1 @c Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999,
2 @c 2000, 2001, 2002, 2003, 2004, 2005 Free Software Foundation, Inc.
3 @c This is part of the GCC manual.
4 @c For copying conditions, see the file gcc.texi.
8 Copyright @copyright{} 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997,
9 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005 Free Software Foundation, Inc.
11 Permission is granted to copy, distribute and/or modify this document
12 under the terms of the GNU Free Documentation License, Version 1.2 or
13 any later version published by the Free Software Foundation; with the
14 Invariant Sections being ``GNU General Public License'' and ``Funding
15 Free Software'', the Front-Cover texts being (a) (see below), and with
16 the Back-Cover Texts being (b) (see below). A copy of the license is
17 included in the gfdl(7) man page.
19 (a) The FSF's Front-Cover Text is:
23 (b) The FSF's Back-Cover Text is:
25 You have freedom to copy and modify this GNU Manual, like GNU
26 software. Copies published by the Free Software Foundation raise
27 funds for GNU development.
29 @c Set file name and title for the man page.
31 @settitle GNU project C and C++ compiler
33 gcc [@option{-c}|@option{-S}|@option{-E}] [@option{-std=}@var{standard}]
34 [@option{-g}] [@option{-pg}] [@option{-O}@var{level}]
35 [@option{-W}@var{warn}@dots{}] [@option{-pedantic}]
36 [@option{-I}@var{dir}@dots{}] [@option{-L}@var{dir}@dots{}]
37 [@option{-D}@var{macro}[=@var{defn}]@dots{}] [@option{-U}@var{macro}]
38 [@option{-f}@var{option}@dots{}] [@option{-m}@var{machine-option}@dots{}]
39 [@option{-o} @var{outfile}] @var{infile}@dots{}
41 Only the most useful options are listed here; see below for the
42 remainder. @samp{g++} accepts mostly the same options as @samp{gcc}.
45 gpl(7), gfdl(7), fsf-funding(7),
46 cpp(1), gcov(1), as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1)
47 and the Info entries for @file{gcc}, @file{cpp}, @file{as},
48 @file{ld}, @file{binutils} and @file{gdb}.
51 For instructions on reporting bugs, see
52 @w{@uref{http://gcc.gnu.org/bugs.html}}.
55 See the Info entry for @command{gcc}, or
56 @w{@uref{http://gcc.gnu.org/onlinedocs/gcc/Contributors.html}},
57 for contributors to GCC@.
62 @chapter GCC Command Options
63 @cindex GCC command options
64 @cindex command options
65 @cindex options, GCC command
67 @c man begin DESCRIPTION
68 When you invoke GCC, it normally does preprocessing, compilation,
69 assembly and linking. The ``overall options'' allow you to stop this
70 process at an intermediate stage. For example, the @option{-c} option
71 says not to run the linker. Then the output consists of object files
72 output by the assembler.
74 Other options are passed on to one stage of processing. Some options
75 control the preprocessor and others the compiler itself. Yet other
76 options control the assembler and linker; most of these are not
77 documented here, since you rarely need to use any of them.
79 @cindex C compilation options
80 Most of the command line options that you can use with GCC are useful
81 for C programs; when an option is only useful with another language
82 (usually C++), the explanation says so explicitly. If the description
83 for a particular option does not mention a source language, you can use
84 that option with all supported languages.
86 @cindex C++ compilation options
87 @xref{Invoking G++,,Compiling C++ Programs}, for a summary of special
88 options for compiling C++ programs.
90 @cindex grouping options
91 @cindex options, grouping
92 The @command{gcc} program accepts options and file names as operands. Many
93 options have multi-letter names; therefore multiple single-letter options
94 may @emph{not} be grouped: @option{-dr} is very different from @w{@samp{-d
97 @cindex order of options
98 @cindex options, order
99 You can mix options and other arguments. For the most part, the order
100 you use doesn't matter. Order does matter when you use several options
101 of the same kind; for example, if you specify @option{-L} more than once,
102 the directories are searched in the order specified.
104 Many options have long names starting with @samp{-f} or with
105 @samp{-W}---for example,
106 @option{-fstrength-reduce}, @option{-Wformat} and so on. Most of
107 these have both positive and negative forms; the negative form of
108 @option{-ffoo} would be @option{-fno-foo}. This manual documents
109 only one of these two forms, whichever one is not the default.
113 @xref{Option Index}, for an index to GCC's options.
116 * Option Summary:: Brief list of all options, without explanations.
117 * Overall Options:: Controlling the kind of output:
118 an executable, object files, assembler files,
119 or preprocessed source.
120 * Invoking G++:: Compiling C++ programs.
121 * C Dialect Options:: Controlling the variant of C language compiled.
122 * C++ Dialect Options:: Variations on C++.
123 * Objective-C and Objective-C++ Dialect Options:: Variations on Objective-C
125 * Language Independent Options:: Controlling how diagnostics should be
127 * Warning Options:: How picky should the compiler be?
128 * Debugging Options:: Symbol tables, measurements, and debugging dumps.
129 * Optimize Options:: How much optimization?
130 * Preprocessor Options:: Controlling header files and macro definitions.
131 Also, getting dependency information for Make.
132 * Assembler Options:: Passing options to the assembler.
133 * Link Options:: Specifying libraries and so on.
134 * Directory Options:: Where to find header files and libraries.
135 Where to find the compiler executable files.
136 * Spec Files:: How to pass switches to sub-processes.
137 * Target Options:: Running a cross-compiler, or an old version of GCC.
138 * Submodel Options:: Specifying minor hardware or convention variations,
139 such as 68010 vs 68020.
140 * Code Gen Options:: Specifying conventions for function calls, data layout
142 * Environment Variables:: Env vars that affect GCC.
143 * Precompiled Headers:: Compiling a header once, and using it many times.
144 * Running Protoize:: Automatically adding or removing function prototypes.
150 @section Option Summary
152 Here is a summary of all the options, grouped by type. Explanations are
153 in the following sections.
156 @item Overall Options
157 @xref{Overall Options,,Options Controlling the Kind of Output}.
158 @gccoptlist{-c -S -E -o @var{file} -combine -pipe -pass-exit-codes @gol
159 -x @var{language} -v -### --help --target-help --version}
161 @item C Language Options
162 @xref{C Dialect Options,,Options Controlling C Dialect}.
163 @gccoptlist{-ansi -std=@var{standard} -aux-info @var{filename} @gol
164 -fno-asm -fno-builtin -fno-builtin-@var{function} @gol
165 -fhosted -ffreestanding -fms-extensions @gol
166 -trigraphs -no-integrated-cpp -traditional -traditional-cpp @gol
167 -fallow-single-precision -fcond-mismatch @gol
168 -fsigned-bitfields -fsigned-char @gol
169 -funsigned-bitfields -funsigned-char}
171 @item C++ Language Options
172 @xref{C++ Dialect Options,,Options Controlling C++ Dialect}.
173 @gccoptlist{-fabi-version=@var{n} -fno-access-control -fcheck-new @gol
174 -fconserve-space -ffriend-injection -fno-const-strings @gol
175 -fno-elide-constructors @gol
176 -fno-enforce-eh-specs @gol
177 -ffor-scope -fno-for-scope -fno-gnu-keywords @gol
178 -fno-implicit-templates @gol
179 -fno-implicit-inline-templates @gol
180 -fno-implement-inlines -fms-extensions @gol
181 -fno-nonansi-builtins -fno-operator-names @gol
182 -fno-optional-diags -fpermissive @gol
183 -frepo -fno-rtti -fstats -ftemplate-depth-@var{n} @gol
184 -fno-threadsafe-statics -fuse-cxa-atexit -fno-weak -nostdinc++ @gol
185 -fno-default-inline -fvisibility-inlines-hidden @gol
186 -Wabi -Wctor-dtor-privacy @gol
187 -Wnon-virtual-dtor -Wreorder @gol
188 -Weffc++ -Wno-deprecated -Wstrict-null-sentinel @gol
189 -Wno-non-template-friend -Wold-style-cast @gol
190 -Woverloaded-virtual -Wno-pmf-conversions @gol
193 @item Objective-C and Objective-C++ Language Options
194 @xref{Objective-C and Objective-C++ Dialect Options,,Options Controlling
195 Objective-C and Objective-C++ Dialects}.
197 -fconstant-string-class=@var{class-name} @gol
198 -fgnu-runtime -fnext-runtime @gol
199 -fno-nil-receivers @gol
200 -fobjc-call-cxx-cdtors @gol
201 -fobjc-direct-dispatch @gol
202 -fobjc-exceptions @gol
204 -freplace-objc-classes @gol
207 -Wassign-intercept @gol
208 -Wno-protocol -Wselector @gol
209 -Wstrict-selector-match @gol
210 -Wundeclared-selector}
212 @item Language Independent Options
213 @xref{Language Independent Options,,Options to Control Diagnostic Messages Formatting}.
214 @gccoptlist{-fmessage-length=@var{n} @gol
215 -fdiagnostics-show-location=@r{[}once@r{|}every-line@r{]}} @gol
216 -fdiagnostics-show-options
218 @item Warning Options
219 @xref{Warning Options,,Options to Request or Suppress Warnings}.
220 @gccoptlist{-fsyntax-only -pedantic -pedantic-errors @gol
221 -w -Wextra -Wall -Waggregate-return -Wno-attributes @gol
222 -Wc++-compat -Wcast-align -Wcast-qual -Wchar-subscripts -Wcomment @gol
223 -Wconversion -Wno-deprecated-declarations @gol
224 -Wdisabled-optimization -Wno-div-by-zero -Wno-endif-labels @gol
225 -Werror -Werror-implicit-function-declaration @gol
226 -Wfatal-errors -Wfloat-equal -Wformat -Wformat=2 @gol
227 -Wno-format-extra-args -Wformat-nonliteral @gol
228 -Wformat-security -Wformat-y2k @gol
229 -Wimplicit -Wimplicit-function-declaration -Wimplicit-int @gol
230 -Wimport -Wno-import -Winit-self -Winline @gol
231 -Wno-int-to-pointer-cast @gol
232 -Wno-invalid-offsetof -Winvalid-pch @gol
233 -Wlarger-than-@var{len} -Wunsafe-loop-optimizations -Wlong-long @gol
234 -Wmain -Wmissing-braces -Wmissing-field-initializers @gol
235 -Wmissing-format-attribute -Wmissing-include-dirs @gol
236 -Wmissing-noreturn @gol
237 -Wno-multichar -Wnonnull -Wpacked -Wpadded @gol
238 -Wparentheses -Wpointer-arith -Wno-pointer-to-int-cast @gol
239 -Wredundant-decls @gol
240 -Wreturn-type -Wsequence-point -Wshadow @gol
241 -Wsign-compare -Wstack-protector @gol
242 -Wstrict-aliasing -Wstrict-aliasing=2 @gol
243 -Wswitch -Wswitch-default -Wswitch-enum @gol
244 -Wsystem-headers -Wtrigraphs -Wundef -Wuninitialized @gol
245 -Wunknown-pragmas -Wno-pragmas -Wunreachable-code @gol
246 -Wunused -Wunused-function -Wunused-label -Wunused-parameter @gol
247 -Wunused-value -Wunused-variable -Wvariadic-macros @gol
250 @item C-only Warning Options
251 @gccoptlist{-Wbad-function-cast -Wmissing-declarations @gol
252 -Wmissing-prototypes -Wnested-externs -Wold-style-definition @gol
253 -Wstrict-prototypes -Wtraditional @gol
254 -Wdeclaration-after-statement -Wno-pointer-sign}
256 @item Debugging Options
257 @xref{Debugging Options,,Options for Debugging Your Program or GCC}.
258 @gccoptlist{-d@var{letters} -dumpspecs -dumpmachine -dumpversion @gol
259 -fdump-unnumbered -fdump-translation-unit@r{[}-@var{n}@r{]} @gol
260 -fdump-class-hierarchy@r{[}-@var{n}@r{]} @gol
261 -fdump-ipa-all -fdump-ipa-cgraph @gol
263 -fdump-tree-original@r{[}-@var{n}@r{]} @gol
264 -fdump-tree-optimized@r{[}-@var{n}@r{]} @gol
265 -fdump-tree-inlined@r{[}-@var{n}@r{]} @gol
266 -fdump-tree-cfg -fdump-tree-vcg -fdump-tree-alias @gol
268 -fdump-tree-ssa@r{[}-@var{n}@r{]} -fdump-tree-pre@r{[}-@var{n}@r{]} @gol
269 -fdump-tree-ccp@r{[}-@var{n}@r{]} -fdump-tree-dce@r{[}-@var{n}@r{]} @gol
270 -fdump-tree-gimple@r{[}-raw@r{]} -fdump-tree-mudflap@r{[}-@var{n}@r{]} @gol
271 -fdump-tree-dom@r{[}-@var{n}@r{]} @gol
272 -fdump-tree-dse@r{[}-@var{n}@r{]} @gol
273 -fdump-tree-phiopt@r{[}-@var{n}@r{]} @gol
274 -fdump-tree-forwprop@r{[}-@var{n}@r{]} @gol
275 -fdump-tree-copyrename@r{[}-@var{n}@r{]} @gol
276 -fdump-tree-nrv -fdump-tree-vect @gol
277 -fdump-tree-sink @gol
278 -fdump-tree-sra@r{[}-@var{n}@r{]} @gol
279 -fdump-tree-salias @gol
280 -fdump-tree-fre@r{[}-@var{n}@r{]} @gol
281 -fdump-tree-vrp@r{[}-@var{n}@r{]} @gol
282 -ftree-vectorizer-verbose=@var{n} @gol
283 -fdump-tree-storeccp@r{[}-@var{n}@r{]} @gol
284 -feliminate-dwarf2-dups -feliminate-unused-debug-types @gol
285 -feliminate-unused-debug-symbols -fmem-report -fprofile-arcs @gol
286 -frandom-seed=@var{string} -fsched-verbose=@var{n} @gol
287 -ftest-coverage -ftime-report -fvar-tracking @gol
288 -g -g@var{level} -gcoff -gdwarf-2 @gol
289 -ggdb -gstabs -gstabs+ -gvms -gxcoff -gxcoff+ @gol
290 -p -pg -print-file-name=@var{library} -print-libgcc-file-name @gol
291 -print-multi-directory -print-multi-lib @gol
292 -print-prog-name=@var{program} -print-search-dirs -Q @gol
295 @item Optimization Options
296 @xref{Optimize Options,,Options that Control Optimization}.
297 @gccoptlist{-falign-functions=@var{n} -falign-jumps=@var{n} @gol
298 -falign-labels=@var{n} -falign-loops=@var{n} @gol
299 -fbounds-check -fmudflap -fmudflapth -fmudflapir @gol
300 -fbranch-probabilities -fprofile-values -fvpt -fbranch-target-load-optimize @gol
301 -fbranch-target-load-optimize2 -fbtr-bb-exclusive @gol
302 -fcaller-saves -fcprop-registers -fcse-follow-jumps @gol
303 -fcse-skip-blocks -fcx-limited-range -fdata-sections @gol
304 -fdelayed-branch -fdelete-null-pointer-checks -fearly-inlining @gol
305 -fexpensive-optimizations -ffast-math -ffloat-store @gol
306 -fforce-addr -ffunction-sections @gol
307 -fgcse -fgcse-lm -fgcse-sm -fgcse-las -fgcse-after-reload @gol
308 -floop-optimize -fcrossjumping -fif-conversion -fif-conversion2 @gol
309 -finline-functions -finline-functions-called-once @gol
310 -finline-limit=@var{n} -fkeep-inline-functions @gol
311 -fkeep-static-consts -fmerge-constants -fmerge-all-constants @gol
312 -fmodulo-sched -fno-branch-count-reg @gol
313 -fno-default-inline -fno-defer-pop -floop-optimize2 -fmove-loop-invariants @gol
314 -fno-function-cse -fno-guess-branch-probability @gol
315 -fno-inline -fno-math-errno -fno-peephole -fno-peephole2 @gol
316 -funsafe-math-optimizations -funsafe-loop-optimizations -ffinite-math-only @gol
317 -fno-trapping-math -fno-zero-initialized-in-bss @gol
318 -fomit-frame-pointer -foptimize-register-move @gol
319 -foptimize-sibling-calls -fprefetch-loop-arrays @gol
320 -fprofile-generate -fprofile-use @gol
321 -fregmove -frename-registers @gol
322 -freorder-blocks -freorder-blocks-and-partition -freorder-functions @gol
323 -frerun-cse-after-loop -frerun-loop-opt @gol
324 -frounding-math -fschedule-insns -fschedule-insns2 @gol
325 -fno-sched-interblock -fno-sched-spec -fsched-spec-load @gol
326 -fsched-spec-load-dangerous @gol
327 -fsched-stalled-insns=@var{n} -fsched-stalled-insns-dep=@var{n} @gol
328 -fsched2-use-superblocks @gol
329 -fsched2-use-traces -freschedule-modulo-scheduled-loops @gol
330 -fsignaling-nans -fsingle-precision-constant @gol
331 -fstack-protector -fstack-protector-all @gol
332 -fstrength-reduce -fstrict-aliasing -ftracer -fthread-jumps @gol
333 -funroll-all-loops -funroll-loops -fpeel-loops @gol
334 -fsplit-ivs-in-unroller -funswitch-loops @gol
335 -fvariable-expansion-in-unroller @gol
336 -ftree-pre -ftree-ccp -ftree-dce -ftree-loop-optimize @gol
337 -ftree-loop-linear -ftree-loop-im -ftree-loop-ivcanon -fivopts @gol
338 -ftree-dominator-opts -ftree-dse -ftree-copyrename -ftree-sink @gol
339 -ftree-ch -ftree-sra -ftree-ter -ftree-lrs -ftree-fre -ftree-vectorize @gol
340 -ftree-vect-loop-version -ftree-salias -fweb @gol
341 -ftree-copy-prop -ftree-store-ccp -ftree-store-copy-prop -fwhole-program @gol
342 --param @var{name}=@var{value}
343 -O -O0 -O1 -O2 -O3 -Os}
345 @item Preprocessor Options
346 @xref{Preprocessor Options,,Options Controlling the Preprocessor}.
347 @gccoptlist{-A@var{question}=@var{answer} @gol
348 -A-@var{question}@r{[}=@var{answer}@r{]} @gol
349 -C -dD -dI -dM -dN @gol
350 -D@var{macro}@r{[}=@var{defn}@r{]} -E -H @gol
351 -idirafter @var{dir} @gol
352 -include @var{file} -imacros @var{file} @gol
353 -iprefix @var{file} -iwithprefix @var{dir} @gol
354 -iwithprefixbefore @var{dir} -isystem @var{dir} @gol
355 -isysroot @var{dir} @gol
356 -M -MM -MF -MG -MP -MQ -MT -nostdinc @gol
357 -P -fworking-directory -remap @gol
358 -trigraphs -undef -U@var{macro} -Wp,@var{option} @gol
359 -Xpreprocessor @var{option}}
361 @item Assembler Option
362 @xref{Assembler Options,,Passing Options to the Assembler}.
363 @gccoptlist{-Wa,@var{option} -Xassembler @var{option}}
366 @xref{Link Options,,Options for Linking}.
367 @gccoptlist{@var{object-file-name} -l@var{library} @gol
368 -nostartfiles -nodefaultlibs -nostdlib -pie -rdynamic @gol
369 -s -static -static-libgcc -shared -shared-libgcc -symbolic @gol
370 -Wl,@var{option} -Xlinker @var{option} @gol
373 @item Directory Options
374 @xref{Directory Options,,Options for Directory Search}.
375 @gccoptlist{-B@var{prefix} -I@var{dir} -iquote@var{dir} -L@var{dir}
376 -specs=@var{file} -I- --sysroot=@var{dir}}
379 @c I wrote this xref this way to avoid overfull hbox. -- rms
380 @xref{Target Options}.
381 @gccoptlist{-V @var{version} -b @var{machine}}
383 @item Machine Dependent Options
384 @xref{Submodel Options,,Hardware Models and Configurations}.
385 @c This list is ordered alphanumerically by subsection name.
386 @c Try and put the significant identifier (CPU or system) first,
387 @c so users have a clue at guessing where the ones they want will be.
390 @gccoptlist{-EB -EL @gol
391 -mmangle-cpu -mcpu=@var{cpu} -mtext=@var{text-section} @gol
392 -mdata=@var{data-section} -mrodata=@var{readonly-data-section}}
395 @gccoptlist{-mapcs-frame -mno-apcs-frame @gol
396 -mabi=@var{name} @gol
397 -mapcs-stack-check -mno-apcs-stack-check @gol
398 -mapcs-float -mno-apcs-float @gol
399 -mapcs-reentrant -mno-apcs-reentrant @gol
400 -msched-prolog -mno-sched-prolog @gol
401 -mlittle-endian -mbig-endian -mwords-little-endian @gol
402 -mfloat-abi=@var{name} -msoft-float -mhard-float -mfpe @gol
403 -mthumb-interwork -mno-thumb-interwork @gol
404 -mcpu=@var{name} -march=@var{name} -mfpu=@var{name} @gol
405 -mstructure-size-boundary=@var{n} @gol
406 -mabort-on-noreturn @gol
407 -mlong-calls -mno-long-calls @gol
408 -msingle-pic-base -mno-single-pic-base @gol
409 -mpic-register=@var{reg} @gol
410 -mnop-fun-dllimport @gol
411 -mcirrus-fix-invalid-insns -mno-cirrus-fix-invalid-insns @gol
412 -mpoke-function-name @gol
414 -mtpcs-frame -mtpcs-leaf-frame @gol
415 -mcaller-super-interworking -mcallee-super-interworking}
418 @gccoptlist{-mmcu=@var{mcu} -msize -minit-stack=@var{n} -mno-interrupts @gol
419 -mcall-prologues -mno-tablejump -mtiny-stack -mint8}
421 @emph{Blackfin Options}
422 @gccoptlist{-momit-leaf-frame-pointer -mno-omit-leaf-frame-pointer @gol
423 -mspecld-anomaly -mno-specld-anomaly -mcsync-anomaly -mno-csync-anomaly @gol
424 -mlow-64k -mno-low64k -mid-shared-library @gol
425 -mno-id-shared-library -mshared-library-id=@var{n} @gol
426 -mlong-calls -mno-long-calls}
429 @gccoptlist{-mcpu=@var{cpu} -march=@var{cpu} -mtune=@var{cpu} @gol
430 -mmax-stack-frame=@var{n} -melinux-stacksize=@var{n} @gol
431 -metrax4 -metrax100 -mpdebug -mcc-init -mno-side-effects @gol
432 -mstack-align -mdata-align -mconst-align @gol
433 -m32-bit -m16-bit -m8-bit -mno-prologue-epilogue -mno-gotplt @gol
434 -melf -maout -melinux -mlinux -sim -sim2 @gol
435 -mmul-bug-workaround -mno-mul-bug-workaround}
438 @gccoptlist{-mmac -mpush-args}
440 @emph{Darwin Options}
441 @gccoptlist{-all_load -allowable_client -arch -arch_errors_fatal @gol
442 -arch_only -bind_at_load -bundle -bundle_loader @gol
443 -client_name -compatibility_version -current_version @gol
445 -dependency-file -dylib_file -dylinker_install_name @gol
446 -dynamic -dynamiclib -exported_symbols_list @gol
447 -filelist -flat_namespace -force_cpusubtype_ALL @gol
448 -force_flat_namespace -headerpad_max_install_names @gol
449 -image_base -init -install_name -keep_private_externs @gol
450 -multi_module -multiply_defined -multiply_defined_unused @gol
451 -noall_load -no_dead_strip_inits_and_terms @gol
452 -nofixprebinding -nomultidefs -noprebind -noseglinkedit @gol
453 -pagezero_size -prebind -prebind_all_twolevel_modules @gol
454 -private_bundle -read_only_relocs -sectalign @gol
455 -sectobjectsymbols -whyload -seg1addr @gol
456 -sectcreate -sectobjectsymbols -sectorder @gol
457 -segaddr -segs_read_only_addr -segs_read_write_addr @gol
458 -seg_addr_table -seg_addr_table_filename -seglinkedit @gol
459 -segprot -segs_read_only_addr -segs_read_write_addr @gol
460 -single_module -static -sub_library -sub_umbrella @gol
461 -twolevel_namespace -umbrella -undefined @gol
462 -unexported_symbols_list -weak_reference_mismatches @gol
463 -whatsloaded -F -gused -gfull -mmacosx-version-min=@var{version} @gol
466 @emph{DEC Alpha Options}
467 @gccoptlist{-mno-fp-regs -msoft-float -malpha-as -mgas @gol
468 -mieee -mieee-with-inexact -mieee-conformant @gol
469 -mfp-trap-mode=@var{mode} -mfp-rounding-mode=@var{mode} @gol
470 -mtrap-precision=@var{mode} -mbuild-constants @gol
471 -mcpu=@var{cpu-type} -mtune=@var{cpu-type} @gol
472 -mbwx -mmax -mfix -mcix @gol
473 -mfloat-vax -mfloat-ieee @gol
474 -mexplicit-relocs -msmall-data -mlarge-data @gol
475 -msmall-text -mlarge-text @gol
476 -mmemory-latency=@var{time}}
478 @emph{DEC Alpha/VMS Options}
479 @gccoptlist{-mvms-return-codes}
482 @gccoptlist{-mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64 @gol
483 -mhard-float -msoft-float @gol
484 -malloc-cc -mfixed-cc -mdword -mno-dword @gol
485 -mdouble -mno-double @gol
486 -mmedia -mno-media -mmuladd -mno-muladd @gol
487 -mfdpic -minline-plt -mgprel-ro -multilib-library-pic @gol
488 -mlinked-fp -mlong-calls -malign-labels @gol
489 -mlibrary-pic -macc-4 -macc-8 @gol
490 -mpack -mno-pack -mno-eflags -mcond-move -mno-cond-move @gol
491 -moptimize-membar -mno-optimize-membar @gol
492 -mscc -mno-scc -mcond-exec -mno-cond-exec @gol
493 -mvliw-branch -mno-vliw-branch @gol
494 -mmulti-cond-exec -mno-multi-cond-exec -mnested-cond-exec @gol
495 -mno-nested-cond-exec -mtomcat-stats @gol
499 @emph{H8/300 Options}
500 @gccoptlist{-mrelax -mh -ms -mn -mint32 -malign-300}
503 @gccoptlist{-march=@var{architecture-type} @gol
504 -mbig-switch -mdisable-fpregs -mdisable-indexing @gol
505 -mfast-indirect-calls -mgas -mgnu-ld -mhp-ld @gol
506 -mfixed-range=@var{register-range} @gol
507 -mjump-in-delay -mlinker-opt -mlong-calls @gol
508 -mlong-load-store -mno-big-switch -mno-disable-fpregs @gol
509 -mno-disable-indexing -mno-fast-indirect-calls -mno-gas @gol
510 -mno-jump-in-delay -mno-long-load-store @gol
511 -mno-portable-runtime -mno-soft-float @gol
512 -mno-space-regs -msoft-float -mpa-risc-1-0 @gol
513 -mpa-risc-1-1 -mpa-risc-2-0 -mportable-runtime @gol
514 -mschedule=@var{cpu-type} -mspace-regs -msio -mwsio @gol
515 -munix=@var{unix-std} -nolibdld -static -threads}
517 @emph{i386 and x86-64 Options}
518 @gccoptlist{-mtune=@var{cpu-type} -march=@var{cpu-type} @gol
519 -mfpmath=@var{unit} @gol
520 -masm=@var{dialect} -mno-fancy-math-387 @gol
521 -mno-fp-ret-in-387 -msoft-float -msvr3-shlib @gol
522 -mno-wide-multiply -mrtd -malign-double @gol
523 -mpreferred-stack-boundary=@var{num} @gol
524 -mmmx -msse -msse2 -msse3 -m3dnow @gol
525 -mthreads -mno-align-stringops -minline-all-stringops @gol
526 -mpush-args -maccumulate-outgoing-args -m128bit-long-double @gol
527 -m96bit-long-double -mregparm=@var{num} -msseregparm @gol
528 -momit-leaf-frame-pointer -mno-red-zone -mno-tls-direct-seg-refs @gol
529 -mcmodel=@var{code-model} @gol
530 -m32 -m64 -mlarge-data-threshold=@var{num}}
533 @gccoptlist{-mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic @gol
534 -mvolatile-asm-stop -mregister-names -mno-sdata @gol
535 -mconstant-gp -mauto-pic -minline-float-divide-min-latency @gol
536 -minline-float-divide-max-throughput @gol
537 -minline-int-divide-min-latency @gol
538 -minline-int-divide-max-throughput @gol
539 -minline-sqrt-min-latency -minline-sqrt-max-throughput @gol
540 -mno-dwarf2-asm -mearly-stop-bits @gol
541 -mfixed-range=@var{register-range} -mtls-size=@var{tls-size} @gol
542 -mtune=@var{cpu-type} -mt -pthread -milp32 -mlp64}
544 @emph{M32R/D Options}
545 @gccoptlist{-m32r2 -m32rx -m32r @gol
547 -malign-loops -mno-align-loops @gol
548 -missue-rate=@var{number} @gol
549 -mbranch-cost=@var{number} @gol
550 -mmodel=@var{code-size-model-type} @gol
551 -msdata=@var{sdata-type} @gol
552 -mno-flush-func -mflush-func=@var{name} @gol
553 -mno-flush-trap -mflush-trap=@var{number} @gol
557 @gccoptlist{-mcpu=@var{cpu} -msim -memregs=@var{number}}
559 @emph{M680x0 Options}
560 @gccoptlist{-m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040 @gol
561 -m68060 -mcpu32 -m5200 -m68881 -mbitfield -mc68000 -mc68020 @gol
562 -mnobitfield -mrtd -mshort -msoft-float -mpcrel @gol
563 -malign-int -mstrict-align -msep-data -mno-sep-data @gol
564 -mshared-library-id=n -mid-shared-library -mno-id-shared-library}
566 @emph{M68hc1x Options}
567 @gccoptlist{-m6811 -m6812 -m68hc11 -m68hc12 -m68hcs12 @gol
568 -mauto-incdec -minmax -mlong-calls -mshort @gol
569 -msoft-reg-count=@var{count}}
572 @gccoptlist{-mhardlit -mno-hardlit -mdiv -mno-div -mrelax-immediates @gol
573 -mno-relax-immediates -mwide-bitfields -mno-wide-bitfields @gol
574 -m4byte-functions -mno-4byte-functions -mcallgraph-data @gol
575 -mno-callgraph-data -mslow-bytes -mno-slow-bytes -mno-lsim @gol
576 -mlittle-endian -mbig-endian -m210 -m340 -mstack-increment}
579 @gccoptlist{-EL -EB -march=@var{arch} -mtune=@var{arch} @gol
580 -mips1 -mips2 -mips3 -mips4 -mips32 -mips32r2 -mips64 @gol
581 -mips16 -mno-mips16 -mabi=@var{abi} -mabicalls -mno-abicalls @gol
582 -mxgot -mno-xgot -mgp32 -mgp64 -mfp32 -mfp64 @gol
583 -mhard-float -msoft-float -msingle-float -mdouble-float @gol
584 -mdsp -mpaired-single -mips3d @gol
585 -mlong64 -mlong32 -msym32 -mno-sym32 @gol
586 -G@var{num} -membedded-data -mno-embedded-data @gol
587 -muninit-const-in-rodata -mno-uninit-const-in-rodata @gol
588 -msplit-addresses -mno-split-addresses @gol
589 -mexplicit-relocs -mno-explicit-relocs @gol
590 -mcheck-zero-division -mno-check-zero-division @gol
591 -mdivide-traps -mdivide-breaks @gol
592 -mmemcpy -mno-memcpy -mlong-calls -mno-long-calls @gol
593 -mmad -mno-mad -mfused-madd -mno-fused-madd -nocpp @gol
594 -mfix-r4000 -mno-fix-r4000 -mfix-r4400 -mno-fix-r4400 @gol
595 -mfix-vr4120 -mno-fix-vr4120 -mfix-vr4130 @gol
596 -mfix-sb1 -mno-fix-sb1 @gol
597 -mflush-func=@var{func} -mno-flush-func @gol
598 -mbranch-likely -mno-branch-likely @gol
599 -mfp-exceptions -mno-fp-exceptions @gol
600 -mvr4130-align -mno-vr4130-align}
603 @gccoptlist{-mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon -mabi=gnu @gol
604 -mabi=mmixware -mzero-extend -mknuthdiv -mtoplevel-symbols @gol
605 -melf -mbranch-predict -mno-branch-predict -mbase-addresses @gol
606 -mno-base-addresses -msingle-exit -mno-single-exit}
608 @emph{MN10300 Options}
609 @gccoptlist{-mmult-bug -mno-mult-bug @gol
610 -mam33 -mno-am33 @gol
611 -mam33-2 -mno-am33-2 @gol
612 -mreturn-pointer-on-d0 @gol
616 @gccoptlist{-mno-crt0 -mmul -mbacc -msim @gol
617 -march=@var{cpu-type} }
619 @emph{PDP-11 Options}
620 @gccoptlist{-mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10 @gol
621 -mbcopy -mbcopy-builtin -mint32 -mno-int16 @gol
622 -mint16 -mno-int32 -mfloat32 -mno-float64 @gol
623 -mfloat64 -mno-float32 -mabshi -mno-abshi @gol
624 -mbranch-expensive -mbranch-cheap @gol
625 -msplit -mno-split -munix-asm -mdec-asm}
627 @emph{PowerPC Options}
628 See RS/6000 and PowerPC Options.
630 @emph{RS/6000 and PowerPC Options}
631 @gccoptlist{-mcpu=@var{cpu-type} @gol
632 -mtune=@var{cpu-type} @gol
633 -mpower -mno-power -mpower2 -mno-power2 @gol
634 -mpowerpc -mpowerpc64 -mno-powerpc @gol
635 -maltivec -mno-altivec @gol
636 -mpowerpc-gpopt -mno-powerpc-gpopt @gol
637 -mpowerpc-gfxopt -mno-powerpc-gfxopt @gol
638 -mnew-mnemonics -mold-mnemonics @gol
639 -mfull-toc -mminimal-toc -mno-fp-in-toc -mno-sum-in-toc @gol
640 -m64 -m32 -mxl-compat -mno-xl-compat -mpe @gol
641 -malign-power -malign-natural @gol
642 -msoft-float -mhard-float -mmultiple -mno-multiple @gol
643 -mstring -mno-string -mupdate -mno-update @gol
644 -mfused-madd -mno-fused-madd -mbit-align -mno-bit-align @gol
645 -mstrict-align -mno-strict-align -mrelocatable @gol
646 -mno-relocatable -mrelocatable-lib -mno-relocatable-lib @gol
647 -mtoc -mno-toc -mlittle -mlittle-endian -mbig -mbig-endian @gol
648 -mdynamic-no-pic -maltivec -mswdiv @gol
649 -mprioritize-restricted-insns=@var{priority} @gol
650 -msched-costly-dep=@var{dependence_type} @gol
651 -minsert-sched-nops=@var{scheme} @gol
652 -mcall-sysv -mcall-netbsd @gol
653 -maix-struct-return -msvr4-struct-return @gol
654 -mabi=@var{abi-type} -msecure-plt -mbss-plt @gol
655 -misel -mno-isel @gol
656 -misel=yes -misel=no @gol
658 -mspe=yes -mspe=no @gol
659 -mvrsave -mno-vrsave @gol
660 -mfloat-gprs=yes -mfloat-gprs=no -mfloat-gprs=single -mfloat-gprs=double @gol
661 -mprototype -mno-prototype @gol
662 -msim -mmvme -mads -myellowknife -memb -msdata @gol
663 -msdata=@var{opt} -mvxworks -mwindiss -G @var{num} -pthread}
665 @emph{S/390 and zSeries Options}
666 @gccoptlist{-mtune=@var{cpu-type} -march=@var{cpu-type} @gol
667 -mhard-float -msoft-float -mbackchain -mno-backchain @gol
668 -mpacked-stack -mno-packed-stack @gol
669 -msmall-exec -mno-small-exec -mmvcle -mno-mvcle @gol
670 -m64 -m31 -mdebug -mno-debug -mesa -mzarch @gol
671 -mtpf-trace -mno-tpf-trace -mfused-madd -mno-fused-madd @gol
672 -mwarn-framesize -mwarn-dynamicstack -mstack-size -mstack-guard}
675 @gccoptlist{-m1 -m2 -m2e -m3 -m3e @gol
676 -m4-nofpu -m4-single-only -m4-single -m4 @gol
677 -m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al @gol
678 -m5-64media -m5-64media-nofpu @gol
679 -m5-32media -m5-32media-nofpu @gol
680 -m5-compact -m5-compact-nofpu @gol
681 -mb -ml -mdalign -mrelax @gol
682 -mbigtable -mfmovd -mhitachi -mrenesas -mno-renesas -mnomacsave @gol
683 -mieee -misize -mpadstruct -mspace @gol
684 -mprefergot -musermode -multcost=@var{number} -mdiv=@var{strategy} @gol
685 -mdivsi3_libfunc=@var{name} @gol
686 -madjust-unroll -mindexed-addressing -mgettrcost=@var{number} -mpt-fixed @gol
690 @gccoptlist{-mcpu=@var{cpu-type} @gol
691 -mtune=@var{cpu-type} @gol
692 -mcmodel=@var{code-model} @gol
693 -m32 -m64 -mapp-regs -mno-app-regs @gol
694 -mfaster-structs -mno-faster-structs @gol
695 -mfpu -mno-fpu -mhard-float -msoft-float @gol
696 -mhard-quad-float -msoft-quad-float @gol
697 -mimpure-text -mno-impure-text -mlittle-endian @gol
698 -mstack-bias -mno-stack-bias @gol
699 -munaligned-doubles -mno-unaligned-doubles @gol
700 -mv8plus -mno-v8plus -mvis -mno-vis
703 @emph{System V Options}
704 @gccoptlist{-Qy -Qn -YP,@var{paths} -Ym,@var{dir}}
706 @emph{TMS320C3x/C4x Options}
707 @gccoptlist{-mcpu=@var{cpu} -mbig -msmall -mregparm -mmemparm @gol
708 -mfast-fix -mmpyi -mbk -mti -mdp-isr-reload @gol
709 -mrpts=@var{count} -mrptb -mdb -mloop-unsigned @gol
710 -mparallel-insns -mparallel-mpy -mpreserve-float}
713 @gccoptlist{-mlong-calls -mno-long-calls -mep -mno-ep @gol
714 -mprolog-function -mno-prolog-function -mspace @gol
715 -mtda=@var{n} -msda=@var{n} -mzda=@var{n} @gol
716 -mapp-regs -mno-app-regs @gol
717 -mdisable-callt -mno-disable-callt @gol
723 @gccoptlist{-mg -mgnu -munix}
725 @emph{x86-64 Options}
726 See i386 and x86-64 Options.
728 @emph{Xstormy16 Options}
731 @emph{Xtensa Options}
732 @gccoptlist{-mconst16 -mno-const16 @gol
733 -mfused-madd -mno-fused-madd @gol
734 -mtext-section-literals -mno-text-section-literals @gol
735 -mtarget-align -mno-target-align @gol
736 -mlongcalls -mno-longcalls}
738 @emph{zSeries Options}
739 See S/390 and zSeries Options.
741 @item Code Generation Options
742 @xref{Code Gen Options,,Options for Code Generation Conventions}.
743 @gccoptlist{-fcall-saved-@var{reg} -fcall-used-@var{reg} @gol
744 -ffixed-@var{reg} -fexceptions @gol
745 -fnon-call-exceptions -funwind-tables @gol
746 -fasynchronous-unwind-tables @gol
747 -finhibit-size-directive -finstrument-functions @gol
748 -fno-common -fno-ident @gol
749 -fpcc-struct-return -fpic -fPIC -fpie -fPIE @gol
750 -fno-jump-tables @gol
751 -freg-struct-return -fshared-data -fshort-enums @gol
752 -fshort-double -fshort-wchar @gol
753 -fverbose-asm -fpack-struct[=@var{n}] -fstack-check @gol
754 -fstack-limit-register=@var{reg} -fstack-limit-symbol=@var{sym} @gol
755 -fargument-alias -fargument-noalias @gol
756 -fargument-noalias-global -fleading-underscore @gol
757 -ftls-model=@var{model} @gol
758 -ftrapv -fwrapv -fbounds-check @gol
763 * Overall Options:: Controlling the kind of output:
764 an executable, object files, assembler files,
765 or preprocessed source.
766 * C Dialect Options:: Controlling the variant of C language compiled.
767 * C++ Dialect Options:: Variations on C++.
768 * Objective-C and Objective-C++ Dialect Options:: Variations on Objective-C
770 * Language Independent Options:: Controlling how diagnostics should be
772 * Warning Options:: How picky should the compiler be?
773 * Debugging Options:: Symbol tables, measurements, and debugging dumps.
774 * Optimize Options:: How much optimization?
775 * Preprocessor Options:: Controlling header files and macro definitions.
776 Also, getting dependency information for Make.
777 * Assembler Options:: Passing options to the assembler.
778 * Link Options:: Specifying libraries and so on.
779 * Directory Options:: Where to find header files and libraries.
780 Where to find the compiler executable files.
781 * Spec Files:: How to pass switches to sub-processes.
782 * Target Options:: Running a cross-compiler, or an old version of GCC.
785 @node Overall Options
786 @section Options Controlling the Kind of Output
788 Compilation can involve up to four stages: preprocessing, compilation
789 proper, assembly and linking, always in that order. GCC is capable of
790 preprocessing and compiling several files either into several
791 assembler input files, or into one assembler input file; then each
792 assembler input file produces an object file, and linking combines all
793 the object files (those newly compiled, and those specified as input)
794 into an executable file.
796 @cindex file name suffix
797 For any given input file, the file name suffix determines what kind of
802 C source code which must be preprocessed.
805 C source code which should not be preprocessed.
808 C++ source code which should not be preprocessed.
811 Objective-C source code. Note that you must link with the @file{libobjc}
812 library to make an Objective-C program work.
815 Objective-C source code which should not be preprocessed.
819 Objective-C++ source code. Note that you must link with the @file{libobjc}
820 library to make an Objective-C++ program work. Note that @samp{.M} refers
821 to a literal capital M@.
824 Objective-C++ source code which should not be preprocessed.
827 C, C++, Objective-C or Objective-C++ header file to be turned into a
832 @itemx @var{file}.cxx
833 @itemx @var{file}.cpp
834 @itemx @var{file}.CPP
835 @itemx @var{file}.c++
837 C++ source code which must be preprocessed. Note that in @samp{.cxx},
838 the last two letters must both be literally @samp{x}. Likewise,
839 @samp{.C} refers to a literal capital C@.
843 Objective-C++ source code which must be preprocessed.
846 Objective-C++ source code which should not be preprocessed.
850 C++ header file to be turned into a precompiled header.
853 @itemx @var{file}.for
854 @itemx @var{file}.FOR
855 Fortran source code which should not be preprocessed.
858 @itemx @var{file}.fpp
859 @itemx @var{file}.FPP
860 Fortran source code which must be preprocessed (with the traditional
864 Fortran source code which must be preprocessed with a RATFOR
865 preprocessor (not included with GCC)@.
868 @itemx @var{file}.f95
869 Fortran 90/95 source code which should not be preprocessed.
872 @itemx @var{file}.F95
873 Fortran 90/95 source code which must be preprocessed (with the
874 traditional preprocessor).
876 @c FIXME: Descriptions of Java file types.
883 Ada source code file which contains a library unit declaration (a
884 declaration of a package, subprogram, or generic, or a generic
885 instantiation), or a library unit renaming declaration (a package,
886 generic, or subprogram renaming declaration). Such files are also
889 @itemx @var{file}.adb
890 Ada source code file containing a library unit body (a subprogram or
891 package body). Such files are also called @dfn{bodies}.
893 @c GCC also knows about some suffixes for languages not yet included:
902 Assembler code which must be preprocessed.
905 An object file to be fed straight into linking.
906 Any file name with no recognized suffix is treated this way.
910 You can specify the input language explicitly with the @option{-x} option:
913 @item -x @var{language}
914 Specify explicitly the @var{language} for the following input files
915 (rather than letting the compiler choose a default based on the file
916 name suffix). This option applies to all following input files until
917 the next @option{-x} option. Possible values for @var{language} are:
919 c c-header c-cpp-output
920 c++ c++-header c++-cpp-output
921 objective-c objective-c-header objective-c-cpp-output
922 objective-c++ objective-c++-header objective-c++-cpp-output
923 assembler assembler-with-cpp
925 f77 f77-cpp-input ratfor
932 Turn off any specification of a language, so that subsequent files are
933 handled according to their file name suffixes (as they are if @option{-x}
934 has not been used at all).
936 @item -pass-exit-codes
937 @opindex pass-exit-codes
938 Normally the @command{gcc} program will exit with the code of 1 if any
939 phase of the compiler returns a non-success return code. If you specify
940 @option{-pass-exit-codes}, the @command{gcc} program will instead return with
941 numerically highest error produced by any phase that returned an error
945 If you only want some of the stages of compilation, you can use
946 @option{-x} (or filename suffixes) to tell @command{gcc} where to start, and
947 one of the options @option{-c}, @option{-S}, or @option{-E} to say where
948 @command{gcc} is to stop. Note that some combinations (for example,
949 @samp{-x cpp-output -E}) instruct @command{gcc} to do nothing at all.
954 Compile or assemble the source files, but do not link. The linking
955 stage simply is not done. The ultimate output is in the form of an
956 object file for each source file.
958 By default, the object file name for a source file is made by replacing
959 the suffix @samp{.c}, @samp{.i}, @samp{.s}, etc., with @samp{.o}.
961 Unrecognized input files, not requiring compilation or assembly, are
966 Stop after the stage of compilation proper; do not assemble. The output
967 is in the form of an assembler code file for each non-assembler input
970 By default, the assembler file name for a source file is made by
971 replacing the suffix @samp{.c}, @samp{.i}, etc., with @samp{.s}.
973 Input files that don't require compilation are ignored.
977 Stop after the preprocessing stage; do not run the compiler proper. The
978 output is in the form of preprocessed source code, which is sent to the
981 Input files which don't require preprocessing are ignored.
983 @cindex output file option
986 Place output in file @var{file}. This applies regardless to whatever
987 sort of output is being produced, whether it be an executable file,
988 an object file, an assembler file or preprocessed C code.
990 If @option{-o} is not specified, the default is to put an executable
991 file in @file{a.out}, the object file for
992 @file{@var{source}.@var{suffix}} in @file{@var{source}.o}, its
993 assembler file in @file{@var{source}.s}, a precompiled header file in
994 @file{@var{source}.@var{suffix}.gch}, and all preprocessed C source on
999 Print (on standard error output) the commands executed to run the stages
1000 of compilation. Also print the version number of the compiler driver
1001 program and of the preprocessor and the compiler proper.
1005 Like @option{-v} except the commands are not executed and all command
1006 arguments are quoted. This is useful for shell scripts to capture the
1007 driver-generated command lines.
1011 Use pipes rather than temporary files for communication between the
1012 various stages of compilation. This fails to work on some systems where
1013 the assembler is unable to read from a pipe; but the GNU assembler has
1018 If you are compiling multiple source files, this option tells the driver
1019 to pass all the source files to the compiler at once (for those
1020 languages for which the compiler can handle this). This will allow
1021 intermodule analysis (IMA) to be performed by the compiler. Currently the only
1022 language for which this is supported is C@. If you pass source files for
1023 multiple languages to the driver, using this option, the driver will invoke
1024 the compiler(s) that support IMA once each, passing each compiler all the
1025 source files appropriate for it. For those languages that do not support
1026 IMA this option will be ignored, and the compiler will be invoked once for
1027 each source file in that language. If you use this option in conjunction
1028 with @option{-save-temps}, the compiler will generate multiple
1030 (one for each source file), but only one (combined) @file{.o} or
1035 Print (on the standard output) a description of the command line options
1036 understood by @command{gcc}. If the @option{-v} option is also specified
1037 then @option{--help} will also be passed on to the various processes
1038 invoked by @command{gcc}, so that they can display the command line options
1039 they accept. If the @option{-Wextra} option is also specified then command
1040 line options which have no documentation associated with them will also
1044 @opindex target-help
1045 Print (on the standard output) a description of target specific command
1046 line options for each tool.
1050 Display the version number and copyrights of the invoked GCC@.
1054 @section Compiling C++ Programs
1056 @cindex suffixes for C++ source
1057 @cindex C++ source file suffixes
1058 C++ source files conventionally use one of the suffixes @samp{.C},
1059 @samp{.cc}, @samp{.cpp}, @samp{.CPP}, @samp{.c++}, @samp{.cp}, or
1060 @samp{.cxx}; C++ header files often use @samp{.hh} or @samp{.H}; and
1061 preprocessed C++ files use the suffix @samp{.ii}. GCC recognizes
1062 files with these names and compiles them as C++ programs even if you
1063 call the compiler the same way as for compiling C programs (usually
1064 with the name @command{gcc}).
1068 However, C++ programs often require class libraries as well as a
1069 compiler that understands the C++ language---and under some
1070 circumstances, you might want to compile programs or header files from
1071 standard input, or otherwise without a suffix that flags them as C++
1072 programs. You might also like to precompile a C header file with a
1073 @samp{.h} extension to be used in C++ compilations. @command{g++} is a
1074 program that calls GCC with the default language set to C++, and
1075 automatically specifies linking against the C++ library. On many
1076 systems, @command{g++} is also installed with the name @command{c++}.
1078 @cindex invoking @command{g++}
1079 When you compile C++ programs, you may specify many of the same
1080 command-line options that you use for compiling programs in any
1081 language; or command-line options meaningful for C and related
1082 languages; or options that are meaningful only for C++ programs.
1083 @xref{C Dialect Options,,Options Controlling C Dialect}, for
1084 explanations of options for languages related to C@.
1085 @xref{C++ Dialect Options,,Options Controlling C++ Dialect}, for
1086 explanations of options that are meaningful only for C++ programs.
1088 @node C Dialect Options
1089 @section Options Controlling C Dialect
1090 @cindex dialect options
1091 @cindex language dialect options
1092 @cindex options, dialect
1094 The following options control the dialect of C (or languages derived
1095 from C, such as C++, Objective-C and Objective-C++) that the compiler
1099 @cindex ANSI support
1103 In C mode, support all ISO C90 programs. In C++ mode,
1104 remove GNU extensions that conflict with ISO C++.
1106 This turns off certain features of GCC that are incompatible with ISO
1107 C90 (when compiling C code), or of standard C++ (when compiling C++ code),
1108 such as the @code{asm} and @code{typeof} keywords, and
1109 predefined macros such as @code{unix} and @code{vax} that identify the
1110 type of system you are using. It also enables the undesirable and
1111 rarely used ISO trigraph feature. For the C compiler,
1112 it disables recognition of C++ style @samp{//} comments as well as
1113 the @code{inline} keyword.
1115 The alternate keywords @code{__asm__}, @code{__extension__},
1116 @code{__inline__} and @code{__typeof__} continue to work despite
1117 @option{-ansi}. You would not want to use them in an ISO C program, of
1118 course, but it is useful to put them in header files that might be included
1119 in compilations done with @option{-ansi}. Alternate predefined macros
1120 such as @code{__unix__} and @code{__vax__} are also available, with or
1121 without @option{-ansi}.
1123 The @option{-ansi} option does not cause non-ISO programs to be
1124 rejected gratuitously. For that, @option{-pedantic} is required in
1125 addition to @option{-ansi}. @xref{Warning Options}.
1127 The macro @code{__STRICT_ANSI__} is predefined when the @option{-ansi}
1128 option is used. Some header files may notice this macro and refrain
1129 from declaring certain functions or defining certain macros that the
1130 ISO standard doesn't call for; this is to avoid interfering with any
1131 programs that might use these names for other things.
1133 Functions which would normally be built in but do not have semantics
1134 defined by ISO C (such as @code{alloca} and @code{ffs}) are not built-in
1135 functions with @option{-ansi} is used. @xref{Other Builtins,,Other
1136 built-in functions provided by GCC}, for details of the functions
1141 Determine the language standard. This option is currently only
1142 supported when compiling C or C++. A value for this option must be
1143 provided; possible values are
1148 ISO C90 (same as @option{-ansi}).
1150 @item iso9899:199409
1151 ISO C90 as modified in amendment 1.
1157 ISO C99. Note that this standard is not yet fully supported; see
1158 @w{@uref{http://gcc.gnu.org/c99status.html}} for more information. The
1159 names @samp{c9x} and @samp{iso9899:199x} are deprecated.
1162 Default, ISO C90 plus GNU extensions (including some C99 features).
1166 ISO C99 plus GNU extensions. When ISO C99 is fully implemented in GCC,
1167 this will become the default. The name @samp{gnu9x} is deprecated.
1170 The 1998 ISO C++ standard plus amendments.
1173 The same as @option{-std=c++98} plus GNU extensions. This is the
1174 default for C++ code.
1177 Even when this option is not specified, you can still use some of the
1178 features of newer standards in so far as they do not conflict with
1179 previous C standards. For example, you may use @code{__restrict__} even
1180 when @option{-std=c99} is not specified.
1182 The @option{-std} options specifying some version of ISO C have the same
1183 effects as @option{-ansi}, except that features that were not in ISO C90
1184 but are in the specified version (for example, @samp{//} comments and
1185 the @code{inline} keyword in ISO C99) are not disabled.
1187 @xref{Standards,,Language Standards Supported by GCC}, for details of
1188 these standard versions.
1190 @item -aux-info @var{filename}
1192 Output to the given filename prototyped declarations for all functions
1193 declared and/or defined in a translation unit, including those in header
1194 files. This option is silently ignored in any language other than C@.
1196 Besides declarations, the file indicates, in comments, the origin of
1197 each declaration (source file and line), whether the declaration was
1198 implicit, prototyped or unprototyped (@samp{I}, @samp{N} for new or
1199 @samp{O} for old, respectively, in the first character after the line
1200 number and the colon), and whether it came from a declaration or a
1201 definition (@samp{C} or @samp{F}, respectively, in the following
1202 character). In the case of function definitions, a K&R-style list of
1203 arguments followed by their declarations is also provided, inside
1204 comments, after the declaration.
1208 Do not recognize @code{asm}, @code{inline} or @code{typeof} as a
1209 keyword, so that code can use these words as identifiers. You can use
1210 the keywords @code{__asm__}, @code{__inline__} and @code{__typeof__}
1211 instead. @option{-ansi} implies @option{-fno-asm}.
1213 In C++, this switch only affects the @code{typeof} keyword, since
1214 @code{asm} and @code{inline} are standard keywords. You may want to
1215 use the @option{-fno-gnu-keywords} flag instead, which has the same
1216 effect. In C99 mode (@option{-std=c99} or @option{-std=gnu99}), this
1217 switch only affects the @code{asm} and @code{typeof} keywords, since
1218 @code{inline} is a standard keyword in ISO C99.
1221 @itemx -fno-builtin-@var{function}
1222 @opindex fno-builtin
1223 @cindex built-in functions
1224 Don't recognize built-in functions that do not begin with
1225 @samp{__builtin_} as prefix. @xref{Other Builtins,,Other built-in
1226 functions provided by GCC}, for details of the functions affected,
1227 including those which are not built-in functions when @option{-ansi} or
1228 @option{-std} options for strict ISO C conformance are used because they
1229 do not have an ISO standard meaning.
1231 GCC normally generates special code to handle certain built-in functions
1232 more efficiently; for instance, calls to @code{alloca} may become single
1233 instructions that adjust the stack directly, and calls to @code{memcpy}
1234 may become inline copy loops. The resulting code is often both smaller
1235 and faster, but since the function calls no longer appear as such, you
1236 cannot set a breakpoint on those calls, nor can you change the behavior
1237 of the functions by linking with a different library. In addition,
1238 when a function is recognized as a built-in function, GCC may use
1239 information about that function to warn about problems with calls to
1240 that function, or to generate more efficient code, even if the
1241 resulting code still contains calls to that function. For example,
1242 warnings are given with @option{-Wformat} for bad calls to
1243 @code{printf}, when @code{printf} is built in, and @code{strlen} is
1244 known not to modify global memory.
