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.
-traditional-cpp -fallow-single-precision -fcond-mis
match -fsigned-bitfields -fsigned-char -fun
signed-bitfields -funsigned-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
-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
-fweb --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 -node
faultlibs -nostdlib -s -static -static-libgcc
-mc68000 -mc68020 -mfpa -mnobitfield -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 -mbroken-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-dou
bles -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
-mwarn-passed-structs
RS/6000 and PowerPC Options
-mcpu=cpu-type -mtune=cpu-type -mpower -mno-power
-mpower2 -mno-power2 -mpowerpc -mpowerpc64
-mno-powerpc -maltivec -mno-altivec -mpowerpc-gpopt
-mno-powerpc-gpopt -mpowerpc-gfxopt -mno-powerpc-gfx
opt -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 -mmultiple -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-relocat
able-lib -mtoc -mno-toc -mlittle -mlittle-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 -mprototype -mno-prototype -msim
-mmvme -mads -myellowknife -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
-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 -mpreferred-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 -mdis
able-fpregs -mdisable-indexing -mfast-indirect-calls
-mgas -mgnu-ld -mhp-ld -mjump-in-delay -mlinker-opt
-mlong-calls -mlong-load-store -mno-big-switch
-mno-disable-fpregs -mno-disable-indexing
-mno-fast-indirect-calls -mno-gas -mno-jump-in-delay
-mno-long-load-store -mno-portable-runtime
-mno-soft-float -mno-space-regs -msoft-float
-mpa-risc-1-0 -mpa-risc-1-1 -mpa-risc-2-0
-mportable-runtime -mschedule=cpu-type -mspace-regs
-msio -mwsio
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-procedures -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 -malpha-as -mgas -mieee
-mieee-with-inexact -mieee-conformant
-mfp-trap-mode=mode -mfp-rounding-mode=mode
-mtrap-precision=mode -mbuild-constants -mcpu=cpu-
type -mtune=cpu-type -mbwx -mmax -mfix -mcix
-mfloat-vax -mfloat-ieee -mexplicit-relocs
-msmall-data -mlarge-data -mmemory-latency=time
DEC Alpha/VMS Options
-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 -mrodata=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 -mparal
lel-insns -mparallel-mpy -mpreserve-float
V850 Options
-mlong-calls -mno-long-calls -mep -mno-ep -mpro
log-function -mno-prolog-function -mspace -mtda=n
-msda=n -mzda=n -mapp-regs -mno-app-regs -mdis
able-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 -mbitfield -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 -mre
lax-immediates -mno-relax-immediates -mwide-bitfields
-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
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 -mdebug -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-pro
logue-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 -mserial
ize-volatile -mno-serialize-volatile -mtext-sec
tion-literals -mno-text-section-literals -mtar
get-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
-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
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
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
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 incom
patible 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
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.
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
-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.
-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
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. The current Working Paper requires that "oper
ator new" never return a null pointer, so this check
is normally unnecessary.
An alternative to using this option is to specify that
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.
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
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
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:
· 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.
· Incorrect handling of tail-padding for virtual
bases. G++ does not use tail padding when laying
out virtual bases. For example:
struct A { virtual void f(); char c1; };
struct B { B(); char c2; };
struct C : public A, public virtual B {};
In this case, G++ will not place "B" into the
tail-padding for "A"; other compilers will. You
can avoid this problem by explicitly padding "A"
so that its size is a multiple of its alignment
(ignoring virtual base classes); that will cause
G++ and other compilers to layout "C" identically.
· Incorrect handling of bit-fields with declared
widths greater than that of their underlying
types, when the bit-fields appear in a union. For
example:
union U { int i : 4096; };
Assuming that an "int" does not have 4096 bits,
G++ will make the union too small by the number of
bits in an "int".
G++ will place the "A" base class of "C" at a
nonzero offset; it should be placed at offset
zero. G++ mistakenly believes that the "A" data
member of "B" is already at offset zero.
· Names of template functions whose types involve
"typename" or template template parameters can be
mangled incorrectly.
template <typename Q>
void f(typename Q::X) {}
template <template <typename> class Q>
void f(typename Q<int>::X) {}
Instantiations of these templates may be mangled
incorrectly.
-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:
must return an object.
and about violations of the following style guidelines
from Scott Meyers' More Effective C++ book:
· Item 6: Distinguish between prefix and postfix
forms of increment and decrement operators.
· Item 7: Never overload "&&", "||", or ",".
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 warn
ing.
-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();
};
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
-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
-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.
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 con
versions 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
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.
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 ();
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
-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
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.
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:
· 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;
}
· 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.
· An unsigned value is compared against zero with <
or <=.
· 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.
· Storage-class specifiers like "static" are not the
· 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.)
· 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 };
· 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
replacement takes place within string literals,
but does not in ISO C.
