R2
processors.
The
register assignments for arguments and return values remain
the same, but each scalar value is passed in a single
64-bit register rather than a pair of 32-bit
registers. For example, scalar floating-point values are
returned in $f0 only, not a $f0/$f1
pair. The set of call-saved registers also remains the same,
but all 64 bits are
saved.
-mabicalls
-mno-abicalls
Generate
(do not generate) code that is suitable for SVR4-style
dynamic objects. -mabicalls is the default for
SVR4-based
systems.
-mshared
-mno-shared
Generate
(do not generate) code that is fully position-independent,
and that can therefore be linked into shared libraries. This
option only affects
-mabicalls.
All
-mabicalls code has traditionally been
position-independent, regardless of options like
-fPIC and -fpic. However, as an
extension, the GNU toolchain allows executables
to use absolute accesses for locally-binding symbols. It can
also use shorter GP initialization sequences and
generate direct calls to locally-defined functions. This
mode is selected by
-mno-shared.
-mno-shared
depends on binutils 2.16 or higher and generates objects
that can only be linked by the GNU linker.
However, the option does not affect the ABI of
the final executable; it only affects the ABI of
relocatable objects. Using -mno-shared
will generally make executables both smaller and
quicker.
-mshared
is the default.
-mplt
-mno-plt
Assume
(do not assume) that the static and dynamic linkers support
PLTs and copy relocations. This option only affects
-mno-shared -mabicalls. For the
n64 ABI , this option has no effect without
-msym32.
You
can make -mplt the default by configuring
GCC with
--with-mips-plt. The default
is -mno-plt
otherwise.
-mxgot
-mno-xgot
Lift
(do not lift) the usual restrictions on the size of the
global offset table.
GCC
normally uses a single instruction to load values from
the GOT . While this is relatively efficient, it
will only work if the GOT is smaller than about
64k. Anything larger will cause the linker to report an
error such as:
relocation truncated to fit: R_MIPS_GOT16 foobar
If
this happens, you should recompile your code with
-mxgot. It should then work with very large
GOTs, although it will also be less efficient, since it will
take three instructions to fetch the value of a global
symbol.
Note
that some linkers can create multiple GOTs. If you have such
a linker, you should only need to use -mxgot
when a single object file accesses more than 64k’s
worth of GOT entries. Very few
do.
These
options have no effect unless GCC is generating
position independent
code.
-mgp32
Assume
that general-purpose registers are 32 bits
wide.
-mgp64
Assume
that general-purpose registers are 64 bits
wide.
-mfp32
Assume
that floating-point registers are 32 bits
wide.
-mfp64
Assume
that floating-point registers are 64 bits
wide.
-mhard-float
Use
floating-point coprocessor
instructions.
-msoft-float
Do
not use floating-point coprocessor instructions. Implement
floating-point calculations using library calls
instead.
-msingle-float
Assume
that the floating-point coprocessor only supports
single-precision
operations.
-mdouble-float
Assume
that the floating-point coprocessor supports
double-precision operations. This is the
default.
-mllsc
-mno-llsc
Use
(do not use) ll, sc, and sync
instructions to implement atomic memory built-in functions.
When neither option is specified, GCC will use
the instructions if the target architecture supports
them.
-mllsc
is useful if the runtime environment can emulate the
instructions and -mno-llsc can be useful
when compiling for nonstandard ISAs. You can make either
option the default by configuring GCC with
--with-llsc and
--without-llsc respectively.
--with-llsc is the default for some
configurations; see the installation documentation for
details.
-mdsp
-mno-dsp
Use
(do not use) revision 1 of the MIPS DSP ASE .
This option defines the preprocessor macro
__mips_dsp. It also defines __mips_dsp_rev to
1.
-mdspr2
-mno-dspr2
Use
(do not use) revision 2 of the MIPS DSP ASE .
This option defines the preprocessor macros
__mips_dsp and __mips_dspr2. It also defines
__mips_dsp_rev to
2.
-msmartmips
-mno-smartmips
Use
(do not use) the MIPS SmartMIPS ASE
.
-mpaired-single
-mno-paired-single
Use
(do not use) paired-single floating-point instructions.
This option requires hardware floating-point support to be
enabled.
-mdmx
-mno-mdmx
Use
(do not use) MIPS Digital Media Extension
instructions. This option can only be used when generating
64-bit code and requires hardware floating-point
support to be
enabled.
-mips3d
-mno-mips3d
Use
(do not use) the MIPS-3D ASE . The option
-mips3d implies
-mpaired-single.
-mmt
-mno-mt
Use
(do not use) MT Multithreading
instructions.
-mlong64
Force
"long" types to be 64 bits wide. See
-mlong32 for an explanation of the default and
the way that the pointer size is
determined.
-mlong32
Force
"long", "int", and
pointer types to be 32 bits
wide.
The
default size of "int"s,
"long"s and pointers depends on the
ABI . All the supported ABIs use 32-bit
"int"s. The n64 ABI uses
64-bit "long"s, as does the
64-bit EABI ; the others use 32-bit
"long"s. Pointers are the same size as
"long"s, or the same size as integer
registers, whichever is
smaller.
-msym32
-mno-sym32
Assume
(do not assume) that all symbols have 32-bit values,
regardless of the selected ABI . This option is
useful in combination with -mabi=64 and
-mno-abicalls because it allows GCC
to generate shorter and faster references to symbolic
addresses.
-G
num
Put
definitions of externally-visible data in a small data
section if that data is no bigger than num
bytes. GCC can then access the data more
efficiently; see -mgpopt for
details.
The
default -G option depends on the
configuration.
-mlocal-sdata
-mno-local-sdata
Extend
(do not extend) the -G behavior to local data
too, such as to static variables in C.
-mlocal-sdata is the default for all
configurations.
If
the linker complains that an application is using too much
small data, you might want to try rebuilding the less
performance-critical parts with
-mno-local-sdata. You might also
want to build large libraries with
-mno-local-sdata, so that the
libraries leave more room for the main
program.
-mextern-sdata
-mno-extern-sdata
Assume
(do not assume) that externally-defined data will be in a
small data section if that data is within the
-G limit. -mextern-sdata is
the default for all
configurations.
If
you compile a module Mod with
-mextern-sdata -G num
-mgpopt, and Mod references a variable
Var that is no bigger than num bytes, you must
make sure that Var is placed in a small data section.
If Var is defined by another module, you must either
compile that module with a high-enough -G
setting or attach a "section" attribute
to Var’s definition. If Var is common,
you must link the application with a high-enough
-G
setting.
The
easiest way of satisfying these restrictions is to compile
and link every module with the same -G option.
However, you may wish to build a library that supports
several different small data limits. You can do this by
compiling the library with the highest supported
-G setting and additionally using
-mno-extern-sdata to stop the
library from making assumptions about externally-defined
data.
-mgpopt
-mno-gpopt
Use
(do not use) GP-relative accesses for symbols that are known
to be in a small data section; see -G,
-mlocal-sdata and
-mextern-sdata. -mgpopt is
the default for all
configurations.
-mno-gpopt
is useful for cases where the $gp register might
not hold the value of "_gp". For example,
if the code is part of a library that might be used in a
boot monitor, programs that call boot monitor routines will
pass an unknown value in $gp. (In such situations,
the boot monitor itself would usually be compiled with
-G0.)
-mno-gpopt
implies -mno-local-sdata and
-mno-extern-sdata.
-membedded-data
-mno-embedded-data
Allocate
variables to the read-only data section first if possible,
then next in the small data section if possible, otherwise
in data. This gives slightly slower code than the default,
but reduces the amount of RAM required when
executing, and thus may be preferred for some embedded
systems.
-muninit-const-in-rodata
-mno-uninit-const-in-rodata
Put
uninitialized "const" variables in the
read-only data section. This option is only meaningful in
conjunction with
-membedded-data.
-mcode-readable=setting
Specify
whether GCC may generate code that reads from
executable sections. There are three possible settings:
-mcode-readable=yes
Instructions
may freely access executable sections. This is the default
setting.
-mcode-readable=pcrel
MIPS16
PC-relative load instructions can access executable
sections, but other instructions must not do so. This option
is useful on 4KSc and 4KSd processors when the code TLBs
have the Read Inhibit bit set. It is also useful on
processors that can be configured to have a dual
instruction/data SRAM interface and that, like
the M4K, automatically redirect PC-relative loads to the
instruction RAM
.
-mcode-readable=no
Instructions
must not access executable sections. This option can be
useful on targets that are configured to have a dual
instruction/data SRAM interface but that (unlike
the M4K) do not automatically redirect PC-relative loads to
the instruction RAM
.
-msplit-addresses
-mno-split-addresses
Enable
(disable) use of the "%hi()" and
"%lo()" assembler relocation operators.
This option has been superseded by
-mexplicit-relocs but is retained for
backwards
compatibility.
-mexplicit-relocs
-mno-explicit-relocs
Use
(do not use) assembler relocation operators when dealing
with symbolic addresses. The alternative, selected by
-mno-explicit-relocs, is to use
assembler macros
instead.
-mexplicit-relocs
is the default if GCC was configured to use an
assembler that supports relocation
operators.
-mcheck-zero-division
-mno-check-zero-division
Trap
(do not trap) on integer division by
zero.
The
default is
-mcheck-zero-division.
-mdivide-traps
-mdivide-breaks
MIPS
systems check for division by zero by generating either
a conditional trap or a break instruction. Using traps
results in smaller code, but is only supported on MIPS
II and later. Also, some versions of the Linux kernel
have a bug that prevents trap from generating the proper
signal ("SIGFPE"). Use
-mdivide-traps to allow conditional traps
on architectures that support them and
-mdivide-breaks to force the use of
breaks.
The
default is usually -mdivide-traps, but
this can be overridden at configure time using
--with-divide=breaks.
Divide-by-zero checks can be completely disabled using
-mno-check-zero-division.
-mmemcpy
-mno-memcpy
Force
(do not force) the use of "memcpy()" for
non-trivial block moves. The default is
-mno-memcpy, which allows GCC
to inline most constant-sized
copies.
-mlong-calls
-mno-long-calls
Disable
(do not disable) use of the "jal"
instruction. Calling functions using
"jal" is more efficient but requires the
caller and callee to be in the same 256 megabyte
segment.
This
option has no effect on abicalls code. The default is
-mno-long-calls.
-mmad
-mno-mad
Enable
(disable) use of the "mad",
"madu" and "mul"
instructions, as provided by the R4650 ISA
.
-mfused-madd
-mno-fused-madd
Enable
(disable) use of the floating-point multiply-accumulate
instructions, when they are available. The default is
-mfused-madd.
When
multiply-accumulate instructions are used, the intermediate
product is calculated to infinite precision and is not
subject to the FCSR Flush to Zero bit. This may
be undesirable in some
circumstances.
-nocpp
Tell
the MIPS assembler to not run its preprocessor
over user assembler files (with a .s suffix) when
assembling them.
-mfix-24k
-mno-fix-24k
Work
around the 24K E48 (lost data on stores during refill)
errata. The workarounds are implemented by the assembler
rather than by GCC
.
-mfix-r4000
-mno-fix-r4000
Work
around certain R4000 CPU
errata:
-
A double-word or a
variable shift may give an incorrect result if executed
immediately after starting an integer
division.
-
A double-word or a
variable shift may give an incorrect result if executed
while an integer multiplication is in
progress.
-
An integer division may
give an incorrect result if started in a delay slot of a
taken branch or a
jump.
-mfix-r4400
-mno-fix-r4400
Work
around certain R4400 CPU
errata:
-
A double-word or a
variable shift may give an incorrect result if executed
immediately after starting an integer
division.
-mfix-r10000
-mno-fix-r10000
Work
around certain R10000
errata:
-
"ll"/"sc"
sequences may not behave atomically on revisions prior to
3.0. They may deadlock on revisions 2.6 and
earlier.
This
option can only be used if the target architecture supports
branch-likely instructions. -mfix-r10000
is the default when -march=r10000 is used;
-mno-fix-r10000 is the default
otherwise.
-mfix-vr4120
-mno-fix-vr4120
Work
around certain VR4120
errata:
-
"dmultu"
does not always produce the correct
result.
-
"div"
and "ddiv" do not always produce the
correct result if one of the operands is
negative.
The
workarounds for the division errata rely on special
functions in libgcc.a. At present, these functions
are only provided by the
"mips64vr*-elf"
configurations.
Other
VR4120 errata require a nop to be inserted between
certain pairs of instructions. These errata are handled by
the assembler, not by GCC
itself.
-mfix-vr4130
Work
around the VR4130
"mflo"/"mfhi"
errata. The workarounds are implemented by the assembler
rather than by GCC , although GCC will
avoid using "mflo" and
"mfhi" if the VR4130
"macc",
"macchi", "dmacc" and
"dmacchi" instructions are available
instead.
-mfix-sb1
-mno-fix-sb1
Work
around certain SB-1 CPU core errata. (This
flag currently works around the SB-1
revision 2 "F1" and "F2"
floating-point
errata.)
-mr10k-cache-barrier=setting
Specify
whether GCC should insert cache barriers to avoid
the side-effects of speculation on R10K
processors.
In
common with many processors, the R10K tries to predict the
outcome of a conditional branch and speculatively executes
instructions from the "taken" branch. It later
aborts these instructions if the predicted outcome was
wrong. However, on the R10K, even aborted instructions can
have side effects.
This
problem only affects kernel stores and, depending on the
system, kernel loads. As an example, a
speculatively-executed store may load the target memory into
cache and mark the cache line as dirty, even if the store
itself is later aborted. If a DMA operation
writes to the same area of memory before the
"dirty" line is flushed, the cached data will
overwrite the DMA-ed data. See the R10K processor manual for
a full description, including other potential
problems.
One
workaround is to insert cache barrier instructions before
every memory access that might be speculatively executed and
that might have side effects even if aborted.
-mr10k-cache-barrier=setting
controls GCC ’s implementation of this
workaround. It assumes that aborted accesses to any byte in
the following regions will not have side
effects:
1.
the memory occupied by the
current function’s stack
frame;
2.
the memory occupied by an
incoming stack
argument;
3.
the memory occupied by an
object with a link-time-constant
address.
It
is the kernel’s responsibility to ensure that
speculative accesses to these regions are indeed
safe.
If
the input program contains a function declaration such
as:
void foo (void);
then
the implementation of "foo" must allow
"j foo" and "jal foo"
to be executed speculatively. GCC honors this
restriction for functions it compiles itself. It expects
non-GCC functions (such as hand-written assembly code) to do
the same.
The
option has three forms:
-mr10k-cache-barrier=load-store
Insert
a cache barrier before a load or store that might be
speculatively executed and that might have side effects even
if aborted.
-mr10k-cache-barrier=store
Insert
a cache barrier before a store that might be speculatively
executed and that might have side effects even if
aborted.
-mr10k-cache-barrier=none
Disable
the insertion of cache barriers. This is the default
setting.
-mflush-func=func
-mno-flush-func
Specifies
the function to call to flush the I and D caches, or to not
call any such function. If called, the function must take
the same arguments as the common
"_flush_func()", that is, the address of
the memory range for which the cache is being flushed, the
size of the memory range, and the number 3 (to flush both
caches). The default depends on the target GCC
was configured for, but commonly is either
_flush_func or
__cpu_flush.
mbranch-cost=num
Set
the cost of branches to roughly num
"simple" instructions. This cost is only a
heuristic and is not guaranteed to produce consistent
results across releases. A zero cost redundantly selects the
default, which is based on the -mtune
setting.
-mbranch-likely
-mno-branch-likely
Enable
or disable use of Branch Likely instructions, regardless of
the default for the selected architecture. By default,
Branch Likely instructions may be generated if they are
supported by the selected architecture. An exception is for
the MIPS32 and MIPS64 architectures
and processors that implement those architectures; for
those, Branch Likely instructions will not be generated by
default because the MIPS32 and MIPS64
architectures specifically deprecate their
use.
-mfp-exceptions
-mno-fp-exceptions
Specifies
whether FP exceptions are enabled. This affects
how we schedule FP instructions for some
processors. The default is that FP exceptions are
enabled.
For
instance, on the SB-1 , if FP
exceptions are disabled, and we are emitting
64-bit code, then we can use both FP pipes.
Otherwise, we can only use one FP
pipe.