1246 With the @option{-fno-builtin-@var{function}} option
1247 only the built-in function @var{function} is
1248 disabled. @var{function} must not begin with @samp{__builtin_}. If a
1249 function is named this is not built-in in this version of GCC, this
1250 option is ignored. There is no corresponding
1251 @option{-fbuiltin-@var{function}} option; if you wish to enable
1252 built-in functions selectively when using @option{-fno-builtin} or
1253 @option{-ffreestanding}, you may define macros such as:
1256 #define abs(n) __builtin_abs ((n))
1257 #define strcpy(d, s) __builtin_strcpy ((d), (s))
1262 @cindex hosted environment
1264 Assert that compilation takes place in a hosted environment. This implies
1265 @option{-fbuiltin}. A hosted environment is one in which the
1266 entire standard library is available, and in which @code{main} has a return
1267 type of @code{int}. Examples are nearly everything except a kernel.
1268 This is equivalent to @option{-fno-freestanding}.
1270 @item -ffreestanding
1271 @opindex ffreestanding
1272 @cindex hosted environment
1274 Assert that compilation takes place in a freestanding environment. This
1275 implies @option{-fno-builtin}. A freestanding environment
1276 is one in which the standard library may not exist, and program startup may
1277 not necessarily be at @code{main}. The most obvious example is an OS kernel.
1278 This is equivalent to @option{-fno-hosted}.
1280 @xref{Standards,,Language Standards Supported by GCC}, for details of
1281 freestanding and hosted environments.
1283 @item -fms-extensions
1284 @opindex fms-extensions
1285 Accept some non-standard constructs used in Microsoft header files.
1287 Some cases of unnamed fields in structures and unions are only
1288 accepted with this option. @xref{Unnamed Fields,,Unnamed struct/union
1289 fields within structs/unions}, for details.
1293 Support ISO C trigraphs. The @option{-ansi} option (and @option{-std}
1294 options for strict ISO C conformance) implies @option{-trigraphs}.
1296 @item -no-integrated-cpp
1297 @opindex no-integrated-cpp
1298 Performs a compilation in two passes: preprocessing and compiling. This
1299 option allows a user supplied "cc1", "cc1plus", or "cc1obj" via the
1300 @option{-B} option. The user supplied compilation step can then add in
1301 an additional preprocessing step after normal preprocessing but before
1302 compiling. The default is to use the integrated cpp (internal cpp)
1304 The semantics of this option will change if "cc1", "cc1plus", and
1305 "cc1obj" are merged.
1307 @cindex traditional C language
1308 @cindex C language, traditional
1310 @itemx -traditional-cpp
1311 @opindex traditional-cpp
1312 @opindex traditional
1313 Formerly, these options caused GCC to attempt to emulate a pre-standard
1314 C compiler. They are now only supported with the @option{-E} switch.
1315 The preprocessor continues to support a pre-standard mode. See the GNU
1316 CPP manual for details.
1318 @item -fcond-mismatch
1319 @opindex fcond-mismatch
1320 Allow conditional expressions with mismatched types in the second and
1321 third arguments. The value of such an expression is void. This option
1322 is not supported for C++.
1324 @item -funsigned-char
1325 @opindex funsigned-char
1326 Let the type @code{char} be unsigned, like @code{unsigned char}.
1328 Each kind of machine has a default for what @code{char} should
1329 be. It is either like @code{unsigned char} by default or like
1330 @code{signed char} by default.
1332 Ideally, a portable program should always use @code{signed char} or
1333 @code{unsigned char} when it depends on the signedness of an object.
1334 But many programs have been written to use plain @code{char} and
1335 expect it to be signed, or expect it to be unsigned, depending on the
1336 machines they were written for. This option, and its inverse, let you
1337 make such a program work with the opposite default.
1339 The type @code{char} is always a distinct type from each of
1340 @code{signed char} or @code{unsigned char}, even though its behavior
1341 is always just like one of those two.
1344 @opindex fsigned-char
1345 Let the type @code{char} be signed, like @code{signed char}.
1347 Note that this is equivalent to @option{-fno-unsigned-char}, which is
1348 the negative form of @option{-funsigned-char}. Likewise, the option
1349 @option{-fno-signed-char} is equivalent to @option{-funsigned-char}.
1351 @item -fsigned-bitfields
1352 @itemx -funsigned-bitfields
1353 @itemx -fno-signed-bitfields
1354 @itemx -fno-unsigned-bitfields
1355 @opindex fsigned-bitfields
1356 @opindex funsigned-bitfields
1357 @opindex fno-signed-bitfields
1358 @opindex fno-unsigned-bitfields
1359 These options control whether a bit-field is signed or unsigned, when the
1360 declaration does not use either @code{signed} or @code{unsigned}. By
1361 default, such a bit-field is signed, because this is consistent: the
1362 basic integer types such as @code{int} are signed types.
1365 @node C++ Dialect Options
1366 @section Options Controlling C++ Dialect
1368 @cindex compiler options, C++
1369 @cindex C++ options, command line
1370 @cindex options, C++
1371 This section describes the command-line options that are only meaningful
1372 for C++ programs; but you can also use most of the GNU compiler options
1373 regardless of what language your program is in. For example, you
1374 might compile a file @code{firstClass.C} like this:
1377 g++ -g -frepo -O -c firstClass.C
1381 In this example, only @option{-frepo} is an option meant
1382 only for C++ programs; you can use the other options with any
1383 language supported by GCC@.
1385 Here is a list of options that are @emph{only} for compiling C++ programs:
1389 @item -fabi-version=@var{n}
1390 @opindex fabi-version
1391 Use version @var{n} of the C++ ABI@. Version 2 is the version of the
1392 C++ ABI that first appeared in G++ 3.4. Version 1 is the version of
1393 the C++ ABI that first appeared in G++ 3.2. Version 0 will always be
1394 the version that conforms most closely to the C++ ABI specification.
1395 Therefore, the ABI obtained using version 0 will change as ABI bugs
1398 The default is version 2.
1400 @item -fno-access-control
1401 @opindex fno-access-control
1402 Turn off all access checking. This switch is mainly useful for working
1403 around bugs in the access control code.
1407 Check that the pointer returned by @code{operator new} is non-null
1408 before attempting to modify the storage allocated. This check is
1409 normally unnecessary because the C++ standard specifies that
1410 @code{operator new} will only return @code{0} if it is declared
1411 @samp{throw()}, in which case the compiler will always check the
1412 return value even without this option. In all other cases, when
1413 @code{operator new} has a non-empty exception specification, memory
1414 exhaustion is signalled by throwing @code{std::bad_alloc}. See also
1415 @samp{new (nothrow)}.
1417 @item -fconserve-space
1418 @opindex fconserve-space
1419 Put uninitialized or runtime-initialized global variables into the
1420 common segment, as C does. This saves space in the executable at the
1421 cost of not diagnosing duplicate definitions. If you compile with this
1422 flag and your program mysteriously crashes after @code{main()} has
1423 completed, you may have an object that is being destroyed twice because
1424 two definitions were merged.
1426 This option is no longer useful on most targets, now that support has
1427 been added for putting variables into BSS without making them common.
1429 @item -ffriend-injection
1430 @opindex ffriend-injection
1431 Inject friend functions into the enclosing namespace, so that they are
1432 visible outside the scope of the class in which they are declared.
1433 Friend functions were documented to work this way in the old Annotated
1434 C++ Reference Manual, and versions of G++ before 4.1 always worked
1435 that way. However, in ISO C++ a friend function which is not declared
1436 in an enclosing scope can only be found using argument dependent
1437 lookup. This option causes friends to be injected as they were in
1440 This option is for compatibility, and may be removed in a future
1443 @item -fno-const-strings
1444 @opindex fno-const-strings
1445 Give string constants type @code{char *} instead of type @code{const
1446 char *}. By default, G++ uses type @code{const char *} as required by
1447 the standard. Even if you use @option{-fno-const-strings}, you cannot
1448 actually modify the value of a string constant.
1450 This option might be removed in a future release of G++. For maximum
1451 portability, you should structure your code so that it works with
1452 string constants that have type @code{const char *}.
1454 @item -fno-elide-constructors
1455 @opindex fno-elide-constructors
1456 The C++ standard allows an implementation to omit creating a temporary
1457 which is only used to initialize another object of the same type.
1458 Specifying this option disables that optimization, and forces G++ to
1459 call the copy constructor in all cases.
1461 @item -fno-enforce-eh-specs
1462 @opindex fno-enforce-eh-specs
1463 Don't generate code to check for violation of exception specifications
1464 at runtime. This option violates the C++ standard, but may be useful
1465 for reducing code size in production builds, much like defining
1466 @samp{NDEBUG}. This does not give user code permission to throw
1467 exceptions in violation of the exception specifications; the compiler
1468 will still optimize based on the specifications, so throwing an
1469 unexpected exception will result in undefined behavior.
1472 @itemx -fno-for-scope
1474 @opindex fno-for-scope
1475 If @option{-ffor-scope} is specified, the scope of variables declared in
1476 a @i{for-init-statement} is limited to the @samp{for} loop itself,
1477 as specified by the C++ standard.
1478 If @option{-fno-for-scope} is specified, the scope of variables declared in
1479 a @i{for-init-statement} extends to the end of the enclosing scope,
1480 as was the case in old versions of G++, and other (traditional)
1481 implementations of C++.
1483 The default if neither flag is given to follow the standard,
1484 but to allow and give a warning for old-style code that would
1485 otherwise be invalid, or have different behavior.
1487 @item -fno-gnu-keywords
1488 @opindex fno-gnu-keywords
1489 Do not recognize @code{typeof} as a keyword, so that code can use this
1490 word as an identifier. You can use the keyword @code{__typeof__} instead.
1491 @option{-ansi} implies @option{-fno-gnu-keywords}.
1493 @item -fno-implicit-templates
1494 @opindex fno-implicit-templates
1495 Never emit code for non-inline templates which are instantiated
1496 implicitly (i.e.@: by use); only emit code for explicit instantiations.
1497 @xref{Template Instantiation}, for more information.
1499 @item -fno-implicit-inline-templates
1500 @opindex fno-implicit-inline-templates
1501 Don't emit code for implicit instantiations of inline templates, either.
1502 The default is to handle inlines differently so that compiles with and
1503 without optimization will need the same set of explicit instantiations.
1505 @item -fno-implement-inlines
1506 @opindex fno-implement-inlines
1507 To save space, do not emit out-of-line copies of inline functions
1508 controlled by @samp{#pragma implementation}. This will cause linker
1509 errors if these functions are not inlined everywhere they are called.
1511 @item -fms-extensions
1512 @opindex fms-extensions
1513 Disable pedantic warnings about constructs used in MFC, such as implicit
1514 int and getting a pointer to member function via non-standard syntax.
1516 @item -fno-nonansi-builtins
1517 @opindex fno-nonansi-builtins
1518 Disable built-in declarations of functions that are not mandated by
1519 ANSI/ISO C@. These include @code{ffs}, @code{alloca}, @code{_exit},
1520 @code{index}, @code{bzero}, @code{conjf}, and other related functions.
1522 @item -fno-operator-names
1523 @opindex fno-operator-names
1524 Do not treat the operator name keywords @code{and}, @code{bitand},
1525 @code{bitor}, @code{compl}, @code{not}, @code{or} and @code{xor} as
1526 synonyms as keywords.
1528 @item -fno-optional-diags
1529 @opindex fno-optional-diags
1530 Disable diagnostics that the standard says a compiler does not need to
1531 issue. Currently, the only such diagnostic issued by G++ is the one for
1532 a name having multiple meanings within a class.
1535 @opindex fpermissive
1536 Downgrade some diagnostics about nonconformant code from errors to
1537 warnings. Thus, using @option{-fpermissive} will allow some
1538 nonconforming code to compile.
1542 Enable automatic template instantiation at link time. This option also
1543 implies @option{-fno-implicit-templates}. @xref{Template
1544 Instantiation}, for more information.
1548 Disable generation of information about every class with virtual
1549 functions for use by the C++ runtime type identification features
1550 (@samp{dynamic_cast} and @samp{typeid}). If you don't use those parts
1551 of the language, you can save some space by using this flag. Note that
1552 exception handling uses the same information, but it will generate it as
1557 Emit statistics about front-end processing at the end of the compilation.
1558 This information is generally only useful to the G++ development team.
1560 @item -ftemplate-depth-@var{n}
1561 @opindex ftemplate-depth
1562 Set the maximum instantiation depth for template classes to @var{n}.
1563 A limit on the template instantiation depth is needed to detect
1564 endless recursions during template class instantiation. ANSI/ISO C++
1565 conforming programs must not rely on a maximum depth greater than 17.
1567 @item -fno-threadsafe-statics
1568 @opindex fno-threadsafe-statics
1569 Do not emit the extra code to use the routines specified in the C++
1570 ABI for thread-safe initialization of local statics. You can use this
1571 option to reduce code size slightly in code that doesn't need to be
1574 @item -fuse-cxa-atexit
1575 @opindex fuse-cxa-atexit
1576 Register destructors for objects with static storage duration with the
1577 @code{__cxa_atexit} function rather than the @code{atexit} function.
1578 This option is required for fully standards-compliant handling of static
1579 destructors, but will only work if your C library supports
1580 @code{__cxa_atexit}.
1582 @item -fvisibility-inlines-hidden
1583 @opindex fvisibility-inlines-hidden
1584 Causes all inlined methods to be marked with
1585 @code{__attribute__ ((visibility ("hidden")))} so that they do not
1586 appear in the export table of a DSO and do not require a PLT indirection
1587 when used within the DSO@. Enabling this option can have a dramatic effect
1588 on load and link times of a DSO as it massively reduces the size of the
1589 dynamic export table when the library makes heavy use of templates. While
1590 it can cause bloating through duplication of code within each DSO where
1591 it is used, often the wastage is less than the considerable space occupied
1592 by a long symbol name in the export table which is typical when using
1593 templates and namespaces. For even more savings, combine with the
1594 @option{-fvisibility=hidden} switch.
1598 Do not use weak symbol support, even if it is provided by the linker.
1599 By default, G++ will use weak symbols if they are available. This
1600 option exists only for testing, and should not be used by end-users;
1601 it will result in inferior code and has no benefits. This option may
1602 be removed in a future release of G++.
1606 Do not search for header files in the standard directories specific to
1607 C++, but do still search the other standard directories. (This option
1608 is used when building the C++ library.)
1611 In addition, these optimization, warning, and code generation options
1612 have meanings only for C++ programs:
1615 @item -fno-default-inline
1616 @opindex fno-default-inline
1617 Do not assume @samp{inline} for functions defined inside a class scope.
1618 @xref{Optimize Options,,Options That Control Optimization}. Note that these
1619 functions will have linkage like inline functions; they just won't be
1622 @item -Wabi @r{(C++ only)}
1624 Warn when G++ generates code that is probably not compatible with the
1625 vendor-neutral C++ ABI@. Although an effort has been made to warn about
1626 all such cases, there are probably some cases that are not warned about,
1627 even though G++ is generating incompatible code. There may also be
1628 cases where warnings are emitted even though the code that is generated
1631 You should rewrite your code to avoid these warnings if you are
1632 concerned about the fact that code generated by G++ may not be binary
1633 compatible with code generated by other compilers.
1635 The known incompatibilities at this point include:
1640 Incorrect handling of tail-padding for bit-fields. G++ may attempt to
1641 pack data into the same byte as a base class. For example:
1644 struct A @{ virtual void f(); int f1 : 1; @};
1645 struct B : public A @{ int f2 : 1; @};
1649 In this case, G++ will place @code{B::f2} into the same byte
1650 as@code{A::f1}; other compilers will not. You can avoid this problem
1651 by explicitly padding @code{A} so that its size is a multiple of the
1652 byte size on your platform; that will cause G++ and other compilers to
1653 layout @code{B} identically.
1656 Incorrect handling of tail-padding for virtual bases. G++ does not use
1657 tail padding when laying out virtual bases. For example:
1660 struct A @{ virtual void f(); char c1; @};
1661 struct B @{ B(); char c2; @};
1662 struct C : public A, public virtual B @{@};
1666 In this case, G++ will not place @code{B} into the tail-padding for
1667 @code{A}; other compilers will. You can avoid this problem by
1668 explicitly padding @code{A} so that its size is a multiple of its
1669 alignment (ignoring virtual base classes); that will cause G++ and other
1670 compilers to layout @code{C} identically.
1673 Incorrect handling of bit-fields with declared widths greater than that
1674 of their underlying types, when the bit-fields appear in a union. For
1678 union U @{ int i : 4096; @};
1682 Assuming that an @code{int} does not have 4096 bits, G++ will make the
1683 union too small by the number of bits in an @code{int}.
1686 Empty classes can be placed at incorrect offsets. For example:
1696 struct C : public B, public A @{@};
1700 G++ will place the @code{A} base class of @code{C} at a nonzero offset;
1701 it should be placed at offset zero. G++ mistakenly believes that the
1702 @code{A} data member of @code{B} is already at offset zero.
1705 Names of template functions whose types involve @code{typename} or
1706 template template parameters can be mangled incorrectly.
1709 template <typename Q>
1710 void f(typename Q::X) @{@}
1712 template <template <typename> class Q>
1713 void f(typename Q<int>::X) @{@}
1717 Instantiations of these templates may be mangled incorrectly.
1721 @item -Wctor-dtor-privacy @r{(C++ only)}
1722 @opindex Wctor-dtor-privacy
1723 Warn when a class seems unusable because all the constructors or
1724 destructors in that class are private, and it has neither friends nor
1725 public static member functions.
1727 @item -Wnon-virtual-dtor @r{(C++ only)}
1728 @opindex Wnon-virtual-dtor
1729 Warn when a class appears to be polymorphic, thereby requiring a virtual
1730 destructor, yet it declares a non-virtual one.
1731 This warning is enabled by @option{-Wall}.
1733 @item -Wreorder @r{(C++ only)}
1735 @cindex reordering, warning
1736 @cindex warning for reordering of member initializers
1737 Warn when the order of member initializers given in the code does not
1738 match the order in which they must be executed. For instance:
1744 A(): j (0), i (1) @{ @}
1748 The compiler will rearrange the member initializers for @samp{i}
1749 and @samp{j} to match the declaration order of the members, emitting
1750 a warning to that effect. This warning is enabled by @option{-Wall}.
1753 The following @option{-W@dots{}} options are not affected by @option{-Wall}.
1756 @item -Weffc++ @r{(C++ only)}
1758 Warn about violations of the following style guidelines from Scott Meyers'
1759 @cite{Effective C++} book:
1763 Item 11: Define a copy constructor and an assignment operator for classes
1764 with dynamically allocated memory.
1767 Item 12: Prefer initialization to assignment in constructors.
1770 Item 14: Make destructors virtual in base classes.
1773 Item 15: Have @code{operator=} return a reference to @code{*this}.
1776 Item 23: Don't try to return a reference when you must return an object.
1780 Also warn about violations of the following style guidelines from
1781 Scott Meyers' @cite{More Effective C++} book:
1785 Item 6: Distinguish between prefix and postfix forms of increment and
1786 decrement operators.
1789 Item 7: Never overload @code{&&}, @code{||}, or @code{,}.
1793 When selecting this option, be aware that the standard library
1794 headers do not obey all of these guidelines; use @samp{grep -v}
1795 to filter out those warnings.
1797 @item -Wno-deprecated @r{(C++ only)}
1798 @opindex Wno-deprecated
1799 Do not warn about usage of deprecated features. @xref{Deprecated Features}.
1801 @item -Wstrict-null-sentinel @r{(C++ only)}
1802 @opindex Wstrict-null-sentinel
1803 Warn also about the use of an uncasted @code{NULL} as sentinel. When
1804 compiling only with GCC this is a valid sentinel, as @code{NULL} is defined
1805 to @code{__null}. Although it is a null pointer constant not a null pointer,
1806 it is guaranteed to of the same size as a pointer. But this use is
1807 not portable across different compilers.
1809 @item -Wno-non-template-friend @r{(C++ only)}
1810 @opindex Wno-non-template-friend
1811 Disable warnings when non-templatized friend functions are declared
1812 within a template. Since the advent of explicit template specification
1813 support in G++, if the name of the friend is an unqualified-id (i.e.,
1814 @samp{friend foo(int)}), the C++ language specification demands that the
1815 friend declare or define an ordinary, nontemplate function. (Section
1816 14.5.3). Before G++ implemented explicit specification, unqualified-ids
1817 could be interpreted as a particular specialization of a templatized
1818 function. Because this non-conforming behavior is no longer the default
1819 behavior for G++, @option{-Wnon-template-friend} allows the compiler to
1820 check existing code for potential trouble spots and is on by default.
1821 This new compiler behavior can be turned off with
1822 @option{-Wno-non-template-friend} which keeps the conformant compiler code
1823 but disables the helpful warning.
1825 @item -Wold-style-cast @r{(C++ only)}
1826 @opindex Wold-style-cast
1827 Warn if an old-style (C-style) cast to a non-void type is used within
1828 a C++ program. The new-style casts (@samp{dynamic_cast},
1829 @samp{static_cast}, @samp{reinterpret_cast}, and @samp{const_cast}) are
1830 less vulnerable to unintended effects and much easier to search for.
1832 @item -Woverloaded-virtual @r{(C++ only)}
1833 @opindex Woverloaded-virtual
1834 @cindex overloaded virtual fn, warning
1835 @cindex warning for overloaded virtual fn
1836 Warn when a function declaration hides virtual functions from a
1837 base class. For example, in:
1844 struct B: public A @{
1849 the @code{A} class version of @code{f} is hidden in @code{B}, and code
1857 will fail to compile.
1859 @item -Wno-pmf-conversions @r{(C++ only)}
1860 @opindex Wno-pmf-conversions
1861 Disable the diagnostic for converting a bound pointer to member function
1864 @item -Wsign-promo @r{(C++ only)}
1865 @opindex Wsign-promo
1866 Warn when overload resolution chooses a promotion from unsigned or
1867 enumerated type to a signed type, over a conversion to an unsigned type of
1868 the same size. Previous versions of G++ would try to preserve
1869 unsignedness, but the standard mandates the current behavior.
1874 A& operator = (int);
1884 In this example, G++ will synthesize a default @samp{A& operator =
1885 (const A&);}, while cfront will use the user-defined @samp{operator =}.
1888 @node Objective-C and Objective-C++ Dialect Options
1889 @section Options Controlling Objective-C and Objective-C++ Dialects
1891 @cindex compiler options, Objective-C and Objective-C++
1892 @cindex Objective-C and Objective-C++ options, command line
1893 @cindex options, Objective-C and Objective-C++
1894 (NOTE: This manual does not describe the Objective-C and Objective-C++
1895 languages themselves. See @xref{Standards,,Language Standards
1896 Supported by GCC}, for references.)
1898 This section describes the command-line options that are only meaningful
1899 for Objective-C and Objective-C++ programs, but you can also use most of
1900 the language-independent GNU compiler options.
1901 For example, you might compile a file @code{some_class.m} like this:
1904 gcc -g -fgnu-runtime -O -c some_class.m
1908 In this example, @option{-fgnu-runtime} is an option meant only for
1909 Objective-C and Objective-C++ programs; you can use the other options with
1910 any language supported by GCC@.
1912 Note that since Objective-C is an extension of the C language, Objective-C
1913 compilations may also use options specific to the C front-end (e.g.,
1914 @option{-Wtraditional}). Similarly, Objective-C++ compilations may use
1915 C++-specific options (e.g., @option{-Wabi}).
1917 Here is a list of options that are @emph{only} for compiling Objective-C
1918 and Objective-C++ programs:
1921 @item -fconstant-string-class=@var{class-name}
1922 @opindex fconstant-string-class
1923 Use @var{class-name} as the name of the class to instantiate for each
1924 literal string specified with the syntax @code{@@"@dots{}"}. The default
1925 class name is @code{NXConstantString} if the GNU runtime is being used, and
1926 @code{NSConstantString} if the NeXT runtime is being used (see below). The
1927 @option{-fconstant-cfstrings} option, if also present, will override the
1928 @option{-fconstant-string-class} setting and cause @code{@@"@dots{}"} literals
1929 to be laid out as constant CoreFoundation strings.
1932 @opindex fgnu-runtime
1933 Generate object code compatible with the standard GNU Objective-C
1934 runtime. This is the default for most types of systems.
1936 @item -fnext-runtime
1937 @opindex fnext-runtime
1938 Generate output compatible with the NeXT runtime. This is the default
1939 for NeXT-based systems, including Darwin and Mac OS X@. The macro
1940 @code{__NEXT_RUNTIME__} is predefined if (and only if) this option is
1943 @item -fno-nil-receivers
1944 @opindex fno-nil-receivers
1945 Assume that all Objective-C message dispatches (e.g.,
1946 @code{[receiver message:arg]}) in this translation unit ensure that the receiver
1947 is not @code{nil}. This allows for more efficient entry points in the runtime
1948 to be used. Currently, this option is only available in conjunction with
1949 the NeXT runtime on Mac OS X 10.3 and later.
1951 @item -fobjc-call-cxx-cdtors
1952 @opindex fobjc-call-cxx-cdtors
1953 For each Objective-C class, check if any of its instance variables is a
1954 C++ object with a non-trivial default constructor. If so, synthesize a
1955 special @code{- (id) .cxx_construct} instance method that will run
1956 non-trivial default constructors on any such instance variables, in order,
1957 and then return @code{self}. Similarly, check if any instance variable
1958 is a C++ object with a non-trivial destructor, and if so, synthesize a
1959 special @code{- (void) .cxx_destruct} method that will run
1960 all such default destructors, in reverse order.
1962 The @code{- (id) .cxx_construct} and/or @code{- (void) .cxx_destruct} methods
1963 thusly generated will only operate on instance variables declared in the
1964 current Objective-C class, and not those inherited from superclasses. It
1965 is the responsibility of the Objective-C runtime to invoke all such methods
1966 in an object's inheritance hierarchy. The @code{- (id) .cxx_construct} methods
1967 will be invoked by the runtime immediately after a new object
1968 instance is allocated; the @code{- (void) .cxx_destruct} methods will
1969 be invoked immediately before the runtime deallocates an object instance.
1971 As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has
1972 support for invoking the @code{- (id) .cxx_construct} and
1973 @code{- (void) .cxx_destruct} methods.
1975 @item -fobjc-direct-dispatch
1976 @opindex fobjc-direct-dispatch
1977 Allow fast jumps to the message dispatcher. On Darwin this is
1978 accomplished via the comm page.
1980 @item -fobjc-exceptions
1981 @opindex fobjc-exceptions
1982 Enable syntactic support for structured exception handling in Objective-C,
1983 similar to what is offered by C++ and Java. Currently, this option is only
1984 available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.
1992 @@catch (AnObjCClass *exc) @{
1999 @@catch (AnotherClass *exc) @{
2002 @@catch (id allOthers) @{
2012 The @code{@@throw} statement may appear anywhere in an Objective-C or
2013 Objective-C++ program; when used inside of a @code{@@catch} block, the
2014 @code{@@throw} may appear without an argument (as shown above), in which case
2015 the object caught by the @code{@@catch} will be rethrown.
2017 Note that only (pointers to) Objective-C objects may be thrown and
2018 caught using this scheme. When an object is thrown, it will be caught
2019 by the nearest @code{@@catch} clause capable of handling objects of that type,
2020 analogously to how @code{catch} blocks work in C++ and Java. A
2021 @code{@@catch(id @dots{})} clause (as shown above) may also be provided to catch
2022 any and all Objective-C exceptions not caught by previous @code{@@catch}
2025 The @code{@@finally} clause, if present, will be executed upon exit from the
2026 immediately preceding @code{@@try @dots{} @@catch} section. This will happen
2027 regardless of whether any exceptions are thrown, caught or rethrown
2028 inside the @code{@@try @dots{} @@catch} section, analogously to the behavior
2029 of the @code{finally} clause in Java.
2031 There are several caveats to using the new exception mechanism:
2035 Although currently designed to be binary compatible with @code{NS_HANDLER}-style
2036 idioms provided by the @code{NSException} class, the new
2037 exceptions can only be used on Mac OS X 10.3 (Panther) and later
2038 systems, due to additional functionality needed in the (NeXT) Objective-C
2042 As mentioned above, the new exceptions do not support handling
2043 types other than Objective-C objects. Furthermore, when used from
2044 Objective-C++, the Objective-C exception model does not interoperate with C++
2045 exceptions at this time. This means you cannot @code{@@throw} an exception
2046 from Objective-C and @code{catch} it in C++, or vice versa
2047 (i.e., @code{throw @dots{} @@catch}).
2050 The @option{-fobjc-exceptions} switch also enables the use of synchronization
2051 blocks for thread-safe execution:
2054 @@synchronized (ObjCClass *guard) @{
2059 Upon entering the @code{@@synchronized} block, a thread of execution shall
2060 first check whether a lock has been placed on the corresponding @code{guard}
2061 object by another thread. If it has, the current thread shall wait until
2062 the other thread relinquishes its lock. Once @code{guard} becomes available,
2063 the current thread will place its own lock on it, execute the code contained in
2064 the @code{@@synchronized} block, and finally relinquish the lock (thereby
2065 making @code{guard} available to other threads).
2067 Unlike Java, Objective-C does not allow for entire methods to be marked
2068 @code{@@synchronized}. Note that throwing exceptions out of
2069 @code{@@synchronized} blocks is allowed, and will cause the guarding object
2070 to be unlocked properly.
2074 Enable garbage collection (GC) in Objective-C and Objective-C++ programs.
2076 @item -freplace-objc-classes
2077 @opindex freplace-objc-classes
2078 Emit a special marker instructing @command{ld(1)} not to statically link in
2079 the resulting object file, and allow @command{dyld(1)} to load it in at
2080 run time instead. This is used in conjunction with the Fix-and-Continue
2081 debugging mode, where the object file in question may be recompiled and
2082 dynamically reloaded in the course of program execution, without the need
2083 to restart the program itself. Currently, Fix-and-Continue functionality
2084 is only available in conjunction with the NeXT runtime on Mac OS X 10.3
2089 When compiling for the NeXT runtime, the compiler ordinarily replaces calls
2090 to @code{objc_getClass("@dots{}")} (when the name of the class is known at
2091 compile time) with static class references that get initialized at load time,
2092 which improves run-time performance. Specifying the @option{-fzero-link} flag
2093 suppresses this behavior and causes calls to @code{objc_getClass("@dots{}")}
2094 to be retained. This is useful in Zero-Link debugging mode, since it allows
2095 for individual class implementations to be modified during program execution.
2099 Dump interface declarations for all classes seen in the source file to a
2100 file named @file{@var{sourcename}.decl}.
2102 @item -Wassign-intercept
2103 @opindex Wassign-intercept
2104 Warn whenever an Objective-C assignment is being intercepted by the
2108 @opindex Wno-protocol
2109 If a class is declared to implement a protocol, a warning is issued for
2110 every method in the protocol that is not implemented by the class. The
2111 default behavior is to issue a warning for every method not explicitly
2112 implemented in the class, even if a method implementation is inherited
2113 from the superclass. If you use the @option{-Wno-protocol} option, then
2114 methods inherited from the superclass are considered to be implemented,
2115 and no warning is issued for them.
2119 Warn if multiple methods of different types for the same selector are
2120 found during compilation. The check is performed on the list of methods
2121 in the final stage of compilation. Additionally, a check is performed
2122 for each selector appearing in a @code{@@selector(@dots{})}
2123 expression, and a corresponding method for that selector has been found
2124 during compilation. Because these checks scan the method table only at
2125 the end of compilation, these warnings are not produced if the final
2126 stage of compilation is not reached, for example because an error is
2127 found during compilation, or because the @option{-fsyntax-only} option is
2130 @item -Wstrict-selector-match
2131 @opindex Wstrict-selector-match
2132 Warn if multiple methods with differing argument and/or return types are
2133 found for a given selector when attempting to send a message using this
2134 selector to a receiver of type @code{id} or @code{Class}. When this flag
2135 is off (which is the default behavior), the compiler will omit such warnings
2136 if any differences found are confined to types which share the same size
2139 @item -Wundeclared-selector
2140 @opindex Wundeclared-selector
2141 Warn if a @code{@@selector(@dots{})} expression referring to an
2142 undeclared selector is found. A selector is considered undeclared if no
2143 method with that name has been declared before the
2144 @code{@@selector(@dots{})} expression, either explicitly in an
2145 @code{@@interface} or @code{@@protocol} declaration, or implicitly in
2146 an @code{@@implementation} section. This option always performs its
2147 checks as soon as a @code{@@selector(@dots{})} expression is found,
2148 while @option{-Wselector} only performs its checks in the final stage of
2149 compilation. This also enforces the coding style convention
2150 that methods and selectors must be declared before being used.
2152 @item -print-objc-runtime-info
2153 @opindex print-objc-runtime-info
2154 Generate C header describing the largest structure that is passed by
2159 @node Language Independent Options
2160 @section Options to Control Diagnostic Messages Formatting
2161 @cindex options to control diagnostics formatting
2162 @cindex diagnostic messages
2163 @cindex message formatting
2165 Traditionally, diagnostic messages have been formatted irrespective of
2166 the output device's aspect (e.g.@: its width, @dots{}). The options described
2167 below can be used to control the diagnostic messages formatting
2168 algorithm, e.g.@: how many characters per line, how often source location
2169 information should be reported. Right now, only the C++ front end can
2170 honor these options. However it is expected, in the near future, that
2171 the remaining front ends would be able to digest them correctly.
2174 @item -fmessage-length=@var{n}
2175 @opindex fmessage-length
2176 Try to format error messages so that they fit on lines of about @var{n}
2177 characters. The default is 72 characters for @command{g++} and 0 for the rest of
2178 the front ends supported by GCC@. If @var{n} is zero, then no
2179 line-wrapping will be done; each error message will appear on a single
2182 @opindex fdiagnostics-show-location
2183 @item -fdiagnostics-show-location=once
2184 Only meaningful in line-wrapping mode. Instructs the diagnostic messages
2185 reporter to emit @emph{once} source location information; that is, in
2186 case the message is too long to fit on a single physical line and has to
2187 be wrapped, the source location won't be emitted (as prefix) again,
2188 over and over, in subsequent continuation lines. This is the default
2191 @item -fdiagnostics-show-location=every-line
2192 Only meaningful in line-wrapping mode. Instructs the diagnostic
2193 messages reporter to emit the same source location information (as
2194 prefix) for physical lines that result from the process of breaking
2195 a message which is too long to fit on a single line.
2197 @item -fdiagnostics-show-options
2198 @opindex fdiagnostics-show-options
2199 This option instructs the diagnostic machinery to add text to each
2200 diagnostic emitted, which indicates which command line option directly
2201 controls that diagnostic, when such an option is known to the
2202 diagnostic machinery.
2206 @node Warning Options
2207 @section Options to Request or Suppress Warnings
2208 @cindex options to control warnings
2209 @cindex warning messages
2210 @cindex messages, warning
2211 @cindex suppressing warnings
2213 Warnings are diagnostic messages that report constructions which
2214 are not inherently erroneous but which are risky or suggest there
2215 may have been an error.
2217 You can request many specific warnings with options beginning @samp{-W},
2218 for example @option{-Wimplicit} to request warnings on implicit
2219 declarations. Each of these specific warning options also has a
2220 negative form beginning @samp{-Wno-} to turn off warnings;
2221 for example, @option{-Wno-implicit}. This manual lists only one of the
2222 two forms, whichever is not the default.
2224 The following options control the amount and kinds of warnings produced
2225 by GCC; for further, language-specific options also refer to
2226 @ref{C++ Dialect Options} and @ref{Objective-C and Objective-C++ Dialect
2230 @cindex syntax checking
2232 @opindex fsyntax-only
2233 Check the code for syntax errors, but don't do anything beyond that.
2237 Issue all the warnings demanded by strict ISO C and ISO C++;
2238 reject all programs that use forbidden extensions, and some other
2239 programs that do not follow ISO C and ISO C++. For ISO C, follows the
2240 version of the ISO C standard specified by any @option{-std} option used.
2242 Valid ISO C and ISO C++ programs should compile properly with or without
2243 this option (though a rare few will require @option{-ansi} or a
2244 @option{-std} option specifying the required version of ISO C)@. However,
2245 without this option, certain GNU extensions and traditional C and C++
2246 features are supported as well. With this option, they are rejected.
2248 @option{-pedantic} does not cause warning messages for use of the
2249 alternate keywords whose names begin and end with @samp{__}. Pedantic
2250 warnings are also disabled in the expression that follows
2251 @code{__extension__}. However, only system header files should use
2252 these escape routes; application programs should avoid them.
2253 @xref{Alternate Keywords}.
2255 Some users try to use @option{-pedantic} to check programs for strict ISO
2256 C conformance. They soon find that it does not do quite what they want:
2257 it finds some non-ISO practices, but not all---only those for which
2258 ISO C @emph{requires} a diagnostic, and some others for which
2259 diagnostics have been added.
2261 A feature to report any failure to conform to ISO C might be useful in
2262 some instances, but would require considerable additional work and would
2263 be quite different from @option{-pedantic}. We don't have plans to
2264 support such a feature in the near future.
2266 Where the standard specified with @option{-std} represents a GNU
2267 extended dialect of C, such as @samp{gnu89} or @samp{gnu99}, there is a
2268 corresponding @dfn{base standard}, the version of ISO C on which the GNU
2269 extended dialect is based. Warnings from @option{-pedantic} are given
2270 where they are required by the base standard. (It would not make sense
2271 for such warnings to be given only for features not in the specified GNU
2272 C dialect, since by definition the GNU dialects of C include all
2273 features the compiler supports with the given option, and there would be
2274 nothing to warn about.)
2276 @item -pedantic-errors
2277 @opindex pedantic-errors
2278 Like @option{-pedantic}, except that errors are produced rather than
2283 Inhibit all warning messages.
2287 Inhibit warning messages about the use of @samp{#import}.
2289 @item -Wchar-subscripts
2290 @opindex Wchar-subscripts
2291 Warn if an array subscript has type @code{char}. This is a common cause
2292 of error, as programmers often forget that this type is signed on some
2294 This warning is enabled by @option{-Wall}.
2298 Warn whenever a comment-start sequence @samp{/*} appears in a @samp{/*}
2299 comment, or whenever a Backslash-Newline appears in a @samp{//} comment.
2300 This warning is enabled by @option{-Wall}.
2302 @item -Wfatal-errors
2303 @opindex Wfatal-errors
2304 This option causes the compiler to abort compilation on the first error
2305 occurred rather than trying to keep going and printing further error
2310 @opindex ffreestanding
2311 @opindex fno-builtin
2312 Check calls to @code{printf} and @code{scanf}, etc., to make sure that
2313 the arguments supplied have types appropriate to the format string
2314 specified, and that the conversions specified in the format string make
2315 sense. This includes standard functions, and others specified by format
2316 attributes (@pxref{Function Attributes}), in the @code{printf},
2317 @code{scanf}, @code{strftime} and @code{strfmon} (an X/Open extension,
2318 not in the C standard) families (or other target-specific families).
2319 Which functions are checked without format attributes having been
2320 specified depends on the standard version selected, and such checks of
2321 functions without the attribute specified are disabled by
2322 @option{-ffreestanding} or @option{-fno-builtin}.
2324 The formats are checked against the format features supported by GNU
2325 libc version 2.2. These include all ISO C90 and C99 features, as well
2326 as features from the Single Unix Specification and some BSD and GNU
2327 extensions. Other library implementations may not support all these
2328 features; GCC does not support warning about features that go beyond a
2329 particular library's limitations. However, if @option{-pedantic} is used
2330 with @option{-Wformat}, warnings will be given about format features not
2331 in the selected standard version (but not for @code{strfmon} formats,
2332 since those are not in any version of the C standard). @xref{C Dialect
2333 Options,,Options Controlling C Dialect}.
2335 Since @option{-Wformat} also checks for null format arguments for
2336 several functions, @option{-Wformat} also implies @option{-Wnonnull}.
2338 @option{-Wformat} is included in @option{-Wall}. For more control over some
2339 aspects of format checking, the options @option{-Wformat-y2k},
2340 @option{-Wno-format-extra-args}, @option{-Wno-format-zero-length},
2341 @option{-Wformat-nonliteral}, @option{-Wformat-security}, and
2342 @option{-Wformat=2} are available, but are not included in @option{-Wall}.
2345 @opindex Wformat-y2k
2346 If @option{-Wformat} is specified, also warn about @code{strftime}
2347 formats which may yield only a two-digit year.
2349 @item -Wno-format-extra-args
2350 @opindex Wno-format-extra-args
2351 If @option{-Wformat} is specified, do not warn about excess arguments to a
2352 @code{printf} or @code{scanf} format function. The C standard specifies
2353 that such arguments are ignored.
2355 Where the unused arguments lie between used arguments that are
2356 specified with @samp{$} operand number specifications, normally
2357 warnings are still given, since the implementation could not know what
2358 type to pass to @code{va_arg} to skip the unused arguments. However,
2359 in the case of @code{scanf} formats, this option will suppress the
2360 warning if the unused arguments are all pointers, since the Single
2361 Unix Specification says that such unused arguments are allowed.
2363 @item -Wno-format-zero-length
2364 @opindex Wno-format-zero-length
2365 If @option{-Wformat} is specified, do not warn about zero-length formats.
2366 The C standard specifies that zero-length formats are allowed.
2368 @item -Wformat-nonliteral
2369 @opindex Wformat-nonliteral
2370 If @option{-Wformat} is specified, also warn if the format string is not a
2371 string literal and so cannot be checked, unless the format function
2372 takes its format arguments as a @code{va_list}.
2374 @item -Wformat-security
2375 @opindex Wformat-security
2376 If @option{-Wformat} is specified, also warn about uses of format
2377 functions that represent possible security problems. At present, this
2378 warns about calls to @code{printf} and @code{scanf} functions where the
2379 format string is not a string literal and there are no format arguments,
2380 as in @code{printf (foo);}. This may be a security hole if the format
2381 string came from untrusted input and contains @samp{%n}. (This is
2382 currently a subset of what @option{-Wformat-nonliteral} warns about, but
2383 in future warnings may be added to @option{-Wformat-security} that are not
2384 included in @option{-Wformat-nonliteral}.)
2388 Enable @option{-Wformat} plus format checks not included in
2389 @option{-Wformat}. Currently equivalent to @samp{-Wformat
2390 -Wformat-nonliteral -Wformat-security -Wformat-y2k}.
2394 Warn about passing a null pointer for arguments marked as
2395 requiring a non-null value by the @code{nonnull} function attribute.
2397 @option{-Wnonnull} is included in @option{-Wall} and @option{-Wformat}. It
2398 can be disabled with the @option{-Wno-nonnull} option.
2400 @item -Winit-self @r{(C, C++, Objective-C and Objective-C++ only)}
2402 Warn about uninitialized variables which are initialized with themselves.
2403 Note this option can only be used with the @option{-Wuninitialized} option,
2404 which in turn only works with @option{-O1} and above.
2406 For example, GCC will warn about @code{i} being uninitialized in the
2407 following snippet only when @option{-Winit-self} has been specified:
2418 @item -Wimplicit-int
2419 @opindex Wimplicit-int
2420 Warn when a declaration does not specify a type.
2421 This warning is enabled by @option{-Wall}.
2423 @item -Wimplicit-function-declaration
2424 @itemx -Werror-implicit-function-declaration
2425 @opindex Wimplicit-function-declaration
2426 @opindex Werror-implicit-function-declaration
2427 Give a warning (or error) whenever a function is used before being
2428 declared. The form @option{-Wno-error-implicit-function-declaration}
2430 This warning is enabled by @option{-Wall} (as a warning, not an error).
2434 Same as @option{-Wimplicit-int} and @option{-Wimplicit-function-declaration}.
2435 This warning is enabled by @option{-Wall}.
2439 Warn if the type of @samp{main} is suspicious. @samp{main} should be a
2440 function with external linkage, returning int, taking either zero
2441 arguments, two, or three arguments of appropriate types.
2442 This warning is enabled by @option{-Wall}.
2444 @item -Wmissing-braces
2445 @opindex Wmissing-braces
2446 Warn if an aggregate or union initializer is not fully bracketed. In
2447 the following example, the initializer for @samp{a} is not fully
2448 bracketed, but that for @samp{b} is fully bracketed.
2451 int a[2][2] = @{ 0, 1, 2, 3 @};
2452 int b[2][2] = @{ @{ 0, 1 @}, @{ 2, 3 @} @};
2455 This warning is enabled by @option{-Wall}.
2457 @item -Wmissing-include-dirs @r{(C, C++, Objective-C and Objective-C++ only)}
2458 @opindex Wmissing-include-dirs
2459 Warn if a user-supplied include directory does not exist.
2462 @opindex Wparentheses
2463 Warn if parentheses are omitted in certain contexts, such
2464 as when there is an assignment in a context where a truth value
2465 is expected, or when operators are nested whose precedence people
2466 often get confused about. Only the warning for an assignment used as
2467 a truth value is supported when compiling C++; the other warnings are
2468 only supported when compiling C@.
2470 Also warn if a comparison like @samp{x<=y<=z} appears; this is
2471 equivalent to @samp{(x<=y ? 1 : 0) <= z}, which is a different
2472 interpretation from that of ordinary mathematical notation.
2474 Also warn about constructions where there may be confusion to which
2475 @code{if} statement an @code{else} branch belongs. Here is an example of
2490 In C, every @code{else} branch belongs to the innermost possible @code{if}
2491 statement, which in this example is @code{if (b)}. This is often not
2492 what the programmer expected, as illustrated in the above example by
2493 indentation the programmer chose. When there is the potential for this
2494 confusion, GCC will issue a warning when this flag is specified.
2495 To eliminate the warning, add explicit braces around the innermost
2496 @code{if} statement so there is no way the @code{else} could belong to
2497 the enclosing @code{if}. The resulting code would look like this:
2513 This warning is enabled by @option{-Wall}.
2515 @item -Wsequence-point
2516 @opindex Wsequence-point
2517 Warn about code that may have undefined semantics because of violations
2518 of sequence point rules in the C standard.
2520 The C standard defines the order in which expressions in a C program are
2521 evaluated in terms of @dfn{sequence points}, which represent a partial
2522 ordering between the execution of parts of the program: those executed
2523 before the sequence point, and those executed after it. These occur
2524 after the evaluation of a full expression (one which is not part of a
2525 larger expression), after the evaluation of the first operand of a
2526 @code{&&}, @code{||}, @code{? :} or @code{,} (comma) operator, before a
2527 function is called (but after the evaluation of its arguments and the
2528 expression denoting the called function), and in certain other places.
2529 Other than as expressed by the sequence point rules, the order of
2530 evaluation of subexpressions of an expression is not specified. All
2531 these rules describe only a partial order rather than a total order,
2532 since, for example, if two functions are called within one expression
2533 with no sequence point between them, the order in which the functions
2534 are called is not specified. However, the standards committee have
2535 ruled that function calls do not overlap.
2537 It is not specified when between sequence points modifications to the
2538 values of objects take effect. Programs whose behavior depends on this
2539 have undefined behavior; the C standard specifies that ``Between the
2540 previous and next sequence point an object shall have its stored value
2541 modified at most once by the evaluation of an expression. Furthermore,
2542 the prior value shall be read only to determine the value to be
2543 stored.''. If a program breaks these rules, the results on any
2544 particular implementation are entirely unpredictable.
2546 Examples of code with undefined behavior are @code{a = a++;}, @code{a[n]
2547 = b[n++]} and @code{a[i++] = i;}. Some more complicated cases are not
2548 diagnosed by this option, and it may give an occasional false positive
2549 result, but in general it has been found fairly effective at detecting
2550 this sort of problem in programs.
2552 The present implementation of this option only works for C programs. A
2553 future implementation may also work for C++ programs.
2555 The C standard is worded confusingly, therefore there is some debate
2556 over the precise meaning of the sequence point rules in subtle cases.
2557 Links to discussions of the problem, including proposed formal
2558 definitions, may be found on the GCC readings page, at
2559 @w{@uref{http://gcc.gnu.org/readings.html}}.
2561 This warning is enabled by @option{-Wall}.
2564 @opindex Wreturn-type
2565 Warn whenever a function is defined with a return-type that defaults to
2566 @code{int}. Also warn about any @code{return} statement with no
2567 return-value in a function whose return-type is not @code{void}.
2569 For C, also warn if the return type of a function has a type qualifier
2570 such as @code{const}. Such a type qualifier has no effect, since the
2571 value returned by a function is not an lvalue. ISO C prohibits
2572 qualified @code{void} return types on function definitions, so such
2573 return types always receive a warning even without this option.
2575 For C++, a function without return type always produces a diagnostic
2576 message, even when @option{-Wno-return-type} is specified. The only
2577 exceptions are @samp{main} and functions defined in system headers.
2579 This warning is enabled by @option{-Wall}.
2583 Warn whenever a @code{switch} statement has an index of enumerated type
2584 and lacks a @code{case} for one or more of the named codes of that
2585 enumeration. (The presence of a @code{default} label prevents this
2586 warning.) @code{case} labels outside the enumeration range also
2587 provoke warnings when this option is used.
2588 This warning is enabled by @option{-Wall}.
2590 @item -Wswitch-default
2591 @opindex Wswitch-switch
2592 Warn whenever a @code{switch} statement does not have a @code{default}
2596 @opindex Wswitch-enum
2597 Warn whenever a @code{switch} statement has an index of enumerated type
2598 and lacks a @code{case} for one or more of the named codes of that
2599 enumeration. @code{case} labels outside the enumeration range also
2600 provoke warnings when this option is used.
2604 Warn if any trigraphs are encountered that might change the meaning of
2605 the program (trigraphs within comments are not warned about).
2606 This warning is enabled by @option{-Wall}.
2608 @item -Wunused-function
2609 @opindex Wunused-function
2610 Warn whenever a static function is declared but not defined or a
2611 non-inline static function is unused.
2612 This warning is enabled by @option{-Wall}.
2614 @item -Wunused-label
2615 @opindex Wunused-label
2616 Warn whenever a label is declared but not used.
2617 This warning is enabled by @option{-Wall}.
2619 To suppress this warning use the @samp{unused} attribute
2620 (@pxref{Variable Attributes}).
2622 @item -Wunused-parameter
2623 @opindex Wunused-parameter
2624 Warn whenever a function parameter is unused aside from its declaration.
2626 To suppress this warning use the @samp{unused} attribute
2627 (@pxref{Variable Attributes}).
2629 @item -Wunused-variable
2630 @opindex Wunused-variable
2631 Warn whenever a local variable or non-constant static variable is unused
2632 aside from its declaration
2633 This warning is enabled by @option{-Wall}.
2635 To suppress this warning use the @samp{unused} attribute
2636 (@pxref{Variable Attributes}).
2638 @item -Wunused-value
2639 @opindex Wunused-value
2640 Warn whenever a statement computes a result that is explicitly not used.
2641 This warning is enabled by @option{-Wall}.
2643 To suppress this warning cast the expression to @samp{void}.
2647 All the above @option{-Wunused} options combined.
2649 In order to get a warning about an unused function parameter, you must
2650 either specify @samp{-Wextra -Wunused} (note that @samp{-Wall} implies
2651 @samp{-Wunused}), or separately specify @option{-Wunused-parameter}.
2653 @item -Wuninitialized
2654 @opindex Wuninitialized
2655 Warn if an automatic variable is used without first being initialized or
2656 if a variable may be clobbered by a @code{setjmp} call.
2658 These warnings are possible only in optimizing compilation,
2659 because they require data flow information that is computed only
2660 when optimizing. If you don't specify @option{-O}, you simply won't
2663 If you want to warn about code which uses the uninitialized value of the
2664 variable in its own initializer, use the @option{-Winit-self} option.
2666 These warnings occur for individual uninitialized or clobbered
2667 elements of structure, union or array variables as well as for
2668 variables which are uninitialized or clobbered as a whole. They do
2669 not occur for variables or elements declared @code{volatile}. Because
2670 these warnings depend on optimization, the exact variables or elements
2671 for which there are warnings will depend on the precise optimization
2672 options and version of GCC used.
2674 Note that there may be no warning about a variable that is used only
2675 to compute a value that itself is never used, because such
2676 computations may be deleted by data flow analysis before the warnings
2679 These warnings are made optional because GCC is not smart
2680 enough to see all the reasons why the code might be correct
2681 despite appearing to have an error. Here is one example of how
2702 If the value of @code{y} is always 1, 2 or 3, then @code{x} is
2703 always initialized, but GCC doesn't know this. Here is
2704 another common case:
2709 if (change_y) save_y = y, y = new_y;
2711 if (change_y) y = save_y;
2716 This has no bug because @code{save_y} is used only if it is set.
2718 @cindex @code{longjmp} warnings
2719 This option also warns when a non-volatile automatic variable might be
2720 changed by a call to @code{longjmp}. These warnings as well are possible
2721 only in optimizing compilation.
2723 The compiler sees only the calls to @code{setjmp}. It cannot know
2724 where @code{longjmp} will be called; in fact, a signal handler could
2725 call it at any point in the code. As a result, you may get a warning
2726 even when there is in fact no problem because @code{longjmp} cannot
2727 in fact be called at the place which would cause a problem.
2729 Some spurious warnings can be avoided if you declare all the functions
2730 you use that never return as @code{noreturn}. @xref{Function
2733 This warning is enabled by @option{-Wall}.
2735 @item -Wunknown-pragmas
2736 @opindex Wunknown-pragmas
2737 @cindex warning for unknown pragmas
2738 @cindex unknown pragmas, warning
2739 @cindex pragmas, warning of unknown
2740 Warn when a #pragma directive is encountered which is not understood by
2741 GCC@. If this command line option is used, warnings will even be issued
2742 for unknown pragmas in system header files. This is not the case if
2743 the warnings were only enabled by the @option{-Wall} command line option.
2746 @opindex Wno-pragmas
2748 Do not warn about misuses of pragmas, such as incorrect parameters,
2749 invalid syntax, or conflicts between pragmas. See also
2750 @samp{-Wunknown-pragmas}.
2752 @item -Wstrict-aliasing
2753 @opindex Wstrict-aliasing
2754 This option is only active when @option{-fstrict-aliasing} is active.
2755 It warns about code which might break the strict aliasing rules that the
2756 compiler is using for optimization. The warning does not catch all
2757 cases, but does attempt to catch the more common pitfalls. It is
2758 included in @option{-Wall}.
2760 @item -Wstrict-aliasing=2
2761 @opindex Wstrict-aliasing=2
2762 This option is only active when @option{-fstrict-aliasing} is active.
2763 It warns about code which might break the strict aliasing rules that the
2764 compiler is using for optimization. This warning catches more cases than
2765 @option{-Wstrict-aliasing}, but it will also give a warning for some ambiguous
2766 cases that are safe.
2770 All of the above @samp{-W} options combined. This enables all the
2771 warnings about constructions that some users consider questionable, and
2772 that are easy to avoid (or modify to prevent the warning), even in
2773 conjunction with macros. This also enables some language-specific
2774 warnings described in @ref{C++ Dialect Options} and
2775 @ref{Objective-C and Objective-C++ Dialect Options}.
2778 The following @option{-W@dots{}} options are not implied by @option{-Wall}.
2779 Some of them warn about constructions that users generally do not
2780 consider questionable, but which occasionally you might wish to check
2781 for; others warn about constructions that are necessary or hard to avoid
2782 in some cases, and there is no simple way to modify the code to suppress
2789 (This option used to be called @option{-W}. The older name is still
2790 supported, but the newer name is more descriptive.) Print extra warning
2791 messages for these events:
2795 A function can return either with or without a value. (Falling
2796 off the end of the function body is considered returning without
2797 a value.) For example, this function would evoke such a
2811 An expression-statement or the left-hand side of a comma expression
2812 contains no side effects.
