GCC(1) GNU GCC(1)NAME
gcc - GNU project C and C++ compiler
SYNOPSIS
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-pedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] infile...
Only the most useful options are listed here; see below
for the remainder. g++ accepts mostly the same options as
gcc.
DESCRIPTION
When you invoke GCC, it normally does preprocessing, com-
pilation, 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 com-
piler 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: -dr is
very different from -d -r.
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 exam-
ple, if you specify -L more than once, the directories are
searched in the order specified.
Many options have long names starting with -f or with
-W---for example, -fforce-mem, -fstrength-reduce, -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 -pipe-pass-exit-codes -x lan-
guage -v -### --help--target-help--version
C Language Options
-ansi -std=standard -aux-info filename -fno-asm
-fno-builtin-fno-builtin-function -fhosted -ffree-
standing -fms-extensions -trigraphs -no-inte-
grated-cpp -traditional-traditional-cpp -fal-
low-single-precision -fcond-mismatch -fsigned-bit-
fields -fsigned-char -funsigned-bitfields -fun-
signed-char -fwritable-strings
C++ Language Options
-fabi-version=n -fno-access-control-fcheck-new
-fconserve-space-fno-const-strings-fdol-
lars-in-identifiers -fno-elide-constructors
-fno-enforce-eh-specs-fexternal-templates
-falt-external-templates -ffor-scope -fno-for-scope
-fno-gnu-keywords -fno-implicit-templates
-fno-implicit-inline-templates -fno-implement-inlines
-fms-extensions -fno-nonansi-builtins -fno-opera-
tor-names -fno-optional-diags-fpermissive -frepo
-fno-rtti-fstats-ftemplate-depth-n
-fuse-cxa-atexit-fvtable-gc-fno-weak -nostdinc++
-fno-default-inline -Wabi -Wctor-dtor-privacy
-Wnon-virtual-dtor-Wreorder -Weffc++ -Wno-depre-
cated -Wno-non-template-friend-Wold-style-cast
-Woverloaded-virtual-Wno-pmf-conversions
-Wsign-promo-Wsynth
Objective-C Language Options
-fconstant-string-class=class-name -fgnu-runtime
-fnext-runtime-gen-decls -Wno-protocol -Wselector
-Wundeclared-selector
Language Independent Options
-fmessage-length=n -fdiagnostics-show-loca-
tion=[once|every-line]
Warning Options
-fsyntax-only-pedantic-pedantic-errors -w -W
-Wall-Waggregate-return -Wcast-align -Wcast-qual
-Wchar-subscripts-Wcomment -Wconversion -Wno-depre-
cated-declarations -Wdisabled-optimization
-Wno-div-by-zero-Werror -Wfloat-equal -Wformat
-Wformat=2 -Wformat-nonliteral-Wformat-security
-Wimplicit-Wimplicit-int -Wimplicit-function-decla-
ration -Werror-implicit-function-declaration -Wimport
-Winline-Wno-endif-labels -Wlarger-than-len
-Wlong-long -Wmain -Wmissing-braces -Wmissing-for-
mat-attribute -Wmissing-noreturn -Wno-multichar
-Wno-format-extra-args-Wno-format-y2k -Wno-import
-Wnonnull-Wpacked -Wpadded -Wparentheses
-Wpointer-arith-Wredundant-decls -Wreturn-type
-Wsequence-point-Wshadow -Wsign-compare
-Wstrict-aliasing -Wswitch -Wswitch-default
-Wswitch-enum -Wsystem-headers -Wtrigraphs -Wundef
-Wuninitialized -Wunknown-pragmas -Wunreachable-code
-Wunused-Wunused-function -Wunused-label
-Wunused-parameter -Wunused-value -Wunused-variable
-Wwrite-strings
C-only Warning Options
-Wbad-function-cast -Wmissing-declarations -Wmiss-
ing-prototypes -Wnested-externs -Wstrict-prototypes
-Wtraditional
Debugging Options
-dletters-dumpspecs-dumpmachine -dumpversion
-fdump-unnumbered -fdump-translation-unit[-n]
-fdump-class-hierarchy[-n] -fdump-tree-original[-n]
-fdump-tree-optimized[-n] -fdump-tree-inlined[-n]
-feliminate-dwarf2-dups-fmem-report -fprofile-arcs
-fsched-verbose=n -ftest-coverage-ftime-report -g
-glevel-gcoff-gdwarf-gdwarf-1 -gdwarf-1+
-gdwarf-2 -ggdb -gstabs -gstabs+ -gvms-gxcoff
-gxcoff+ -p -pg -print-file-name=library
-print-libgcc-file-name -print-multi-directory
-print-multi-lib -print-prog-name=program
-print-search-dirs-Q -save-temps -time
Optimization Options
-falign-functions=n -falign-jumps=n -falign-labels=n
-falign-loops=n -fbranch-probabilities-fcaller-saves
-fcprop-registers -fcse-follow-jumps
-fcse-skip-blocks-fdata-sections -fdelayed-branch
-fdelete-null-pointer-checks -fexpensive-optimizations
-ffast-math -ffloat-store -fforce-addr-fforce-mem
-ffunction-sections -fgcse -fgcse-lm-fgcse-sm
-floop-optimize-fcrossjumping -fif-conversion
-fif-conversion2 -finline-functions -finline-limit=n
-fkeep-inline-functions -fkeep-static-consts
-fmerge-constants-fmerge-all-constants
-fmove-all-movables -fnew-ra -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 -fun-
safe-math-optimizations -ffinite-math-only -fno-trap-
ping-math -fno-zero-initialized-in-bss
-fomit-frame-pointer-foptimize-register-move -fopti-
mize-sibling-calls -fprefetch-loop-arrays -fre-
duce-all-givs -fregmove-frename-registers -fre-
order-blocks -freorder-functions -fre-
run-cse-after-loop -frerun-loop-opt -fschedule-insns
-fschedule-insns2 -fno-sched-interblock
-fno-sched-spec-fsched-spec-load
-fsched-spec-load-dangerous -fsignaling-nans -fsin-
gle-precision-constant -fssa-fssa-ccp-fssa-dce
-fstrength-reduce-fstrict-aliasing-ftracer
-fthread-jumps -funroll-all-loops -funroll-loops
--param name=value -O-O0-O1-O2-O3-Os
Preprocessor Options
-$ -Aquestion=answer -A-question[=answer] -C-dD
-dI -dM -dN -Dmacro[=defn] -E-H -idirafter dir
-include file -imacros file -iprefix file -iwithpre-
fix dir -iwithprefixbefore dir -isystem dir -M-MM
-MF -MG -MP-MQ-MT-nostdinc-P-remap -tri-
graphs -undef-Umacro-Wp,option
Assembler Option
-Wa,option
Linker Options
object-file-name -llibrary -nostartfiles -nodefault-
libs -nostdlib -s -static -static-libgcc -shared
-shared-libgcc-symbolic -Wl,option -Xlinker option
-u symbol
Directory Options
-Bprefix-Idir-I--Ldir -specs=file
Target Options
-V version -b machine
Machine Dependent Options
M680x0 Options -m68000-m68020-m68020-40
-m68020-60-m68030 -m68040 -m68060-mcpu32-m5200
-m68881-mbitfield -mc68000 -mc68020 -mfpa -mno-
bitfield -mrtd-mshort-msoft-float-mpcrel
-malign-int -mstrict-align
M68hc1x Options -m6811-m6812-m68hc11-m68hc12
-m68hcs12 -mauto-incdec -minmax-mlong-calls
-mshort -msoft-reg-count=count
VAX Options -mg-mgnu-munix
SPARC Options -mcpu=cpu-type -mtune=cpu-type
-mcmodel=code-model -m32-m64 -mapp-regs -mbro-
ken-saverestore -mcypress -mfaster-structs -mflat
-mfpu-mhard-float -mhard-quad-float -mimpure-text
-mlive-g0-mno-app-regs -mno-faster-structs
-mno-flat-mno-fpu -mno-impure-text -mno-stack-bias
-mno-unaligned-doubles -msoft-float -msoft-quad-float
-msparclite -mstack-bias -msupersparc
-munaligned-doubles -mv8
ARM Options -mapcs-frame-mno-apcs-frame -mapcs-26
-mapcs-32 -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
-malignment-traps-mno-alignment-traps -msoft-float
-mhard-float-mfpe -mthumb-interwork
-mno-thumb-interwork -mcpu=name -march=name
-mfpe=name -mstructure-size-boundary=n
-mabort-on-noreturn -mlong-calls -mno-long-calls
-msingle-pic-base-mno-single-pic-base -mpic-regis-
ter=reg -mnop-fun-dllimport -mpoke-function-name
-mthumb-marm -mtpcs-frame -mtpcs-leaf-frame
-mcaller-super-interworking -mcallee-super-interwork-
ing
MN10200 Options -mrelax
MN10300 Options -mmult-bug-mno-mult-bug -mam33
-mno-am33 -mno-crt0 -mrelax
M32R/D Options -m32rx-m32r -mcode-model=model-type
-msdata=sdata-type -G num
M88K Options -m88000-m88100-m88110-mbig-pic
-mcheck-zero-division-mhandle-large-shift -miden-
tify-revision -mno-check-zero-division
-mno-ocs-debug-info -mno-ocs-frame-position
-mno-optimize-arg-area-mno-serialize-volatile
-mno-underscores-mocs-debug-info -mocs-frame-posi-
tion -moptimize-arg-area -mserialize-volatile
-mshort-data-num-msvr3 -msvr4 -mtrap-large-shift
-muse-div-instruction -mversion-03.00
-mwarn-passed-structs
RS/6000 and PowerPC Options -mcpu=cpu-type -mtune=cpu-
type -mpower-mno-power-mpower2-mno-power2 -mpow-
erpc -mpowerpc64-mno-powerpc -maltivec
-mno-altivec -mpowerpc-gpopt -mno-powerpc-gpopt
-mpowerpc-gfxopt-mno-powerpc-gfxopt -mnew-mnemonics
-mold-mnemonics -mfull-toc -mminimal-toc
-mno-fp-in-toc-mno-sum-in-toc -m64 -m32-mxl-call
-mno-xl-call-mpe -msoft-float -mhard-float-mmul-
tiple -mno-multiple -mstring -mno-string-mupdate
-mno-update -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-mlit-
tle-endian -mbig-mbig-endian -mcall-aix
-mcall-sysv -mcall-netbsd -maix-struct-return
-msvr4-struct-return -mabi=altivec -mabi=no-altivec
-mabi=spe -mabi=no-spe -misel=yes -misel=no -mproto-
type -mno-prototype -msim -mmvme-mads-myel-
lowknife -memb-msdata -msdata=opt -mvxworks
-mwindiss-G num -pthread
Darwin Options
-all_load -allowable_client -arch -arch_errors_fatal
-arch_only -bind_at_load -bundle -bundle_loader
-client_name -compatibility_version -current_version
-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
-image_base -init -install_name -keep_private_externs
-multi_module -multiply_defined -multi-
ply_defined_unused -noall_load -nomultidefs -noprebind
-noseglinkedit -pagezero_size -prebind -pre-
bind_all_twolevel_modules -private_bundle
-read_only_relocs -sectalign -sectobjectsymbols -why-
load -seg1addr -sectcreate -sectobjectsymbols -sec-
torder -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
RT Options -mcall-lib-mul-mfp-arg-in-fpregs
-mfp-arg-in-gregs -mfull-fp-blocks -mhc-struct-return
-min-line-mul -mminimum-fp-blocks
-mnohc-struct-return
MIPS Options -mabicalls -march=cpu-type
-mtune=cpu=type -mcpu=cpu-type -membedded-data
-muninit-const-in-rodata -membedded-pic -mfp32
-mfp64-mfused-madd-mno-fused-madd -mgas -mgp32
-mgp64 -mgpopt -mhalf-pic-mhard-float-mint64
-mips1 -mips2 -mips3-mips4-mlong64-mlong32
-mlong-calls-mmemcpy -mmips-as -mmips-tfile
-mno-abicalls -mno-embedded-data
-mno-uninit-const-in-rodata -mno-embedded-pic
-mno-gpopt-mno-long-calls -mno-memcpy
-mno-mips-tfile-mno-rnames-mno-stats -mrnames
-msoft-float -m4650 -msingle-float -mmad -mstats
-EL -EB -G num -nocpp -mabi=32 -mabi=n32 -mabi=64
-mabi=eabi -mfix7000-mno-crt0 -mflush-func=func
-mno-flush-func -mbranch-likely -mno-branch-likely
i386 and x86-64 Options -mcpu=cpu-type -march=cpu-
type -mfpmath=unit -masm=dialect -mno-fancy-math-387
-mno-fp-ret-in-387-msoft-float-msvr3-shlib
-mno-wide-multiply-mrtd-malign-double -mpre-
ferred-stack-boundary=num -mmmx-msse-msse2
-m3dnow -mthreads -mno-align-stringops
-minline-all-stringops -mpush-args -maccumulate-out-
going-args -m128bit-long-double -m96bit-long-double
-mregparm=num -momit-leaf-frame-pointer -mno-red-zone
-mcmodel=code-model -m32-m64
HPPA Options -march=architecture-type -mbig-switch
-mdisable-fpregs-mdisable-indexing -mfast-indi-
rect-calls -mgas-mgnu-ld -mhp-ld -mjump-in-delay
-mlinker-opt-mlong-calls -mlong-load-store
-mno-big-switch-mno-disable-fpregs -mno-dis-
able-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 -nolibdld-static
-threads
Intel 960 Options -mcpu-type-masm-compat
-mclean-linkage -mcode-align -mcomplex-addr
-mleaf-procedures -mic-compat -mic2.0-compat
-mic3.0-compat -mintel-asm -mno-clean-linkage
-mno-code-align -mno-complex-addr -mno-leaf-proce-
dures -mno-old-align-mno-strict-align
-mno-tail-call -mnumerics -mold-align-msoft-float
-mstrict-align -mtail-call
DEC Alpha Options -mno-fp-regs-msoft-float-mal-
pha-as -mgas -mieee -mieee-with-inexact-mieee-con-
formant -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 -mmemory-latency=time
DEC Alpha/VMS Options -mvms-return-codes
H8/300 Options -mrelax-mh -ms -mn-mint32
-malign-300
SH Options -m1-m2 -m3 -m3e -m4-nofpu -m4-sin-
gle-only -m4-single-m4 -m5-64media
-m5-64media-nofpu -m5-32media -m5-32media-nofpu
-m5-compact -m5-compact-nofpu -mb-ml-mdalign
-mrelax -mbigtable -mfmovd -mhitachi -mnomacsave
-mieee-misize-mpadstruct-mspace -mprefergot
-musermode
System V Options -Qy-Qn-YP,paths -Ym,dir
ARC Options -EB-EL -mmangle-cpu -mcpu=cpu
-mtext=text-section -mdata=data-section -mro-
data=readonly-data-section
TMS320C3x/C4x Options -mcpu=cpu -mbig-msmall
-mregparm-mmemparm -mfast-fix -mmpyi-mbk-mti
-mdp-isr-reload -mrpts=count -mrptb-mdb
-mloop-unsigned -mparallel-insns -mparallel-mpy
-mpreserve-float
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
-mv850e -mv850 -mbig-switch
NS32K Options -m32032-m32332-m32532-m32081
-m32381 -mmult-add -mnomult-add-msoft-float-mrtd
-mnortd -mregparam -mnoregparam-msb-mnosb -mbit-
field -mnobitfield -mhimem -mnohimem
AVR Options -mmcu=mcu -msize -minit-stack=n
-mno-interrupts -mcall-prologues -mno-tablejump
-mtiny-stack
MCore Options -mhardlit-mno-hardlit-mdiv-mno-div
-mrelax-immediates -mno-relax-immediates -mwide-bit-
fields -mno-wide-bitfields -m4byte-functions
-mno-4byte-functions-mcallgraph-data -mno-call-
graph-data -mslow-bytes-mno-slow-bytes-mno-lsim
-mlittle-endian-mbig-endian-m210-m340
-mstack-increment
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
IA-64 Options -mbig-endian-mlittle-endian -mgnu-as
-mgnu-ld-mno-pic -mvolatile-asm-stop -mb-step
-mregister-names-mno-sdata -mconstant-gp -mauto-pic
-minline-float-divide-min-latency -min-
line-float-divide-max-throughput -min-
line-int-divide-min-latency -min-
line-int-divide-max-throughput -mno-dwarf2-asm
-mfixed-range=register-range
D30V Options -mextmem-mextmemory-monchip
-mno-asm-optimize -masm-optimize -mbranch-cost=n
-mcond-exec=n
S/390 and zSeries Options -mhard-float-msoft-float
-mbackchain -mno-backchain -msmall-exec
-mno-small-exec-mmvcle-mno-mvcle -m64 -m31-mde-
bug -mno-debug
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
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 -msplit
-mno-split-munix-asm-mdec-asm
Xstormy16 Options -msim
Xtensa Options -mbig-endian -mlittle-endian -mdensity
-mno-density -mmac16 -mno-mac16 -mmul16 -mno-mul16
-mmul32-mno-mul32 -mnsa -mno-nsa -mminmax
-mno-minmax -msext -mno-sext -mbooleans
-mno-booleans -mhard-float -msoft-float -mfused-madd
-mno-fused-madd -mserialize-volatile -mno-serial-
ize-volatile -mtext-section-literals-mno-text-sec-
tion-literals -mtarget-align-mno-target-align
-mlongcalls -mno-longcalls
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-mlibrary-pic
-macc-4-macc-8-mpack-mno-pack -mno-eflags
-mcond-move -mno-cond-move -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 -mcpu=cpu
Code Generation Options
-fcall-saved-reg-fcall-used-reg -ffixed-reg -fex-
ceptions -fnon-call-exceptions-funwind-tables
-fasynchronous-unwind-tables -finhibit-size-directive
-finstrument-functions -fno-common -fno-ident
-fno-gnu-linker -fpcc-struct-return -fpic-fPIC
-freg-struct-return -fshared-data -fshort-enums
-fshort-double-fshort-wchar-fvolatile
-fvolatile-global-fvolatile-static -fverbose-asm
-fpack-struct-fstack-check -fstack-limit-regis-
ter=reg -fstack-limit-symbol=sym -fargument-alias
-fargument-noalias -fargument-noalias-global -flead-
ing-underscore -ftls-model=model -ftrapv
-fbounds-check
Options Controlling the Kind of Output
Compilation can involve up to four stages: preprocessing,
compilation proper, assembly and linking, always in that
order. The first three stages apply to an individual
source file, and end by producing an object file; 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 which must be preprocessed.
file.i
C source code which should not be preprocessed.
file.ii
C++ source code which should not be preprocessed.
file.m
Objective-C source code. Note that you must link with
the library libobjc.a to make an Objective-C program
work.
file.mi
Objective-C source code which should not be prepro-
cessed.
file.h
C header file (not to be compiled or linked).
file.cc
file.cp
file.cxx
file.cpp
file.c++
file.C
C++ source code which 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.f
file.for
file.FOR
Fortran source code which should not be preprocessed.
file.F
file.fpp
file.FPP
Fortran source code which must be preprocessed (with
the traditional preprocessor).
file.r
Fortran source code which must be preprocessed with a
RATFOR preprocessor (not included with GCC).
file.ads
Ada source code file which 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 sub-
program 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
Assembler code which 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++-cpp-output
objective-c objc-cpp-output
assembler assembler-with-cpp
ada
f77 f77-cpp-input ratfor
java
treelang
-x none
Turn off any specification of a language, so that sub-
sequent 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.