-mvr4130-align
-mno-vr4130-align
The
VR4130 pipeline is two-way superscalar, but can only
issue two instructions together if the first one is
8-byte aligned. When this option is enabled, GCC
will align pairs of instructions that it thinks should
execute in parallel.
This
option only has an effect when optimizing for the
VR4130 . It normally makes code faster, but at the
expense of making it bigger. It is enabled by default at
optimization level
-O3.
-msynci
-mno-synci
Enable
(disable) generation of "synci"
instructions on architectures that support it. The
"synci" instructions (if enabled) will be
generated when
"__builtin___clear_cache()" is
compiled.
This
option defaults to
"-mno-synci", but the default
can be overridden by configuring with
"--with-synci".
When
compiling code for single processor systems, it is generally
safe to use "synci". However, on many
multi-core ( SMP ) systems, it will not
invalidate the instruction caches on all cores and may lead
to undefined
behavior.
-mrelax-pic-calls
-mno-relax-pic-calls
Try
to turn PIC calls that are normally dispatched
via register $25 into direct calls. This is only
possible if the linker can resolve the destination at
link-time and if the destination is within range for a
direct call.
-mrelax-pic-calls
is the default if GCC was configured to use an
assembler and a linker that supports the
".reloc" assembly directive and
"-mexplicit-relocs" is in
effect. With
"-mno-explicit-relocs",
this optimization can be performed by the assembler and the
linker alone without help from the
compiler.
-mmcount-ra-address
-mno-mcount-ra-address
Emit
(do not emit) code that allows "_mcount"
to modify the calling function’s return address. When
enabled, this option extends the usual
"_mcount" interface with a new
ra-address parameter, which has type
"intptr_t *" and is passed in register
$12. "_mcount" can then modify
the return address by doing both of the
following:
•
Returning the new address
in register
$31.
•
Storing the new address in
"*ra-address", if
ra-address is
nonnull.
The
default is
-mno-mcount-ra-address.
MMIX
Options
These
options are defined for the MMIX:
-mlibfuncs
-mno-libfuncs
Specify
that intrinsic library functions are being compiled, passing
all values in registers, no matter the
size.
-mepsilon
-mno-epsilon
Generate
floating-point comparison instructions that compare with
respect to the "rE" epsilon
register.
-mabi=mmixware
-mabi=gnu
Generate
code that passes function parameters and return values that
(in the called function) are seen as registers $0
and up, as opposed to the GNU ABI which uses
global registers $231 and
up.
-mzero-extend
-mno-zero-extend
When
reading data from memory in sizes shorter than 64 bits, use
(do not use) zero-extending load instructions by default,
rather than sign-extending
ones.
-mknuthdiv
-mno-knuthdiv
Make
the result of a division yielding a remainder have the same
sign as the divisor. With the default,
-mno-knuthdiv, the sign of the remainder
follows the sign of the dividend. Both methods are
arithmetically valid, the latter being almost exclusively
used.
-mtoplevel-symbols
-mno-toplevel-symbols
Prepend
(do not prepend) a : to all global symbols, so the
assembly code can be used with the
"PREFIX" assembly
directive.
-melf
Generate
an executable in the ELF format, rather than the
default mmo format used by the mmix
simulator.
-mbranch-predict
-mno-branch-predict
Use
(do not use) the probable-branch instructions, when static
branch prediction indicates a probable
branch.
-mbase-addresses
-mno-base-addresses
Generate
(do not generate) code that uses base addresses.
Using a base address automatically generates a request
(handled by the assembler and the linker) for a constant to
be set up in a global register. The register is used for one
or more base address requests within the range 0 to 255 from
the value held in the register. The generally leads to short
and fast code, but the number of different data items that
can be addressed is limited. This means that a program that
uses lots of static data may require
-mno-base-addresses.
-msingle-exit
-mno-single-exit
Force
(do not force) generated code to have a single exit point in
each function.
MN10300
Options
These
-m options are defined for Matsushita
MN10300 architectures:
-mmult-bug
Generate
code to avoid bugs in the multiply instructions for
the MN10300 processors. This is the
default.
-mno-mult-bug
Do
not generate code to avoid bugs in the multiply instructions
for the MN10300
processors.
-mam33
Generate
code using features specific to the AM33
processor.
-mno-am33
Do
not generate code using features specific to the AM33
processor. This is the
default.
-mam33-2
Generate
code using features specific to the AM33/2 .0
processor.
-mam34
Generate
code using features specific to the AM34
processor.
-mtune=cpu-type
Use
the timing characteristics of the indicated CPU
type when scheduling instructions. This does not change
the targeted processor type. The CPU type must be
one of mn10300, am33, am33-2 or
am34.
-mreturn-pointer-on-d0
When
generating a function that returns a pointer, return the
pointer in both "a0" and
"d0". Otherwise, the pointer is returned
only in a0, and attempts to call such functions without a
prototype would result in errors. Note that this option is
on by default; use
-mno-return-pointer-on-d0
to disable it.
-mno-crt0
Do
not link in the C run-time initialization object
file.
-mrelax
Indicate
to the linker that it should perform a relaxation
optimization pass to shorten branches, calls and absolute
memory addresses. This option only has an effect when used
on the command line for the final link
step.
This
option makes symbolic debugging
impossible.
-mliw
Allow
the compiler to generate Long Instruction Word
instructions if the target is the AM33 or
later. This is the default. This option defines the
preprocessor macro
__LIW__.
-mnoliw
Do
not allow the compiler to generate Long Instruction
Word instructions. This option defines the preprocessor
macro
__NO_LIW__.
-msetlb
Allow
the compiler to generate the SETLB and
Lcc instructions if the target is the
AM33 or later. This is the default. This option
defines the preprocessor macro
__SETLB__.
-mnosetlb
Do
not allow the compiler to generate SETLB
or Lcc instructions. This option defines the
preprocessor macro
__NO_SETLB__.
PDP-11
Options
These
options are defined for the PDP-11:
-mfpu
Use
hardware FPP floating point. This is the default.
( FIS floating point on the PDP-11/40
is not
supported.)
-msoft-float
Do
not use hardware floating
point.
-mac0
Return
floating-point results in ac0 (fr0 in Unix assembler
syntax).
-mno-ac0
Return
floating-point results in memory. This is the
default.
-m40
Generate
code for a PDP-11/40
.
-m45
Generate
code for a PDP-11/45 . This is the
default.
-m10
Generate
code for a PDP-11/10
.
-mbcopy-builtin
Use
inline "movmemhi" patterns for copying
memory. This is the
default.
-mbcopy
Do
not use inline "movmemhi" patterns for
copying memory.
-mint16
-mno-int32
Use
16-bit "int". This is the
default.
-mint32
-mno-int16
Use
32-bit
"int".
-mfloat64
-mno-float32
Use
64-bit "float". This is the
default.
-mfloat32
-mno-float64
Use
32-bit
"float".
-mabshi
Use
"abshi2" pattern. This is the
default.
-mno-abshi
Do
not use "abshi2"
pattern.
-mbranch-expensive
Pretend
that branches are expensive. This is for experimenting with
code generation
only.
-mbranch-cheap
Do
not pretend that branches are expensive. This is the
default.
-munix-asm
Use
Unix assembler syntax. This is the default when configured
for
pdp11-*-bsd.
-mdec-asm
Use
DEC assembler syntax. This is the default when
configured for any PDP-11 target other than
pdp11-*-bsd.
picoChip
Options
These
-m options are defined for picoChip
implementations:
-mae=ae_type
Set
the instruction set, register set, and instruction
scheduling parameters for array element type ae_type.
Supported values for ae_type are ANY
, MUL , and MAC
.
-mae=ANY
selects a completely generic AE type. Code
generated with this option will run on any of the
other AE types. The code will not be as efficient
as it would be if compiled for a specific AE
type, and some types of operation (e.g.,
multiplication) will not work properly on all types of
AE .
-mae=MUL
selects a MUL AE type. This is the most
useful AE type for compiled code, and is the
default.
-mae=MAC
selects a DSP-style MAC AE . Code compiled with
this option may suffer from poor performance of byte (char)
manipulation, since the DSP AE does not provide
hardware support for byte
load/stores.
-msymbol-as-address
Enable
the compiler to directly use a symbol name as an address in
a load/store instruction, without first loading it into a
register. Typically, the use of this option will generate
larger programs, which run faster than when the option
isn’t used. However, the results vary from program to
program, so it is left as a user option, rather than being
permanently enabled.
-mno-inefficient-warnings
Disables
warnings about the generation of inefficient code. These
warnings can be generated, for example, when compiling code
that performs byte-level memory operations on the MAC
AE type. The MAC AE has no hardware support
for byte-level memory operations, so all byte load/stores
must be synthesized from word load/store operations. This is
inefficient and a warning will be generated indicating to
the programmer that they should rewrite the code to avoid
byte operations, or to target an AE type that has
the necessary hardware support. This option enables the
warning to be turned
off.
PowerPC
Options
These
are listed under
RL78
Options
-msim
Links
in additional target libraries to support operation within a
simulator.
-mmul=none
-mmul=g13
-mmul=rl78
Specifies
the type of hardware multiplication support to be used. The
default is "none", which uses software
multiplication functions. The "g13"
option is for the hardware multiply/divide peripheral only
on the RL78/G13 targets. The
"rl78" option is for the standard
hardware multiplication defined in the RL78
software
manual.
IBM
RS/6000 and PowerPC
Options
These
-m options are defined for the IBM
RS/6000 and PowerPC:
-mpower
-mno-power
-mpower2
-mno-power2
-mpowerpc
-mno-powerpc
-mpowerpc-gpopt
-mno-powerpc-gpopt
-mpowerpc-gfxopt
-mno-powerpc-gfxopt
-mpowerpc64
-mno-powerpc64
-mmfcrf
-mno-mfcrf
-mpopcntb
-mno-popcntb
-mpopcntd
-mno-popcntd
-mfprnd
-mno-fprnd
-mcmpb
-mno-cmpb
-mmfpgpr
-mno-mfpgpr
-mhard-dfp
-mno-hard-dfp
GCC
supports two related instruction set architectures for
the RS/6000 and PowerPC. The POWER
instruction set are those instructions supported by the
rios chip set used in the original RS/6000
systems and the PowerPC instruction set is the
architecture of the Freescale MPC5xx, MPC6xx, MPC8xx
microprocessors, and the IBM 4xx, 6xx, and
follow-on
microprocessors.
Neither
architecture is a subset of the other. However there is a
large common subset of instructions supported by both.
An MQ register is included in processors
supporting the POWER
architecture.
You
use these options to specify which instructions are
available on the processor you are using. The default value
of these options is determined when configuring GCC
. Specifying the -mcpu=cpu_type
overrides the specification of these options. We recommend
you use the -mcpu=cpu_type option rather
than the options listed
above.
The
-mpower option allows GCC to
generate instructions that are found only in the POWER
architecture and to use the MQ register.
Specifying -mpower2 implies -power
and also allows GCC to generate instructions that
are present in the POWER2 architecture but not
the original POWER
architecture.
The
-mpowerpc option allows GCC to
generate instructions that are found only in the
32-bit subset of the PowerPC architecture. Specifying
-mpowerpc-gpopt implies
-mpowerpc and also allows GCC to use
the optional PowerPC architecture instructions in the
General Purpose group, including floating-point square root.
Specifying -mpowerpc-gfxopt implies
-mpowerpc and also allows GCC to use
the optional PowerPC architecture instructions in the
Graphics group, including floating-point
select.
The
-mmfcrf option allows GCC to
generate the move from condition register field instruction
implemented on the POWER4 processor and other
processors that support the PowerPC V2.01 architecture. The
-mpopcntb option allows GCC to
generate the popcount and double-precision FP
reciprocal estimate instruction implemented on
the POWER5 processor and other processors that
support the PowerPC V2.02 architecture. The
-mpopcntd option allows GCC to
generate the popcount instruction implemented on the
POWER7 processor and other processors that support the
PowerPC V2.06 architecture. The -mfprnd option
allows GCC to generate the FP round to
integer instructions implemented on the POWER5+
processor and other processors that support the PowerPC
V2.03 architecture. The -mcmpb option
allows GCC to generate the compare bytes
instruction implemented on the POWER6 processor
and other processors that support the PowerPC V2.05
architecture. The -mmfpgpr option allows
GCC to generate the FP move to/from
general-purpose register instructions implemented on
the POWER6X processor and other processors that
support the extended PowerPC V2.05 architecture. The
-mhard-dfp option allows GCC
to generate the decimal floating-point instructions
implemented on some POWER
processors.
The
-mpowerpc64 option allows GCC to
generate the additional 64-bit instructions that are
found in the full PowerPC64 architecture and to treat GPRs
as 64-bit, doubleword quantities. GCC
defaults to
-mno-powerpc64.
If
you specify both -mno-power and
-mno-powerpc, GCC will use
only the instructions in the common subset of both
architectures plus some special AIX common-mode
calls, and will not use the MQ register.
Specifying both -mpower and
-mpowerpc permits GCC to use any
instruction from either architecture and to allow use of
the MQ register; specify this for the
Motorola MPC601
.
-mnew-mnemonics
-mold-mnemonics
Select
which mnemonics to use in the generated assembler code. With
-mnew-mnemonics, GCC uses the
assembler mnemonics defined for the PowerPC architecture.
With -mold-mnemonics it uses the
assembler mnemonics defined for the POWER
architecture. Instructions defined in only one
architecture have only one mnemonic; GCC uses
that mnemonic irrespective of which of these options is
specified.
GCC
defaults to the mnemonics appropriate for the
architecture in use. Specifying
-mcpu=cpu_type sometimes overrides the
value of these option. Unless you are building a
cross-compiler, you should normally not specify either
-mnew-mnemonics or
-mold-mnemonics, but should instead
accept the default.
-mcpu=cpu_type
Set
architecture type, register usage, choice of mnemonics, and
instruction scheduling parameters for machine type
cpu_type. Supported values for cpu_type are
401, 403, 405, 405fp,
440, 440fp, 464, 464fp,
476, 476fp, 505, 601,
602, 603, 603e, 604,
604e, 620, 630, 740,
7400, 7450, 750, 801,
821, 823, 860, 970, 8540,
a2, e300c2, e300c3, e500mc,
e500mc64, ec603e, G3, G4,
G5, titan, power, power2,
power3, power4, power5, power5+,
power6, power6x, power7, common,
powerpc, powerpc64, rios, rios1,
rios2, rsc, and
rs64.
-mcpu=common
selects a completely generic processor. Code generated under
this option will run on any POWER or PowerPC
processor. GCC will use only the instructions in
the common subset of both architectures, and will not use
the MQ register. GCC assumes a generic
processor model for scheduling
purposes.
-mcpu=power,
-mcpu=power2, -mcpu=powerpc, and
-mcpu=powerpc64 specify generic POWER
, POWER2 , pure 32-bit PowerPC (i.e.,
not MPC601 ), and 64-bit PowerPC
architecture machine types, with an appropriate, generic
processor model assumed for scheduling
purposes.
The
other options specify a specific processor. Code generated
under those options will run best on that processor, and may
not run at all on
others.
The
-mcpu options automatically enable or disable
the following
options:
-maltivec
-mfprnd -mhard-float -mmfcrf
-mmultiple -mnew-mnemonics -mpopcntb
-mpopcntd -mpower -mpower2
-mpowerpc64 -mpowerpc-gpopt
-mpowerpc-gfxopt -msingle-float
-mdouble-float -msimple-fpu
-mstring -mmulhw -mdlmzb -mmfpgpr
-mvsx
The
particular options set for any particular CPU
will vary between compiler versions, depending on what
setting seems to produce optimal code for that CPU
; it doesn’t necessarily reflect the actual
hardware’s capabilities. If you wish to set an
individual option to a particular value, you may specify it
after the -mcpu option, like -mcpu=970
-mno-altivec.
On
AIX , the -maltivec and
-mpowerpc64 options are not enabled or disabled
by the -mcpu option at present because
AIX does not have full support for these options. You
may still enable or disable them individually if
you’re sure it’ll work in your
environment.