2813 To suppress the warning, cast the unused expression to void.
2814 For example, an expression such as @samp{x[i,j]} will cause a warning,
2815 but @samp{x[(void)i,j]} will not.
2818 An unsigned value is compared against zero with @samp{<} or @samp{>=}.
2821 Storage-class specifiers like @code{static} are not the first things in
2822 a declaration. According to the C Standard, this usage is obsolescent.
2825 If @option{-Wall} or @option{-Wunused} is also specified, warn about unused
2829 A comparison between signed and unsigned values could produce an
2830 incorrect result when the signed value is converted to unsigned.
2831 (But don't warn if @option{-Wno-sign-compare} is also specified.)
2834 An aggregate has an initializer which does not initialize all members.
2835 This warning can be independently controlled by
2836 @option{-Wmissing-field-initializers}.
2839 A function parameter is declared without a type specifier in K&R-style
2847 An empty body occurs in an @samp{if} or @samp{else} statement.
2850 A pointer is compared against integer zero with @samp{<}, @samp{<=},
2851 @samp{>}, or @samp{>=}.
2854 A variable might be changed by @samp{longjmp} or @samp{vfork}.
2857 Any of several floating-point events that often indicate errors, such as
2858 overflow, underflow, loss of precision, etc.
2860 @item @r{(C++ only)}
2861 An enumerator and a non-enumerator both appear in a conditional expression.
2863 @item @r{(C++ only)}
2864 A non-static reference or non-static @samp{const} member appears in a
2865 class without constructors.
2867 @item @r{(C++ only)}
2868 Ambiguous virtual bases.
2870 @item @r{(C++ only)}
2871 Subscripting an array which has been declared @samp{register}.
2873 @item @r{(C++ only)}
2874 Taking the address of a variable which has been declared @samp{register}.
2876 @item @r{(C++ only)}
2877 A base class is not initialized in a derived class' copy constructor.
2880 @item -Wno-div-by-zero
2881 @opindex Wno-div-by-zero
2882 @opindex Wdiv-by-zero
2883 Do not warn about compile-time integer division by zero. Floating point
2884 division by zero is not warned about, as it can be a legitimate way of
2885 obtaining infinities and NaNs.
2887 @item -Wsystem-headers
2888 @opindex Wsystem-headers
2889 @cindex warnings from system headers
2890 @cindex system headers, warnings from
2891 Print warning messages for constructs found in system header files.
2892 Warnings from system headers are normally suppressed, on the assumption
2893 that they usually do not indicate real problems and would only make the
2894 compiler output harder to read. Using this command line option tells
2895 GCC to emit warnings from system headers as if they occurred in user
2896 code. However, note that using @option{-Wall} in conjunction with this
2897 option will @emph{not} warn about unknown pragmas in system
2898 headers---for that, @option{-Wunknown-pragmas} must also be used.
2901 @opindex Wfloat-equal
2902 Warn if floating point values are used in equality comparisons.
2904 The idea behind this is that sometimes it is convenient (for the
2905 programmer) to consider floating-point values as approximations to
2906 infinitely precise real numbers. If you are doing this, then you need
2907 to compute (by analyzing the code, or in some other way) the maximum or
2908 likely maximum error that the computation introduces, and allow for it
2909 when performing comparisons (and when producing output, but that's a
2910 different problem). In particular, instead of testing for equality, you
2911 would check to see whether the two values have ranges that overlap; and
2912 this is done with the relational operators, so equality comparisons are
2915 @item -Wtraditional @r{(C only)}
2916 @opindex Wtraditional
2917 Warn about certain constructs that behave differently in traditional and
2918 ISO C@. Also warn about ISO C constructs that have no traditional C
2919 equivalent, and/or problematic constructs which should be avoided.
2923 Macro parameters that appear within string literals in the macro body.
2924 In traditional C macro replacement takes place within string literals,
2925 but does not in ISO C@.
2928 In traditional C, some preprocessor directives did not exist.
2929 Traditional preprocessors would only consider a line to be a directive
2930 if the @samp{#} appeared in column 1 on the line. Therefore
2931 @option{-Wtraditional} warns about directives that traditional C
2932 understands but would ignore because the @samp{#} does not appear as the
2933 first character on the line. It also suggests you hide directives like
2934 @samp{#pragma} not understood by traditional C by indenting them. Some
2935 traditional implementations would not recognize @samp{#elif}, so it
2936 suggests avoiding it altogether.
2939 A function-like macro that appears without arguments.
2942 The unary plus operator.
2945 The @samp{U} integer constant suffix, or the @samp{F} or @samp{L} floating point
2946 constant suffixes. (Traditional C does support the @samp{L} suffix on integer
2947 constants.) Note, these suffixes appear in macros defined in the system
2948 headers of most modern systems, e.g.@: the @samp{_MIN}/@samp{_MAX} macros in @code{<limits.h>}.
2949 Use of these macros in user code might normally lead to spurious
2950 warnings, however GCC's integrated preprocessor has enough context to
2951 avoid warning in these cases.
2954 A function declared external in one block and then used after the end of
2958 A @code{switch} statement has an operand of type @code{long}.
2961 A non-@code{static} function declaration follows a @code{static} one.
2962 This construct is not accepted by some traditional C compilers.
2965 The ISO type of an integer constant has a different width or
2966 signedness from its traditional type. This warning is only issued if
2967 the base of the constant is ten. I.e.@: hexadecimal or octal values, which
2968 typically represent bit patterns, are not warned about.
2971 Usage of ISO string concatenation is detected.
2974 Initialization of automatic aggregates.
2977 Identifier conflicts with labels. Traditional C lacks a separate
2978 namespace for labels.
2981 Initialization of unions. If the initializer is zero, the warning is
2982 omitted. This is done under the assumption that the zero initializer in
2983 user code appears conditioned on e.g.@: @code{__STDC__} to avoid missing
2984 initializer warnings and relies on default initialization to zero in the
2988 Conversions by prototypes between fixed/floating point values and vice
2989 versa. The absence of these prototypes when compiling with traditional
2990 C would cause serious problems. This is a subset of the possible
2991 conversion warnings, for the full set use @option{-Wconversion}.
2994 Use of ISO C style function definitions. This warning intentionally is
2995 @emph{not} issued for prototype declarations or variadic functions
2996 because these ISO C features will appear in your code when using
2997 libiberty's traditional C compatibility macros, @code{PARAMS} and
2998 @code{VPARAMS}. This warning is also bypassed for nested functions
2999 because that feature is already a GCC extension and thus not relevant to
3000 traditional C compatibility.
3003 @item -Wdeclaration-after-statement @r{(C only)}
3004 @opindex Wdeclaration-after-statement
3005 Warn when a declaration is found after a statement in a block. This
3006 construct, known from C++, was introduced with ISO C99 and is by default
3007 allowed in GCC@. It is not supported by ISO C90 and was not supported by
3008 GCC versions before GCC 3.0. @xref{Mixed Declarations}.
3012 Warn if an undefined identifier is evaluated in an @samp{#if} directive.
3014 @item -Wno-endif-labels
3015 @opindex Wno-endif-labels
3016 @opindex Wendif-labels
3017 Do not warn whenever an @samp{#else} or an @samp{#endif} are followed by text.
3021 Warn whenever a local variable shadows another local variable, parameter or
3022 global variable or whenever a built-in function is shadowed.
3024 @item -Wlarger-than-@var{len}
3025 @opindex Wlarger-than
3026 Warn whenever an object of larger than @var{len} bytes is defined.
3028 @item -Wunsafe-loop-optimizations
3029 @opindex Wunsafe-loop-optimizations
3030 Warn if the loop cannot be optimized because the compiler could not
3031 assume anything on the bounds of the loop indices. With
3032 @option{-funsafe-loop-optimizations} warn if the compiler made
3035 @item -Wpointer-arith
3036 @opindex Wpointer-arith
3037 Warn about anything that depends on the ``size of'' a function type or
3038 of @code{void}. GNU C assigns these types a size of 1, for
3039 convenience in calculations with @code{void *} pointers and pointers
3042 @item -Wbad-function-cast @r{(C only)}
3043 @opindex Wbad-function-cast
3044 Warn whenever a function call is cast to a non-matching type.
3045 For example, warn if @code{int malloc()} is cast to @code{anything *}.
3048 Warn about ISO C constructs that are outside of the common subset of
3049 ISO C and ISO C++, e.g.@: request for implicit conversion from
3050 @code{void *} to a pointer to non-@code{void} type.
3054 Warn whenever a pointer is cast so as to remove a type qualifier from
3055 the target type. For example, warn if a @code{const char *} is cast
3056 to an ordinary @code{char *}.
3059 @opindex Wcast-align
3060 Warn whenever a pointer is cast such that the required alignment of the
3061 target is increased. For example, warn if a @code{char *} is cast to
3062 an @code{int *} on machines where integers can only be accessed at
3063 two- or four-byte boundaries.
3065 @item -Wwrite-strings
3066 @opindex Wwrite-strings
3067 When compiling C, give string constants the type @code{const
3068 char[@var{length}]} so that
3069 copying the address of one into a non-@code{const} @code{char *}
3070 pointer will get a warning; when compiling C++, warn about the
3071 deprecated conversion from string constants to @code{char *}.
3072 These warnings will help you find at
3073 compile time code that can try to write into a string constant, but
3074 only if you have been very careful about using @code{const} in
3075 declarations and prototypes. Otherwise, it will just be a nuisance;
3076 this is why we did not make @option{-Wall} request these warnings.
3079 @opindex Wconversion
3080 Warn if a prototype causes a type conversion that is different from what
3081 would happen to the same argument in the absence of a prototype. This
3082 includes conversions of fixed point to floating and vice versa, and
3083 conversions changing the width or signedness of a fixed point argument
3084 except when the same as the default promotion.
3086 Also, warn if a negative integer constant expression is implicitly
3087 converted to an unsigned type. For example, warn about the assignment
3088 @code{x = -1} if @code{x} is unsigned. But do not warn about explicit
3089 casts like @code{(unsigned) -1}.
3091 @item -Wsign-compare
3092 @opindex Wsign-compare
3093 @cindex warning for comparison of signed and unsigned values
3094 @cindex comparison of signed and unsigned values, warning
3095 @cindex signed and unsigned values, comparison warning
3096 Warn when a comparison between signed and unsigned values could produce
3097 an incorrect result when the signed value is converted to unsigned.
3098 This warning is also enabled by @option{-Wextra}; to get the other warnings
3099 of @option{-Wextra} without this warning, use @samp{-Wextra -Wno-sign-compare}.
3101 @item -Waggregate-return
3102 @opindex Waggregate-return
3103 Warn if any functions that return structures or unions are defined or
3104 called. (In languages where you can return an array, this also elicits
3107 @item -Wno-attributes
3108 @opindex Wno-attributes
3109 @opindex Wattributes
3110 Do not warn if an unexpected @code{__attribute__} is used, such as
3111 unrecognized attributes, function attributes applied to variables,
3112 etc. This will not stop errors for incorrect use of supported
3115 @item -Wstrict-prototypes @r{(C only)}
3116 @opindex Wstrict-prototypes
3117 Warn if a function is declared or defined without specifying the
3118 argument types. (An old-style function definition is permitted without
3119 a warning if preceded by a declaration which specifies the argument
3122 @item -Wold-style-definition @r{(C only)}
3123 @opindex Wold-style-definition
3124 Warn if an old-style function definition is used. A warning is given
3125 even if there is a previous prototype.
3127 @item -Wmissing-prototypes @r{(C only)}
3128 @opindex Wmissing-prototypes
3129 Warn if a global function is defined without a previous prototype
3130 declaration. This warning is issued even if the definition itself
3131 provides a prototype. The aim is to detect global functions that fail
3132 to be declared in header files.
3134 @item -Wmissing-declarations @r{(C only)}
3135 @opindex Wmissing-declarations
3136 Warn if a global function is defined without a previous declaration.
3137 Do so even if the definition itself provides a prototype.
3138 Use this option to detect global functions that are not declared in
3141 @item -Wmissing-field-initializers
3142 @opindex Wmissing-field-initializers
3145 Warn if a structure's initializer has some fields missing. For
3146 example, the following code would cause such a warning, because
3147 @code{x.h} is implicitly zero:
3150 struct s @{ int f, g, h; @};
3151 struct s x = @{ 3, 4 @};
3154 This option does not warn about designated initializers, so the following
3155 modification would not trigger a warning:
3158 struct s @{ int f, g, h; @};
3159 struct s x = @{ .f = 3, .g = 4 @};
3162 This warning is included in @option{-Wextra}. To get other @option{-Wextra}
3163 warnings without this one, use @samp{-Wextra -Wno-missing-field-initializers}.
3165 @item -Wmissing-noreturn
3166 @opindex Wmissing-noreturn
3167 Warn about functions which might be candidates for attribute @code{noreturn}.
3168 Note these are only possible candidates, not absolute ones. Care should
3169 be taken to manually verify functions actually do not ever return before
3170 adding the @code{noreturn} attribute, otherwise subtle code generation
3171 bugs could be introduced. You will not get a warning for @code{main} in
3172 hosted C environments.
3174 @item -Wmissing-format-attribute
3175 @opindex Wmissing-format-attribute
3177 Warn about function pointers which might be candidates for @code{format}
3178 attributes. Note these are only possible candidates, not absolute ones.
3179 GCC will guess that function pointers with @code{format} attributes that
3180 are used in assignment, initialization, parameter passing or return
3181 statements should have a corresponding @code{format} attribute in the
3182 resulting type. I.e.@: the left-hand side of the assignment or
3183 initialization, the type of the parameter variable, or the return type
3184 of the containing function respectively should also have a @code{format}
3185 attribute to avoid the warning.
3187 GCC will also warn about function definitions which might be
3188 candidates for @code{format} attributes. Again, these are only
3189 possible candidates. GCC will guess that @code{format} attributes
3190 might be appropriate for any function that calls a function like
3191 @code{vprintf} or @code{vscanf}, but this might not always be the
3192 case, and some functions for which @code{format} attributes are
3193 appropriate may not be detected.
3195 @item -Wno-multichar
3196 @opindex Wno-multichar
3198 Do not warn if a multicharacter constant (@samp{'FOOF'}) is used.
3199 Usually they indicate a typo in the user's code, as they have
3200 implementation-defined values, and should not be used in portable code.
3202 @item -Wnormalized=<none|id|nfc|nfkc>
3203 @opindex Wnormalized
3206 @cindex character set, input normalization
3207 In ISO C and ISO C++, two identifiers are different if they are
3208 different sequences of characters. However, sometimes when characters
3209 outside the basic ASCII character set are used, you can have two
3210 different character sequences that look the same. To avoid confusion,
3211 the ISO 10646 standard sets out some @dfn{normalization rules} which
3212 when applied ensure that two sequences that look the same are turned into
3213 the same sequence. GCC can warn you if you are using identifiers which
3214 have not been normalized; this option controls that warning.
3216 There are four levels of warning that GCC supports. The default is
3217 @option{-Wnormalized=nfc}, which warns about any identifier which is
3218 not in the ISO 10646 ``C'' normalized form, @dfn{NFC}. NFC is the
3219 recommended form for most uses.
3221 Unfortunately, there are some characters which ISO C and ISO C++ allow
3222 in identifiers that when turned into NFC aren't allowable as
3223 identifiers. That is, there's no way to use these symbols in portable
3224 ISO C or C++ and have all your identifiers in NFC.
3225 @option{-Wnormalized=id} suppresses the warning for these characters.
3226 It is hoped that future versions of the standards involved will correct
3227 this, which is why this option is not the default.
3229 You can switch the warning off for all characters by writing
3230 @option{-Wnormalized=none}. You would only want to do this if you
3231 were using some other normalization scheme (like ``D''), because
3232 otherwise you can easily create bugs that are literally impossible to see.
3234 Some characters in ISO 10646 have distinct meanings but look identical
3235 in some fonts or display methodologies, especially once formatting has
3236 been applied. For instance @code{\u207F}, ``SUPERSCRIPT LATIN SMALL
3237 LETTER N'', will display just like a regular @code{n} which has been
3238 placed in a superscript. ISO 10646 defines the @dfn{NFKC}
3239 normalisation scheme to convert all these into a standard form as
3240 well, and GCC will warn if your code is not in NFKC if you use
3241 @option{-Wnormalized=nfkc}. This warning is comparable to warning
3242 about every identifier that contains the letter O because it might be
3243 confused with the digit 0, and so is not the default, but may be
3244 useful as a local coding convention if the programming environment is
3245 unable to be fixed to display these characters distinctly.
3247 @item -Wno-deprecated-declarations
3248 @opindex Wno-deprecated-declarations
3249 Do not warn about uses of functions, variables, and types marked as
3250 deprecated by using the @code{deprecated} attribute.
3251 (@pxref{Function Attributes}, @pxref{Variable Attributes},
3252 @pxref{Type Attributes}.)
3256 Warn if a structure is given the packed attribute, but the packed
3257 attribute has no effect on the layout or size of the structure.
3258 Such structures may be mis-aligned for little benefit. For
3259 instance, in this code, the variable @code{f.x} in @code{struct bar}
3260 will be misaligned even though @code{struct bar} does not itself
3261 have the packed attribute:
3268 @} __attribute__((packed));
3278 Warn if padding is included in a structure, either to align an element
3279 of the structure or to align the whole structure. Sometimes when this
3280 happens it is possible to rearrange the fields of the structure to
3281 reduce the padding and so make the structure smaller.
3283 @item -Wredundant-decls
3284 @opindex Wredundant-decls
3285 Warn if anything is declared more than once in the same scope, even in
3286 cases where multiple declaration is valid and changes nothing.
3288 @item -Wnested-externs @r{(C only)}
3289 @opindex Wnested-externs
3290 Warn if an @code{extern} declaration is encountered within a function.
3292 @item -Wunreachable-code
3293 @opindex Wunreachable-code
3294 Warn if the compiler detects that code will never be executed.
3296 This option is intended to warn when the compiler detects that at
3297 least a whole line of source code will never be executed, because
3298 some condition is never satisfied or because it is after a
3299 procedure that never returns.
3301 It is possible for this option to produce a warning even though there
3302 are circumstances under which part of the affected line can be executed,
3303 so care should be taken when removing apparently-unreachable code.
3305 For instance, when a function is inlined, a warning may mean that the
3306 line is unreachable in only one inlined copy of the function.
3308 This option is not made part of @option{-Wall} because in a debugging
3309 version of a program there is often substantial code which checks
3310 correct functioning of the program and is, hopefully, unreachable
3311 because the program does work. Another common use of unreachable
3312 code is to provide behavior which is selectable at compile-time.
3316 Warn if a function can not be inlined and it was declared as inline.
3317 Even with this option, the compiler will not warn about failures to
3318 inline functions declared in system headers.
3320 The compiler uses a variety of heuristics to determine whether or not
3321 to inline a function. For example, the compiler takes into account
3322 the size of the function being inlined and the amount of inlining
3323 that has already been done in the current function. Therefore,
3324 seemingly insignificant changes in the source program can cause the
3325 warnings produced by @option{-Winline} to appear or disappear.
3327 @item -Wno-invalid-offsetof @r{(C++ only)}
3328 @opindex Wno-invalid-offsetof
3329 Suppress warnings from applying the @samp{offsetof} macro to a non-POD
3330 type. According to the 1998 ISO C++ standard, applying @samp{offsetof}
3331 to a non-POD type is undefined. In existing C++ implementations,
3332 however, @samp{offsetof} typically gives meaningful results even when
3333 applied to certain kinds of non-POD types. (Such as a simple
3334 @samp{struct} that fails to be a POD type only by virtue of having a
3335 constructor.) This flag is for users who are aware that they are
3336 writing nonportable code and who have deliberately chosen to ignore the
3339 The restrictions on @samp{offsetof} may be relaxed in a future version
3340 of the C++ standard.
3342 @item -Wno-int-to-pointer-cast @r{(C only)}
3343 @opindex Wno-int-to-pointer-cast
3344 Suppress warnings from casts to pointer type of an integer of a
3347 @item -Wno-pointer-to-int-cast @r{(C only)}
3348 @opindex Wno-pointer-to-int-cast
3349 Suppress warnings from casts from a pointer to an integer type of a
3353 @opindex Winvalid-pch
3354 Warn if a precompiled header (@pxref{Precompiled Headers}) is found in
3355 the search path but can't be used.
3359 @opindex Wno-long-long
3360 Warn if @samp{long long} type is used. This is default. To inhibit
3361 the warning messages, use @option{-Wno-long-long}. Flags
3362 @option{-Wlong-long} and @option{-Wno-long-long} are taken into account
3363 only when @option{-pedantic} flag is used.
3365 @item -Wvariadic-macros
3366 @opindex Wvariadic-macros
3367 @opindex Wno-variadic-macros
3368 Warn if variadic macros are used in pedantic ISO C90 mode, or the GNU
3369 alternate syntax when in pedantic ISO C99 mode. This is default.
3370 To inhibit the warning messages, use @option{-Wno-variadic-macros}.
3372 @item -Wdisabled-optimization
3373 @opindex Wdisabled-optimization
3374 Warn if a requested optimization pass is disabled. This warning does
3375 not generally indicate that there is anything wrong with your code; it
3376 merely indicates that GCC's optimizers were unable to handle the code
3377 effectively. Often, the problem is that your code is too big or too
3378 complex; GCC will refuse to optimize programs when the optimization
3379 itself is likely to take inordinate amounts of time.
3381 @item -Wno-pointer-sign
3382 @opindex Wno-pointer-sign
3383 Don't warn for pointer argument passing or assignment with different signedness.
3384 Only useful in the negative form since this warning is enabled by default.
3385 This option is only supported for C and Objective-C@.
3389 Make all warnings into errors.
3391 @item -Wstack-protector
3392 This option is only active when @option{-fstack-protector} is active. It
3393 warns about functions that will not be protected against stack smashing.
3397 @node Debugging Options
3398 @section Options for Debugging Your Program or GCC
3399 @cindex options, debugging
3400 @cindex debugging information options
3402 GCC has various special options that are used for debugging
3403 either your program or GCC:
3408 Produce debugging information in the operating system's native format
3409 (stabs, COFF, XCOFF, or DWARF 2)@. GDB can work with this debugging
3412 On most systems that use stabs format, @option{-g} enables use of extra
3413 debugging information that only GDB can use; this extra information
3414 makes debugging work better in GDB but will probably make other debuggers
3416 refuse to read the program. If you want to control for certain whether
3417 to generate the extra information, use @option{-gstabs+}, @option{-gstabs},
3418 @option{-gxcoff+}, @option{-gxcoff}, or @option{-gvms} (see below).
3420 GCC allows you to use @option{-g} with
3421 @option{-O}. The shortcuts taken by optimized code may occasionally
3422 produce surprising results: some variables you declared may not exist
3423 at all; flow of control may briefly move where you did not expect it;
3424 some statements may not be executed because they compute constant
3425 results or their values were already at hand; some statements may
3426 execute in different places because they were moved out of loops.
3428 Nevertheless it proves possible to debug optimized output. This makes
3429 it reasonable to use the optimizer for programs that might have bugs.
3431 The following options are useful when GCC is generated with the
3432 capability for more than one debugging format.
3436 Produce debugging information for use by GDB@. This means to use the
3437 most expressive format available (DWARF 2, stabs, or the native format
3438 if neither of those are supported), including GDB extensions if at all
3443 Produce debugging information in stabs format (if that is supported),
3444 without GDB extensions. This is the format used by DBX on most BSD
3445 systems. On MIPS, Alpha and System V Release 4 systems this option
3446 produces stabs debugging output which is not understood by DBX or SDB@.
3447 On System V Release 4 systems this option requires the GNU assembler.
3449 @item -feliminate-unused-debug-symbols
3450 @opindex feliminate-unused-debug-symbols
3451 Produce debugging information in stabs format (if that is supported),
3452 for only symbols that are actually used.
3456 Produce debugging information in stabs format (if that is supported),
3457 using GNU extensions understood only by the GNU debugger (GDB)@. The
3458 use of these extensions is likely to make other debuggers crash or
3459 refuse to read the program.
3463 Produce debugging information in COFF format (if that is supported).
3464 This is the format used by SDB on most System V systems prior to
3469 Produce debugging information in XCOFF format (if that is supported).
3470 This is the format used by the DBX debugger on IBM RS/6000 systems.
3474 Produce debugging information in XCOFF format (if that is supported),
3475 using GNU extensions understood only by the GNU debugger (GDB)@. The
3476 use of these extensions is likely to make other debuggers crash or
3477 refuse to read the program, and may cause assemblers other than the GNU
3478 assembler (GAS) to fail with an error.
3482 Produce debugging information in DWARF version 2 format (if that is
3483 supported). This is the format used by DBX on IRIX 6. With this
3484 option, GCC uses features of DWARF version 3 when they are useful;
3485 version 3 is upward compatible with version 2, but may still cause
3486 problems for older debuggers.
3490 Produce debugging information in VMS debug format (if that is
3491 supported). This is the format used by DEBUG on VMS systems.
3494 @itemx -ggdb@var{level}
3495 @itemx -gstabs@var{level}
3496 @itemx -gcoff@var{level}
3497 @itemx -gxcoff@var{level}
3498 @itemx -gvms@var{level}
3499 Request debugging information and also use @var{level} to specify how
3500 much information. The default level is 2.
3502 Level 1 produces minimal information, enough for making backtraces in
3503 parts of the program that you don't plan to debug. This includes
3504 descriptions of functions and external variables, but no information
3505 about local variables and no line numbers.
3507 Level 3 includes extra information, such as all the macro definitions
3508 present in the program. Some debuggers support macro expansion when
3509 you use @option{-g3}.
3511 @option{-gdwarf-2} does not accept a concatenated debug level, because
3512 GCC used to support an option @option{-gdwarf} that meant to generate
3513 debug information in version 1 of the DWARF format (which is very
3514 different from version 2), and it would have been too confusing. That
3515 debug format is long obsolete, but the option cannot be changed now.
3516 Instead use an additional @option{-g@var{level}} option to change the
3517 debug level for DWARF2.
3519 @item -feliminate-dwarf2-dups
3520 @opindex feliminate-dwarf2-dups
3521 Compress DWARF2 debugging information by eliminating duplicated
3522 information about each symbol. This option only makes sense when
3523 generating DWARF2 debugging information with @option{-gdwarf-2}.
3525 @cindex @command{prof}
3528 Generate extra code to write profile information suitable for the
3529 analysis program @command{prof}. You must use this option when compiling
3530 the source files you want data about, and you must also use it when
3533 @cindex @command{gprof}
3536 Generate extra code to write profile information suitable for the
3537 analysis program @command{gprof}. You must use this option when compiling
3538 the source files you want data about, and you must also use it when
3543 Makes the compiler print out each function name as it is compiled, and
3544 print some statistics about each pass when it finishes.
3547 @opindex ftime-report
3548 Makes the compiler print some statistics about the time consumed by each
3549 pass when it finishes.
3552 @opindex fmem-report
3553 Makes the compiler print some statistics about permanent memory
3554 allocation when it finishes.
3556 @item -fprofile-arcs
3557 @opindex fprofile-arcs
3558 Add code so that program flow @dfn{arcs} are instrumented. During
3559 execution the program records how many times each branch and call is
3560 executed and how many times it is taken or returns. When the compiled
3561 program exits it saves this data to a file called
3562 @file{@var{auxname}.gcda} for each source file. The data may be used for
3563 profile-directed optimizations (@option{-fbranch-probabilities}), or for
3564 test coverage analysis (@option{-ftest-coverage}). Each object file's
3565 @var{auxname} is generated from the name of the output file, if
3566 explicitly specified and it is not the final executable, otherwise it is
3567 the basename of the source file. In both cases any suffix is removed
3568 (e.g.@: @file{foo.gcda} for input file @file{dir/foo.c}, or
3569 @file{dir/foo.gcda} for output file specified as @option{-o dir/foo.o}).
3570 @xref{Cross-profiling}.
3572 @cindex @command{gcov}
3576 This option is used to compile and link code instrumented for coverage
3577 analysis. The option is a synonym for @option{-fprofile-arcs}
3578 @option{-ftest-coverage} (when compiling) and @option{-lgcov} (when
3579 linking). See the documentation for those options for more details.
3584 Compile the source files with @option{-fprofile-arcs} plus optimization
3585 and code generation options. For test coverage analysis, use the
3586 additional @option{-ftest-coverage} option. You do not need to profile
3587 every source file in a program.
3590 Link your object files with @option{-lgcov} or @option{-fprofile-arcs}
3591 (the latter implies the former).
3594 Run the program on a representative workload to generate the arc profile
3595 information. This may be repeated any number of times. You can run
3596 concurrent instances of your program, and provided that the file system
3597 supports locking, the data files will be correctly updated. Also
3598 @code{fork} calls are detected and correctly handled (double counting
3602 For profile-directed optimizations, compile the source files again with
3603 the same optimization and code generation options plus
3604 @option{-fbranch-probabilities} (@pxref{Optimize Options,,Options that
3605 Control Optimization}).
3608 For test coverage analysis, use @command{gcov} to produce human readable
3609 information from the @file{.gcno} and @file{.gcda} files. Refer to the
3610 @command{gcov} documentation for further information.
3614 With @option{-fprofile-arcs}, for each function of your program GCC
3615 creates a program flow graph, then finds a spanning tree for the graph.
3616 Only arcs that are not on the spanning tree have to be instrumented: the
3617 compiler adds code to count the number of times that these arcs are
3618 executed. When an arc is the only exit or only entrance to a block, the
3619 instrumentation code can be added to the block; otherwise, a new basic
3620 block must be created to hold the instrumentation code.
3623 @item -ftest-coverage
3624 @opindex ftest-coverage
3625 Produce a notes file that the @command{gcov} code-coverage utility
3626 (@pxref{Gcov,, @command{gcov}---a Test Coverage Program}) can use to
3627 show program coverage. Each source file's note file is called
3628 @file{@var{auxname}.gcno}. Refer to the @option{-fprofile-arcs} option
3629 above for a description of @var{auxname} and instructions on how to
3630 generate test coverage data. Coverage data will match the source files
3631 more closely, if you do not optimize.
3633 @item -d@var{letters}
3634 @item -fdump-rtl-@var{pass}
3636 Says to make debugging dumps during compilation at times specified by
3637 @var{letters}. This is used for debugging the RTL-based passes of the
3638 compiler. The file names for most of the dumps are made by appending a
3639 pass number and a word to the @var{dumpname}. @var{dumpname} is generated
3640 from the name of the output file, if explicitly specified and it is not
3641 an executable, otherwise it is the basename of the source file.
3643 Most debug dumps can be enabled either passing a letter to the @option{-d}
3644 option, or with a long @option{-fdump-rtl} switch; here are the possible
3645 letters for use in @var{letters} and @var{pass}, and their meanings:
3650 Annotate the assembler output with miscellaneous debugging information.
3653 @itemx -fdump-rtl-bp
3655 @opindex fdump-rtl-bp
3656 Dump after computing branch probabilities, to @file{@var{file}.09.bp}.
3659 @itemx -fdump-rtl-bbro
3661 @opindex fdump-rtl-bbro
3662 Dump after block reordering, to @file{@var{file}.30.bbro}.
3665 @itemx -fdump-rtl-combine
3667 @opindex fdump-rtl-combine
3668 Dump after instruction combination, to the file @file{@var{file}.17.combine}.
3671 @itemx -fdump-rtl-ce1
3672 @itemx -fdump-rtl-ce2
3674 @opindex fdump-rtl-ce1
3675 @opindex fdump-rtl-ce2
3676 @option{-dC} and @option{-fdump-rtl-ce1} enable dumping after the
3677 first if conversion, to the file @file{@var{file}.11.ce1}. @option{-dC}
3678 and @option{-fdump-rtl-ce2} enable dumping after the second if
3679 conversion, to the file @file{@var{file}.18.ce2}.
3682 @itemx -fdump-rtl-btl
3683 @itemx -fdump-rtl-dbr
3685 @opindex fdump-rtl-btl
3686 @opindex fdump-rtl-dbr
3687 @option{-dd} and @option{-fdump-rtl-btl} enable dumping after branch
3688 target load optimization, to @file{@var{file}.31.btl}. @option{-dd}
3689 and @option{-fdump-rtl-dbr} enable dumping after delayed branch
3690 scheduling, to @file{@var{file}.36.dbr}.
3694 Dump all macro definitions, at the end of preprocessing, in addition to
3698 @itemx -fdump-rtl-ce3
3700 @opindex fdump-rtl-ce3
3701 Dump after the third if conversion, to @file{@var{file}.28.ce3}.
3704 @itemx -fdump-rtl-cfg
3705 @itemx -fdump-rtl-life
3707 @opindex fdump-rtl-cfg
3708 @opindex fdump-rtl-life
3709 @option{-df} and @option{-fdump-rtl-cfg} enable dumping after control
3710 and data flow analysis, to @file{@var{file}.08.cfg}. @option{-df}
3711 and @option{-fdump-rtl-cfg} enable dumping dump after life analysis,
3712 to @file{@var{file}.16.life}.
3715 @itemx -fdump-rtl-greg
3717 @opindex fdump-rtl-greg
3718 Dump after global register allocation, to @file{@var{file}.23.greg}.
3721 @itemx -fdump-rtl-gcse
3722 @itemx -fdump-rtl-bypass
3724 @opindex fdump-rtl-gcse
3725 @opindex fdump-rtl-bypass
3726 @option{-dG} and @option{-fdump-rtl-gcse} enable dumping after GCSE, to
3727 @file{@var{file}.05.gcse}. @option{-dG} and @option{-fdump-rtl-bypass}
3728 enable dumping after jump bypassing and control flow optimizations, to
3729 @file{@var{file}.07.bypass}.
3732 @itemx -fdump-rtl-eh
3734 @opindex fdump-rtl-eh
3735 Dump after finalization of EH handling code, to @file{@var{file}.02.eh}.
3738 @itemx -fdump-rtl-sibling
3740 @opindex fdump-rtl-sibling
3741 Dump after sibling call optimizations, to @file{@var{file}.01.sibling}.
3744 @itemx -fdump-rtl-jump
3746 @opindex fdump-rtl-jump
3747 Dump after the first jump optimization, to @file{@var{file}.03.jump}.
3750 @itemx -fdump-rtl-stack
3752 @opindex fdump-rtl-stack
3753 Dump after conversion from registers to stack, to @file{@var{file}.33.stack}.
3756 @itemx -fdump-rtl-lreg
3758 @opindex fdump-rtl-lreg
3759 Dump after local register allocation, to @file{@var{file}.22.lreg}.
3762 @itemx -fdump-rtl-loop
3763 @itemx -fdump-rtl-loop2
3765 @opindex fdump-rtl-loop
3766 @opindex fdump-rtl-loop2
3767 @option{-dL} and @option{-fdump-rtl-loop} enable dumping after the first
3768 loop optimization pass, to @file{@var{file}.06.loop}. @option{-dL} and
3769 @option{-fdump-rtl-loop2} enable dumping after the second pass, to
3770 @file{@var{file}.13.loop2}.
3773 @itemx -fdump-rtl-sms
3775 @opindex fdump-rtl-sms
3776 Dump after modulo scheduling, to @file{@var{file}.20.sms}.
3779 @itemx -fdump-rtl-mach
3781 @opindex fdump-rtl-mach
3782 Dump after performing the machine dependent reorganization pass, to
3783 @file{@var{file}.35.mach}.
3786 @itemx -fdump-rtl-rnreg
3788 @opindex fdump-rtl-rnreg
3789 Dump after register renumbering, to @file{@var{file}.29.rnreg}.
3792 @itemx -fdump-rtl-regmove
3794 @opindex fdump-rtl-regmove
3795 Dump after the register move pass, to @file{@var{file}.19.regmove}.
3798 @itemx -fdump-rtl-postreload
3800 @opindex fdump-rtl-postreload
3801 Dump after post-reload optimizations, to @file{@var{file}.24.postreload}.
3804 @itemx -fdump-rtl-expand
3806 @opindex fdump-rtl-expand
3807 Dump after RTL generation, to @file{@var{file}.00.expand}.
3810 @itemx -fdump-rtl-sched2
3812 @opindex fdump-rtl-sched2
3813 Dump after the second scheduling pass, to @file{@var{file}.32.sched2}.
3816 @itemx -fdump-rtl-cse
3818 @opindex fdump-rtl-cse
3819 Dump after CSE (including the jump optimization that sometimes follows
3820 CSE), to @file{@var{file}.04.cse}.
3823 @itemx -fdump-rtl-sched
3825 @opindex fdump-rtl-sched
3826 Dump after the first scheduling pass, to @file{@var{file}.21.sched}.
3829 @itemx -fdump-rtl-cse2
3831 @opindex fdump-rtl-cse2
3832 Dump after the second CSE pass (including the jump optimization that
3833 sometimes follows CSE), to @file{@var{file}.15.cse2}.
3836 @itemx -fdump-rtl-tracer
3838 @opindex fdump-rtl-tracer
3839 Dump after running tracer, to @file{@var{file}.12.tracer}.
3842 @itemx -fdump-rtl-vpt
3843 @itemx -fdump-rtl-vartrack
3845 @opindex fdump-rtl-vpt
3846 @opindex fdump-rtl-vartrack
3847 @option{-dV} and @option{-fdump-rtl-vpt} enable dumping after the value
3848 profile transformations, to @file{@var{file}.10.vpt}. @option{-dV}
3849 and @option{-fdump-rtl-vartrack} enable dumping after variable tracking,
3850 to @file{@var{file}.34.vartrack}.
3853 @itemx -fdump-rtl-flow2
3855 @opindex fdump-rtl-flow2
3856 Dump after the second flow pass, to @file{@var{file}.26.flow2}.
3859 @itemx -fdump-rtl-peephole2
3861 @opindex fdump-rtl-peephole2
3862 Dump after the peephole pass, to @file{@var{file}.27.peephole2}.
3865 @itemx -fdump-rtl-web
3867 @opindex fdump-rtl-web
3868 Dump after live range splitting, to @file{@var{file}.14.web}.
3871 @itemx -fdump-rtl-all
3873 @opindex fdump-rtl-all
3874 Produce all the dumps listed above.
3878 Produce a core dump whenever an error occurs.
3882 Print statistics on memory usage, at the end of the run, to
3887 Annotate the assembler output with a comment indicating which
3888 pattern and alternative was used. The length of each instruction is
3893 Dump the RTL in the assembler output as a comment before each instruction.
3894 Also turns on @option{-dp} annotation.
3898 For each of the other indicated dump files (either with @option{-d} or
3899 @option{-fdump-rtl-@var{pass}}), dump a representation of the control flow
3900 graph suitable for viewing with VCG to @file{@var{file}.@var{pass}.vcg}.
3904 Just generate RTL for a function instead of compiling it. Usually used
3905 with @samp{r} (@option{-fdump-rtl-expand}).
3909 Dump debugging information during parsing, to standard error.
3912 @item -fdump-unnumbered
3913 @opindex fdump-unnumbered
3914 When doing debugging dumps (see @option{-d} option above), suppress instruction
3915 numbers and line number note output. This makes it more feasible to
3916 use diff on debugging dumps for compiler invocations with different
3917 options, in particular with and without @option{-g}.
3919 @item -fdump-translation-unit @r{(C++ only)}
3920 @itemx -fdump-translation-unit-@var{options} @r{(C++ only)}
3921 @opindex fdump-translation-unit
3922 Dump a representation of the tree structure for the entire translation
3923 unit to a file. The file name is made by appending @file{.tu} to the
3924 source file name. If the @samp{-@var{options}} form is used, @var{options}
3925 controls the details of the dump as described for the
3926 @option{-fdump-tree} options.
3928 @item -fdump-class-hierarchy @r{(C++ only)}
3929 @itemx -fdump-class-hierarchy-@var{options} @r{(C++ only)}
3930 @opindex fdump-class-hierarchy
3931 Dump a representation of each class's hierarchy and virtual function
3932 table layout to a file. The file name is made by appending @file{.class}
3933 to the source file name. If the @samp{-@var{options}} form is used,
3934 @var{options} controls the details of the dump as described for the
3935 @option{-fdump-tree} options.
3937 @item -fdump-ipa-@var{switch}
3939 Control the dumping at various stages of inter-procedural analysis
3940 language tree to a file. The file name is generated by appending a switch
3941 specific suffix to the source file name. The following dumps are possible:
3945 Enables all inter-procedural analysis dumps; currently the only produced
3946 dump is the @samp{cgraph} dump.
3949 Dumps information about call-graph optimization, unused function removal,
3950 and inlining decisions.
3953 @item -fdump-tree-@var{switch} @r{(C and C++ only)}
3954 @itemx -fdump-tree-@var{switch}-@var{options} @r{(C and C++ only)}
3956 Control the dumping at various stages of processing the intermediate
3957 language tree to a file. The file name is generated by appending a switch
3958 specific suffix to the source file name. If the @samp{-@var{options}}
3959 form is used, @var{options} is a list of @samp{-} separated options that
3960 control the details of the dump. Not all options are applicable to all
3961 dumps, those which are not meaningful will be ignored. The following
3962 options are available
3966 Print the address of each node. Usually this is not meaningful as it
3967 changes according to the environment and source file. Its primary use
3968 is for tying up a dump file with a debug environment.
3970 Inhibit dumping of members of a scope or body of a function merely
3971 because that scope has been reached. Only dump such items when they
3972 are directly reachable by some other path. When dumping pretty-printed
3973 trees, this option inhibits dumping the bodies of control structures.
3975 Print a raw representation of the tree. By default, trees are
3976 pretty-printed into a C-like representation.
3978 Enable more detailed dumps (not honored by every dump option).
3980 Enable dumping various statistics about the pass (not honored by every dump
3983 Enable showing basic block boundaries (disabled in raw dumps).
3985 Enable showing virtual operands for every statement.
3987 Enable showing line numbers for statements.
3989 Enable showing the unique ID (@code{DECL_UID}) for each variable.
3991 Turn on all options, except @option{raw}, @option{slim} and @option{lineno}.
3994 The following tree dumps are possible:
3998 Dump before any tree based optimization, to @file{@var{file}.original}.
4001 Dump after all tree based optimization, to @file{@var{file}.optimized}.
4004 Dump after function inlining, to @file{@var{file}.inlined}.
4007 @opindex fdump-tree-gimple
4008 Dump each function before and after the gimplification pass to a file. The
4009 file name is made by appending @file{.gimple} to the source file name.
4012 @opindex fdump-tree-cfg
4013 Dump the control flow graph of each function to a file. The file name is
4014 made by appending @file{.cfg} to the source file name.
4017 @opindex fdump-tree-vcg
4018 Dump the control flow graph of each function to a file in VCG format. The
4019 file name is made by appending @file{.vcg} to the source file name. Note
4020 that if the file contains more than one function, the generated file cannot
4021 be used directly by VCG@. You will need to cut and paste each function's
4022 graph into its own separate file first.
4025 @opindex fdump-tree-ch
4026 Dump each function after copying loop headers. The file name is made by
4027 appending @file{.ch} to the source file name.
4030 @opindex fdump-tree-ssa
4031 Dump SSA related information to a file. The file name is made by appending
4032 @file{.ssa} to the source file name.
4035 @opindex fdump-tree-salias
4036 Dump structure aliasing variable information to a file. This file name
4037 is made by appending @file{.salias} to the source file name.
4040 @opindex fdump-tree-alias
4041 Dump aliasing information for each function. The file name is made by
4042 appending @file{.alias} to the source file name.
4045 @opindex fdump-tree-ccp
4046 Dump each function after CCP@. The file name is made by appending
4047 @file{.ccp} to the source file name.
4050 @opindex fdump-tree-storeccp
4051 Dump each function after STORE-CCP. The file name is made by appending
4052 @file{.storeccp} to the source file name.
4055 @opindex fdump-tree-pre
4056 Dump trees after partial redundancy elimination. The file name is made
4057 by appending @file{.pre} to the source file name.
4060 @opindex fdump-tree-fre
4061 Dump trees after full redundancy elimination. The file name is made
4062 by appending @file{.fre} to the source file name.
4065 @opindex fdump-tree-copyprop
4066 Dump trees after copy propagation. The file name is made
4067 by appending @file{.copyprop} to the source file name.
4069 @item store_copyprop
4070 @opindex fdump-tree-store_copyprop
4071 Dump trees after store copy-propagation. The file name is made
4072 by appending @file{.store_copyprop} to the source file name.
4075 @opindex fdump-tree-dce
4076 Dump each function after dead code elimination. The file name is made by
4077 appending @file{.dce} to the source file name.
4080 @opindex fdump-tree-mudflap
4081 Dump each function after adding mudflap instrumentation. The file name is
4082 made by appending @file{.mudflap} to the source file name.
4085 @opindex fdump-tree-sra
4086 Dump each function after performing scalar replacement of aggregates. The
4087 file name is made by appending @file{.sra} to the source file name.
4090 @opindex fdump-tree-sink
4091 Dump each function after performing code sinking. The file name is made
4092 by appending @file{.sink} to the source file name.
4095 @opindex fdump-tree-dom
4096 Dump each function after applying dominator tree optimizations. The file
4097 name is made by appending @file{.dom} to the source file name.
4100 @opindex fdump-tree-dse
4101 Dump each function after applying dead store elimination. The file
4102 name is made by appending @file{.dse} to the source file name.
4105 @opindex fdump-tree-phiopt
4106 Dump each function after optimizing PHI nodes into straightline code. The file
4107 name is made by appending @file{.phiopt} to the source file name.
4110 @opindex fdump-tree-forwprop
4111 Dump each function after forward propagating single use variables. The file
4112 name is made by appending @file{.forwprop} to the source file name.
4115 @opindex fdump-tree-copyrename
4116 Dump each function after applying the copy rename optimization. The file
4117 name is made by appending @file{.copyrename} to the source file name.
4120 @opindex fdump-tree-nrv
4121 Dump each function after applying the named return value optimization on
4122 generic trees. The file name is made by appending @file{.nrv} to the source
4126 @opindex fdump-tree-vect
4127 Dump each function after applying vectorization of loops. The file name is
4128 made by appending @file{.vect} to the source file name.
4131 @opindex fdump-tree-vrp
4132 Dump each function after Value Range Propagation (VRP). The file name
4133 is made by appending @file{.vrp} to the source file name.
4136 @opindex fdump-tree-all
4137 Enable all the available tree dumps with the flags provided in this option.
4140 @item -ftree-vectorizer-verbose=@var{n}
4141 @opindex ftree-vectorizer-verbose
4142 This option controls the amount of debugging output the vectorizer prints.
4143 This information is written to standard error, unless @option{-fdump-tree-all}
4144 or @option{-fdump-tree-vect} is specified, in which case it is output to the
4145 usual dump listing file, @file{.vect}.
4147 @item -frandom-seed=@var{string}
4148 @opindex frandom-string
4149 This option provides a seed that GCC uses when it would otherwise use
4150 random numbers. It is used to generate certain symbol names
4151 that have to be different in every compiled file. It is also used to
4152 place unique stamps in coverage data files and the object files that
4153 produce them. You can use the @option{-frandom-seed} option to produce
4154 reproducibly identical object files.
4156 The @var{string} should be different for every file you compile.
4158 @item -fsched-verbose=@var{n}
4159 @opindex fsched-verbose
4160 On targets that use instruction scheduling, this option controls the
4161 amount of debugging output the scheduler prints. This information is
4162 written to standard error, unless @option{-dS} or @option{-dR} is
4163 specified, in which case it is output to the usual dump
4164 listing file, @file{.sched} or @file{.sched2} respectively. However
4165 for @var{n} greater than nine, the output is always printed to standard
4168 For @var{n} greater than zero, @option{-fsched-verbose} outputs the
4169 same information as @option{-dRS}. For @var{n} greater than one, it
4170 also output basic block probabilities, detailed ready list information
4171 and unit/insn info. For @var{n} greater than two, it includes RTL
4172 at abort point, control-flow and regions info. And for @var{n} over
4173 four, @option{-fsched-verbose} also includes dependence info.
4177 Store the usual ``temporary'' intermediate files permanently; place them
4178 in the current directory and name them based on the source file. Thus,
4179 compiling @file{foo.c} with @samp{-c -save-temps} would produce files
4180 @file{foo.i} and @file{foo.s}, as well as @file{foo.o}. This creates a
4181 preprocessed @file{foo.i} output file even though the compiler now
4182 normally uses an integrated preprocessor.
4184 When used in combination with the @option{-x} command line option,
4185 @option{-save-temps} is sensible enough to avoid over writing an
4186 input source file with the same extension as an intermediate file.
4187 The corresponding intermediate file may be obtained by renaming the
4188 source file before using @option{-save-temps}.
4192 Report the CPU time taken by each subprocess in the compilation
4193 sequence. For C source files, this is the compiler proper and assembler
4194 (plus the linker if linking is done). The output looks like this:
4201 The first number on each line is the ``user time'', that is time spent
4202 executing the program itself. The second number is ``system time'',
4203 time spent executing operating system routines on behalf of the program.
4204 Both numbers are in seconds.
4206 @item -fvar-tracking
4207 @opindex fvar-tracking
4208 Run variable tracking pass. It computes where variables are stored at each
4209 position in code. Better debugging information is then generated
4210 (if the debugging information format supports this information).
4212 It is enabled by default when compiling with optimization (@option{-Os},
4213 @option{-O}, @option{-O2}, ...), debugging information (@option{-g}) and
4214 the debug info format supports it.
4216 @item -print-file-name=@var{library}
4217 @opindex print-file-name
4218 Print the full absolute name of the library file @var{library} that
4219 would be used when linking---and don't do anything else. With this
4220 option, GCC does not compile or link anything; it just prints the
4223 @item -print-multi-directory
4224 @opindex print-multi-directory
4225 Print the directory name corresponding to the multilib selected by any
4226 other switches present in the command line. This directory is supposed
4227 to exist in @env{GCC_EXEC_PREFIX}.
4229 @item -print-multi-lib
4230 @opindex print-multi-lib
4231 Print the mapping from multilib directory names to compiler switches
4232 that enable them. The directory name is separated from the switches by
4233 @samp{;}, and each switch starts with an @samp{@@} instead of the
4234 @samp{-}, without spaces between multiple switches. This is supposed to
4235 ease shell-processing.
4237 @item -print-prog-name=@var{program}
4238 @opindex print-prog-name
4239 Like @option{-print-file-name}, but searches for a program such as @samp{cpp}.
4241 @item -print-libgcc-file-name
4242 @opindex print-libgcc-file-name
4243 Same as @option{-print-file-name=libgcc.a}.
4245 This is useful when you use @option{-nostdlib} or @option{-nodefaultlibs}
4246 but you do want to link with @file{libgcc.a}. You can do
4249 gcc -nostdlib @var{files}@dots{} `gcc -print-libgcc-file-name`
4252 @item -print-search-dirs
4253 @opindex print-search-dirs
4254 Print the name of the configured installation directory and a list of
4255 program and library directories @command{gcc} will search---and don't do anything else.
4257 This is useful when @command{gcc} prints the error message
4258 @samp{installation problem, cannot exec cpp0: No such file or directory}.
4259 To resolve this you either need to put @file{cpp0} and the other compiler
4260 components where @command{gcc} expects to find them, or you can set the environment
4261 variable @env{GCC_EXEC_PREFIX} to the directory where you installed them.
4262 Don't forget the trailing @samp{/}.
4263 @xref{Environment Variables}.
4266 @opindex dumpmachine
4267 Print the compiler's target machine (for example,
4268 @samp{i686-pc-linux-gnu})---and don't do anything else.
4271 @opindex dumpversion
4272 Print the compiler version (for example, @samp{3.0})---and don't do
4277 Print the compiler's built-in specs---and don't do anything else. (This
4278 is used when GCC itself is being built.) @xref{Spec Files}.
4280 @item -feliminate-unused-debug-types
4281 @opindex feliminate-unused-debug-types
4282 Normally, when producing DWARF2 output, GCC will emit debugging
4283 information for all types declared in a compilation
4284 unit, regardless of whether or not they are actually used
4285 in that compilation unit. Sometimes this is useful, such as
4286 if, in the debugger, you want to cast a value to a type that is
4287 not actually used in your program (but is declared). More often,
4288 however, this results in a significant amount of wasted space.
4289 With this option, GCC will avoid producing debug symbol output
4290 for types that are nowhere used in the source file being compiled.
4293 @node Optimize Options
4294 @section Options That Control Optimization
4295 @cindex optimize options
4296 @cindex options, optimization
4298 These options control various sorts of optimizations.
4300 Without any optimization option, the compiler's goal is to reduce the
4301 cost of compilation and to make debugging produce the expected
4302 results. Statements are independent: if you stop the program with a
4303 breakpoint between statements, you can then assign a new value to any
4304 variable or change the program counter to any other statement in the
4305 function and get exactly the results you would expect from the source
4308 Turning on optimization flags makes the compiler attempt to improve
4309 the performance and/or code size at the expense of compilation time
4310 and possibly the ability to debug the program.
4312 The compiler performs optimization based on the knowledge it has of
4313 the program. Optimization levels @option{-O2} and above, in
4314 particular, enable @emph{unit-at-a-time} mode, which allows the
4315 compiler to consider information gained from later functions in
4316 the file when compiling a function. Compiling multiple files at
4317 once to a single output file in @emph{unit-at-a-time} mode allows
4318 the compiler to use information gained from all of the files when
4319 compiling each of them.
4321 Not all optimizations are controlled directly by a flag. Only
4322 optimizations that have a flag are listed.
4329 Optimize. Optimizing compilation takes somewhat more time, and a lot
4330 more memory for a large function.
4332 With @option{-O}, the compiler tries to reduce code size and execution
4333 time, without performing any optimizations that take a great deal of
4336 @option{-O} turns on the following optimization flags:
4337 @gccoptlist{-fdefer-pop @gol
4338 -fdelayed-branch @gol
4339 -fguess-branch-probability @gol
4340 -fcprop-registers @gol
4341 -floop-optimize @gol
4342 -fif-conversion @gol
4343 -fif-conversion2 @gol
4346 -ftree-dominator-opts @gol
4351 -ftree-copyrename @gol
4356 @option{-O} also turns on @option{-fomit-frame-pointer} on machines
4357 where doing so does not interfere with debugging.
4361 Optimize even more. GCC performs nearly all supported optimizations
4362 that do not involve a space-speed tradeoff. The compiler does not
4363 perform loop unrolling or function inlining when you specify @option{-O2}.
4364 As compared to @option{-O}, this option increases both compilation time
4365 and the performance of the generated code.
4367 @option{-O2} turns on all optimization flags specified by @option{-O}. It
4368 also turns on the following optimization flags:
4369 @gccoptlist{-fthread-jumps @gol
4371 -foptimize-sibling-calls @gol
4372 -fcse-follow-jumps -fcse-skip-blocks @gol
4373 -fgcse -fgcse-lm @gol
4374 -fexpensive-optimizations @gol
4375 -fstrength-reduce @gol
4376 -frerun-cse-after-loop -frerun-loop-opt @gol
4379 -fschedule-insns -fschedule-insns2 @gol
4380 -fsched-interblock -fsched-spec @gol
4382 -fstrict-aliasing @gol
4383 -fdelete-null-pointer-checks @gol
4384 -freorder-blocks -freorder-functions @gol
4385 -funit-at-a-time @gol
4386 -falign-functions -falign-jumps @gol
4387 -falign-loops -falign-labels @gol
4391 Please note the warning under @option{-fgcse} about
4392 invoking @option{-O2} on programs that use computed gotos.
4396 Optimize yet more. @option{-O3} turns on all optimizations specified by
4397 @option{-O2} and also turns on the @option{-finline-functions},
4398 @option{-funswitch-loops} and @option{-fgcse-after-reload} options.