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 prepro-
cessed source code, which is sent to the standard out-
put.
Input files which 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.
Since only one output file can be specified, it does
not make sense to use -o when compiling more than one
input file, unless you are producing an executable
file as output.
If -o is not specified, the default is to put an exe-
cutable file in a.out, the object file for source.suf-
fix in source.o, its assembler file in source.s, 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 ver-
sion number of the compiler driver program and of the
preprocessor and the compiler proper.
-###
Like -v except the commands are not executed and all
command arguments are quoted. This is useful for
shell scripts to capture the driver-generated command
lines.
-pipe
Use pipes rather than temporary files for communica-
tion 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 -W option is also specified then com-
mand line options which have no documentation associ-
ated with them will also be displayed.
--target-help
Print (on the standard output) a description of target
specific command line options for each tool.
--version
Display the version number and copyrights of the
invoked GCC.
Compiling C++ Programs
C++ source files conventionally use one of the suffixes
.C, .cc, .cpp, .c++, .cp, or .cxx; 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, C++ programs often require class libraries as
well as a compiler that understands the C++ language---and
under some circumstances, you might want to compile pro-
grams from standard input, or otherwise without a suffix
that flags them as C++ programs. g++ is a program that
calls GCC with the default language set to C++, and auto-
matically specifies linking against the C++ library. 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 pro-
grams in any language; or command-line options meaningful
for C and related languages; or options that are meaning-
ful only for C++ programs.
Options Controlling C Dialect
The following options control the dialect of C (or lan-
guages derived from C, such as C++ and Objective-C) that
the compiler accepts:
-ansi
In C mode, support all ISO C90 programs. In C++ mode,
remove GNU extensions that conflict with ISO C++.
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 com-
piler, it disables recognition of C++ style // com-
ments 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 func-
tions 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 which would normally be built in but do not
have semantics defined by ISO C (such as "alloca" and
"ffs") are not built-in functions with -ansi is used.
-std=
Determine the language standard. This option is cur-
rently only supported when compiling C or C++. A
value for this option must be provided; possible val-
ues are
c89
iso9899:1990
ISO C90 (same as -ansi).
iso9899:199409
ISO C90 as modified in amendment 1.
c99
c9x
iso9899:1999
iso9899:199x
ISO C99. Note that this standard is not yet fully
supported; see <http://gcc.gnu.org/gcc-3.3/c99sta-
tus.html> for more information. The names c9x and
iso9899:199x are deprecated.
gnu89
Default, ISO C90 plus GNU extensions (including
some C99 features).
gnu99
gnu9x
ISO C99 plus GNU extensions. When ISO C99 is
fully implemented in GCC, this will become the
default. The name gnu9x is deprecated.
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.
Even when this option is not specified, you can still
use some of the features of newer standards in so far
as they do not conflict with previous C standards.
For example, you may use "__restrict__" even when
-std=c99 is not specified.
The -std options specifying some version of ISO C have
the same effects as -ansi, except that features that
were not in ISO C90 but are in the specified version
(for example, // comments and the "inline" keyword in
ISO C99) are not disabled.
-aux-info filename
Output to the given filename prototyped declarations
for all functions declared and/or defined in a trans-
lation 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 declara-
tions is also provided, inside comments, after the
declaration.
-fno-asm
Do not recognize "asm", "inline" or "typeof" as a key-
word, 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 key-
word 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 that
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.
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 this 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 envi-
ronment. This implies -fbuiltin. A hosted environ-
ment 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 ker-
nel. This is equivalent to -fno-freestanding.
-ffreestanding
Assert that compilation takes place in a freestanding
environment. This implies -fno-builtin. A freestand-
ing 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 ker-
nel. This is equivalent to -fno-hosted.
-fms-extensions
Accept some non-standard constructs used in Microsoft
header files.
-trigraphs
Support ISO C trigraphs. The -ansi option (and -std
options for strict ISO C conformance) implies -tri-
graphs.
-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 preprocess-
ing 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 emu-
late a pre-standard C compiler. They are now only
supported with the -E switch. The preprocessor con-
tinues 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++.
-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. Like-
wise, 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 inte-
ger types such as "int" are signed types.
-fwritable-strings
Store string constants in the writable data segment
and don't uniquize them. This is for compatibility
with old programs which assume they can write into
string constants.
Writing into string constants is a very bad idea;
``constants'' should be constant.
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 lan-
guage 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 lan-
guage 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 1 is the ver-
sion 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 1.
-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 allo-
cated. 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 with-
out 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 runtime-initialized global vari-
ables 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.
-fno-const-strings
Give string constants type "char *" instead of type
"const char *". By default, G++ uses type "const char
*" as required by the standard. Even if you use
-fno-const-strings, you cannot actually modify the
value of a string constant, unless you also use
-fwritable-strings.
This option might be removed in a future release of
G++. For maximum portability, you should structure
your code so that it works with string constants that
have type "const char *".
-fdollars-in-identifiers
Accept $ in identifiers. You can also explicitly pro-
hibit use of $ with the option -fno-dollars-in-identi-
fiers. (GNU C allows $ by default on most target sys-
tems, but there are a few exceptions.) Traditional C
allowed the character $ to form part of identifiers.
However, ISO C and C++ forbid $ in identifiers.
-fno-elide-constructors
The C++ standard allows an implementation to omit cre-
ating a temporary which 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 check for violation of exception specifications
at runtime. This option violates the C++ standard,
but may be useful for reducing code size in production
builds, much like defining NDEBUG. The compiler will
still optimize based on the exception specifications.
-fexternal-templates
Cause #pragma interface and implementation to apply to
template instantiation; template instances are emitted
or not according to the location of the template defi-
nition.
This option is deprecated.
-falt-external-templates
Similar to -fexternal-templates, but template
instances are emitted or not according to the place
where they are first instantiated.
This option is deprecated.
-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 which 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 opti-
mization will need the same set of explicit instantia-
tions.
-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.
-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 messages about nonconformant code from
errors to warnings. By default, G++ effectively sets
-pedantic-errors without -pedantic; this option
reverses that. This behavior and this option are
superseded by -pedantic, which works as it does for
GNU C.
-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++ runtime 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 excep-
tion handling uses the same information, but it will
generate it as needed.
-fstats
Emit statistics about front-end processing at the end
of the compilation. This information is generally
only useful to the G++ development team.
-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.
-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 destruc-
tors, but will only work if your C library supports
"__cxa_atexit".
-fvtable-gc
Emit special relocations for vtables and virtual func-
tion references so that the linker can identify unused
virtual functions and zero out vtable slots that refer
to them. This is most useful with -ffunction-sections
and -Wl,--gc-sections, in order to also discard the
functions themselves.
This optimization requires GNU as and GNU ld. Not all
systems support this option. -Wl,--gc-sections is
ignored without -static.
-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 direc-
tories 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 genera-
tion 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++ only)
Warn when G++ generates code that is probably not com-
patible 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 gener-
ated by G++ may not be binary compatible with code
generated by other compilers.
The known incompatibilities at this point include:
o 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 com-
pilers to layout "B" identically.
o 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.
o 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".
o 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.
o 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.
-Wctor-dtor-privacy (C++ only)
Warn when a class seems unusable, because all the con-
structors or destructors in a class are private and
the class has no friends or public static member func-
tions. This warning is enabled by default.
-Wnon-virtual-dtor (C++ only)
Warn when a class declares a non-virtual destructor
that should probably be virtual, because it looks like
the class will be used polymorphically. This warning
is enabled by -Wall.
-Wreorder (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) { }
};
Here the compiler will warn that the member initializ-
ers for i and j will be rearranged to match the decla-
ration order of the members. This warning is enabled
by -Wall.
The following -W... options are not affected by -Wall.
-Weffc++ (C++ only)
Warn about violations of the following style guide-
lines from Scott Meyers' Effective C++ book:
o Item 11: Define a copy constructor and an assign-
ment operator for classes with dynamically allo-
cated memory.
o Item 12: Prefer initialization to assignment in
constructors.
o Item 14: Make destructors virtual in base
classes.
o Item 15: Have "operator=" return a reference to
*this.
o Item 23: Don't try to return a reference when you
must return an object.
and about violations of the following style guidelines
from Scott Meyers' More Effective C++ book:
o Item 6: Distinguish between prefix and postfix
forms of increment and decrement operators.
o Item 7: Never overload "&&", "||", or ",".
If you use this option, you should be aware that the
standard library headers do not obey all of these
guidelines; you can use grep -v to filter out those
warnings.
-Wno-deprecated (C++ only)
Do not warn about usage of deprecated features.
-Wno-non-template-friend (C++ only)
Disable warnings when non-templatized friend functions
are declared within a template. With 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 tem-
platized function. Because this non-conforming behav-
ior 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 con-
formant compiler code but disables the helpful
warning.
-Wold-style-cast (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
(static_cast, reinterpret_cast, and const_cast) are
less vulnerable to unintended effects, and much easier
to grep for.
-Woverloaded-virtual (C++ only)
Warn when a function declaration hides virtual func-
tions 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 this:
B* b;
b->f();
will fail to compile.
-Wno-pmf-conversions (C++ only)
Disable the diagnostic for converting a bound pointer
to member function to a plain pointer.
-Wsign-promo (C++ only)
Warn when overload resolution chooses a promotion from
unsigned or enumeral type to a signed type over a con-
version to an unsigned type of the same size. Previ-
ous versions of G++ would try to preserve unsigned-
ness, but the standard mandates the current behavior.
-Wsynth (C++ only)
Warn when G++'s synthesis behavior does not match that
of cfront. For instance:
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 Dialect
This section describes the command-line options that are
only meaningful for Objective-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 com-
pile a file "some_class.m" like this:
gcc -g -fgnu-runtime -O -c some_class.m
In this example, only -fgnu-runtime is an option meant
only for Objective-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
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 "NXCon-
stantString".
-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.
-gen-decls
Dump interface declarations for all classes seen in
the source file to a file named sourcename.decl.
-Wno-protocol
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
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
that for each selector appearing in a "@selector(...)"
expression, a corresponding method with that selector
has been found during compilation. Because these
checks scan the method table only at the end of compi-
lation, 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.
-Wundeclared-selector
Warn if a "@selector(...)" expression referring to an
undeclared selector is found. A selector is consid-
ered undeclared if no method with that name has been
declared (explicitly, in an @interface or @protocol
declaration, or implicitly, in an @implementation sec-
tion) before the "@selector(...)" expression. This
option always performs its checks as soon as a
"@selector(...)" expression is found (while "-Wselec-
tor" only performs its checks in the final stage of
compilation), and so additionally enforces the coding
style convention that methods and selectors must be
declared before being used.
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 loca-
tion 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 pre-
fix) 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.
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions
which are not inherently erroneous but which are risky or
suggest there may have been an error.
You can request many specific warnings with options begin-
ning -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.
The following options control the amount and kinds of
warnings produced by GCC; for further, language-specific
options also refer to @ref{C++ Dialect Options} and
@ref{Objective-C Dialect Options}.
-fsyntax-only
Check the code for syntax errors, but don't do any-
thing beyond that.
-pedantic
Issue all the warnings demanded by strict ISO C and
ISO C++; reject all programs that use forbidden exten-
sions, 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 prop-
erly 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 diag-
nostics 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 dif-
ferent 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 gnu89 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 defini-
tion 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.
-w Inhibit all warning messages.
-Wno-import
Inhibit warning messages about the use of #import.
-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.
-Wcomment
Warn whenever a comment-start sequence /* appears in a
/* comment, or whenever a Backslash-Newline appears in
a // comment.
-Wformat
Check calls to "printf" and "scanf", etc., to make
sure that the arguments supplied have types appropri-
ate 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", "strf-
time" and "strfmon" (an X/Open extension, not in the C
standard) families.
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 exten-
sions. Other library implementations may not support
all these features; GCC does not support warning about
features that go beyond a particular library's limita-
tions. 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 -Wnon-
null.
-Wformat is included in -Wall. For more control over
some aspects of format checking, the options -Wno-for-
mat-y2k, -Wno-format-extra-args, -Wno-for-
mat-zero-length, -Wformat-nonliteral, -Wformat-secu-
rity, and -Wformat=2 are available, but are not
included in -Wall.
-Wno-format-y2k
If -Wformat is specified, do not warn about "strftime"
formats which may yield only a two-digit year.
-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 specifica-
tions, 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 for-
mat functions that represent possible security prob-
lems. 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 con-
tains %n. (This is currently a subset of what -Wfor-
mat-nonliteral warns about, but in future warnings may
be added to -Wformat-security that are not included in
-Wformat-nonliteral.)
-Wformat=2
Enable -Wformat plus format checks not included in
-Wformat. Currently equivalent to -Wformat -Wfor-
mat-nonliteral -Wformat-security.
-Wnonnull
Enable warning about passing a null pointer for argu-
ments 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.
-Wimplicit-int
Warn when a declaration does not specify a type.
-Wimplicit-function-declaration
-Werror-implicit-function-declaration
Give a warning (or error) whenever a function is used
before being declared.
-Wimplicit
Same as -Wimplicit-int and -Wimplicit-function-decla-
ration.
-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.
-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 } };
-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 about constructions where there may be con-
fusion 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, 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 enclos-
ing "if". The resulting code would look like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
-Wsequence-point
Warn about code that may have undefined semantics
because of violations of sequence point rules in the C
standard.
The C standard defines the order in which expressions
in a 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 expres-
sion), 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 func-
tion), 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 modi-
fications to the values of objects take effect. Pro-
grams whose behavior depends on this have undefined
behavior; the C standard specifies that ``Between the
previous and next sequence point an object shall have
its stored value modified at most once by the evalua-
tion 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 present implementation of this option only works
for C programs. A future implementation may also work
for C++ programs.
The C standard is worded confusingly, therefore there
is some debate over the precise meaning of the
sequence point rules in subtle cases. Links to dis-
cussions of the problem, including proposed formal
definitions, may be found on our readings page, at
<http://gcc.gnu.org/readings.html>.
-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".
For C++, a function without return type always pro-
duces a diagnostic message, even when -Wno-return-type
is specified. The only exceptions are main and func-
tions defined in system headers.
-Wswitch
Warn whenever a "switch" statement has an index of
enumeral 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.
-Wswitch-default
Warn whenever a "switch" statement does not have a
"default" case.
-Wswitch-enum
Warn whenever a "switch" statement has an index of
enumeral 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.
-Wtrigraphs
Warn if any trigraphs are encountered that might
change the meaning of the program (trigraphs within
comments are not warned about).
-Wunused-function
Warn whenever a static function is declared but not
defined or a non\-inline static function is unused.
-Wunused-label
Warn whenever a label is declared but not used.
To suppress this warning use the unused attribute.
-Wunused-parameter
Warn whenever a function parameter is unused aside
from its declaration.
To suppress this warning use the unused attribute.
-Wunused-variable
Warn whenever a local variable or non-constant static
variable is unused aside from its declaration
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 expression to void.
-Wunused
All the above -Wunused options combined.
In order to get a warning about an unused function
parameter, you must either specify -W -Wunused or sep-
arately 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.
These warnings are possible only in optimizing compi-
lation, because they require data flow information
that is computed only when optimizing. If you don't
specify -O, you simply won't get these warnings.
These warnings occur only for variables that are can-
didates for register allocation. Therefore, they do
not occur for a variable that is declared "volatile",
or whose address is taken, or whose size is other than
1, 2, 4 or 8 bytes. Also, they do not occur for
structures, unions or arrays, even when they are in
registers.
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.
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. Here
is another common case:
{
int save_y;
if (change_y) save_y = y, y = new_y;
...
if (change_y) y = save_y;
}
This has no bug because "save_y" is used only if it is
set.
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 can-
not 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 which would cause a problem.
Some spurious warnings can be avoided if you declare
all the functions you use that never return as "nore-
turn".
-Wunknown-pragmas
Warn when a #pragma directive is encountered which 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.