-mtune=cpu_type
Set
the instruction scheduling parameters for machine type
cpu_type, but do not set the architecture type,
register usage, or choice of mnemonics, as
-mcpu=cpu_type would. The same values
for cpu_type are used for -mtune as for
-mcpu. If both are specified, the code
generated will use the architecture, registers, and
mnemonics set by -mcpu, but the scheduling
parameters set by
-mtune.
-mcmodel=small
Generate
PowerPC64 code for the small model: The TOC is
limited to 64k.
-mcmodel=medium
Generate
PowerPC64 code for the medium model: The TOC and
other static data may be up to a total of 4G in
size.
-mcmodel=large
Generate
PowerPC64 code for the large model: The TOC may
be up to 4G in size. Other data and code is only limited by
the 64-bit address
space.
-maltivec
-mno-altivec
Generate
code that uses (does not use) AltiVec instructions, and also
enable the use of built-in functions that allow more direct
access to the AltiVec instruction set. You may also need to
set -mabi=altivec to adjust the current
ABI with AltiVec ABI
enhancements.
-mvrsave
-mno-vrsave
Generate
VRSAVE instructions when generating AltiVec
code.
-mgen-cell-microcode
Generate
Cell microcode
instructions
-mwarn-cell-microcode
Warning
when a Cell microcode instruction is going to emitted. An
example of a Cell microcode instruction is a variable
shift.
-msecure-plt
Generate
code that allows ld and ld.so to build executables and
shared libraries with non-exec .plt and .got sections. This
is a PowerPC 32-bit SYSV ABI
option.
-mbss-plt
Generate
code that uses a BSS .plt section that ld.so
fills in, and requires .plt and .got sections that are both
writable and executable. This is a PowerPC
32-bit SYSV ABI
option.
-misel
-mno-isel
This
switch enables or disables the generation of ISEL
instructions.
-misel=yes/no
This
switch has been deprecated. Use -misel and
-mno-isel
instead.
-mspe
-mno-spe
This
switch enables or disables the generation of SPE
simd
instructions.
-mpaired
-mno-paired
This
switch enables or disables the generation of PAIRED
simd
instructions.
-mspe=yes/no
This
option has been deprecated. Use -mspe and
-mno-spe
instead.
-mvsx
-mno-vsx
Generate
code that uses (does not use) vector/scalar ( VSX
) instructions, and also enable the use of built-in
functions that allow more direct access to the VSX
instruction
set.
-mfloat-gprs=yes/single/double/no
-mfloat-gprs
This
switch enables or disables the generation of floating-point
operations on the general-purpose registers for
architectures that support
it.
The
argument yes or single enables the use of
single-precision floating-point
operations.
The
argument double enables the use of single and
double-precision floating-point
operations.
The
argument no disables floating-point operations on the
general-purpose
registers.
This
option is currently only available on the
MPC854x.
-m32
-m64
Generate
code for 32-bit or 64-bit environments of Darwin
and SVR4 targets (including GNU/Linux). The
32-bit environment sets int, long and pointer to 32
bits and generates code that runs on any PowerPC variant.
The 64-bit environment sets int to 32 bits and long
and pointer to 64 bits, and generates code for PowerPC64, as
for
-mpowerpc64.
-mfull-toc
-mno-fp-in-toc
-mno-sum-in-toc
-mminimal-toc
Modify
generation of the TOC (Table Of Contents), which
is created for every executable file. The
-mfull-toc option is selected by default.
In that case, GCC will allocate at least
one TOC entry for each unique non-automatic
variable reference in your program. GCC will also
place floating-point constants in the TOC .
However, only 16,384 entries are available in the TOC
.
If
you receive a linker error message that saying you have
overflowed the available TOC space, you can
reduce the amount of TOC space used with the
-mno-fp-in-toc and
-mno-sum-in-toc options.
-mno-fp-in-toc prevents
GCC from putting floating-point constants in the
TOC and -mno-sum-in-toc
forces GCC to generate code to calculate the sum
of an address and a constant at run time instead of putting
that sum into the TOC . You may specify one or
both of these options. Each causes GCC to produce
very slightly slower and larger code at the expense of
conserving TOC
space.
If
you still run out of space in the TOC even when
you specify both of these options, specify
-mminimal-toc instead. This option
causes GCC to make only one TOC entry
for every file. When you specify this option, GCC
will produce code that is slower and larger but which
uses extremely little TOC space. You may wish to
use this option only on files that contain less frequently
executed code.
-maix64
-maix32
Enable
64-bit AIX ABI and calling convention:
64-bit pointers, 64-bit
"long" type, and the infrastructure
needed to support them. Specifying -maix64
implies -mpowerpc64 and -mpowerpc,
while -maix32 disables the 64-bit
ABI and implies
-mno-powerpc64. GCC defaults
to
-maix32.
-mxl-compat
-mno-xl-compat
Produce
code that conforms more closely to IBM XL
compiler semantics when using AIX-compatible ABI
. Pass floating-point arguments to prototyped functions
beyond the register save area ( RSA ) on the
stack in addition to argument FPRs. Do not assume that most
significant double in 128-bit long double value is
properly rounded when comparing values and converting to
double. Use XL symbol names for long double
support routines.
The
AIX calling convention was extended but not initially
documented to handle an obscure K&R C case of calling a
function that takes the address of its arguments with fewer
arguments than declared. IBM XL compilers access
floating-point arguments that do not fit in the RSA
from the stack when a subroutine is compiled without
optimization. Because always storing floating-point
arguments on the stack is inefficient and rarely needed,
this option is not enabled by default and only is necessary
when calling subroutines compiled by IBM XL
compilers without
optimization.
-mpe
Support
IBM RS/6000 SP Parallel Environment
( PE ). Link an application written to use
message passing with special startup code to enable the
application to run. The system must have PE
installed in the standard location
(/usr/lpp/ppe.poe/), or the specs file must be
overridden with the -specs= option to specify
the appropriate directory location. The Parallel Environment
does not support threads, so the -mpe option
and the -pthread option are
incompatible.
-malign-natural
-malign-power
On
AIX , 32-bit Darwin, and 64-bit PowerPC
GNU/Linux, the option -malign-natural
overrides the ABI-defined alignment of larger types, such as
floating-point doubles, on their natural size-based
boundary. The option -malign-power
instructs GCC to follow the ABI-specified
alignment rules. GCC defaults to the standard
alignment defined in the ABI
.
On
64-bit Darwin, natural alignment is the default, and
-malign-power is not
supported.
-msoft-float
-mhard-float
Generate
code that does not use (uses) the floating-point register
set. Software floating-point emulation is provided if you
use the -msoft-float option, and pass the
option to GCC when
linking.
-msingle-float
-mdouble-float
Generate
code for single- or double-precision floating-point
operations. -mdouble-float implies
-msingle-float.
-msimple-fpu
Do
not generate sqrt and div instructions for hardware
floating-point unit.
-mfpu
Specify
type of floating-point unit. Valid values are sp_lite
(equivalent to -msingle-float
-msimple-fpu), dp_lite (equivalent to
-mdouble-float -msimple-fpu),
sp_full (equivalent to -msingle-float),
and dp_full (equivalent to
-mdouble-float).
-mxilinx-fpu
Perform
optimizations for the floating-point unit on Xilinx
PPC 405/440.
-mmultiple
-mno-multiple
Generate
code that uses (does not use) the load multiple word
instructions and the store multiple word instructions. These
instructions are generated by default on POWER
systems, and not generated on PowerPC systems. Do not
use -mmultiple on little-endian PowerPC
systems, since those instructions do not work when the
processor is in little-endian mode. The exceptions are
PPC740 and PPC750 which permit these
instructions in little-endian
mode.
-mstring
-mno-string
Generate
code that uses (does not use) the load string instructions
and the store string word instructions to save multiple
registers and do small block moves. These instructions are
generated by default on POWER systems, and not
generated on PowerPC systems. Do not use
-mstring on little-endian PowerPC systems,
since those instructions do not work when the processor is
in little-endian mode. The exceptions are PPC740
and PPC750 which permit these instructions
in little-endian
mode.
-mupdate
-mno-update
Generate
code that uses (does not use) the load or store instructions
that update the base register to the address of the
calculated memory location. These instructions are generated
by default. If you use -mno-update, there
is a small window between the time that the stack pointer is
updated and the address of the previous frame is stored,
which means code that walks the stack frame across
interrupts or signals may get corrupted
data.
-mavoid-indexed-addresses
-mno-avoid-indexed-addresses
Generate
code that tries to avoid (not avoid) the use of indexed load
or store instructions. These instructions can incur a
performance penalty on Power6 processors in certain
situations, such as when stepping through large arrays that
cross a 16M boundary. This option is enabled by default when
targetting Power6 and disabled
otherwise.
-mfused-madd
-mno-fused-madd
Generate
code that uses (does not use) the floating-point multiply
and accumulate instructions. These instructions are
generated by default if hardware floating point is used. The
machine-dependent -mfused-madd option is
now mapped to the machine-independent
-ffp-contract=fast option, and
-mno-fused-madd is mapped to
-ffp-contract=off.
-mmulhw
-mno-mulhw
Generate
code that uses (does not use) the half-word multiply and
multiply-accumulate instructions on the IBM 405,
440, 464 and 476 processors. These instructions are
generated by default when targetting those
processors.
-mdlmzb
-mno-dlmzb
Generate
code that uses (does not use) the string-search dlmzb
instruction on the IBM 405, 440, 464 and 476
processors. This instruction is generated by default when
targetting those
processors.
-mno-bit-align
-mbit-align
On
System V.4 and embedded PowerPC systems do not (do) force
structures and unions that contain bit-fields to be aligned
to the base type of the
bit-field.
For
example, by default a structure containing nothing but 8
"unsigned" bit-fields of length 1 is
aligned to a 4-byte boundary and has a size of 4
bytes. By using -mno-bit-align, the
structure is aligned to a 1-byte boundary and is 1
byte in size.
-mno-strict-align
-mstrict-align
On
System V.4 and embedded PowerPC systems do not (do) assume
that unaligned memory references will be handled by the
system.
-mrelocatable
-mno-relocatable
Generate
code that allows (does not allow) a static executable to be
relocated to a different address at run time. A simple
embedded PowerPC system loader should relocate the entire
contents of ".got2" and 4-byte
locations listed in the ".fixup" section,
a table of 32-bit addresses generated by this option.
For this to work, all objects linked together must be
compiled with -mrelocatable or
-mrelocatable-lib.
-mrelocatable code aligns the stack to an
8-byte
boundary.
-mrelocatable-lib
-mno-relocatable-lib
Like
-mrelocatable,
-mrelocatable-lib generates a
".fixup" section to allow static
executables to be relocated at run time, but
-mrelocatable-lib does not use the
smaller stack alignment of -mrelocatable.
Objects compiled with -mrelocatable-lib
may be linked with objects compiled with any combination of
the -mrelocatable
options.
-mno-toc
-mtoc
On
System V.4 and embedded PowerPC systems do not (do) assume
that register 2 contains a pointer to a global area pointing
to the addresses used in the
program.
-mlittle
-mlittle-endian
On
System V.4 and embedded PowerPC systems compile code for the
processor in little-endian mode. The
-mlittle-endian option is the same as
-mlittle.
-mbig
-mbig-endian
On
System V.4 and embedded PowerPC systems compile code for the
processor in big-endian mode. The
-mbig-endian option is the same as
-mbig.
-mdynamic-no-pic
On
Darwin and Mac OS X systems, compile code so that
it is not relocatable, but that its external references are
relocatable. The resulting code is suitable for
applications, but not shared
libraries.
-msingle-pic-base
Treat
the register used for PIC addressing as
read-only, rather than loading it in the prologue for each
function. The runtime system is responsible for initializing
this register with an appropriate value before execution
begins.
-mprioritize-restricted-insns=priority
This
option controls the priority that is assigned to
dispatch-slot restricted instructions during the second
scheduling pass. The argument priority takes the
value 0/1/2 to assign
no/highest/second-highest priority to dispatch
slot restricted
instructions.
-msched-costly-dep=dependence_type
This
option controls which dependences are considered costly by
the target during instruction scheduling. The argument
dependence_type takes one of the following values:
no: no dependence is costly, all: all
dependences are costly, true_store_to_load: a true
dependence from store to load is costly,
store_to_load: any dependence from store to load is
costly, number: any dependence for which latency
>= number is
costly.
-minsert-sched-nops=scheme
This
option controls which nop insertion scheme will be used
during the second scheduling pass. The argument
scheme takes one of the following values: no:
Don’t insert nops. pad: Pad with nops any
dispatch group that has vacant issue slots, according to the
scheduler’s grouping. regroup_exact: Insert
nops to force costly dependent insns into separate groups.
Insert exactly as many nops as needed to force an insn to a
new group, according to the estimated processor grouping.
number: Insert nops to force costly dependent insns
into separate groups. Insert number nops to force an
insn to a new group.
-mcall-sysv
On
System V.4 and embedded PowerPC systems compile code using
calling conventions that adheres to the March 1995 draft of
the System V Application Binary Interface, PowerPC processor
supplement. This is the default unless you configured
GCC using
powerpc-*-eabiaix.
-mcall-sysv-eabi
-mcall-eabi
Specify
both -mcall-sysv and -meabi
options.
-mcall-sysv-noeabi
Specify
both -mcall-sysv and
-mno-eabi
options.
-mcall-aixdesc
On
System V.4 and embedded PowerPC systems compile code for
the AIX operating
system.
-mcall-linux
On
System V.4 and embedded PowerPC systems compile code for the
Linux-based GNU
system.
-mcall-freebsd
On
System V.4 and embedded PowerPC systems compile code for the
FreeBSD operating
system.
-mcall-netbsd
On
System V.4 and embedded PowerPC systems compile code for the
NetBSD operating
system.
-mcall-openbsd
On
System V.4 and embedded PowerPC systems compile code for the
OpenBSD operating
system.
-maix-struct-return
Return
all structures in memory (as specified by the AIX ABI
).
-msvr4-struct-return
Return
structures smaller than 8 bytes in registers (as specified
by the SVR4 ABI
).
-mabi=abi-type
Extend
the current ABI with a particular extension, or
remove such extension. Valid values are altivec,
no-altivec, spe, no-spe,
ibmlongdouble,
ieeelongdouble.
-mabi=spe
Extend
the current ABI with SPE ABI
extensions. This does not change the default ABI
, instead it adds the SPE ABI extensions to
the current ABI
.
-mabi=no-spe
Disable
Booke SPE ABI extensions for the current
ABI .
-mabi=ibmlongdouble
Change
the current ABI to use IBM
extended-precision long double. This is a PowerPC
32-bit SYSV ABI
option.
-mabi=ieeelongdouble
Change
the current ABI to use IEEE
extended-precision long double. This is a PowerPC
32-bit Linux ABI
option.
-mprototype
-mno-prototype
On
System V.4 and embedded PowerPC systems assume that all
calls to variable argument functions are properly
prototyped. Otherwise, the compiler must insert an
instruction before every non prototyped call to set or clear
bit 6 of the condition code register ( CR
) to indicate whether floating-point values were passed
in the floating-point registers in case the function takes
variable arguments. With -mprototype, only
calls to prototyped variable argument functions will set or
clear the bit.
-msim
On
embedded PowerPC systems, assume that the startup module is
called sim-crt0.o and that the standard C
libraries are libsim.a and libc.a. This is the
default for powerpc-*-eabisim
configurations.
-mmvme
On
embedded PowerPC systems, assume that the startup module is
called crt0.o and the standard C libraries are
libmvme.a and
libc.a.
-mads
On
embedded PowerPC systems, assume that the startup module is
called crt0.o and the standard C libraries are
libads.a and
libc.a.
-myellowknife
On
embedded PowerPC systems, assume that the startup module is
called crt0.o and the standard C libraries are
libyk.a and
libc.a.
-mvxworks
On
System V.4 and embedded PowerPC systems, specify that you
are compiling for a VxWorks
system.
-memb
On
embedded PowerPC systems, set the PPC_EMB
bit in the ELF flags header to indicate that
eabi extended relocations are
used.
-meabi
-mno-eabi
On
System V.4 and embedded PowerPC systems do (do not) adhere
to the Embedded Applications Binary Interface (eabi) which
is a set of modifications to the System V.4 specifications.