4402 Do not optimize. This is the default.
4406 Optimize for size. @option{-Os} enables all @option{-O2} optimizations that
4407 do not typically increase code size. It also performs further
4408 optimizations designed to reduce code size.
4410 @option{-Os} disables the following optimization flags:
4411 @gccoptlist{-falign-functions -falign-jumps -falign-loops @gol
4412 -falign-labels -freorder-blocks -freorder-blocks-and-partition @gol
4413 -fprefetch-loop-arrays -ftree-vect-loop-version}
4415 If you use multiple @option{-O} options, with or without level numbers,
4416 the last such option is the one that is effective.
4419 Options of the form @option{-f@var{flag}} specify machine-independent
4420 flags. Most flags have both positive and negative forms; the negative
4421 form of @option{-ffoo} would be @option{-fno-foo}. In the table
4422 below, only one of the forms is listed---the one you typically will
4423 use. You can figure out the other form by either removing @samp{no-}
4426 The following options control specific optimizations. They are either
4427 activated by @option{-O} options or are related to ones that are. You
4428 can use the following flags in the rare cases when ``fine-tuning'' of
4429 optimizations to be performed is desired.
4432 @item -fno-default-inline
4433 @opindex fno-default-inline
4434 Do not make member functions inline by default merely because they are
4435 defined inside the class scope (C++ only). Otherwise, when you specify
4436 @w{@option{-O}}, member functions defined inside class scope are compiled
4437 inline by default; i.e., you don't need to add @samp{inline} in front of
4438 the member function name.
4440 @item -fno-defer-pop
4441 @opindex fno-defer-pop
4442 Always pop the arguments to each function call as soon as that function
4443 returns. For machines which must pop arguments after a function call,
4444 the compiler normally lets arguments accumulate on the stack for several
4445 function calls and pops them all at once.
4447 Disabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4451 Force memory operands to be copied into registers before doing
4452 arithmetic on them. This produces better code by making all memory
4453 references potential common subexpressions. When they are not common
4454 subexpressions, instruction combination should eliminate the separate
4455 register-load. This option is now a nop and will be removed in 4.2.
4458 @opindex fforce-addr
4459 Force memory address constants to be copied into registers before
4460 doing arithmetic on them.
4462 @item -fomit-frame-pointer
4463 @opindex fomit-frame-pointer
4464 Don't keep the frame pointer in a register for functions that
4465 don't need one. This avoids the instructions to save, set up and
4466 restore frame pointers; it also makes an extra register available
4467 in many functions. @strong{It also makes debugging impossible on
4470 On some machines, such as the VAX, this flag has no effect, because
4471 the standard calling sequence automatically handles the frame pointer
4472 and nothing is saved by pretending it doesn't exist. The
4473 machine-description macro @code{FRAME_POINTER_REQUIRED} controls
4474 whether a target machine supports this flag. @xref{Registers,,Register
4475 Usage, gccint, GNU Compiler Collection (GCC) Internals}.
4477 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4479 @item -foptimize-sibling-calls
4480 @opindex foptimize-sibling-calls
4481 Optimize sibling and tail recursive calls.
4483 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4487 Don't pay attention to the @code{inline} keyword. Normally this option
4488 is used to keep the compiler from expanding any functions inline.
4489 Note that if you are not optimizing, no functions can be expanded inline.
4491 @item -finline-functions
4492 @opindex finline-functions
4493 Integrate all simple functions into their callers. The compiler
4494 heuristically decides which functions are simple enough to be worth
4495 integrating in this way.
4497 If all calls to a given function are integrated, and the function is
4498 declared @code{static}, then the function is normally not output as
4499 assembler code in its own right.
4501 Enabled at level @option{-O3}.
4503 @item -finline-functions-called-once
4504 @opindex finline-functions-called-once
4505 Consider all @code{static} functions called once for inlining into their
4506 caller even if they are not marked @code{inline}. If a call to a given
4507 function is integrated, then the function is not output as assembler code
4510 Enabled if @option{-funit-at-a-time} is enabled.
4512 @item -fearly-inlining
4513 @opindex fearly-inlining
4514 Inline functions marked by @code{always_inline} and functions whose body seems
4515 smaller than the function call overhead early before doing
4516 @option{-fprofile-generate} instrumentation and real inlining pass. Doing so
4517 makes profiling significantly cheaper and usually inlining faster on programs
4518 having large chains of nested wrapper functions.
4522 @item -finline-limit=@var{n}
4523 @opindex finline-limit
4524 By default, GCC limits the size of functions that can be inlined. This flag
4525 allows the control of this limit for functions that are explicitly marked as
4526 inline (i.e., marked with the inline keyword or defined within the class
4527 definition in c++). @var{n} is the size of functions that can be inlined in
4528 number of pseudo instructions (not counting parameter handling). The default
4529 value of @var{n} is 600.
4530 Increasing this value can result in more inlined code at
4531 the cost of compilation time and memory consumption. Decreasing usually makes
4532 the compilation faster and less code will be inlined (which presumably
4533 means slower programs). This option is particularly useful for programs that
4534 use inlining heavily such as those based on recursive templates with C++.
4536 Inlining is actually controlled by a number of parameters, which may be
4537 specified individually by using @option{--param @var{name}=@var{value}}.
4538 The @option{-finline-limit=@var{n}} option sets some of these parameters
4542 @item max-inline-insns-single
4543 is set to @var{n}/2.
4544 @item max-inline-insns-auto
4545 is set to @var{n}/2.
4546 @item min-inline-insns
4547 is set to 130 or @var{n}/4, whichever is smaller.
4548 @item max-inline-insns-rtl
4552 See below for a documentation of the individual
4553 parameters controlling inlining.
4555 @emph{Note:} pseudo instruction represents, in this particular context, an
4556 abstract measurement of function's size. In no way does it represent a count
4557 of assembly instructions and as such its exact meaning might change from one
4558 release to an another.
4560 @item -fkeep-inline-functions
4561 @opindex fkeep-inline-functions
4562 In C, emit @code{static} functions that are declared @code{inline}
4563 into the object file, even if the function has been inlined into all
4564 of its callers. This switch does not affect functions using the
4565 @code{extern inline} extension in GNU C@. In C++, emit any and all
4566 inline functions into the object file.
4568 @item -fkeep-static-consts
4569 @opindex fkeep-static-consts
4570 Emit variables declared @code{static const} when optimization isn't turned
4571 on, even if the variables aren't referenced.
4573 GCC enables this option by default. If you want to force the compiler to
4574 check if the variable was referenced, regardless of whether or not
4575 optimization is turned on, use the @option{-fno-keep-static-consts} option.
4577 @item -fmerge-constants
4578 Attempt to merge identical constants (string constants and floating point
4579 constants) across compilation units.
4581 This option is the default for optimized compilation if the assembler and
4582 linker support it. Use @option{-fno-merge-constants} to inhibit this
4585 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4587 @item -fmerge-all-constants
4588 Attempt to merge identical constants and identical variables.
4590 This option implies @option{-fmerge-constants}. In addition to
4591 @option{-fmerge-constants} this considers e.g.@: even constant initialized
4592 arrays or initialized constant variables with integral or floating point
4593 types. Languages like C or C++ require each non-automatic variable to
4594 have distinct location, so using this option will result in non-conforming
4597 @item -fmodulo-sched
4598 @opindex fmodulo-sched
4599 Perform swing modulo scheduling immediately before the first scheduling
4600 pass. This pass looks at innermost loops and reorders their
4601 instructions by overlapping different iterations.
4603 @item -fno-branch-count-reg
4604 @opindex fno-branch-count-reg
4605 Do not use ``decrement and branch'' instructions on a count register,
4606 but instead generate a sequence of instructions that decrement a
4607 register, compare it against zero, then branch based upon the result.
4608 This option is only meaningful on architectures that support such
4609 instructions, which include x86, PowerPC, IA-64 and S/390.
4611 The default is @option{-fbranch-count-reg}, enabled when
4612 @option{-fstrength-reduce} is enabled.
4614 @item -fno-function-cse
4615 @opindex fno-function-cse
4616 Do not put function addresses in registers; make each instruction that
4617 calls a constant function contain the function's address explicitly.
4619 This option results in less efficient code, but some strange hacks
4620 that alter the assembler output may be confused by the optimizations
4621 performed when this option is not used.
4623 The default is @option{-ffunction-cse}
4625 @item -fno-zero-initialized-in-bss
4626 @opindex fno-zero-initialized-in-bss
4627 If the target supports a BSS section, GCC by default puts variables that
4628 are initialized to zero into BSS@. This can save space in the resulting
4631 This option turns off this behavior because some programs explicitly
4632 rely on variables going to the data section. E.g., so that the
4633 resulting executable can find the beginning of that section and/or make
4634 assumptions based on that.
4636 The default is @option{-fzero-initialized-in-bss}.
4638 @item -fbounds-check
4639 @opindex fbounds-check
4640 For front-ends that support it, generate additional code to check that
4641 indices used to access arrays are within the declared range. This is
4642 currently only supported by the Java and Fortran front-ends, where
4643 this option defaults to true and false respectively.
4645 @item -fmudflap -fmudflapth -fmudflapir
4649 @cindex bounds checking
4651 For front-ends that support it (C and C++), instrument all risky
4652 pointer/array dereferencing operations, some standard library
4653 string/heap functions, and some other associated constructs with
4654 range/validity tests. Modules so instrumented should be immune to
4655 buffer overflows, invalid heap use, and some other classes of C/C++
4656 programming errors. The instrumentation relies on a separate runtime
4657 library (@file{libmudflap}), which will be linked into a program if
4658 @option{-fmudflap} is given at link time. Run-time behavior of the
4659 instrumented program is controlled by the @env{MUDFLAP_OPTIONS}
4660 environment variable. See @code{env MUDFLAP_OPTIONS=-help a.out}
4663 Use @option{-fmudflapth} instead of @option{-fmudflap} to compile and to
4664 link if your program is multi-threaded. Use @option{-fmudflapir}, in
4665 addition to @option{-fmudflap} or @option{-fmudflapth}, if
4666 instrumentation should ignore pointer reads. This produces less
4667 instrumentation (and therefore faster execution) and still provides
4668 some protection against outright memory corrupting writes, but allows
4669 erroneously read data to propagate within a program.
4671 @item -fstrength-reduce
4672 @opindex fstrength-reduce
4673 Perform the optimizations of loop strength reduction and
4674 elimination of iteration variables.
4676 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4678 @item -fthread-jumps
4679 @opindex fthread-jumps
4680 Perform optimizations where we check to see if a jump branches to a
4681 location where another comparison subsumed by the first is found. If
4682 so, the first branch is redirected to either the destination of the
4683 second branch or a point immediately following it, depending on whether
4684 the condition is known to be true or false.
4686 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4688 @item -fcse-follow-jumps
4689 @opindex fcse-follow-jumps
4690 In common subexpression elimination, scan through jump instructions
4691 when the target of the jump is not reached by any other path. For
4692 example, when CSE encounters an @code{if} statement with an
4693 @code{else} clause, CSE will follow the jump when the condition
4696 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4698 @item -fcse-skip-blocks
4699 @opindex fcse-skip-blocks
4700 This is similar to @option{-fcse-follow-jumps}, but causes CSE to
4701 follow jumps which conditionally skip over blocks. When CSE
4702 encounters a simple @code{if} statement with no else clause,
4703 @option{-fcse-skip-blocks} causes CSE to follow the jump around the
4704 body of the @code{if}.
4706 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4708 @item -frerun-cse-after-loop
4709 @opindex frerun-cse-after-loop
4710 Re-run common subexpression elimination after loop optimizations has been
4713 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4715 @item -frerun-loop-opt
4716 @opindex frerun-loop-opt
4717 Run the loop optimizer twice.
4719 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4723 Perform a global common subexpression elimination pass.
4724 This pass also performs global constant and copy propagation.
4726 @emph{Note:} When compiling a program using computed gotos, a GCC
4727 extension, you may get better runtime performance if you disable
4728 the global common subexpression elimination pass by adding
4729 @option{-fno-gcse} to the command line.
4731 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4735 When @option{-fgcse-lm} is enabled, global common subexpression elimination will
4736 attempt to move loads which are only killed by stores into themselves. This
4737 allows a loop containing a load/store sequence to be changed to a load outside
4738 the loop, and a copy/store within the loop.
4740 Enabled by default when gcse is enabled.
4744 When @option{-fgcse-sm} is enabled, a store motion pass is run after
4745 global common subexpression elimination. This pass will attempt to move
4746 stores out of loops. When used in conjunction with @option{-fgcse-lm},
4747 loops containing a load/store sequence can be changed to a load before
4748 the loop and a store after the loop.
4750 Not enabled at any optimization level.
4754 When @option{-fgcse-las} is enabled, the global common subexpression
4755 elimination pass eliminates redundant loads that come after stores to the
4756 same memory location (both partial and full redundancies).
4758 Not enabled at any optimization level.
4760 @item -fgcse-after-reload
4761 @opindex fgcse-after-reload
4762 When @option{-fgcse-after-reload} is enabled, a redundant load elimination
4763 pass is performed after reload. The purpose of this pass is to cleanup
4766 @item -floop-optimize
4767 @opindex floop-optimize
4768 Perform loop optimizations: move constant expressions out of loops, simplify
4769 exit test conditions and optionally do strength-reduction as well.
4771 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4773 @item -floop-optimize2
4774 @opindex floop-optimize2
4775 Perform loop optimizations using the new loop optimizer. The optimizations
4776 (loop unrolling, peeling and unswitching, loop invariant motion) are enabled
4779 @item -funsafe-loop-optimizations
4780 @opindex funsafe-loop-optimizations
4781 If given, the loop optimizer will assume that loop indices do not
4782 overflow, and that the loops with nontrivial exit condition are not
4783 infinite. This enables a wider range of loop optimizations even if
4784 the loop optimizer itself cannot prove that these assumptions are valid.
4785 Using @option{-Wunsafe-loop-optimizations}, the compiler will warn you
4786 if it finds this kind of loop.
4788 @item -fcrossjumping
4789 @opindex crossjumping
4790 Perform cross-jumping transformation. This transformation unifies equivalent code and save code size. The
4791 resulting code may or may not perform better than without cross-jumping.
4793 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4795 @item -fif-conversion
4796 @opindex if-conversion
4797 Attempt to transform conditional jumps into branch-less equivalents. This
4798 include use of conditional moves, min, max, set flags and abs instructions, and
4799 some tricks doable by standard arithmetics. The use of conditional execution
4800 on chips where it is available is controlled by @code{if-conversion2}.
4802 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4804 @item -fif-conversion2
4805 @opindex if-conversion2
4806 Use conditional execution (where available) to transform conditional jumps into
4807 branch-less equivalents.
4809 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4811 @item -fdelete-null-pointer-checks
4812 @opindex fdelete-null-pointer-checks
4813 Use global dataflow analysis to identify and eliminate useless checks
4814 for null pointers. The compiler assumes that dereferencing a null
4815 pointer would have halted the program. If a pointer is checked after
4816 it has already been dereferenced, it cannot be null.
4818 In some environments, this assumption is not true, and programs can
4819 safely dereference null pointers. Use
4820 @option{-fno-delete-null-pointer-checks} to disable this optimization
4821 for programs which depend on that behavior.
4823 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4825 @item -fexpensive-optimizations
4826 @opindex fexpensive-optimizations
4827 Perform a number of minor optimizations that are relatively expensive.
4829 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4831 @item -foptimize-register-move
4833 @opindex foptimize-register-move
4835 Attempt to reassign register numbers in move instructions and as
4836 operands of other simple instructions in order to maximize the amount of
4837 register tying. This is especially helpful on machines with two-operand
4840 Note @option{-fregmove} and @option{-foptimize-register-move} are the same
4843 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4845 @item -fdelayed-branch
4846 @opindex fdelayed-branch
4847 If supported for the target machine, attempt to reorder instructions
4848 to exploit instruction slots available after delayed branch
4851 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4853 @item -fschedule-insns
4854 @opindex fschedule-insns
4855 If supported for the target machine, attempt to reorder instructions to
4856 eliminate execution stalls due to required data being unavailable. This
4857 helps machines that have slow floating point or memory load instructions
4858 by allowing other instructions to be issued until the result of the load
4859 or floating point instruction is required.
4861 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4863 @item -fschedule-insns2
4864 @opindex fschedule-insns2
4865 Similar to @option{-fschedule-insns}, but requests an additional pass of
4866 instruction scheduling after register allocation has been done. This is
4867 especially useful on machines with a relatively small number of
4868 registers and where memory load instructions take more than one cycle.
4870 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4872 @item -fno-sched-interblock
4873 @opindex fno-sched-interblock
4874 Don't schedule instructions across basic blocks. This is normally
4875 enabled by default when scheduling before register allocation, i.e.@:
4876 with @option{-fschedule-insns} or at @option{-O2} or higher.
4878 @item -fno-sched-spec
4879 @opindex fno-sched-spec
4880 Don't allow speculative motion of non-load instructions. This is normally
4881 enabled by default when scheduling before register allocation, i.e.@:
4882 with @option{-fschedule-insns} or at @option{-O2} or higher.
4884 @item -fsched-spec-load
4885 @opindex fsched-spec-load
4886 Allow speculative motion of some load instructions. This only makes
4887 sense when scheduling before register allocation, i.e.@: with
4888 @option{-fschedule-insns} or at @option{-O2} or higher.
4890 @item -fsched-spec-load-dangerous
4891 @opindex fsched-spec-load-dangerous
4892 Allow speculative motion of more load instructions. This only makes
4893 sense when scheduling before register allocation, i.e.@: with
4894 @option{-fschedule-insns} or at @option{-O2} or higher.
4896 @item -fsched-stalled-insns=@var{n}
4897 @opindex fsched-stalled-insns
4898 Define how many insns (if any) can be moved prematurely from the queue
4899 of stalled insns into the ready list, during the second scheduling pass.
4901 @item -fsched-stalled-insns-dep=@var{n}
4902 @opindex fsched-stalled-insns-dep
4903 Define how many insn groups (cycles) will be examined for a dependency
4904 on a stalled insn that is candidate for premature removal from the queue
4905 of stalled insns. Has an effect only during the second scheduling pass,
4906 and only if @option{-fsched-stalled-insns} is used and its value is not zero.
4908 @item -fsched2-use-superblocks
4909 @opindex fsched2-use-superblocks
4910 When scheduling after register allocation, do use superblock scheduling
4911 algorithm. Superblock scheduling allows motion across basic block boundaries
4912 resulting on faster schedules. This option is experimental, as not all machine
4913 descriptions used by GCC model the CPU closely enough to avoid unreliable
4914 results from the algorithm.
4916 This only makes sense when scheduling after register allocation, i.e.@: with
4917 @option{-fschedule-insns2} or at @option{-O2} or higher.
4919 @item -fsched2-use-traces
4920 @opindex fsched2-use-traces
4921 Use @option{-fsched2-use-superblocks} algorithm when scheduling after register
4922 allocation and additionally perform code duplication in order to increase the
4923 size of superblocks using tracer pass. See @option{-ftracer} for details on
4926 This mode should produce faster but significantly longer programs. Also
4927 without @option{-fbranch-probabilities} the traces constructed may not
4928 match the reality and hurt the performance. This only makes
4929 sense when scheduling after register allocation, i.e.@: with
4930 @option{-fschedule-insns2} or at @option{-O2} or higher.
4932 @item -freschedule-modulo-scheduled-loops
4933 @opindex fscheduling-in-modulo-scheduled-loops
4934 The modulo scheduling comes before the traditional scheduling, if a loop was modulo scheduled
4935 we may want to prevent the later scheduling passes from changing its schedule, we use this
4936 option to control that.
4938 @item -fcaller-saves
4939 @opindex fcaller-saves
4940 Enable values to be allocated in registers that will be clobbered by
4941 function calls, by emitting extra instructions to save and restore the
4942 registers around such calls. Such allocation is done only when it
4943 seems to result in better code than would otherwise be produced.
4945 This option is always enabled by default on certain machines, usually
4946 those which have no call-preserved registers to use instead.
4948 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4951 Perform Partial Redundancy Elimination (PRE) on trees. This flag is
4952 enabled by default at @option{-O2} and @option{-O3}.
4955 Perform Full Redundancy Elimination (FRE) on trees. The difference
4956 between FRE and PRE is that FRE only considers expressions
4957 that are computed on all paths leading to the redundant computation.
4958 This analysis faster than PRE, though it exposes fewer redundancies.
4959 This flag is enabled by default at @option{-O} and higher.
4961 @item -ftree-copy-prop
4962 Perform copy propagation on trees. This pass eliminates unnecessary
4963 copy operations. This flag is enabled by default at @option{-O} and
4966 @item -ftree-store-copy-prop
4967 Perform copy propagation of memory loads and stores. This pass
4968 eliminates unnecessary copy operations in memory references
4969 (structures, global variables, arrays, etc). This flag is enabled by
4970 default at @option{-O2} and higher.
4973 Perform structural alias analysis on trees. This flag
4974 is enabled by default at @option{-O} and higher.
4977 Perform forward store motion on trees. This flag is
4978 enabled by default at @option{-O} and higher.
4981 Perform sparse conditional constant propagation (CCP) on trees. This
4982 pass only operates on local scalar variables and is enabled by default
4983 at @option{-O} and higher.
4985 @item -ftree-store-ccp
4986 Perform sparse conditional constant propagation (CCP) on trees. This
4987 pass operates on both local scalar variables and memory stores and
4988 loads (global variables, structures, arrays, etc). This flag is
4989 enabled by default at @option{-O2} and higher.
4992 Perform dead code elimination (DCE) on trees. This flag is enabled by
4993 default at @option{-O} and higher.
4995 @item -ftree-dominator-opts
4996 Perform a variety of simple scalar cleanups (constant/copy
4997 propagation, redundancy elimination, range propagation and expression
4998 simplification) based on a dominator tree traversal. This also
4999 performs jump threading (to reduce jumps to jumps). This flag is
5000 enabled by default at @option{-O} and higher.
5003 Perform loop header copying on trees. This is beneficial since it increases
5004 effectiveness of code motion optimizations. It also saves one jump. This flag
5005 is enabled by default at @option{-O} and higher. It is not enabled
5006 for @option{-Os}, since it usually increases code size.
5008 @item -ftree-loop-optimize
5009 Perform loop optimizations on trees. This flag is enabled by default
5010 at @option{-O} and higher.
5012 @item -ftree-loop-linear
5013 Perform linear loop transformations on tree. This flag can improve cache
5014 performance and allow further loop optimizations to take place.
5016 @item -ftree-loop-im
5017 Perform loop invariant motion on trees. This pass moves only invariants that
5018 would be hard to handle at RTL level (function calls, operations that expand to
5019 nontrivial sequences of insns). With @option{-funswitch-loops} it also moves
5020 operands of conditions that are invariant out of the loop, so that we can use
5021 just trivial invariantness analysis in loop unswitching. The pass also includes
5024 @item -ftree-loop-ivcanon
5025 Create a canonical counter for number of iterations in the loop for that
5026 determining number of iterations requires complicated analysis. Later
5027 optimizations then may determine the number easily. Useful especially
5028 in connection with unrolling.
5031 Perform induction variable optimizations (strength reduction, induction
5032 variable merging and induction variable elimination) on trees.
5035 Perform scalar replacement of aggregates. This pass replaces structure
5036 references with scalars to prevent committing structures to memory too
5037 early. This flag is enabled by default at @option{-O} and higher.
5039 @item -ftree-copyrename
5040 Perform copy renaming on trees. This pass attempts to rename compiler
5041 temporaries to other variables at copy locations, usually resulting in
5042 variable names which more closely resemble the original variables. This flag
5043 is enabled by default at @option{-O} and higher.
5046 Perform temporary expression replacement during the SSA->normal phase. Single
5047 use/single def temporaries are replaced at their use location with their
5048 defining expression. This results in non-GIMPLE code, but gives the expanders
5049 much more complex trees to work on resulting in better RTL generation. This is
5050 enabled by default at @option{-O} and higher.
5053 Perform live range splitting during the SSA->normal phase. Distinct live
5054 ranges of a variable are split into unique variables, allowing for better
5055 optimization later. This is enabled by default at @option{-O} and higher.
5057 @item -ftree-vectorize
5058 Perform loop vectorization on trees.
5060 @item -ftree-vect-loop-version
5061 @opindex ftree-vect-loop-version
5062 Perform loop versioning when doing loop vectorization on trees. When a loop
5063 appears to be vectorizable except that data alignment or data dependence cannot
5064 be determined at compile time then vectorized and non-vectorized versions of
5065 the loop are generated along with runtime checks for alignment or dependence
5066 to control which version is executed. This option is enabled by default
5067 except at level @option{-Os} where it is disabled.
5070 Perform Value Range Propagation on trees. This is similar to the
5071 constant propagation pass, but instead of values, ranges of values are
5072 propagated. This allows the optimizers to remove unnecessary range
5073 checks like array bound checks and null pointer checks. This is
5074 enabled by default at @option{-O2} and higher. Null pointer check
5075 elimination is only done if @option{-fdelete-null-pointer-checks} is
5080 Perform tail duplication to enlarge superblock size. This transformation
5081 simplifies the control flow of the function allowing other optimizations to do
5084 @item -funroll-loops
5085 @opindex funroll-loops
5086 Unroll loops whose number of iterations can be determined at compile
5087 time or upon entry to the loop. @option{-funroll-loops} implies both
5088 @option{-fstrength-reduce} and @option{-frerun-cse-after-loop}. This
5089 option makes code larger, and may or may not make it run faster.
5091 @item -funroll-all-loops
5092 @opindex funroll-all-loops
5093 Unroll all loops, even if their number of iterations is uncertain when
5094 the loop is entered. This usually makes programs run more slowly.
5095 @option{-funroll-all-loops} implies the same options as
5096 @option{-funroll-loops},
5098 @item -fsplit-ivs-in-unroller
5099 @opindex -fsplit-ivs-in-unroller
5100 Enables expressing of values of induction variables in later iterations
5101 of the unrolled loop using the value in the first iteration. This breaks
5102 long dependency chains, thus improving efficiency of the scheduling passes.
5104 Combination of @option{-fweb} and CSE is often sufficient to obtain the
5105 same effect. However in cases the loop body is more complicated than
5106 a single basic block, this is not reliable. It also does not work at all
5107 on some of the architectures due to restrictions in the CSE pass.
5109 This optimization is enabled by default.
5111 @item -fvariable-expansion-in-unroller
5112 @opindex -fvariable-expansion-in-unroller
5113 With this option, the compiler will create multiple copies of some
5114 local variables when unrolling a loop which can result in superior code.
5116 @item -fprefetch-loop-arrays
5117 @opindex fprefetch-loop-arrays
5118 If supported by the target machine, generate instructions to prefetch
5119 memory to improve the performance of loops that access large arrays.
5121 These options may generate better or worse code; results are highly
5122 dependent on the structure of loops within the source code.
5125 @itemx -fno-peephole2
5126 @opindex fno-peephole
5127 @opindex fno-peephole2
5128 Disable any machine-specific peephole optimizations. The difference
5129 between @option{-fno-peephole} and @option{-fno-peephole2} is in how they
5130 are implemented in the compiler; some targets use one, some use the
5131 other, a few use both.
5133 @option{-fpeephole} is enabled by default.
5134 @option{-fpeephole2} enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
5136 @item -fno-guess-branch-probability
5137 @opindex fno-guess-branch-probability
5138 Do not guess branch probabilities using heuristics.
5140 GCC will use heuristics to guess branch probabilities if they are
5141 not provided by profiling feedback (@option{-fprofile-arcs}). These
5142 heuristics are based on the control flow graph. If some branch probabilities
5143 are specified by @samp{__builtin_expect}, then the heuristics will be
5144 used to guess branch probabilities for the rest of the control flow graph,
5145 taking the @samp{__builtin_expect} info into account. The interactions
5146 between the heuristics and @samp{__builtin_expect} can be complex, and in
5147 some cases, it may be useful to disable the heuristics so that the effects
5148 of @samp{__builtin_expect} are easier to understand.
5150 The default is @option{-fguess-branch-probability} at levels
5151 @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
5153 @item -freorder-blocks
5154 @opindex freorder-blocks
5155 Reorder basic blocks in the compiled function in order to reduce number of
5156 taken branches and improve code locality.
5158 Enabled at levels @option{-O2}, @option{-O3}.
5160 @item -freorder-blocks-and-partition
5161 @opindex freorder-blocks-and-partition
5162 In addition to reordering basic blocks in the compiled function, in order
5163 to reduce number of taken branches, partitions hot and cold basic blocks
5164 into separate sections of the assembly and .o files, to improve
5165 paging and cache locality performance.
5167 This optimization is automatically turned off in the presence of
5168 exception handling, for linkonce sections, for functions with a user-defined
5169 section attribute and on any architecture that does not support named
5172 @item -freorder-functions
5173 @opindex freorder-functions
5174 Reorder functions in the object file in order to
5175 improve code locality. This is implemented by using special
5176 subsections @code{.text.hot} for most frequently executed functions and
5177 @code{.text.unlikely} for unlikely executed functions. Reordering is done by
5178 the linker so object file format must support named sections and linker must
5179 place them in a reasonable way.
5181 Also profile feedback must be available in to make this option effective. See
5182 @option{-fprofile-arcs} for details.
5184 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
5186 @item -fstrict-aliasing
5187 @opindex fstrict-aliasing
5188 Allows the compiler to assume the strictest aliasing rules applicable to
5189 the language being compiled. For C (and C++), this activates
5190 optimizations based on the type of expressions. In particular, an
5191 object of one type is assumed never to reside at the same address as an
5192 object of a different type, unless the types are almost the same. For
5193 example, an @code{unsigned int} can alias an @code{int}, but not a
5194 @code{void*} or a @code{double}. A character type may alias any other
5197 Pay special attention to code like this:
5210 The practice of reading from a different union member than the one most
5211 recently written to (called ``type-punning'') is common. Even with
5212 @option{-fstrict-aliasing}, type-punning is allowed, provided the memory
5213 is accessed through the union type. So, the code above will work as
5214 expected. However, this code might not:
5225 Every language that wishes to perform language-specific alias analysis
5226 should define a function that computes, given an @code{tree}
5227 node, an alias set for the node. Nodes in different alias sets are not
5228 allowed to alias. For an example, see the C front-end function
5229 @code{c_get_alias_set}.
5231 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
5233 @item -falign-functions
5234 @itemx -falign-functions=@var{n}
5235 @opindex falign-functions
5236 Align the start of functions to the next power-of-two greater than
5237 @var{n}, skipping up to @var{n} bytes. For instance,
5238 @option{-falign-functions=32} aligns functions to the next 32-byte
5239 boundary, but @option{-falign-functions=24} would align to the next
5240 32-byte boundary only if this can be done by skipping 23 bytes or less.
5242 @option{-fno-align-functions} and @option{-falign-functions=1} are
5243 equivalent and mean that functions will not be aligned.
5245 Some assemblers only support this flag when @var{n} is a power of two;
5246 in that case, it is rounded up.
5248 If @var{n} is not specified or is zero, use a machine-dependent default.
5250 Enabled at levels @option{-O2}, @option{-O3}.
5252 @item -falign-labels
5253 @itemx -falign-labels=@var{n}
5254 @opindex falign-labels
5255 Align all branch targets to a power-of-two boundary, skipping up to
5256 @var{n} bytes like @option{-falign-functions}. This option can easily
5257 make code slower, because it must insert dummy operations for when the
5258 branch target is reached in the usual flow of the code.
5260 @option{-fno-align-labels} and @option{-falign-labels=1} are
5261 equivalent and mean that labels will not be aligned.
5263 If @option{-falign-loops} or @option{-falign-jumps} are applicable and
5264 are greater than this value, then their values are used instead.
5266 If @var{n} is not specified or is zero, use a machine-dependent default
5267 which is very likely to be @samp{1}, meaning no alignment.
5269 Enabled at levels @option{-O2}, @option{-O3}.
5272 @itemx -falign-loops=@var{n}
5273 @opindex falign-loops
5274 Align loops to a power-of-two boundary, skipping up to @var{n} bytes
5275 like @option{-falign-functions}. The hope is that the loop will be
5276 executed many times, which will make up for any execution of the dummy
5279 @option{-fno-align-loops} and @option{-falign-loops=1} are
5280 equivalent and mean that loops will not be aligned.
5282 If @var{n} is not specified or is zero, use a machine-dependent default.
5284 Enabled at levels @option{-O2}, @option{-O3}.
5287 @itemx -falign-jumps=@var{n}
5288 @opindex falign-jumps
5289 Align branch targets to a power-of-two boundary, for branch targets
5290 where the targets can only be reached by jumping, skipping up to @var{n}
5291 bytes like @option{-falign-functions}. In this case, no dummy operations
5294 @option{-fno-align-jumps} and @option{-falign-jumps=1} are
5295 equivalent and mean that loops will not be aligned.
5297 If @var{n} is not specified or is zero, use a machine-dependent default.
5299 Enabled at levels @option{-O2}, @option{-O3}.
5301 @item -funit-at-a-time
5302 @opindex funit-at-a-time
5303 Parse the whole compilation unit before starting to produce code.
5304 This allows some extra optimizations to take place but consumes
5305 more memory (in general). There are some compatibility issues
5306 with @emph{unit-at-at-time} mode:
5309 enabling @emph{unit-at-a-time} mode may change the order
5310 in which functions, variables, and top-level @code{asm} statements
5311 are emitted, and will likely break code relying on some particular
5312 ordering. The majority of such top-level @code{asm} statements,
5313 though, can be replaced by @code{section} attributes.
5316 @emph{unit-at-a-time} mode removes unreferenced static variables
5317 and functions. This may result in undefined references
5318 when an @code{asm} statement refers directly to variables or functions
5319 that are otherwise unused. In that case either the variable/function
5320 shall be listed as an operand of the @code{asm} statement operand or,
5321 in the case of top-level @code{asm} statements the attribute @code{used}
5322 shall be used on the declaration.
5325 Static functions now can use non-standard passing conventions that
5326 may break @code{asm} statements calling functions directly. Again,
5327 attribute @code{used} will prevent this behavior.
5330 As a temporary workaround, @option{-fno-unit-at-a-time} can be used,
5331 but this scheme may not be supported by future releases of GCC@.
5333 Enabled at levels @option{-O2}, @option{-O3}.
5337 Constructs webs as commonly used for register allocation purposes and assign
5338 each web individual pseudo register. This allows the register allocation pass
5339 to operate on pseudos directly, but also strengthens several other optimization
5340 passes, such as CSE, loop optimizer and trivial dead code remover. It can,
5341 however, make debugging impossible, since variables will no longer stay in a
5344 Enabled by default with @option{-funroll-loops}.
5346 @item -fwhole-program
5347 @opindex fwhole-program
5348 Assume that the current compilation unit represents whole program being
5349 compiled. All public functions and variables with the exception of @code{main}
5350 and those merged by attribute @code{externally_visible} become static functions
5351 and in a affect gets more aggressively optimized by interprocedural optimizers.
5352 While this option is equivalent to proper use of @code{static} keyword for
5353 programs consisting of single file, in combination with option
5354 @option{--combine} this flag can be used to compile most of smaller scale C
5355 programs since the functions and variables become local for the whole combined
5356 compilation unit, not for the single source file itself.
5359 @item -fno-cprop-registers
5360 @opindex fno-cprop-registers
5361 After register allocation and post-register allocation instruction splitting,
5362 we perform a copy-propagation pass to try to reduce scheduling dependencies
5363 and occasionally eliminate the copy.
5365 Disabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
5367 @item -fprofile-generate
5368 @opindex fprofile-generate
5370 Enable options usually used for instrumenting application to produce
5371 profile useful for later recompilation with profile feedback based
5372 optimization. You must use @option{-fprofile-generate} both when
5373 compiling and when linking your program.
5375 The following options are enabled: @code{-fprofile-arcs}, @code{-fprofile-values}, @code{-fvpt}.
5378 @opindex fprofile-use
5379 Enable profile feedback directed optimizations, and optimizations
5380 generally profitable only with profile feedback available.
5382 The following options are enabled: @code{-fbranch-probabilities}, @code{-fvpt},
5383 @code{-funroll-loops}, @code{-fpeel-loops}, @code{-ftracer},
5384 @code{-fno-loop-optimize}.
5388 The following options control compiler behavior regarding floating
5389 point arithmetic. These options trade off between speed and
5390 correctness. All must be specifically enabled.
5394 @opindex ffloat-store
5395 Do not store floating point variables in registers, and inhibit other
5396 options that might change whether a floating point value is taken from a
5399 @cindex floating point precision
5400 This option prevents undesirable excess precision on machines such as
5401 the 68000 where the floating registers (of the 68881) keep more
5402 precision than a @code{double} is supposed to have. Similarly for the
5403 x86 architecture. For most programs, the excess precision does only
5404 good, but a few programs rely on the precise definition of IEEE floating
5405 point. Use @option{-ffloat-store} for such programs, after modifying
5406 them to store all pertinent intermediate computations into variables.
5410 Sets @option{-fno-math-errno}, @option{-funsafe-math-optimizations}, @*
5411 @option{-fno-trapping-math}, @option{-ffinite-math-only},
5412 @option{-fno-rounding-math}, @option{-fno-signaling-nans}
5413 and @option{fcx-limited-range}.
5415 This option causes the preprocessor macro @code{__FAST_MATH__} to be defined.
5417 This option should never be turned on by any @option{-O} option since
5418 it can result in incorrect output for programs which depend on
5419 an exact implementation of IEEE or ISO rules/specifications for
5422 @item -fno-math-errno
5423 @opindex fno-math-errno
5424 Do not set ERRNO after calling math functions that are executed
5425 with a single instruction, e.g., sqrt. A program that relies on
5426 IEEE exceptions for math error handling may want to use this flag
5427 for speed while maintaining IEEE arithmetic compatibility.
5429 This option should never be turned on by any @option{-O} option since
5430 it can result in incorrect output for programs which depend on
5431 an exact implementation of IEEE or ISO rules/specifications for
5434 The default is @option{-fmath-errno}.
5436 On Darwin systems, the math library never sets @code{errno}. There is therefore
5437 no reason for the compiler to consider the possibility that it might,
5438 and @option{-fno-math-errno} is the default.
5440 @item -funsafe-math-optimizations
5441 @opindex funsafe-math-optimizations
5442 Allow optimizations for floating-point arithmetic that (a) assume
5443 that arguments and results are valid and (b) may violate IEEE or
5444 ANSI standards. When used at link-time, it may include libraries
5445 or startup files that change the default FPU control word or other
5446 similar optimizations.
5448 This option should never be turned on by any @option{-O} option since
5449 it can result in incorrect output for programs which depend on
5450 an exact implementation of IEEE or ISO rules/specifications for
5453 The default is @option{-fno-unsafe-math-optimizations}.
5455 @item -ffinite-math-only
5456 @opindex ffinite-math-only
5457 Allow optimizations for floating-point arithmetic that assume
5458 that arguments and results are not NaNs or +-Infs.
5460 This option should never be turned on by any @option{-O} option since
5461 it can result in incorrect output for programs which depend on
5462 an exact implementation of IEEE or ISO rules/specifications.
5464 The default is @option{-fno-finite-math-only}.
5466 @item -fno-trapping-math
5467 @opindex fno-trapping-math
5468 Compile code assuming that floating-point operations cannot generate
5469 user-visible traps. These traps include division by zero, overflow,
5470 underflow, inexact result and invalid operation. This option implies
5471 @option{-fno-signaling-nans}. Setting this option may allow faster
5472 code if one relies on ``non-stop'' IEEE arithmetic, for example.
5474 This option should never be turned on by any @option{-O} option since
5475 it can result in incorrect output for programs which depend on
5476 an exact implementation of IEEE or ISO rules/specifications for
5479 The default is @option{-ftrapping-math}.
5481 @item -frounding-math
5482 @opindex frounding-math
5483 Disable transformations and optimizations that assume default floating
5484 point rounding behavior. This is round-to-zero for all floating point
5485 to integer conversions, and round-to-nearest for all other arithmetic
5486 truncations. This option should be specified for programs that change
5487 the FP rounding mode dynamically, or that may be executed with a
5488 non-default rounding mode. This option disables constant folding of
5489 floating point expressions at compile-time (which may be affected by
5490 rounding mode) and arithmetic transformations that are unsafe in the
5491 presence of sign-dependent rounding modes.
5493 The default is @option{-fno-rounding-math}.
5495 This option is experimental and does not currently guarantee to
5496 disable all GCC optimizations that are affected by rounding mode.
5497 Future versions of GCC may provide finer control of this setting
5498 using C99's @code{FENV_ACCESS} pragma. This command line option
5499 will be used to specify the default state for @code{FENV_ACCESS}.
5501 @item -fsignaling-nans
5502 @opindex fsignaling-nans
5503 Compile code assuming that IEEE signaling NaNs may generate user-visible
5504 traps during floating-point operations. Setting this option disables
5505 optimizations that may change the number of exceptions visible with
5506 signaling NaNs. This option implies @option{-ftrapping-math}.
5508 This option causes the preprocessor macro @code{__SUPPORT_SNAN__} to
5511 The default is @option{-fno-signaling-nans}.
5513 This option is experimental and does not currently guarantee to
5514 disable all GCC optimizations that affect signaling NaN behavior.
5516 @item -fsingle-precision-constant
5517 @opindex fsingle-precision-constant
5518 Treat floating point constant as single precision constant instead of
5519 implicitly converting it to double precision constant.
5521 @item -fcx-limited-range
5522 @itemx -fno-cx-limited-range
5523 @opindex fcx-limited-range
5524 @opindex fno-cx-limited-range
5525 When enabled, this option states that a range reduction step is not
5526 needed when performing complex division. The default is
5527 @option{-fno-cx-limited-range}, but is enabled by @option{-ffast-math}.
5529 This option controls the default setting of the ISO C99
5530 @code{CX_LIMITED_RANGE} pragma. Nevertheless, the option applies to
5535 The following options control optimizations that may improve
5536 performance, but are not enabled by any @option{-O} options. This
5537 section includes experimental options that may produce broken code.
5540 @item -fbranch-probabilities
5541 @opindex fbranch-probabilities
5542 After running a program compiled with @option{-fprofile-arcs}
5543 (@pxref{Debugging Options,, Options for Debugging Your Program or
5544 @command{gcc}}), you can compile it a second time using
5545 @option{-fbranch-probabilities}, to improve optimizations based on
5546 the number of times each branch was taken. When the program
5547 compiled with @option{-fprofile-arcs} exits it saves arc execution
5548 counts to a file called @file{@var{sourcename}.gcda} for each source
5549 file The information in this data file is very dependent on the
5550 structure of the generated code, so you must use the same source code
5551 and the same optimization options for both compilations.
5553 With @option{-fbranch-probabilities}, GCC puts a
5554 @samp{REG_BR_PROB} note on each @samp{JUMP_INSN} and @samp{CALL_INSN}.
5555 These can be used to improve optimization. Currently, they are only
5556 used in one place: in @file{reorg.c}, instead of guessing which path a
5557 branch is mostly to take, the @samp{REG_BR_PROB} values are used to
5558 exactly determine which path is taken more often.
5560 @item -fprofile-values
5561 @opindex fprofile-values
5562 If combined with @option{-fprofile-arcs}, it adds code so that some
5563 data about values of expressions in the program is gathered.
5565 With @option{-fbranch-probabilities}, it reads back the data gathered
5566 from profiling values of expressions and adds @samp{REG_VALUE_PROFILE}
5567 notes to instructions for their later usage in optimizations.
5569 Enabled with @option{-fprofile-generate} and @option{-fprofile-use}.
5573 If combined with @option{-fprofile-arcs}, it instructs the compiler to add
5574 a code to gather information about values of expressions.
5576 With @option{-fbranch-probabilities}, it reads back the data gathered
5577 and actually performs the optimizations based on them.
5578 Currently the optimizations include specialization of division operation
5579 using the knowledge about the value of the denominator.
5581 @item -frename-registers
5582 @opindex frename-registers
5583 Attempt to avoid false dependencies in scheduled code by making use
5584 of registers left over after register allocation. This optimization
5585 will most benefit processors with lots of registers. Depending on the
5586 debug information format adopted by the target, however, it can
5587 make debugging impossible, since variables will no longer stay in
5588 a ``home register''.
5590 Enabled by default with @option{-funroll-loops}.
5594 Perform tail duplication to enlarge superblock size. This transformation
5595 simplifies the control flow of the function allowing other optimizations to do
5598 Enabled with @option{-fprofile-use}.
5600 @item -funroll-loops
5601 @opindex funroll-loops
5602 Unroll loops whose number of iterations can be determined at compile time or
5603 upon entry to the loop. @option{-funroll-loops} implies
5604 @option{-frerun-cse-after-loop}, @option{-fweb} and @option{-frename-registers}.
5605 It also turns on complete loop peeling (i.e.@: complete removal of loops with
5606 small constant number of iterations). This option makes code larger, and may
5607 or may not make it run faster.
5609 Enabled with @option{-fprofile-use}.
5611 @item -funroll-all-loops
5612 @opindex funroll-all-loops
5613 Unroll all loops, even if their number of iterations is uncertain when
5614 the loop is entered. This usually makes programs run more slowly.
5615 @option{-funroll-all-loops} implies the same options as
5616 @option{-funroll-loops}.
5619 @opindex fpeel-loops
5620 Peels the loops for that there is enough information that they do not
5621 roll much (from profile feedback). It also turns on complete loop peeling
5622 (i.e.@: complete removal of loops with small constant number of iterations).
5624 Enabled with @option{-fprofile-use}.
5626 @item -fmove-loop-invariants
5627 @opindex fmove-loop-invariants
5628 Enables the loop invariant motion pass in the new loop optimizer. Enabled
5629 at level @option{-O1}
5631 @item -funswitch-loops
5632 @opindex funswitch-loops
5633 Move branches with loop invariant conditions out of the loop, with duplicates
5634 of the loop on both branches (modified according to result of the condition).
5636 @item -fprefetch-loop-arrays
5637 @opindex fprefetch-loop-arrays
5638 If supported by the target machine, generate instructions to prefetch
5639 memory to improve the performance of loops that access large arrays.
5641 Disabled at level @option{-Os}.
5643 @item -ffunction-sections
5644 @itemx -fdata-sections
5645 @opindex ffunction-sections
5646 @opindex fdata-sections
5647 Place each function or data item into its own section in the output
5648 file if the target supports arbitrary sections. The name of the
5649 function or the name of the data item determines the section's name
5652 Use these options on systems where the linker can perform optimizations
5653 to improve locality of reference in the instruction space. Most systems
5654 using the ELF object format and SPARC processors running Solaris 2 have
5655 linkers with such optimizations. AIX may have these optimizations in
5658 Only use these options when there are significant benefits from doing
5659 so. When you specify these options, the assembler and linker will
5660 create larger object and executable files and will also be slower.
5661 You will not be able to use @code{gprof} on all systems if you
5662 specify this option and you may have problems with debugging if
5663 you specify both this option and @option{-g}.
5665 @item -fbranch-target-load-optimize
5666 @opindex fbranch-target-load-optimize
5667 Perform branch target register load optimization before prologue / epilogue
5669 The use of target registers can typically be exposed only during reload,
5670 thus hoisting loads out of loops and doing inter-block scheduling needs
5671 a separate optimization pass.
5673 @item -fbranch-target-load-optimize2
5674 @opindex fbranch-target-load-optimize2
5675 Perform branch target register load optimization after prologue / epilogue
5678 @item -fbtr-bb-exclusive
5679 @opindex fbtr-bb-exclusive
5680 When performing branch target register load optimization, don't reuse
5681 branch target registers in within any basic block.
5683 @item -fstack-protector
5684 Emit extra code to check for buffer overflows, such as stack smashing
5685 attacks. This is done by adding a guard variable to functions with
5686 vulnerable objects. This includes functions that call alloca, and
5687 functions with buffers larger than 8 bytes. The guards are initialized
5688 when a function is entered and then checked when the function exits.
5689 If a guard check fails, an error message is printed and the program exits.
5691 @item -fstack-protector-all
5692 Like @option{-fstack-protector} except that all functions are protected.
5694 @item --param @var{name}=@var{value}
5696 In some places, GCC uses various constants to control the amount of
5697 optimization that is done. For example, GCC will not inline functions
5698 that contain more that a certain number of instructions. You can
5699 control some of these constants on the command-line using the
5700 @option{--param} option.
5702 The names of specific parameters, and the meaning of the values, are
5703 tied to the internals of the compiler, and are subject to change
5704 without notice in future releases.
5706 In each case, the @var{value} is an integer. The allowable choices for
5707 @var{name} are given in the following table:
5710 @item salias-max-implicit-fields
5711 The maximum number of fields in a variable without direct
5712 structure accesses for which structure aliasing will consider trying
5713 to track each field. The default is 5
5715 @item sra-max-structure-size
5716 The maximum structure size, in bytes, at which the scalar replacement
5717 of aggregates (SRA) optimization will perform block copies. The
5718 default value, 0, implies that GCC will select the most appropriate
5721 @item sra-field-structure-ratio
5722 The threshold ratio (as a percentage) between instantiated fields and
5723 the complete structure size. We say that if the ratio of the number
5724 of bytes in instantiated fields to the number of bytes in the complete
5725 structure exceeds this parameter, then block copies are not used. The
5728 @item max-crossjump-edges
5729 The maximum number of incoming edges to consider for crossjumping.
5730 The algorithm used by @option{-fcrossjumping} is @math{O(N^2)} in
5731 the number of edges incoming to each block. Increasing values mean
5732 more aggressive optimization, making the compile time increase with
5733 probably small improvement in executable size.
5735 @item min-crossjump-insns
5736 The minimum number of instructions which must be matched at the end
5737 of two blocks before crossjumping will be performed on them. This
5738 value is ignored in the case where all instructions in the block being
5739 crossjumped from are matched. The default value is 5.
5741 @item max-goto-duplication-insns
5742 The maximum number of instructions to duplicate to a block that jumps
5743 to a computed goto. To avoid @math{O(N^2)} behavior in a number of
5744 passes, GCC factors computed gotos early in the compilation process,
5745 and unfactors them as late as possible. Only computed jumps at the
5746 end of a basic blocks with no more than max-goto-duplication-insns are
5747 unfactored. The default value is 8.
5749 @item max-delay-slot-insn-search
5750 The maximum number of instructions to consider when looking for an
5751 instruction to fill a delay slot. If more than this arbitrary number of
5752 instructions is searched, the time savings from filling the delay slot
5753 will be minimal so stop searching. Increasing values mean more
5754 aggressive optimization, making the compile time increase with probably
5755 small improvement in executable run time.
5757 @item max-delay-slot-live-search
5758 When trying to fill delay slots, the maximum number of instructions to
5759 consider when searching for a block with valid live register
5760 information. Increasing this arbitrarily chosen value means more
5761 aggressive optimization, increasing the compile time. This parameter
5762 should be removed when the delay slot code is rewritten to maintain the
5765 @item max-gcse-memory
5766 The approximate maximum amount of memory that will be allocated in
5767 order to perform the global common subexpression elimination
5768 optimization. If more memory than specified is required, the
5769 optimization will not be done.
5771 @item max-gcse-passes
5772 The maximum number of passes of GCSE to run. The default is 1.
5774 @item max-pending-list-length
5775 The maximum number of pending dependencies scheduling will allow
5776 before flushing the current state and starting over. Large functions
5777 with few branches or calls can create excessively large lists which
5778 needlessly consume memory and resources.
5780 @item max-inline-insns-single
5781 Several parameters control the tree inliner used in gcc.
5782 This number sets the maximum number of instructions (counted in GCC's
5783 internal representation) in a single function that the tree inliner
5784 will consider for inlining. This only affects functions declared
5785 inline and methods implemented in a class declaration (C++).
5786 The default value is 450.
5788 @item max-inline-insns-auto
5789 When you use @option{-finline-functions} (included in @option{-O3}),
5790 a lot of functions that would otherwise not be considered for inlining
5791 by the compiler will be investigated. To those functions, a different
5792 (more restrictive) limit compared to functions declared inline can
5794 The default value is 90.
5796 @item large-function-insns
5797 The limit specifying really large functions. For functions larger than this
5798 limit after inlining inlining is constrained by
5799 @option{--param large-function-growth}. This parameter is useful primarily
5800 to avoid extreme compilation time caused by non-linear algorithms used by the
5802 This parameter is ignored when @option{-funit-at-a-time} is not used.
5803 The default value is 2700.
5805 @item large-function-growth
5806 Specifies maximal growth of large function caused by inlining in percents.
5807 This parameter is ignored when @option{-funit-at-a-time} is not used.
5808 The default value is 100 which limits large function growth to 2.0 times
5811 @item inline-unit-growth
5812 Specifies maximal overall growth of the compilation unit caused by inlining.
5813 This parameter is ignored when @option{-funit-at-a-time} is not used.
5814 The default value is 50 which limits unit growth to 1.5 times the original
5817 @item max-inline-insns-recursive
5818 @itemx max-inline-insns-recursive-auto
5819 Specifies maximum number of instructions out-of-line copy of self recursive inline
5820 function can grow into by performing recursive inlining.
5822 For functions declared inline @option{--param max-inline-insns-recursive} is
5823 taken into acount. For function not declared inline, recursive inlining
5824 happens only when @option{-finline-functions} (included in @option{-O3}) is
5825 enabled and @option{--param max-inline-insns-recursive-auto} is used. The
5826 default value is 450.
5828 @item max-inline-recursive-depth
5829 @itemx max-inline-recursive-depth-auto
5830 Specifies maximum recursion depth used by the recursive inlining.
5832 For functions declared inline @option{--param max-inline-recursive-depth} is
5833 taken into acount. For function not declared inline, recursive inlining
5834 happens only when @option{-finline-functions} (included in @option{-O3}) is
5835 enabled and @option{--param max-inline-recursive-depth-auto} is used. The
5836 default value is 450.
5838 @item min-inline-recursive-probability
5839 Recursive inlining is profitable only for function having deep recursion
5840 in average and can hurt for function having little recursion depth by
5841 increasing the prologue size or complexity of function body to other
5844 When profile feedback is available (see @option{-fprofile-generate}) the actual
5845 recursion depth can be guessed from probability that function will recurse via
5846 given call expression. This parameter limits inlining only to call expression
5847 whose probability exceeds given threshold (in percents). The default value is
5850 @item inline-call-cost
5851 Specify cost of call instruction relative to simple arithmetics operations
5852 (having cost of 1). Increasing this cost disqualifies inlining of non-leaf
5853 functions and at the same time increases size of leaf function that is believed to
5854 reduce function size by being inlined. In effect it increases amount of
5855 inlining for code having large abstraction penalty (many functions that just
5856 pass the arguments to other functions) and decrease inlining for code with low
5857 abstraction penalty. The default value is 16.
5859 @item max-unrolled-insns
5860 The maximum number of instructions that a loop should have if that loop
5861 is unrolled, and if the loop is unrolled, it determines how many times
5862 the loop code is unrolled.
5864 @item max-average-unrolled-insns
5865 The maximum number of instructions biased by probabilities of their execution
5866 that a loop should have if that loop is unrolled, and if the loop is unrolled,
5867 it determines how many times the loop code is unrolled.
5869 @item max-unroll-times
5870 The maximum number of unrollings of a single loop.
5872 @item max-peeled-insns
5873 The maximum number of instructions that a loop should have if that loop
5874 is peeled, and if the loop is peeled, it determines how many times
5875 the loop code is peeled.
5877 @item max-peel-times
5878 The maximum number of peelings of a single loop.
5880 @item max-completely-peeled-insns
5881 The maximum number of insns of a completely peeled loop.
5883 @item max-completely-peel-times
5884 The maximum number of iterations of a loop to be suitable for complete peeling.
5886 @item max-unswitch-insns
5887 The maximum number of insns of an unswitched loop.
5889 @item max-unswitch-level
5890 The maximum number of branches unswitched in a single loop.
5893 The minimum cost of an expensive expression in the loop invariant motion.
5895 @item iv-consider-all-candidates-bound
5896 Bound on number of candidates for induction variables below that
5897 all candidates are considered for each use in induction variable
5898 optimizations. Only the most relevant candidates are considered
5899 if there are more candidates, to avoid quadratic time complexity.