-Wstrict-aliasing
This option is only active when -fstrict-aliasing is
active. It warns about code which 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.
-Wall
All of the above -W options combined. 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 @ref{C++ Dialect Options} and
@ref{Objective-C Dialect Options}.
The following -W... options 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.
-W Print extra warning messages for these events:
o A function can return either with or without a
value. (Falling off the end of the function body
is considered returning without a value.) For
example, this function would evoke such a warning:
foo (a)
{
if (a > 0)
return a;
}
o An expression-statement or the left-hand side of a
comma expression contains no side effects. To
suppress the warning, cast the unused expression
to void. For example, an expression such as
x[i,j] will cause a warning, but x[(void)i,j] will
not.
o An unsigned value is compared against zero with <
or <=.
o A comparison like x<=y<=z appears; this is equiva-
lent to (x<=y ? 1 : 0) <= z, which is a different
interpretation from that of ordinary mathematical
notation.
o Storage-class specifiers like "static" are not the
first things in a declaration. According to the C
Standard, this usage is obsolescent.
o The return type of a function has a type qualifier
such as "const". Such a type qualifier has no
effect, since the value returned by a function is
not an lvalue. (But don't warn about the GNU
extension of "volatile void" return types. That
extension will be warned about if -pedantic is
specified.)
o If -Wall or -Wunused is also specified, warn about
unused arguments.
o A comparison between signed and unsigned values
could produce an incorrect result when the signed
value is converted to unsigned. (But don't warn
if -Wno-sign-compare is also specified.)
o An aggregate has a partly bracketed initializer.
For example, the following code would evoke such a
warning, because braces are missing around the
initializer for "x.h":
struct s { int f, g; };
struct t { struct s h; int i; };
struct t x = { 1, 2, 3 };
o An aggregate has an initializer which does not
initialize all members. For example, the follow-
ing code would cause such a warning, because "x.h"
would be implicitly initialized to zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
-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 nor-
mally 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 head-
ers---for that, -Wunknown-pragmas must also be used.
-Wfloat-equal
Warn if floating point values are used in equality
comparisons.
The idea behind this is that sometimes it is conve-
nient (for the programmer) to consider floating-point
values as approximations to infinitely precise real
numbers. If you are doing this, then you need to com-
pute (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 compar-
isons (and when producing output, but that's a differ-
ent problem). In particular, instead of testing for
equality, you would check to see whether the two val-
ues have ranges that overlap; and this is done with
the relational operators, so equality comparisons are
probably mistaken.
-Wtraditional (C only)
Warn about certain constructs that behave differently
in traditional and ISO C. Also warn about ISO C con-
structs that have no traditional C equivalent, and/or
problematic constructs which should be avoided.
o Macro parameters that appear within string liter-
als in the macro body. In traditional C macro
replacement takes place within string literals,
but does not in ISO C.
o 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 tradi-
tional 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 indent-
ing them. Some traditional implementations would
not recognize #elif, so it suggests avoiding it
altogether.
o A function-like macro that appears without argu-
ments.
o The unary plus operator.
o 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 pre-
processor has enough context to avoid warning in
these cases.
o A function declared external in one block and then
used after the end of the block.
o A "switch" statement has an operand of type
"long".
o A non-"static" function declaration follows a
"static" one. This construct is not accepted by
some traditional C compilers.
o The ISO type of an integer constant has a differ-
ent 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 val-
ues, which typically represent bit patterns, are
not warned about.
o Usage of ISO string concatenation is detected.
o Initialization of automatic aggregates.
o Identifier conflicts with labels. Traditional C
lacks a separate namespace for labels.
o 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.
o 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
-Wconversion.
o 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.
-Wundef
Warn if an undefined identifier is evaluated in an #if
directive.
-Wendif-labels
Warn whenever an #else or an #endif are followed by
text.
-Wshadow
Warn whenever a local variable shadows another local
variable, parameter or global variable or whenever a
built-in function is shadowed.
-Wlarger-than-len
Warn whenever an object of larger than len bytes is
defined.
-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.
-Wbad-function-cast (C only)
Warn whenever a function call is cast to a non-match-
ing type. For example, warn if "int malloc()" is cast
to "anything *".
-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 *".
-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; when compiling C++, warn about the deprecated
conversion from string constants to "char *". 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 dec-
larations and prototypes. Otherwise, it will just be
a nuisance; this is why we did not make -Wall request
these warnings.
-Wconversion
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 conver-
sions 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.
Also, warn if a negative integer constant expression
is implicitly converted to an unsigned type. For
example, warn about the assignment "x = -1" if "x" is
unsigned. But do not warn about explicit casts like
"(unsigned) -1".
-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 enabled by -W, and by -Wall in C++ only.
-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.)
-Wstrict-prototypes (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 which specifies the argument types.)
-Wmissing-prototypes (C only)
Warn if a global function is defined without a previ-
ous prototype declaration. This warning is issued
even if the definition itself provides a prototype.
The aim is to detect global functions that fail to be
declared in header files.
-Wmissing-declarations
Warn if a global function is defined without a previ-
ous 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.
-Wmissing-noreturn
Warn about functions which might be candidates for
attribute "noreturn". Note these are only possible
candidates, not absolute ones. Care should be taken
to manually verify functions actually do not ever
return before adding the "noreturn" attribute, other-
wise subtle code generation bugs could be introduced.
You will not get a warning for "main" in hosted C
environments.
-Wmissing-format-attribute
If -Wformat is enabled, also warn about functions
which might be candidates for "format" attributes.
Note these are only possible candidates, not absolute
ones. 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. This
option has no effect unless -Wformat is enabled (pos-
sibly by -Wall).
-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.
-Wno-deprecated-declarations
Do not warn about uses of functions, variables, and
types marked as deprecated by using the "deprecated"
attribute. (@pxref{Function Attributes}, @pxref{Vari-
able Attributes}, @pxref{Type Attributes}.)
-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 mis-
aligned 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;
};
-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 only)
Warn if an "extern" declaration is encountered within
a function.
-Wunreachable-code
Warn if the compiler detects that code will never be
executed.
This option is intended to warn when the compiler
detects that at least a whole line of source code will
never be executed, because some condition is never
satisfied or because it is after a procedure that
never returns.
It is possible for this option to produce a warning
even though there are circumstances under which part
of the affected line can be executed, so care should
be taken when removing apparently-unreachable code.
For instance, when a function is inlined, a warning
may mean that the line is unreachable in only one
inlined copy of the function.
This option is not made part of -Wall because in a
debugging version of a program there is often substan-
tial code which checks correct functioning of the pro-
gram and is, hopefully, unreachable because the pro-
gram does work. Another common use of unreachable
code is to provide behavior which is selectable at
compile-time.
-Winline
Warn if a function can not be inlined and it was
declared as inline.
-Wlong-long
Warn if long long type is used. This is default. To
inhibit the warning messages, use -Wno-long-long.
Flags -Wlong-long and -Wno-long-long are taken into
account only when -pedantic flag is used.
-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.
-Werror
Make all warnings into errors.
Options for Debugging Your Program or GCC
GCC has various special options that are used for debug-
ging either your program or GCC:
-g Produce debugging information in the operating sys-
tem's native format (stabs, COFF, XCOFF, or DWARF).
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, -gdwarf-1+,
-gdwarf-1, or -gvms (see below).
Unlike most other C compilers, GCC allows you to use
-g with -O. The shortcuts taken by optimized code may
occasionally produce surprising results: some vari-
ables you declared may not exist at all; flow of con-
trol may briefly move where you did not expect it;
some statements may not be executed because they com-
pute 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 for-
mat.
-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 pro-
duces stabs debugging output which is not understood
by DBX or SDB. On System V Release 4 systems this
option requires the GNU assembler.
-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
Produce debugging information in DWARF version 1 for-
mat (if that is supported). This is the format used
by SDB on most System V Release 4 systems.
This option is deprecated.
-gdwarf+
Produce debugging information in DWARF version 1 for-
mat (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.
This option is deprecated.
-gdwarf-2
Produce debugging information in DWARF version 2 for-
mat (if that is supported). This is the format used
by DBX on IRIX 6.
-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 1 produces minimal information, enough for mak-
ing backtraces in parts of the program that you don't
plan to debug. This includes descriptions of func-
tions 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 debug-
gers support macro expansion when you use -g3.
Note that in order to avoid confusion between DWARF1
debug level 2, and DWARF2, neither -gdwarf nor
-gdwarf-2 accept a concatenated debug level. Instead
use an additional -glevel option to change the debug
level for DWARF1 or DWARF2.
-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.
-p Generate extra code to write profile information suit-
able 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 suit-
able 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 perma-
nent memory allocation when it finishes.
-fprofile-arcs
Instrument arcs during compilation to generate cover-
age data or for profile-directed block ordering. Dur-
ing execution the program records how many times each
branch is executed and how many times it is taken.
When the compiled program exits it saves this data to
a file called auxname.da for each source file. aux-
name is generated from the name of the output file, if
explicitly specified and it is not the final exe-
cutable, otherwise it is the basename of the source
file. In both cases any suffix is removed (e.g.
foo.da for input file dir/foo.c, or dir/foo.da for
output file specified as -o dir/foo.o).
For profile-directed block ordering, compile the pro-
gram with -fprofile-arcs plus optimization and code
generation options, generate the arc profile informa-
tion by running the program on a selected workload,
and then compile the program again with the same opti-
mization and code generation options plus
-fbranch-probabilities.
The other use of -fprofile-arcs is for use with gcov,
when it is used with the -ftest-coverage option.
With -fprofile-arcs, for each function of your program
GCC creates a program flow graph, then finds a span-
ning tree for the graph. Only arcs that are not on
the spanning tree have to be instrumented: the com-
piler 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
Create data files for the gcov code-coverage utility.
See -fprofile-arcs option above for a description of
auxname.
auxname.bb
A mapping from basic blocks to line numbers, which
gcov uses to associate basic block execution
counts with line numbers.
auxname.bbg
A list of all arcs in the program flow graph.
This allows gcov to reconstruct the program flow
graph, so that it can compute all basic block and
arc execution counts from the information in the
auxname.da file.
Use -ftest-coverage with -fprofile-arcs; the latter
option adds instrumentation to the program, which then
writes execution counts to another data file:
auxname.da
Runtime arc execution counts, used in conjunction
with the arc information in the file auxname.bbg.
Coverage data will map better to the source files if
-ftest-coverage is used without optimization.
-dletters
Says to make debugging dumps during compilation at
times specified by letters. This is used for debug-
ging the compiler. The file names for most of the
dumps are made by appending a pass number and a word
to the dumpname. 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. In both cases any suffix is removed (e.g.
foo.00.rtl or foo.01.sibling). Here are the possible
letters for use in letters, and their meanings:
A Annotate the assembler output with miscellaneous
debugging information.
b Dump after computing branch probabilities, to
file.14.bp.
B Dump after block reordering, to file.32.bbro.
c Dump after instruction combination, to the file
file.19.combine.
C Dump after the first if conversion, to the file
file.15.ce1.
d Dump after delayed branch scheduling, to
file.34.dbr.
D Dump all macro definitions, at the end of prepro-
cessing, in addition to normal output.
e Dump after SSA optimizations, to file.04.ssa and
file.07.ussa.
E Dump after the second if conversion, to
file.29.ce3.
f Dump after life analysis, to file.18.life.
F Dump after purging "ADDRESSOF" codes, to
file.10.addressof.
g Dump after global register allocation, to
file.24.greg.
h Dump after finalization of EH handling code, to
file.02.eh.
k Dump after reg-to-stack conversion, to
file.31.stack.
o Dump after post-reload optimizations, to
file.25.postreload.
G Dump after GCSE, to file.11.gcse.
i Dump after sibling call optimizations, to
file.01.sibling.
j Dump after the first jump optimization, to
file.03.jump.
k Dump after conversion from registers to stack, to
file.31.stack.
l Dump after local register allocation, to
file.23.lreg.
L Dump after loop optimization, to file.12.loop.
M Dump after performing the machine dependent reor-
ganization pass, to file.33.mach.
n Dump after register renumbering, to file.28.rnreg.
N Dump after the register move pass, to file.21.reg-
move.
r Dump after RTL generation, to file.00.rtl.
R Dump after the second scheduling pass, to
file.30.sched2.
s Dump after CSE (including the jump optimization
that sometimes follows CSE), to file.09.cse.
S Dump after the first scheduling pass, to
file.22.sched.
t Dump after the second CSE pass (including the jump
optimization that sometimes follows CSE), to
file.17.cse2.
u Dump after null pointer elimination pass to
file.08.null.
w Dump after the second flow pass, to file.26.flow2.
X Dump after SSA dead code elimination, to
file.06.ssadce.
z Dump after the peephole pass, to file.27.peep-
hole2.
a Produce all the dumps listed above.
m Print statistics on memory usage, at the end of
the run, to standard error.
p Annotate the assembler output with a comment indi-
cating which pattern and alternative was used.
The length of each instruction is also printed.
P Dump the RTL in the assembler output as a comment
before each instruction. Also turns on -dp anno-
tation.
v For each of the other indicated dump files (except
for file.00.rtl), dump a representation of the
control flow graph suitable for viewing with VCG
to file.pass.vcg.
x Just generate RTL for a function instead of com-
piling it. Usually used with r.
y Dump debugging information during parsing, to
standard error.
-fdump-unnumbered
When doing debugging dumps (see -d option above), sup-
press instruction numbers and line number note 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-translation-unit (C and C++ only)
-fdump-translation-unit-options (C and 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. 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. If the -options form is used, options controls
the details of the dump as described for the
-fdump-tree options.
-fdump-tree-switch (C++ only)
-fdump-tree-switch-options (C++ only)
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 suf-
fix to the source file name. If the -options form is
used, options is a list of - separated options that
control the details of the dump. Not all options are
applicable to all dumps, those which are not meaning-
ful will be ignored. The following options are avail-
able
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.
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.
all Turn on all options.
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.
inlined
Dump after function inlining, to file.inlined.
-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 -dS or -dR is specified, in
which case it is output to the usual dump listing
file, .sched 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 -dRS. 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-ver-
bose also includes dependence info.
-save-temps
Store the usual ``temporary'' intermediate files per-
manently; 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 nor-
mally uses an integrated preprocessor.
-time
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). 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.
-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 @samp{-, with-
out spaces between multiple switches. This is sup-
posed to ease shell-processing.
-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 -nodefault-
libs 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 direc-
tory 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 '/'.
-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.)
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 state-
ments, 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.
Not all optimizations are controlled directly by a flag.
Only optimizations that have a flag are listed.
-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: -fde-
fer-pop -fmerge-constants -fthread-jumps -floop-opti-
mize -fcrossjumping -fif-conversion -fif-conversion2
-fdelayed-branch -fguess-branch-probability
-fcprop-registers
-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 trade-
off. The compiler does not perform loop unrolling or
function inlining when you specify -O2. 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:
-fforce-mem -foptimize-sibling-calls -fstrength-reduce
-fcse-follow-jumps-fcse-skip-blocks -fre-
run-cse-after-loop -frerun-loop-opt -fgcse
-fgcse-lm-fgcse-sm -fdelete-null-pointer-checks
-fexpensive-optimizations -fregmove -fschedule-insns
-fschedule-insns2 -fsched-interblock -fsched-spec
-fcaller-saves -fpeephole2 -freorder-blocks -fre-
order-functions -fstrict-aliasing -falign-functions
-falign-jumps -falign-loops -falign-labels
Please note the warning under -fgcse about invoking
-O2 on programs that use computed gotos.
-O3 Optimize yet more. -O3 turns on all optimizations
specified by -O2 and also turns on the -finline-func-
tions and -frename-registers options.
-O0 Do not optimize. 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
-fprefetch-loop-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 func-
tions 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 which 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.
-fforce-mem
Force memory operands to be copied into registers
before doing arithmetic on them. This produces better
code by making all memory references potential common
subexpressions. When they are not common subexpres-
sions, instruction combination should eliminate the
separate register-load.
Enabled at levels -O2, -O3, -Os.
-fforce-addr
Force memory address constants to be copied into reg-
isters before doing arithmetic on them. This may pro-
duce better code just as -fforce-mem may.
-fomit-frame-pointer
Don't keep the frame pointer in a register for func-
tions that don't need one. This avoids the instruc-
tions to save, set up and restore frame pointers; it
also makes an extra register available in many func-
tions. 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 automat-
ically handles the frame pointer and nothing is saved
by pretending it doesn't exist. The machine-descrip-
tion macro "FRAME_POINTER_REQUIRED" controls whether a
target machine supports this flag.
Enabled at levels -O, -O2, -O3, -Os.
-foptimize-sibling-calls
Optimize sibling and tail recursive calls.
Enabled at levels -O2, -O3, -Os.
-fno-inline
Don't pay attention to the "inline" keyword. Normally
this option is used to keep the compiler from expand-
ing any functions inline. Note that if you are not
optimizing, no functions can be expanded inline.
-finline-functions
Integrate all simple functions into their callers.
The compiler heuristically decides which functions are
simple enough to be 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-limit=n
By default, gcc limits the size of functions that can
be inlined. This flag allows the control of this
limit for functions that are explicitly marked as
inline (i.e., marked with the inline keyword or
defined within the class definition in c++). n is the
size of functions that can be inlined in number of
pseudo instructions (not counting parameter handling).
The default value of n is 600. Increasing this value
can result in more inlined code at the cost of compi-
lation time and memory consumption. Decreasing usu-
ally makes the compilation faster and less code will
be inlined (which presumably means slower programs).
This option is particularly useful for programs that
use inlining heavily such as those based on recursive
templates with C++.