Selecting -meabi means that the stack is
aligned to an 8-byte boundary, a function
"__eabi" is called to from
"main" to set up the eabi environment,
and the -msdata option can use both
"r2" and "r13" to
point to two separate small data areas. Selecting
-mno-eabi means that the stack is aligned
to a 16-byte boundary, do not call an initialization
function from "main", and the
-msdata option will only use
"r13" to point to a single small data
area. The -meabi option is on by default if you
configured GCC using one of the
powerpc*-*-eabi*
options.
-msdata=eabi
On
System V.4 and embedded PowerPC systems, put small
initialized "const" global and static
data in the .sdata2 section, which is pointed to by
register "r2". Put small initialized
non-"const" global and static data
in the .sdata section, which is pointed to by
register "r13". Put small uninitialized
global and static data in the .sbss section, which is
adjacent to the .sdata section. The
-msdata=eabi option is incompatible with the
-mrelocatable option. The
-msdata=eabi option also sets the
-memb
option.
-msdata=sysv
On
System V.4 and embedded PowerPC systems, put small global
and static data in the .sdata section, which is
pointed to by register "r13". Put small
uninitialized global and static data in the .sbss
section, which is adjacent to the .sdata section. The
-msdata=sysv option is incompatible with the
-mrelocatable
option.
-msdata=default
-msdata
On
System V.4 and embedded PowerPC systems, if
-meabi is used, compile code the same as
-msdata=eabi, otherwise compile code the same
as
-msdata=sysv.
-msdata=data
On
System V.4 and embedded PowerPC systems, put small global
data in the .sdata section. Put small uninitialized
global data in the .sbss section. Do not use register
"r13" to address small data however. This
is the default behavior unless other -msdata
options are used.
-msdata=none
-mno-sdata
On
embedded PowerPC systems, put all initialized global and
static data in the .data section, and all
uninitialized data in the .bss
section.
-mblock-move-inline-limit=num
Inline
all block moves (such as calls to
"memcpy" or structure copies) less than
or equal to num bytes. The minimum value for
num is 32 bytes on 32-bit targets and 64 bytes
on 64-bit targets. The default value is
target-specific.
-G
num
On
embedded PowerPC systems, put global and static items less
than or equal to num bytes into the small data or bss
sections instead of the normal data or bss section. By
default, num is 8. The -G num
switch is also passed to the linker. All modules should be
compiled with the same -G num
value.
-mregnames
-mno-regnames
On
System V.4 and embedded PowerPC systems do (do not) emit
register names in the assembly language output using
symbolic forms.
-mlongcall
-mno-longcall
By
default assume that all calls are far away so that a longer
more expensive calling sequence is required. This is
required for calls further than 32 megabytes (33,554,432
bytes) from the current location. A short call will be
generated if the compiler knows the call cannot be that far
away. This setting can be overridden by the
"shortcall" function attribute, or by
"#pragma
longcall(0)".
Some
linkers are capable of detecting out-of-range calls and
generating glue code on the fly. On these systems, long
calls are unnecessary and generate slower code. As of this
writing, the AIX linker can do this, as can
the GNU linker for PowerPC/64. It is planned to
add this feature to the GNU linker for
32-bit PowerPC systems as
well.
On
Darwin/PPC systems, "#pragma longcall"
will generate "jbsr callee, L42", plus a
"branch island" (glue code). The two target
addresses represent the callee and the "branch
island". The Darwin/PPC linker will prefer the first
address and generate a "bl callee" if the
PPC "bl" instruction will reach the callee
directly; otherwise, the linker will generate "bl
L42" to call the "branch island". The
"branch island" is appended to the body of the
calling function; it computes the full 32-bit address
of the callee and jumps to
it.
On
Mach-O (Darwin) systems, this option directs the compiler
emit to the glue for every direct call, and the Darwin
linker decides whether to use or discard
it.
In
the future, we may cause GCC to ignore all
longcall specifications when the linker is known to generate
glue.
-mtls-markers
-mno-tls-markers
Mark
(do not mark) calls to "__tls_get_addr"
with a relocation specifying the function argument. The
relocation allows ld to reliably associate function call
with argument setup instructions for TLS
optimization, which in turn allows gcc to better
schedule the
sequence.
-pthread
Adds
support for multithreading with the pthreads library.
This option sets flags for both the preprocessor and
linker.
-mrecip
-mno-recip
This
option will enable GCC to use the reciprocal
estimate and reciprocal square root estimate instructions
with additional Newton-Raphson steps to increase precision
instead of doing a divide or square root and divide for
floating-point arguments. You should use the
-ffast-math option when using
-mrecip (or at least
-funsafe-math-optimizations,
-finite-math-only,
-freciprocal-math and
-fno-trapping-math). Note that
while the throughput of the sequence is generally higher
than the throughput of the non-reciprocal instruction, the
precision of the sequence can be decreased by up to 2 ulp
(i.e. the inverse of 1.0 equals 0.99999994) for reciprocal
square roots.
-mrecip=opt
This
option allows to control which reciprocal estimate
instructions may be used. opt is a comma separated
list of options, which may be preceded by a
"!" to invert the option:
"all": enable all estimate instructions,
"default": enable the default
instructions, equivalent to -mrecip,
"none": disable all estimate
instructions, equivalent to -mno-recip;
"div": enable the reciprocal
approximation instructions for both single and double
precision; "divf": enable the
single-precision reciprocal approximation instructions;
"divd": enable the double-precision
reciprocal approximation instructions;
"rsqrt": enable the reciprocal square
root approximation instructions for both single and double
precision; "rsqrtf": enable the
single-precision reciprocal square root approximation
instructions; "rsqrtd": enable the
double-precision reciprocal square root approximation
instructions;
So
for example, -mrecip=all,!rsqrtd would enable
the all of the reciprocal estimate instructions, except for
the "FRSQRTE",
"XSRSQRTEDP", and
"XVRSQRTEDP" instructions which handle
the double-precision reciprocal square root
calculations.
-mrecip-precision
-mno-recip-precision
Assume
(do not assume) that the reciprocal estimate instructions
provide higher-precision estimates than is mandated by the
PowerPC ABI . Selecting -mcpu=power6
or -mcpu=power7 automatically selects
-mrecip-precision. The double-precision
square root estimate instructions are not generated by
default on low-precision machines, since they do not provide
an estimate that converges after three
steps.
-mveclibabi=type
Specifies
the ABI type to use for vectorizing intrinsics
using an external library. The only type supported at
present is "mass", which specifies to
use IBM ’s Mathematical Acceleration
Subsystem ( MASS ) libraries for vectorizing
intrinsics using external libraries. GCC will
currently emit calls to "acosd2",
"acosf4", "acoshd2",
"acoshf4", "asind2",
"asinf4", "asinhd2",
"asinhf4", "atan2d2",
"atan2f4", "atand2",
"atanf4", "atanhd2",
"atanhf4", "cbrtd2",
"cbrtf4", "cosd2",
"cosf4", "coshd2",
"coshf4", "erfcd2",
"erfcf4", "erfd2",
"erff4", "exp2d2",
"exp2f4", "expd2",
"expf4", "expm1d2",
"expm1f4", "hypotd2",
"hypotf4", "lgammad2",
"lgammaf4", "log10d2",
"log10f4", "log1pd2",
"log1pf4", "log2d2",
"log2f4", "logd2",
"logf4", "powd2",
"powf4", "sind2",
"sinf4", "sinhd2",
"sinhf4", "sqrtd2",
"sqrtf4", "tand2",
"tanf4", "tanhd2", and
"tanhf4" when generating code for power7.
Both -ftree-vectorize and
-funsafe-math-optimizations have to
be enabled. The MASS libraries will have to be
specified at link
time.
-mfriz
-mno-friz
Generate
(do not generate) the "friz" instruction
when the
-funsafe-math-optimizations option
is used to optimize rounding of floating-point values to
64-bit integer and back to floating point. The
"friz" instruction does not return the
same value if the floating-point number is too large to fit
in an integer.
-mpointers-to-nested-functions
-mno-pointers-to-nested-functions
Generate
(do not generate) code to load up the static chain register
(r11) when calling through a pointer on AIX
and 64-bit Linux systems where a function pointer
points to a 3-word descriptor giving the function
address, TOC value to be loaded in register
r2, and static chain value to be loaded in register
r11. The
-mpointers-to-nested-functions
is on by default. You will not be able to call through
pointers to nested functions or pointers to functions
compiled in other languages that use the static chain if you
use the
-mno-pointers-to-nested-functions.
-msave-toc-indirect
-mno-save-toc-indirect
Generate
(do not generate) code to save the TOC value in
the reserved stack location in the function prologue if the
function calls through a pointer on AIX and
64-bit Linux systems. If the TOC value is
not saved in the prologue, it is saved just before the call
through the pointer. The
-mno-save-toc-indirect option
is the default.
RX
Options
These
command-line options are defined for RX targets:
-m64bit-doubles
-m32bit-doubles
Make
the "double" data type be 64 bits
(-m64bit-doubles) or 32 bits
(-m32bit-doubles) in size. The default is
-m32bit-doubles. Note RX
floating-point hardware only works on 32-bit
values, which is why the default is
-m32bit-doubles.
-fpu
-nofpu
Enables
(-fpu) or disables (-nofpu) the
use of RX floating-point hardware. The default is
enabled for the RX600 series and disabled
for the RX200
series.
Floating-point
instructions will only be generated for 32-bit
floating-point values however, so if the
-m64bit-doubles option is in use then
the FPU hardware will not be used for
doubles.
Note
If the -fpu option is enabled then
-funsafe-math-optimizations is also
enabled automatically. This is because the RX FPU
instructions are themselves
unsafe.
-mcpu=name
Selects
the type of RX CPU to be targeted. Currently
three types are supported, the generic RX600
and RX200 series hardware and the
specific RX610 CPU . The default is
RX600 .
The
only difference between RX600 and
RX610 is that the RX610 does
not support the "MVTIPL"
instruction.
The
RX200 series does not have a hardware
floating-point unit and so -nofpu is enabled by
default when this type is
selected.
-mbig-endian-data
-mlittle-endian-data
Store
data (but not code) in the big-endian format. The default is
-mlittle-endian-data, i.e. to store
data in the little-endian
format.
-msmall-data-limit=N
Specifies
the maximum size in bytes of global and static variables
which can be placed into the small data area. Using the
small data area can lead to smaller and faster code, but the
size of area is limited and it is up to the programmer to
ensure that the area does not overflow. Also when the small
data area is used one of the RX ’s
registers (usually "r13") is reserved for
use pointing to this area, so it is no longer available for
use by the compiler. This could result in slower and/or
larger code if variables which once could have been held in
the reserved register are now pushed onto the
stack.
Note,
common variables (variables that have not been initialized)
and constants are not placed into the small data area as
they are assigned to other sections in the output
executable.
The
default value is zero, which disables this feature. Note,
this feature is not enabled by default with higher
optimization levels (-O2 etc) because of the
potentially detrimental effects of reserving a register. It
is up to the programmer to experiment and discover whether
this feature is of benefit to their program. See the
description of the -mpid option for a
description of how the actual register to hold the small
data area pointer is
chosen.
-msim
-mno-sim
Use
the simulator runtime. The default is to use the libgloss
board specific
runtime.
-mas100-syntax
-mno-as100-syntax
When
generating assembler output use a syntax that is compatible
with Renesas’s AS100 assembler. This syntax
can also be handled by the GAS assembler but it
has some restrictions so generating it is not the default
option.
-mmax-constant-size=N
Specifies
the maximum size, in bytes, of a constant that can be used
as an operand in a RX instruction. Although
the RX instruction set does allow constants of up
to 4 bytes in length to be used in instructions, a longer
value equates to a longer instruction. Thus in some
circumstances it can be beneficial to restrict the size of
constants that are used in instructions. Constants that are
too big are instead placed into a constant pool and
referenced via register
indirection.
The
value N can be between 0 and 4. A value of 0 (the
default) or 4 means that constants of any size are
allowed.
-mrelax
Enable
linker relaxation. Linker relaxation is a process whereby
the linker will attempt to reduce the size of a program by
finding shorter versions of various instructions. Disabled
by default.
-mint-register=N
Specify
the number of registers to reserve for fast interrupt
handler functions. The value N can be between 0 and
4. A value of 1 means that register "r13"
will be reserved for the exclusive use of fast interrupt
handlers. A value of 2 reserves "r13" and
"r12". A value of 3 reserves
"r13", "r12" and
"r11", and a value of 4 reserves
"r13" through "r10". A
value of 0, the default, does not reserve any
registers.
-msave-acc-in-interrupts
Specifies
that interrupt handler functions should preserve the
accumulator register. This is only necessary if normal code
might use the accumulator register, for example because it
performs 64-bit multiplications. The default is to
ignore the accumulator as this makes the interrupt handlers
faster.
-mpid
-mno-pid
Enables
the generation of position independent data. When enabled
any access to constant data will done via an offset from a
base address held in a register. This allows the location of
constant data to be determined at run time without requiring
the executable to be relocated, which is a benefit to
embedded applications with tight memory constraints. Data
that can be modified is not affected by this
option.
Note,
using this feature reserves a register, usually
"r13", for the constant data base
address. This can result in slower and/or larger code,
especially in complicated
functions.
The
actual register chosen to hold the constant data base
address depends upon whether the
-msmall-data-limit and/or the
-mint-register command-line options are
enabled. Starting with register "r13" and
proceeding downwards, registers are allocated first to
satisfy the requirements of
-mint-register, then -mpid
and finally -msmall-data-limit.
Thus it is possible for the small data area register to be
"r8" if both
-mint-register=4 and -mpid
are specified on the command
line.
By
default this feature is not enabled. The default can be
restored via the -mno-pid command-line
option.
Note:
The generic GCC command-line option
-ffixed-reg has special
significance to the RX port when used with the
"interrupt" function attribute. This
attribute indicates a function intended to process fast
interrupts. GCC will will ensure that it only
uses the registers "r10",
"r11", "r12" and/or
"r13" and only provided that the normal
use of the corresponding registers have been restricted via
the -ffixed-reg or
-mint-register command-line
options.
S/390
and zSeries
Options
These
are the -m options defined for the S/390 and
zSeries architecture.
-mhard-float
-msoft-float
Use
(do not use) the hardware floating-point instructions and
registers for floating-point operations. When
-msoft-float is specified, functions in
libgcc.a will be used to perform floating-point
operations. When -mhard-float is
specified, the compiler generates IEEE
floating-point instructions. This is the
default.
-mhard-dfp
-mno-hard-dfp
Use
(do not use) the hardware decimal-floating-point
instructions for decimal-floating-point operations. When
-mno-hard-dfp is specified,
functions in libgcc.a will be used to perform
decimal-floating-point operations. When
-mhard-dfp is specified, the compiler
generates decimal-floating-point hardware instructions. This
is the default for -march=z9-ec or
higher.
-mlong-double-64
-mlong-double-128
These
switches control the size of "long
double" type. A size of 64 bits makes the
"long double" type equivalent to the
"double" type. This is the
default.
-mbackchain
-mno-backchain
Store
(do not store) the address of the caller’s frame as
backchain pointer into the callee’s stack frame. A
backchain may be needed to allow debugging using tools that
do not understand DWARF-2 call frame
information. When -mno-packed-stack
is in effect, the backchain pointer is stored at the bottom
of the stack frame; when -mpacked-stack
is in effect, the backchain is placed into the topmost word
of the 96/160 byte register save
area.
In
general, code compiled with -mbackchain is
call-compatible with code compiled with
-mmo-backchain; however, use of the
backchain for debugging purposes usually requires that the
whole binary is built with -mbackchain. Note
that the combination of -mbackchain,
-mpacked-stack and
-mhard-float is not supported. In order
to build a linux kernel use
-msoft-float.
The
default is to not maintain the
backchain.
-mpacked-stack
-mno-packed-stack
Use
(do not use) the packed stack layout. When
-mno-packed-stack is specified, the
compiler uses the all fields of the 96/160 byte register
save area only for their default purpose; unused fields
still take up stack space. When
-mpacked-stack is specified, register
save slots are densely packed at the top of the register
save area; unused space is reused for other purposes,
allowing for more efficient use of the available stack
space. However, when -mbackchain is also in
effect, the topmost word of the save area is always used to
store the backchain, and the return address register is
always saved two words below the
backchain.