5901 @item iv-max-considered-uses
5902 The induction variable optimizations give up on loops that contain more
5903 induction variable uses.
5905 @item iv-always-prune-cand-set-bound
5906 If number of candidates in the set is smaller than this value,
5907 we always try to remove unnecessary ivs from the set during its
5908 optimization when a new iv is added to the set.
5910 @item scev-max-expr-size
5911 Bound on size of expressions used in the scalar evolutions analyzer.
5912 Large expressions slow the analyzer.
5914 @item vect-max-version-checks
5915 The maximum number of runtime checks that can be performed when doing
5916 loop versioning in the vectorizer. See option ftree-vect-loop-version
5917 for more information.
5919 @item max-iterations-to-track
5921 The maximum number of iterations of a loop the brute force algorithm
5922 for analysis of # of iterations of the loop tries to evaluate.
5924 @item hot-bb-count-fraction
5925 Select fraction of the maximal count of repetitions of basic block in program
5926 given basic block needs to have to be considered hot.
5928 @item hot-bb-frequency-fraction
5929 Select fraction of the maximal frequency of executions of basic block in
5930 function given basic block needs to have to be considered hot
5932 @item tracer-dynamic-coverage
5933 @itemx tracer-dynamic-coverage-feedback
5935 This value is used to limit superblock formation once the given percentage of
5936 executed instructions is covered. This limits unnecessary code size
5939 The @option{tracer-dynamic-coverage-feedback} is used only when profile
5940 feedback is available. The real profiles (as opposed to statically estimated
5941 ones) are much less balanced allowing the threshold to be larger value.
5943 @item tracer-max-code-growth
5944 Stop tail duplication once code growth has reached given percentage. This is
5945 rather hokey argument, as most of the duplicates will be eliminated later in
5946 cross jumping, so it may be set to much higher values than is the desired code
5949 @item tracer-min-branch-ratio
5951 Stop reverse growth when the reverse probability of best edge is less than this
5952 threshold (in percent).
5954 @item tracer-min-branch-ratio
5955 @itemx tracer-min-branch-ratio-feedback
5957 Stop forward growth if the best edge do have probability lower than this
5960 Similarly to @option{tracer-dynamic-coverage} two values are present, one for
5961 compilation for profile feedback and one for compilation without. The value
5962 for compilation with profile feedback needs to be more conservative (higher) in
5963 order to make tracer effective.
5965 @item max-cse-path-length
5967 Maximum number of basic blocks on path that cse considers. The default is 10.
5969 @item global-var-threshold
5971 Counts the number of function calls (@var{n}) and the number of
5972 call-clobbered variables (@var{v}). If @var{n}x@var{v} is larger than this limit, a
5973 single artificial variable will be created to represent all the
5974 call-clobbered variables at function call sites. This artificial
5975 variable will then be made to alias every call-clobbered variable.
5976 (done as @code{int * size_t} on the host machine; beware overflow).
5978 @item max-aliased-vops
5980 Maximum number of virtual operands allowed to represent aliases
5981 before triggering the alias grouping heuristic. Alias grouping
5982 reduces compile times and memory consumption needed for aliasing at
5983 the expense of precision loss in alias information.
5985 @item ggc-min-expand
5987 GCC uses a garbage collector to manage its own memory allocation. This
5988 parameter specifies the minimum percentage by which the garbage
5989 collector's heap should be allowed to expand between collections.
5990 Tuning this may improve compilation speed; it has no effect on code
5993 The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when
5994 RAM >= 1GB@. If @code{getrlimit} is available, the notion of "RAM" is
5995 the smallest of actual RAM and @code{RLIMIT_DATA} or @code{RLIMIT_AS}. If
5996 GCC is not able to calculate RAM on a particular platform, the lower
5997 bound of 30% is used. Setting this parameter and
5998 @option{ggc-min-heapsize} to zero causes a full collection to occur at
5999 every opportunity. This is extremely slow, but can be useful for
6002 @item ggc-min-heapsize
6004 Minimum size of the garbage collector's heap before it begins bothering
6005 to collect garbage. The first collection occurs after the heap expands
6006 by @option{ggc-min-expand}% beyond @option{ggc-min-heapsize}. Again,
6007 tuning this may improve compilation speed, and has no effect on code
6010 The default is the smaller of RAM/8, RLIMIT_RSS, or a limit which
6011 tries to ensure that RLIMIT_DATA or RLIMIT_AS are not exceeded, but
6012 with a lower bound of 4096 (four megabytes) and an upper bound of
6013 131072 (128 megabytes). If GCC is not able to calculate RAM on a
6014 particular platform, the lower bound is used. Setting this parameter
6015 very large effectively disables garbage collection. Setting this
6016 parameter and @option{ggc-min-expand} to zero causes a full collection
6017 to occur at every opportunity.
6019 @item max-reload-search-insns
6020 The maximum number of instruction reload should look backward for equivalent
6021 register. Increasing values mean more aggressive optimization, making the
6022 compile time increase with probably slightly better performance. The default
6025 @item max-cselib-memory-location
6026 The maximum number of memory locations cselib should take into acount.
6027 Increasing values mean more aggressive optimization, making the compile time
6028 increase with probably slightly better performance. The default value is 500.
6030 @item reorder-blocks-duplicate
6031 @itemx reorder-blocks-duplicate-feedback
6033 Used by basic block reordering pass to decide whether to use unconditional
6034 branch or duplicate the code on its destination. Code is duplicated when its
6035 estimated size is smaller than this value multiplied by the estimated size of
6036 unconditional jump in the hot spots of the program.
6038 The @option{reorder-block-duplicate-feedback} is used only when profile
6039 feedback is available and may be set to higher values than
6040 @option{reorder-block-duplicate} since information about the hot spots is more
6043 @item max-sched-region-blocks
6044 The maximum number of blocks in a region to be considered for
6045 interblock scheduling. The default value is 10.
6047 @item max-sched-region-insns
6048 The maximum number of insns in a region to be considered for
6049 interblock scheduling. The default value is 100.
6051 @item min-sched-prob
6052 The minimum probability of reaching a source block for interblock
6053 speculative scheduling. The default value is 40.
6055 @item max-last-value-rtl
6057 The maximum size measured as number of RTLs that can be recorded in an expression
6058 in combiner for a pseudo register as last known value of that register. The default
6061 @item integer-share-limit
6062 Small integer constants can use a shared data structure, reducing the
6063 compiler's memory usage and increasing its speed. This sets the maximum
6064 value of a shared integer constant's. The default value is 256.
6066 @item min-virtual-mappings
6067 Specifies the minimum number of virtual mappings in the incremental
6068 SSA updater that should be registered to trigger the virtual mappings
6069 heuristic defined by virtual-mappings-ratio. The default value is
6072 @item virtual-mappings-ratio
6073 If the number of virtual mappings is virtual-mappings-ratio bigger
6074 than the number of virtual symbols to be updated, then the incremental
6075 SSA updater switches to a full update for those symbols. The default
6078 @item ssp-buffer-size
6079 The minimum size of buffers (i.e. arrays) that will receive stack smashing
6080 protection when @option{-fstack-protection} is used.
6085 @node Preprocessor Options
6086 @section Options Controlling the Preprocessor
6087 @cindex preprocessor options
6088 @cindex options, preprocessor
6090 These options control the C preprocessor, which is run on each C source
6091 file before actual compilation.
6093 If you use the @option{-E} option, nothing is done except preprocessing.
6094 Some of these options make sense only together with @option{-E} because
6095 they cause the preprocessor output to be unsuitable for actual
6100 You can use @option{-Wp,@var{option}} to bypass the compiler driver
6101 and pass @var{option} directly through to the preprocessor. If
6102 @var{option} contains commas, it is split into multiple options at the
6103 commas. However, many options are modified, translated or interpreted
6104 by the compiler driver before being passed to the preprocessor, and
6105 @option{-Wp} forcibly bypasses this phase. The preprocessor's direct
6106 interface is undocumented and subject to change, so whenever possible
6107 you should avoid using @option{-Wp} and let the driver handle the
6110 @item -Xpreprocessor @var{option}
6111 @opindex preprocessor
6112 Pass @var{option} as an option to the preprocessor. You can use this to
6113 supply system-specific preprocessor options which GCC does not know how to
6116 If you want to pass an option that takes an argument, you must use
6117 @option{-Xpreprocessor} twice, once for the option and once for the argument.
6120 @include cppopts.texi
6122 @node Assembler Options
6123 @section Passing Options to the Assembler
6125 @c prevent bad page break with this line
6126 You can pass options to the assembler.
6129 @item -Wa,@var{option}
6131 Pass @var{option} as an option to the assembler. If @var{option}
6132 contains commas, it is split into multiple options at the commas.
6134 @item -Xassembler @var{option}
6136 Pass @var{option} as an option to the assembler. You can use this to
6137 supply system-specific assembler options which GCC does not know how to
6140 If you want to pass an option that takes an argument, you must use
6141 @option{-Xassembler} twice, once for the option and once for the argument.
6146 @section Options for Linking
6147 @cindex link options
6148 @cindex options, linking
6150 These options come into play when the compiler links object files into
6151 an executable output file. They are meaningless if the compiler is
6152 not doing a link step.
6156 @item @var{object-file-name}
6157 A file name that does not end in a special recognized suffix is
6158 considered to name an object file or library. (Object files are
6159 distinguished from libraries by the linker according to the file
6160 contents.) If linking is done, these object files are used as input
6169 If any of these options is used, then the linker is not run, and
6170 object file names should not be used as arguments. @xref{Overall
6174 @item -l@var{library}
6175 @itemx -l @var{library}
6177 Search the library named @var{library} when linking. (The second
6178 alternative with the library as a separate argument is only for
6179 POSIX compliance and is not recommended.)
6181 It makes a difference where in the command you write this option; the
6182 linker searches and processes libraries and object files in the order they
6183 are specified. Thus, @samp{foo.o -lz bar.o} searches library @samp{z}
6184 after file @file{foo.o} but before @file{bar.o}. If @file{bar.o} refers
6185 to functions in @samp{z}, those functions may not be loaded.
6187 The linker searches a standard list of directories for the library,
6188 which is actually a file named @file{lib@var{library}.a}. The linker
6189 then uses this file as if it had been specified precisely by name.
6191 The directories searched include several standard system directories
6192 plus any that you specify with @option{-L}.
6194 Normally the files found this way are library files---archive files
6195 whose members are object files. The linker handles an archive file by
6196 scanning through it for members which define symbols that have so far
6197 been referenced but not defined. But if the file that is found is an
6198 ordinary object file, it is linked in the usual fashion. The only
6199 difference between using an @option{-l} option and specifying a file name
6200 is that @option{-l} surrounds @var{library} with @samp{lib} and @samp{.a}
6201 and searches several directories.
6205 You need this special case of the @option{-l} option in order to
6206 link an Objective-C or Objective-C++ program.
6209 @opindex nostartfiles
6210 Do not use the standard system startup files when linking.
6211 The standard system libraries are used normally, unless @option{-nostdlib}
6212 or @option{-nodefaultlibs} is used.
6214 @item -nodefaultlibs
6215 @opindex nodefaultlibs
6216 Do not use the standard system libraries when linking.
6217 Only the libraries you specify will be passed to the linker.
6218 The standard startup files are used normally, unless @option{-nostartfiles}
6219 is used. The compiler may generate calls to @code{memcmp},
6220 @code{memset}, @code{memcpy} and @code{memmove}.
6221 These entries are usually resolved by entries in
6222 libc. These entry points should be supplied through some other
6223 mechanism when this option is specified.
6227 Do not use the standard system startup files or libraries when linking.
6228 No startup files and only the libraries you specify will be passed to
6229 the linker. The compiler may generate calls to @code{memcmp}, @code{memset},
6230 @code{memcpy} and @code{memmove}.
6231 These entries are usually resolved by entries in
6232 libc. These entry points should be supplied through some other
6233 mechanism when this option is specified.
6235 @cindex @option{-lgcc}, use with @option{-nostdlib}
6236 @cindex @option{-nostdlib} and unresolved references
6237 @cindex unresolved references and @option{-nostdlib}
6238 @cindex @option{-lgcc}, use with @option{-nodefaultlibs}
6239 @cindex @option{-nodefaultlibs} and unresolved references
6240 @cindex unresolved references and @option{-nodefaultlibs}
6241 One of the standard libraries bypassed by @option{-nostdlib} and
6242 @option{-nodefaultlibs} is @file{libgcc.a}, a library of internal subroutines
6243 that GCC uses to overcome shortcomings of particular machines, or special
6244 needs for some languages.
6245 (@xref{Interface,,Interfacing to GCC Output,gccint,GNU Compiler
6246 Collection (GCC) Internals},
6247 for more discussion of @file{libgcc.a}.)
6248 In most cases, you need @file{libgcc.a} even when you want to avoid
6249 other standard libraries. In other words, when you specify @option{-nostdlib}
6250 or @option{-nodefaultlibs} you should usually specify @option{-lgcc} as well.
6251 This ensures that you have no unresolved references to internal GCC
6252 library subroutines. (For example, @samp{__main}, used to ensure C++
6253 constructors will be called; @pxref{Collect2,,@code{collect2}, gccint,
6254 GNU Compiler Collection (GCC) Internals}.)
6258 Produce a position independent executable on targets which support it.
6259 For predictable results, you must also specify the same set of options
6260 that were used to generate code (@option{-fpie}, @option{-fPIE},
6261 or model suboptions) when you specify this option.
6265 Pass the flag @option{-export-dynamic} to the ELF linker, on targets
6266 that support it. This instructs the linker to add all symbols, not
6267 only used ones, to the dynamic symbol table. This option is needed
6268 for some uses of @code{dlopen} or to allow obtaining backtraces
6269 from within a program.
6273 Remove all symbol table and relocation information from the executable.
6277 On systems that support dynamic linking, this prevents linking with the shared
6278 libraries. On other systems, this option has no effect.
6282 Produce a shared object which can then be linked with other objects to
6283 form an executable. Not all systems support this option. For predictable
6284 results, you must also specify the same set of options that were used to
6285 generate code (@option{-fpic}, @option{-fPIC}, or model suboptions)
6286 when you specify this option.@footnote{On some systems, @samp{gcc -shared}
6287 needs to build supplementary stub code for constructors to work. On
6288 multi-libbed systems, @samp{gcc -shared} must select the correct support
6289 libraries to link against. Failing to supply the correct flags may lead
6290 to subtle defects. Supplying them in cases where they are not necessary
6293 @item -shared-libgcc
6294 @itemx -static-libgcc
6295 @opindex shared-libgcc
6296 @opindex static-libgcc
6297 On systems that provide @file{libgcc} as a shared library, these options
6298 force the use of either the shared or static version respectively.
6299 If no shared version of @file{libgcc} was built when the compiler was
6300 configured, these options have no effect.
6302 There are several situations in which an application should use the
6303 shared @file{libgcc} instead of the static version. The most common
6304 of these is when the application wishes to throw and catch exceptions
6305 across different shared libraries. In that case, each of the libraries
6306 as well as the application itself should use the shared @file{libgcc}.
6308 Therefore, the G++ and GCJ drivers automatically add
6309 @option{-shared-libgcc} whenever you build a shared library or a main
6310 executable, because C++ and Java programs typically use exceptions, so
6311 this is the right thing to do.
6313 If, instead, you use the GCC driver to create shared libraries, you may
6314 find that they will not always be linked with the shared @file{libgcc}.
6315 If GCC finds, at its configuration time, that you have a non-GNU linker
6316 or a GNU linker that does not support option @option{--eh-frame-hdr},
6317 it will link the shared version of @file{libgcc} into shared libraries
6318 by default. Otherwise, it will take advantage of the linker and optimize
6319 away the linking with the shared version of @file{libgcc}, linking with
6320 the static version of libgcc by default. This allows exceptions to
6321 propagate through such shared libraries, without incurring relocation
6322 costs at library load time.
6324 However, if a library or main executable is supposed to throw or catch
6325 exceptions, you must link it using the G++ or GCJ driver, as appropriate
6326 for the languages used in the program, or using the option
6327 @option{-shared-libgcc}, such that it is linked with the shared
6332 Bind references to global symbols when building a shared object. Warn
6333 about any unresolved references (unless overridden by the link editor
6334 option @samp{-Xlinker -z -Xlinker defs}). Only a few systems support
6337 @item -Xlinker @var{option}
6339 Pass @var{option} as an option to the linker. You can use this to
6340 supply system-specific linker options which GCC does not know how to
6343 If you want to pass an option that takes an argument, you must use
6344 @option{-Xlinker} twice, once for the option and once for the argument.
6345 For example, to pass @option{-assert definitions}, you must write
6346 @samp{-Xlinker -assert -Xlinker definitions}. It does not work to write
6347 @option{-Xlinker "-assert definitions"}, because this passes the entire
6348 string as a single argument, which is not what the linker expects.
6350 @item -Wl,@var{option}
6352 Pass @var{option} as an option to the linker. If @var{option} contains
6353 commas, it is split into multiple options at the commas.
6355 @item -u @var{symbol}
6357 Pretend the symbol @var{symbol} is undefined, to force linking of
6358 library modules to define it. You can use @option{-u} multiple times with
6359 different symbols to force loading of additional library modules.
6362 @node Directory Options
6363 @section Options for Directory Search
6364 @cindex directory options
6365 @cindex options, directory search
6368 These options specify directories to search for header files, for
6369 libraries and for parts of the compiler:
6374 Add the directory @var{dir} to the head of the list of directories to be
6375 searched for header files. This can be used to override a system header
6376 file, substituting your own version, since these directories are
6377 searched before the system header file directories. However, you should
6378 not use this option to add directories that contain vendor-supplied
6379 system header files (use @option{-isystem} for that). If you use more than
6380 one @option{-I} option, the directories are scanned in left-to-right
6381 order; the standard system directories come after.
6383 If a standard system include directory, or a directory specified with
6384 @option{-isystem}, is also specified with @option{-I}, the @option{-I}
6385 option will be ignored. The directory will still be searched but as a
6386 system directory at its normal position in the system include chain.
6387 This is to ensure that GCC's procedure to fix buggy system headers and
6388 the ordering for the include_next directive are not inadvertently changed.
6389 If you really need to change the search order for system directories,
6390 use the @option{-nostdinc} and/or @option{-isystem} options.
6392 @item -iquote@var{dir}
6394 Add the directory @var{dir} to the head of the list of directories to
6395 be searched for header files only for the case of @samp{#include
6396 "@var{file}"}; they are not searched for @samp{#include <@var{file}>},
6397 otherwise just like @option{-I}.
6401 Add directory @var{dir} to the list of directories to be searched
6404 @item -B@var{prefix}
6406 This option specifies where to find the executables, libraries,
6407 include files, and data files of the compiler itself.
6409 The compiler driver program runs one or more of the subprograms
6410 @file{cpp}, @file{cc1}, @file{as} and @file{ld}. It tries
6411 @var{prefix} as a prefix for each program it tries to run, both with and
6412 without @samp{@var{machine}/@var{version}/} (@pxref{Target Options}).
6414 For each subprogram to be run, the compiler driver first tries the
6415 @option{-B} prefix, if any. If that name is not found, or if @option{-B}
6416 was not specified, the driver tries two standard prefixes, which are
6417 @file{/usr/lib/gcc/} and @file{/usr/local/lib/gcc/}. If neither of
6418 those results in a file name that is found, the unmodified program
6419 name is searched for using the directories specified in your
6420 @env{PATH} environment variable.
6422 The compiler will check to see if the path provided by the @option{-B}
6423 refers to a directory, and if necessary it will add a directory
6424 separator character at the end of the path.
6426 @option{-B} prefixes that effectively specify directory names also apply
6427 to libraries in the linker, because the compiler translates these
6428 options into @option{-L} options for the linker. They also apply to
6429 includes files in the preprocessor, because the compiler translates these
6430 options into @option{-isystem} options for the preprocessor. In this case,
6431 the compiler appends @samp{include} to the prefix.
6433 The run-time support file @file{libgcc.a} can also be searched for using
6434 the @option{-B} prefix, if needed. If it is not found there, the two
6435 standard prefixes above are tried, and that is all. The file is left
6436 out of the link if it is not found by those means.
6438 Another way to specify a prefix much like the @option{-B} prefix is to use
6439 the environment variable @env{GCC_EXEC_PREFIX}. @xref{Environment
6442 As a special kludge, if the path provided by @option{-B} is
6443 @file{[dir/]stage@var{N}/}, where @var{N} is a number in the range 0 to
6444 9, then it will be replaced by @file{[dir/]include}. This is to help
6445 with boot-strapping the compiler.
6447 @item -specs=@var{file}
6449 Process @var{file} after the compiler reads in the standard @file{specs}
6450 file, in order to override the defaults that the @file{gcc} driver
6451 program uses when determining what switches to pass to @file{cc1},
6452 @file{cc1plus}, @file{as}, @file{ld}, etc. More than one
6453 @option{-specs=@var{file}} can be specified on the command line, and they
6454 are processed in order, from left to right.
6456 @item --sysroot=@var{dir}
6458 Use @var{dir} as the logical root directory for headers and libraries.
6459 For example, if the compiler would normally search for headers in
6460 @file{/usr/include} and libraries in @file{/usr/lib}, it will instead
6461 search @file{@var{dir}/usr/include} and @file{@var{dir}/usr/lib}.
6463 If you use both this option and the @option{-isysroot} option, then
6464 the @option{--sysroot} option will apply to libraries, but the
6465 @option{-isysroot} option will apply to header files.
6467 The GNU linker (beginning with version 2.16) has the necessary support
6468 for this option. If your linker does not support this option, the
6469 header file aspect of @option{--sysroot} will still work, but the
6470 library aspect will not.
6474 This option has been deprecated. Please use @option{-iquote} instead for
6475 @option{-I} directories before the @option{-I-} and remove the @option{-I-}.
6476 Any directories you specify with @option{-I} options before the @option{-I-}
6477 option are searched only for the case of @samp{#include "@var{file}"};
6478 they are not searched for @samp{#include <@var{file}>}.
6480 If additional directories are specified with @option{-I} options after
6481 the @option{-I-}, these directories are searched for all @samp{#include}
6482 directives. (Ordinarily @emph{all} @option{-I} directories are used
6485 In addition, the @option{-I-} option inhibits the use of the current
6486 directory (where the current input file came from) as the first search
6487 directory for @samp{#include "@var{file}"}. There is no way to
6488 override this effect of @option{-I-}. With @option{-I.} you can specify
6489 searching the directory which was current when the compiler was
6490 invoked. That is not exactly the same as what the preprocessor does
6491 by default, but it is often satisfactory.
6493 @option{-I-} does not inhibit the use of the standard system directories
6494 for header files. Thus, @option{-I-} and @option{-nostdinc} are
6501 @section Specifying subprocesses and the switches to pass to them
6504 @command{gcc} is a driver program. It performs its job by invoking a
6505 sequence of other programs to do the work of compiling, assembling and
6506 linking. GCC interprets its command-line parameters and uses these to
6507 deduce which programs it should invoke, and which command-line options
6508 it ought to place on their command lines. This behavior is controlled
6509 by @dfn{spec strings}. In most cases there is one spec string for each
6510 program that GCC can invoke, but a few programs have multiple spec
6511 strings to control their behavior. The spec strings built into GCC can
6512 be overridden by using the @option{-specs=} command-line switch to specify
6515 @dfn{Spec files} are plaintext files that are used to construct spec
6516 strings. They consist of a sequence of directives separated by blank
6517 lines. The type of directive is determined by the first non-whitespace
6518 character on the line and it can be one of the following:
6521 @item %@var{command}
6522 Issues a @var{command} to the spec file processor. The commands that can
6526 @item %include <@var{file}>
6528 Search for @var{file} and insert its text at the current point in the
6531 @item %include_noerr <@var{file}>
6532 @cindex %include_noerr
6533 Just like @samp{%include}, but do not generate an error message if the include
6534 file cannot be found.
6536 @item %rename @var{old_name} @var{new_name}
6538 Rename the spec string @var{old_name} to @var{new_name}.
6542 @item *[@var{spec_name}]:
6543 This tells the compiler to create, override or delete the named spec
6544 string. All lines after this directive up to the next directive or
6545 blank line are considered to be the text for the spec string. If this
6546 results in an empty string then the spec will be deleted. (Or, if the
6547 spec did not exist, then nothing will happened.) Otherwise, if the spec
6548 does not currently exist a new spec will be created. If the spec does
6549 exist then its contents will be overridden by the text of this
6550 directive, unless the first character of that text is the @samp{+}
6551 character, in which case the text will be appended to the spec.
6553 @item [@var{suffix}]:
6554 Creates a new @samp{[@var{suffix}] spec} pair. All lines after this directive
6555 and up to the next directive or blank line are considered to make up the
6556 spec string for the indicated suffix. When the compiler encounters an
6557 input file with the named suffix, it will processes the spec string in
6558 order to work out how to compile that file. For example:
6565 This says that any input file whose name ends in @samp{.ZZ} should be
6566 passed to the program @samp{z-compile}, which should be invoked with the
6567 command-line switch @option{-input} and with the result of performing the
6568 @samp{%i} substitution. (See below.)
6570 As an alternative to providing a spec string, the text that follows a
6571 suffix directive can be one of the following:
6574 @item @@@var{language}
6575 This says that the suffix is an alias for a known @var{language}. This is
6576 similar to using the @option{-x} command-line switch to GCC to specify a
6577 language explicitly. For example:
6584 Says that .ZZ files are, in fact, C++ source files.
6587 This causes an error messages saying:
6590 @var{name} compiler not installed on this system.
6594 GCC already has an extensive list of suffixes built into it.
6595 This directive will add an entry to the end of the list of suffixes, but
6596 since the list is searched from the end backwards, it is effectively
6597 possible to override earlier entries using this technique.
6601 GCC has the following spec strings built into it. Spec files can
6602 override these strings or create their own. Note that individual
6603 targets can also add their own spec strings to this list.
6606 asm Options to pass to the assembler
6607 asm_final Options to pass to the assembler post-processor
6608 cpp Options to pass to the C preprocessor
6609 cc1 Options to pass to the C compiler
6610 cc1plus Options to pass to the C++ compiler
6611 endfile Object files to include at the end of the link
6612 link Options to pass to the linker
6613 lib Libraries to include on the command line to the linker
6614 libgcc Decides which GCC support library to pass to the linker
6615 linker Sets the name of the linker
6616 predefines Defines to be passed to the C preprocessor
6617 signed_char Defines to pass to CPP to say whether @code{char} is signed
6619 startfile Object files to include at the start of the link
6622 Here is a small example of a spec file:
6628 --start-group -lgcc -lc -leval1 --end-group %(old_lib)
6631 This example renames the spec called @samp{lib} to @samp{old_lib} and
6632 then overrides the previous definition of @samp{lib} with a new one.
6633 The new definition adds in some extra command-line options before
6634 including the text of the old definition.
6636 @dfn{Spec strings} are a list of command-line options to be passed to their
6637 corresponding program. In addition, the spec strings can contain
6638 @samp{%}-prefixed sequences to substitute variable text or to
6639 conditionally insert text into the command line. Using these constructs
6640 it is possible to generate quite complex command lines.
6642 Here is a table of all defined @samp{%}-sequences for spec
6643 strings. Note that spaces are not generated automatically around the
6644 results of expanding these sequences. Therefore you can concatenate them
6645 together or combine them with constant text in a single argument.
6649 Substitute one @samp{%} into the program name or argument.
6652 Substitute the name of the input file being processed.
6655 Substitute the basename of the input file being processed.
6656 This is the substring up to (and not including) the last period
6657 and not including the directory.
6660 This is the same as @samp{%b}, but include the file suffix (text after
6664 Marks the argument containing or following the @samp{%d} as a
6665 temporary file name, so that that file will be deleted if GCC exits
6666 successfully. Unlike @samp{%g}, this contributes no text to the
6669 @item %g@var{suffix}
6670 Substitute a file name that has suffix @var{suffix} and is chosen
6671 once per compilation, and mark the argument in the same way as
6672 @samp{%d}. To reduce exposure to denial-of-service attacks, the file
6673 name is now chosen in a way that is hard to predict even when previously
6674 chosen file names are known. For example, @samp{%g.s @dots{} %g.o @dots{} %g.s}
6675 might turn into @samp{ccUVUUAU.s ccXYAXZ12.o ccUVUUAU.s}. @var{suffix} matches
6676 the regexp @samp{[.A-Za-z]*} or the special string @samp{%O}, which is
6677 treated exactly as if @samp{%O} had been preprocessed. Previously, @samp{%g}
6678 was simply substituted with a file name chosen once per compilation,
6679 without regard to any appended suffix (which was therefore treated
6680 just like ordinary text), making such attacks more likely to succeed.
6682 @item %u@var{suffix}
6683 Like @samp{%g}, but generates a new temporary file name even if
6684 @samp{%u@var{suffix}} was already seen.
6686 @item %U@var{suffix}
6687 Substitutes the last file name generated with @samp{%u@var{suffix}}, generating a
6688 new one if there is no such last file name. In the absence of any
6689 @samp{%u@var{suffix}}, this is just like @samp{%g@var{suffix}}, except they don't share
6690 the same suffix @emph{space}, so @samp{%g.s @dots{} %U.s @dots{} %g.s @dots{} %U.s}
6691 would involve the generation of two distinct file names, one
6692 for each @samp{%g.s} and another for each @samp{%U.s}. Previously, @samp{%U} was
6693 simply substituted with a file name chosen for the previous @samp{%u},
6694 without regard to any appended suffix.
6696 @item %j@var{suffix}
6697 Substitutes the name of the @code{HOST_BIT_BUCKET}, if any, and if it is
6698 writable, and if save-temps is off; otherwise, substitute the name
6699 of a temporary file, just like @samp{%u}. This temporary file is not
6700 meant for communication between processes, but rather as a junk
6703 @item %|@var{suffix}
6704 @itemx %m@var{suffix}
6705 Like @samp{%g}, except if @option{-pipe} is in effect. In that case
6706 @samp{%|} substitutes a single dash and @samp{%m} substitutes nothing at
6707 all. These are the two most common ways to instruct a program that it
6708 should read from standard input or write to standard output. If you
6709 need something more elaborate you can use an @samp{%@{pipe:@code{X}@}}
6710 construct: see for example @file{f/lang-specs.h}.
6712 @item %.@var{SUFFIX}
6713 Substitutes @var{.SUFFIX} for the suffixes of a matched switch's args
6714 when it is subsequently output with @samp{%*}. @var{SUFFIX} is
6715 terminated by the next space or %.
6718 Marks the argument containing or following the @samp{%w} as the
6719 designated output file of this compilation. This puts the argument
6720 into the sequence of arguments that @samp{%o} will substitute later.
6723 Substitutes the names of all the output files, with spaces
6724 automatically placed around them. You should write spaces
6725 around the @samp{%o} as well or the results are undefined.
6726 @samp{%o} is for use in the specs for running the linker.
6727 Input files whose names have no recognized suffix are not compiled
6728 at all, but they are included among the output files, so they will
6732 Substitutes the suffix for object files. Note that this is
6733 handled specially when it immediately follows @samp{%g, %u, or %U},
6734 because of the need for those to form complete file names. The
6735 handling is such that @samp{%O} is treated exactly as if it had already
6736 been substituted, except that @samp{%g, %u, and %U} do not currently
6737 support additional @var{suffix} characters following @samp{%O} as they would
6738 following, for example, @samp{.o}.
6741 Substitutes the standard macro predefinitions for the
6742 current target machine. Use this when running @code{cpp}.
6745 Like @samp{%p}, but puts @samp{__} before and after the name of each
6746 predefined macro, except for macros that start with @samp{__} or with
6747 @samp{_@var{L}}, where @var{L} is an uppercase letter. This is for ISO
6751 Substitute any of @option{-iprefix} (made from @env{GCC_EXEC_PREFIX}),
6752 @option{-isysroot} (made from @env{TARGET_SYSTEM_ROOT}), and
6753 @option{-isystem} (made from @env{COMPILER_PATH} and @option{-B} options)
6757 Current argument is the name of a library or startup file of some sort.
6758 Search for that file in a standard list of directories and substitute
6759 the full name found.
6762 Print @var{str} as an error message. @var{str} is terminated by a newline.
6763 Use this when inconsistent options are detected.
6766 Substitute the contents of spec string @var{name} at this point.
6769 Like @samp{%(@dots{})} but put @samp{__} around @option{-D} arguments.
6771 @item %x@{@var{option}@}
6772 Accumulate an option for @samp{%X}.
6775 Output the accumulated linker options specified by @option{-Wl} or a @samp{%x}
6779 Output the accumulated assembler options specified by @option{-Wa}.
6782 Output the accumulated preprocessor options specified by @option{-Wp}.
6785 Process the @code{asm} spec. This is used to compute the
6786 switches to be passed to the assembler.
6789 Process the @code{asm_final} spec. This is a spec string for
6790 passing switches to an assembler post-processor, if such a program is
6794 Process the @code{link} spec. This is the spec for computing the
6795 command line passed to the linker. Typically it will make use of the
6796 @samp{%L %G %S %D and %E} sequences.
6799 Dump out a @option{-L} option for each directory that GCC believes might
6800 contain startup files. If the target supports multilibs then the
6801 current multilib directory will be prepended to each of these paths.
6804 Process the @code{lib} spec. This is a spec string for deciding which
6805 libraries should be included on the command line to the linker.
6808 Process the @code{libgcc} spec. This is a spec string for deciding
6809 which GCC support library should be included on the command line to the linker.
6812 Process the @code{startfile} spec. This is a spec for deciding which
6813 object files should be the first ones passed to the linker. Typically
6814 this might be a file named @file{crt0.o}.
6817 Process the @code{endfile} spec. This is a spec string that specifies
6818 the last object files that will be passed to the linker.
6821 Process the @code{cpp} spec. This is used to construct the arguments
6822 to be passed to the C preprocessor.
6825 Process the @code{cc1} spec. This is used to construct the options to be
6826 passed to the actual C compiler (@samp{cc1}).
6829 Process the @code{cc1plus} spec. This is used to construct the options to be
6830 passed to the actual C++ compiler (@samp{cc1plus}).
6833 Substitute the variable part of a matched option. See below.
6834 Note that each comma in the substituted string is replaced by
6838 Remove all occurrences of @code{-S} from the command line. Note---this
6839 command is position dependent. @samp{%} commands in the spec string
6840 before this one will see @code{-S}, @samp{%} commands in the spec string
6841 after this one will not.
6843 @item %:@var{function}(@var{args})
6844 Call the named function @var{function}, passing it @var{args}.
6845 @var{args} is first processed as a nested spec string, then split
6846 into an argument vector in the usual fashion. The function returns
6847 a string which is processed as if it had appeared literally as part
6848 of the current spec.
6850 The following built-in spec functions are provided:
6853 @item @code{if-exists}
6854 The @code{if-exists} spec function takes one argument, an absolute
6855 pathname to a file. If the file exists, @code{if-exists} returns the
6856 pathname. Here is a small example of its usage:
6860 crt0%O%s %:if-exists(crti%O%s) crtbegin%O%s
6863 @item @code{if-exists-else}
6864 The @code{if-exists-else} spec function is similar to the @code{if-exists}
6865 spec function, except that it takes two arguments. The first argument is
6866 an absolute pathname to a file. If the file exists, @code{if-exists-else}
6867 returns the pathname. If it does not exist, it returns the second argument.
6868 This way, @code{if-exists-else} can be used to select one file or another,
6869 based on the existence of the first. Here is a small example of its usage:
6873 crt0%O%s %:if-exists(crti%O%s) \
6874 %:if-exists-else(crtbeginT%O%s crtbegin%O%s)
6877 @item @code{replace-outfile}
6878 The @code{replace-outfile} spec function takes two arguments. It looks for the
6879 first argument in the outfiles array and replaces it with the second argument. Here
6880 is a small example of its usage:
6883 %@{fgnu-runtime:%:replace-outfile(-lobjc -lobjc-gnu)@}
6889 Substitutes the @code{-S} switch, if that switch was given to GCC@.
6890 If that switch was not specified, this substitutes nothing. Note that
6891 the leading dash is omitted when specifying this option, and it is
6892 automatically inserted if the substitution is performed. Thus the spec
6893 string @samp{%@{foo@}} would match the command-line option @option{-foo}
6894 and would output the command line option @option{-foo}.
6896 @item %W@{@code{S}@}
6897 Like %@{@code{S}@} but mark last argument supplied within as a file to be
6900 @item %@{@code{S}*@}
6901 Substitutes all the switches specified to GCC whose names start
6902 with @code{-S}, but which also take an argument. This is used for
6903 switches like @option{-o}, @option{-D}, @option{-I}, etc.
6904 GCC considers @option{-o foo} as being
6905 one switch whose names starts with @samp{o}. %@{o*@} would substitute this
6906 text, including the space. Thus two arguments would be generated.
6908 @item %@{@code{S}*&@code{T}*@}
6909 Like %@{@code{S}*@}, but preserve order of @code{S} and @code{T} options
6910 (the order of @code{S} and @code{T} in the spec is not significant).
6911 There can be any number of ampersand-separated variables; for each the
6912 wild card is optional. Useful for CPP as @samp{%@{D*&U*&A*@}}.
6914 @item %@{@code{S}:@code{X}@}
6915 Substitutes @code{X}, if the @samp{-S} switch was given to GCC@.
6917 @item %@{!@code{S}:@code{X}@}
6918 Substitutes @code{X}, if the @samp{-S} switch was @emph{not} given to GCC@.
6920 @item %@{@code{S}*:@code{X}@}
6921 Substitutes @code{X} if one or more switches whose names start with
6922 @code{-S} are specified to GCC@. Normally @code{X} is substituted only
6923 once, no matter how many such switches appeared. However, if @code{%*}
6924 appears somewhere in @code{X}, then @code{X} will be substituted once
6925 for each matching switch, with the @code{%*} replaced by the part of
6926 that switch that matched the @code{*}.
6928 @item %@{.@code{S}:@code{X}@}
6929 Substitutes @code{X}, if processing a file with suffix @code{S}.
6931 @item %@{!.@code{S}:@code{X}@}
6932 Substitutes @code{X}, if @emph{not} processing a file with suffix @code{S}.
6934 @item %@{@code{S}|@code{P}:@code{X}@}
6935 Substitutes @code{X} if either @code{-S} or @code{-P} was given to GCC@.
6936 This may be combined with @samp{!}, @samp{.}, and @code{*} sequences as well,
6937 although they have a stronger binding than the @samp{|}. If @code{%*}
6938 appears in @code{X}, all of the alternatives must be starred, and only
6939 the first matching alternative is substituted.
6941 For example, a spec string like this:
6944 %@{.c:-foo@} %@{!.c:-bar@} %@{.c|d:-baz@} %@{!.c|d:-boggle@}
6947 will output the following command-line options from the following input
6948 command-line options:
6953 -d fred.c -foo -baz -boggle
6954 -d jim.d -bar -baz -boggle
6957 @item %@{S:X; T:Y; :D@}
6959 If @code{S} was given to GCC, substitutes @code{X}; else if @code{T} was
6960 given to GCC, substitutes @code{Y}; else substitutes @code{D}. There can
6961 be as many clauses as you need. This may be combined with @code{.},
6962 @code{!}, @code{|}, and @code{*} as needed.
6967 The conditional text @code{X} in a %@{@code{S}:@code{X}@} or similar
6968 construct may contain other nested @samp{%} constructs or spaces, or
6969 even newlines. They are processed as usual, as described above.
6970 Trailing white space in @code{X} is ignored. White space may also
6971 appear anywhere on the left side of the colon in these constructs,
6972 except between @code{.} or @code{*} and the corresponding word.
6974 The @option{-O}, @option{-f}, @option{-m}, and @option{-W} switches are
6975 handled specifically in these constructs. If another value of
6976 @option{-O} or the negated form of a @option{-f}, @option{-m}, or
6977 @option{-W} switch is found later in the command line, the earlier
6978 switch value is ignored, except with @{@code{S}*@} where @code{S} is
6979 just one letter, which passes all matching options.
6981 The character @samp{|} at the beginning of the predicate text is used to
6982 indicate that a command should be piped to the following command, but
6983 only if @option{-pipe} is specified.
6985 It is built into GCC which switches take arguments and which do not.
6986 (You might think it would be useful to generalize this to allow each
6987 compiler's spec to say which switches take arguments. But this cannot
6988 be done in a consistent fashion. GCC cannot even decide which input
6989 files have been specified without knowing which switches take arguments,
6990 and it must know which input files to compile in order to tell which
6993 GCC also knows implicitly that arguments starting in @option{-l} are to be
6994 treated as compiler output files, and passed to the linker in their
6995 proper position among the other output files.
6997 @c man begin OPTIONS
6999 @node Target Options
7000 @section Specifying Target Machine and Compiler Version
7001 @cindex target options
7002 @cindex cross compiling
7003 @cindex specifying machine version
7004 @cindex specifying compiler version and target machine
7005 @cindex compiler version, specifying
7006 @cindex target machine, specifying
7008 The usual way to run GCC is to run the executable called @file{gcc}, or
7009 @file{<machine>-gcc} when cross-compiling, or
7010 @file{<machine>-gcc-<version>} to run a version other than the one that
7011 was installed last. Sometimes this is inconvenient, so GCC provides
7012 options that will switch to another cross-compiler or version.
7015 @item -b @var{machine}
7017 The argument @var{machine} specifies the target machine for compilation.
7019 The value to use for @var{machine} is the same as was specified as the
7020 machine type when configuring GCC as a cross-compiler. For
7021 example, if a cross-compiler was configured with @samp{configure
7022 arm-elf}, meaning to compile for an arm processor with elf binaries,
7023 then you would specify @option{-b arm-elf} to run that cross compiler.
7024 Because there are other options beginning with @option{-b}, the
7025 configuration must contain a hyphen.
7027 @item -V @var{version}
7029 The argument @var{version} specifies which version of GCC to run.
7030 This is useful when multiple versions are installed. For example,
7031 @var{version} might be @samp{4.0}, meaning to run GCC version 4.0.
7034 The @option{-V} and @option{-b} options work by running the
7035 @file{<machine>-gcc-<version>} executable, so there's no real reason to
7036 use them if you can just run that directly.
7038 @node Submodel Options
7039 @section Hardware Models and Configurations
7040 @cindex submodel options
7041 @cindex specifying hardware config
7042 @cindex hardware models and configurations, specifying
7043 @cindex machine dependent options
7045 Earlier we discussed the standard option @option{-b} which chooses among
7046 different installed compilers for completely different target
7047 machines, such as VAX vs.@: 68000 vs.@: 80386.
7049 In addition, each of these target machine types can have its own
7050 special options, starting with @samp{-m}, to choose among various
7051 hardware models or configurations---for example, 68010 vs 68020,
7052 floating coprocessor or none. A single installed version of the
7053 compiler can compile for any model or configuration, according to the
7056 Some configurations of the compiler also support additional special
7057 options, usually for compatibility with other compilers on the same
7060 @c This list is ordered alphanumerically by subsection name.
7061 @c It should be the same order and spelling as these options are listed
7062 @c in Machine Dependent Options
7068 * Blackfin Options::
7072 * DEC Alpha Options::
7073 * DEC Alpha/VMS Options::
7077 * i386 and x86-64 Options::
7090 * RS/6000 and PowerPC Options::
7091 * S/390 and zSeries Options::
7094 * System V Options::
7095 * TMS320C3x/C4x Options::
7099 * Xstormy16 Options::
7105 @subsection ARC Options
7108 These options are defined for ARC implementations:
7113 Compile code for little endian mode. This is the default.
7117 Compile code for big endian mode.
7120 @opindex mmangle-cpu
7121 Prepend the name of the cpu to all public symbol names.
7122 In multiple-processor systems, there are many ARC variants with different
7123 instruction and register set characteristics. This flag prevents code
7124 compiled for one cpu to be linked with code compiled for another.
7125 No facility exists for handling variants that are ``almost identical''.
7126 This is an all or nothing option.
7128 @item -mcpu=@var{cpu}
7130 Compile code for ARC variant @var{cpu}.
7131 Which variants are supported depend on the configuration.
7132 All variants support @option{-mcpu=base}, this is the default.
7134 @item -mtext=@var{text-section}
7135 @itemx -mdata=@var{data-section}
7136 @itemx -mrodata=@var{readonly-data-section}
7140 Put functions, data, and readonly data in @var{text-section},
7141 @var{data-section}, and @var{readonly-data-section} respectively
7142 by default. This can be overridden with the @code{section} attribute.
7143 @xref{Variable Attributes}.
7148 @subsection ARM Options
7151 These @samp{-m} options are defined for Advanced RISC Machines (ARM)
7155 @item -mabi=@var{name}
7157 Generate code for the specified ABI@. Permissible values are: @samp{apcs-gnu},
7158 @samp{atpcs}, @samp{aapcs} and @samp{iwmmxt}.
7161 @opindex mapcs-frame
7162 Generate a stack frame that is compliant with the ARM Procedure Call
7163 Standard for all functions, even if this is not strictly necessary for
7164 correct execution of the code. Specifying @option{-fomit-frame-pointer}
7165 with this option will cause the stack frames not to be generated for
7166 leaf functions. The default is @option{-mno-apcs-frame}.
7170 This is a synonym for @option{-mapcs-frame}.
7173 @c not currently implemented
7174 @item -mapcs-stack-check
7175 @opindex mapcs-stack-check
7176 Generate code to check the amount of stack space available upon entry to
7177 every function (that actually uses some stack space). If there is
7178 insufficient space available then either the function
7179 @samp{__rt_stkovf_split_small} or @samp{__rt_stkovf_split_big} will be
7180 called, depending upon the amount of stack space required. The run time
7181 system is required to provide these functions. The default is
7182 @option{-mno-apcs-stack-check}, since this produces smaller code.
7184 @c not currently implemented
7186 @opindex mapcs-float
7187 Pass floating point arguments using the float point registers. This is
7188 one of the variants of the APCS@. This option is recommended if the
7189 target hardware has a floating point unit or if a lot of floating point
7190 arithmetic is going to be performed by the code. The default is
7191 @option{-mno-apcs-float}, since integer only code is slightly increased in
7192 size if @option{-mapcs-float} is used.
7194 @c not currently implemented
7195 @item -mapcs-reentrant
7196 @opindex mapcs-reentrant
7197 Generate reentrant, position independent code. The default is
7198 @option{-mno-apcs-reentrant}.
7201 @item -mthumb-interwork
7202 @opindex mthumb-interwork
7203 Generate code which supports calling between the ARM and Thumb
7204 instruction sets. Without this option the two instruction sets cannot
7205 be reliably used inside one program. The default is
7206 @option{-mno-thumb-interwork}, since slightly larger code is generated
7207 when @option{-mthumb-interwork} is specified.
7209 @item -mno-sched-prolog
7210 @opindex mno-sched-prolog
7211 Prevent the reordering of instructions in the function prolog, or the
7212 merging of those instruction with the instructions in the function's
7213 body. This means that all functions will start with a recognizable set
7214 of instructions (or in fact one of a choice from a small set of
7215 different function prologues), and this information can be used to
7216 locate the start if functions inside an executable piece of code. The
7217 default is @option{-msched-prolog}.
7220 @opindex mhard-float
7221 Generate output containing floating point instructions. This is the
7225 @opindex msoft-float
7226 Generate output containing library calls for floating point.
7227 @strong{Warning:} the requisite libraries are not available for all ARM
7228 targets. Normally the facilities of the machine's usual C compiler are
7229 used, but this cannot be done directly in cross-compilation. You must make
7230 your own arrangements to provide suitable library functions for
7233 @option{-msoft-float} changes the calling convention in the output file;
7234 therefore, it is only useful if you compile @emph{all} of a program with
7235 this option. In particular, you need to compile @file{libgcc.a}, the
7236 library that comes with GCC, with @option{-msoft-float} in order for
7239 @item -mfloat-abi=@var{name}
7241 Specifies which ABI to use for floating point values. Permissible values
7242 are: @samp{soft}, @samp{softfp} and @samp{hard}.
7244 @samp{soft} and @samp{hard} are equivalent to @option{-msoft-float}
7245 and @option{-mhard-float} respectively. @samp{softfp} allows the generation
7246 of floating point instructions, but still uses the soft-float calling
7249 @item -mlittle-endian
7250 @opindex mlittle-endian
7251 Generate code for a processor running in little-endian mode. This is
7252 the default for all standard configurations.
7255 @opindex mbig-endian
7256 Generate code for a processor running in big-endian mode; the default is
7257 to compile code for a little-endian processor.
7259 @item -mwords-little-endian
7260 @opindex mwords-little-endian
7261 This option only applies when generating code for big-endian processors.
7262 Generate code for a little-endian word order but a big-endian byte
7263 order. That is, a byte order of the form @samp{32107654}. Note: this
7264 option should only be used if you require compatibility with code for
7265 big-endian ARM processors generated by versions of the compiler prior to
7268 @item -mcpu=@var{name}
7270 This specifies the name of the target ARM processor. GCC uses this name
7271 to determine what kind of instructions it can emit when generating
7272 assembly code. Permissible names are: @samp{arm2}, @samp{arm250},
7273 @samp{arm3}, @samp{arm6}, @samp{arm60}, @samp{arm600}, @samp{arm610},
7274 @samp{arm620}, @samp{arm7}, @samp{arm7m}, @samp{arm7d}, @samp{arm7dm},
7275 @samp{arm7di}, @samp{arm7dmi}, @samp{arm70}, @samp{arm700},
7276 @samp{arm700i}, @samp{arm710}, @samp{arm710c}, @samp{arm7100},
7277 @samp{arm7500}, @samp{arm7500fe}, @samp{arm7tdmi}, @samp{arm7tdmi-s},
7278 @samp{arm8}, @samp{strongarm}, @samp{strongarm110}, @samp{strongarm1100},
7279 @samp{arm8}, @samp{arm810}, @samp{arm9}, @samp{arm9e}, @samp{arm920},
7280 @samp{arm920t}, @samp{arm922t}, @samp{arm946e-s}, @samp{arm966e-s},
7281 @samp{arm968e-s}, @samp{arm926ej-s}, @samp{arm940t}, @samp{arm9tdmi},
7282 @samp{arm10tdmi}, @samp{arm1020t}, @samp{arm1026ej-s},
7283 @samp{arm10e}, @samp{arm1020e}, @samp{arm1022e},
7284 @samp{arm1136j-s}, @samp{arm1136jf-s}, @samp{mpcore}, @samp{mpcorenovfp},
7285 @samp{arm1176jz-s}, @samp{arm1176jzf-s}, @samp{xscale}, @samp{iwmmxt},
7288 @itemx -mtune=@var{name}
7290 This option is very similar to the @option{-mcpu=} option, except that
7291 instead of specifying the actual target processor type, and hence
7292 restricting which instructions can be used, it specifies that GCC should
7293 tune the performance of the code as if the target were of the type
7294 specified in this option, but still choosing the instructions that it
7295 will generate based on the cpu specified by a @option{-mcpu=} option.
7296 For some ARM implementations better performance can be obtained by using
7299 @item -march=@var{name}
7301 This specifies the name of the target ARM architecture. GCC uses this
7302 name to determine what kind of instructions it can emit when generating
7303 assembly code. This option can be used in conjunction with or instead
7304 of the @option{-mcpu=} option. Permissible names are: @samp{armv2},
7305 @samp{armv2a}, @samp{armv3}, @samp{armv3m}, @samp{armv4}, @samp{armv4t},
7306 @samp{armv5}, @samp{armv5t}, @samp{armv5te}, @samp{armv6}, @samp{armv6j},
7307 @samp{iwmmxt}, @samp{ep9312}.
7309 @item -mfpu=@var{name}
7310 @itemx -mfpe=@var{number}
7311 @itemx -mfp=@var{number}
7315 This specifies what floating point hardware (or hardware emulation) is
7316 available on the target. Permissible names are: @samp{fpa}, @samp{fpe2},
7317 @samp{fpe3}, @samp{maverick}, @samp{vfp}. @option{-mfp} and @option{-mfpe}
7318 are synonyms for @option{-mfpu}=@samp{fpe}@var{number}, for compatibility
7319 with older versions of GCC@.
7321 If @option{-msoft-float} is specified this specifies the format of
7322 floating point values.
7324 @item -mstructure-size-boundary=@var{n}
7325 @opindex mstructure-size-boundary
7326 The size of all structures and unions will be rounded up to a multiple
7327 of the number of bits set by this option. Permissible values are 8, 32
7328 and 64. The default value varies for different toolchains. For the COFF
7329 targeted toolchain the default value is 8. A value of 64 is only allowed
7330 if the underlying ABI supports it.
7332 Specifying the larger number can produce faster, more efficient code, but
7333 can also increase the size of the program. Different values are potentially
7334 incompatible. Code compiled with one value cannot necessarily expect to
7335 work with code or libraries compiled with another value, if they exchange
7336 information using structures or unions.
7338 @item -mabort-on-noreturn
7339 @opindex mabort-on-noreturn
7340 Generate a call to the function @code{abort} at the end of a
7341 @code{noreturn} function. It will be executed if the function tries to
7345 @itemx -mno-long-calls
7346 @opindex mlong-calls
7347 @opindex mno-long-calls
7348 Tells the compiler to perform function calls by first loading the
7349 address of the function into a register and then performing a subroutine
7350 call on this register. This switch is needed if the target function
7351 will lie outside of the 64 megabyte addressing range of the offset based
7352 version of subroutine call instruction.
7354 Even if this switch is enabled, not all function calls will be turned
7355 into long calls. The heuristic is that static functions, functions
7356 which have the @samp{short-call} attribute, functions that are inside
7357 the scope of a @samp{#pragma no_long_calls} directive and functions whose
7358 definitions have already been compiled within the current compilation
7359 unit, will not be turned into long calls. The exception to this rule is
7360 that weak function definitions, functions with the @samp{long-call}
7361 attribute or the @samp{section} attribute, and functions that are within
7362 the scope of a @samp{#pragma long_calls} directive, will always be
7363 turned into long calls.
7365 This feature is not enabled by default. Specifying
7366 @option{-mno-long-calls} will restore the default behavior, as will
7367 placing the function calls within the scope of a @samp{#pragma
7368 long_calls_off} directive. Note these switches have no effect on how
7369 the compiler generates code to handle function calls via function
7372 @item -mnop-fun-dllimport
7373 @opindex mnop-fun-dllimport
7374 Disable support for the @code{dllimport} attribute.
7376 @item -msingle-pic-base
7377 @opindex msingle-pic-base
7378 Treat the register used for PIC addressing as read-only, rather than
7379 loading it in the prologue for each function. The run-time system is
7380 responsible for initializing this register with an appropriate value
7381 before execution begins.
7383 @item -mpic-register=@var{reg}
7384 @opindex mpic-register
7385 Specify the register to be used for PIC addressing. The default is R10
7386 unless stack-checking is enabled, when R9 is used.
7388 @item -mcirrus-fix-invalid-insns
7389 @opindex mcirrus-fix-invalid-insns
7390 @opindex mno-cirrus-fix-invalid-insns
7391 Insert NOPs into the instruction stream to in order to work around
7392 problems with invalid Maverick instruction combinations. This option
7393 is only valid if the @option{-mcpu=ep9312} option has been used to
7394 enable generation of instructions for the Cirrus Maverick floating
7395 point co-processor. This option is not enabled by default, since the
7396 problem is only present in older Maverick implementations. The default
7397 can be re-enabled by use of the @option{-mno-cirrus-fix-invalid-insns}
7400 @item -mpoke-function-name
7401 @opindex mpoke-function-name
7402 Write the name of each function into the text section, directly
7403 preceding the function prologue. The generated code is similar to this:
7407 .ascii "arm_poke_function_name", 0
7410 .word 0xff000000 + (t1 - t0)
7411 arm_poke_function_name
7413 stmfd sp!, @{fp, ip, lr, pc@}
7417 When performing a stack backtrace, code can inspect the value of
7418 @code{pc} stored at @code{fp + 0}. If the trace function then looks at
7419 location @code{pc - 12} and the top 8 bits are set, then we know that
7420 there is a function name embedded immediately preceding this location
7421 and has length @code{((pc[-3]) & 0xff000000)}.