Inlining is actually controlled by a number of parame-
ters, which may be specified individually by using
--param name=value. The -finline-limit=n option sets
some of these parameters as follows:
@table @gcctabopt
@item max-inline-insns
is set to I<n>.
@item max-inline-insns-single
is set to I<n>/2.
@item max-inline-insns-auto
is set to I<n>/2.
@item min-inline-insns
is set to 130 or I<n>/4, whichever is smaller.
@item max-inline-insns-rtl
is set to I<n>.
@end table
Using -finline-limit=600 thus results in the default
settings for these parameters. See below for a docu-
mentation of the individual parameters controlling
inlining.
Note: pseudo instruction represents, in this particu-
lar context, an abstract measurement of function's
size. In no way, it represents a count of assembly
instructions and as such its exact meaning might
change from one release to an another.
-fkeep-inline-functions
Even if all calls to a given function are integrated,
and the function is declared "static", nevertheless
output a separate run-time callable version of the
function. This switch does not affect "extern inline"
functions.
-fkeep-static-consts
Emit variables declared "static const" when optimiza-
tion 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 refer-
enced, 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 non-automatic variable to have
distinct location, so using this option will result in
non-conforming behavior.
-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 sup-
port such instructions, which include x86, PowerPC,
IA-64 and S/390.
The default is -fbranch-count-reg, enabled when
-fstrength-reduce is enabled.
-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 pro-
grams 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 assump-
tions based on that.
The default is -fzero-initialized-in-bss.
-fstrength-reduce
Perform the optimizations of loop strength reduction
and elimination of iteration variables.
Enabled at levels -O2, -O3, -Os.
-fthread-jumps
Perform optimizations where we check to see if a jump
branches to a location where another comparison sub-
sumed 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 -O, -O2, -O3, -Os.
-fcse-follow-jumps
In common subexpression elimination, 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 which 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.
-frerun-loop-opt
Run the loop optimizer twice.
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 runtime performance
if you disable the global common subexpression elimi-
nation 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 which 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.
Enabled by default when gcse is enabled.
-floop-optimize
Perform loop optimizations: move constant expressions
out of loops, simplify exit test conditions and
optionally do strength-reduction and loop unrolling as
well.
Enabled at levels -O, -O2, -O3, -Os.
-fcrossjumping
Perform cross-jumping transformation. This transforma-
tion unifies equivalent code and save code size. The
resulting code may or may not perform better than
without cross-jumping.
Enabled at levels -O, -O2, -O3, -Os.
-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 avail-
able is controlled by "if-conversion2".
Enabled at levels -O, -O2, -O3, -Os.
-fif-conversion2
Use conditional execution (where available) to trans-
form conditional jumps into branch-less equivalents.
Enabled at levels -O, -O2, -O3, -Os.
-fdelete-null-pointer-checks
Use global dataflow analysis to identify and eliminate
useless checks for null pointers. The compiler
assumes that dereferencing a null pointer would have
halted the program. If a pointer is checked after it
has already been dereferenced, it cannot be null.
In some environments, this assumption is not true, and
programs can safely dereference null pointers. Use
-fno-delete-null-pointer-checks to disable this opti-
mization for programs which depend on that behavior.
Enabled at levels -O2, -O3, -Os.
-fexpensive-optimizations
Perform a number of minor optimizations that are rela-
tively expensive.
Enabled at levels -O2, -O3, -Os.
-foptimize-register-move
-fregmove
Attempt to reassign register numbers in move instruc-
tions 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.
-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, -Os.
-fschedule-insns2
Similar to -fschedule-insns, but requests an addi-
tional pass of instruction scheduling after register
allocation has been done. This is especially useful
on machines with a relatively small number of regis-
ters 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 instruc-
tions. This is normally enabled by default when
scheduling before register allocation, i.e. with
-fschedule-insns or at -O2 or higher.
-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.
-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.
-fmove-all-movables
Forces all invariant computations in loops to be moved
outside the loop.
-freduce-all-givs
Forces all general-induction variables in loops to be
strength-reduced.
Note: When compiling programs written in Fortran,
-fmove-all-movables and -freduce-all-givs are enabled
by default when you use the optimizer.
These options may generate better or worse code;
results are highly dependent on the structure of loops
within the source code.
These two options are intended to be removed someday,
once they have helped determine the efficacy of vari-
ous approaches to improving loop optimizations.
Please let us (<gcc@gcc.gnu.org> and <for-
tran@gnu.org>) know how use of these options affects
the performance of your production code. We're very
interested in code that runs slower when these options
are enabled.
-fno-peephole
-fno-peephole2
Disable any machine-specific peephole optimizations.
The difference between -fno-peephole and -fno-peep-
hole2 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.
-fbranch-probabilities
-fno-guess-branch-probability
Do not guess branch probabilities using a randomized
model.
Sometimes gcc will opt to use a randomized model to
guess branch probabilities, when none are available
from either profiling feedback (-fprofile-arcs) or
__builtin_expect. This means that different runs of
the compiler on the same program may produce different
object code.
In a hard real-time system, people don't want differ-
ent runs of the compiler to produce code that has dif-
ferent behavior; minimizing non-determinism is of
paramount import. This switch allows users to reduce
non-determinism, possibly at the expense of inferior
optimization.
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, -Os.
-freorder-functions
Reorder basic blocks in the compiled function in order
to reduce number of taken branches and improve code
locality. This is implemented by using special subsec-
tions "text.hot" for most frequently executed func-
tions and "text.unlikely" for unlikely executed func-
tions. 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
Allows 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() {
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() {
a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
Every language that wishes to perform language-spe-
cific alias analysis should define a function that
computes, given an "tree" node, an alias set for the
node. Nodes in different alias sets are not allowed
to alias. For an example, see the C front-end func-
tion "c_get_alias_set".
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, 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.
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, 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.
If n is not specified, 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-func-
tions. In this case, no dummy operations need be exe-
cuted.
If n is not specified, use a machine-dependent
default.
Enabled at levels -O2, -O3.
-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 pro-
cessors with lots of registers. It can, however, make
debugging impossible, since variables will no longer
stay in a ``home register''.
Enabled at levels -O3.
-fno-cprop-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.
Disabled at levels -O, -O2, -O3, -Os.
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 mem-
ory.
This option prevents undesirable excess precision on
machines such as the 68000 where the floating regis-
ters (of the 68881) keep more precision than a "dou-
ble" 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.
-ffast-math
Sets -fno-math-errno, -funsafe-math-optimizations,
-fno-trapping-math, -ffinite-math-only and -fno-sig-
naling-nans.
This option causes the preprocessor macro
"__FAST_MATH__" to be defined.
This option should never be turned on by any -O option
since it can result in incorrect output for programs
which depend on an exact implementation of IEEE or ISO
rules/specifications for math functions.
-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 should never be turned on by any -O option
since it can result in incorrect output for programs
which depend on an exact implementation of IEEE or ISO
rules/specifications for math functions.
The default is -fmath-errno.
-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 sim-
ilar optimizations.
This option should never be turned on by any -O option
since it can result in incorrect output for programs
which depend on an exact implementation of IEEE or ISO
rules/specifications for math functions.
The default is -fno-unsafe-math-optimizations.
-ffinite-math-only
Allow optimizations for floating-point arithmetic that
assume that arguments and results are not NaNs or
+-Infs.
This option should never be turned on by any -O option
since it can result in incorrect output for programs
which depend on an exact implementation of IEEE or ISO
rules/specifications.
The default is -fno-finite-math-only.
-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 implies
-fno-signaling-nans. Setting this option may allow
faster code if one relies on ``non-stop'' IEEE arith-
metic, for example.
This option should never be turned on by any -O option
since it can result in incorrect output for programs
which depend on an exact implementation of IEEE or ISO
rules/specifications for math functions.
The default is -ftrapping-math.
-fsignaling-nans
Compile code assuming that IEEE signaling NaNs may
generate user-visible traps during floating-point
operations. Setting this option disables optimiza-
tions that may change the number of exceptions visible
with signaling NaNs. This option implies -ftrap-
ping-math.
This option causes the preprocessor macro "__SUP-
PORT_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 constant as single precision con-
stant instead of implicitly converting it to double
precision constant.
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-proba-
bilities, to improve optimizations based on the number
of times each branch was taken. When the program com-
piled with -fprofile-arcs exits it saves arc execution
counts to a file called sourcename.da for each source
file The information in this data file is very depen-
dent on the structure of the generated code, so you
must use the same source code and the same optimiza-
tion 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 guess-
ing which path a branch is mostly to take, the
REG_BR_PROB values are used to exactly determine which
path is taken more often.
-fnew-ra
Use a graph coloring register allocator. Currently
this option is meant for testing, so we are interested
to hear about miscompilations with -fnew-ra.
-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 deter-
mined at compile time or upon entry to the loop.
-funroll-loops implies both -fstrength-reduce and
-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,
-fprefetch-loop-arrays
If supported by the target machine, generate instruc-
tions to prefetch memory to improve the performance of
loops that access large arrays.
Disabled at level -Os.
-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 per-
form 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 ben-
efits 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 spec-
ify this option and you may have problems with debug-
ging if you specify both this option and -g.
-fssa
Perform optimizations in static single assignment
form. Each function's flow graph is translated into
SSA form, optimizations are performed, and the flow
graph is translated back from SSA form. Users should
not specify this option, since it is not yet ready for
production use.
-fssa-ccp
Perform Sparse Conditional Constant Propagation in SSA
form. Requires -fssa. Like -fssa, this is an experi-
mental feature.
-fssa-dce
Perform aggressive dead-code elimination in SSA form.
Requires -fssa. Like -fssa, this is an experimental
feature.
--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 that a
certain number of instructions. You can control some
of these constants on the command-line using the
--param option.
In each case, the value is an integer. The allowable
choices for name are given in the following table:
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 compile
time increase with probably small improvement in
executable size.
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 compile time increase
with probably small improvement in executable run
time.
max-delay-slot-live-search
When trying to fill delay slots, the maximum num-
ber 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 com-
pile 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 optimiza-
tion will not be done.
max-gcse-passes
The maximum number of passes of GCSE to run.
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-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 represen-
tation) 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 300.
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 300.
max-inline-insns
The tree inliner does decrease the allowable size
for single functions to be inlined after we
already inlined the number of instructions given
here by repeated inlining. This number should be
a factor of two or more larger than the single
function limit. Higher numbers result in better
runtime performance, but incur higher compile-time
resource (CPU time, memory) requirements and
result in larger binaries. Very high values are
not advisable, as too large binaries may adversely
affect runtime performance. The default value is
600.
max-inline-slope
After exceeding the maximum number of inlined
instructions by repeated inlining, a linear func-
tion is used to decrease the allowable size for
single functions. The slope of that function is
the negative reciprocal of the number specified
here. The default value is 32.
min-inline-insns
The repeated inlining is throttled more and more
by the linear function after exceeding the limit.
To avoid too much throttling, a minimum for this
function is specified here to allow repeated
inlining for very small functions even when a lot
of repeated inlining already has been done. The
default value is 130.
max-inline-insns-rtl
For languages that use the RTL inliner (this hap-
pens at a later stage than tree inlining), you can
set the maximum allowable size (counted in RTL
instructions) for the RTL inliner with this param-
eter. The default value is 600.
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.
hot-bb-count-fraction
Select fraction of the maximal count of repeti-
tions of basic block in program given basic block
needs to have to be considered hot.
hot-bb-frequency-fraction
Select fraction of the maximal frequency of execu-
tions of basic block in function given basic block
needs to have to be considered hot
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 pro-
files (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 per-
cent).
tracer-min-branch-ratio
tracer-min-branch-ratio-feedback
Stop forward growth if the best edge do have prob-
ability lower than this threshold.
Similarly to tracer-dynamic-coverage two values
are present, one for compilation for profile feed-
back and one for compilation without. The value
for compilation with profile feedback needs to be
more conservative (higher) in order to make tracer
effective.
ggc-min-expand
GCC uses a garbage collector to manage its own
memory allocation. This parameter specifies the
minimum percentage by which the garbage collec-
tor'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, RLIMIT_RSS, RLIMIT_DATA and RLIMIT_AS.
If GCC is not able to calculate RAM on a particu-
lar 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 RAM/8, with a lower bound of 4096
(four megabytes) and an upper bound of 131072 (128
megabytes). If "getrlimit" is available, the
notion of "RAM" is the smallest of actual RAM,
RLIMIT_RSS, RLIMIT_DATA and RLIMIT_AS. If GCC is
not able to calculate RAM on a particular plat-
form, 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.
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 prepro-
cessing. Some of these options make sense only together
with -E because they cause the preprocessor output to be
unsuitable for actual compilation.
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, trans-
lated or interpreted by the compiler driver before being
passed to the preprocessor, and -Wp forcibly bypasses this
phase. The preprocessor's direct interface is undocu-
mented and subject to change, so whenever possible you
should avoid using -Wp and let the driver handle the
options instead.
-D name
Predefine name as a macro, with definition 1.
-D name=definition
Predefine name as a macro, with definition definition.
There are no restrictions on the contents of defini-
tion, but 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). Paren-
theses 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 macros. The com-
mon 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 direc-
tories. 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 .
-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 and -Wtri-
graphs. 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
Warn if any trigraphs are encountered. This option
used to take effect only if -trigraphs was also speci-
fied, but now works independently. Warnings are not
given for trigraphs within comments, as they do not
affect the meaning of the program.
-Wtraditional
Warn about certain constructs that behave differently
in traditional and ISO C. Also warn about ISO C con-
structs that have no traditional C equivalent, and
problematic constructs which should be avoided.
-Wimport
Warn the first time #import is used.
-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 -pedan-
tic but treats as warnings.
-M Instead of outputting the result of preprocessing,
output a rule suitable for make describing the depen-
dencies 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 basename of the
source file with any suffix replaced with object file
suffix. 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 out-
put 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
@anchor{-MF} 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 depen-
dency list without raising an error. The dependency
filename is taken directly from the "#include" direc-
tive 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 make-
files.
-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, including any path, deletes any file suf-
fix 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 take the basename of the input file 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 @pxref{-MF}), 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 compi-
lation process.
-MMD
Like -MD except mention only user header files, not
system -header files.
-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 con-
formance 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:
"iso9899:1990"
"c89"
The ISO C standard from 1990. c89 is the custom-
ary shorthand for this version of the standard.
The -ansi option is equivalent to -std=c89.
"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.
"gnu89"
The 1990 C standard plus GNU extensions. This is
the default.
"gnu99"
"gnu9x"
The 1999 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 head-
ers requested with "#include "file""; they are not
searched for "#include <file>". If additional direc-
tories are specified with -I options after the -I-,
those directories are searched for all #include direc-
tives.
In addition, -I- inhibits the use of the directory of
the current file directory as the first search direc-
tory for "#include "file"".
-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 preproces-
sor's working directory instead of the directory con-
taining 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 dec-
larations.
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.
-iprefix prefix
Specify prefix as the prefix for subsequent -iwithpre-
fix 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.
Use of these options is discouraged.
-isystem dir
Search dir for header files, after all directories
specified by -I but before the standard system direc-
tories. Mark it as a system directory, so that it
gets the same special treatment as is applied to the
standard system directories.
-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 com-
ments, 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 tok-
enizer 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 cre-
ated 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.
-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 -Apredicate(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 dur-
ing the execution of the preprocessor, including
predefined macros. This gives you a way of find-
ing 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.
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 preprocess-
ing. Both kinds of output go to the standard out-
put file.
N Like D, but emit only the macro names, not their
expansions.
I Output #include directives in addition to the
result of preprocessing.
-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 expan-
sion. 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 com-
ments.
-gcc
Define the macros __GNUC__, __GNUC_MINOR__ and
__GNUC_PATCHLEVEL__. These are defined automatically
when you use gcc -E; you can turn them off in that
case with -no-gcc.
-traditional-cpp
Try to imitate the behavior of old-fashioned C prepro-
cessors, 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.
-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.
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 con-
tents.) 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 rec-
ommended.)
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 spec-
ified. Thus, foo.o -lz bar.o searches library z after
file foo.o but before bar.o. If bar.o refers to func-
tions 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 libli-
brary.a. The linker then uses this file as if it had
been specified precisely by name.
The directories searched include several standard sys-
tem 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 program.
-nostartfiles
Do not use the standard system startup files when
linking. The standard system libraries are used nor-
mally, 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. The standard startup files are used normally,
unless -nostartfiles is used. The compiler may gener-
ate calls to memcmp, memset, and memcpy for System V
(and ISO C) environments or to bcopy and bzero for BSD
environments. These entries are usually resolved by
entries in libc. These entry points should be sup-
plied 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.
The compiler may generate calls to memcmp, memset, and
memcpy for System V (and ISO C) environments or to
bcopy and bzero for BSD environments. 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 that GCC uses to overcome shortcomings of
particular machines, or special needs for some lan-
guages.
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.)
-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 subop-
tions) 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 appli-
cation 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 typi-
cally 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 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 propa-
gate 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 lan-
guages used in the program, or using the option
-shared-libgcc, such that it is linked with the shared
libgcc.
-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.
-Xlinker option
Pass option as an option to the linker. You can use
this to supply system-specific linker options which
GCC does not know how to recognize.
If you want to pass an option that takes an 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.
-Wl,option
Pass option as an option to the linker. If option
contains commas, it is split into multiple options at
the commas.
-u symbol
Pretend the symbol symbol is undefined, to force link-
ing 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 direc-
tive are not inadvertently changed. If you really
need to change the search order for system directo-
ries, use the -nostdinc and/or -isystem options.
-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
which 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.
-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 com-
piler 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, which are /usr/lib/gcc/
and /usr/local/lib/gcc-lib/. 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 pre-
fix.
The run-time 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 pre-
fix 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 that 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.
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. Sometimes this is inconve-
nient, so GCC provides options that will switch to another
cross-compiler or version.