As
long as the stack frame backchain is not used, code
generated with -mpacked-stack is
call-compatible with code generated with
-mno-packed-stack. Note that some
non-FSF releases of GCC 2.95 for S/390 or zSeries
generated code that uses the stack frame backchain at run
time, not just for debugging purposes. Such code is not
call-compatible with code compiled with
-mpacked-stack. Also, note that the
combination of -mbackchain,
-mpacked-stack and
-mhard-float is not supported. In order
to build a linux kernel use
-msoft-float.
The
default is to not use the packed stack
layout.
-msmall-exec
-mno-small-exec
Generate
(or do not generate) code using the
"bras" instruction to do subroutine
calls. This only works reliably if the total executable size
does not exceed 64k. The default is to use the
"basr" instruction instead, which does
not have this
limitation.
-m64
-m31
When
-m31 is specified, generate code compliant to
the GNU/Linux for S/390 ABI . When
-m64 is specified, generate code compliant to
the GNU/Linux for zSeries ABI . This allows
GCC in particular to generate 64-bit
instructions. For the s390 targets, the default is
-m31, while the s390x targets default to
-m64.
-mzarch
-mesa
When
-mzarch is specified, generate code using the
instructions available on z/Architecture. When
-mesa is specified, generate code using the
instructions available on ESA/390 . Note that
-mesa is not possible with -m64.
When generating code compliant to the GNU/Linux for
S/390 ABI , the default is -mesa.
When generating code compliant to the GNU/Linux for
zSeries ABI , the default is
-mzarch.
-mmvcle
-mno-mvcle
Generate
(or do not generate) code using the
"mvcle" instruction to perform block
moves. When -mno-mvcle is specified, use
a "mvc" loop instead. This is the default
unless optimizing for
size.
-mdebug
-mno-debug
Print
(or do not print) additional debug information when
compiling. The default is to not print debug
information.
-march=cpu-type
Generate
code that will run on cpu-type, which is the name of
a system representing a certain processor type. Possible
values for cpu-type are g5, g6,
z900, z990, z9-109,
z9-ec and z10. When generating code
using the instructions available on z/Architecture, the
default is -march=z900. Otherwise, the default
is
-march=g5.
-mtune=cpu-type
Tune
to cpu-type everything applicable about the generated
code, except for the ABI and the set of available
instructions. The list of cpu-type values is the same
as for -march. The default is the value used
for
-march.
-mtpf-trace
-mno-tpf-trace
Generate
code that adds (does not add) in TPF OS specific
branches to trace routines in the operating system. This
option is off by default, even when compiling for the
TPF OS .
-mfused-madd
-mno-fused-madd
Generate
code that uses (does not use) the floating-point multiply
and accumulate instructions. These instructions are
generated by default if hardware floating point is
used.
-mwarn-framesize=framesize
Emit
a warning if the current function exceeds the given frame
size. Because this is a compile-time check it doesn’t
need to be a real problem when the program runs. It is
intended to identify functions that most probably cause a
stack overflow. It is useful to be used in an environment
with limited stack size e.g. the linux
kernel.
-mwarn-dynamicstack
Emit
a warning if the function calls alloca or uses dynamically
sized arrays. This is generally a bad idea with a limited
stack size.
-mstack-guard=stack-guard
-mstack-size=stack-size
If
these options are provided the s390 back end emits
additional instructions in the function prologue which
trigger a trap if the stack size is stack-guard bytes
above the stack-size (remember that the stack on s390
grows downward). If the stack-guard option is omitted
the smallest power of 2 larger than the frame size of the
compiled function is chosen. These options are intended to
be used to help debugging stack overflow problems. The
additionally emitted code causes only little overhead and
hence can also be used in production like systems without
greater performance degradation. The given values have to be
exact powers of 2 and stack-size has to be greater
than stack-guard without exceeding 64k. In order to
be efficient the extra code makes the assumption that the
stack starts at an address aligned to the value given by
stack-size. The stack-guard option can only be
used in conjunction with
stack-size.
Score
Options
These
options are defined for Score implementations:
-meb
Compile
code for big-endian mode. This is the
default.
-mel
Compile
code for little-endian
mode.
-mnhwloop
Disable
generate bcnz
instruction.
-muls
Enable
generate unaligned load and store
instruction.
-mmac
Enable
the use of multiply-accumulate instructions. Disabled by
default.
-mscore5
Specify
the SCORE5 as the target
architecture.
-mscore5u
Specify
the SCORE5U of the target
architecture.
-mscore7
Specify
the SCORE7 as the target architecture. This is
the default.
-mscore7d
Specify
the SCORE7D as the target
architecture.
SH
Options
These
-m options are defined for the SH
implementations:
-m1
Generate code for
the SH1 .
-m2
Generate code for
the SH2 .
-m2e
Generate
code for the SH2e.
-m2a-nofpu
Generate
code for the SH2a without FPU , or for a
SH2a-FPU in such a way that the floating-point unit is
not used.
-m2a-single-only
Generate
code for the SH2a-FPU, in such a way that no
double-precision floating-point operations are
used.
-m2a-single
Generate
code for the SH2a-FPU assuming the floating-point unit
is in single-precision mode by
default.
-m2a
Generate
code for the SH2a-FPU assuming the floating-point unit
is in double-precision mode by
default.
-m3
Generate code for
the SH3 .
-m3e
Generate
code for the SH3e.
-m4-nofpu
Generate
code for the SH4 without a floating-point
unit.
-m4-single-only
Generate
code for the SH4 with a floating-point unit that
only supports single-precision
arithmetic.
-m4-single
Generate
code for the SH4 assuming the floating-point unit
is in single-precision mode by
default.
-m4
Generate code for
the SH4 .
-m4a-nofpu
Generate
code for the SH4al-dsp, or for a SH4a in such a way
that the floating-point unit is not
used.
-m4a-single-only
Generate
code for the SH4a, in such a way that no double-precision
floating-point operations are
used.
-m4a-single
Generate
code for the SH4a assuming the floating-point unit is in
single-precision mode by
default.
-m4a
Generate
code for the SH4a.
-m4al
Same
as -m4a-nofpu, except that it implicitly
passes -dsp to the assembler. GCC
doesn’t generate any DSP instructions
at the moment.
-mb
Compile code for the
processor in big-endian
mode.
-ml
Compile code for the
processor in little-endian
mode.
-mdalign
Align
doubles at 64-bit boundaries. Note that this changes
the calling conventions, and thus some functions from the
standard C library will not work unless you recompile it
first with
-mdalign.
-mrelax
Shorten
some address references at link time, when possible; uses
the linker option
-relax.
-mbigtable
Use
32-bit offsets in "switch" tables.
The default is to use 16-bit
offsets.
-mbitops
Enable
the use of bit manipulation instructions on SH2A
.
-mfmovd
Enable
the use of the instruction "fmovd". Check
-mdalign for alignment
constraints.
-mhitachi
Comply
with the calling conventions defined by
Renesas.
-mrenesas
Comply
with the calling conventions defined by
Renesas.
-mno-renesas
Comply
with the calling conventions defined for GCC
before the Renesas conventions were available. This
option is the default for all targets of the SH
toolchain.
-mnomacsave
Mark
the "MAC" register as call-clobbered,
even if -mhitachi is
given.
-mieee
-mno-ieee
Control
the IEEE compliance of floating-point
comparisons, which affects the handling of cases where the
result of a comparison is unordered. By default
-mieee is implicitly enabled. If
-ffinite-math-only is enabled
-mno-ieee is implicitly set, which
results in faster floating-point greater-equal and
less-equal comparisons. The implcit settings can be
overridden by specifying either -mieee or
-mno-ieee.
-minline-ic_invalidate
Inline
code to invalidate instruction cache entries after setting
up nested function trampolines. This option has no effect if
-musermode is in effect and the selected code
generation option (e.g. -m4) does not allow the use of
the icbi instruction. If the selected code generation option
does not allow the use of the icbi instruction, and
-musermode is not in effect, the inlined code will
manipulate the instruction cache address array directly with
an associative write. This not only requires privileged
mode, but it will also fail if the cache line had been
mapped via the TLB and has become
unmapped.
-misize
Dump
instruction size and location in the assembly
code.
-mpadstruct
This
option is deprecated. It pads structures to multiple of 4
bytes, which is incompatible with the SH ABI
.
-msoft-atomic
Generate
GNU/Linux compatible gUSA software atomic sequences for the
atomic built-in functions. The generated atomic sequences
require support from the interrupt / exception handling code
of the system and are only suitable for single-core systems.
They will not perform correctly on multi-core systems. This
option is enabled by default when the target is
"sh-*-linux*". For details on
the atomic built-in functions see __atomic
Builtins.
-mspace
Optimize
for space instead of speed. Implied by
-Os.
-mprefergot
When
generating position-independent code, emit function calls
using the Global Offset Table instead of the Procedure
Linkage Table.
-musermode
Don’t
generate privileged mode only code; implies
-mno-inline-ic_invalidate if the inlined
code would not work in user mode. This is the default when
the target is
"sh-*-linux*".
-multcost=number
Set
the cost to assume for a multiply
insn.
-mdiv=strategy
Set
the division strategy to be used for integer division
operations. For SHmedia strategy can be one
of:
fp
Performs the operation in
floating point. This has a very high latency, but needs only
a few instructions, so it might be a good choice if your
code has enough easily-exploitable ILP to allow
the compiler to schedule the floating-point instructions
together with other instructions. Division by zero causes a
floating-point
exception.
inv
Uses integer operations to
calculate the inverse of the divisor, and then multiplies
the dividend with the inverse. This strategy allows
CSE and hoisting of the inverse calculation. Division
by zero calculates an unspecified result, but does not
trap.
inv:minlat
A
variant of inv where, if no CSE or
hoisting opportunities have been found, or if the entire
operation has been hoisted to the same place, the last
stages of the inverse calculation are intertwined with the
final multiply to reduce the overall latency, at the expense
of using a few more instructions, and thus offering fewer
scheduling opportunities with other
code.
call
Calls
a library function that usually implements the
inv:minlat strategy. This gives high code density for
"m5-*media-nofpu"
compilations.
call2
Uses
a different entry point of the same library function, where
it assumes that a pointer to a lookup table has already been
set up, which exposes the pointer load to CSE and
code hoisting
optimizations.
inv:call
inv:call2
inv:fp
Use
the inv algorithm for initial code generation, but if
the code stays unoptimized, revert to the call,
call2, or fp strategies, respectively. Note
that the potentially-trapping side effect of division by
zero is carried by a separate instruction, so it is possible
that all the integer instructions are hoisted out, but the
marker for the side effect stays where it is. A
recombination to floating-point operations or a call is not
possible in that
case.
inv20u
inv20l
Variants
of the inv:minlat strategy. In the case that the
inverse calculation is not separated from the multiply, they
speed up division where the dividend fits into 20 bits (plus
sign where applicable) by inserting a test to skip a number
of operations in this case; this test slows down the case of
larger dividends. inv20u assumes the case of a such a
small dividend to be unlikely, and inv20l assumes it
to be likely.
For
targets other than SHmedia strategy can be one of:
call-div1
Calls
a library function that uses the single-step division
instruction "div1" to perform the
operation. Division by zero calculates an unspecified result
and does not trap. This is the default except for SH4
, SH2A and
SHcompact.
call-fp
Calls
a library function that performs the operation in double
precision floating point. Division by zero causes a
floating-point exception. This is the default for SHcompact
with FPU . Specifying this for targets that do
not have a double precision FPU will default to
"call-div1".
call-table
Calls
a library function that uses a lookup table for small
divisors and the "div1" instruction with
case distinction for larger divisors. Division by zero
calculates an unspecified result and does not trap. This is
the default for SH4 . Specifying this for targets
that do not have dynamic shift instructions will default to
"call-div1".
When
a division strategy has not been specified the default
strategy will be selected based on the current target.
For SH2A the default strategy is to use the
"divs" and "divu"
instructions instead of library function
calls.
-maccumulate-outgoing-args
Reserve
space once for outgoing arguments in the function prologue
rather than around each call. Generally beneficial for
performance and size. Also needed for unwinding to avoid
changing the stack frame around conditional
code.
-mdivsi3_libfunc=name
Set
the name of the library function used for 32-bit
signed division to name. This only affect the name
used in the call and inv:call division strategies, and the
compiler will still expect the same sets of
input/output/clobbered registers as if this option was not
present.
-mfixed-range=register-range
Generate
code treating the given register range as fixed registers. A
fixed register is one that the register allocator can not
use. This is useful when compiling kernel code. A register
range is specified as two registers separated by a dash.
Multiple register ranges can be specified separated by a
comma.
-madjust-unroll
Throttle
unrolling to avoid thrashing target registers. This option
only has an effect if the gcc code base supports the
TARGET_ADJUST_UNROLL_MAX target
hook.
-mindexed-addressing
Enable
the use of the indexed addressing mode for
SHmedia32/SHcompact. This is only safe if the hardware
and/or OS implement 32-bit wrap-around
semantics for the indexed addressing mode. The architecture
allows the implementation of processors with
64-bit MMU , which the OS could
use to get 32-bit addressing, but since no current
hardware implementation supports this or any other way to
make the indexed addressing mode safe to use in the
32-bit ABI , the default is
-mno-indexed-addressing.
-mgettrcost=number
Set
the cost assumed for the gettr instruction to number.
The default is 2 if -mpt-fixed is in
effect, 100
otherwise.
-mpt-fixed
Assume
pt* instructions won’t trap. This will generally
generate better scheduled code, but is unsafe on current
hardware. The current architecture definition says that
ptabs and ptrel trap when the target anded with 3 is 3. This
has the unintentional effect of making it unsafe to schedule
ptabs / ptrel before a branch, or hoist it out of a loop.
For example, __do_global_ctors, a part of libgcc that runs
constructors at program startup, calls functions in a list
which is delimited by -1. With the
-mpt-fixed option, the ptabs will be done before
testing against -1. That means that all the
constructors will be run a bit quicker, but when the loop
comes to the end of the list, the program crashes because
ptabs loads -1 into a target register. Since this
option is unsafe for any hardware implementing the current
architecture specification, the default is
-mno-pt-fixed. Unless the user specifies a
specific cost with -mgettrcost,
-mno-pt-fixed also implies
-mgettrcost=100; this deters register
allocation using target registers for storing ordinary
integers.
-minvalid-symbols
Assume
symbols might be invalid. Ordinary function symbols
generated by the compiler will always be valid to load with
movi/shori/ptabs or movi/shori/ptrel, but with assembler
and/or linker tricks it is possible to generate symbols that
will cause ptabs / ptrel to trap. This option is only
meaningful when -mno-pt-fixed is in
effect. It will then prevent cross-basic-block cse, hoisting
and most scheduling of symbol loads. The default is
-mno-invalid-symbols.
-mbranch-cost=num
Assume
num to be the cost for a branch instruction. Higher
numbers will make the compiler try to generate more
branch-free code if possible. If not specified the value is
selected depending on the processor type that is being
compiled for.
-mcbranchdi
Enable
the "cbranchdi4" instruction
pattern.
-mcmpeqdi
Emit
the "cmpeqdi_t" instruction pattern even
when -mcbranchdi is in
effect.
-mfused-madd
Allow
the usage of the "fmac" instruction
(floating-point multiply-accumulate) if the processor type
supports it. Enabling this option might generate code that
produces different numeric floating-point results compared
to strict IEEE 754
arithmetic.
-mpretend-cmove
Prefer
zero-displacement conditional branches for conditional move
instruction patterns. This can result in faster code on
the SH4
processor.
Solaris
2 Options
These
-m options are supported on Solaris 2:
-mimpure-text
-mimpure-text,
used in addition to -shared, tells the compiler
to not pass -z text to the linker when linking
a shared object. Using this option, you can link
position-dependent code into a shared
object.
-mimpure-text
suppresses the "relocations remain against allocatable
but non-writable sections" linker error message.
However, the necessary relocations will trigger
copy-on-write, and the shared object is not actually shared
across processes. Instead of using
-mimpure-text, you should compile all
source code with -fpic or
-fPIC.
These
switches are supported in addition to the above on Solaris
2:
-pthreads
Add
support for multithreading using the POSIX
threads library. This option sets flags for both the
preprocessor and linker. This option does not affect the
thread safety of object code produced by the compiler or
that of libraries supplied with
it.