· In traditional C, some preprocessor directives did
not exist. Traditional preprocessors would only
consider a line to be a directive if the #
appeared in column 1 on the line. Therefore
-Wtraditional warns about directives that 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.
· A function-like macro that appears without argu
ments.
· The unary plus operator.
· The U integer constant suffix, or the F or L
floating point constant suffixes. (Traditional C
does support the L suffix on integer constants.)
Note, these suffixes appear in macros defined in
the system headers of most modern systems, e.g.
the _MIN/_MAX macros in "<limits.h>". Use of
these macros in user code might normally lead to
spurious warnings, however gcc's integrated pre
processor has enough context to avoid warning in
these cases.
· A function declared external in one block and then
used after the end of the block.
· A "switch" statement has an operand of type
"long".
· A non-"static" function declaration follows a
"static" one. This construct is not accepted by
some traditional C compilers.
· The ISO type of an integer constant has a 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.
· Usage of ISO string concatenation is detected.
· Initialization of automatic aggregates.
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.
· 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
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 also enabled by -W; to get the other warnings of -W
without this warning, use -W -Wno-sign-compare.
-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
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
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).
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.
-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
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
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
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
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.
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.
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 pro
duce the expected results. Statements are independent: if
you stop the program with a breakpoint between statements,
you can then assign a new value to any variable or change
the program counter to any other statement in the function
and get exactly the results you would expect from the
source code.
Turning on optimization flags makes the compiler attempt
to improve the performance and/or code size at the expense
of compilation time and possibly the ability to debug the
program.
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
-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, -fweb, -funit-at-a-time, -ftracer,
-funswitch-loops 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
-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
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:
@item max-inline-insns
is set to I<n>.
@item max-inline-insns-single
is set to I<n>/2.
@item max-inline-insns-single-auto
is set to I<n>*2/5.
@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>.
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
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
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
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
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
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
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".
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.
optimizer and trivial dead code remover. It can, how
ever, 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
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.
-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
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-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
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 240.
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.
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
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(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 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.
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 dou
ble 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 seman
tics 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
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
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.
"..."" 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
-fno-show-column
Do not print column numbers in diagnostics. This may
be necessary if diagnostics are being scanned by a
program that does not understand the column numbers,
such as dejagnu.
-A predicate=answer
Make an assertion with the predicate predicate and
answer answer. This form is preferred to the older
form -A predicate(answer), which is still supported,
because it does not use shell special characters.
-A -predicate=answer
Cancel an assertion with the predicate predicate and
answer answer.
-A- Cancel all predefined assertions and all assertions
preceding it on the command line. Also, undefine all
predefined macros and all macros preceding it on the
command line. (This is a historical wart and may
change in the future.)
-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
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
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.
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.
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
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.
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_PRE
FIX.
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
-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 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
-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
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
-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 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.
-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
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.
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
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
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 instruc
tions. 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
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,
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
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.
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.
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.
-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.
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.
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
-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
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
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
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
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
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.
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)
-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
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
-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
-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
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.
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.
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
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.
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.
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 gen
erated code, except for the ABI and the set of avail
able instructions. The choices for cpu-type are i386,
i486, i586, i686, pentium, pentium-mmx, pentiumpro,
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
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
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
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
-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
-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
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.
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
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.
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)
-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.
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).
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
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.
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.
-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.
-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*".
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
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
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"
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
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
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
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.
-mdiv
-mdiv
-mno-div
Use the divide instruction. (Enabled by default).
-mrelax-immediate
-mrelax-immediate
-mno-relax-immediate
Allow arbitrary sized immediates in bit operations.
-mwide-bitfields
-mwide-bitfields
-mno-wide-bitfields
Always treat bit-fields as int-sized.
-m4byte-functions
-m4byte-functions
-mno-4byte-functions
Force all functions to be aligned to a four byte
boundary.
-mcallgraph-data
-mcallgraph-data
-mno-callgraph-data
Emit callgraph information.
-mslow-bytes
-mslow-bytes
-mno-slow-bytes
Prefer word access when reading byte quantities.
-mlittle-endian
-mlittle-endian
-mbig-endian
Generate code for a little endian target.
-m210
-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
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
-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
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.
-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
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
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
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.
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
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
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.
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
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 func
tions in other source files. Specifically, the assem
bler translates a direct "CALL" instruction into an
"L32R" followed by a "CALLX" instruction. The default
is -mno-longcalls. This option should be used in pro
grams where the call target can potentially be out of
range. This option is implemented in the assembler,
not the compiler, so the assembly code generated by
GCC will still show direct call instructions---look at
the disassembled object code to see the actual
instructions. Note that the assembler will use an
indirect call for every cross-file call, not just
those that really will be out of range.
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
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.
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
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,
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
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 func
tions inline, you might have gotten away without pro
viding 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
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
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
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
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 ALSO
gpl(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.
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