-b machine
The argument machine specifies the target machine for
compilation.
The value to use for machine is the same as was speci-
fied as the machine type when configuring GCC as a
cross-compiler. For example, if a cross-compiler was
configured with configure i386v, meaning to compile
for an 80386 running System V, then you would specify
-b i386v to run that cross compiler.
-V version
The argument version specifies which version of GCC to
run. This is useful when multiple versions are
installed. For example, version might be 2.0, meaning
to run GCC version 2.0.
The -V and -b options work by running the
<machine>-gcc-<version> executable, so there's no real
reason to use them if you can just run that directly.
Hardware Models and Configurations
Earlier we discussed the standard option -b which chooses
among different installed compilers for completely differ-
ent target machines, such as VAX vs. 68000 vs. 80386.
In addition, each of these 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 speci-
fied.
Some configurations of the compiler also support addi-
tional special options, usually for compatibility with
other compilers on the same platform.
These options are defined by the macro "TARGET_SWITCHES"
in the machine description. The default for the options
is also defined by that macro, which enables you to change
the defaults.
M680x0 Options
These are the -m options defined for the 68000 series.
The default values for these options depends on which
style of 68000 was selected when the compiler was config-
ured; the defaults for the most common choices are given
below.
-m68000
-mc68000
Generate output for a 68000. This is the default when
the compiler is configured for 68000-based systems.
Use this option for microcontrollers with a 68000 or
EC000 core, including the 68008, 68302, 68306, 68307,
68322, 68328 and 68356.
-m68020
-mc68020
Generate output for a 68020. This is the default when
the compiler is configured for 68020-based systems.
-m68881
Generate output containing 68881 instructions for
floating point. This is the default for most 68020
systems unless --nfp was specified when the compiler
was configured.
-m68030
Generate output for a 68030. This is the default when
the compiler is configured for 68030-based systems.
-m68040
Generate output for a 68040. This is the default when
the compiler is configured for 68040-based systems.
This option inhibits the use of 68881/68882 instruc-
tions 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.
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.
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'' family cpu.
This is the default when the compiler is configured
for 520X-based systems.
Use this option for microcontroller with a 5200 core,
including the MCF5202, MCF5203, MCF5204 and MCF5202.
-m68020-40
Generate output for a 68040, without using any of the
new instructions. This results in code which 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.
-m68020-60
Generate output for a 68060, without using any of the
new instructions. This results in code which 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.
-mfpa
Generate output containing Sun FPA instructions for
floating point.
-msoft-float
Generate output containing library calls for floating
point. Warning: the requisite libraries are not
available for all m68k targets. Normally the facili-
ties 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. The embedded
targets m68k-*-aout and m68k-*-coff do provide soft-
ware floating point support.
-mshort
Consider type "int" to be 16 bits wide, like "short
int".
-mnobitfield
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 com-
piler.
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. (Nor-
mally, 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.
-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 bound-
aries produces code that runs somewhat faster on pro-
cessors with 32-bit busses at the expense of more mem-
ory.
Warning: if you use the -malign-int switch, GCC will
align structures containing the above types differ-
ently 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.
M68hc1x Options
These are the -m options defined for the 68hc11 and 68hc12
microcontrollers. The default values for these options
depends on which style of microcontroller was selected
when the compiler was configured; the defaults for the
most common choices are given below.
-m6811
-m68hc11
Generate output for a 68HC11. This is the default
when the compiler is configured for 68HC11-based sys-
tems.
-m6812
-m68hc12
Generate output for a 68HC12. This is the default
when the compiler is configured for 68HC12-based sys-
tems.
-m68S12
-m68hcs12
Generate output for a 68HCS12.
-mauto-incdec
Enable the use of 68HC12 pre and post auto-increment
and auto-decrement addressing modes.
-minmax
-nominmax
Enable the use of 68HC12 min and max instructions.
-mlong-calls
-mno-long-calls
Treat all calls as being far away (near). If calls
are assumed to be far away, the compiler will use the
"call" instruction to call a function and the "rtc"
instruction for returning.
-mshort
Consider type "int" to be 16 bits wide, like "short
int".
-msoft-reg-count=count
Specify the number of pseudo-soft registers which are
used for the code generation. The maximum number is
32. Using more pseudo-soft register may or may not
result in better code depending on the program. The
default is 4 for 68HC11 and 2 for 68HC12.
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 han-
dle 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.
SPARC Options
These -m switches 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.
-mfpu
-mhard-float
Generate output containing floating point instruc-
tions. 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 facili-
ties 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 suit-
able 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 partic-
ular, 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 func-
tions 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 han-
dler 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-flat
-mflat
With -mflat, the compiler does not generate
save/restore instructions and will use a ``flat'' or
single register window calling convention. This model
uses %i7 as the frame pointer and is compatible with
the normal register window model. Code from either
may be intermixed. The local registers and the input
registers (0--5) are still treated as ``call saved''
registers and will be saved on the stack as necessary.
With -mno-flat (the default), the compiler emits
save/restore instructions (except for leaf functions)
and is the normal mode of operation.
-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.
-mv8
-msparclite
These two options select variations on the SPARC
architecture.
By default (unless specifically configured for the
Fujitsu SPARClite), GCC generates code for the v7
variant of the SPARC architecture.
-mv8 will give you SPARC v8 code. 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.
-msparclite will give you SPARClite code. This adds
the integer multiply, integer divide step and scan
("ffs") instructions which exist in SPARClite but not
in SPARC v7.
These options are deprecated and will be deleted in a
future GCC release. They have been replaced with
-mcpu=xxx.
-mcypress
-msupersparc
These two options select the processor for which the
code is optimized.
With -mcypress (the default), the compiler optimizes
code 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 -msupersparc the compiler optimizes code for the
SuperSPARC cpu, as used in the SPARCStation 10, 1000
and 2000 series. This flag also enables use of the
full SPARC v8 instruction set.
These options are deprecated and will be deleted in a
future GCC release. They have been replaced with
-mcpu=xxx.
-mcpu=cpu_type
Set the instruction set, register set, and instruction
scheduling parameters for machine type cpu_type. Sup-
ported values for cpu_type are v7, cypress, v8, super-
sparc, sparclite, hypersparc, sparclite86x, f930,
f934, sparclet, tsc701, v9, ultrasparc, and ultra-
sparc3.
Default instruction scheduling parameters are used for
values that select an architecture and not an imple-
mentation. These are v7, v8, sparclite, sparclet, v9.
Here is a list of each supported architecture and
their supported implementations.
v7: cypress
v8: supersparc, hypersparc
sparclite: f930, f934, sparclite86x
sparclet: tsc701
v9: ultrasparc, ultrasparc3
-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, f930, f934, spar-
clite86x, tsc701, ultrasparc, and ultrasparc3.
These -m switches are supported in addition to the above
on the SPARCLET processor.
-mlittle-endian
Generate code for a processor running in little-endian
mode.
-mlive-g0
Treat register %g0 as a normal register. GCC will
continue to clobber it as necessary but will not
assume it always reads as 0.
-mbroken-saverestore
Generate code that does not use non-trivial forms of
the "save" and "restore" instructions. Early versions
of the SPARCLET processor do not correctly handle
"save" and "restore" instructions used with arguments.
They correctly handle them used without arguments. A
"save" instruction used without arguments increments
the current window pointer but does not allocate a new
stack frame. It is assumed that the window overflow
trap handler will properly handle this case as will
interrupt handlers.
These -m switches are supported in addition to the above
on SPARC V9 processors in 64-bit environments.
-mlittle-endian
Generate code for a processor running in little-endian
mode.
-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=medlow
Generate code for the Medium/Low code model: the pro-
gram must be linked in the low 32 bits of the address
space. Pointers are 64 bits. Programs can be stati-
cally or dynamically linked.
-mcmodel=medmid
Generate code for the Medium/Middle code model: the
program must be linked in the low 44 bits of the
address space, the text segment must be less than 2G
bytes, and data segment must be within 2G of the text
segment. Pointers are 64 bits.
-mcmodel=medany
Generate code for the Medium/Anywhere code model: the
program may be linked anywhere in the address space,
the text segment must be less than 2G bytes, and data
segment must be within 2G of the text segment. Point-
ers are 64 bits.
-mcmodel=embmedany
Generate code for the Medium/Anywhere code model for
embedded systems: assume a 32-bit text and a 32-bit
data segment, both starting anywhere (determined at
link time). Register %g4 points to the base of the
data segment. Pointers are still 64 bits. Programs
are statically linked, PIC is not supported.
-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 ref-
erences. Otherwise, assume no such offset is present.
ARM Options
These -m options are defined for Advanced RISC Machines
(ARM) architectures:
-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 gen-
erated for leaf functions. The default is
-mno-apcs-frame.
-mapcs
This is a synonym for -mapcs-frame.
-mapcs-26
Generate code for a processor running with a 26-bit
program counter, and conforming to the function call-
ing standards for the APCS 26-bit option. This option
replaces the -m2 and -m3 options of previous releases
of the compiler.
-mapcs-32
Generate code for a processor running with a 32-bit
program counter, and conforming to the function call-
ing standards for the APCS 32-bit option. This option
replaces the -m6 option of previous releases of the
compiler.
-mthumb-interwork
Generate code which supports calling between the ARM
and Thumb instruction sets. Without this option 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.
-mno-sched-prolog
Prevent the reordering of instructions in the function
prolog, 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 infor-
mation can be used to locate the start if functions
inside an executable piece of code. The default is
-msched-prolog.
-mhard-float
Generate output containing floating point
instructions. This is the default.
-msoft-float
Generate output containing library calls for floating
point. Warning: the requisite libraries are not
available for all ARM targets. Normally the facili-
ties 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 suit-
able library functions for cross-compilation.
-msoft-float changes the calling convention in the
output file; therefore, it is only useful if you com-
pile all of a program with this option. In particu-
lar, you need to compile libgcc.a, the library that
comes with GCC, with -msoft-float in order for this to
work.
-mlittle-endian
Generate code for a processor running in little-endian
mode. This is the default for all standard configura-
tions.
-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 ver-
sions of the compiler prior to 2.8.
-malignment-traps
Generate code that will not trap if the MMU has align-
ment traps enabled. On ARM architectures prior to
ARMv4, there were no instructions to access half-word
objects stored in memory. However, when reading from
memory a feature of the ARM architecture allows a word
load to be used, even if the address is unaligned, and
the processor core will rotate the data as it is being
loaded. This option tells the compiler that such mis-
aligned accesses will cause a MMU trap and that it
should instead synthesize the access as a series of
byte accesses. The compiler can still use word
accesses to load half-word data if it knows that the
address is aligned to a word boundary.
This option is ignored when compiling for ARM archi-
tecture 4 or later, since these processors have
instructions to directly access half-word objects in
memory.
-mno-alignment-traps
Generate code that assumes that the MMU will not trap
unaligned accesses. This produces better code when
the target instruction set does not have half-word
memory operations (i.e. implementations prior to
ARMv4).
Note that you cannot use this option to access
unaligned word objects, since the processor will only
fetch one 32-bit aligned object from memory.
The default setting for most targets is -mno-align-
ment-traps, since this produces better code when there
are no half-word memory instructions available.
-mshort-load-bytes
-mno-short-load-words
These are deprecated aliases for -malignment-traps.
-mno-short-load-bytes
-mshort-load-words
This are deprecated aliases for -mno-alignment-traps.
-mcpu=name
This specifies the name of the target ARM processor.
GCC uses this name to determine what kind of instruc-
tions it can emit when generating assembly code. Per-
missible names are: arm2, arm250, arm3, arm6, arm60,
arm600, arm610, arm620, arm7, arm7m, arm7d, arm7dm,
arm7di, arm7dmi, arm70, arm700, arm700i, arm710,
arm710c, arm7100, arm7500, arm7500fe, arm7tdmi, arm8,
strongarm, strongarm110, strongarm1100, arm8, arm810,
arm9, arm9e, arm920, arm920t, arm940t, arm9tdmi,
arm10tdmi, arm1020t, xscale.
-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 instruc-
tions 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 imple-
mentations better performance can be obtained by using
this option.
-march=name
This specifies the name of the target ARM architec-
ture. 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, armv5te.
-mfpe=number
-mfp=number
This specifies the version of the floating point emu-
lation available on the target. Permissible values
are 2 and 3. -mfp= is a synonym for -mfpe=, for com-
patibility with older versions of GCC.
-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 and 32. The default
value varies for different toolchains. For the COFF
targeted toolchain the default value is 8. Specifying
the larger number can produce faster, more efficient
code, but can also increase the size of the program.
The two values are potentially incompatible. Code
compiled with one value cannot necessarily expect to
work with code or libraries compiled with the other
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 regis-
ter. 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 which 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 cur-
rent compilation unit, will not be turned into long
calls. The exception to this rule is that weak func-
tion definitions, functions with the long-call
attribute or the section attribute, and functions that
are within the scope of a #pragma long_calls direc-
tive, 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.
-mnop-fun-dllimport
Disable support for the "dllimport" attribute.
-msingle-pic-base
Treat the register used for PIC addressing as
read-only, rather than loading it in the prologue for
each function. The run-time 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.
-mpoke-function-name
Write the name of each function into the text section,
directly preceding the function prologue. The gener-
ated 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
Generate code for the 16-bit Thumb instruction set.
The default is to use the 32-bit ARM instruction set.
-mtpcs-frame
Generate a stack frame that is compliant with the
Thumb Procedure Call Standard for all non-leaf func-
tions. (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.
-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 execut-
ing a function pointer if this option is enabled.
MN10200 Options
These -m options are defined for Matsushita MN10200 archi-
tectures:
-mrelax
Indicate to the linker that it should perform a relax-
ation 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.
MN10300 Options
These -m options are defined for Matsushita MN10300 archi-
tectures:
-mmult-bug
Generate code to avoid bugs in the multiply instruc-
tions 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 which uses features specific to the AM33
processor.
-mno-am33
Do not generate code which uses features specific to
the AM33 processor. This is the default.
-mno-crt0
Do not link in the C run-time initialization object
file.
-mrelax
Indicate to the linker that it should perform a relax-
ation 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.
M32R/D Options
These -m options are defined for Mitsubishi M32R/D archi-
tectures:
-m32rx
Generate code for the M32R/X.
-m32r
Generate code for the M32R. This is the default.
-mcode-model=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.
-mcode-model=medium
Assume objects may be anywhere in the 32-bit address
space (the compiler will generate "seth/add3" instruc-
tions to load their addresses), and assume all subrou-
tines are reachable with the "bl" instruction.
-mcode-model=large
Assume objects may be anywhere in the 32-bit address
space (the compiler will generate "seth/add3" instruc-
tions 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.
M88K Options
These -m options are defined for Motorola 88k architec-
tures:
-m88000
Generate code that works well on both the m88100 and
the m88110.
-m88100
Generate code that works best for the m88100, but that
also runs on the m88110.
-m88110
Generate code that works best for the m88110, and may
not run on the m88100.
-mbig-pic
Obsolete option to be removed from the next revision.
Use -fPIC.
-midentify-revision
Include an "ident" directive in the assembler output
recording the source file name, compiler name and ver-
sion, timestamp, and compilation flags used.
-mno-underscores
In assembler output, emit symbol names without adding
an underscore character at the beginning of each name.
The default is to use an underscore as prefix on each
name.
-mocs-debug-info
-mno-ocs-debug-info
Include (or omit) additional debugging information
(about registers used in each stack frame) as speci-
fied in the 88open Object Compatibility Standard,
``OCS''. This extra information allows debugging of
code that has had the frame pointer eliminated. The
default for SVr4 and Delta 88 SVr3.2 is to include
this information; other 88k configurations omit this
information by default.
-mocs-frame-position
When emitting COFF debugging information for automatic
variables and parameters stored on the stack, use the
offset from the canonical frame address, which is the
stack pointer (register 31) on entry to the function.
The SVr4 and Delta88 SVr3.2, and BCS configurations
use -mocs-frame-position; other 88k configurations
have the default -mno-ocs-frame-position.
-mno-ocs-frame-position
When emitting COFF debugging information for automatic
variables and parameters stored on the stack, use the
offset from the frame pointer register (register 30).
When this option is in effect, the frame pointer is
not eliminated when debugging information is selected
by the -g switch.
-moptimize-arg-area
Save space by reorganizing the stack frame. This
option generates code that does not agree with the
88open specifications, but uses less memory.
-mno-optimize-arg-area
Do not reorganize the stack frame to save space. This
is the default. The generated conforms to the speci-
fication, but uses more memory.
-mshort-data-num
Generate smaller data references by making them rela-
tive to "r0", which allows loading a value using a
single instruction (rather than the usual two). You
control which data references are affected by specify-
ing num with this option. For example, if you specify
-mshort-data-512, then the data references affected
are those involving displacements of less than 512
bytes. -mshort-data-num is not effective for num
greater than 64k.
-mserialize-volatile
-mno-serialize-volatile
Do, or don't, generate code to guarantee sequential
consistency of volatile memory references. By
default, consistency is guaranteed.
The order of memory references made by the MC88110
processor does not always match the order of the
instructions requesting those references. In particu-
lar, a load instruction may execute before a preceding
store instruction. Such reordering violates sequen-
tial consistency of volatile memory references, when
there are multiple processors. When consistency must
be guaranteed, GCC generates special instructions, as
needed, to force execution in the proper order.
The MC88100 processor does not reorder memory refer-
ences and so always provides sequential consistency.
However, by default, GCC generates the special
instructions to guarantee consistency even when you
use -m88100, so that the code may be run on an MC88110
processor. If you intend to run your code only on the
MC88100 processor, you may use -mno-serial-
ize-volatile.