-pthread
This
is a synonym for
-pthreads.
SPARC
Options
These
-m options are supported on the SPARC:
-mno-app-regs
-mapp-regs
Specify
-mapp-regs to generate output using the
global registers 2 through 4, which the SPARC SVR4 ABI
reserves for applications. This is the
default.
To
be fully SVR4 ABI compliant at the cost of some
performance loss, specify
-mno-app-regs. You should compile
libraries and system software with this
option.
-mflat
-mno-flat
With
-mflat, the compiler does not generate
save/restore instructions and uses a "flat" or
single register window model. This model is compatible with
the regular register window model. The local registers and
the input registers (0--5) are still treated as
"call-saved" registers and will be saved on the
stack as needed.
With
-mno-flat (the default), the compiler
generates save/restore instructions (except for leaf
functions). This is the normal operating
mode.
-mfpu
-mhard-float
Generate
output containing floating-point instructions. This is the
default.
-mno-fpu
-msoft-float
Generate
output containing library calls for floating point.
Warning: the requisite libraries are not available
for all SPARC targets. Normally the facilities of
the machine’s usual C compiler are used, but this
cannot be done directly in cross-compilation. You must make
your own arrangements to provide suitable library functions
for cross-compilation. The embedded targets
sparc-*-aout and
sparclite-*-* do provide software
floating-point
support.
-msoft-float
changes the calling convention in the output file;
therefore, it is only useful if you compile all of a
program with this option. In particular, you need to compile
libgcc.a, the library that comes with GCC
, with -msoft-float in order for
this to work.
-mhard-quad-float
Generate
output containing quad-word (long double) floating-point
instructions.
-msoft-quad-float
Generate
output containing library calls for quad-word (long double)
floating-point instructions. The functions called are those
specified in the SPARC ABI . This is the
default.
As
of this writing, there are no SPARC
implementations that have hardware support for the
quad-word floating-point instructions. They all invoke a
trap handler for one of these instructions, and then the
trap handler emulates the effect of the instruction. Because
of the trap handler overhead, this is much slower than
calling the ABI library routines. Thus the
-msoft-quad-float option is the
default.
-mno-unaligned-doubles
-munaligned-doubles
Assume
that doubles have 8-byte alignment. This is the
default.
With
-munaligned-doubles, GCC
assumes that doubles have 8-byte alignment only
if they are contained in another type, or if they have an
absolute address. Otherwise, it assumes they have
4-byte alignment. Specifying this option avoids some
rare compatibility problems with code generated by other
compilers. It is not the default because it results in a
performance loss, especially for floating-point
code.
-mno-faster-structs
-mfaster-structs
With
-mfaster-structs, the compiler assumes
that structures should have 8-byte alignment. This
enables the use of pairs of "ldd" and
"std" instructions for copies in
structure assignment, in place of twice as many
"ld" and "st" pairs.
However, the use of this changed alignment directly violates
the SPARC ABI . Thus, it’s intended only
for use on targets where the developer acknowledges that
their resulting code will not be directly in line with the
rules of the ABI
.
-mcpu=cpu_type
Set
the instruction set, register set, and instruction
scheduling parameters for machine type cpu_type.
Supported values for cpu_type are v7,
cypress, v8, supersparc,
hypersparc, leon, sparclite,
f930, f934, sparclite86x,
sparclet, tsc701, v9,
ultrasparc, ultrasparc3, niagara,
niagara2, niagara3, and
niagara4.
Native
Solaris and GNU/Linux toolchains also support the value
native, which selects the best architecture option
for the host processor. -mcpu=native has no
effect if GCC does not recognize the
processor.
Default
instruction scheduling parameters are used for values that
select an architecture and not an implementation. These are
v7, v8, sparclite, sparclet,
v9.
Here
is a list of each supported architecture and their supported
implementations.
v7
cypress
v8
supersparc, hypersparc,
leon
sparclite
f930,
f934, sparclite86x
sparclet
tsc701
v9
ultrasparc, ultrasparc3,
niagara, niagara2, niagara3,
niagara4
By
default (unless configured otherwise), GCC
generates code for the V7 variant of the SPARC
architecture. With -mcpu=cypress, the
compiler additionally optimizes it for the Cypress
CY7C602 chip, as used in the SPARCStation/SPARCServer
3xx series. This is also appropriate for the older
SPARCStation 1, 2, IPX
etc.
With
-mcpu=v8, GCC generates code for the
V8 variant of the SPARC architecture. The only
difference from V7 code is that the compiler emits the
integer multiply and integer divide instructions which exist
in SPARC-V8 but not in SPARC-V7
. With -mcpu=supersparc, the compiler
additionally optimizes it for the SuperSPARC chip, as used
in the SPARCStation 10, 1000 and 2000
series.
With
-mcpu=sparclite, GCC generates code
for the SPARClite variant of the SPARC
architecture. This adds the integer multiply, integer
divide step and scan ("ffs") instructions
which exist in SPARClite but not in SPARC-V7
. With -mcpu=f930, the compiler
additionally optimizes it for the Fujitsu MB86930
chip, which is the original SPARClite, with no
FPU . With -mcpu=f934, the compiler
additionally optimizes it for the Fujitsu MB86934
chip, which is the more recent SPARClite with FPU
.
With
-mcpu=sparclet, GCC generates code
for the SPARClet variant of the SPARC
architecture. This adds the integer multiply,
multiply/accumulate, integer divide step and scan
("ffs") instructions which exist in
SPARClet but not in SPARC-V7 . With
-mcpu=tsc701, the compiler additionally
optimizes it for the TEMIC SPARClet
chip.
With
-mcpu=v9, GCC generates code for the
V9 variant of the SPARC architecture. This adds
64-bit integer and floating-point move instructions, 3
additional floating-point condition code registers and
conditional move instructions. With
-mcpu=ultrasparc, the compiler additionally
optimizes it for the Sun UltraSPARC I/II/IIi chips. With
-mcpu=ultrasparc3, the compiler additionally
optimizes it for the Sun UltraSPARC
III/III+/IIIi/IIIi+/IV/IV+ chips. With
-mcpu=niagara, the compiler additionally
optimizes it for Sun UltraSPARC T1 chips. With
-mcpu=niagara2, the compiler additionally
optimizes it for Sun UltraSPARC T2 chips. With
-mcpu=niagara3, the compiler additionally
optimizes it for Sun UltraSPARC T3 chips. With
-mcpu=niagara4, the compiler additionally
optimizes it for Sun UltraSPARC T4
chips.
-mtune=cpu_type
Set
the instruction scheduling parameters for machine type
cpu_type, but do not set the instruction set or
register set that the option
-mcpu=cpu_type
would.
The
same values for -mcpu=cpu_type can be
used for -mtune=cpu_type, but the only
useful values are those that select a particular CPU
implementation. Those are cypress,
supersparc, hypersparc, leon,
f930, f934, sparclite86x,
tsc701, ultrasparc, ultrasparc3,
niagara, niagara2, niagara3 and
niagara4. With native Solaris and GNU/Linux
toolchains, native can also be
used.
-mv8plus
-mno-v8plus
With
-mv8plus, GCC generates code for
the SPARC-V8+ ABI . The difference from the
V8 ABI is that the global and out registers are
considered 64 bits wide. This is enabled by default on
Solaris in 32-bit mode for all SPARC-V9
processors.
-mvis
-mno-vis
With
-mvis, GCC generates code that takes
advantage of the UltraSPARC Visual Instruction Set
extensions. The default is
-mno-vis.
-mvis2
-mno-vis2
With
-mvis2, GCC generates code that
takes advantage of version 2.0 of the UltraSPARC Visual
Instruction Set extensions. The default is
-mvis2 when targetting a cpu that supports such
instructions, such as UltraSPARC-III and later. Setting
-mvis2 also sets
-mvis.
-mvis3
-mno-vis3
With
-mvis3, GCC generates code that
takes advantage of version 3.0 of the UltraSPARC Visual
Instruction Set extensions. The default is
-mvis3 when targetting a cpu that supports such
instructions, such as niagara-3 and later. Setting
-mvis3 also sets -mvis2 and
-mvis.
-mpopc
-mno-popc
With
-mpopc, GCC generates code that
takes advantage of the UltraSPARC population count
instruction. The default is -mpopc when
targetting a cpu that supports such instructions, such as
Niagara-2 and
later.
-mfmaf
-mno-fmaf
With
-mfmaf, GCC generates code that
takes advantage of the UltraSPARC Fused Multiply-Add
Floating-point extensions. The default is
-mfmaf when targetting a cpu that supports such
instructions, such as Niagara-3 and
later.
-mfix-at697f
Enable
the documented workaround for the single erratum of the
Atmel AT697F processor (which corresponds to
erratum #13 of the AT697E
processor).
These
-m options are supported in addition to the
above on SPARC-V9 processors in
64-bit environments:
-m32
-m64
Generate
code for a 32-bit or 64-bit environment. The
32-bit environment sets int, long and pointer to 32
bits. The 64-bit environment sets int to 32 bits and
long and pointer to 64
bits.
-mcmodel=which
Set
the code model to one of
medlow
The
Medium/Low code model: 64-bit addresses, programs must
be linked in the low 32 bits of memory. Programs can be
statically or dynamically
linked.
medmid
The
Medium/Middle code model: 64-bit addresses, programs
must be linked in the low 44 bits of memory, the text and
data segments must be less than 2GB in size and the data
segment must be located within 2GB of the text
segment.
medany
The
Medium/Anywhere code model: 64-bit addresses, programs
may be linked anywhere in memory, the text and data segments
must be less than 2GB in size and the data segment must be
located within 2GB of the text
segment.
embmedany
The
Medium/Anywhere code model for embedded systems:
64-bit addresses, the text and data segments must be
less than 2GB in size, both starting anywhere in memory
(determined at link time). The global register %g4
points to the base of the data segment. Programs are
statically linked and PIC is not
supported.
-mmemory-model=mem-model
Set
the memory model in force on the processor to one of
default
The
default memory model for the processor and operating
system.
rmo
Relaxed Memory
Order
pso
Partial Store
Order
tso
Total Store
Order
sc
Sequential
Consistency
These
memory models are formally defined in Appendix D of the
Sparc V9 architecture manual, as set in the
processor’s "PSTATE.MM"
field.
-mstack-bias
-mno-stack-bias
With
-mstack-bias, GCC assumes that
the stack pointer, and frame pointer if present, are offset
by -2047 which must be added back when making stack
frame references. This is the default in 64-bit mode.
Otherwise, assume no such offset is
present.
SPU
Options
These
-m options are supported on the SPU:
-mwarn-reloc
-merror-reloc
The
loader for SPU does not handle dynamic
relocations. By default, GCC will give an error
when it generates code that requires a dynamic relocation.
-mno-error-reloc disables the
error, -mwarn-reloc will generate a
warning instead.
-msafe-dma
-munsafe-dma
Instructions
that initiate or test completion of DMA must not
be reordered with respect to loads and stores of the memory
that is being accessed. Users typically address this problem
using the volatile keyword, but that can lead to inefficient
code in places where the memory is known to not change.
Rather than mark the memory as volatile we treat the
DMA instructions as potentially effecting all memory.
With -munsafe-dma users must use the
volatile keyword to protect memory
accesses.
-mbranch-hints
By
default, GCC will generate a branch hint
instruction to avoid pipeline stalls for always taken or
probably taken branches. A hint will not be generated closer
than 8 instructions away from its branch. There is little
reason to disable them, except for debugging purposes, or to
make an object a little bit
smaller.
-msmall-mem
-mlarge-mem
By
default, GCC generates code assuming that
addresses are never larger than 18 bits. With
-mlarge-mem code is generated that
assumes a full 32-bit
address.
-mstdmain
By
default, GCC links against startup code that
assumes the SPU-style main function interface (which has an
unconventional parameter list). With
-mstdmain, GCC will link your
program against startup code that assumes a C99-style
interface to "main", including a local
copy of "argv"
strings.
-mfixed-range=register-range
Generate
code treating the given register range as fixed registers. A
fixed register is one that the register allocator can not
use. This is useful when compiling kernel code. A register
range is specified as two registers separated by a dash.
Multiple register ranges can be specified separated by a
comma.
-mea32
-mea64
Compile
code assuming that pointers to the PPU address
space accessed via the "__ea" named
address space qualifier are either 32 or 64 bits wide. The
default is 32 bits. As this is an ABI changing
option, all object code in an executable must be compiled
with the same
setting.
-maddress-space-conversion
-mno-address-space-conversion
Allow/disallow
treating the "__ea" address space as
superset of the generic address space. This enables explicit
type casts between "__ea" and generic
pointer as well as implicit conversions of generic pointers
to "__ea" pointers. The default is to
allow address space pointer
conversions.
-mcache-size=cache-size
This
option controls the version of libgcc that the compiler
links to an executable and selects a software-managed cache
for accessing variables in the "__ea"
address space with a particular cache size. Possible options
for cache-size are 8, 16, 32,
64 and 128. The default cache size is
64KB.
-matomic-updates
-mno-atomic-updates
This
option controls the version of libgcc that the compiler
links to an executable and selects whether atomic updates to
the software-managed cache of PPU-side variables are used.
If you use atomic updates, changes to a PPU
variable from SPU code using the
"__ea" named address space qualifier will
not interfere with changes to other PPU variables
residing in the same cache line from PPU code. If
you do not use atomic updates, such interference may occur;
however, writing back cache lines will be more efficient.
The default behavior is to use atomic
updates.
-mdual-nops
-mdual-nops=n
By
default, GCC will insert nops to increase dual
issue when it expects it to increase performance. n
can be a value from 0 to 10. A smaller n will insert
fewer nops. 10 is the default, 0 is the same as
-mno-dual-nops. Disabled with
-Os.
-mhint-max-nops=n
Maximum
number of nops to insert for a branch hint. A branch hint
must be at least 8 instructions away from the branch it is
effecting. GCC will insert up to n nops to
enforce this, otherwise it will not generate the branch
hint.
-mhint-max-distance=n
The
encoding of the branch hint instruction limits the hint to
be within 256 instructions of the branch it is effecting. By
default, GCC makes sure it is within
125.
-msafe-hints
Work
around a hardware bug that causes the SPU to
stall indefinitely. By default, GCC will insert
the "hbrp" instruction to make sure this
stall won’t
happen.
Options
for System V
These
additional options are available on System V Release 4 for
compatibility with other compilers on those
systems:
-G
Create a shared object. It
is recommended that -symbolic or
-shared be used
instead.
-Qy
Identify the versions of
each tool used by the compiler, in a
".ident" assembler directive in the
output.
-Qn
Refrain from adding
".ident" directives to the output file
(this is the
default).
-YP,dirs
Search
the directories dirs, and no others, for libraries
specified with
-l.
-Ym,dir
Look
in the directory dir to find the M4 preprocessor. The
assembler uses this
option.
TILE-Gx
Options
These
-m options are supported on the TILE-Gx:
-mcpu=name
Selects
the type of CPU to be targeted. Currently the
only supported type is
tilegx.
-m32
-m64
Generate
code for a 32-bit or 64-bit environment. The
32-bit environment sets int, long, and pointer to 32
bits. The 64-bit environment sets int to 32 bits and
long and pointer to 64
bits.
TILEPro
Options
These
-m options are supported on the TILEPro:
-mcpu=name
Selects
the type of CPU to be targeted. Currently the
only supported type is
tilepro.
-m32
Generate
code for a 32-bit environment, which sets int, long,
and pointer to 32 bits. This is the only supported behavior
so the flag is essentially
ignored.
V850
Options
These
-m options are defined for V850
implementations:
-mlong-calls
-mno-long-calls
Treat
all calls as being far away (near). If calls are assumed to
be far away, the compiler will always load the functions
address up into a register, and call indirect through the
pointer.
-mno-ep
-mep
Do
not optimize (do optimize) basic blocks that use the same
index pointer 4 or more times to copy pointer into the
"ep" register, and use the shorter
"sld" and "sst"
instructions. The -mep option is on by default
if you optimize.
-mno-prolog-function
-mprolog-function
Do
not use (do use) external functions to save and restore
registers at the prologue and epilogue of a function. The
external functions are slower, but use less code space if
more than one function saves the same number of registers.
The -mprolog-function option is on by
default if you
optimize.