7425 Generate code for the 16-bit Thumb instruction set. The default is to
7426 use the 32-bit ARM instruction set.
7429 @opindex mtpcs-frame
7430 Generate a stack frame that is compliant with the Thumb Procedure Call
7431 Standard for all non-leaf functions. (A leaf function is one that does
7432 not call any other functions.) The default is @option{-mno-tpcs-frame}.
7434 @item -mtpcs-leaf-frame
7435 @opindex mtpcs-leaf-frame
7436 Generate a stack frame that is compliant with the Thumb Procedure Call
7437 Standard for all leaf functions. (A leaf function is one that does
7438 not call any other functions.) The default is @option{-mno-apcs-leaf-frame}.
7440 @item -mcallee-super-interworking
7441 @opindex mcallee-super-interworking
7442 Gives all externally visible functions in the file being compiled an ARM
7443 instruction set header which switches to Thumb mode before executing the
7444 rest of the function. This allows these functions to be called from
7445 non-interworking code.
7447 @item -mcaller-super-interworking
7448 @opindex mcaller-super-interworking
7449 Allows calls via function pointers (including virtual functions) to
7450 execute correctly regardless of whether the target code has been
7451 compiled for interworking or not. There is a small overhead in the cost
7452 of executing a function pointer if this option is enabled.
7457 @subsection AVR Options
7460 These options are defined for AVR implementations:
7463 @item -mmcu=@var{mcu}
7465 Specify ATMEL AVR instruction set or MCU type.
7467 Instruction set avr1 is for the minimal AVR core, not supported by the C
7468 compiler, only for assembler programs (MCU types: at90s1200, attiny10,
7469 attiny11, attiny12, attiny15, attiny28).
7471 Instruction set avr2 (default) is for the classic AVR core with up to
7472 8K program memory space (MCU types: at90s2313, at90s2323, attiny22,
7473 at90s2333, at90s2343, at90s4414, at90s4433, at90s4434, at90s8515,
7474 at90c8534, at90s8535).
7476 Instruction set avr3 is for the classic AVR core with up to 128K program
7477 memory space (MCU types: atmega103, atmega603, at43usb320, at76c711).
7479 Instruction set avr4 is for the enhanced AVR core with up to 8K program
7480 memory space (MCU types: atmega8, atmega83, atmega85).
7482 Instruction set avr5 is for the enhanced AVR core with up to 128K program
7483 memory space (MCU types: atmega16, atmega161, atmega163, atmega32, atmega323,
7484 atmega64, atmega128, at43usb355, at94k).
7488 Output instruction sizes to the asm file.
7490 @item -minit-stack=@var{N}
7491 @opindex minit-stack
7492 Specify the initial stack address, which may be a symbol or numeric value,
7493 @samp{__stack} is the default.
7495 @item -mno-interrupts
7496 @opindex mno-interrupts
7497 Generated code is not compatible with hardware interrupts.
7498 Code size will be smaller.
7500 @item -mcall-prologues
7501 @opindex mcall-prologues
7502 Functions prologues/epilogues expanded as call to appropriate
7503 subroutines. Code size will be smaller.
7505 @item -mno-tablejump
7506 @opindex mno-tablejump
7507 Do not generate tablejump insns which sometimes increase code size.
7510 @opindex mtiny-stack
7511 Change only the low 8 bits of the stack pointer.
7515 Assume int to be 8 bit integer. This affects the sizes of all types: A
7516 char will be 1 byte, an int will be 1 byte, an long will be 2 bytes
7517 and long long will be 4 bytes. Please note that this option does not
7518 comply to the C standards, but it will provide you with smaller code
7522 @node Blackfin Options
7523 @subsection Blackfin Options
7524 @cindex Blackfin Options
7527 @item -momit-leaf-frame-pointer
7528 @opindex momit-leaf-frame-pointer
7529 Don't keep the frame pointer in a register for leaf functions. This
7530 avoids the instructions to save, set up and restore frame pointers and
7531 makes an extra register available in leaf functions. The option
7532 @option{-fomit-frame-pointer} removes the frame pointer for all functions
7533 which might make debugging harder.
7535 @item -mspecld-anomaly
7536 @opindex mspecld-anomaly
7537 When enabled, the compiler will ensure that the generated code does not
7538 contain speculative loads after jump instructions. This option is enabled
7541 @item -mno-specld-anomaly
7542 @opindex mno-specld-anomaly
7543 Don't generate extra code to prevent speculative loads from occurring.
7545 @item -mcsync-anomaly
7546 @opindex mcsync-anomaly
7547 When enabled, the compiler will ensure that the generated code does not
7548 contain CSYNC or SSYNC instructions too soon after conditional branches.
7549 This option is enabled by default.
7551 @item -mno-csync-anomaly
7552 @opindex mno-csync-anomaly
7553 Don't generate extra code to prevent CSYNC or SSYNC instructions from
7554 occurring too soon after a conditional branch.
7558 When enabled, the compiler is free to take advantage of the knowledge that
7559 the entire program fits into the low 64k of memory.
7562 @opindex mno-low-64k
7563 Assume that the program is arbitrarily large. This is the default.
7565 @item -mid-shared-library
7566 @opindex mid-shared-library
7567 Generate code that supports shared libraries via the library ID method.
7568 This allows for execute in place and shared libraries in an environment
7569 without virtual memory management. This option implies @option{-fPIC}.
7571 @item -mno-id-shared-library
7572 @opindex mno-id-shared-library
7573 Generate code that doesn't assume ID based shared libraries are being used.
7574 This is the default.
7576 @item -mshared-library-id=n
7577 @opindex mshared-library-id
7578 Specified the identification number of the ID based shared library being
7579 compiled. Specifying a value of 0 will generate more compact code, specifying
7580 other values will force the allocation of that number to the current
7581 library but is no more space or time efficient than omitting this option.
7584 @itemx -mno-long-calls
7585 @opindex mlong-calls
7586 @opindex mno-long-calls
7587 Tells the compiler to perform function calls by first loading the
7588 address of the function into a register and then performing a subroutine
7589 call on this register. This switch is needed if the target function
7590 will lie outside of the 24 bit addressing range of the offset based
7591 version of subroutine call instruction.
7593 This feature is not enabled by default. Specifying
7594 @option{-mno-long-calls} will restore the default behavior. Note these
7595 switches have no effect on how the compiler generates code to handle
7596 function calls via function pointers.
7600 @subsection CRIS Options
7601 @cindex CRIS Options
7603 These options are defined specifically for the CRIS ports.
7606 @item -march=@var{architecture-type}
7607 @itemx -mcpu=@var{architecture-type}
7610 Generate code for the specified architecture. The choices for
7611 @var{architecture-type} are @samp{v3}, @samp{v8} and @samp{v10} for
7612 respectively ETRAX@w{ }4, ETRAX@w{ }100, and ETRAX@w{ }100@w{ }LX@.
7613 Default is @samp{v0} except for cris-axis-linux-gnu, where the default is
7616 @item -mtune=@var{architecture-type}
7618 Tune to @var{architecture-type} everything applicable about the generated
7619 code, except for the ABI and the set of available instructions. The
7620 choices for @var{architecture-type} are the same as for
7621 @option{-march=@var{architecture-type}}.
7623 @item -mmax-stack-frame=@var{n}
7624 @opindex mmax-stack-frame
7625 Warn when the stack frame of a function exceeds @var{n} bytes.
7627 @item -melinux-stacksize=@var{n}
7628 @opindex melinux-stacksize
7629 Only available with the @samp{cris-axis-aout} target. Arranges for
7630 indications in the program to the kernel loader that the stack of the
7631 program should be set to @var{n} bytes.
7637 The options @option{-metrax4} and @option{-metrax100} are synonyms for
7638 @option{-march=v3} and @option{-march=v8} respectively.
7640 @item -mmul-bug-workaround
7641 @itemx -mno-mul-bug-workaround
7642 @opindex mmul-bug-workaround
7643 @opindex mno-mul-bug-workaround
7644 Work around a bug in the @code{muls} and @code{mulu} instructions for CPU
7645 models where it applies. This option is active by default.
7649 Enable CRIS-specific verbose debug-related information in the assembly
7650 code. This option also has the effect to turn off the @samp{#NO_APP}
7651 formatted-code indicator to the assembler at the beginning of the
7656 Do not use condition-code results from previous instruction; always emit
7657 compare and test instructions before use of condition codes.
7659 @item -mno-side-effects
7660 @opindex mno-side-effects
7661 Do not emit instructions with side-effects in addressing modes other than
7665 @itemx -mno-stack-align
7667 @itemx -mno-data-align
7668 @itemx -mconst-align
7669 @itemx -mno-const-align
7670 @opindex mstack-align
7671 @opindex mno-stack-align
7672 @opindex mdata-align
7673 @opindex mno-data-align
7674 @opindex mconst-align
7675 @opindex mno-const-align
7676 These options (no-options) arranges (eliminate arrangements) for the
7677 stack-frame, individual data and constants to be aligned for the maximum
7678 single data access size for the chosen CPU model. The default is to
7679 arrange for 32-bit alignment. ABI details such as structure layout are
7680 not affected by these options.
7688 Similar to the stack- data- and const-align options above, these options
7689 arrange for stack-frame, writable data and constants to all be 32-bit,
7690 16-bit or 8-bit aligned. The default is 32-bit alignment.
7692 @item -mno-prologue-epilogue
7693 @itemx -mprologue-epilogue
7694 @opindex mno-prologue-epilogue
7695 @opindex mprologue-epilogue
7696 With @option{-mno-prologue-epilogue}, the normal function prologue and
7697 epilogue that sets up the stack-frame are omitted and no return
7698 instructions or return sequences are generated in the code. Use this
7699 option only together with visual inspection of the compiled code: no
7700 warnings or errors are generated when call-saved registers must be saved,
7701 or storage for local variable needs to be allocated.
7707 With @option{-fpic} and @option{-fPIC}, don't generate (do generate)
7708 instruction sequences that load addresses for functions from the PLT part
7709 of the GOT rather than (traditional on other architectures) calls to the
7710 PLT@. The default is @option{-mgotplt}.
7714 Legacy no-op option only recognized with the cris-axis-aout target.
7718 Legacy no-op option only recognized with the cris-axis-elf and
7719 cris-axis-linux-gnu targets.
7723 Only recognized with the cris-axis-aout target, where it selects a
7724 GNU/linux-like multilib, include files and instruction set for
7729 Legacy no-op option only recognized with the cris-axis-linux-gnu target.
7733 This option, recognized for the cris-axis-aout and cris-axis-elf arranges
7734 to link with input-output functions from a simulator library. Code,
7735 initialized data and zero-initialized data are allocated consecutively.
7739 Like @option{-sim}, but pass linker options to locate initialized data at
7740 0x40000000 and zero-initialized data at 0x80000000.
7744 @subsection CRX Options
7747 These options are defined specifically for the CRX ports.
7753 Enable the use of multiply-accumulate instructions. Disabled by default.
7757 Push instructions will be used to pass outgoing arguments when functions
7758 are called. Enabled by default.
7761 @node Darwin Options
7762 @subsection Darwin Options
7763 @cindex Darwin options
7765 These options are defined for all architectures running the Darwin operating
7768 FSF GCC on Darwin does not create ``fat'' object files; it will create
7769 an object file for the single architecture that it was built to
7770 target. Apple's GCC on Darwin does create ``fat'' files if multiple
7771 @option{-arch} options are used; it does so by running the compiler or
7772 linker multiple times and joining the results together with
7775 The subtype of the file created (like @samp{ppc7400} or @samp{ppc970} or
7776 @samp{i686}) is determined by the flags that specify the ISA
7777 that GCC is targetting, like @option{-mcpu} or @option{-march}. The
7778 @option{-force_cpusubtype_ALL} option can be used to override this.
7780 The Darwin tools vary in their behavior when presented with an ISA
7781 mismatch. The assembler, @file{as}, will only permit instructions to
7782 be used that are valid for the subtype of the file it is generating,
7783 so you cannot put 64-bit instructions in an @samp{ppc750} object file.
7784 The linker for shared libraries, @file{/usr/bin/libtool}, will fail
7785 and print an error if asked to create a shared library with a less
7786 restrictive subtype than its input files (for instance, trying to put
7787 a @samp{ppc970} object file in a @samp{ppc7400} library). The linker
7788 for executables, @file{ld}, will quietly give the executable the most
7789 restrictive subtype of any of its input files.
7794 Add the framework directory @var{dir} to the head of the list of
7795 directories to be searched for header files. These directories are
7796 interleaved with those specified by @option{-I} options and are
7797 scanned in a left-to-right order.
7799 A framework directory is a directory with frameworks in it. A
7800 framework is a directory with a @samp{"Headers"} and/or
7801 @samp{"PrivateHeaders"} directory contained directly in it that ends
7802 in @samp{".framework"}. The name of a framework is the name of this
7803 directory excluding the @samp{".framework"}. Headers associated with
7804 the framework are found in one of those two directories, with
7805 @samp{"Headers"} being searched first. A subframework is a framework
7806 directory that is in a framework's @samp{"Frameworks"} directory.
7807 Includes of subframework headers can only appear in a header of a
7808 framework that contains the subframework, or in a sibling subframework
7809 header. Two subframeworks are siblings if they occur in the same
7810 framework. A subframework should not have the same name as a
7811 framework, a warning will be issued if this is violated. Currently a
7812 subframework cannot have subframeworks, in the future, the mechanism
7813 may be extended to support this. The standard frameworks can be found
7814 in @samp{"/System/Library/Frameworks"} and
7815 @samp{"/Library/Frameworks"}. An example include looks like
7816 @code{#include <Framework/header.h>}, where @samp{Framework} denotes
7817 the name of the framework and header.h is found in the
7818 @samp{"PrivateHeaders"} or @samp{"Headers"} directory.
7822 Emit debugging information for symbols that are used. For STABS
7823 debugging format, this enables @option{-feliminate-unused-debug-symbols}.
7824 This is by default ON@.
7828 Emit debugging information for all symbols and types.
7830 @item -mmacosx-version-min=@var{version}
7831 The earliest version of MacOS X that this executable will run on
7832 is @var{version}. Typical values of @var{version} include @code{10.1},
7833 @code{10.2}, and @code{10.3.9}.
7835 The default for this option is to make choices that seem to be most
7838 @item -mone-byte-bool
7839 @opindex -mone-byte-bool
7840 Override the defaults for @samp{bool} so that @samp{sizeof(bool)==1}.
7841 By default @samp{sizeof(bool)} is @samp{4} when compiling for
7842 Darwin/PowerPC and @samp{1} when compiling for Darwin/x86, so this
7843 option has no effect on x86.
7845 @strong{Warning:} The @option{-mone-byte-bool} switch causes GCC
7846 to generate code that is not binary compatible with code generated
7847 without that switch. Using this switch may require recompiling all
7848 other modules in a program, including system libraries. Use this
7849 switch to conform to a non-default data model.
7851 @item -mfix-and-continue
7852 @itemx -ffix-and-continue
7853 @itemx -findirect-data
7854 @opindex mfix-and-continue
7855 @opindex ffix-and-continue
7856 @opindex findirect-data
7857 Generate code suitable for fast turn around development. Needed to
7858 enable gdb to dynamically load @code{.o} files into already running
7859 programs. @option{-findirect-data} and @option{-ffix-and-continue}
7860 are provided for backwards compatibility.
7864 Loads all members of static archive libraries.
7865 See man ld(1) for more information.
7867 @item -arch_errors_fatal
7868 @opindex arch_errors_fatal
7869 Cause the errors having to do with files that have the wrong architecture
7873 @opindex bind_at_load
7874 Causes the output file to be marked such that the dynamic linker will
7875 bind all undefined references when the file is loaded or launched.
7879 Produce a Mach-o bundle format file.
7880 See man ld(1) for more information.
7882 @item -bundle_loader @var{executable}
7883 @opindex bundle_loader
7884 This option specifies the @var{executable} that will be loading the build
7885 output file being linked. See man ld(1) for more information.
7888 @opindex -dynamiclib
7889 When passed this option, GCC will produce a dynamic library instead of
7890 an executable when linking, using the Darwin @file{libtool} command.
7892 @item -force_cpusubtype_ALL
7893 @opindex -force_cpusubtype_ALL
7894 This causes GCC's output file to have the @var{ALL} subtype, instead of
7895 one controlled by the @option{-mcpu} or @option{-march} option.
7897 @item -allowable_client @var{client_name}
7899 @itemx -compatibility_version
7900 @itemx -current_version
7902 @itemx -dependency-file
7904 @itemx -dylinker_install_name
7906 @itemx -exported_symbols_list
7908 @itemx -flat_namespace
7909 @itemx -force_flat_namespace
7910 @itemx -headerpad_max_install_names
7913 @itemx -install_name
7914 @itemx -keep_private_externs
7915 @itemx -multi_module
7916 @itemx -multiply_defined
7917 @itemx -multiply_defined_unused
7919 @itemx -no_dead_strip_inits_and_terms
7920 @itemx -nofixprebinding
7923 @itemx -noseglinkedit
7924 @itemx -pagezero_size
7926 @itemx -prebind_all_twolevel_modules
7927 @itemx -private_bundle
7928 @itemx -read_only_relocs
7930 @itemx -sectobjectsymbols
7934 @itemx -sectobjectsymbols
7937 @itemx -segs_read_only_addr
7938 @itemx -segs_read_write_addr
7939 @itemx -seg_addr_table
7940 @itemx -seg_addr_table_filename
7943 @itemx -segs_read_only_addr
7944 @itemx -segs_read_write_addr
7945 @itemx -single_module
7948 @itemx -sub_umbrella
7949 @itemx -twolevel_namespace
7952 @itemx -unexported_symbols_list
7953 @itemx -weak_reference_mismatches
7956 @opindex allowable_client
7957 @opindex client_name
7958 @opindex compatibility_version
7959 @opindex current_version
7961 @opindex dependency-file
7963 @opindex dylinker_install_name
7965 @opindex exported_symbols_list
7967 @opindex flat_namespace
7968 @opindex force_flat_namespace
7969 @opindex headerpad_max_install_names
7972 @opindex install_name
7973 @opindex keep_private_externs
7974 @opindex multi_module
7975 @opindex multiply_defined
7976 @opindex multiply_defined_unused
7978 @opindex no_dead_strip_inits_and_terms
7979 @opindex nofixprebinding
7980 @opindex nomultidefs
7982 @opindex noseglinkedit
7983 @opindex pagezero_size
7985 @opindex prebind_all_twolevel_modules
7986 @opindex private_bundle
7987 @opindex read_only_relocs
7989 @opindex sectobjectsymbols
7993 @opindex sectobjectsymbols
7996 @opindex segs_read_only_addr
7997 @opindex segs_read_write_addr
7998 @opindex seg_addr_table
7999 @opindex seg_addr_table_filename
8000 @opindex seglinkedit
8002 @opindex segs_read_only_addr
8003 @opindex segs_read_write_addr
8004 @opindex single_module
8006 @opindex sub_library
8007 @opindex sub_umbrella
8008 @opindex twolevel_namespace
8011 @opindex unexported_symbols_list
8012 @opindex weak_reference_mismatches
8013 @opindex whatsloaded
8015 These options are passed to the Darwin linker. The Darwin linker man page
8016 describes them in detail.
8019 @node DEC Alpha Options
8020 @subsection DEC Alpha Options
8022 These @samp{-m} options are defined for the DEC Alpha implementations:
8025 @item -mno-soft-float
8027 @opindex mno-soft-float
8028 @opindex msoft-float
8029 Use (do not use) the hardware floating-point instructions for
8030 floating-point operations. When @option{-msoft-float} is specified,
8031 functions in @file{libgcc.a} will be used to perform floating-point
8032 operations. Unless they are replaced by routines that emulate the
8033 floating-point operations, or compiled in such a way as to call such
8034 emulations routines, these routines will issue floating-point
8035 operations. If you are compiling for an Alpha without floating-point
8036 operations, you must ensure that the library is built so as not to call
8039 Note that Alpha implementations without floating-point operations are
8040 required to have floating-point registers.
8045 @opindex mno-fp-regs
8046 Generate code that uses (does not use) the floating-point register set.
8047 @option{-mno-fp-regs} implies @option{-msoft-float}. If the floating-point
8048 register set is not used, floating point operands are passed in integer
8049 registers as if they were integers and floating-point results are passed
8050 in @code{$0} instead of @code{$f0}. This is a non-standard calling sequence,
8051 so any function with a floating-point argument or return value called by code
8052 compiled with @option{-mno-fp-regs} must also be compiled with that
8055 A typical use of this option is building a kernel that does not use,
8056 and hence need not save and restore, any floating-point registers.
8060 The Alpha architecture implements floating-point hardware optimized for
8061 maximum performance. It is mostly compliant with the IEEE floating
8062 point standard. However, for full compliance, software assistance is
8063 required. This option generates code fully IEEE compliant code
8064 @emph{except} that the @var{inexact-flag} is not maintained (see below).
8065 If this option is turned on, the preprocessor macro @code{_IEEE_FP} is
8066 defined during compilation. The resulting code is less efficient but is
8067 able to correctly support denormalized numbers and exceptional IEEE
8068 values such as not-a-number and plus/minus infinity. Other Alpha
8069 compilers call this option @option{-ieee_with_no_inexact}.
8071 @item -mieee-with-inexact
8072 @opindex mieee-with-inexact
8073 This is like @option{-mieee} except the generated code also maintains
8074 the IEEE @var{inexact-flag}. Turning on this option causes the
8075 generated code to implement fully-compliant IEEE math. In addition to
8076 @code{_IEEE_FP}, @code{_IEEE_FP_EXACT} is defined as a preprocessor
8077 macro. On some Alpha implementations the resulting code may execute
8078 significantly slower than the code generated by default. Since there is
8079 very little code that depends on the @var{inexact-flag}, you should
8080 normally not specify this option. Other Alpha compilers call this
8081 option @option{-ieee_with_inexact}.
8083 @item -mfp-trap-mode=@var{trap-mode}
8084 @opindex mfp-trap-mode
8085 This option controls what floating-point related traps are enabled.
8086 Other Alpha compilers call this option @option{-fptm @var{trap-mode}}.
8087 The trap mode can be set to one of four values:
8091 This is the default (normal) setting. The only traps that are enabled
8092 are the ones that cannot be disabled in software (e.g., division by zero
8096 In addition to the traps enabled by @samp{n}, underflow traps are enabled
8100 Like @samp{su}, but the instructions are marked to be safe for software
8101 completion (see Alpha architecture manual for details).
8104 Like @samp{su}, but inexact traps are enabled as well.
8107 @item -mfp-rounding-mode=@var{rounding-mode}
8108 @opindex mfp-rounding-mode
8109 Selects the IEEE rounding mode. Other Alpha compilers call this option
8110 @option{-fprm @var{rounding-mode}}. The @var{rounding-mode} can be one
8115 Normal IEEE rounding mode. Floating point numbers are rounded towards
8116 the nearest machine number or towards the even machine number in case
8120 Round towards minus infinity.
8123 Chopped rounding mode. Floating point numbers are rounded towards zero.
8126 Dynamic rounding mode. A field in the floating point control register
8127 (@var{fpcr}, see Alpha architecture reference manual) controls the
8128 rounding mode in effect. The C library initializes this register for
8129 rounding towards plus infinity. Thus, unless your program modifies the
8130 @var{fpcr}, @samp{d} corresponds to round towards plus infinity.
8133 @item -mtrap-precision=@var{trap-precision}
8134 @opindex mtrap-precision
8135 In the Alpha architecture, floating point traps are imprecise. This
8136 means without software assistance it is impossible to recover from a
8137 floating trap and program execution normally needs to be terminated.
8138 GCC can generate code that can assist operating system trap handlers
8139 in determining the exact location that caused a floating point trap.
8140 Depending on the requirements of an application, different levels of
8141 precisions can be selected:
8145 Program precision. This option is the default and means a trap handler
8146 can only identify which program caused a floating point exception.
8149 Function precision. The trap handler can determine the function that
8150 caused a floating point exception.
8153 Instruction precision. The trap handler can determine the exact
8154 instruction that caused a floating point exception.
8157 Other Alpha compilers provide the equivalent options called
8158 @option{-scope_safe} and @option{-resumption_safe}.
8160 @item -mieee-conformant
8161 @opindex mieee-conformant
8162 This option marks the generated code as IEEE conformant. You must not
8163 use this option unless you also specify @option{-mtrap-precision=i} and either
8164 @option{-mfp-trap-mode=su} or @option{-mfp-trap-mode=sui}. Its only effect
8165 is to emit the line @samp{.eflag 48} in the function prologue of the
8166 generated assembly file. Under DEC Unix, this has the effect that
8167 IEEE-conformant math library routines will be linked in.
8169 @item -mbuild-constants
8170 @opindex mbuild-constants
8171 Normally GCC examines a 32- or 64-bit integer constant to
8172 see if it can construct it from smaller constants in two or three
8173 instructions. If it cannot, it will output the constant as a literal and
8174 generate code to load it from the data segment at runtime.
8176 Use this option to require GCC to construct @emph{all} integer constants
8177 using code, even if it takes more instructions (the maximum is six).
8179 You would typically use this option to build a shared library dynamic
8180 loader. Itself a shared library, it must relocate itself in memory
8181 before it can find the variables and constants in its own data segment.
8187 Select whether to generate code to be assembled by the vendor-supplied
8188 assembler (@option{-malpha-as}) or by the GNU assembler @option{-mgas}.
8206 Indicate whether GCC should generate code to use the optional BWX,
8207 CIX, FIX and MAX instruction sets. The default is to use the instruction
8208 sets supported by the CPU type specified via @option{-mcpu=} option or that
8209 of the CPU on which GCC was built if none was specified.
8214 @opindex mfloat-ieee
8215 Generate code that uses (does not use) VAX F and G floating point
8216 arithmetic instead of IEEE single and double precision.
8218 @item -mexplicit-relocs
8219 @itemx -mno-explicit-relocs
8220 @opindex mexplicit-relocs
8221 @opindex mno-explicit-relocs
8222 Older Alpha assemblers provided no way to generate symbol relocations
8223 except via assembler macros. Use of these macros does not allow
8224 optimal instruction scheduling. GNU binutils as of version 2.12
8225 supports a new syntax that allows the compiler to explicitly mark
8226 which relocations should apply to which instructions. This option
8227 is mostly useful for debugging, as GCC detects the capabilities of
8228 the assembler when it is built and sets the default accordingly.
8232 @opindex msmall-data
8233 @opindex mlarge-data
8234 When @option{-mexplicit-relocs} is in effect, static data is
8235 accessed via @dfn{gp-relative} relocations. When @option{-msmall-data}
8236 is used, objects 8 bytes long or smaller are placed in a @dfn{small data area}
8237 (the @code{.sdata} and @code{.sbss} sections) and are accessed via
8238 16-bit relocations off of the @code{$gp} register. This limits the
8239 size of the small data area to 64KB, but allows the variables to be
8240 directly accessed via a single instruction.
8242 The default is @option{-mlarge-data}. With this option the data area
8243 is limited to just below 2GB@. Programs that require more than 2GB of
8244 data must use @code{malloc} or @code{mmap} to allocate the data in the
8245 heap instead of in the program's data segment.
8247 When generating code for shared libraries, @option{-fpic} implies
8248 @option{-msmall-data} and @option{-fPIC} implies @option{-mlarge-data}.
8252 @opindex msmall-text
8253 @opindex mlarge-text
8254 When @option{-msmall-text} is used, the compiler assumes that the
8255 code of the entire program (or shared library) fits in 4MB, and is
8256 thus reachable with a branch instruction. When @option{-msmall-data}
8257 is used, the compiler can assume that all local symbols share the
8258 same @code{$gp} value, and thus reduce the number of instructions
8259 required for a function call from 4 to 1.
8261 The default is @option{-mlarge-text}.
8263 @item -mcpu=@var{cpu_type}
8265 Set the instruction set and instruction scheduling parameters for
8266 machine type @var{cpu_type}. You can specify either the @samp{EV}
8267 style name or the corresponding chip number. GCC supports scheduling
8268 parameters for the EV4, EV5 and EV6 family of processors and will
8269 choose the default values for the instruction set from the processor
8270 you specify. If you do not specify a processor type, GCC will default
8271 to the processor on which the compiler was built.
8273 Supported values for @var{cpu_type} are
8279 Schedules as an EV4 and has no instruction set extensions.
8283 Schedules as an EV5 and has no instruction set extensions.
8287 Schedules as an EV5 and supports the BWX extension.
8292 Schedules as an EV5 and supports the BWX and MAX extensions.
8296 Schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
8300 Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.
8303 @item -mtune=@var{cpu_type}
8305 Set only the instruction scheduling parameters for machine type
8306 @var{cpu_type}. The instruction set is not changed.
8308 @item -mmemory-latency=@var{time}
8309 @opindex mmemory-latency
8310 Sets the latency the scheduler should assume for typical memory
8311 references as seen by the application. This number is highly
8312 dependent on the memory access patterns used by the application
8313 and the size of the external cache on the machine.
8315 Valid options for @var{time} are
8319 A decimal number representing clock cycles.
8325 The compiler contains estimates of the number of clock cycles for
8326 ``typical'' EV4 & EV5 hardware for the Level 1, 2 & 3 caches
8327 (also called Dcache, Scache, and Bcache), as well as to main memory.
8328 Note that L3 is only valid for EV5.
8333 @node DEC Alpha/VMS Options
8334 @subsection DEC Alpha/VMS Options
8336 These @samp{-m} options are defined for the DEC Alpha/VMS implementations:
8339 @item -mvms-return-codes
8340 @opindex mvms-return-codes
8341 Return VMS condition codes from main. The default is to return POSIX
8342 style condition (e.g.@ error) codes.
8346 @subsection FRV Options
8353 Only use the first 32 general purpose registers.
8358 Use all 64 general purpose registers.
8363 Use only the first 32 floating point registers.
8368 Use all 64 floating point registers
8371 @opindex mhard-float
8373 Use hardware instructions for floating point operations.
8376 @opindex msoft-float
8378 Use library routines for floating point operations.
8383 Dynamically allocate condition code registers.
8388 Do not try to dynamically allocate condition code registers, only
8389 use @code{icc0} and @code{fcc0}.
8394 Change ABI to use double word insns.
8399 Do not use double word instructions.
8404 Use floating point double instructions.
8409 Do not use floating point double instructions.
8414 Use media instructions.
8419 Do not use media instructions.
8424 Use multiply and add/subtract instructions.
8429 Do not use multiply and add/subtract instructions.
8434 Select the FDPIC ABI, that uses function descriptors to represent
8435 pointers to functions. Without any PIC/PIE-related options, it
8436 implies @option{-fPIE}. With @option{-fpic} or @option{-fpie}, it
8437 assumes GOT entries and small data are within a 12-bit range from the
8438 GOT base address; with @option{-fPIC} or @option{-fPIE}, GOT offsets
8439 are computed with 32 bits.
8442 @opindex minline-plt
8444 Enable inlining of PLT entries in function calls to functions that are
8445 not known to bind locally. It has no effect without @option{-mfdpic}.
8446 It's enabled by default if optimizing for speed and compiling for
8447 shared libraries (i.e., @option{-fPIC} or @option{-fpic}), or when an
8448 optimization option such as @option{-O3} or above is present in the
8454 Assume a large TLS segment when generating thread-local code.
8459 Do not assume a large TLS segment when generating thread-local code.
8464 Enable the use of @code{GPREL} relocations in the FDPIC ABI for data
8465 that is known to be in read-only sections. It's enabled by default,
8466 except for @option{-fpic} or @option{-fpie}: even though it may help
8467 make the global offset table smaller, it trades 1 instruction for 4.
8468 With @option{-fPIC} or @option{-fPIE}, it trades 3 instructions for 4,
8469 one of which may be shared by multiple symbols, and it avoids the need
8470 for a GOT entry for the referenced symbol, so it's more likely to be a
8471 win. If it is not, @option{-mno-gprel-ro} can be used to disable it.
8473 @item -multilib-library-pic
8474 @opindex multilib-library-pic
8476 Link with the (library, not FD) pic libraries. It's implied by
8477 @option{-mlibrary-pic}, as well as by @option{-fPIC} and
8478 @option{-fpic} without @option{-mfdpic}. You should never have to use
8484 Follow the EABI requirement of always creating a frame pointer whenever
8485 a stack frame is allocated. This option is enabled by default and can
8486 be disabled with @option{-mno-linked-fp}.
8489 @opindex mlong-calls
8491 Use indirect addressing to call functions outside the current
8492 compilation unit. This allows the functions to be placed anywhere
8493 within the 32-bit address space.
8495 @item -malign-labels
8496 @opindex malign-labels
8498 Try to align labels to an 8-byte boundary by inserting nops into the
8499 previous packet. This option only has an effect when VLIW packing
8500 is enabled. It doesn't create new packets; it merely adds nops to
8504 @opindex mlibrary-pic
8506 Generate position-independent EABI code.
8511 Use only the first four media accumulator registers.
8516 Use all eight media accumulator registers.
8521 Pack VLIW instructions.
8526 Do not pack VLIW instructions.
8531 Do not mark ABI switches in e_flags.
8536 Enable the use of conditional-move instructions (default).
8538 This switch is mainly for debugging the compiler and will likely be removed
8539 in a future version.
8541 @item -mno-cond-move
8542 @opindex mno-cond-move
8544 Disable the use of conditional-move instructions.
8546 This switch is mainly for debugging the compiler and will likely be removed
8547 in a future version.
8552 Enable the use of conditional set instructions (default).
8554 This switch is mainly for debugging the compiler and will likely be removed
8555 in a future version.
8560 Disable the use of conditional set instructions.
8562 This switch is mainly for debugging the compiler and will likely be removed
8563 in a future version.
8568 Enable the use of conditional execution (default).
8570 This switch is mainly for debugging the compiler and will likely be removed
8571 in a future version.
8573 @item -mno-cond-exec
8574 @opindex mno-cond-exec
8576 Disable the use of conditional execution.
8578 This switch is mainly for debugging the compiler and will likely be removed
8579 in a future version.
8582 @opindex mvliw-branch
8584 Run a pass to pack branches into VLIW instructions (default).
8586 This switch is mainly for debugging the compiler and will likely be removed
8587 in a future version.
8589 @item -mno-vliw-branch
8590 @opindex mno-vliw-branch
8592 Do not run a pass to pack branches into VLIW instructions.
8594 This switch is mainly for debugging the compiler and will likely be removed
8595 in a future version.
8597 @item -mmulti-cond-exec
8598 @opindex mmulti-cond-exec
8600 Enable optimization of @code{&&} and @code{||} in conditional execution
8603 This switch is mainly for debugging the compiler and will likely be removed
8604 in a future version.
8606 @item -mno-multi-cond-exec
8607 @opindex mno-multi-cond-exec
8609 Disable optimization of @code{&&} and @code{||} in conditional execution.
8611 This switch is mainly for debugging the compiler and will likely be removed
8612 in a future version.
8614 @item -mnested-cond-exec
8615 @opindex mnested-cond-exec
8617 Enable nested conditional execution optimizations (default).
8619 This switch is mainly for debugging the compiler and will likely be removed
8620 in a future version.
8622 @item -mno-nested-cond-exec
8623 @opindex mno-nested-cond-exec
8625 Disable nested conditional execution optimizations.
8627 This switch is mainly for debugging the compiler and will likely be removed
8628 in a future version.
8630 @item -moptimize-membar
8631 @opindex moptimize-membar
8633 This switch removes redundant @code{membar} instructions from the
8634 compiler generated code. It is enabled by default.
8636 @item -mno-optimize-membar
8637 @opindex mno-optimize-membar
8639 This switch disables the automatic removal of redundant @code{membar}
8640 instructions from the generated code.
8642 @item -mtomcat-stats
8643 @opindex mtomcat-stats
8645 Cause gas to print out tomcat statistics.
8647 @item -mcpu=@var{cpu}
8650 Select the processor type for which to generate code. Possible values are
8651 @samp{frv}, @samp{fr550}, @samp{tomcat}, @samp{fr500}, @samp{fr450},
8652 @samp{fr405}, @samp{fr400}, @samp{fr300} and @samp{simple}.
8656 @node H8/300 Options
8657 @subsection H8/300 Options
8659 These @samp{-m} options are defined for the H8/300 implementations:
8664 Shorten some address references at link time, when possible; uses the
8665 linker option @option{-relax}. @xref{H8/300,, @code{ld} and the H8/300,
8666 ld, Using ld}, for a fuller description.
8670 Generate code for the H8/300H@.
8674 Generate code for the H8S@.
8678 Generate code for the H8S and H8/300H in the normal mode. This switch
8679 must be used either with @option{-mh} or @option{-ms}.
8683 Generate code for the H8S/2600. This switch must be used with @option{-ms}.
8687 Make @code{int} data 32 bits by default.
8691 On the H8/300H and H8S, use the same alignment rules as for the H8/300.
8692 The default for the H8/300H and H8S is to align longs and floats on 4
8694 @option{-malign-300} causes them to be aligned on 2 byte boundaries.
8695 This option has no effect on the H8/300.
8699 @subsection HPPA Options
8700 @cindex HPPA Options
8702 These @samp{-m} options are defined for the HPPA family of computers:
8705 @item -march=@var{architecture-type}
8707 Generate code for the specified architecture. The choices for
8708 @var{architecture-type} are @samp{1.0} for PA 1.0, @samp{1.1} for PA
8709 1.1, and @samp{2.0} for PA 2.0 processors. Refer to
8710 @file{/usr/lib/sched.models} on an HP-UX system to determine the proper
8711 architecture option for your machine. Code compiled for lower numbered
8712 architectures will run on higher numbered architectures, but not the
8716 @itemx -mpa-risc-1-1
8717 @itemx -mpa-risc-2-0
8718 @opindex mpa-risc-1-0
8719 @opindex mpa-risc-1-1
8720 @opindex mpa-risc-2-0
8721 Synonyms for @option{-march=1.0}, @option{-march=1.1}, and @option{-march=2.0} respectively.
8724 @opindex mbig-switch
8725 Generate code suitable for big switch tables. Use this option only if
8726 the assembler/linker complain about out of range branches within a switch
8729 @item -mjump-in-delay
8730 @opindex mjump-in-delay
8731 Fill delay slots of function calls with unconditional jump instructions
8732 by modifying the return pointer for the function call to be the target
8733 of the conditional jump.
8735 @item -mdisable-fpregs
8736 @opindex mdisable-fpregs
8737 Prevent floating point registers from being used in any manner. This is
8738 necessary for compiling kernels which perform lazy context switching of
8739 floating point registers. If you use this option and attempt to perform
8740 floating point operations, the compiler will abort.
8742 @item -mdisable-indexing
8743 @opindex mdisable-indexing
8744 Prevent the compiler from using indexing address modes. This avoids some
8745 rather obscure problems when compiling MIG generated code under MACH@.
8747 @item -mno-space-regs
8748 @opindex mno-space-regs
8749 Generate code that assumes the target has no space registers. This allows
8750 GCC to generate faster indirect calls and use unscaled index address modes.
8752 Such code is suitable for level 0 PA systems and kernels.
8754 @item -mfast-indirect-calls
8755 @opindex mfast-indirect-calls
8756 Generate code that assumes calls never cross space boundaries. This
8757 allows GCC to emit code which performs faster indirect calls.
8759 This option will not work in the presence of shared libraries or nested
8762 @item -mfixed-range=@var{register-range}
8763 @opindex mfixed-range
8764 Generate code treating the given register range as fixed registers.
8765 A fixed register is one that the register allocator can not use. This is
8766 useful when compiling kernel code. A register range is specified as
8767 two registers separated by a dash. Multiple register ranges can be
8768 specified separated by a comma.
8770 @item -mlong-load-store
8771 @opindex mlong-load-store
8772 Generate 3-instruction load and store sequences as sometimes required by
8773 the HP-UX 10 linker. This is equivalent to the @samp{+k} option to
8776 @item -mportable-runtime
8777 @opindex mportable-runtime
8778 Use the portable calling conventions proposed by HP for ELF systems.
8782 Enable the use of assembler directives only GAS understands.
8784 @item -mschedule=@var{cpu-type}
8786 Schedule code according to the constraints for the machine type
8787 @var{cpu-type}. The choices for @var{cpu-type} are @samp{700}
8788 @samp{7100}, @samp{7100LC}, @samp{7200}, @samp{7300} and @samp{8000}. Refer
8789 to @file{/usr/lib/sched.models} on an HP-UX system to determine the
8790 proper scheduling option for your machine. The default scheduling is
8794 @opindex mlinker-opt
8795 Enable the optimization pass in the HP-UX linker. Note this makes symbolic
8796 debugging impossible. It also triggers a bug in the HP-UX 8 and HP-UX 9
8797 linkers in which they give bogus error messages when linking some programs.
8800 @opindex msoft-float
8801 Generate output containing library calls for floating point.
8802 @strong{Warning:} the requisite libraries are not available for all HPPA
8803 targets. Normally the facilities of the machine's usual C compiler are
8804 used, but this cannot be done directly in cross-compilation. You must make
8805 your own arrangements to provide suitable library functions for
8806 cross-compilation. The embedded target @samp{hppa1.1-*-pro}
8807 does provide software floating point support.
8809 @option{-msoft-float} changes the calling convention in the output file;
8810 therefore, it is only useful if you compile @emph{all} of a program with
8811 this option. In particular, you need to compile @file{libgcc.a}, the
8812 library that comes with GCC, with @option{-msoft-float} in order for
8817 Generate the predefine, @code{_SIO}, for server IO@. The default is
8818 @option{-mwsio}. This generates the predefines, @code{__hp9000s700},
8819 @code{__hp9000s700__} and @code{_WSIO}, for workstation IO@. These
8820 options are available under HP-UX and HI-UX@.
8824 Use GNU ld specific options. This passes @option{-shared} to ld when
8825 building a shared library. It is the default when GCC is configured,
8826 explicitly or implicitly, with the GNU linker. This option does not
8827 have any affect on which ld is called, it only changes what parameters
8828 are passed to that ld. The ld that is called is determined by the
8829 @option{--with-ld} configure option, GCC's program search path, and
8830 finally by the user's @env{PATH}. The linker used by GCC can be printed
8831 using @samp{which `gcc -print-prog-name=ld`}. This option is only available
8832 on the 64 bit HP-UX GCC, i.e. configured with @samp{hppa*64*-*-hpux*}.
8836 Use HP ld specific options. This passes @option{-b} to ld when building
8837 a shared library and passes @option{+Accept TypeMismatch} to ld on all
8838 links. It is the default when GCC is configured, explicitly or
8839 implicitly, with the HP linker. This option does not have any affect on
8840 which ld is called, it only changes what parameters are passed to that
8841 ld. The ld that is called is determined by the @option{--with-ld}
8842 configure option, GCC's program search path, and finally by the user's
8843 @env{PATH}. The linker used by GCC can be printed using @samp{which
8844 `gcc -print-prog-name=ld`}. This option is only available on the 64 bit
8845 HP-UX GCC, i.e. configured with @samp{hppa*64*-*-hpux*}.
8848 @opindex mno-long-calls
8849 Generate code that uses long call sequences. This ensures that a call
8850 is always able to reach linker generated stubs. The default is to generate
8851 long calls only when the distance from the call site to the beginning
8852 of the function or translation unit, as the case may be, exceeds a
8853 predefined limit set by the branch type being used. The limits for
8854 normal calls are 7,600,000 and 240,000 bytes, respectively for the
8855 PA 2.0 and PA 1.X architectures. Sibcalls are always limited at
8858 Distances are measured from the beginning of functions when using the
8859 @option{-ffunction-sections} option, or when using the @option{-mgas}
8860 and @option{-mno-portable-runtime} options together under HP-UX with
8863 It is normally not desirable to use this option as it will degrade
8864 performance. However, it may be useful in large applications,
8865 particularly when partial linking is used to build the application.
8867 The types of long calls used depends on the capabilities of the
8868 assembler and linker, and the type of code being generated. The
8869 impact on systems that support long absolute calls, and long pic
8870 symbol-difference or pc-relative calls should be relatively small.
8871 However, an indirect call is used on 32-bit ELF systems in pic code
8872 and it is quite long.
8874 @item -munix=@var{unix-std}
8876 Generate compiler predefines and select a startfile for the specified
8877 UNIX standard. The choices for @var{unix-std} are @samp{93}, @samp{95}
8878 and @samp{98}. @samp{93} is supported on all HP-UX versions. @samp{95}
8879 is available on HP-UX 10.10 and later. @samp{98} is available on HP-UX
8880 11.11 and later. The default values are @samp{93} for HP-UX 10.00,
8881 @samp{95} for HP-UX 10.10 though to 11.00, and @samp{98} for HP-UX 11.11
8884 @option{-munix=93} provides the same predefines as GCC 3.3 and 3.4.
8885 @option{-munix=95} provides additional predefines for @code{XOPEN_UNIX}
8886 and @code{_XOPEN_SOURCE_EXTENDED}, and the startfile @file{unix95.o}.
8887 @option{-munix=98} provides additional predefines for @code{_XOPEN_UNIX},
8888 @code{_XOPEN_SOURCE_EXTENDED}, @code{_INCLUDE__STDC_A1_SOURCE} and
8889 @code{_INCLUDE_XOPEN_SOURCE_500}, and the startfile @file{unix98.o}.
8891 It is @emph{important} to note that this option changes the interfaces
8892 for various library routines. It also affects the operational behavior
8893 of the C library. Thus, @emph{extreme} care is needed in using this
8896 Library code that is intended to operate with more than one UNIX
8897 standard must test, set and restore the variable @var{__xpg4_extended_mask}
8898 as appropriate. Most GNU software doesn't provide this capability.
8902 Suppress the generation of link options to search libdld.sl when the
8903 @option{-static} option is specified on HP-UX 10 and later.
8907 The HP-UX implementation of setlocale in libc has a dependency on
8908 libdld.sl. There isn't an archive version of libdld.sl. Thus,
8909 when the @option{-static} option is specified, special link options
8910 are needed to resolve this dependency.
8912 On HP-UX 10 and later, the GCC driver adds the necessary options to
8913 link with libdld.sl when the @option{-static} option is specified.
8914 This causes the resulting binary to be dynamic. On the 64-bit port,
8915 the linkers generate dynamic binaries by default in any case. The
8916 @option{-nolibdld} option can be used to prevent the GCC driver from
8917 adding these link options.
8921 Add support for multithreading with the @dfn{dce thread} library
8922 under HP-UX@. This option sets flags for both the preprocessor and
8926 @node i386 and x86-64 Options
8927 @subsection Intel 386 and AMD x86-64 Options
8928 @cindex i386 Options
8929 @cindex x86-64 Options
8930 @cindex Intel 386 Options
8931 @cindex AMD x86-64 Options
8933 These @samp{-m} options are defined for the i386 and x86-64 family of
8937 @item -mtune=@var{cpu-type}
8939 Tune to @var{cpu-type} everything applicable about the generated code, except
8940 for the ABI and the set of available instructions. The choices for
8944 Original Intel's i386 CPU@.
8946 Intel's i486 CPU@. (No scheduling is implemented for this chip.)
8948 Intel Pentium CPU with no MMX support.
8950 Intel PentiumMMX CPU based on Pentium core with MMX instruction set support.
8951 @item i686, pentiumpro
8952 Intel PentiumPro CPU@.
8954 Intel Pentium2 CPU based on PentiumPro core with MMX instruction set support.
8955 @item pentium3, pentium3m
8956 Intel Pentium3 CPU based on PentiumPro core with MMX and SSE instruction set
8959 Low power version of Intel Pentium3 CPU with MMX, SSE and SSE2 instruction set
8960 support. Used by Centrino notebooks.
8961 @item pentium4, pentium4m
8962 Intel Pentium4 CPU with MMX, SSE and SSE2 instruction set support.
8964 Improved version of Intel Pentium4 CPU with MMX, SSE, SSE2 and SSE3 instruction
8967 Improved version of Intel Pentium4 CPU with 64-bit extensions, MMX, SSE,
8968 SSE2 and SSE3 instruction set support.
8970 AMD K6 CPU with MMX instruction set support.
8972 Improved versions of AMD K6 CPU with MMX and 3dNOW! instruction set support.
8973 @item athlon, athlon-tbird
8974 AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and SSE prefetch instructions
8976 @item athlon-4, athlon-xp, athlon-mp
8977 Improved AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and full SSE
8978 instruction set support.
8979 @item k8, opteron, athlon64, athlon-fx
8980 AMD K8 core based CPUs with x86-64 instruction set support. (This supersets
8981 MMX, SSE, SSE2, 3dNOW!, enhanced 3dNOW! and 64-bit instruction set extensions.)
8983 IDT Winchip C6 CPU, dealt in same way as i486 with additional MMX instruction
8986 IDT Winchip2 CPU, dealt in same way as i486 with additional MMX and 3dNOW!
8987 instruction set support.
8989 Via C3 CPU with MMX and 3dNOW! instruction set support. (No scheduling is
8990 implemented for this chip.)
8992 Via C3-2 CPU with MMX and SSE instruction set support. (No scheduling is
8993 implemented for this chip.)
8996 While picking a specific @var{cpu-type} will schedule things appropriately
8997 for that particular chip, the compiler will not generate any code that
8998 does not run on the i386 without the @option{-march=@var{cpu-type}} option
9001 @item -march=@var{cpu-type}
9003 Generate instructions for the machine type @var{cpu-type}. The choices
9004 for @var{cpu-type} are the same as for @option{-mtune}. Moreover,
9005 specifying @option{-march=@var{cpu-type}} implies @option{-mtune=@var{cpu-type}}.
9007 @item -mcpu=@var{cpu-type}
9009 A deprecated synonym for @option{-mtune}.
9018 @opindex mpentiumpro
9019 These options are synonyms for @option{-mtune=i386}, @option{-mtune=i486},
9020 @option{-mtune=pentium}, and @option{-mtune=pentiumpro} respectively.
9021 These synonyms are deprecated.
9023 @item -mfpmath=@var{unit}
9025 Generate floating point arithmetics for selected unit @var{unit}. The choices
9030 Use the standard 387 floating point coprocessor present majority of chips and
9031 emulated otherwise. Code compiled with this option will run almost everywhere.
9032 The temporary results are computed in 80bit precision instead of precision
9033 specified by the type resulting in slightly different results compared to most
9034 of other chips. See @option{-ffloat-store} for more detailed description.
9036 This is the default choice for i386 compiler.
9039 Use scalar floating point instructions present in the SSE instruction set.
9040 This instruction set is supported by Pentium3 and newer chips, in the AMD line
9041 by Athlon-4, Athlon-xp and Athlon-mp chips. The earlier version of SSE
9042 instruction set supports only single precision arithmetics, thus the double and
9043 extended precision arithmetics is still done using 387. Later version, present
9044 only in Pentium4 and the future AMD x86-64 chips supports double precision
9047 For the i386 compiler, you need to use @option{-march=@var{cpu-type}}, @option{-msse}
9048 or @option{-msse2} switches to enable SSE extensions and make this option
9049 effective. For the x86-64 compiler, these extensions are enabled by default.
9051 The resulting code should be considerably faster in the majority of cases and avoid
9052 the numerical instability problems of 387 code, but may break some existing
9053 code that expects temporaries to be 80bit.
9055 This is the default choice for the x86-64 compiler.
9058 Attempt to utilize both instruction sets at once. This effectively double the
9059 amount of available registers and on chips with separate execution units for
9060 387 and SSE the execution resources too. Use this option with care, as it is
9061 still experimental, because the GCC register allocator does not model separate
9062 functional units well resulting in instable performance.
9065 @item -masm=@var{dialect}
9066 @opindex masm=@var{dialect}
9067 Output asm instructions using selected @var{dialect}. Supported choices are
9068 @samp{intel} or @samp{att} (the default one).
9073 @opindex mno-ieee-fp
9074 Control whether or not the compiler uses IEEE floating point
9075 comparisons. These handle correctly the case where the result of a
9076 comparison is unordered.
9079 @opindex msoft-float
9080 Generate output containing library calls for floating point.
9081 @strong{Warning:} the requisite libraries are not part of GCC@.
9082 Normally the facilities of the machine's usual C compiler are used, but
9083 this can't be done directly in cross-compilation. You must make your
9084 own arrangements to provide suitable library functions for
9087 On machines where a function returns floating point results in the 80387
9088 register stack, some floating point opcodes may be emitted even if
9089 @option{-msoft-float} is used.
9091 @item -mno-fp-ret-in-387
9092 @opindex mno-fp-ret-in-387
9093 Do not use the FPU registers for return values of functions.
9095 The usual calling convention has functions return values of types
9096 @code{float} and @code{double} in an FPU register, even if there
9097 is no FPU@. The idea is that the operating system should emulate
9100 The option @option{-mno-fp-ret-in-387} causes such values to be returned
9101 in ordinary CPU registers instead.
9103 @item -mno-fancy-math-387
9104 @opindex mno-fancy-math-387
9105 Some 387 emulators do not support the @code{sin}, @code{cos} and
9106 @code{sqrt} instructions for the 387. Specify this option to avoid
9107 generating those instructions. This option is the default on FreeBSD,
9108 OpenBSD and NetBSD@. This option is overridden when @option{-march}
9109 indicates that the target cpu will always have an FPU and so the
9110 instruction will not need emulation. As of revision 2.6.1, these
9111 instructions are not generated unless you also use the
9112 @option{-funsafe-math-optimizations} switch.
9114 @item -malign-double
9115 @itemx -mno-align-double
9116 @opindex malign-double
9117 @opindex mno-align-double
9118 Control whether GCC aligns @code{double}, @code{long double}, and
9119 @code{long long} variables on a two word boundary or a one word
9120 boundary. Aligning @code{double} variables on a two word boundary will
9121 produce code that runs somewhat faster on a @samp{Pentium} at the
9122 expense of more memory.
9124 @strong{Warning:} if you use the @option{-malign-double} switch,
9125 structures containing the above types will be aligned differently than
9126 the published application binary interface specifications for the 386
9127 and will not be binary compatible with structures in code compiled
9128 without that switch.
9130 @item -m96bit-long-double
9131 @itemx -m128bit-long-double
9132 @opindex m96bit-long-double
9133 @opindex m128bit-long-double
9134 These switches control the size of @code{long double} type. The i386
9135 application binary interface specifies the size to be 96 bits,
9136 so @option{-m96bit-long-double} is the default in 32 bit mode.
9138 Modern architectures (Pentium and newer) would prefer @code{long double}
9139 to be aligned to an 8 or 16 byte boundary. In arrays or structures
9140 conforming to the ABI, this would not be possible. So specifying a
9141 @option{-m128bit-long-double} will align @code{long double}
9142 to a 16 byte boundary by padding the @code{long double} with an additional
9145 In the x86-64 compiler, @option{-m128bit-long-double} is the default choice as
9146 its ABI specifies that @code{long double} is to be aligned on 16 byte boundary.
9148 Notice that neither of these options enable any extra precision over the x87
9149 standard of 80 bits for a @code{long double}.
9151 @strong{Warning:} if you override the default value for your target ABI, the
9152 structures and arrays containing @code{long double} variables will change
9153 their size as well as function calling convention for function taking
9154 @code{long double} will be modified. Hence they will not be binary
9155 compatible with arrays or structures in code compiled without that switch.
9157 @item -mmlarge-data-threshold=@var{number}
9158 @opindex mlarge-data-threshold=@var{number}
9159 When @option{-mcmodel=medium} is specified, the data greater than
9160 @var{threshold} are placed in large data section. This value must be the
9161 same across all object linked into the binary and defaults to 65535.