The extra code generated to guarantee consistency may
affect the performance of your application. If you
know that you can safely forgo this guarantee, you may
use -mno-serialize-volatile.
-msvr4
-msvr3
Turn on (-msvr4) or off (-msvr3) compiler extensions
related to System V release 4 (SVr4). This controls
the following:
1. Which variant of the assembler syntax to emit.
2. -msvr4 makes the C preprocessor recognize #pragma
weak that is used on System V release 4.
3. -msvr4 makes GCC issue additional declaration
directives used in SVr4.
-msvr4 is the default for the m88k-motorola-sysv4 con-
figuration. -msvr3 is the default for all other m88k
configurations.
-mversion-03.00
This option is obsolete, and is ignored.
-mno-check-zero-division
-mcheck-zero-division
Do, or don't, generate code to guarantee that integer
division by zero will be detected. By default, detec-
tion is guaranteed.
Some models of the MC88100 processor fail to trap upon
integer division by zero under certain conditions. By
default, when compiling code that might be run on such
a processor, GCC generates code that explicitly checks
for zero-valued divisors and traps with exception num-
ber 503 when one is detected. Use of
-mno-check-zero-division suppresses such checking for
code generated to run on an MC88100 processor.
GCC assumes that the MC88110 processor correctly
detects all instances of integer division by zero.
When -m88110 is specified, no explicit checks for
zero-valued divisors are generated, and both
-mcheck-zero-division and -mno-check-zero-division are
ignored.
-muse-div-instruction
Use the div instruction for signed integer division on
the MC88100 processor. By default, the div instruc-
tion is not used.
On the MC88100 processor the signed integer division
instruction div) traps to the operating system on a
negative operand. The operating system transparently
completes the operation, but at a large cost in execu-
tion time. By default, when compiling code that might
be run on an MC88100 processor, GCC emulates signed
integer division using the unsigned integer division
instruction divu), thereby avoiding the large penalty
of a trap to the operating system. Such emulation has
its own, smaller, execution cost in both time and
space. To the extent that your code's important
signed integer division operations are performed on
two nonnegative operands, it may be desirable to use
the div instruction directly.
On the MC88110 processor the div instruction (also
known as the divs instruction) processes negative
operands without trapping to the operating system.
When -m88110 is specified, -muse-div-instruction is
ignored, and the div instruction is used for signed
integer division.
Note that the result of dividing "INT_MIN" by -1 is
undefined. In particular, the behavior of such a
division with and without -muse-div-instruction may
differ.
-mtrap-large-shift
-mhandle-large-shift
Include code to detect bit-shifts of more than 31
bits; respectively, trap such shifts or emit code to
handle them properly. By default GCC makes no special
provision for large bit shifts.
-mwarn-passed-structs
Warn when a function passes a struct as an argument or
result. Structure-passing conventions have changed
during the evolution of the C language, and are often
the source of portability problems. By default, GCC
issues no such warning.
IBM RS/6000 and PowerPC Options
These -m options are defined for the IBM RS/6000 and Pow-
erPC:
-mpower
-mno-power
-mpower2
-mno-power2
-mpowerpc
-mno-powerpc
-mpowerpc-gpopt
-mno-powerpc-gpopt
-mpowerpc-gfxopt
-mno-powerpc-gfxopt
-mpowerpc64
-mno-powerpc64
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 Pow-
erPC instruction set is the architecture of the
Motorola MPC5xx, MPC6xx, MPC8xx microprocessors, and
the IBM 4xx microprocessors.
Neither architecture is a subset of the other. How-
ever there is a large common subset of instructions
supported by both. An MQ register is included in pro-
cessors 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 con-
figuring 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 instruc-
tions 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 Gen-
eral Purpose group, including floating-point square
root. Specifying -mpowerpc-gfxopt implies -mpowerpc
and also allows GCC to use the optional PowerPC archi-
tecture instructions in the Graphics group, including
floating-point select.
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 assem-
bler code. With -mnew-mnemonics, GCC uses the assem-
bler 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 some-
times 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 rios, rios1, rsc, rios2, rs64a, 601, 602, 603,
603e, 604, 604e, 620, 630, 740, 7400, 7450, 750,
power, power2, powerpc, 403, 505, 801, 821, 823, and
860 and common.
-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 instruc-
tions 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 pur-
poses.
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
other -m options as follows:
common
-mno-power, -mno-powerpc
power
power2
rios1
rios2
rsc -mpower, -mno-powerpc, -mno-new-mnemonics
powerpc
rs64a
602
603
603e
604
620
630
740
7400
7450
750
505 -mno-power, -mpowerpc, -mnew-mnemonics
601 -mpower, -mpowerpc, -mnew-mnemonics
403
821
860 -mno-power, -mpowerpc, -mnew-mnemonics,
-msoft-float
-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 speci-
fied, the code generated will use the architecture,
registers, and mnemonics set by -mcpu, but the
scheduling parameters set by -mtune.
-maltivec
-mno-altivec
These switches enable or disable the use of built-in
functions that allow access to the AltiVec instruction
set. You may also need to set -mabi=altivec to adjust
the current ABI with AltiVec ABI enhancements.
-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.
-misel=yes/no
-misel
This switch enables or disables the generation of ISEL
instructions.
-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 avail-
able 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 con-
serving 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-call
-mno-xl-call
On AIX, pass floating-point arguments to prototyped
functions beyond the register save area (RSA) on the
stack in addition to argument FPRs. 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. AIX XL compilers access
floating point arguments which 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 AIX
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 stan-
dard 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.
-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.
-mmultiple
-mno-multiple
Generate code that uses (does not use) the load multi-
ple 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 Pow-
erPC systems, since those instructions do not work
when the processor is in little endian mode. The
exceptions are PPC740 and PPC750 which permit the
instructions usage 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 the instructions usage 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.
-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 is used.
-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 would be
aligned to a 4 byte boundary and have a size of 4
bytes. By using -mno-bit-align, the structure would
be aligned to a 1 byte boundary and be one 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 han-
dled by the system.
-mrelocatable
-mno-relocatable
On embedded PowerPC systems generate code that allows
(does not allow) the program to be relocated to a dif-
ferent address at runtime. If you use -mrelocatable
on any module, all objects linked together must be
compiled with -mrelocatable or -mrelocatable-lib.
-mrelocatable-lib
-mno-relocatable-lib
On embedded PowerPC systems generate code that allows
(does not allow) the program to be relocated to a dif-
ferent address at runtime. Modules compiled with
-mrelocatable-lib can be linked with either modules
compiled without -mrelocatable and -mrelocatable-lib
or with modules compiled with 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.
-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 pow-
erpc-*-eabiaix.
-mcall-sysv-eabi
Specify both -mcall-sysv and -meabi options.
-mcall-sysv-noeabi
Specify both -mcall-sysv and -mno-eabi options.
-mcall-aix
On System V.4 and embedded PowerPC systems compile
code using calling conventions that are similar to
those used on AIX. This is the default if you config-
ured GCC using powerpc-*-eabiaix.
-mcall-solaris
On System V.4 and embedded PowerPC systems compile
code for the Solaris operating system.
-mcall-linux
On System V.4 and embedded PowerPC systems compile
code for the Linux-based GNU system.
-mcall-gnu
On System V.4 and embedded PowerPC systems compile
code for the Hurd-based GNU system.
-mcall-netbsd
On System V.4 and embedded PowerPC systems compile
code for the NetBSD 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=altivec
Extend the current ABI with AltiVec ABI extensions.
This does not change the default ABI, instead it adds
the AltiVec ABI extensions to the current ABI.
-mabi=no-altivec
Disable AltiVec ABI extensions for the current ABI.
-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 a 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.
-mwindiss
Specify that you are compiling for the WindISS simula-
tion environment.
-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 incom-
patible 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, other-
wise compile code the same as -msdata=sysv.
-msdata-data
On System V.4 and embedded PowerPC systems, put small
global and static data in the .sdata section. Put
small uninitialized global and static 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.
-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 com-
piled 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
Default to making all function calls via pointers, so
that functions which reside further than 64 megabytes
(67,108,864 bytes) from the current location can be
called. This setting can be overridden by the "short-
call" 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.
In the future, we may cause GCC to ignore all longcall
specifications when the linker is known to generate
glue.
-pthread
Adds support for multithreading with the pthreads
library. This option sets flags for both the prepro-
cessor and linker.
Darwin Options
These options are defined for all architectures running
the Darwin operating system. These are useful for compat-
ibility with other Mac OS compilers.
-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 specifies the executable that will be loading the
build output file being linked. See man ld(1) for more
information.
-allowable_client client_name
-arch_only
-client_name
-compatibility_version
-current_version
-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
-image_base
-init
-install_name
-keep_private_externs
-multi_module
-multiply_defined
-multiply_defined_unused
-noall_load
-nomultidefs
-noprebind
-noseglinkedit
-pagezero_size
-prebind
-prebind_all_twolevel_modules
-private_bundle
-read_only_relocs
-sectalign
-sectobjectsymbols
-whyload
-seg1addr
-sectcreate
-sectobjectsymbols
-sectorder
-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
This options are available for Darwin linker. Darwin
linker man page describes them in detail.
IBM RT Options
These -m options are defined for the IBM RT PC:
-min-line-mul
Use an in-line code sequence for integer multiplies.
This is the default.
-mcall-lib-mul
Call "lmul$$" for integer multiples.
-mfull-fp-blocks
Generate full-size floating point data blocks, includ-
ing the minimum amount of scratch space recommended by
IBM. This is the default.
-mminimum-fp-blocks
Do not include extra scratch space in floating point
data blocks. This results in smaller code, but slower
execution, since scratch space must be allocated
dynamically.
-mfp-arg-in-fpregs
Use a calling sequence incompatible with the IBM call-
ing convention in which floating point arguments are
passed in floating point registers. Note that
"stdarg.h" will not work with floating point operands
if this option is specified.
-mfp-arg-in-gregs
Use the normal calling convention for floating point
arguments. This is the default.
-mhc-struct-return
Return structures of more than one word in memory,
rather than in a register. This provides compatibil-
ity with the MetaWare HighC (hc) compiler. Use the
option -fpcc-struct-return for compatibility with the
Portable C Compiler (pcc).
-mnohc-struct-return
Return some structures of more than one word in regis-
ters, when convenient. This is the default. For com-
patibility with the IBM-supplied compilers, use the
option -fpcc-struct-return or the option
-mhc-struct-return.
MIPS Options
These -m options are defined for the MIPS family of com-
puters:
-march=arch
Generate code that will run on arch, which can be the
name of a generic MIPS ISA, or the name of a particu-
lar processor. The ISA names are: mips1, mips2,
mips3, mips4, mips32 and mips64. The processor names
are: r2000, r3000, r3900, r4000, vr4100, vr4300,
r4400, r4600, r4650, vr5000, r6000, r8000, 4kc, 4kp,
5kc, 20kc, orion, and sb1. 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).
In processor names, a final 000 can be abbreviated as
k (for example, -march=r2k). Prefixes are optional,
and vr may be written r.
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 archi-
tecture (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.
-mips64
Equivalent to -march=mips64.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating
point multiply and accumulate instructions, when they
are available. These instructions are generated by
default if they are available, but this may be unde-
sirable if the extra precision causes problems or on
certain chips in the mode where denormals are rounded
to zero where denormals generated by multiply and
accumulate instructions cause exceptions anyway.
-mfp32
Assume that floating point registers are 32 bits wide.
-mfp64
Assume that floating point registers are 64 bits wide.
-mgp32
Assume that general purpose registers are 32 bits
wide.
-mgp64
Assume that general purpose registers are 64 bits
wide.
-mint64
Force int and long types to be 64 bits wide. See
-mlong32 for an explanation of the default, and the
width of pointers.
-mlong64
Force long types to be 64 bits wide. See -mlong32 for
an explanation of the default, and the width of point-
ers.
-mlong32
Force long, int, and pointer types to be 32 bits wide.
The default size of ints, longs and pointers depends
on the ABI. All the supported ABIs use 32-bit ints.
The n64 ABI uses 64-bit longs, as does the 64-bit
Cygnus EABI; the others use 32-bit longs. Pointers
are the same size as longs, or the same size as inte-
ger registers, whichever is smaller.
-mabi=32
-mabi=o64
-mabi=n32
-mabi=64
-mabi=eabi
-mabi=meabi
Generate code for the given ABI.
Note that there are two embedded ABIs: -mabi=eabi
selects the one defined by Cygnus while -meabi=meabi
selects the one defined by MIPS. Both these ABIs have
32-bit and 64-bit variants. Normally, GCC will gener-
ate 64-bit code when you select a 64-bit architecture,
but you can use -mgp32 to get 32-bit code instead.
-mmips-as
Generate code for the MIPS assembler, and invoke mips-
tfile to add normal debug information. This is the
default for all platforms except for the OSF/1 refer-
ence platform, using the OSF/rose object format. If
the either of the -gstabs or -gstabs+ switches are
used, the mips-tfile program will encapsulate the
stabs within MIPS ECOFF.
-mgas
Generate code for the GNU assembler. This is the
default on the OSF/1 reference platform, using the
OSF/rose object format. Also, this is the default if
the configure option --with-gnu-as is used.
-msplit-addresses
-mno-split-addresses
Generate code to load the high and low parts of
address constants separately. This allows GCC to
optimize away redundant loads of the high order bits
of addresses. This optimization requires GNU as and
GNU ld. This optimization is enabled by default for
some embedded targets where GNU as and GNU ld are
standard.
-mrnames
-mno-rnames
The -mrnames switch says to output code using the MIPS
software names for the registers, instead of the hard-
ware names (ie, a0 instead of $4). The only known
assembler that supports this option is the Algorith-
mics assembler.
-mgpopt
-mno-gpopt
The -mgpopt switch says to write all of the data dec-
larations before the instructions in the text section,
this allows the MIPS assembler to generate one word
memory references instead of using two words for short
global or static data items. This is on by default if
optimization is selected.
-mstats
-mno-stats
For each non-inline function processed, the -mstats
switch causes the compiler to emit one line to the
standard error file to print statistics about the pro-
gram (number of registers saved, stack size, etc.).
-mmemcpy
-mno-memcpy
The -mmemcpy switch makes all block moves call the
appropriate string function (memcpy or bcopy) instead
of possibly generating inline code.
-mmips-tfile
-mno-mips-tfile
The -mno-mips-tfile switch causes the compiler not
postprocess the object file with the mips-tfile pro-
gram, after the MIPS assembler has generated it to add
debug support. If mips-tfile is not run, then no
local variables will be available to the debugger. In
addition, stage2 and stage3 objects will have the tem-
porary file names passed to the assembler embedded in
the object file, which means the objects will not com-
pare the same. The -mno-mips-tfile switch should only
be used when there are bugs in the mips-tfile program
that prevents compilation.
-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.
-mhard-float
Generate output containing floating point instruc-
tions. This is the default if you use the unmodified
sources.
-mabicalls
-mno-abicalls
Emit (or do not emit) the pseudo operations .abicalls,
.cpload, and .cprestore that some System V.4 ports use
for position independent code.
-mlong-calls
-mno-long-calls
Do all calls with the JALR instruction, which requires
loading up a function's address into a register before
the call. You need to use this switch, if you call
outside of the current 512 megabyte segment to func-
tions that are not through pointers.
-mhalf-pic
-mno-half-pic
Put pointers to extern references into the data sec-
tion and load them up, rather than put the references
in the text section.
-membedded-pic
-mno-embedded-pic
Generate PIC code suitable for some embedded systems.
All calls are made using PC relative address, and all
data is addressed using the $gp register. No more
than 65536 bytes of global data may be used. This
requires GNU as and GNU ld which do most of the work.
This currently only works on targets which use ECOFF;
it does not work with ELF.
-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 pre-
ferred for some embedded systems.
-muninit-const-in-rodata
-mno-uninit-const-in-rodata
When used together with -membedded-data, it will
always store uninitialized const variables in the
read-only data section.
-msingle-float
-mdouble-float
The -msingle-float switch tells gcc to assume that the
floating point coprocessor only supports single preci-
sion operations, as on the r4650 chip. The -mdou-
ble-float switch permits gcc to use double precision
operations. This is the default.
-mmad
-mno-mad
Permit use of the mad, madu and mul instructions, as
on the r4650 chip.
-m4650
Turns on -msingle-float, -mmad, and, at least for now,
-mcpu=r4650.
-mips16
-mno-mips16
Enable 16-bit instructions.
-mentry
Use the entry and exit pseudo ops. This option can
only be used with -mips16.
-EL Compile code for the processor in little endian mode.
The requisite libraries are assumed to exist.
-EB Compile code for the processor in big endian mode.
The requisite libraries are assumed to exist.
-G num
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. This allows the
assembler to emit one word memory reference instruc-
tions based on the global pointer (gp or $28), instead
of the normal two words used. By default, num is 8
when the MIPS assembler is used, and 0 when the GNU
assembler is used. The -G num switch is also passed
to the assembler and linker. All modules should be
compiled with the same -G num value.
-nocpp
Tell the MIPS assembler to not run its preprocessor
over user assembler files (with a .s suffix) when
assembling them.
-mfix7000
Pass an option to gas which will cause nops to be
inserted if the read of the destination register of an
mfhi or mflo instruction occurs in the following two
instructions.
-no-crt0
Do not include the default crt0.
-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 com-
mon "_flush_func()", that is, the address of the mem-
ory 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-likely
-mno-branch-likely
Enable or disable use of Branch Likely instructions,
regardless of the default for the selected architec-
ture. By default, Branch Likely instructions may be
generated if they are supported by the selected archi-
tecture. An exception is for the MIPS32 and MIPS64
architectures and processors which implement those
architectures; for those, Branch Likely instructions
will not be generated by default because the MIPS32
and MIPS64 architectures specifically deprecate their
use.