-mspace
Try
to make the code as small as possible. At present, this just
turns on the -mep and
-mprolog-function
options.
-mtda=n
Put
static or global variables whose size is n bytes or
less into the tiny data area that register
"ep" points to. The tiny data area can
hold up to 256 bytes in total (128 bytes for byte
references).
-msda=n
Put
static or global variables whose size is n bytes or
less into the small data area that register
"gp" points to. The small data area can
hold up to 64
kilobytes.
-mzda=n
Put
static or global variables whose size is n bytes or
less into the first 32 kilobytes of
memory.
-mv850
Specify
that the target processor is the
V850.
-mbig-switch
Generate
code suitable for big switch tables. Use this option only if
the assembler/linker complain about out of range branches
within a switch
table.
-mapp-regs
This
option will cause r2 and r5 to be used in the code generated
by the compiler. This setting is the
default.
-mno-app-regs
This
option will cause r2 and r5 to be treated as fixed
registers.
-mv850e2v3
Specify
that the target processor is the V850E2V3. The preprocessor
constants __v850e2v3__ will be defined if this option
is used.
-mv850e2
Specify
that the target processor is the V850E2. The preprocessor
constants __v850e2__ will be defined if this option
is used.
-mv850e1
Specify
that the target processor is the V850E1. The preprocessor
constants __v850e1__ and __v850e__ will be
defined if this option is
used.
-mv850es
Specify
that the target processor is the V850ES. This is an alias
for the -mv850e1
option.
-mv850e
Specify
that the target processor is the V850E. The preprocessor
constant __v850e__ will be defined if this option is
used.
If
neither -mv850 nor -mv850e nor
-mv850e1 nor -mv850e2 nor
-mv850e2v3 are defined then a default target
processor will be chosen and the relevant __v850*__
preprocessor constant will be
defined.
The
preprocessor constants __v850 and __v851__ are
always defined, regardless of which processor variant is the
target.
-mdisable-callt
This
option will suppress generation of the CALLT
instruction for the v850e, v850e1, v850e2 and v850e2v3
flavors of the v850 architecture. The default is
-mno-disable-callt which allows
the CALLT instruction to be
used.
VAX
Options
These
-m options are defined for the VAX:
-munix
Do
not output certain jump instructions
("aobleq" and so on) that the Unix
assembler for the VAX cannot handle across long
ranges.
-mgnu
Do
output those jump instructions, on the assumption that you
will assemble with the GNU
assembler.
-mg
Output code for
G-format floating-point numbers instead of
D-format.
VxWorks
Options
The
options in this section are defined for all VxWorks targets.
Options specific to the target hardware are listed with the
other options for that target.
-mrtp
GCC
can generate code for both VxWorks kernels and real
time processes (RTPs). This option switches from the former
to the latter. It also defines the preprocessor macro
"__RTP__".
-non-static
Link
an RTP executable against shared libraries rather
than static libraries. The options -static and
-shared can also be used for RTPs;
-static is the
default.
-Bstatic
-Bdynamic
These
options are passed down to the linker. They are defined for
compatibility with
Diab.
-Xbind-lazy
Enable
lazy binding of function calls. This option is equivalent to
-Wl,-z,now and is defined for
compatibility with
Diab.
-Xbind-now
Disable
lazy binding of function calls. This option is the default
and is defined for compatibility with
Diab.
x86-64
Options
These
are listed under
Xstormy16
Options
These
options are defined for Xstormy16:
-msim
Choose
startup files and linker script suitable for the
simulator.
Xtensa
Options
These
options are supported for Xtensa targets:
-mconst16
-mno-const16
Enable
or disable use of "CONST16" instructions
for loading constant values. The
"CONST16" instruction is currently not a
standard option from Tensilica. When enabled,
"CONST16" instructions are always used in
place of the standard "L32R"
instructions. The use of "CONST16" is
enabled by default only if the "L32R"
instruction is not
available.
-mfused-madd
-mno-fused-madd
Enable
or disable use of fused multiply/add and multiply/subtract
instructions in the floating-point option. This has no
effect if the floating-point option is not also enabled.
Disabling fused multiply/add and multiply/subtract
instructions forces the compiler to use separate
instructions for the multiply and add/subtract operations.
This may be desirable in some cases where strict IEEE
754-compliant results are required: the fused
multiply add/subtract instructions do not round the
intermediate result, thereby producing results with
more bits of precision than specified by the
IEEE standard. Disabling fused multiply add/subtract
instructions also ensures that the program output is not
sensitive to the compiler’s ability to combine
multiply and add/subtract
operations.
-mserialize-volatile
-mno-serialize-volatile
When
this option is enabled, GCC inserts
"MEMW" instructions before
"volatile" memory references to guarantee
sequential consistency. The default is
-mserialize-volatile. Use
-mno-serialize-volatile to omit the
"MEMW"
instructions.
-mforce-no-pic
For
targets, like GNU/Linux, where all user-mode Xtensa code
must be position-independent code ( PIC ), this
option disables PIC for compiling kernel
code.
-mtext-section-literals
-mno-text-section-literals
Control
the treatment of literal pools. The default is
-mno-text-section-literals,
which places literals in a separate section in the output
file. This allows the literal pool to be placed in a
data RAM/ROM , and it also allows the linker to
combine literal pools from separate object files to remove
redundant literals and improve code size. With
-mtext-section-literals, the
literals are interspersed in the text section in order to
keep them as close as possible to their references. This may
be necessary for large assembly
files.
-mtarget-align
-mno-target-align
When
this option is enabled, GCC instructs the
assembler to automatically align instructions to reduce
branch penalties at the expense of some code density. The
assembler attempts to widen density instructions to align
branch targets and the instructions following call
instructions. If there are not enough preceding safe density
instructions to align a target, no widening will be
performed. The default is -mtarget-align.
These options do not affect the treatment of auto-aligned
instructions like "LOOP", which the
assembler will always align, either by widening density
instructions or by inserting no-op
instructions.
-mlongcalls
-mno-longcalls
When
this option is enabled, GCC instructs the
assembler to translate direct calls to indirect calls unless
it can determine that the target of a direct call is in the
range allowed by the call instruction. This translation
typically occurs for calls to functions in other source
files. Specifically, the assembler translates a direct
"CALL" instruction into an
"L32R" followed by a
"CALLX" instruction. The default is
-mno-longcalls. This option should be
used in programs where the call target can potentially be
out of range. This option is implemented in the assembler,
not the compiler, so the assembly code generated by
GCC will still show direct call
instructions---look at the disassembled
object code to see the actual instructions. Note that the
assembler will use an indirect call for every cross-file
call, not just those that really will be out of
range.
zSeries
Options
These
are listed under
Options
for Code Generation Conventions
These machine-independent options control the interface
conventions used in code
generation.
Most
of them have both positive and negative forms; the negative
form of -ffoo would be
-fno-foo. In the table below, only one of
the forms is listed---the one that is not
the default. You can figure out the other form by either
removing no- or adding it.
-fbounds-check
For
front ends that support it, generate additional code to
check that indices used to access arrays are within the
declared range. This is currently only supported by the Java
and Fortran front ends, where this option defaults to true
and false
respectively.
-ftrapv
This
option generates traps for signed overflow on addition,
subtraction, multiplication
operations.
-fwrapv
This
option instructs the compiler to assume that signed
arithmetic overflow of addition, subtraction and
multiplication wraps around using twos-complement
representation. This flag enables some optimizations and
disables others. This option is enabled by default for the
Java front end, as required by the Java language
specification.
-fexceptions
Enable
exception handling. Generates extra code needed to propagate
exceptions. For some targets, this implies GCC
will generate frame unwind information for all
functions, which can produce significant data size overhead,
although it does not affect execution. If you do not specify
this option, GCC will enable it by default for
languages like C ++ that normally require
exception handling, and disable it for languages like C that
do not normally require it. However, you may need to enable
this option when compiling C code that needs to interoperate
properly with exception handlers written in C ++
. You may also wish to disable this option if you are
compiling older C ++ programs that don’t
use exception
handling.
-fnon-call-exceptions
Generate
code that allows trapping instructions to throw exceptions.
Note that this requires platform-specific runtime support
that does not exist everywhere. Moreover, it only allows
trapping instructions to throw exceptions, i.e.
memory references or floating-point instructions. It does
not allow exceptions to be thrown from arbitrary signal
handlers such as
"SIGALRM".
-funwind-tables
Similar
to -fexceptions, except that it will just
generate any needed static data, but will not affect the
generated code in any other way. You will normally not
enable this option; instead, a language processor that needs
this handling would enable it on your
behalf.
-fasynchronous-unwind-tables
Generate
unwind table in dwarf2 format, if supported by target
machine. The table is exact at each instruction boundary, so
it can be used for stack unwinding from asynchronous events
(such as debugger or garbage
collector).
-fpcc-struct-return
Return
"short" "struct" and
"union" values in memory like longer
ones, rather than in registers. This convention is less
efficient, but it has the advantage of allowing
intercallability between GCC-compiled files and files
compiled with other compilers, particularly the Portable C
Compiler (pcc).
The
precise convention for returning structures in memory
depends on the target configuration
macros.
Short
structures and unions are those whose size and alignment
match that of some integer
type.
Warning:
code compiled with the
-fpcc-struct-return switch is not
binary compatible with code compiled with the
-freg-struct-return switch. Use it
to conform to a non-default application binary
interface.
-freg-struct-return
Return
"struct" and "union"
values in registers when possible. This is more efficient
for small structures than
-fpcc-struct-return.
If
you specify neither
-fpcc-struct-return nor
-freg-struct-return, GCC
defaults to whichever convention is standard for the
target. If there is no standard convention, GCC
defaults to
-fpcc-struct-return, except on
targets where GCC is the principal compiler. In
those cases, we can choose the standard, and we chose the
more efficient register return
alternative.
Warning:
code compiled with the
-freg-struct-return switch is not
binary compatible with code compiled with the
-fpcc-struct-return switch. Use it
to conform to a non-default application binary
interface.
-fshort-enums
Allocate
to an "enum" type only as many bytes as
it needs for the declared range of possible values.
Specifically, the "enum" type will be
equivalent to the smallest integer type that has enough
room.
Warning:
the -fshort-enums switch causes GCC
to generate code that is not binary compatible with
code generated without that switch. Use it to conform to a
non-default application binary
interface.
-fshort-double
Use
the same size for "double" as for
"float".
Warning:
the -fshort-double switch causes
GCC to generate code that is not binary compatible with
code generated without that switch. Use it to conform to a
non-default application binary
interface.
-fshort-wchar
Override
the underlying type for wchar_t to be short
unsigned int instead of the default for the target. This
option is useful for building programs to run under
WINE .
Warning:
the -fshort-wchar switch causes GCC
to generate code that is not binary compatible with
code generated without that switch. Use it to conform to a
non-default application binary
interface.
-fno-common
In
C code, controls the placement of uninitialized global
variables. Unix C compilers have traditionally permitted
multiple definitions of such variables in different
compilation units by placing the variables in a common
block. This is the behavior specified by
-fcommon, and is the default for GCC
on most targets. On the other hand, this behavior is
not required by ISO C, and on some targets may
carry a speed or code size penalty on variable references.
The -fno-common option specifies that the
compiler should place uninitialized global variables in the
data section of the object file, rather than generating them
as common blocks. This has the effect that if the same
variable is declared (without "extern")
in two different compilations, you will get a
multiple-definition error when you link them. In this case,
you must compile with -fcommon instead.
Compiling with -fno-common is useful on
targets for which it provides better performance, or if you
wish to verify that the program will work on other systems
that always treat uninitialized variable declarations this
way.
-fno-ident
Ignore
the #ident
directive.
-finhibit-size-directive
Don’t
output a ".size" assembler directive, or
anything else that would cause trouble if the function is
split in the middle, and the two halves are placed at
locations far apart in memory. This option is used when
compiling crtstuff.c; you should not need to use it
for anything else.
-fverbose-asm
Put
extra commentary information in the generated assembly code
to make it more readable. This option is generally only of
use to those who actually need to read the generated
assembly code (perhaps while debugging the compiler
itself).
-fno-verbose-asm,
the default, causes the extra information to be omitted and
is useful when comparing two assembler
files.
-frecord-gcc-switches
This
switch causes the command line that was used to invoke the
compiler to be recorded into the object file that is being
created. This switch is only implemented on some targets and
the exact format of the recording is target and binary file
format dependent, but it usually takes the form of a section
containing ASCII text. This switch is related to
the -fverbose-asm switch, but that switch
only records information in the assembler output file as
comments, so it never reaches the object file. See also
-grecord-gcc-switches for another
way of storing compiler options into the object
file.
-fpic
Generate
position-independent code ( PIC ) suitable for
use in a shared library, if supported for the target
machine. Such code accesses all constant addresses through a
global offset table ( GOT ). The dynamic loader
resolves the GOT entries when the program starts
(the dynamic loader is not part of GCC ; it is
part of the operating system). If the GOT size
for the linked executable exceeds a machine-specific maximum
size, you get an error message from the linker indicating
that -fpic does not work; in that case,
recompile with -fPIC instead. (These maximums
are 8k on the SPARC and 32k on the m68k and
RS/6000 . The 386 has no such
limit.)
Position-independent
code requires special support, and therefore works only on
certain machines. For the 386, GCC supports
PIC for System V but not for the Sun 386i. Code
generated for the IBM RS/6000 is always
position-independent.
When
this flag is set, the macros "__pic__"
and "__PIC__" are defined to
1.
-fPIC
If
supported for the target machine, emit position-independent
code, suitable for dynamic linking and avoiding any limit on
the size of the global offset table. This option makes a
difference on the m68k, PowerPC and SPARC
.
Position-independent
code requires special support, and therefore works only on
certain machines.
When
this flag is set, the macros "__pic__"
and "__PIC__" are defined to
2.
-fpie
-fPIE
These
options are similar to -fpic and
-fPIC, but generated position independent code
can be only linked into executables. Usually these options
are used when -pie GCC option will
be used during
linking.
-fpie
and -fPIE both define the macros
"__pie__" and
"__PIE__". The macros have the value 1
for -fpie and 2 for
-fPIE.
-fno-jump-tables
Do
not use jump tables for switch statements even where it
would be more efficient than other code generation
strategies. This option is of use in conjunction with
-fpic or -fPIC for building code
that forms part of a dynamic linker and cannot reference the
address of a jump table. On some targets, jump tables do not
require a GOT and this option is not
needed.
-ffixed-reg
Treat
the register named reg as a fixed register; generated
code should never refer to it (except perhaps as a stack
pointer, frame pointer or in some other fixed
role).
reg
must be the name of a register. The register names accepted
are machine-specific and are defined in the
"REGISTER_NAMES" macro in the machine
description macro
file.
This
flag does not have a negative form, because it specifies a
three-way choice.
-fcall-used-reg
Treat
the register named reg as an allocable register that
is clobbered by function calls. It may be allocated for
temporaries or variables that do not live across a call.
Functions compiled this way will not save and restore the
register reg.
It
is an error to used this flag with the frame pointer or
stack pointer. Use of this flag for other registers that
have fixed pervasive roles in the machine’s execution
model will produce disastrous
results.
This
flag does not have a negative form, because it specifies a
three-way choice.
-fcall-saved-reg
Treat
the register named reg as an allocable register saved
by functions. It may be allocated even for temporaries or
variables that live across a call. Functions compiled this
way will save and restore the register reg if they
use it.
It
is an error to used this flag with the frame pointer or
stack pointer. Use of this flag for other registers that
have fixed pervasive roles in the machine’s execution
model will produce disastrous
results.
A
different sort of disaster will result from the use of this
flag for a register in which function values may be
returned.
This
flag does not have a negative form, because it specifies a
three-way choice.
-fpack-struct[=n]
Without
a value specified, pack all structure members together
without holes. When a value is specified (which must be a
small power of two), pack structure members according to
this value, representing the maximum alignment (that is,
objects with default alignment requirements larger than this
will be output potentially unaligned at the next fitting
location.
Warning:
the -fpack-struct switch causes GCC
to generate code that is not binary compatible with
code generated without that switch. Additionally, it makes
the code suboptimal. Use it to conform to a non-default
application binary
interface.