9164 @itemx -mno-svr3-shlib
9165 @opindex msvr3-shlib
9166 @opindex mno-svr3-shlib
9167 Control whether GCC places uninitialized local variables into the
9168 @code{bss} or @code{data} segments. @option{-msvr3-shlib} places them
9169 into @code{bss}. These options are meaningful only on System V Release 3.
9173 Use a different function-calling convention, in which functions that
9174 take a fixed number of arguments return with the @code{ret} @var{num}
9175 instruction, which pops their arguments while returning. This saves one
9176 instruction in the caller since there is no need to pop the arguments
9179 You can specify that an individual function is called with this calling
9180 sequence with the function attribute @samp{stdcall}. You can also
9181 override the @option{-mrtd} option by using the function attribute
9182 @samp{cdecl}. @xref{Function Attributes}.
9184 @strong{Warning:} this calling convention is incompatible with the one
9185 normally used on Unix, so you cannot use it if you need to call
9186 libraries compiled with the Unix compiler.
9188 Also, you must provide function prototypes for all functions that
9189 take variable numbers of arguments (including @code{printf});
9190 otherwise incorrect code will be generated for calls to those
9193 In addition, seriously incorrect code will result if you call a
9194 function with too many arguments. (Normally, extra arguments are
9195 harmlessly ignored.)
9197 @item -mregparm=@var{num}
9199 Control how many registers are used to pass integer arguments. By
9200 default, no registers are used to pass arguments, and at most 3
9201 registers can be used. You can control this behavior for a specific
9202 function by using the function attribute @samp{regparm}.
9203 @xref{Function Attributes}.
9205 @strong{Warning:} if you use this switch, and
9206 @var{num} is nonzero, then you must build all modules with the same
9207 value, including any libraries. This includes the system libraries and
9211 @opindex msseregparm
9212 Use SSE register passing conventions for float and double arguments
9213 and return values. You can control this behavior for a specific
9214 function by using the function attribute @samp{sseregparm}.
9215 @xref{Function Attributes}.
9217 @strong{Warning:} if you use this switch then you must build all
9218 modules with the same value, including any libraries. This includes
9219 the system libraries and startup modules.
9221 @item -mpreferred-stack-boundary=@var{num}
9222 @opindex mpreferred-stack-boundary
9223 Attempt to keep the stack boundary aligned to a 2 raised to @var{num}
9224 byte boundary. If @option{-mpreferred-stack-boundary} is not specified,
9225 the default is 4 (16 bytes or 128 bits), except when optimizing for code
9226 size (@option{-Os}), in which case the default is the minimum correct
9227 alignment (4 bytes for x86, and 8 bytes for x86-64).
9229 On Pentium and PentiumPro, @code{double} and @code{long double} values
9230 should be aligned to an 8 byte boundary (see @option{-malign-double}) or
9231 suffer significant run time performance penalties. On Pentium III, the
9232 Streaming SIMD Extension (SSE) data type @code{__m128} suffers similar
9233 penalties if it is not 16 byte aligned.
9235 To ensure proper alignment of this values on the stack, the stack boundary
9236 must be as aligned as that required by any value stored on the stack.
9237 Further, every function must be generated such that it keeps the stack
9238 aligned. Thus calling a function compiled with a higher preferred
9239 stack boundary from a function compiled with a lower preferred stack
9240 boundary will most likely misalign the stack. It is recommended that
9241 libraries that use callbacks always use the default setting.
9243 This extra alignment does consume extra stack space, and generally
9244 increases code size. Code that is sensitive to stack space usage, such
9245 as embedded systems and operating system kernels, may want to reduce the
9246 preferred alignment to @option{-mpreferred-stack-boundary=2}.
9264 These switches enable or disable the use of instructions in the MMX,
9265 SSE, SSE2 or 3DNow! extended instruction sets. These extensions are
9266 also available as built-in functions: see @ref{X86 Built-in Functions},
9267 for details of the functions enabled and disabled by these switches.
9269 To have SSE/SSE2 instructions generated automatically from floating-point
9270 code (as opposed to 387 instructions), see @option{-mfpmath=sse}.
9272 These options will enable GCC to use these extended instructions in
9273 generated code, even without @option{-mfpmath=sse}. Applications which
9274 perform runtime CPU detection must compile separate files for each
9275 supported architecture, using the appropriate flags. In particular,
9276 the file containing the CPU detection code should be compiled without
9280 @itemx -mno-push-args
9282 @opindex mno-push-args
9283 Use PUSH operations to store outgoing parameters. This method is shorter
9284 and usually equally fast as method using SUB/MOV operations and is enabled
9285 by default. In some cases disabling it may improve performance because of
9286 improved scheduling and reduced dependencies.
9288 @item -maccumulate-outgoing-args
9289 @opindex maccumulate-outgoing-args
9290 If enabled, the maximum amount of space required for outgoing arguments will be
9291 computed in the function prologue. This is faster on most modern CPUs
9292 because of reduced dependencies, improved scheduling and reduced stack usage
9293 when preferred stack boundary is not equal to 2. The drawback is a notable
9294 increase in code size. This switch implies @option{-mno-push-args}.
9298 Support thread-safe exception handling on @samp{Mingw32}. Code that relies
9299 on thread-safe exception handling must compile and link all code with the
9300 @option{-mthreads} option. When compiling, @option{-mthreads} defines
9301 @option{-D_MT}; when linking, it links in a special thread helper library
9302 @option{-lmingwthrd} which cleans up per thread exception handling data.
9304 @item -mno-align-stringops
9305 @opindex mno-align-stringops
9306 Do not align destination of inlined string operations. This switch reduces
9307 code size and improves performance in case the destination is already aligned,
9308 but GCC doesn't know about it.
9310 @item -minline-all-stringops
9311 @opindex minline-all-stringops
9312 By default GCC inlines string operations only when destination is known to be
9313 aligned at least to 4 byte boundary. This enables more inlining, increase code
9314 size, but may improve performance of code that depends on fast memcpy, strlen
9315 and memset for short lengths.
9317 @item -momit-leaf-frame-pointer
9318 @opindex momit-leaf-frame-pointer
9319 Don't keep the frame pointer in a register for leaf functions. This
9320 avoids the instructions to save, set up and restore frame pointers and
9321 makes an extra register available in leaf functions. The option
9322 @option{-fomit-frame-pointer} removes the frame pointer for all functions
9323 which might make debugging harder.
9325 @item -mtls-direct-seg-refs
9326 @itemx -mno-tls-direct-seg-refs
9327 @opindex mtls-direct-seg-refs
9328 Controls whether TLS variables may be accessed with offsets from the
9329 TLS segment register (@code{%gs} for 32-bit, @code{%fs} for 64-bit),
9330 or whether the thread base pointer must be added. Whether or not this
9331 is legal depends on the operating system, and whether it maps the
9332 segment to cover the entire TLS area.
9334 For systems that use GNU libc, the default is on.
9337 These @samp{-m} switches are supported in addition to the above
9338 on AMD x86-64 processors in 64-bit environments.
9345 Generate code for a 32-bit or 64-bit environment.
9346 The 32-bit environment sets int, long and pointer to 32 bits and
9347 generates code that runs on any i386 system.
9348 The 64-bit environment sets int to 32 bits and long and pointer
9349 to 64 bits and generates code for AMD's x86-64 architecture.
9352 @opindex no-red-zone
9353 Do not use a so called red zone for x86-64 code. The red zone is mandated
9354 by the x86-64 ABI, it is a 128-byte area beyond the location of the
9355 stack pointer that will not be modified by signal or interrupt handlers
9356 and therefore can be used for temporary data without adjusting the stack
9357 pointer. The flag @option{-mno-red-zone} disables this red zone.
9359 @item -mcmodel=small
9360 @opindex mcmodel=small
9361 Generate code for the small code model: the program and its symbols must
9362 be linked in the lower 2 GB of the address space. Pointers are 64 bits.
9363 Programs can be statically or dynamically linked. This is the default
9366 @item -mcmodel=kernel
9367 @opindex mcmodel=kernel
9368 Generate code for the kernel code model. The kernel runs in the
9369 negative 2 GB of the address space.
9370 This model has to be used for Linux kernel code.
9372 @item -mcmodel=medium
9373 @opindex mcmodel=medium
9374 Generate code for the medium model: The program is linked in the lower 2
9375 GB of the address space but symbols can be located anywhere in the
9376 address space. Programs can be statically or dynamically linked, but
9377 building of shared libraries are not supported with the medium model.
9379 @item -mcmodel=large
9380 @opindex mcmodel=large
9381 Generate code for the large model: This model makes no assumptions
9382 about addresses and sizes of sections. Currently GCC does not implement
9387 @subsection IA-64 Options
9388 @cindex IA-64 Options
9390 These are the @samp{-m} options defined for the Intel IA-64 architecture.
9394 @opindex mbig-endian
9395 Generate code for a big endian target. This is the default for HP-UX@.
9397 @item -mlittle-endian
9398 @opindex mlittle-endian
9399 Generate code for a little endian target. This is the default for AIX5
9406 Generate (or don't) code for the GNU assembler. This is the default.
9407 @c Also, this is the default if the configure option @option{--with-gnu-as}
9414 Generate (or don't) code for the GNU linker. This is the default.
9415 @c Also, this is the default if the configure option @option{--with-gnu-ld}
9420 Generate code that does not use a global pointer register. The result
9421 is not position independent code, and violates the IA-64 ABI@.
9423 @item -mvolatile-asm-stop
9424 @itemx -mno-volatile-asm-stop
9425 @opindex mvolatile-asm-stop
9426 @opindex mno-volatile-asm-stop
9427 Generate (or don't) a stop bit immediately before and after volatile asm
9430 @item -mregister-names
9431 @itemx -mno-register-names
9432 @opindex mregister-names
9433 @opindex mno-register-names
9434 Generate (or don't) @samp{in}, @samp{loc}, and @samp{out} register names for
9435 the stacked registers. This may make assembler output more readable.
9441 Disable (or enable) optimizations that use the small data section. This may
9442 be useful for working around optimizer bugs.
9445 @opindex mconstant-gp
9446 Generate code that uses a single constant global pointer value. This is
9447 useful when compiling kernel code.
9451 Generate code that is self-relocatable. This implies @option{-mconstant-gp}.
9452 This is useful when compiling firmware code.
9454 @item -minline-float-divide-min-latency
9455 @opindex minline-float-divide-min-latency
9456 Generate code for inline divides of floating point values
9457 using the minimum latency algorithm.
9459 @item -minline-float-divide-max-throughput
9460 @opindex minline-float-divide-max-throughput
9461 Generate code for inline divides of floating point values
9462 using the maximum throughput algorithm.
9464 @item -minline-int-divide-min-latency
9465 @opindex minline-int-divide-min-latency
9466 Generate code for inline divides of integer values
9467 using the minimum latency algorithm.
9469 @item -minline-int-divide-max-throughput
9470 @opindex minline-int-divide-max-throughput
9471 Generate code for inline divides of integer values
9472 using the maximum throughput algorithm.
9474 @item -minline-sqrt-min-latency
9475 @opindex minline-sqrt-min-latency
9476 Generate code for inline square roots
9477 using the minimum latency algorithm.
9479 @item -minline-sqrt-max-throughput
9480 @opindex minline-sqrt-max-throughput
9481 Generate code for inline square roots
9482 using the maximum throughput algorithm.
9484 @item -mno-dwarf2-asm
9486 @opindex mno-dwarf2-asm
9487 @opindex mdwarf2-asm
9488 Don't (or do) generate assembler code for the DWARF2 line number debugging
9489 info. This may be useful when not using the GNU assembler.
9491 @item -mearly-stop-bits
9492 @itemx -mno-early-stop-bits
9493 @opindex mearly-stop-bits
9494 @opindex mno-early-stop-bits
9495 Allow stop bits to be placed earlier than immediately preceding the
9496 instruction that triggered the stop bit. This can improve instruction
9497 scheduling, but does not always do so.
9499 @item -mfixed-range=@var{register-range}
9500 @opindex mfixed-range
9501 Generate code treating the given register range as fixed registers.
9502 A fixed register is one that the register allocator can not use. This is
9503 useful when compiling kernel code. A register range is specified as
9504 two registers separated by a dash. Multiple register ranges can be
9505 specified separated by a comma.
9507 @item -mtls-size=@var{tls-size}
9509 Specify bit size of immediate TLS offsets. Valid values are 14, 22, and
9512 @item -mtune-arch=@var{cpu-type}
9514 Tune the instruction scheduling for a particular CPU, Valid values are
9515 itanium, itanium1, merced, itanium2, and mckinley.
9521 Add support for multithreading using the POSIX threads library. This
9522 option sets flags for both the preprocessor and linker. It does
9523 not affect the thread safety of object code produced by the compiler or
9524 that of libraries supplied with it. These are HP-UX specific flags.
9530 Generate code for a 32-bit or 64-bit environment.
9531 The 32-bit environment sets int, long and pointer to 32 bits.
9532 The 64-bit environment sets int to 32 bits and long and pointer
9533 to 64 bits. These are HP-UX specific flags.
9538 @subsection M32C Options
9539 @cindex M32C options
9542 @item -mcpu=@var{name}
9544 Select the CPU for which code is generated. @var{name} may be one of
9545 @samp{r8c} for the R8C/Tiny series, @samp{m16c} for the M16C (up to
9546 /60) series, @samp{m32cm} for the M16C/80 series, or @samp{m32c} for
9551 Specifies that the program will be run on the simulator. This causes
9552 an alternate runtime library to be linked in which supports, for
9553 example, file I/O. You must not use this option when generating
9554 programs that will run on real hardware; you must provide your own
9555 runtime library for whatever I/O functions are needed.
9557 @item -memregs=@var{number}
9559 Specifies the number of memory-based pseudo-registers GCC will use
9560 during code generation. These pseudo-registers will be used like real
9561 registers, so there is a tradeoff between GCC's ability to fit the
9562 code into available registers, and the performance penalty of using
9563 memory instead of registers. Note that all modules in a program must
9564 be compiled with the same value for this option. Because of that, you
9565 must not use this option with the default runtime libraries gcc
9570 @node M32R/D Options
9571 @subsection M32R/D Options
9572 @cindex M32R/D options
9574 These @option{-m} options are defined for Renesas M32R/D architectures:
9579 Generate code for the M32R/2@.
9583 Generate code for the M32R/X@.
9587 Generate code for the M32R@. This is the default.
9590 @opindex mmodel=small
9591 Assume all objects live in the lower 16MB of memory (so that their addresses
9592 can be loaded with the @code{ld24} instruction), and assume all subroutines
9593 are reachable with the @code{bl} instruction.
9594 This is the default.
9596 The addressability of a particular object can be set with the
9597 @code{model} attribute.
9599 @item -mmodel=medium
9600 @opindex mmodel=medium
9601 Assume objects may be anywhere in the 32-bit address space (the compiler
9602 will generate @code{seth/add3} instructions to load their addresses), and
9603 assume all subroutines are reachable with the @code{bl} instruction.
9606 @opindex mmodel=large
9607 Assume objects may be anywhere in the 32-bit address space (the compiler
9608 will generate @code{seth/add3} instructions to load their addresses), and
9609 assume subroutines may not be reachable with the @code{bl} instruction
9610 (the compiler will generate the much slower @code{seth/add3/jl}
9611 instruction sequence).
9614 @opindex msdata=none
9615 Disable use of the small data area. Variables will be put into
9616 one of @samp{.data}, @samp{bss}, or @samp{.rodata} (unless the
9617 @code{section} attribute has been specified).
9618 This is the default.
9620 The small data area consists of sections @samp{.sdata} and @samp{.sbss}.
9621 Objects may be explicitly put in the small data area with the
9622 @code{section} attribute using one of these sections.
9625 @opindex msdata=sdata
9626 Put small global and static data in the small data area, but do not
9627 generate special code to reference them.
9631 Put small global and static data in the small data area, and generate
9632 special instructions to reference them.
9636 @cindex smaller data references
9637 Put global and static objects less than or equal to @var{num} bytes
9638 into the small data or bss sections instead of the normal data or bss
9639 sections. The default value of @var{num} is 8.
9640 The @option{-msdata} option must be set to one of @samp{sdata} or @samp{use}
9641 for this option to have any effect.
9643 All modules should be compiled with the same @option{-G @var{num}} value.
9644 Compiling with different values of @var{num} may or may not work; if it
9645 doesn't the linker will give an error message---incorrect code will not be
9650 Makes the M32R specific code in the compiler display some statistics
9651 that might help in debugging programs.
9654 @opindex malign-loops
9655 Align all loops to a 32-byte boundary.
9657 @item -mno-align-loops
9658 @opindex mno-align-loops
9659 Do not enforce a 32-byte alignment for loops. This is the default.
9661 @item -missue-rate=@var{number}
9662 @opindex missue-rate=@var{number}
9663 Issue @var{number} instructions per cycle. @var{number} can only be 1
9666 @item -mbranch-cost=@var{number}
9667 @opindex mbranch-cost=@var{number}
9668 @var{number} can only be 1 or 2. If it is 1 then branches will be
9669 preferred over conditional code, if it is 2, then the opposite will
9672 @item -mflush-trap=@var{number}
9673 @opindex mflush-trap=@var{number}
9674 Specifies the trap number to use to flush the cache. The default is
9675 12. Valid numbers are between 0 and 15 inclusive.
9677 @item -mno-flush-trap
9678 @opindex mno-flush-trap
9679 Specifies that the cache cannot be flushed by using a trap.
9681 @item -mflush-func=@var{name}
9682 @opindex mflush-func=@var{name}
9683 Specifies the name of the operating system function to call to flush
9684 the cache. The default is @emph{_flush_cache}, but a function call
9685 will only be used if a trap is not available.
9687 @item -mno-flush-func
9688 @opindex mno-flush-func
9689 Indicates that there is no OS function for flushing the cache.
9693 @node M680x0 Options
9694 @subsection M680x0 Options
9695 @cindex M680x0 options
9697 These are the @samp{-m} options defined for the 68000 series. The default
9698 values for these options depends on which style of 68000 was selected when
9699 the compiler was configured; the defaults for the most common choices are
9707 Generate output for a 68000. This is the default
9708 when the compiler is configured for 68000-based systems.
9710 Use this option for microcontrollers with a 68000 or EC000 core,
9711 including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.
9717 Generate output for a 68020. This is the default
9718 when the compiler is configured for 68020-based systems.
9722 Generate output containing 68881 instructions for floating point.
9723 This is the default for most 68020 systems unless @option{--nfp} was
9724 specified when the compiler was configured.
9728 Generate output for a 68030. This is the default when the compiler is
9729 configured for 68030-based systems.
9733 Generate output for a 68040. This is the default when the compiler is
9734 configured for 68040-based systems.
9736 This option inhibits the use of 68881/68882 instructions that have to be
9737 emulated by software on the 68040. Use this option if your 68040 does not
9738 have code to emulate those instructions.
9742 Generate output for a 68060. This is the default when the compiler is
9743 configured for 68060-based systems.
9745 This option inhibits the use of 68020 and 68881/68882 instructions that
9746 have to be emulated by software on the 68060. Use this option if your 68060
9747 does not have code to emulate those instructions.
9751 Generate output for a CPU32. This is the default
9752 when the compiler is configured for CPU32-based systems.
9754 Use this option for microcontrollers with a
9755 CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334,
9756 68336, 68340, 68341, 68349 and 68360.
9760 Generate output for a 520X ``coldfire'' family cpu. This is the default
9761 when the compiler is configured for 520X-based systems.
9763 Use this option for microcontroller with a 5200 core, including
9764 the MCF5202, MCF5203, MCF5204 and MCF5202.
9769 Generate output for a 68040, without using any of the new instructions.
9770 This results in code which can run relatively efficiently on either a
9771 68020/68881 or a 68030 or a 68040. The generated code does use the
9772 68881 instructions that are emulated on the 68040.
9776 Generate output for a 68060, without using any of the new instructions.
9777 This results in code which can run relatively efficiently on either a
9778 68020/68881 or a 68030 or a 68040. The generated code does use the
9779 68881 instructions that are emulated on the 68060.
9782 @opindex msoft-float
9783 Generate output containing library calls for floating point.
9784 @strong{Warning:} the requisite libraries are not available for all m68k
9785 targets. Normally the facilities of the machine's usual C compiler are
9786 used, but this can't be done directly in cross-compilation. You must
9787 make your own arrangements to provide suitable library functions for
9788 cross-compilation. The embedded targets @samp{m68k-*-aout} and
9789 @samp{m68k-*-coff} do provide software floating point support.
9793 Consider type @code{int} to be 16 bits wide, like @code{short int}.
9794 Additionally, parameters passed on the stack are also aligned to a
9795 16-bit boundary even on targets whose API mandates promotion to 32-bit.
9798 @opindex mnobitfield
9799 Do not use the bit-field instructions. The @option{-m68000}, @option{-mcpu32}
9800 and @option{-m5200} options imply @w{@option{-mnobitfield}}.
9804 Do use the bit-field instructions. The @option{-m68020} option implies
9805 @option{-mbitfield}. This is the default if you use a configuration
9806 designed for a 68020.
9810 Use a different function-calling convention, in which functions
9811 that take a fixed number of arguments return with the @code{rtd}
9812 instruction, which pops their arguments while returning. This
9813 saves one instruction in the caller since there is no need to pop
9814 the arguments there.
9816 This calling convention is incompatible with the one normally
9817 used on Unix, so you cannot use it if you need to call libraries
9818 compiled with the Unix compiler.
9820 Also, you must provide function prototypes for all functions that
9821 take variable numbers of arguments (including @code{printf});
9822 otherwise incorrect code will be generated for calls to those
9825 In addition, seriously incorrect code will result if you call a
9826 function with too many arguments. (Normally, extra arguments are
9827 harmlessly ignored.)
9829 The @code{rtd} instruction is supported by the 68010, 68020, 68030,
9830 68040, 68060 and CPU32 processors, but not by the 68000 or 5200.
9833 @itemx -mno-align-int
9835 @opindex mno-align-int
9836 Control whether GCC aligns @code{int}, @code{long}, @code{long long},
9837 @code{float}, @code{double}, and @code{long double} variables on a 32-bit
9838 boundary (@option{-malign-int}) or a 16-bit boundary (@option{-mno-align-int}).
9839 Aligning variables on 32-bit boundaries produces code that runs somewhat
9840 faster on processors with 32-bit busses at the expense of more memory.
9842 @strong{Warning:} if you use the @option{-malign-int} switch, GCC will
9843 align structures containing the above types differently than
9844 most published application binary interface specifications for the m68k.
9848 Use the pc-relative addressing mode of the 68000 directly, instead of
9849 using a global offset table. At present, this option implies @option{-fpic},
9850 allowing at most a 16-bit offset for pc-relative addressing. @option{-fPIC} is
9851 not presently supported with @option{-mpcrel}, though this could be supported for
9852 68020 and higher processors.
9854 @item -mno-strict-align
9855 @itemx -mstrict-align
9856 @opindex mno-strict-align
9857 @opindex mstrict-align
9858 Do not (do) assume that unaligned memory references will be handled by
9862 Generate code that allows the data segment to be located in a different
9863 area of memory from the text segment. This allows for execute in place in
9864 an environment without virtual memory management. This option implies
9868 Generate code that assumes that the data segment follows the text segment.
9869 This is the default.
9871 @item -mid-shared-library
9872 Generate code that supports shared libraries via the library ID method.
9873 This allows for execute in place and shared libraries in an environment
9874 without virtual memory management. This option implies @option{-fPIC}.
9876 @item -mno-id-shared-library
9877 Generate code that doesn't assume ID based shared libraries are being used.
9878 This is the default.
9880 @item -mshared-library-id=n
9881 Specified the identification number of the ID based shared library being
9882 compiled. Specifying a value of 0 will generate more compact code, specifying
9883 other values will force the allocation of that number to the current
9884 library but is no more space or time efficient than omitting this option.
9888 @node M68hc1x Options
9889 @subsection M68hc1x Options
9890 @cindex M68hc1x options
9892 These are the @samp{-m} options defined for the 68hc11 and 68hc12
9893 microcontrollers. The default values for these options depends on
9894 which style of microcontroller was selected when the compiler was configured;
9895 the defaults for the most common choices are given below.
9902 Generate output for a 68HC11. This is the default
9903 when the compiler is configured for 68HC11-based systems.
9909 Generate output for a 68HC12. This is the default
9910 when the compiler is configured for 68HC12-based systems.
9916 Generate output for a 68HCS12.
9919 @opindex mauto-incdec
9920 Enable the use of 68HC12 pre and post auto-increment and auto-decrement
9927 Enable the use of 68HC12 min and max instructions.
9930 @itemx -mno-long-calls
9931 @opindex mlong-calls
9932 @opindex mno-long-calls
9933 Treat all calls as being far away (near). If calls are assumed to be
9934 far away, the compiler will use the @code{call} instruction to
9935 call a function and the @code{rtc} instruction for returning.
9939 Consider type @code{int} to be 16 bits wide, like @code{short int}.
9941 @item -msoft-reg-count=@var{count}
9942 @opindex msoft-reg-count
9943 Specify the number of pseudo-soft registers which are used for the
9944 code generation. The maximum number is 32. Using more pseudo-soft
9945 register may or may not result in better code depending on the program.
9946 The default is 4 for 68HC11 and 2 for 68HC12.
9951 @subsection MCore Options
9952 @cindex MCore options
9954 These are the @samp{-m} options defined for the Motorola M*Core
9962 @opindex mno-hardlit
9963 Inline constants into the code stream if it can be done in two
9964 instructions or less.
9970 Use the divide instruction. (Enabled by default).
9972 @item -mrelax-immediate
9973 @itemx -mno-relax-immediate
9974 @opindex mrelax-immediate
9975 @opindex mno-relax-immediate
9976 Allow arbitrary sized immediates in bit operations.
9978 @item -mwide-bitfields
9979 @itemx -mno-wide-bitfields
9980 @opindex mwide-bitfields
9981 @opindex mno-wide-bitfields
9982 Always treat bit-fields as int-sized.
9984 @item -m4byte-functions
9985 @itemx -mno-4byte-functions
9986 @opindex m4byte-functions
9987 @opindex mno-4byte-functions
9988 Force all functions to be aligned to a four byte boundary.
9990 @item -mcallgraph-data
9991 @itemx -mno-callgraph-data
9992 @opindex mcallgraph-data
9993 @opindex mno-callgraph-data
9994 Emit callgraph information.
9997 @itemx -mno-slow-bytes
9998 @opindex mslow-bytes
9999 @opindex mno-slow-bytes
10000 Prefer word access when reading byte quantities.
10002 @item -mlittle-endian
10003 @itemx -mbig-endian
10004 @opindex mlittle-endian
10005 @opindex mbig-endian
10006 Generate code for a little endian target.
10012 Generate code for the 210 processor.
10016 @subsection MIPS Options
10017 @cindex MIPS options
10023 Generate big-endian code.
10027 Generate little-endian code. This is the default for @samp{mips*el-*-*}
10030 @item -march=@var{arch}
10032 Generate code that will run on @var{arch}, which can be the name of a
10033 generic MIPS ISA, or the name of a particular processor.
10035 @samp{mips1}, @samp{mips2}, @samp{mips3}, @samp{mips4},
10036 @samp{mips32}, @samp{mips32r2}, and @samp{mips64}.
10037 The processor names are:
10038 @samp{4kc}, @samp{4km}, @samp{4kp},
10039 @samp{5kc}, @samp{5kf},
10041 @samp{24k}, @samp{24kc}, @samp{24kf}, @samp{24kx},
10044 @samp{r2000}, @samp{r3000}, @samp{r3900}, @samp{r4000}, @samp{r4400},
10045 @samp{r4600}, @samp{r4650}, @samp{r6000}, @samp{r8000},
10046 @samp{rm7000}, @samp{rm9000},
10049 @samp{vr4100}, @samp{vr4111}, @samp{vr4120}, @samp{vr4130}, @samp{vr4300},
10050 @samp{vr5000}, @samp{vr5400} and @samp{vr5500}.
10051 The special value @samp{from-abi} selects the
10052 most compatible architecture for the selected ABI (that is,
10053 @samp{mips1} for 32-bit ABIs and @samp{mips3} for 64-bit ABIs)@.
10055 In processor names, a final @samp{000} can be abbreviated as @samp{k}
10056 (for example, @samp{-march=r2k}). Prefixes are optional, and
10057 @samp{vr} may be written @samp{r}.
10059 GCC defines two macros based on the value of this option. The first
10060 is @samp{_MIPS_ARCH}, which gives the name of target architecture, as
10061 a string. The second has the form @samp{_MIPS_ARCH_@var{foo}},
10062 where @var{foo} is the capitalized value of @samp{_MIPS_ARCH}@.
10063 For example, @samp{-march=r2000} will set @samp{_MIPS_ARCH}
10064 to @samp{"r2000"} and define the macro @samp{_MIPS_ARCH_R2000}.
10066 Note that the @samp{_MIPS_ARCH} macro uses the processor names given
10067 above. In other words, it will have the full prefix and will not
10068 abbreviate @samp{000} as @samp{k}. In the case of @samp{from-abi},
10069 the macro names the resolved architecture (either @samp{"mips1"} or
10070 @samp{"mips3"}). It names the default architecture when no
10071 @option{-march} option is given.
10073 @item -mtune=@var{arch}
10075 Optimize for @var{arch}. Among other things, this option controls
10076 the way instructions are scheduled, and the perceived cost of arithmetic
10077 operations. The list of @var{arch} values is the same as for
10080 When this option is not used, GCC will optimize for the processor
10081 specified by @option{-march}. By using @option{-march} and
10082 @option{-mtune} together, it is possible to generate code that will
10083 run on a family of processors, but optimize the code for one
10084 particular member of that family.
10086 @samp{-mtune} defines the macros @samp{_MIPS_TUNE} and
10087 @samp{_MIPS_TUNE_@var{foo}}, which work in the same way as the
10088 @samp{-march} ones described above.
10092 Equivalent to @samp{-march=mips1}.
10096 Equivalent to @samp{-march=mips2}.
10100 Equivalent to @samp{-march=mips3}.
10104 Equivalent to @samp{-march=mips4}.
10108 Equivalent to @samp{-march=mips32}.
10112 Equivalent to @samp{-march=mips32r2}.
10116 Equivalent to @samp{-march=mips64}.
10121 @opindex mno-mips16
10122 Generate (do not generate) MIPS16 code. If GCC is targetting a
10123 MIPS32 or MIPS64 architecture, it will make use of the MIPS16e ASE@.
10135 Generate code for the given ABI@.
10137 Note that the EABI has a 32-bit and a 64-bit variant. GCC normally
10138 generates 64-bit code when you select a 64-bit architecture, but you
10139 can use @option{-mgp32} to get 32-bit code instead.
10141 For information about the O64 ABI, see
10142 @w{@uref{http://gcc.gnu.org/projects/mipso64-abi.html}}.
10145 @itemx -mno-abicalls
10147 @opindex mno-abicalls
10148 Generate (do not generate) SVR4-style position-independent code.
10149 @option{-mabicalls} is the default for SVR4-based systems.
10155 Lift (do not lift) the usual restrictions on the size of the global
10158 GCC normally uses a single instruction to load values from the GOT@.
10159 While this is relatively efficient, it will only work if the GOT
10160 is smaller than about 64k. Anything larger will cause the linker
10161 to report an error such as:
10163 @cindex relocation truncated to fit (MIPS)
10165 relocation truncated to fit: R_MIPS_GOT16 foobar
10168 If this happens, you should recompile your code with @option{-mxgot}.
10169 It should then work with very large GOTs, although it will also be
10170 less efficient, since it will take three instructions to fetch the
10171 value of a global symbol.
10173 Note that some linkers can create multiple GOTs. If you have such a
10174 linker, you should only need to use @option{-mxgot} when a single object
10175 file accesses more than 64k's worth of GOT entries. Very few do.
10177 These options have no effect unless GCC is generating position
10182 Assume that general-purpose registers are 32 bits wide.
10186 Assume that general-purpose registers are 64 bits wide.
10190 Assume that floating-point registers are 32 bits wide.
10194 Assume that floating-point registers are 64 bits wide.
10197 @opindex mhard-float
10198 Use floating-point coprocessor instructions.
10201 @opindex msoft-float
10202 Do not use floating-point coprocessor instructions. Implement
10203 floating-point calculations using library calls instead.
10205 @item -msingle-float
10206 @opindex msingle-float
10207 Assume that the floating-point coprocessor only supports single-precision
10210 @itemx -mdouble-float
10211 @opindex mdouble-float
10212 Assume that the floating-point coprocessor supports double-precision
10213 operations. This is the default.
10219 Use (do not use) the MIPS DSP ASE. @xref{MIPS DSP Built-in Functions}.
10221 @itemx -mpaired-single
10222 @itemx -mno-paired-single
10223 @opindex mpaired-single
10224 @opindex mno-paired-single
10225 Use (do not use) paired-single floating-point instructions.
10226 @xref{MIPS Paired-Single Support}. This option can only be used
10227 when generating 64-bit code and requires hardware floating-point
10228 support to be enabled.
10233 @opindex mno-mips3d
10234 Use (do not use) the MIPS-3D ASE@. @xref{MIPS-3D Built-in Functions}.
10235 The option @option{-mips3d} implies @option{-mpaired-single}.
10239 Force @code{long} types to be 64 bits wide. See @option{-mlong32} for
10240 an explanation of the default and the way that the pointer size is
10245 Force @code{long}, @code{int}, and pointer types to be 32 bits wide.
10247 The default size of @code{int}s, @code{long}s and pointers depends on
10248 the ABI@. All the supported ABIs use 32-bit @code{int}s. The n64 ABI
10249 uses 64-bit @code{long}s, as does the 64-bit EABI; the others use
10250 32-bit @code{long}s. Pointers are the same size as @code{long}s,
10251 or the same size as integer registers, whichever is smaller.
10257 Assume (do not assume) that all symbols have 32-bit values, regardless
10258 of the selected ABI@. This option is useful in combination with
10259 @option{-mabi=64} and @option{-mno-abicalls} because it allows GCC
10260 to generate shorter and faster references to symbolic addresses.
10264 @cindex smaller data references (MIPS)
10265 @cindex gp-relative references (MIPS)
10266 Put global and static items less than or equal to @var{num} bytes into
10267 the small data or bss section instead of the normal data or bss section.
10268 This allows the data to be accessed using a single instruction.
10270 All modules should be compiled with the same @option{-G @var{num}}
10273 @item -membedded-data
10274 @itemx -mno-embedded-data
10275 @opindex membedded-data
10276 @opindex mno-embedded-data
10277 Allocate variables to the read-only data section first if possible, then
10278 next in the small data section if possible, otherwise in data. This gives
10279 slightly slower code than the default, but reduces the amount of RAM required
10280 when executing, and thus may be preferred for some embedded systems.
10282 @item -muninit-const-in-rodata
10283 @itemx -mno-uninit-const-in-rodata
10284 @opindex muninit-const-in-rodata
10285 @opindex mno-uninit-const-in-rodata
10286 Put uninitialized @code{const} variables in the read-only data section.
10287 This option is only meaningful in conjunction with @option{-membedded-data}.
10289 @item -msplit-addresses
10290 @itemx -mno-split-addresses
10291 @opindex msplit-addresses
10292 @opindex mno-split-addresses
10293 Enable (disable) use of the @code{%hi()} and @code{%lo()} assembler
10294 relocation operators. This option has been superseded by
10295 @option{-mexplicit-relocs} but is retained for backwards compatibility.
10297 @item -mexplicit-relocs
10298 @itemx -mno-explicit-relocs
10299 @opindex mexplicit-relocs
10300 @opindex mno-explicit-relocs
10301 Use (do not use) assembler relocation operators when dealing with symbolic
10302 addresses. The alternative, selected by @option{-mno-explicit-relocs},
10303 is to use assembler macros instead.
10305 @option{-mexplicit-relocs} is the default if GCC was configured
10306 to use an assembler that supports relocation operators.
10308 @item -mcheck-zero-division
10309 @itemx -mno-check-zero-division
10310 @opindex mcheck-zero-division
10311 @opindex mno-check-zero-division
10312 Trap (do not trap) on integer division by zero. The default is
10313 @option{-mcheck-zero-division}.
10315 @item -mdivide-traps
10316 @itemx -mdivide-breaks
10317 @opindex mdivide-traps
10318 @opindex mdivide-breaks
10319 MIPS systems check for division by zero by generating either a
10320 conditional trap or a break instruction. Using traps results in
10321 smaller code, but is only supported on MIPS II and later. Also, some
10322 versions of the Linux kernel have a bug that prevents trap from
10323 generating the proper signal (@code{SIGFPE}). Use @option{-mdivide-traps} to
10324 allow conditional traps on architectures that support them and
10325 @option{-mdivide-breaks} to force the use of breaks.
10327 The default is usually @option{-mdivide-traps}, but this can be
10328 overridden at configure time using @option{--with-divide=breaks}.
10329 Divide-by-zero checks can be completely disabled using
10330 @option{-mno-check-zero-division}.
10335 @opindex mno-memcpy
10336 Force (do not force) the use of @code{memcpy()} for non-trivial block
10337 moves. The default is @option{-mno-memcpy}, which allows GCC to inline
10338 most constant-sized copies.
10341 @itemx -mno-long-calls
10342 @opindex mlong-calls
10343 @opindex mno-long-calls
10344 Disable (do not disable) use of the @code{jal} instruction. Calling
10345 functions using @code{jal} is more efficient but requires the caller
10346 and callee to be in the same 256 megabyte segment.
10348 This option has no effect on abicalls code. The default is
10349 @option{-mno-long-calls}.
10355 Enable (disable) use of the @code{mad}, @code{madu} and @code{mul}
10356 instructions, as provided by the R4650 ISA@.
10359 @itemx -mno-fused-madd
10360 @opindex mfused-madd
10361 @opindex mno-fused-madd
10362 Enable (disable) use of the floating point multiply-accumulate
10363 instructions, when they are available. The default is
10364 @option{-mfused-madd}.
10366 When multiply-accumulate instructions are used, the intermediate
10367 product is calculated to infinite precision and is not subject to
10368 the FCSR Flush to Zero bit. This may be undesirable in some
10373 Tell the MIPS assembler to not run its preprocessor over user
10374 assembler files (with a @samp{.s} suffix) when assembling them.
10377 @itemx -mno-fix-r4000
10378 @opindex mfix-r4000
10379 @opindex mno-fix-r4000
10380 Work around certain R4000 CPU errata:
10383 A double-word or a variable shift may give an incorrect result if executed
10384 immediately after starting an integer division.
10386 A double-word or a variable shift may give an incorrect result if executed
10387 while an integer multiplication is in progress.
10389 An integer division may give an incorrect result if started in a delay slot
10390 of a taken branch or a jump.
10394 @itemx -mno-fix-r4400
10395 @opindex mfix-r4400
10396 @opindex mno-fix-r4400
10397 Work around certain R4400 CPU errata:
10400 A double-word or a variable shift may give an incorrect result if executed
10401 immediately after starting an integer division.
10405 @itemx -mno-fix-vr4120
10406 @opindex mfix-vr4120
10407 Work around certain VR4120 errata:
10410 @code{dmultu} does not always produce the correct result.
10412 @code{div} and @code{ddiv} do not always produce the correct result if one
10413 of the operands is negative.
10415 The workarounds for the division errata rely on special functions in
10416 @file{libgcc.a}. At present, these functions are only provided by
10417 the @code{mips64vr*-elf} configurations.
10419 Other VR4120 errata require a nop to be inserted between certain pairs of
10420 instructions. These errata are handled by the assembler, not by GCC itself.
10423 @opindex mfix-vr4130
10424 Work around the VR4130 @code{mflo}/@code{mfhi} errata. The
10425 workarounds are implemented by the assembler rather than by GCC,
10426 although GCC will avoid using @code{mflo} and @code{mfhi} if the
10427 VR4130 @code{macc}, @code{macchi}, @code{dmacc} and @code{dmacchi}
10428 instructions are available instead.
10431 @itemx -mno-fix-sb1
10433 Work around certain SB-1 CPU core errata.
10434 (This flag currently works around the SB-1 revision 2
10435 ``F1'' and ``F2'' floating point errata.)
10437 @item -mflush-func=@var{func}
10438 @itemx -mno-flush-func
10439 @opindex mflush-func
10440 Specifies the function to call to flush the I and D caches, or to not
10441 call any such function. If called, the function must take the same
10442 arguments as the common @code{_flush_func()}, that is, the address of the
10443 memory range for which the cache is being flushed, the size of the
10444 memory range, and the number 3 (to flush both caches). The default
10445 depends on the target GCC was configured for, but commonly is either
10446 @samp{_flush_func} or @samp{__cpu_flush}.
10448 @item -mbranch-likely
10449 @itemx -mno-branch-likely
10450 @opindex mbranch-likely
10451 @opindex mno-branch-likely
10452 Enable or disable use of Branch Likely instructions, regardless of the
10453 default for the selected architecture. By default, Branch Likely
10454 instructions may be generated if they are supported by the selected
10455 architecture. An exception is for the MIPS32 and MIPS64 architectures
10456 and processors which implement those architectures; for those, Branch
10457 Likely instructions will not be generated by default because the MIPS32
10458 and MIPS64 architectures specifically deprecate their use.
10460 @item -mfp-exceptions
10461 @itemx -mno-fp-exceptions
10462 @opindex mfp-exceptions
10463 Specifies whether FP exceptions are enabled. This affects how we schedule
10464 FP instructions for some processors. The default is that FP exceptions are
10467 For instance, on the SB-1, if FP exceptions are disabled, and we are emitting
10468 64-bit code, then we can use both FP pipes. Otherwise, we can only use one
10471 @item -mvr4130-align
10472 @itemx -mno-vr4130-align
10473 @opindex mvr4130-align
10474 The VR4130 pipeline is two-way superscalar, but can only issue two
10475 instructions together if the first one is 8-byte aligned. When this
10476 option is enabled, GCC will align pairs of instructions that it
10477 thinks should execute in parallel.
10479 This option only has an effect when optimizing for the VR4130.
10480 It normally makes code faster, but at the expense of making it bigger.
10481 It is enabled by default at optimization level @option{-O3}.
10485 @subsection MMIX Options
10486 @cindex MMIX Options
10488 These options are defined for the MMIX:
10492 @itemx -mno-libfuncs
10494 @opindex mno-libfuncs
10495 Specify that intrinsic library functions are being compiled, passing all
10496 values in registers, no matter the size.
10499 @itemx -mno-epsilon
10501 @opindex mno-epsilon
10502 Generate floating-point comparison instructions that compare with respect
10503 to the @code{rE} epsilon register.
10505 @item -mabi=mmixware
10507 @opindex mabi-mmixware
10509 Generate code that passes function parameters and return values that (in
10510 the called function) are seen as registers @code{$0} and up, as opposed to
10511 the GNU ABI which uses global registers @code{$231} and up.
10513 @item -mzero-extend
10514 @itemx -mno-zero-extend
10515 @opindex mzero-extend
10516 @opindex mno-zero-extend
10517 When reading data from memory in sizes shorter than 64 bits, use (do not
10518 use) zero-extending load instructions by default, rather than
10519 sign-extending ones.
10522 @itemx -mno-knuthdiv
10524 @opindex mno-knuthdiv
10525 Make the result of a division yielding a remainder have the same sign as
10526 the divisor. With the default, @option{-mno-knuthdiv}, the sign of the
10527 remainder follows the sign of the dividend. Both methods are
10528 arithmetically valid, the latter being almost exclusively used.
10530 @item -mtoplevel-symbols
10531 @itemx -mno-toplevel-symbols
10532 @opindex mtoplevel-symbols
10533 @opindex mno-toplevel-symbols
10534 Prepend (do not prepend) a @samp{:} to all global symbols, so the assembly
10535 code can be used with the @code{PREFIX} assembly directive.
10539 Generate an executable in the ELF format, rather than the default
10540 @samp{mmo} format used by the @command{mmix} simulator.
10542 @item -mbranch-predict
10543 @itemx -mno-branch-predict
10544 @opindex mbranch-predict
10545 @opindex mno-branch-predict
10546 Use (do not use) the probable-branch instructions, when static branch
10547 prediction indicates a probable branch.
10549 @item -mbase-addresses
10550 @itemx -mno-base-addresses
10551 @opindex mbase-addresses
10552 @opindex mno-base-addresses
10553 Generate (do not generate) code that uses @emph{base addresses}. Using a
10554 base address automatically generates a request (handled by the assembler
10555 and the linker) for a constant to be set up in a global register. The
10556 register is used for one or more base address requests within the range 0
10557 to 255 from the value held in the register. The generally leads to short
10558 and fast code, but the number of different data items that can be
10559 addressed is limited. This means that a program that uses lots of static
10560 data may require @option{-mno-base-addresses}.
10562 @item -msingle-exit
10563 @itemx -mno-single-exit
10564 @opindex msingle-exit
10565 @opindex mno-single-exit
10566 Force (do not force) generated code to have a single exit point in each
10570 @node MN10300 Options
10571 @subsection MN10300 Options
10572 @cindex MN10300 options
10574 These @option{-m} options are defined for Matsushita MN10300 architectures:
10579 Generate code to avoid bugs in the multiply instructions for the MN10300
10580 processors. This is the default.
10582 @item -mno-mult-bug
10583 @opindex mno-mult-bug
10584 Do not generate code to avoid bugs in the multiply instructions for the
10585 MN10300 processors.
10589 Generate code which uses features specific to the AM33 processor.
10593 Do not generate code which uses features specific to the AM33 processor. This
10596 @item -mreturn-pointer-on-d0
10597 @opindex mreturn-pointer-on-d0
10598 When generating a function which returns a pointer, return the pointer
10599 in both @code{a0} and @code{d0}. Otherwise, the pointer is returned
10600 only in a0, and attempts to call such functions without a prototype
10601 would result in errors. Note that this option is on by default; use
10602 @option{-mno-return-pointer-on-d0} to disable it.
10606 Do not link in the C run-time initialization object file.
10610 Indicate to the linker that it should perform a relaxation optimization pass
10611 to shorten branches, calls and absolute memory addresses. This option only
10612 has an effect when used on the command line for the final link step.
10614 This option makes symbolic debugging impossible.
10618 @subsection MS1 Options
10619 @cindex MS1 options
10621 These @option{-m} options are defined for Morpho MS1 architectures:
10625 @item -march=@var{cpu-type}
10627 Generate code that will run on @var{cpu-type}, which is the name of a system
10628 representing a certain processor type. Possible values for
10629 @var{cpu-type} are @samp{MS1-64-001}, @samp{MS1-16-002}, and
10632 When this option is not used, the default is @option{-march=MS1-16-003}.
10636 Generate multiply instructions.
10640 Do not generate multiply instructions.
10644 Use byte loads and stores when generating code.
10648 Do not use byte loads and stores when generating code.
10652 Use simulator runtime
10656 Do not link in the C run-time initialization object file
10657 @file{crti.o}. Other run-time initialization and termination files
10658 such as @file{startup.o} and @file{exit.o} are still included on the
10659 linker command line.
10663 @node PDP-11 Options
10664 @subsection PDP-11 Options
10665 @cindex PDP-11 Options
10667 These options are defined for the PDP-11:
10672 Use hardware FPP floating point. This is the default. (FIS floating
10673 point on the PDP-11/40 is not supported.)
10676 @opindex msoft-float
10677 Do not use hardware floating point.
10681 Return floating-point results in ac0 (fr0 in Unix assembler syntax).
10685 Return floating-point results in memory. This is the default.
10689 Generate code for a PDP-11/40.
10693 Generate code for a PDP-11/45. This is the default.
10697 Generate code for a PDP-11/10.
10699 @item -mbcopy-builtin
10700 @opindex bcopy-builtin
10701 Use inline @code{movmemhi} patterns for copying memory. This is the
10706 Do not use inline @code{movmemhi} patterns for copying memory.
10712 Use 16-bit @code{int}. This is the default.
10718 Use 32-bit @code{int}.
10721 @itemx -mno-float32
10723 @opindex mno-float32
10724 Use 64-bit @code{float}. This is the default.
10727 @itemx -mno-float64
10729 @opindex mno-float64
10730 Use 32-bit @code{float}.
10734 Use @code{abshi2} pattern. This is the default.
10738 Do not use @code{abshi2} pattern.
10740 @item -mbranch-expensive
10741 @opindex mbranch-expensive
10742 Pretend that branches are expensive. This is for experimenting with
10743 code generation only.
10745 @item -mbranch-cheap
10746 @opindex mbranch-cheap
10747 Do not pretend that branches are expensive. This is the default.
10751 Generate code for a system with split I&D@.
10755 Generate code for a system without split I&D@. This is the default.
10759 Use Unix assembler syntax. This is the default when configured for
10760 @samp{pdp11-*-bsd}.
10764 Use DEC assembler syntax. This is the default when configured for any
10765 PDP-11 target other than @samp{pdp11-*-bsd}.
10768 @node PowerPC Options
10769 @subsection PowerPC Options
10770 @cindex PowerPC options
10772 These are listed under @xref{RS/6000 and PowerPC Options}.
10774 @node RS/6000 and PowerPC Options
10775 @subsection IBM RS/6000 and PowerPC Options
10776 @cindex RS/6000 and PowerPC Options
10777 @cindex IBM RS/6000 and PowerPC Options
10779 These @samp{-m} options are defined for the IBM RS/6000 and PowerPC:
10786 @itemx -mno-powerpc
10787 @itemx -mpowerpc-gpopt
10788 @itemx -mno-powerpc-gpopt
10789 @itemx -mpowerpc-gfxopt
10790 @itemx -mno-powerpc-gfxopt
10792 @itemx -mno-powerpc64
10796 @opindex mno-power2
10798 @opindex mno-powerpc
10799 @opindex mpowerpc-gpopt
10800 @opindex mno-powerpc-gpopt
10801 @opindex mpowerpc-gfxopt
10802 @opindex mno-powerpc-gfxopt
10803 @opindex mpowerpc64
10804 @opindex mno-powerpc64
10805 GCC supports two related instruction set architectures for the
10806 RS/6000 and PowerPC@. The @dfn{POWER} instruction set are those
10807 instructions supported by the @samp{rios} chip set used in the original
10808 RS/6000 systems and the @dfn{PowerPC} instruction set is the
10809 architecture of the Motorola MPC5xx, MPC6xx, MPC8xx microprocessors, and
10810 the IBM 4xx microprocessors.
10812 Neither architecture is a subset of the other. However there is a
10813 large common subset of instructions supported by both. An MQ
10814 register is included in processors supporting the POWER architecture.
10816 You use these options to specify which instructions are available on the
10817 processor you are using. The default value of these options is
10818 determined when configuring GCC@. Specifying the
10819 @option{-mcpu=@var{cpu_type}} overrides the specification of these
10820 options. We recommend you use the @option{-mcpu=@var{cpu_type}} option
10821 rather than the options listed above.
10823 The @option{-mpower} option allows GCC to generate instructions that
10824 are found only in the POWER architecture and to use the MQ register.
10825 Specifying @option{-mpower2} implies @option{-power} and also allows GCC
10826 to generate instructions that are present in the POWER2 architecture but
10827 not the original POWER architecture.
10829 The @option{-mpowerpc} option allows GCC to generate instructions that
10830 are found only in the 32-bit subset of the PowerPC architecture.
10831 Specifying @option{-mpowerpc-gpopt} implies @option{-mpowerpc} and also allows
10832 GCC to use the optional PowerPC architecture instructions in the
10833 General Purpose group, including floating-point square root. Specifying
10834 @option{-mpowerpc-gfxopt} implies @option{-mpowerpc} and also allows GCC to
10835 use the optional PowerPC architecture instructions in the Graphics
10836 group, including floating-point select.
10838 The @option{-mpowerpc64} option allows GCC to generate the additional
10839 64-bit instructions that are found in the full PowerPC64 architecture
10840 and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to
10841 @option{-mno-powerpc64}.
10843 If you specify both @option{-mno-power} and @option{-mno-powerpc}, GCC
10844 will use only the instructions in the common subset of both
10845 architectures plus some special AIX common-mode calls, and will not use
10846 the MQ register. Specifying both @option{-mpower} and @option{-mpowerpc}
10847 permits GCC to use any instruction from either architecture and to
10848 allow use of the MQ register; specify this for the Motorola MPC601.
10850 @item -mnew-mnemonics
10851 @itemx -mold-mnemonics
10852 @opindex mnew-mnemonics
10853 @opindex mold-mnemonics
10854 Select which mnemonics to use in the generated assembler code. With
10855 @option{-mnew-mnemonics}, GCC uses the assembler mnemonics defined for
10856 the PowerPC architecture. With @option{-mold-mnemonics} it uses the
10857 assembler mnemonics defined for the POWER architecture. Instructions
10858 defined in only one architecture have only one mnemonic; GCC uses that
10859 mnemonic irrespective of which of these options is specified.
10861 GCC defaults to the mnemonics appropriate for the architecture in
10862 use. Specifying @option{-mcpu=@var{cpu_type}} sometimes overrides the
10863 value of these option. Unless you are building a cross-compiler, you
10864 should normally not specify either @option{-mnew-mnemonics} or
10865 @option{-mold-mnemonics}, but should instead accept the default.
10867 @item -mcpu=@var{cpu_type}
10869 Set architecture type, register usage, choice of mnemonics, and
10870 instruction scheduling parameters for machine type @var{cpu_type}.
10871 Supported values for @var{cpu_type} are @samp{401}, @samp{403},
10872 @samp{405}, @samp{405fp}, @samp{440}, @samp{440fp}, @samp{505},
10873 @samp{601}, @samp{602}, @samp{603}, @samp{603e}, @samp{604},
10874 @samp{604e}, @samp{620}, @samp{630}, @samp{740}, @samp{7400},
10875 @samp{7450}, @samp{750}, @samp{801}, @samp{821}, @samp{823},
10876 @samp{860}, @samp{970}, @samp{8540}, @samp{common}, @samp{ec603e}, @samp{G3},
10877 @samp{G4}, @samp{G5}, @samp{power}, @samp{power2}, @samp{power3},
10878 @samp{power4}, @samp{power5}, @samp{powerpc}, @samp{powerpc64},
10879 @samp{rios}, @samp{rios1}, @samp{rios2}, @samp{rsc}, and @samp{rs64}.
10881 @option{-mcpu=common} selects a completely generic processor. Code
10882 generated under this option will run on any POWER or PowerPC processor.
10883 GCC will use only the instructions in the common subset of both
10884 architectures, and will not use the MQ register. GCC assumes a generic
10885 processor model for scheduling purposes.
10887 @option{-mcpu=power}, @option{-mcpu=power2}, @option{-mcpu=powerpc}, and
10888 @option{-mcpu=powerpc64} specify generic POWER, POWER2, pure 32-bit
10889 PowerPC (i.e., not MPC601), and 64-bit PowerPC architecture machine
10890 types, with an appropriate, generic processor model assumed for
10891 scheduling purposes.
10893 The other options specify a specific processor. Code generated under
10894 those options will run best on that processor, and may not run at all on
10897 The @option{-mcpu} options automatically enable or disable the
10898 following options: @option{-maltivec}, @option{-mhard-float},
10899 @option{-mmfcrf}, @option{-mmultiple}, @option{-mnew-mnemonics},
10900 @option{-mpower}, @option{-mpower2}, @option{-mpowerpc64},
10901 @option{-mpowerpc-gpopt}, @option{-mpowerpc-gfxopt},
10902 @option{-mstring}. The particular options set for any particular CPU
10903 will vary between compiler versions, depending on what setting seems
10904 to produce optimal code for that CPU; it doesn't necessarily reflect
10905 the actual hardware's capabilities. If you wish to set an individual
10906 option to a particular value, you may specify it after the
10907 @option{-mcpu} option, like @samp{-mcpu=970 -mno-altivec}.