Intel 386 and AMD x86-64 Options
These -m options are defined for the i386 and x86-64 fam-
ily of computers:
-mcpu=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
i386, i486, i586, i686, pentium, pentium-mmx, pen-
tiumpro, pentium2, pentium3, pentium4, k6, k6-2, k6-3,
athlon, athlon-tbird, athlon-4, athlon-xp, athlon-mp,
winchip-c6, winchip2 and c3.
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
i386 without the -march=cpu-type option being used.
i586 is equivalent to pentium and i686 is equivalent
to pentiumpro. k6 and athlon are the AMD chips as
opposed to the Intel ones.
-march=cpu-type
Generate instructions for the machine type cpu-type.
The choices for cpu-type are the same as for -mcpu.
Moreover, specifying -march=cpu-type implies
-mcpu=cpu-type.
-m386
-m486
-mpentium
-mpentiumpro
These options are synonyms for -mcpu=i386, -mcpu=i486,
-mcpu=pentium, and -mcpu=pentiumpro respectively.
These synonyms are deprecated.
-mfpmath=unit
generate floating point arithmetics for selected unit
unit. the choices for unit are:
387 Use the standard 387 floating point coprocessor
present majority of chips and emulated otherwise.
Code compiled with this option will run almost
everywhere. The temporary results are computed in
80bit precision instead of 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 sup-
ports only single precision arithmetics, thus the
double and extended precision arithmetics is still
done using 387. Later version, present only in
Pentium4 and the future AMD x86-64 chips supports
double precision arithmetics too.
For i387 you need to use -march=cpu-type, -msse or
-msse2 switches to enable SSE extensions and make
this option effective. For x86-64 compiler, these
extensions are enabled by default.
The resulting code should be considerably faster
in majority of cases and avoid the numerical
instability problems of 387 code, but may break
some existing code that expects temporaries to be
80bit.
This is the default choice for x86-64 compiler.
sse,387
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 experi-
mental, because gcc register allocator does not
model separate functional units well resulting in
instable performance.
-masm=dialect
Output asm instructions using selected dialect. Sup-
ported choices are intel or att (the default one).
-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 val-
ues of types "float" and "double" in an FPU register,
even if there is no FPU. The idea is that the operat-
ing 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 instruc-
tion 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 will produce code that runs somewhat
faster on a Pentium at the expense of more memory.
Warning: if you use the -malign-double switch, struc-
tures containing the above types will be aligned dif-
ferently than the published application binary inter-
face specifications for the 386 and will not be binary
compatible with structures in code compiled without
that switch.
-m128bit-long-double
Control the size of "long double" type. i386 applica-
tion binary interface specify the size to be 12 bytes,
while modern architectures (Pentium and newer) prefer
"long double" aligned to 8 or 16 byte boundary. This
is impossible to reach with 12 byte long doubles in
the array accesses.
Warning: if you use the -m128bit-long-double switch,
the structures and arrays containing "long double"
will change their size as well as function calling
convention for function taking "long double" will be
modified.
-m96bit-long-double
Set the size of "long double" to 96 bits as required
by the i386 application binary interface. This is the
default.
-msvr3-shlib
-mno-svr3-shlib
Control whether GCC places uninitialized local vari-
ables into the "bss" or "data" segments. -msvr3-shlib
places them into "bss". These options are meaningful
only on System V Release 3.
-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 argu-
ments 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 com-
piler.
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. (Nor-
mally, 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.
-mpreferred-stack-boundary=num
Attempt to keep the stack boundary aligned to a 2
raised to num byte boundary. If -mpre-
ferred-stack-boundary is not specified, the default is
4 (16 bytes or 128 bits), except when optimizing for
code size (-Os), in which case the default is the min-
imum correct alignment (4 bytes for x86, and 8 bytes
for x86-64).
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 perfor-
mance penalties. On Pentium III, the Streaming SIMD
Extension (SSE) data type "__m128" suffers similar
penalties 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 sen-
sitive 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
-m3dnow
-mno-3dnow
These switches enable or disable the use of built-in
functions that allow direct access to the MMX, SSE and
3Dnow extensions of the instruction set.
To have SSE/SSE2 instructions generated automatically
from floating-point code, see -mfpmath=sse.
-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 per-
formance 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 han-
dling 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
don't know about it.
-minline-all-stringops
By default GCC inlines string operations only when
destination is known to be aligned at least to 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.
-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 reg-
ister available in leaf functions. The option
-fomit-frame-pointer removes the frame pointer for all
functions which might make debugging harder.
These -m switches are supported in addition to the above
on AMD x86-64 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 and generates code that runs on any i386 system.
The 64-bit environment sets int to 32 bits and long
and pointer to 64 bits and generates code for AMD's
x86-64 architecture.
-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 han-
dlers 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 but sym-
bols can be located anywhere in the address space.
Programs can be statically or dynamically linked, but
building of shared libraries are not supported with
the medium model.
-mcmodel=large
Generate code for the large model: This model makes no
assumptions about addresses and sizes of sections.
Currently GCC does not implement this model.
HPPA Options
These -m options are defined for the HPPA family of com-
puters:
-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.
PA 2.0 support currently requires gas snapshot
19990413 or later. The next release of binutils (cur-
rent is 2.9.1) will probably contain PA 2.0 support.
-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
which perform lazy context switching of floating point
registers. If you use this option and attempt to per-
form floating point operations, the compiler will
abort.
-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 indi-
rect calls and use unscaled index address modes.
Such code is suitable for level 0 PA systems and ker-
nels.
-mfast-indirect-calls
Generate code that assumes calls never cross space
boundaries. This allows GCC to emit code which per-
forms faster indirect calls.
This option will not work in the presence of shared
libraries or nested functions.
-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 under-
stands.
-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 facili-
ties 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 suit-
able library functions for cross-compilation. The
embedded target hppa1.1-*-pro does provide software
floating point support.
-msoft-float changes the calling convention in the
output file; therefore, it is only useful if you com-
pile all of a program with this option. In particu-
lar, 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`.
-mhp-ld
Use HP ld specific options. This passes -b to ld when
building a shared library and passes +Accept TypeMis-
match 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 deter-
mined 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`.
-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 link-
ing is used to build the application.
The types of long calls used depends on the capabili-
ties 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. How-
ever, an indirect call is used on 32-bit ELF systems
in pic code and it is quite long.
-nolibdld
Suppress the generation of link options to search lib-
dld.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 neces-
sary 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 gener-
ate 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 960 Options
These -m options are defined for the Intel 960 implementa-
tions:
-mcpu-type
Assume the defaults for the machine type cpu-type for
some of the other options, including instruction
scheduling, floating point support, and addressing
modes. The choices for cpu-type are ka, kb, mc, ca,
cf, sa, and sb. The default is kb.
-mnumerics
-msoft-float
The -mnumerics option indicates that the processor
does support floating-point instructions. The
-msoft-float option indicates that floating-point sup-
port should not be assumed.
-mleaf-procedures
-mno-leaf-procedures
Do (or do not) attempt to alter leaf procedures to be
callable with the "bal" instruction as well as "call".
This will result in more efficient code for explicit
calls when the "bal" instruction can be substituted by
the assembler or linker, but less efficient code in
other cases, such as calls via function pointers, or
using a linker that doesn't support this optimization.
-mtail-call
-mno-tail-call
Do (or do not) make additional attempts (beyond those
of the machine-independent portions of the compiler)
to optimize tail-recursive calls into branches. You
may not want to do this because the detection of cases
where this is not valid is not totally complete. The
default is -mno-tail-call.
-mcomplex-addr
-mno-complex-addr
Assume (or do not assume) that the use of a complex
addressing mode is a win on this implementation of the
i960. Complex addressing modes may not be worthwhile
on the K-series, but they definitely are on the
C-series. The default is currently -mcomplex-addr for
all processors except the CB and CC.
-mcode-align
-mno-code-align
Align code to 8-byte boundaries for faster fetching
(or don't bother). Currently turned on by default for
C-series implementations only.
-mic-compat
-mic2.0-compat
-mic3.0-compat
Enable compatibility with iC960 v2.0 or v3.0.
-masm-compat
-mintel-asm
Enable compatibility with the iC960 assembler.
-mstrict-align
-mno-strict-align
Do not permit (do permit) unaligned accesses.
-mold-align
Enable structure-alignment compatibility with Intel's
gcc release version 1.3 (based on gcc 1.37). This
option implies -mstrict-align.
-mlong-double-64
Implement type long double as 64-bit floating point
numbers. Without the option long double is imple-
mented by 80-bit floating point numbers. The only
reason we have it because there is no 128-bit long
double support in fp-bit.c yet. So it is only useful
for people using soft-float targets. Otherwise, we
should recommend against use of it.
DEC Alpha Options
These -m options are defined for the DEC Alpha implementa-
tions:
-mno-soft-float
-msoft-float
Use (do not use) the hardware floating-point instruc-
tions 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 float-
ing-point operations, or compiled in such a way as to
call such emulations routines, these routines will
issue floating-point operations. If you are compil-
ing 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 regis-
ters.
-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 inte-
ger 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 com-
piled 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 hard-
ware optimized for maximum performance. It is mostly
compliant with the IEEE floating point standard. How-
ever, for full compliance, software assistance is
required. This option generates code fully IEEE com-
pliant code except that the inexact-flag is not main-
tained (see below). If this option is turned on, the
preprocessor macro "_IEEE_FP" is defined during compi-
lation. 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 su, but the instructions are marked to be
safe for software completion (see Alpha architec-
ture 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 architec-
ture reference manual) controls the rounding mode
in effect. The C library initializes this regis-
ter for rounding towards plus infinity. Thus,
unless your program modifies the fpcr, d corre-
sponds 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 pro-
gram 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 pro-
gram caused a floating point exception.
f Function precision. The trap handler can deter-
mine 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 confor-
mant. 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 runtime.
Use this option to require GCC to construct all inte-
ger 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 instruc-
tions. 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 reloca-
tions off of the $gp register. This limits the size
of the small data area to 64KB, but allows the vari-
ables to be directly accessed via a single instruc-
tion.
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.
-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 pro-
cessor type, GCC will default to the processor on
which the compiler was built.
Supported values for cpu_type are
ev4
ev45
21064
Schedules as an EV4 and has no instruction set
extensions.
ev5
21164
Schedules as an EV5 and has no instruction set
extensions.
ev56
21164a
Schedules as an EV5 and supports the BWX exten-
sion.
pca56
21164pc
21164PC
Schedules as an EV5 and supports the BWX and MAX
extensions.
ev6
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.
-mtune=cpu_type
Set only the instruction scheduling parameters for
machine type cpu_type. The instruction set is not
changed.
-mmemory-latency=time
Sets the latency the scheduler should assume for typi-
cal 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.
L1
L2
L3
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 imple-
mentations:
-mvms-return-codes
Return VMS condition codes from main. The default is
to return POSIX style condition (e.g. error) codes.
H8/300 Options
These -m options are defined for the H8/300 implementa-
tions:
-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 bound-
aries. This option has no effect on the H8/300.
SH Options
These -m options are defined for the SH implementations:
-m1 Generate code for the SH1.
-m2 Generate code for the SH2.
-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.
-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 func-
tions 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.
-mfmovd
Enable the use of the instruction "fmovd".
-mhitachi
Comply with the calling conventions defined by
Hitachi.
-mnomacsave
Mark the "MAC" register as call-clobbered, even if
-mhitachi is given.
-mieee
Increase IEEE-compliance of floating-point code.
-misize
Dump instruction size and location in the assembly
code.
-mpadstruct
This option is deprecated. It pads structures to mul-
tiple of 4 bytes, which is incompatible with the SH
ABI.
-mspace
Optimize for space instead of speed. Implied by -Os.
-mprefergot
When generating position-independent code, emit func-
tion calls using the Global Offset Table instead of
the Procedure Linkage Table.
-musermode
Generate a library function call to invalidate
instruction cache entries, after fixing up a trampo-
line. This library function call doesn't assume it
can write to the whole memory address space. This is
the default when the target is "sh-*-linux*".
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 -sym-
bolic or -shared be used instead.
-Qy Identify the versions of each tool used by the com-
piler, in a ".ident" assembler directive in the out-
put.
-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.
TMS320C3x/C4x Options
These -m options are defined for TMS320C3x/C4x implementa-
tions:
-mcpu=cpu_type
Set the instruction set, register set, and instruction
scheduling parameters for machine type cpu_type. Sup-
ported values for cpu_type are c30, c31, c32, c40, and
c44. The default is c40 to generate code for the
TMS320C40.
-mbig-memory
-mbig
-msmall-memory
-msmall
Generates code for the big or small memory model. The
small memory model assumed that all data fits into one
64K word page. At run-time the data page (DP) regis-
ter must be set to point to the 64K page containing
the .bss and .data program sections. The big memory
model is the default and requires reloading of the DP
register for every direct memory access.
-mbk
-mno-bk
Allow (disallow) allocation of general integer
operands into the block count register BK.
-mdb
-mno-db
Enable (disable) generation of code using decrement
and branch, DBcond(D), instructions. This is enabled
by default for the C4x. To be on the safe side, this
is disabled for the C3x, since the maximum iteration
count on the C3x is 2^{23 + 1} (but who iterates loops
more than 2^{23} times on the C3x?). Note that GCC
will try to reverse a loop so that it can utilize the
decrement and branch instruction, but will give up if
there is more than one memory reference in the loop.
Thus a loop where the loop counter is decremented can
generate slightly more efficient code, in cases where
the RPTB instruction cannot be utilized.
-mdp-isr-reload
-mparanoid
Force the DP register to be saved on entry to an
interrupt service routine (ISR), reloaded to point to
the data section, and restored on exit from the ISR.
This should not be required unless someone has vio-
lated the small memory model by modifying the DP reg-
ister, say within an object library.
-mmpyi
-mno-mpyi
For the C3x use the 24-bit MPYI instruction for inte-
ger multiplies instead of a library call to guarantee
32-bit results. Note that if one of the operands is a
constant, then the multiplication will be performed
using shifts and adds. If the -mmpyi option is not
specified for the C3x, then squaring operations are
performed inline instead of a library call.
-mfast-fix
-mno-fast-fix
The C3x/C4x FIX instruction to convert a floating
point value to an integer value chooses the nearest
integer less than or equal to the floating point value
rather than to the nearest integer. Thus if the
floating point number is negative, the result will be
incorrectly truncated an additional code is necessary
to detect and correct this case. This option can be
used to disable generation of the additional code
required to correct the result.
-mrptb
-mno-rptb
Enable (disable) generation of repeat block sequences
using the RPTB instruction for zero overhead looping.
The RPTB construct is only used for innermost loops
that do not call functions or jump across the loop
boundaries. There is no advantage having nested RPTB
loops due to the overhead required to save and restore
the RC, RS, and RE registers. This is enabled by
default with -O2.
-mrpts=count
-mno-rpts
Enable (disable) the use of the single instruction
repeat instruction RPTS. If a repeat block contains a
single instruction, and the loop count can be guaran-
teed to be less than the value count, GCC will emit a
RPTS instruction instead of a RPTB. If no value is
specified, then a RPTS will be emitted even if the
loop count cannot be determined at compile time. Note
that the repeated instruction following RPTS does not
have to be reloaded from memory each iteration, thus
freeing up the CPU buses for operands. However, since
interrupts are blocked by this instruction, it is dis-
abled by default.
-mloop-unsigned
-mno-loop-unsigned
The maximum iteration count when using RPTS and RPTB
(and DB on the C40) is 2^{31 + 1} since these instruc-
tions test if the iteration count is negative to ter-
minate the loop. If the iteration count is unsigned
there is a possibility than the 2^{31 + 1} maximum
iteration count may be exceeded. This switch allows
an unsigned iteration count.
-mti
Try to emit an assembler syntax that the TI assembler
(asm30) is happy with. This also enforces compatibil-
ity with the API employed by the TI C3x C compiler.
For example, long doubles are passed as structures
rather than in floating point registers.
-mregparm
-mmemparm
Generate code that uses registers (stack) for passing
arguments to functions. By default, arguments are
passed in registers where possible rather than by
pushing arguments on to the stack.
-mparallel-insns
-mno-parallel-insns
Allow the generation of parallel instructions. This
is enabled by default with -O2.
-mparallel-mpy
-mno-parallel-mpy
Allow the generation of MPY||ADD and MPY||SUB parallel
instructions, provided -mparallel-insns is also speci-
fied. These instructions have tight register con-
straints which can pessimize the code generation of
large functions.
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 prolog and epilog of a func-
tion. 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 pre-
sent, this just turns on the -mep and -mprolog-func-
tion 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.
-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 are defined then a
default target processor will be chosen and the rele-
vant __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 flavors of the v850 archi-
tecture. The default is -mno-disable-callt which
allows the CALLT instruction to be used.
ARC Options
These options are defined for ARC implementations:
-EL Compile code for little endian mode. This is the
default.
-EB Compile code for big endian mode.
-mmangle-cpu
Prepend the name of the cpu to all public symbol
names. In multiple-processor systems, there are many
ARC variants with different instruction and register
set characteristics. This flag prevents code compiled
for one cpu to be linked with code compiled for
another. No facility exists for handling variants
that are ``almost identical''. This is an all or
nothing option.
-mcpu=cpu
Compile code for ARC variant cpu. Which variants are
supported depend on the configuration. All variants
support -mcpu=base, this is the default.
-mtext=text-section
-mdata=data-section
-mrodata=readonly-data-section
Put functions, data, and readonly data in text-sec-
tion, data-section, and readonly-data-section respec-
tively by default. This can be overridden with the
"section" attribute.
NS32K Options
These are the -m options defined for the 32000 series.