-finstrument-functions
Generate
instrumentation calls for entry and exit to functions. Just
after function entry and just before function exit, the
following profiling functions will be called with the
address of the current function and its call site. (On some
platforms, "__builtin_return_address"
does not work beyond the current function, so the call site
information may not be available to the profiling functions
otherwise.)
void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);
The
first argument is the address of the start of the current
function, which may be looked up exactly in the symbol
table.
This
instrumentation is also done for functions expanded inline
in other functions. The profiling calls will indicate where,
conceptually, the inline function is entered and exited.
This means that addressable versions of such functions must
be available. If all your uses of a function are expanded
inline, this may mean an additional expansion of code size.
If you use extern inline in your C code, an
addressable version of such functions must be provided.
(This is normally the case anyways, but if you get lucky and
the optimizer always expands the functions inline, you might
have gotten away without providing static
copies.)
A
function may be given the attribute
"no_instrument_function", in which case
this instrumentation will not be done. This can be used, for
example, for the profiling functions listed above,
high-priority interrupt routines, and any functions from
which the profiling functions cannot safely be called
(perhaps signal handlers, if the profiling routines generate
output or allocate
memory).
-finstrument-functions-exclude-file-list=file,file,...
Set
the list of functions that are excluded from instrumentation
(see the description of
"-finstrument-functions"). If
the file that contains a function definition matches with
one of file, then that function is not instrumented.
The match is done on substrings: if the file
parameter is a substring of the file name, it is considered
to be a match.
For
example:
-finstrument-functions-exclude-file-list=/bits/stl,include/sys
will
exclude any inline function defined in files whose pathnames
contain "/bits/stl" or
"include/sys".
If,
for some reason, you want to include letter ',' in
one of sym, write ','. For example,
"-finstrument-functions-exclude-file-list=',,tmp'"
(note the single quote surrounding the
option).
-finstrument-functions-exclude-function-list=sym,sym,...
This
is similar to
"-finstrument-functions-exclude-file-list",
but this option sets the list of function names to be
excluded from instrumentation. The function name to be
matched is its user-visible name, such as
"vector<int> blah(const vector<int>
&)", not the internal mangled name (e.g.,
"_Z4blahRSt6vectorIiSaIiEE"). The match
is done on substrings: if the sym parameter is a
substring of the function name, it is considered to be a
match. For C99 and C ++ extended identifiers, the
function name must be given in UTF-8 , not
using universal character
names.
-fstack-check
Generate
code to verify that you do not go beyond the boundary of the
stack. You should specify this flag if you are running in an
environment with multiple threads, but only rarely need to
specify it in a single-threaded environment since stack
overflow is automatically detected on nearly all systems if
there is only one
stack.
Note
that this switch does not actually cause checking to be
done; the operating system or the language runtime must do
that. The switch causes generation of code to ensure that
they see the stack being
extended.
You
can additionally specify a string parameter:
"no" means no checking,
"generic" means force the use of
old-style checking, "specific" means use
the best checking method and is equivalent to bare
-fstack-check.
Old-style
checking is a generic mechanism that requires no specific
target support in the compiler but comes with the following
drawbacks:
1.
Modified allocation
strategy for large objects: they will always be allocated
dynamically if their size exceeds a fixed
threshold.
2.
Fixed limit on the size of
the static frame of functions: when it is topped by a
particular function, stack checking is not reliable and a
warning is issued by the
compiler.
3.
Inefficiency: because of
both the modified allocation strategy and the generic
implementation, the performances of the code are
hampered.
Note
that old-style stack checking is also the fallback method
for "specific" if no target support has
been added in the
compiler.
-fstack-limit-register=reg
-fstack-limit-symbol=sym
-fno-stack-limit
Generate
code to ensure that the stack does not grow beyond a certain
value, either the value of a register or the address of a
symbol. If the stack would grow beyond the value, a signal
is raised. For most targets, the signal is raised before the
stack overruns the boundary, so it is possible to catch the
signal without taking special
precautions.
For
instance, if the stack starts at absolute address
0x80000000 and grows downwards, you can use the flags
-fstack-limit-symbol=__stack_limit
and
-Wl,--defsym,__stack_limit=0x7ffe0000
to enforce a stack limit of 128KB. Note that this may only
work with the GNU
linker.
-fsplit-stack
Generate
code to automatically split the stack before it overflows.
The resulting program has a discontiguous stack which can
only overflow if the program is unable to allocate any more
memory. This is most useful when running threaded programs,
as it is no longer necessary to calculate a good stack size
to use for each thread. This is currently only implemented
for the i386 and x86_64 back ends running
GNU/Linux.
When
code compiled with -fsplit-stack calls
code compiled without -fsplit-stack,
there may not be much stack space available for the latter
code to run. If compiling all code, including library code,
with -fsplit-stack is not an option, then
the linker can fix up these calls so that the code compiled
without -fsplit-stack always has a large
stack. Support for this is implemented in the gold linker
in GNU binutils release 2.21 and
later.
-fleading-underscore
This
option and its counterpart,
-fno-leading-underscore, forcibly
change the way C symbols are represented in the object file.
One use is to help link with legacy assembly
code.
Warning:
the -fleading-underscore switch
causes GCC to generate code that is not binary
compatible with code generated without that switch. Use it
to conform to a non-default application binary interface.
Not all targets provide complete support for this
switch.
-ftls-model=model
Alter
the thread-local storage model to be used. The model
argument should be one of
"global-dynamic",
"local-dynamic",
"initial-exec" or
"local-exec".
The
default without -fpic is
"initial-exec"; with
-fpic the default is
"global-dynamic".
-fvisibility=default|internal|hidden|protected
Set
the default ELF image symbol visibility to the
specified option---all symbols will be
marked with this unless overridden within the code. Using
this feature can very substantially improve linking and load
times of shared object libraries, produce more optimized
code, provide near-perfect API export and prevent
symbol clashes. It is strongly recommended that you
use this in any shared objects you
distribute.
Despite
the nomenclature, "default" always means
public; i.e., available to be linked against from outside
the shared object. "protected" and
"internal" are pretty useless in
real-world usage so the only other commonly used option will
be "hidden". The default if
-fvisibility isn’t specified is
"default", i.e., make every symbol
public---this causes the same behavior as
previous versions of GCC
.
A
good explanation of the benefits offered by ensuring
ELF symbols have the correct visibility is given by
"How To Write Shared Libraries" by Ulrich Drepper
(which can be found at
<http://people.redhat.com/~drepper/>)---however
a superior solution made possible by this option to marking
things hidden when the default is public is to make the
default hidden and mark things public. This is the norm
with DLL ’s on Windows and with
-fvisibility=hidden and "__attribute__
((visibility("default")))" instead of
"__declspec(dllexport)" you get almost
identical semantics with identical syntax. This is a great
boon to those working with cross-platform
projects.
For
those adding visibility support to existing code, you may
find #pragma GCC visibility
of use. This works by you enclosing the declarations
you wish to set visibility for with (for example)
#pragma GCC visibility
push(hidden) and #pragma GCC
visibility pop. Bear in mind that symbol
visibility should be viewed as part of the
API interface contract and thus all
new code should always specify visibility when it is not the
default; i.e., declarations only for use within the
local DSO should always be marked
explicitly as hidden as so to avoid PLT
indirection overheads---making this
abundantly clear also aids readability and
self-documentation of the code. Note that due to ISO
C ++ specification requirements, operator
new and operator delete must always be of default
visibility.
Be
aware that headers from outside your project, in particular
system headers and headers from any other library you use,
may not be expecting to be compiled with visibility other
than the default. You may need to explicitly say
#pragma GCC visibility
push(default) before including any such
headers.
extern
declarations are not affected by -fvisibility,
so a lot of code can be recompiled with
-fvisibility=hidden with no modifications.
However, this means that calls to extern functions
with no explicit visibility will use the PLT , so
it is more effective to use __attribute
((visibility)) and/or #pragma GCC
visibility to tell the compiler which
extern declarations should be treated as
hidden.
Note
that -fvisibility does affect C ++
vague linkage entities. This means that, for instance,
an exception class that will be thrown between DSOs must be
explicitly marked with default visibility so that the
type_info nodes will be unified between the
DSOs.
An
overview of these techniques, their benefits and how to use
them is at
<http://gcc.gnu.org/wiki/Visibility>.
-fstrict-volatile-bitfields
This
option should be used if accesses to volatile bit-fields (or
other structure fields, although the compiler usually honors
those types anyway) should use a single access of the width
of the field’s type, aligned to a natural alignment if
possible. For example, targets with memory-mapped peripheral
registers might require all such accesses to be 16 bits
wide; with this flag the user could declare all peripheral
bit-fields as "unsigned short" (assuming short is
16 bits on these targets) to force GCC to use
16-bit accesses instead of, perhaps, a more efficient
32-bit access.
If
this option is disabled, the compiler will use the most
efficient instruction. In the previous example, that might
be a 32-bit load instruction, even though that will
access bytes that do not contain any portion of the
bit-field, or memory-mapped registers unrelated to the one
being updated.
If
the target requires strict alignment, and honoring the field
type would require violating this alignment, a warning is
issued. If the field has "packed"
attribute, the access is done without honoring the field
type. If the field doesn’t have
"packed" attribute, the access is done
honoring the field type. In both cases, GCC
assumes that the user knows something about the target
hardware that it is unaware
of.
The
default value of this option is determined by the
application binary interface for the target
processor.
copyright
Copyright (c) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997,
1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008,
2009, 2010, 2011, 2012 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License,
Version 1.3 or any later version published by the Free Software
Foundation; with the Invariant Sections being " GNU General
Public License" and "Funding Free Software", the Front-Cover
texts being (a) (see below), and with the Back-Cover Texts being
(b) (see below). A copy of the license is included in the
gfdl(7) man page.
(a) The FSF ’s Front-Cover Text is:
A GNU Manual
(b) The FSF ’s Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.
environment
This section describes several environment variables that affect
how GCC operates. Some of them work by specifying directories or
prefixes to use when searching for various kinds of files. Some
are used to specify other aspects of the compilation environment.
Note that you can also specify places to search using options
such as -B, -I and -L. These take precedence
over places specified using environment variables, which in turn
take precedence over those specified by the configuration of GCC
.
LANG
LC_CTYPE
LC_MESSAGES
LC_ALL
These environment variables control the way that GCC uses
localization information which allows GCC to work with different
national conventions. GCC inspects the locale categories
LC_CTYPE and LC_MESSAGES if it has been configured
to do so. These locale categories can be set to any value
supported by your installation. A typical value is
en_GB.UTF-8 for English in the United Kingdom encoded in
UTF-8 .
The LC_CTYPE environment variable specifies character
classification. GCC uses it to determine the character boundaries
in a string; this is needed for some multibyte encodings that
contain quote and escape characters that would otherwise be
interpreted as a string end or escape.
The LC_MESSAGES environment variable specifies the
language to use in diagnostic messages.
If the LC_ALL environment variable is set, it overrides
the value of LC_CTYPE and LC_MESSAGES ; otherwise,
LC_CTYPE and LC_MESSAGES default to the value of
the LANG environment variable. If none of these variables
are set, GCC defaults to traditional C English behavior.
TMPDIR
If TMPDIR is set, it specifies the directory to use for
temporary files. GCC uses temporary files to hold the output of
one stage of compilation which is to be used as input to the next
stage: for example, the output of the preprocessor, which is the
input to the compiler proper.
GCC_COMPARE_DEBUG
Setting GCC_COMPARE_DEBUG is nearly equivalent to passing
-fcompare-debug to the compiler driver. See the
documentation of this option for more details.
GCC_EXEC_PREFIX
If GCC_EXEC_PREFIX is set, it specifies a prefix to use in
the names of the subprograms executed by the compiler. No slash
is added when this prefix is combined with the name of a
subprogram, but you can specify a prefix that ends with a slash
if you wish.
If GCC_EXEC_PREFIX is not set, GCC will attempt to figure
out an appropriate prefix to use based on the pathname it was
invoked with.
If GCC cannot find the subprogram using the specified prefix, it
tries looking in the usual places for the subprogram.
The default value of GCC_EXEC_PREFIX is
prefix/lib/gcc/ where prefix is the prefix to the
installed compiler. In many cases prefix is the value of
"prefix" when you ran the configure script.
Other prefixes specified with -B take precedence over this
prefix.
This prefix is also used for finding files such as crt0.o
that are used for linking.
In addition, the prefix is used in an unusual way in finding the
directories to search for header files. For each of the standard
directories whose name normally begins with
/usr/local/lib/gcc (more precisely, with the value of
GCC_INCLUDE_DIR ), GCC tries replacing that beginning with
the specified prefix to produce an alternate directory name.
Thus, with -Bfoo/, GCC will search foo/bar where it
would normally search /usr/local/lib/bar. These alternate
directories are searched first; the standard directories come
next. If a standard directory begins with the configured
prefix then the value of prefix is replaced by
GCC_EXEC_PREFIX when looking for header files.
COMPILER_PATH
The value of COMPILER_PATH is a colon-separated list of
directories, much like PATH . GCC tries the directories
thus specified when searching for subprograms, if it can’t find
the subprograms using GCC_EXEC_PREFIX .
LIBRARY_PATH
The value of LIBRARY_PATH is a colon-separated list of
directories, much like PATH . When configured as a native
compiler, GCC tries the directories thus specified when searching
for special linker files, if it can’t find them using
GCC_EXEC_PREFIX . Linking using GCC also uses these
directories when searching for ordinary libraries for the
-l option (but directories specified with -L come
first).
LANG
This variable is used to pass locale information to the compiler.
One way in which this information is used is to determine the
character set to be used when character literals, string literals
and comments are parsed in C and C ++ . When the compiler is
configured to allow multibyte characters, the following values
for LANG are recognized:
C-JIS
Recognize JIS characters.
C-SJIS
Recognize SJIS characters.
C-EUCJP
Recognize EUCJP characters.
If LANG is not defined, or if it has some other value,
then the compiler will use mblen and mbtowc as defined by the
default locale to recognize and translate multibyte characters.
Some additional environments variables affect the behavior of the
preprocessor.
CPATH
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
Each variable’s value is a list of directories separated by a
special character, much like PATH , in which to look for
header files. The special character, "PATH_SEPARATOR",
is target-dependent and determined at GCC build time. For
Microsoft Windows-based targets it is a semicolon, and for almost
all other targets it is a colon.
CPATH specifies a list of directories to be searched as if
specified with -I, but after any paths given with
-I options on the command line. This environment variable
is used regardless of which language is being preprocessed.
The remaining environment variables apply only when preprocessing
the particular language indicated. Each specifies a list of
directories to be searched as if specified with -isystem,
but after any paths given with -isystem options on the
command line.
In all these variables, an empty element instructs the compiler
to search its current working directory. Empty elements can
appear at the beginning or end of a path. For instance, if the
value of CPATH is ":/special/include", that has
the same effect as -I. -I/special/include.
DEPENDENCIES_OUTPUT
If this variable is set, its value specifies how to output
dependencies for Make based on the non-system header files
processed by the compiler. System header files are ignored in the
dependency output.
The value of DEPENDENCIES_OUTPUT can be just a file name,
in which case the Make rules are written to that file, guessing
the target name from the source file name. Or the value can have
the form file target, in which case the rules are written
to file file using target as the target name.
In other words, this environment variable is equivalent to
combining the options -MM and -MF, with an optional
-MT switch too.
SUNPRO_DEPENDENCIES
This variable is the same as DEPENDENCIES_OUTPUT (see
above), except that system header files are not ignored, so it
implies -M rather than -MM. However, the dependence
on the main input file is omitted.
footnotes
1.
On some systems, gcc -shared needs to build supplementary
stub code for constructors to work. On multi-libbed systems,
gcc -shared must select the correct support libraries to
link against. Failing to supply the correct flags may lead to
subtle defects. Supplying them in cases where they are not
necessary is innocuous.
bugs
For
instructions on reporting bugs, see
<file:///usr/share/doc/gcc-4.7/README.Bugs>.
see also
gpl,
gfdl, fsf-funding, cpp ,
gcov , as , ld , gdb ,
adb, dbx, sdb and the Info
entries for gcc, cpp, as, ld,
binutils and
gdb.
author
See
the Info entry for gcc, or
<http://gcc.gnu.org/onlinedocs/gcc/Contributors.html>,
for contributors to GCC
.