The default values for these options depends on which
style of 32000 was selected when the compiler was config-
ured; the defaults for the most common choices are given
below.
-m32032
-m32032
Generate output for a 32032. This is the default when
the compiler is configured for 32032 and 32016 based
systems.
-m32332
-m32332
Generate output for a 32332. This is the default when
the compiler is configured for 32332-based systems.
-m32532
-m32532
Generate output for a 32532. This is the default when
the compiler is configured for 32532-based systems.
-m32081
Generate output containing 32081 instructions for
floating point. This is the default for all systems.
-m32381
Generate output containing 32381 instructions for
floating point. This also implies -m32081. The 32381
is only compatible with the 32332 and 32532 cpus.
This is the default for the pc532-netbsd configura-
tion.
-mmulti-add
Try and generate multiply-add floating point instruc-
tions "polyF" and "dotF". This option is only avail-
able if the -m32381 option is in effect. Using these
instructions requires changes to register allocation
which generally has a negative impact on performance.
This option should only be enabled when compiling code
particularly likely to make heavy use of multiply-add
instructions.
-mnomulti-add
Do not try and generate multiply-add floating point
instructions "polyF" and "dotF". This is the default
on all platforms.
-msoft-float
Generate output containing library calls for floating
point. Warning: the requisite libraries may not be
available.
-mieee-compare
-mno-ieee-compare
Control whether or not the compiler uses IEEE floating
point comparisons. These handle correctly the case
where the result of a comparison is unordered. Warn-
ing: the requisite kernel support may not be avail-
able.
-mnobitfield
Do not use the bit-field instructions. On some
machines it is faster to use shifting and masking
operations. This is the default for the pc532.
-mbitfield
Do use the bit-field instructions. This is the
default for all platforms except the pc532.
-mrtd
Use a different function-calling convention, in which
functions that take a fixed number of arguments return
pop their arguments on return with the "ret" instruc-
tion.
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 com-
piler.
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. (Nor-
mally, extra arguments are harmlessly ignored.)
This option takes its name from the 680x0 "rtd"
instruction.
-mregparam
Use a different function-calling convention where the
first two arguments are passed in registers.
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 com-
piler.
-mnoregparam
Do not pass any arguments in registers. This is the
default for all targets.
-msb
It is OK to use the sb as an index register which is
always loaded with zero. This is the default for the
pc532-netbsd target.
-mnosb
The sb register is not available for use or has not
been initialized to zero by the run time system. This
is the default for all targets except the
pc532-netbsd. It is also implied whenever -mhimem or
-fpic is set.
-mhimem
Many ns32000 series addressing modes use displacements
of up to 512MB. If an address is above 512MB then
displacements from zero can not be used. This option
causes code to be generated which can be loaded above
512MB. This may be useful for operating systems or
ROM code.
-mnohimem
Assume code will be loaded in the first 512MB of vir-
tual address space. This is the default for all plat-
forms.
AVR Options
These options are defined for AVR implementations:
-mmcu=mcu
Specify ATMEL AVR instruction set or MCU type.
Instruction set avr1 is for the minimal AVR core, not
supported by the C compiler, only for assembler pro-
grams (MCU types: at90s1200, attiny10, attiny11,
attiny12, attiny15, attiny28).
Instruction set avr2 (default) is for the classic AVR
core with up to 8K program memory space (MCU types:
at90s2313, at90s2323, attiny22, at90s2333, at90s2343,
at90s4414, at90s4433, at90s4434, at90s8515, at90c8534,
at90s8535).
Instruction set avr3 is for the classic AVR core with
up to 128K program memory space (MCU types: atmega103,
atmega603, at43usb320, at76c711).
Instruction set avr4 is for the enhanced AVR core with
up to 8K program memory space (MCU types: atmega8,
atmega83, atmega85).
Instruction set avr5 is for the enhanced AVR core with
up to 128K program memory space (MCU types: atmega16,
atmega161, atmega163, atmega32, atmega323, atmega64,
atmega128, at43usb355, at94k).
-msize
Output instruction sizes to the asm file.
-minit-stack=N
Specify the initial stack address, which may be a sym-
bol or numeric value, __stack is the default.
-mno-interrupts
Generated code is not compatible with hardware inter-
rupts. Code size will be smaller.
-mcall-prologues
Functions prologues/epilogues expanded as call to
appropriate subroutines. Code size will be smaller.
-mno-tablejump
Do not generate tablejump insns which sometimes
increase code size.
-mtiny-stack
Change only the low 8 bits of the stack pointer.
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 four 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.
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 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 reg-
ister. 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.
-mb-step
Generate code that works around Itanium B step errata.
-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.
-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-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.
-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 regis-
ter ranges can be specified separated by a comma.
D30V Options
These -m options are defined for D30V implementations:
-mextmem
Link the .text, .data, .bss, .strings, .rodata,
.rodata1, .data1 sections into external memory, which
starts at location 0x80000000.
-mextmemory
Same as the -mextmem switch.
-monchip
Link the .text section into onchip text memory, which
starts at location 0x0. Also link .data, .bss,
.strings, .rodata, .rodata1, .data1 sections into
onchip data memory, which starts at location
0x20000000.
-mno-asm-optimize
-masm-optimize
Disable (enable) passing -O to the assembler when
optimizing. The assembler uses the -O option to auto-
matically parallelize adjacent short instructions
where possible.
-mbranch-cost=n
Increase the internal costs of branches to n. Higher
costs means that the compiler will issue more instruc-
tions to avoid doing a branch. The default is 2.
-mcond-exec=n
Specify the maximum number of conditionally executed
instructions that replace a branch. The default is 4.
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 instruc-
tions 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.
-mbackchain
-mno-backchain
Generate (or do not generate) code which maintains an
explicit backchain within the stack frame that points
to the caller's frame. This is currently needed to
allow debugging. The default is to generate the
backchain.
-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
Linux for S/390 ABI. When -m64 is specified, generate
code compliant to the Linux for zSeries ABI. This
allows GCC in particular to generate 64-bit instruc-
tions. For the s390 targets, the default is -m31,
while the s390x targets default to -m64.
-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.
-mdebug
-mno-debug
Print (or do not print) additional debug information
when compiling. The default is to not print debug
information.
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.
-melinux-stacksize=n
Only available with the cris-axis-aout target.
Arranges for indications in the program to the kernel
loader that the stack of the program should be set to
n bytes.
-metrax4
-metrax100
The options -metrax4 and -metrax100 are synonyms for
-march=v3 and -march=v8 respectively.
-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 address-
ing 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 pro-
logue and epilogue that sets 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 func-
tions from the PLT part of the GOT rather than (tradi-
tional on other architectures) calls to the PLT. The
default is -mgotplt.
-maout
Legacy no-op option only recognized with the cris-
axis-aout target.
-melf
Legacy no-op option only recognized with the cris-
axis-elf and cris-axis-linux-gnu targets.
-melinux
Only recognized with the cris-axis-aout target, where
it selects a GNU/linux-like multilib, include files
and instruction set for -march=v8.
-mlinux
Legacy no-op option only recognized with the cris-
axis-linux-gnu target.
-sim
This option, recognized for the cris-axis-aout and
cris-axis-elf arranges to link with input-output func-
tions from a simulator library. Code, initialized
data and zero-initialized data are allocated consecu-
tively.
-sim2
Like -sim, but pass linker options to locate initial-
ized data at 0x40000000 and zero-initialized data at
0x80000000.
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 instruc-
tions 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" assem-
bly 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 gener-
ates a request (handled by the assembler and the
linker) for a constant to be set up in a global regis-
ter. 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.
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 sup-
ported.)
-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 "movstrhi" patterns for copying memory.
This is the default.
-mbcopy
Do not use inline "movstrhi" patterns for copying mem-
ory.
-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.
-msplit
Generate code for a system with split I&D.
-mno-split
Generate code for a system without split I&D. 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.
Xstormy16 Options
These options are defined for Xstormy16:
-msim
Choose startup files and linker script suitable for
the simulator.
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 opera-
tions.
-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 reg-
isters, 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.
-mlibrary-pic
Enable PIC support for building libraries
-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 instruc-
tions.
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.
-mtomcat-stats
Cause gas to print out tomcat statistics.
-mcpu=cpu
Select the processor type for which to generate code.
Possible values are simple, tomcat, fr500, fr400,
fr300, frv.
Xtensa Options
The Xtensa architecture is designed to support many dif-
ferent configurations. The compiler's default options can
be set to match a particular Xtensa configuration by copy-
ing a configuration file into the GCC sources when build-
ing GCC. The options below may be used to override the
default options.
-mbig-endian
-mlittle-endian
Specify big-endian or little-endian byte ordering for
the target Xtensa processor.
-mdensity
-mno-density
Enable or disable use of the optional Xtensa code den-
sity instructions.
-mmac16
-mno-mac16
Enable or disable use of the Xtensa MAC16 option.
When enabled, GCC will generate MAC16 instructions
from standard C code, with the limitation that it will
use neither the MR register file nor any instruction
that operates on the MR registers. When this option
is disabled, GCC will translate 16-bit multiply/accu-
mulate operations to a combination of core instruc-
tions and library calls, depending on whether any
other multiplier options are enabled.
-mmul16
-mno-mul16
Enable or disable use of the 16-bit integer multiplier
option. When enabled, the compiler will generate
16-bit multiply instructions for multiplications of 16
bits or smaller in standard C code. When this option
is disabled, the compiler will either use 32-bit mul-
tiply or MAC16 instructions if they are available or
generate library calls to perform the multiply opera-
tions using shifts and adds.
-mmul32
-mno-mul32
Enable or disable use of the 32-bit integer multiplier
option. When enabled, the compiler will generate
32-bit multiply instructions for multiplications of 32
bits or smaller in standard C code. When this option
is disabled, the compiler will generate library calls
to perform the multiply operations using either shifts
and adds or 16-bit multiply instructions if they are
available.
-mnsa
-mno-nsa
Enable or disable use of the optional normalization
shift amount ("NSA") instructions to implement the
built-in "ffs" function.
-mminmax
-mno-minmax
Enable or disable use of the optional minimum and max-
imum value instructions.
-msext
-mno-sext
Enable or disable use of the optional sign extend
("SEXT") instruction.
-mbooleans
-mno-booleans
Enable or disable support for the boolean register
file used by Xtensa coprocessors. This is not typi-
cally useful by itself but may be required for other
options that make use of the boolean registers (e.g.,
the floating-point option).
-mhard-float
-msoft-float
Enable or disable use of the floating-point option.
When enabled, GCC generates floating-point instruc-
tions for 32-bit "float" operations. When this option
is disabled, GCC generates library calls to emulate
32-bit floating-point operations using integer
instructions. Regardless of this option, 64-bit "dou-
ble" operations are always emulated with calls to
library functions.
-mfused-madd
-mno-fused-madd
Enable or disable use of fused multiply/add and multi-
ply/subtract instructions in the floating-point
option. This has no effect if the floating-point
option is not also enabled. Disabling fused multi-
ply/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 produc-
ing results with more bits of precision than specified
by the IEEE standard. Disabling fused multiply
add/subtract instructions also ensures that the pro-
gram 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.
-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 liter-
als and improve code size. With -mtext-section-liter-
als, 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 assem-
bler 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 widen-
ing 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 instruc-
tions or by inserting no-op instructions.
-mlongcalls
-mno-longcalls
When this option is enabled, GCC instructs the assem-
bler 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 gen-
erated by GCC will still show direct call instruc-
tions---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.
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 which 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 77 front-ends, where
this option defaults to true and false respectively.
-ftrapv
This option generates traps for signed overflow on
addition, subtraction, multiplication operations.
-fexceptions
Enable exception handling. Generates extra code
needed to propagate exceptions. For some targets,
this implies GCC will generate frame unwind informa-
tion for all functions, which can produce significant
data size overhead, although it does not affect execu-
tion. If you do not specify this option, GCC will
enable it by default for languages like C++ which nor-
mally 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 com-
piling 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 every-
where. Moreover, it only allows trapping instructions
to throw exceptions, i.e. memory references or float-
ing 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 gen-
erate 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 con-
vention 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 con-
form 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 conven-
tion is standard for the target. If there is no stan-
dard 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 con-
form 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 which has enough room.
Warning: the -fshort-enums switch causes GCC to gener-
ate code that is not binary compatible with code gen-
erated 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 gen-
erate 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 gener-
ate code that is not binary compatible with code gen-
erated without that switch. Use it to conform to a
non-default application binary interface.
-fshared-data
Requests that the data and non-"const" variables of
this compilation be shared data rather than private
data. The distinction makes sense only on certain
operating systems, where shared data is shared between
processes running the same program, while private data
exists in one copy per process.
-fno-common
In C, allocate even uninitialized global variables in
the data section of the object file, rather than gen-
erating them as common blocks. This has the effect
that if the same variable is declared (without
"extern") in two different compilations, you will get
an error when you link them. The only reason this
might be useful is if you wish to verify that the pro-
gram will work on other systems which always work this
way.
-fno-ident
Ignore the #ident directive.
-fno-gnu-linker
Do not output global initializations (such as C++ con-
structors and destructors) in the form used by the GNU
linker (on systems where the GNU linker is the stan-
dard method of handling them). Use this option when
you want to use a non-GNU linker, which also requires
using the collect2 program to make sure the system
linker includes constructors and destructors. (col-
lect2 is included in the GCC distribution.) For sys-
tems which must use collect2, the compiler driver gcc
is configured to do this automatically.
-finhibit-size-directive
Don't output a ".size" assembler directive, or any-
thing 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 debug-
ging the compiler itself).
-fno-verbose-asm, the default, causes the extra infor-
mation to be omitted and is useful when comparing two
assembler files.
-fvolatile
Consider all memory references through pointers to be
volatile.
-fvolatile-global
Consider all memory references to extern and global
data items to be volatile. GCC does not consider
static data items to be volatile because of this
switch.
-fvolatile-static
Consider all memory references to static data to be
volatile.
-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 maxi-
mum 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 16k
on the m88k, 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.
-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,
m88k, and the SPARC.
Position-independent code requires special support,
and therefore works only on certain machines.
-ffixed-reg
Treat the register named reg as a fixed register; gen-
erated 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 allo-
cated 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 tem-
poraries or variables that live across a call. Func-
tions compiled this way will save and restore the reg-
ister 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
Pack all structure members together without holes.
Warning: the -fpack-struct switch causes GCC to gener-
ate code that is not binary compatible with code gen-
erated 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 avail-
able. 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 pro-
vided. (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_instru-
ment_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).
-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 sin-
gle-threaded environment since stack overflow is auto-
matically 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 must do that. The
switch causes generation of code to ensure that the
operating system sees the stack being extended.
-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 tar-
gets, 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,--def-
sym,__stack_limit=0x7ffe0000 to enforce a stack limit
of 128KB. Note that this may only work with the GNU
linker.
-fargument-alias
-fargument-noalias
-fargument-noalias-global
Specify the possible relationships among parameters
and between parameters and global data.
-fargument-alias specifies that arguments (parameters)
may alias each other and may alias global stor-
age.-fargument-noalias specifies that arguments do not
alias each other, but may alias global storage.-fargu-
ment-noalias-global specifies that arguments do not
alias each other and do not alias global storage.
Each language will automatically use whatever option
is required by the language standard. You should not
need to use these options yourself.
-fleading-underscore
This option and its counterpart, -fno-leading-under-
score, forcibly change the way C symbols are repre-
sented 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".
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 configu-
ration of GCC.
LANG
LC_CTYPE
LC_MESSAGES
LC_ALL
These environment variables control the way that GCC
uses localization information that allow 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_UK for English in the United King-
dom.
The LC_CTYPE environment variable specifies character
classification. GCC uses it to determine the charac-
ter 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 over-
rides the value of LC_CTYPE and LC_MESSAGES; other-
wise, LC_CTYPE and LC_MESSAGES default to the value of
the LANG environment variable. If none of these vari-
ables 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 out-
put of the preprocessor, which is the input to the
compiler proper.
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 com-
bined with the name of a subprogram, but you can spec-
ify 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 pre-
fix/lib/gcc-lib/ where 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 nor-
mally begins with /usr/local/lib/gcc-lib (more pre-
cisely, 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 nor-
mally search /usr/local/lib/bar. These alternate
directories are searched first; the standard directo-
ries come next.
COMPILER_PATH
The value of COMPILER_PATH is a colon-separated list
of directories, much like PATH. GCC tries the direc-
tories thus specified when searching for subprograms,
if it can't find the subprograms using GCC_EXEC_PRE-
FIX.
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 speci-
fied 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 sepa-
rated 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 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 ":/spe-
cial/include", that has the same effect as -I. -I/spe-
cial/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 equiva-
lent 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.
BUGS
For instructions on reporting bugs, see
<http://gcc.gnu.org/bugs.html>. Use of the gccbug script
to report bugs is recommended.
FOOTNOTES
1. On some systems, gcc -shared needs to build supplemen-
tary 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. Supply-
ing them in cases where they are not necessary is
innocuous.
SEE ALSOgpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), g77(1),
as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1) and the Info
entries for gcc, cpp, g77, as, ld, binutils and gdb.
AUTHOR
See the Info entry for gcc, or <http://gcc.gnu.org/online-
docs/gcc/Contributors.html>, for contributors to GCC.
COPYRIGHT
Copyright (c) 1988, 1989, 1992, 1993, 1994, 1995, 1996,
1997, 1998, 1999, 2000, 2001, 2002, 2003 Free Software
Foundation, Inc.
Permission is granted to copy, distribute and/or modify
this document under the terms of the GNU Free Documenta-
tion License, Version 1.2 or any later version published
by the Free Software Foundation; with the Invariant Sec-
tions 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.
gcc-3.3 2003-05-14 GCC(1)