10909 On AIX, the @option{-maltivec} and @option{-mpowerpc64} options are
10910 not enabled or disabled by the @option{-mcpu} option at present, since
10911 AIX does not have full support for these options. You may still
10912 enable or disable them individually if you're sure it'll work in your
10915 @item -mtune=@var{cpu_type}
10917 Set the instruction scheduling parameters for machine type
10918 @var{cpu_type}, but do not set the architecture type, register usage, or
10919 choice of mnemonics, as @option{-mcpu=@var{cpu_type}} would. The same
10920 values for @var{cpu_type} are used for @option{-mtune} as for
10921 @option{-mcpu}. If both are specified, the code generated will use the
10922 architecture, registers, and mnemonics set by @option{-mcpu}, but the
10923 scheduling parameters set by @option{-mtune}.
10929 Generate code to compute division as reciprocal estimate and iterative
10930 refinement, creating opportunities for increased throughput. This
10931 feature requires: optional PowerPC Graphics instruction set for single
10932 precision and FRE instruction for double precision, assuming divides
10933 cannot generate user-visible traps, and the domain values not include
10934 Infinities, denormals or zero denominator.
10937 @itemx -mno-altivec
10939 @opindex mno-altivec
10940 Generate code that uses (does not use) AltiVec instructions, and also
10941 enable the use of built-in functions that allow more direct access to
10942 the AltiVec instruction set. You may also need to set
10943 @option{-mabi=altivec} to adjust the current ABI with AltiVec ABI
10949 @opindex mno-vrsave
10950 Generate VRSAVE instructions when generating AltiVec code.
10954 Extend the current ABI with SPE ABI extensions. This does not change
10955 the default ABI, instead it adds the SPE ABI extensions to the current
10959 @opindex mabi=no-spe
10960 Disable Booke SPE ABI extensions for the current ABI@.
10963 @opindex msecure-plt
10964 Generate code that allows ld and ld.so to build executables and shared
10965 libraries with non-exec .plt and .got sections. This is a PowerPC
10966 32-bit SYSV ABI option.
10970 Generate code that uses a BSS .plt section that ld.so fills in, and
10971 requires .plt and .got sections that are both writable and executable.
10972 This is a PowerPC 32-bit SYSV ABI option.
10978 This switch enables or disables the generation of ISEL instructions.
10980 @item -misel=@var{yes/no}
10981 This switch has been deprecated. Use @option{-misel} and
10982 @option{-mno-isel} instead.
10988 This switch enables or disables the generation of SPE simd
10991 @item -mspe=@var{yes/no}
10992 This option has been deprecated. Use @option{-mspe} and
10993 @option{-mno-spe} instead.
10995 @item -mfloat-gprs=@var{yes/single/double/no}
10996 @itemx -mfloat-gprs
10997 @opindex mfloat-gprs
10998 This switch enables or disables the generation of floating point
10999 operations on the general purpose registers for architectures that
11002 The argument @var{yes} or @var{single} enables the use of
11003 single-precision floating point operations.
11005 The argument @var{double} enables the use of single and
11006 double-precision floating point operations.
11008 The argument @var{no} disables floating point operations on the
11009 general purpose registers.
11011 This option is currently only available on the MPC854x.
11017 Generate code for 32-bit or 64-bit environments of Darwin and SVR4
11018 targets (including GNU/Linux). The 32-bit environment sets int, long
11019 and pointer to 32 bits and generates code that runs on any PowerPC
11020 variant. The 64-bit environment sets int to 32 bits and long and
11021 pointer to 64 bits, and generates code for PowerPC64, as for
11022 @option{-mpowerpc64}.
11025 @itemx -mno-fp-in-toc
11026 @itemx -mno-sum-in-toc
11027 @itemx -mminimal-toc
11029 @opindex mno-fp-in-toc
11030 @opindex mno-sum-in-toc
11031 @opindex mminimal-toc
11032 Modify generation of the TOC (Table Of Contents), which is created for
11033 every executable file. The @option{-mfull-toc} option is selected by
11034 default. In that case, GCC will allocate at least one TOC entry for
11035 each unique non-automatic variable reference in your program. GCC
11036 will also place floating-point constants in the TOC@. However, only
11037 16,384 entries are available in the TOC@.
11039 If you receive a linker error message that saying you have overflowed
11040 the available TOC space, you can reduce the amount of TOC space used
11041 with the @option{-mno-fp-in-toc} and @option{-mno-sum-in-toc} options.
11042 @option{-mno-fp-in-toc} prevents GCC from putting floating-point
11043 constants in the TOC and @option{-mno-sum-in-toc} forces GCC to
11044 generate code to calculate the sum of an address and a constant at
11045 run-time instead of putting that sum into the TOC@. You may specify one
11046 or both of these options. Each causes GCC to produce very slightly
11047 slower and larger code at the expense of conserving TOC space.
11049 If you still run out of space in the TOC even when you specify both of
11050 these options, specify @option{-mminimal-toc} instead. This option causes
11051 GCC to make only one TOC entry for every file. When you specify this
11052 option, GCC will produce code that is slower and larger but which
11053 uses extremely little TOC space. You may wish to use this option
11054 only on files that contain less frequently executed code.
11060 Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit
11061 @code{long} type, and the infrastructure needed to support them.
11062 Specifying @option{-maix64} implies @option{-mpowerpc64} and
11063 @option{-mpowerpc}, while @option{-maix32} disables the 64-bit ABI and
11064 implies @option{-mno-powerpc64}. GCC defaults to @option{-maix32}.
11067 @itemx -mno-xl-compat
11068 @opindex mxl-compat
11069 @opindex mno-xl-compat
11070 Produce code that conforms more closely to IBM XLC semantics when using
11071 AIX-compatible ABI. Pass floating-point arguments to prototyped
11072 functions beyond the register save area (RSA) on the stack in addition
11073 to argument FPRs. Do not assume that most significant double in 128
11074 bit long double value is properly rounded when comparing values.
11076 The AIX calling convention was extended but not initially documented to
11077 handle an obscure K&R C case of calling a function that takes the
11078 address of its arguments with fewer arguments than declared. AIX XL
11079 compilers access floating point arguments which do not fit in the
11080 RSA from the stack when a subroutine is compiled without
11081 optimization. Because always storing floating-point arguments on the
11082 stack is inefficient and rarely needed, this option is not enabled by
11083 default and only is necessary when calling subroutines compiled by AIX
11084 XL compilers without optimization.
11088 Support @dfn{IBM RS/6000 SP} @dfn{Parallel Environment} (PE)@. Link an
11089 application written to use message passing with special startup code to
11090 enable the application to run. The system must have PE installed in the
11091 standard location (@file{/usr/lpp/ppe.poe/}), or the @file{specs} file
11092 must be overridden with the @option{-specs=} option to specify the
11093 appropriate directory location. The Parallel Environment does not
11094 support threads, so the @option{-mpe} option and the @option{-pthread}
11095 option are incompatible.
11097 @item -malign-natural
11098 @itemx -malign-power
11099 @opindex malign-natural
11100 @opindex malign-power
11101 On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option
11102 @option{-malign-natural} overrides the ABI-defined alignment of larger
11103 types, such as floating-point doubles, on their natural size-based boundary.
11104 The option @option{-malign-power} instructs GCC to follow the ABI-specified
11105 alignment rules. GCC defaults to the standard alignment defined in the ABI@.
11107 On 64-bit Darwin, natural alignment is the default, and @option{-malign-power}
11111 @itemx -mhard-float
11112 @opindex msoft-float
11113 @opindex mhard-float
11114 Generate code that does not use (uses) the floating-point register set.
11115 Software floating point emulation is provided if you use the
11116 @option{-msoft-float} option, and pass the option to GCC when linking.
11119 @itemx -mno-multiple
11121 @opindex mno-multiple
11122 Generate code that uses (does not use) the load multiple word
11123 instructions and the store multiple word instructions. These
11124 instructions are generated by default on POWER systems, and not
11125 generated on PowerPC systems. Do not use @option{-mmultiple} on little
11126 endian PowerPC systems, since those instructions do not work when the
11127 processor is in little endian mode. The exceptions are PPC740 and
11128 PPC750 which permit the instructions usage in little endian mode.
11133 @opindex mno-string
11134 Generate code that uses (does not use) the load string instructions
11135 and the store string word instructions to save multiple registers and
11136 do small block moves. These instructions are generated by default on
11137 POWER systems, and not generated on PowerPC systems. Do not use
11138 @option{-mstring} on little endian PowerPC systems, since those
11139 instructions do not work when the processor is in little endian mode.
11140 The exceptions are PPC740 and PPC750 which permit the instructions
11141 usage in little endian mode.
11146 @opindex mno-update
11147 Generate code that uses (does not use) the load or store instructions
11148 that update the base register to the address of the calculated memory
11149 location. These instructions are generated by default. If you use
11150 @option{-mno-update}, there is a small window between the time that the
11151 stack pointer is updated and the address of the previous frame is
11152 stored, which means code that walks the stack frame across interrupts or
11153 signals may get corrupted data.
11156 @itemx -mno-fused-madd
11157 @opindex mfused-madd
11158 @opindex mno-fused-madd
11159 Generate code that uses (does not use) the floating point multiply and
11160 accumulate instructions. These instructions are generated by default if
11161 hardware floating is used.
11163 @item -mno-bit-align
11165 @opindex mno-bit-align
11166 @opindex mbit-align
11167 On System V.4 and embedded PowerPC systems do not (do) force structures
11168 and unions that contain bit-fields to be aligned to the base type of the
11171 For example, by default a structure containing nothing but 8
11172 @code{unsigned} bit-fields of length 1 would be aligned to a 4 byte
11173 boundary and have a size of 4 bytes. By using @option{-mno-bit-align},
11174 the structure would be aligned to a 1 byte boundary and be one byte in
11177 @item -mno-strict-align
11178 @itemx -mstrict-align
11179 @opindex mno-strict-align
11180 @opindex mstrict-align
11181 On System V.4 and embedded PowerPC systems do not (do) assume that
11182 unaligned memory references will be handled by the system.
11184 @item -mrelocatable
11185 @itemx -mno-relocatable
11186 @opindex mrelocatable
11187 @opindex mno-relocatable
11188 On embedded PowerPC systems generate code that allows (does not allow)
11189 the program to be relocated to a different address at runtime. If you
11190 use @option{-mrelocatable} on any module, all objects linked together must
11191 be compiled with @option{-mrelocatable} or @option{-mrelocatable-lib}.
11193 @item -mrelocatable-lib
11194 @itemx -mno-relocatable-lib
11195 @opindex mrelocatable-lib
11196 @opindex mno-relocatable-lib
11197 On embedded PowerPC systems generate code that allows (does not allow)
11198 the program to be relocated to a different address at runtime. Modules
11199 compiled with @option{-mrelocatable-lib} can be linked with either modules
11200 compiled without @option{-mrelocatable} and @option{-mrelocatable-lib} or
11201 with modules compiled with the @option{-mrelocatable} options.
11207 On System V.4 and embedded PowerPC systems do not (do) assume that
11208 register 2 contains a pointer to a global area pointing to the addresses
11209 used in the program.
11212 @itemx -mlittle-endian
11214 @opindex mlittle-endian
11215 On System V.4 and embedded PowerPC systems compile code for the
11216 processor in little endian mode. The @option{-mlittle-endian} option is
11217 the same as @option{-mlittle}.
11220 @itemx -mbig-endian
11222 @opindex mbig-endian
11223 On System V.4 and embedded PowerPC systems compile code for the
11224 processor in big endian mode. The @option{-mbig-endian} option is
11225 the same as @option{-mbig}.
11227 @item -mdynamic-no-pic
11228 @opindex mdynamic-no-pic
11229 On Darwin and Mac OS X systems, compile code so that it is not
11230 relocatable, but that its external references are relocatable. The
11231 resulting code is suitable for applications, but not shared
11234 @item -mprioritize-restricted-insns=@var{priority}
11235 @opindex mprioritize-restricted-insns
11236 This option controls the priority that is assigned to
11237 dispatch-slot restricted instructions during the second scheduling
11238 pass. The argument @var{priority} takes the value @var{0/1/2} to assign
11239 @var{no/highest/second-highest} priority to dispatch slot restricted
11242 @item -msched-costly-dep=@var{dependence_type}
11243 @opindex msched-costly-dep
11244 This option controls which dependences are considered costly
11245 by the target during instruction scheduling. The argument
11246 @var{dependence_type} takes one of the following values:
11247 @var{no}: no dependence is costly,
11248 @var{all}: all dependences are costly,
11249 @var{true_store_to_load}: a true dependence from store to load is costly,
11250 @var{store_to_load}: any dependence from store to load is costly,
11251 @var{number}: any dependence which latency >= @var{number} is costly.
11253 @item -minsert-sched-nops=@var{scheme}
11254 @opindex minsert-sched-nops
11255 This option controls which nop insertion scheme will be used during
11256 the second scheduling pass. The argument @var{scheme} takes one of the
11258 @var{no}: Don't insert nops.
11259 @var{pad}: Pad with nops any dispatch group which has vacant issue slots,
11260 according to the scheduler's grouping.
11261 @var{regroup_exact}: Insert nops to force costly dependent insns into
11262 separate groups. Insert exactly as many nops as needed to force an insn
11263 to a new group, according to the estimated processor grouping.
11264 @var{number}: Insert nops to force costly dependent insns into
11265 separate groups. Insert @var{number} nops to force an insn to a new group.
11268 @opindex mcall-sysv
11269 On System V.4 and embedded PowerPC systems compile code using calling
11270 conventions that adheres to the March 1995 draft of the System V
11271 Application Binary Interface, PowerPC processor supplement. This is the
11272 default unless you configured GCC using @samp{powerpc-*-eabiaix}.
11274 @item -mcall-sysv-eabi
11275 @opindex mcall-sysv-eabi
11276 Specify both @option{-mcall-sysv} and @option{-meabi} options.
11278 @item -mcall-sysv-noeabi
11279 @opindex mcall-sysv-noeabi
11280 Specify both @option{-mcall-sysv} and @option{-mno-eabi} options.
11282 @item -mcall-solaris
11283 @opindex mcall-solaris
11284 On System V.4 and embedded PowerPC systems compile code for the Solaris
11288 @opindex mcall-linux
11289 On System V.4 and embedded PowerPC systems compile code for the
11290 Linux-based GNU system.
11294 On System V.4 and embedded PowerPC systems compile code for the
11295 Hurd-based GNU system.
11297 @item -mcall-netbsd
11298 @opindex mcall-netbsd
11299 On System V.4 and embedded PowerPC systems compile code for the
11300 NetBSD operating system.
11302 @item -maix-struct-return
11303 @opindex maix-struct-return
11304 Return all structures in memory (as specified by the AIX ABI)@.
11306 @item -msvr4-struct-return
11307 @opindex msvr4-struct-return
11308 Return structures smaller than 8 bytes in registers (as specified by the
11311 @item -mabi=@var{abi-type}
11313 Extend the current ABI with a particular extension, or remove such extension.
11314 Valid values are @var{altivec}, @var{no-altivec}, @var{spe},
11318 @itemx -mno-prototype
11319 @opindex mprototype
11320 @opindex mno-prototype
11321 On System V.4 and embedded PowerPC systems assume that all calls to
11322 variable argument functions are properly prototyped. Otherwise, the
11323 compiler must insert an instruction before every non prototyped call to
11324 set or clear bit 6 of the condition code register (@var{CR}) to
11325 indicate whether floating point values were passed in the floating point
11326 registers in case the function takes a variable arguments. With
11327 @option{-mprototype}, only calls to prototyped variable argument functions
11328 will set or clear the bit.
11332 On embedded PowerPC systems, assume that the startup module is called
11333 @file{sim-crt0.o} and that the standard C libraries are @file{libsim.a} and
11334 @file{libc.a}. This is the default for @samp{powerpc-*-eabisim}.
11339 On embedded PowerPC systems, assume that the startup module is called
11340 @file{crt0.o} and the standard C libraries are @file{libmvme.a} and
11345 On embedded PowerPC systems, assume that the startup module is called
11346 @file{crt0.o} and the standard C libraries are @file{libads.a} and
11349 @item -myellowknife
11350 @opindex myellowknife
11351 On embedded PowerPC systems, assume that the startup module is called
11352 @file{crt0.o} and the standard C libraries are @file{libyk.a} and
11357 On System V.4 and embedded PowerPC systems, specify that you are
11358 compiling for a VxWorks system.
11362 Specify that you are compiling for the WindISS simulation environment.
11366 On embedded PowerPC systems, set the @var{PPC_EMB} bit in the ELF flags
11367 header to indicate that @samp{eabi} extended relocations are used.
11373 On System V.4 and embedded PowerPC systems do (do not) adhere to the
11374 Embedded Applications Binary Interface (eabi) which is a set of
11375 modifications to the System V.4 specifications. Selecting @option{-meabi}
11376 means that the stack is aligned to an 8 byte boundary, a function
11377 @code{__eabi} is called to from @code{main} to set up the eabi
11378 environment, and the @option{-msdata} option can use both @code{r2} and
11379 @code{r13} to point to two separate small data areas. Selecting
11380 @option{-mno-eabi} means that the stack is aligned to a 16 byte boundary,
11381 do not call an initialization function from @code{main}, and the
11382 @option{-msdata} option will only use @code{r13} to point to a single
11383 small data area. The @option{-meabi} option is on by default if you
11384 configured GCC using one of the @samp{powerpc*-*-eabi*} options.
11387 @opindex msdata=eabi
11388 On System V.4 and embedded PowerPC systems, put small initialized
11389 @code{const} global and static data in the @samp{.sdata2} section, which
11390 is pointed to by register @code{r2}. Put small initialized
11391 non-@code{const} global and static data in the @samp{.sdata} section,
11392 which is pointed to by register @code{r13}. Put small uninitialized
11393 global and static data in the @samp{.sbss} section, which is adjacent to
11394 the @samp{.sdata} section. The @option{-msdata=eabi} option is
11395 incompatible with the @option{-mrelocatable} option. The
11396 @option{-msdata=eabi} option also sets the @option{-memb} option.
11399 @opindex msdata=sysv
11400 On System V.4 and embedded PowerPC systems, put small global and static
11401 data in the @samp{.sdata} section, which is pointed to by register
11402 @code{r13}. Put small uninitialized global and static data in the
11403 @samp{.sbss} section, which is adjacent to the @samp{.sdata} section.
11404 The @option{-msdata=sysv} option is incompatible with the
11405 @option{-mrelocatable} option.
11407 @item -msdata=default
11409 @opindex msdata=default
11411 On System V.4 and embedded PowerPC systems, if @option{-meabi} is used,
11412 compile code the same as @option{-msdata=eabi}, otherwise compile code the
11413 same as @option{-msdata=sysv}.
11416 @opindex msdata-data
11417 On System V.4 and embedded PowerPC systems, put small global and static
11418 data in the @samp{.sdata} section. Put small uninitialized global and
11419 static data in the @samp{.sbss} section. Do not use register @code{r13}
11420 to address small data however. This is the default behavior unless
11421 other @option{-msdata} options are used.
11425 @opindex msdata=none
11427 On embedded PowerPC systems, put all initialized global and static data
11428 in the @samp{.data} section, and all uninitialized data in the
11429 @samp{.bss} section.
11433 @cindex smaller data references (PowerPC)
11434 @cindex .sdata/.sdata2 references (PowerPC)
11435 On embedded PowerPC systems, put global and static items less than or
11436 equal to @var{num} bytes into the small data or bss sections instead of
11437 the normal data or bss section. By default, @var{num} is 8. The
11438 @option{-G @var{num}} switch is also passed to the linker.
11439 All modules should be compiled with the same @option{-G @var{num}} value.
11442 @itemx -mno-regnames
11444 @opindex mno-regnames
11445 On System V.4 and embedded PowerPC systems do (do not) emit register
11446 names in the assembly language output using symbolic forms.
11449 @itemx -mno-longcall
11451 @opindex mno-longcall
11452 Default to making all function calls indirectly, using a register, so
11453 that functions which reside further than 32 megabytes (33,554,432
11454 bytes) from the current location can be called. This setting can be
11455 overridden by the @code{shortcall} function attribute, or by
11456 @code{#pragma longcall(0)}.
11458 Some linkers are capable of detecting out-of-range calls and generating
11459 glue code on the fly. On these systems, long calls are unnecessary and
11460 generate slower code. As of this writing, the AIX linker can do this,
11461 as can the GNU linker for PowerPC/64. It is planned to add this feature
11462 to the GNU linker for 32-bit PowerPC systems as well.
11464 On Darwin/PPC systems, @code{#pragma longcall} will generate ``jbsr
11465 callee, L42'', plus a ``branch island'' (glue code). The two target
11466 addresses represent the callee and the ``branch island''. The
11467 Darwin/PPC linker will prefer the first address and generate a ``bl
11468 callee'' if the PPC ``bl'' instruction will reach the callee directly;
11469 otherwise, the linker will generate ``bl L42'' to call the ``branch
11470 island''. The ``branch island'' is appended to the body of the
11471 calling function; it computes the full 32-bit address of the callee
11474 On Mach-O (Darwin) systems, this option directs the compiler emit to
11475 the glue for every direct call, and the Darwin linker decides whether
11476 to use or discard it.
11478 In the future, we may cause GCC to ignore all longcall specifications
11479 when the linker is known to generate glue.
11483 Adds support for multithreading with the @dfn{pthreads} library.
11484 This option sets flags for both the preprocessor and linker.
11488 @node S/390 and zSeries Options
11489 @subsection S/390 and zSeries Options
11490 @cindex S/390 and zSeries Options
11492 These are the @samp{-m} options defined for the S/390 and zSeries architecture.
11496 @itemx -msoft-float
11497 @opindex mhard-float
11498 @opindex msoft-float
11499 Use (do not use) the hardware floating-point instructions and registers
11500 for floating-point operations. When @option{-msoft-float} is specified,
11501 functions in @file{libgcc.a} will be used to perform floating-point
11502 operations. When @option{-mhard-float} is specified, the compiler
11503 generates IEEE floating-point instructions. This is the default.
11506 @itemx -mno-backchain
11507 @opindex mbackchain
11508 @opindex mno-backchain
11509 Store (do not store) the address of the caller's frame as backchain pointer
11510 into the callee's stack frame.
11511 A backchain may be needed to allow debugging using tools that do not understand
11512 DWARF-2 call frame information.
11513 When @option{-mno-packed-stack} is in effect, the backchain pointer is stored
11514 at the bottom of the stack frame; when @option{-mpacked-stack} is in effect,
11515 the backchain is placed into the topmost word of the 96/160 byte register
11518 In general, code compiled with @option{-mbackchain} is call-compatible with
11519 code compiled with @option{-mmo-backchain}; however, use of the backchain
11520 for debugging purposes usually requires that the whole binary is built with
11521 @option{-mbackchain}. Note that the combination of @option{-mbackchain},
11522 @option{-mpacked-stack} and @option{-mhard-float} is not supported. In order
11523 to build a linux kernel use @option{-msoft-float}.
11525 The default is to not maintain the backchain.
11527 @item -mpacked-stack
11528 @item -mno-packed-stack
11529 @opindex mpacked-stack
11530 @opindex mno-packed-stack
11531 Use (do not use) the packed stack layout. When @option{-mno-packed-stack} is
11532 specified, the compiler uses the all fields of the 96/160 byte register save
11533 area only for their default purpose; unused fields still take up stack space.
11534 When @option{-mpacked-stack} is specified, register save slots are densely
11535 packed at the top of the register save area; unused space is reused for other
11536 purposes, allowing for more efficient use of the available stack space.
11537 However, when @option{-mbackchain} is also in effect, the topmost word of
11538 the save area is always used to store the backchain, and the return address
11539 register is always saved two words below the backchain.
11541 As long as the stack frame backchain is not used, code generated with
11542 @option{-mpacked-stack} is call-compatible with code generated with
11543 @option{-mno-packed-stack}. Note that some non-FSF releases of GCC 2.95 for
11544 S/390 or zSeries generated code that uses the stack frame backchain at run
11545 time, not just for debugging purposes. Such code is not call-compatible
11546 with code compiled with @option{-mpacked-stack}. Also, note that the
11547 combination of @option{-mbackchain},
11548 @option{-mpacked-stack} and @option{-mhard-float} is not supported. In order
11549 to build a linux kernel use @option{-msoft-float}.
11551 The default is to not use the packed stack layout.
11554 @itemx -mno-small-exec
11555 @opindex msmall-exec
11556 @opindex mno-small-exec
11557 Generate (or do not generate) code using the @code{bras} instruction
11558 to do subroutine calls.
11559 This only works reliably if the total executable size does not
11560 exceed 64k. The default is to use the @code{basr} instruction instead,
11561 which does not have this limitation.
11567 When @option{-m31} is specified, generate code compliant to the
11568 GNU/Linux for S/390 ABI@. When @option{-m64} is specified, generate
11569 code compliant to the GNU/Linux for zSeries ABI@. This allows GCC in
11570 particular to generate 64-bit instructions. For the @samp{s390}
11571 targets, the default is @option{-m31}, while the @samp{s390x}
11572 targets default to @option{-m64}.
11578 When @option{-mzarch} is specified, generate code using the
11579 instructions available on z/Architecture.
11580 When @option{-mesa} is specified, generate code using the
11581 instructions available on ESA/390. Note that @option{-mesa} is
11582 not possible with @option{-m64}.
11583 When generating code compliant to the GNU/Linux for S/390 ABI,
11584 the default is @option{-mesa}. When generating code compliant
11585 to the GNU/Linux for zSeries ABI, the default is @option{-mzarch}.
11591 Generate (or do not generate) code using the @code{mvcle} instruction
11592 to perform block moves. When @option{-mno-mvcle} is specified,
11593 use a @code{mvc} loop instead. This is the default unless optimizing for
11600 Print (or do not print) additional debug information when compiling.
11601 The default is to not print debug information.
11603 @item -march=@var{cpu-type}
11605 Generate code that will run on @var{cpu-type}, which is the name of a system
11606 representing a certain processor type. Possible values for
11607 @var{cpu-type} are @samp{g5}, @samp{g6}, @samp{z900}, and @samp{z990}.
11608 When generating code using the instructions available on z/Architecture,
11609 the default is @option{-march=z900}. Otherwise, the default is
11610 @option{-march=g5}.
11612 @item -mtune=@var{cpu-type}
11614 Tune to @var{cpu-type} everything applicable about the generated code,
11615 except for the ABI and the set of available instructions.
11616 The list of @var{cpu-type} values is the same as for @option{-march}.
11617 The default is the value used for @option{-march}.
11620 @itemx -mno-tpf-trace
11621 @opindex mtpf-trace
11622 @opindex mno-tpf-trace
11623 Generate code that adds (does not add) in TPF OS specific branches to trace
11624 routines in the operating system. This option is off by default, even
11625 when compiling for the TPF OS@.
11628 @itemx -mno-fused-madd
11629 @opindex mfused-madd
11630 @opindex mno-fused-madd
11631 Generate code that uses (does not use) the floating point multiply and
11632 accumulate instructions. These instructions are generated by default if
11633 hardware floating point is used.
11635 @item -mwarn-framesize=@var{framesize}
11636 @opindex mwarn-framesize
11637 Emit a warning if the current function exceeds the given frame size. Because
11638 this is a compile time check it doesn't need to be a real problem when the program
11639 runs. It is intended to identify functions which most probably cause
11640 a stack overflow. It is useful to be used in an environment with limited stack
11641 size e.g.@: the linux kernel.
11643 @item -mwarn-dynamicstack
11644 @opindex mwarn-dynamicstack
11645 Emit a warning if the function calls alloca or uses dynamically
11646 sized arrays. This is generally a bad idea with a limited stack size.
11648 @item -mstack-guard=@var{stack-guard}
11649 @item -mstack-size=@var{stack-size}
11650 @opindex mstack-guard
11651 @opindex mstack-size
11652 These arguments always have to be used in conjunction. If they are present the s390
11653 back end emits additional instructions in the function prologue which trigger a trap
11654 if the stack size is @var{stack-guard} bytes above the @var{stack-size}
11655 (remember that the stack on s390 grows downward). These options are intended to
11656 be used to help debugging stack overflow problems. The additionally emitted code
11657 cause only little overhead and hence can also be used in production like systems
11658 without greater performance degradation. The given values have to be exact
11659 powers of 2 and @var{stack-size} has to be greater than @var{stack-guard}.
11660 In order to be efficient the extra code makes the assumption that the stack starts
11661 at an address aligned to the value given by @var{stack-size}.
11665 @subsection SH Options
11667 These @samp{-m} options are defined for the SH implementations:
11672 Generate code for the SH1.
11676 Generate code for the SH2.
11679 Generate code for the SH2e.
11683 Generate code for the SH3.
11687 Generate code for the SH3e.
11691 Generate code for the SH4 without a floating-point unit.
11693 @item -m4-single-only
11694 @opindex m4-single-only
11695 Generate code for the SH4 with a floating-point unit that only
11696 supports single-precision arithmetic.
11700 Generate code for the SH4 assuming the floating-point unit is in
11701 single-precision mode by default.
11705 Generate code for the SH4.
11709 Generate code for the SH4al-dsp, or for a SH4a in such a way that the
11710 floating-point unit is not used.
11712 @item -m4a-single-only
11713 @opindex m4a-single-only
11714 Generate code for the SH4a, in such a way that no double-precision
11715 floating point operations are used.
11718 @opindex m4a-single
11719 Generate code for the SH4a assuming the floating-point unit is in
11720 single-precision mode by default.
11724 Generate code for the SH4a.
11728 Same as @option{-m4a-nofpu}, except that it implicitly passes
11729 @option{-dsp} to the assembler. GCC doesn't generate any DSP
11730 instructions at the moment.
11734 Compile code for the processor in big endian mode.
11738 Compile code for the processor in little endian mode.
11742 Align doubles at 64-bit boundaries. Note that this changes the calling
11743 conventions, and thus some functions from the standard C library will
11744 not work unless you recompile it first with @option{-mdalign}.
11748 Shorten some address references at link time, when possible; uses the
11749 linker option @option{-relax}.
11753 Use 32-bit offsets in @code{switch} tables. The default is to use
11758 Enable the use of the instruction @code{fmovd}.
11762 Comply with the calling conventions defined by Renesas.
11766 Comply with the calling conventions defined by Renesas.
11770 Comply with the calling conventions defined for GCC before the Renesas
11771 conventions were available. This option is the default for all
11772 targets of the SH toolchain except for @samp{sh-symbianelf}.
11775 @opindex mnomacsave
11776 Mark the @code{MAC} register as call-clobbered, even if
11777 @option{-mhitachi} is given.
11781 Increase IEEE-compliance of floating-point code.
11782 At the moment, this is equivalent to @option{-fno-finite-math-only}.
11783 When generating 16 bit SH opcodes, getting IEEE-conforming results for
11784 comparisons of NANs / infinities incurs extra overhead in every
11785 floating point comparison, therefore the default is set to
11786 @option{-ffinite-math-only}.
11790 Dump instruction size and location in the assembly code.
11793 @opindex mpadstruct
11794 This option is deprecated. It pads structures to multiple of 4 bytes,
11795 which is incompatible with the SH ABI@.
11799 Optimize for space instead of speed. Implied by @option{-Os}.
11802 @opindex mprefergot
11803 When generating position-independent code, emit function calls using
11804 the Global Offset Table instead of the Procedure Linkage Table.
11808 Generate a library function call to invalidate instruction cache
11809 entries, after fixing up a trampoline. This library function call
11810 doesn't assume it can write to the whole memory address space. This
11811 is the default when the target is @code{sh-*-linux*}.
11813 @item -multcost=@var{number}
11814 @opindex multcost=@var{number}
11815 Set the cost to assume for a multiply insn.
11817 @item -mdiv=@var{strategy}
11818 @opindex mdiv=@var{strategy}
11819 Set the division strategy to use for SHmedia code. @var{strategy} must be
11820 one of: call, call2, fp, inv, inv:minlat, inv20u, inv20l, inv:call,
11821 inv:call2, inv:fp .
11822 "fp" performs the operation in floating point. This has a very high latency,
11823 but needs only a few instructions, so it might be a good choice if
11824 your code has enough easily exploitable ILP to allow the compiler to
11825 schedule the floating point instructions together with other instructions.
11826 Division by zero causes a floating point exception.
11827 "inv" uses integer operations to calculate the inverse of the divisor,
11828 and then multiplies the dividend with the inverse. This strategy allows
11829 cse and hoisting of the inverse calculation. Division by zero calculates
11830 an unspecified result, but does not trap.
11831 "inv:minlat" is a variant of "inv" where if no cse / hoisting opportunities
11832 have been found, or if the entire operation has been hoisted to the same
11833 place, the last stages of the inverse calculation are intertwined with the
11834 final multiply to reduce the overall latency, at the expense of using a few
11835 more instructions, and thus offering fewer scheduling opportunities with
11837 "call" calls a library function that usually implements the inv:minlat
11839 This gives high code density for m5-*media-nofpu compilations.
11840 "call2" uses a different entry point of the same library function, where it
11841 assumes that a pointer to a lookup table has already been set up, which
11842 exposes the pointer load to cse / code hoisting optimizations.
11843 "inv:call", "inv:call2" and "inv:fp" all use the "inv" algorithm for initial
11844 code generation, but if the code stays unoptimized, revert to the "call",
11845 "call2", or "fp" strategies, respectively. Note that the
11846 potentially-trapping side effect of division by zero is carried by a
11847 separate instruction, so it is possible that all the integer instructions
11848 are hoisted out, but the marker for the side effect stays where it is.
11849 A recombination to fp operations or a call is not possible in that case.
11850 "inv20u" and "inv20l" are variants of the "inv:minlat" strategy. In the case
11851 that the inverse calculation was nor separated from the multiply, they speed
11852 up division where the dividend fits into 20 bits (plus sign where applicable),
11853 by inserting a test to skip a number of operations in this case; this test
11854 slows down the case of larger dividends. inv20u assumes the case of a such
11855 a small dividend to be unlikely, and inv20l assumes it to be likely.
11857 @item -mdivsi3_libfunc=@var{name}
11858 @opindex mdivsi3_libfunc=@var{name}
11859 Set the name of the library function used for 32 bit signed division to
11860 @var{name}. This only affect the name used in the call and inv:call
11861 division strategies, and the compiler will still expect the same
11862 sets of input/output/clobbered registers as if this option was not present.
11864 @item -madjust-unroll
11865 @opindex madjust-unroll
11866 Throttle unrolling to avoid thrashing target registers.
11867 This option only has an effect if the gcc code base supports the
11868 TARGET_ADJUST_UNROLL_MAX target hook.
11870 @item -mindexed-addressing
11871 @opindex mindexed-addressing
11872 Enable the use of the indexed addressing mode for SHmedia32/SHcompact.
11873 This is only safe if the hardware and/or OS implement 32 bit wrap-around
11874 semantics for the indexed addressing mode. The architecture allows the
11875 implementation of processors with 64 bit MMU, which the OS could use to
11876 get 32 bit addressing, but since no current hardware implementation supports
11877 this or any other way to make the indexed addressing mode safe to use in
11878 the 32 bit ABI, the default is -mno-indexed-addressing.
11880 @item -mgettrcost=@var{number}
11881 @opindex mgettrcost=@var{number}
11882 Set the cost assumed for the gettr instruction to @var{number}.
11883 The default is 2 if @option{-mpt-fixed} is in effect, 100 otherwise.
11887 Assume pt* instructions won't trap. This will generally generate better
11888 scheduled code, but is unsafe on current hardware. The current architecture
11889 definition says that ptabs and ptrel trap when the target anded with 3 is 3.
11890 This has the unintentional effect of making it unsafe to schedule ptabs /
11891 ptrel before a branch, or hoist it out of a loop. For example,
11892 __do_global_ctors, a part of libgcc that runs constructors at program
11893 startup, calls functions in a list which is delimited by -1. With the
11894 -mpt-fixed option, the ptabs will be done before testing against -1.
11895 That means that all the constructors will be run a bit quicker, but when
11896 the loop comes to the end of the list, the program crashes because ptabs
11897 loads -1 into a target register. Since this option is unsafe for any
11898 hardware implementing the current architecture specification, the default
11899 is -mno-pt-fixed. Unless the user specifies a specific cost with
11900 @option{-mgettrcost}, -mno-pt-fixed also implies @option{-mgettrcost=100};
11901 this deters register allocation using target registers for storing
11904 @item -minvalid-symbols
11905 @opindex minvalid-symbols
11906 Assume symbols might be invalid. Ordinary function symbols generated by
11907 the compiler will always be valid to load with movi/shori/ptabs or
11908 movi/shori/ptrel, but with assembler and/or linker tricks it is possible
11909 to generate symbols that will cause ptabs / ptrel to trap.
11910 This option is only meaningful when @option{-mno-pt-fixed} is in effect.
11911 It will then prevent cross-basic-block cse, hoisting and most scheduling
11912 of symbol loads. The default is @option{-mno-invalid-symbols}.
11915 @node SPARC Options
11916 @subsection SPARC Options
11917 @cindex SPARC options
11919 These @samp{-m} options are supported on the SPARC:
11922 @item -mno-app-regs
11924 @opindex mno-app-regs
11926 Specify @option{-mapp-regs} to generate output using the global registers
11927 2 through 4, which the SPARC SVR4 ABI reserves for applications. This
11930 To be fully SVR4 ABI compliant at the cost of some performance loss,
11931 specify @option{-mno-app-regs}. You should compile libraries and system
11932 software with this option.
11935 @itemx -mhard-float
11937 @opindex mhard-float
11938 Generate output containing floating point instructions. This is the
11942 @itemx -msoft-float
11944 @opindex msoft-float
11945 Generate output containing library calls for floating point.
11946 @strong{Warning:} the requisite libraries are not available for all SPARC
11947 targets. Normally the facilities of the machine's usual C compiler are
11948 used, but this cannot be done directly in cross-compilation. You must make
11949 your own arrangements to provide suitable library functions for
11950 cross-compilation. The embedded targets @samp{sparc-*-aout} and
11951 @samp{sparclite-*-*} do provide software floating point support.
11953 @option{-msoft-float} changes the calling convention in the output file;
11954 therefore, it is only useful if you compile @emph{all} of a program with
11955 this option. In particular, you need to compile @file{libgcc.a}, the
11956 library that comes with GCC, with @option{-msoft-float} in order for
11959 @item -mhard-quad-float
11960 @opindex mhard-quad-float
11961 Generate output containing quad-word (long double) floating point
11964 @item -msoft-quad-float
11965 @opindex msoft-quad-float
11966 Generate output containing library calls for quad-word (long double)
11967 floating point instructions. The functions called are those specified
11968 in the SPARC ABI@. This is the default.
11970 As of this writing, there are no SPARC implementations that have hardware
11971 support for the quad-word floating point instructions. They all invoke
11972 a trap handler for one of these instructions, and then the trap handler
11973 emulates the effect of the instruction. Because of the trap handler overhead,
11974 this is much slower than calling the ABI library routines. Thus the
11975 @option{-msoft-quad-float} option is the default.
11977 @item -mno-unaligned-doubles
11978 @itemx -munaligned-doubles
11979 @opindex mno-unaligned-doubles
11980 @opindex munaligned-doubles
11981 Assume that doubles have 8 byte alignment. This is the default.
11983 With @option{-munaligned-doubles}, GCC assumes that doubles have 8 byte
11984 alignment only if they are contained in another type, or if they have an
11985 absolute address. Otherwise, it assumes they have 4 byte alignment.
11986 Specifying this option avoids some rare compatibility problems with code
11987 generated by other compilers. It is not the default because it results
11988 in a performance loss, especially for floating point code.
11990 @item -mno-faster-structs
11991 @itemx -mfaster-structs
11992 @opindex mno-faster-structs
11993 @opindex mfaster-structs
11994 With @option{-mfaster-structs}, the compiler assumes that structures
11995 should have 8 byte alignment. This enables the use of pairs of
11996 @code{ldd} and @code{std} instructions for copies in structure
11997 assignment, in place of twice as many @code{ld} and @code{st} pairs.
11998 However, the use of this changed alignment directly violates the SPARC
11999 ABI@. Thus, it's intended only for use on targets where the developer
12000 acknowledges that their resulting code will not be directly in line with
12001 the rules of the ABI@.
12003 @item -mimpure-text
12004 @opindex mimpure-text
12005 @option{-mimpure-text}, used in addition to @option{-shared}, tells
12006 the compiler to not pass @option{-z text} to the linker when linking a
12007 shared object. Using this option, you can link position-dependent
12008 code into a shared object.
12010 @option{-mimpure-text} suppresses the ``relocations remain against
12011 allocatable but non-writable sections'' linker error message.
12012 However, the necessary relocations will trigger copy-on-write, and the
12013 shared object is not actually shared across processes. Instead of
12014 using @option{-mimpure-text}, you should compile all source code with
12015 @option{-fpic} or @option{-fPIC}.
12017 This option is only available on SunOS and Solaris.
12019 @item -mcpu=@var{cpu_type}
12021 Set the instruction set, register set, and instruction scheduling parameters
12022 for machine type @var{cpu_type}. Supported values for @var{cpu_type} are
12023 @samp{v7}, @samp{cypress}, @samp{v8}, @samp{supersparc}, @samp{sparclite},
12024 @samp{f930}, @samp{f934}, @samp{hypersparc}, @samp{sparclite86x},
12025 @samp{sparclet}, @samp{tsc701}, @samp{v9}, @samp{ultrasparc}, and
12026 @samp{ultrasparc3}.
12028 Default instruction scheduling parameters are used for values that select
12029 an architecture and not an implementation. These are @samp{v7}, @samp{v8},
12030 @samp{sparclite}, @samp{sparclet}, @samp{v9}.
12032 Here is a list of each supported architecture and their supported
12037 v8: supersparc, hypersparc
12038 sparclite: f930, f934, sparclite86x
12040 v9: ultrasparc, ultrasparc3
12043 By default (unless configured otherwise), GCC generates code for the V7
12044 variant of the SPARC architecture. With @option{-mcpu=cypress}, the compiler
12045 additionally optimizes it for the Cypress CY7C602 chip, as used in the
12046 SPARCStation/SPARCServer 3xx series. This is also appropriate for the older
12047 SPARCStation 1, 2, IPX etc.
12049 With @option{-mcpu=v8}, GCC generates code for the V8 variant of the SPARC
12050 architecture. The only difference from V7 code is that the compiler emits
12051 the integer multiply and integer divide instructions which exist in SPARC-V8
12052 but not in SPARC-V7. With @option{-mcpu=supersparc}, the compiler additionally
12053 optimizes it for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and
12056 With @option{-mcpu=sparclite}, GCC generates code for the SPARClite variant of
12057 the SPARC architecture. This adds the integer multiply, integer divide step
12058 and scan (@code{ffs}) instructions which exist in SPARClite but not in SPARC-V7.
12059 With @option{-mcpu=f930}, the compiler additionally optimizes it for the
12060 Fujitsu MB86930 chip, which is the original SPARClite, with no FPU@. With
12061 @option{-mcpu=f934}, the compiler additionally optimizes it for the Fujitsu
12062 MB86934 chip, which is the more recent SPARClite with FPU@.
12064 With @option{-mcpu=sparclet}, GCC generates code for the SPARClet variant of
12065 the SPARC architecture. This adds the integer multiply, multiply/accumulate,
12066 integer divide step and scan (@code{ffs}) instructions which exist in SPARClet
12067 but not in SPARC-V7. With @option{-mcpu=tsc701}, the compiler additionally
12068 optimizes it for the TEMIC SPARClet chip.
12070 With @option{-mcpu=v9}, GCC generates code for the V9 variant of the SPARC
12071 architecture. This adds 64-bit integer and floating-point move instructions,
12072 3 additional floating-point condition code registers and conditional move
12073 instructions. With @option{-mcpu=ultrasparc}, the compiler additionally
12074 optimizes it for the Sun UltraSPARC I/II chips. With
12075 @option{-mcpu=ultrasparc3}, the compiler additionally optimizes it for the
12076 Sun UltraSPARC III chip.
12078 @item -mtune=@var{cpu_type}
12080 Set the instruction scheduling parameters for machine type
12081 @var{cpu_type}, but do not set the instruction set or register set that the
12082 option @option{-mcpu=@var{cpu_type}} would.
12084 The same values for @option{-mcpu=@var{cpu_type}} can be used for
12085 @option{-mtune=@var{cpu_type}}, but the only useful values are those
12086 that select a particular cpu implementation. Those are @samp{cypress},
12087 @samp{supersparc}, @samp{hypersparc}, @samp{f930}, @samp{f934},
12088 @samp{sparclite86x}, @samp{tsc701}, @samp{ultrasparc}, and
12089 @samp{ultrasparc3}.
12094 @opindex mno-v8plus
12095 With @option{-mv8plus}, GCC generates code for the SPARC-V8+ ABI@. The
12096 difference from the V8 ABI is that the global and out registers are
12097 considered 64-bit wide. This is enabled by default on Solaris in 32-bit
12098 mode for all SPARC-V9 processors.
12104 With @option{-mvis}, GCC generates code that takes advantage of the UltraSPARC
12105 Visual Instruction Set extensions. The default is @option{-mno-vis}.
12108 These @samp{-m} options are supported in addition to the above
12109 on SPARC-V9 processors in 64-bit environments:
12112 @item -mlittle-endian
12113 @opindex mlittle-endian
12114 Generate code for a processor running in little-endian mode. It is only
12115 available for a few configurations and most notably not on Solaris and Linux.
12121 Generate code for a 32-bit or 64-bit environment.
12122 The 32-bit environment sets int, long and pointer to 32 bits.
12123 The 64-bit environment sets int to 32 bits and long and pointer
12126 @item -mcmodel=medlow
12127 @opindex mcmodel=medlow
12128 Generate code for the Medium/Low code model: 64-bit addresses, programs
12129 must be linked in the low 32 bits of memory. Programs can be statically
12130 or dynamically linked.
12132 @item -mcmodel=medmid
12133 @opindex mcmodel=medmid
12134 Generate code for the Medium/Middle code model: 64-bit addresses, programs
12135 must be linked in the low 44 bits of memory, the text and data segments must
12136 be less than 2GB in size and the data segment must be located within 2GB of
12139 @item -mcmodel=medany
12140 @opindex mcmodel=medany
12141 Generate code for the Medium/Anywhere code model: 64-bit addresses, programs
12142 may be linked anywhere in memory, the text and data segments must be less
12143 than 2GB in size and the data segment must be located within 2GB of the
12146 @item -mcmodel=embmedany
12147 @opindex mcmodel=embmedany
12148 Generate code for the Medium/Anywhere code model for embedded systems:
12149 64-bit addresses, the text and data segments must be less than 2GB in
12150 size, both starting anywhere in memory (determined at link time). The
12151 global register %g4 points to the base of the data segment. Programs
12152 are statically linked and PIC is not supported.
12155 @itemx -mno-stack-bias
12156 @opindex mstack-bias
12157 @opindex mno-stack-bias
12158 With @option{-mstack-bias}, GCC assumes that the stack pointer, and
12159 frame pointer if present, are offset by @minus{}2047 which must be added back
12160 when making stack frame references. This is the default in 64-bit mode.
12161 Otherwise, assume no such offset is present.
12164 These switches are supported in addition to the above on Solaris:
12169 Add support for multithreading using the Solaris threads library. This
12170 option sets flags for both the preprocessor and linker. This option does
12171 not affect the thread safety of object code produced by the compiler or
12172 that of libraries supplied with it.
12176 Add support for multithreading using the POSIX threads library. This
12177 option sets flags for both the preprocessor and linker. This option does
12178 not affect the thread safety of object code produced by the compiler or
12179 that of libraries supplied with it.
12182 @node System V Options
12183 @subsection Options for System V
12185 These additional options are available on System V Release 4 for
12186 compatibility with other compilers on those systems:
12191 Create a shared object.
12192 It is recommended that @option{-symbolic} or @option{-shared} be used instead.
12196 Identify the versions of each tool used by the compiler, in a
12197 @code{.ident} assembler directive in the output.
12201 Refrain from adding @code{.ident} directives to the output file (this is
12204 @item -YP,@var{dirs}
12206 Search the directories @var{dirs}, and no others, for libraries
12207 specified with @option{-l}.
12209 @item -Ym,@var{dir}
12211 Look in the directory @var{dir} to find the M4 preprocessor.
12212 The assembler uses this option.
12213 @c This is supposed to go with a -Yd for predefined M4 macro files, but
12214 @c the generic assembler that comes with Solaris takes just -Ym.
12217 @node TMS320C3x/C4x Options
12218 @subsection TMS320C3x/C4x Options
12219 @cindex TMS320C3x/C4x Options
12221 These @samp{-m} options are defined for TMS320C3x/C4x implementations:
12225 @item -mcpu=@var{cpu_type}
12227 Set the instruction set, register set, and instruction scheduling
12228 parameters for machine type @var{cpu_type}. Supported values for
12229 @var{cpu_type} are @samp{c30}, @samp{c31}, @samp{c32}, @samp{c40}, and
12230 @samp{c44}. The default is @samp{c40} to generate code for the
12235 @itemx -msmall-memory
12237 @opindex mbig-memory
12239 @opindex msmall-memory
12241 Generates code for the big or small memory model. The small memory
12242 model assumed that all data fits into one 64K word page. At run-time
12243 the data page (DP) register must be set to point to the 64K page
12244 containing the .bss and .data program sections. The big memory model is
12245 the default and requires reloading of the DP register for every direct
12252 Allow (disallow) allocation of general integer operands into the block
12253 count register BK@.
12259 Enable (disable) generation of code using decrement and branch,
12260 DBcond(D), instructions. This is enabled by default for the C4x. To be
12261 on the safe side, this is disabled for the C3x, since the maximum
12262 iteration count on the C3x is @math{2^{23} + 1} (but who iterates loops more than
12263 @math{2^{23}} times on the C3x?). Note that GCC will try to reverse a loop so
12264 that it can utilize the decrement and branch instruction, but will give
12265 up if there is more than one memory reference in the loop. Thus a loop
12266 where the loop counter is decremented can generate slightly more
12267 efficient code, in cases where the RPTB instruction cannot be utilized.
12269 @item -mdp-isr-reload
12271 @opindex mdp-isr-reload
12273 Force the DP register to be saved on entry to an interrupt service
12274 routine (ISR), reloaded to point to the data section, and restored on
12275 exit from the ISR@. This should not be required unless someone has
12276 violated the small memory model by modifying the DP register, say within
12283 For the C3x use the 24-bit MPYI instruction for integer multiplies
12284 instead of a library call to guarantee 32-bit results. Note that if one
12285 of the operands is a constant, then the multiplication will be performed
12286 using shifts and adds. If the @option{-mmpyi} option is not specified for the C3x,
12287 then squaring operations are performed inline instead of a library call.
12290 @itemx -mno-fast-fix
12292 @opindex mno-fast-fix
12293 The C3x/C4x FIX instruction to convert a floating point value to an
12294 integer value chooses the nearest integer less than or equal to the
12295 floating point value rather than to the nearest integer. Thus if the
12296 floating point number is negative, the result will be incorrectly
12297 truncated an additional code is necessary to detect and correct this
12298 case. This option can be used to disable generation of the additional
12299 code required to correct the result.
12305 Enable (disable) generation of repeat block sequences using the RPTB
12306 instruction for zero overhead looping. The RPTB construct is only used
12307 for innermost loops that do not call functions or jump across the loop
12308 boundaries. There is no advantage having nested RPTB loops due to the
12309 overhead required to save and restore the RC, RS, and RE registers.
12310 This is enabled by default with @option{-O2}.
12312 @item -mrpts=@var{count}
12316 Enable (disable) the use of the single instruction repeat instruction
12317 RPTS@. If a repeat block contains a single instruction, and the loop
12318 count can be guaranteed to be less than the value @var{count}, GCC will
12319 emit a RPTS instruction instead of a RPTB@. If no value is specified,
12320 then a RPTS will be emitted even if the loop count cannot be determined
12321 at compile time. Note that the repeated instruction following RPTS does
12322 not have to be reloaded from memory each iteration, thus freeing up the
12323 CPU buses for operands. However, since interrupts are blocked by this
12324 instruction, it is disabled by default.
12326 @item -mloop-unsigned
12327 @itemx -mno-loop-unsigned
12328 @opindex mloop-unsigned
12329 @opindex mno-loop-unsigned
12330 The maximum iteration count when using RPTS and RPTB (and DB on the C40)
12331 is @math{2^{31} + 1} since these instructions test if the iteration count is
12332 negative to terminate the loop. If the iteration count is unsigned
12333 there is a possibility than the @math{2^{31} + 1} maximum iteration count may be
12334 exceeded. This switch allows an unsigned iteration count.
12338 Try to emit an assembler syntax that the TI assembler (asm30) is happy
12339 with. This also enforces compatibility with the API employed by the TI
12340 C3x C compiler. For example, long doubles are passed as structures
12341 rather than in floating point registers.
12347 Generate code that uses registers (stack) for passing arguments to functions.
12348 By default, arguments are passed in registers where possible rather
12349 than by pushing arguments on to the stack.
12351 @item -mparallel-insns
12352 @itemx -mno-parallel-insns
12353 @opindex mparallel-insns
12354 @opindex mno-parallel-insns
12355 Allow the generation of parallel instructions. This is enabled by
12356 default with @option{-O2}.
12358 @item -mparallel-mpy
12359 @itemx -mno-parallel-mpy
12360 @opindex mparallel-mpy
12361 @opindex mno-parallel-mpy
12362 Allow the generation of MPY||ADD and MPY||SUB parallel instructions,
12363 provided @option{-mparallel-insns} is also specified. These instructions have
12364 tight register constraints which can pessimize the code generation
12365 of large functions.
12370 @subsection V850 Options
12371 @cindex V850 Options
12373 These @samp{-m} options are defined for V850 implementations:
12377 @itemx -mno-long-calls
12378 @opindex mlong-calls
12379 @opindex mno-long-calls
12380 Treat all calls as being far away (near). If calls are assumed to be
12381 far away, the compiler will always load the functions address up into a
12382 register, and call indirect through the pointer.
12388 Do not optimize (do optimize) basic blocks that use the same index
12389 pointer 4 or more times to copy pointer into the @code{ep} register, and
12390 use the shorter @code{sld} and @code{sst} instructions. The @option{-mep}
12391 option is on by default if you optimize.
12393 @item -mno-prolog-function
12394 @itemx -mprolog-function
12395 @opindex mno-prolog-function
12396 @opindex mprolog-function
12397 Do not use (do use) external functions to save and restore registers
12398 at the prologue and epilogue of a function. The external functions
12399 are slower, but use less code space if more than one function saves
12400 the same number of registers. The @option{-mprolog-function} option
12401 is on by default if you optimize.
12405 Try to make the code as small as possible. At present, this just turns
12406 on the @option{-mep} and @option{-mprolog-function} options.
12408 @item -mtda=@var{n}
12410 Put static or global variables whose size is @var{n} bytes or less into
12411 the tiny data area that register @code{ep} points to. The tiny data
12412 area can hold up to 256 bytes in total (128 bytes for byte references).
12414 @item -msda=@var{n}
12416 Put static or global variables whose size is @var{n} bytes or less into
12417 the small data area that register @code{gp} points to. The small data
12418 area can hold up to 64 kilobytes.
12420 @item -mzda=@var{n}
12422 Put static or global variables whose size is @var{n} bytes or less into
12423 the first 32 kilobytes of memory.
12427 Specify that the target processor is the V850.
12430 @opindex mbig-switch
12431 Generate code suitable for big switch tables. Use this option only if
12432 the assembler/linker complain about out of range branches within a switch
12437 This option will cause r2 and r5 to be used in the code generated by
12438 the compiler. This setting is the default.
12440 @item -mno-app-regs
12441 @opindex mno-app-regs
12442 This option will cause r2 and r5 to be treated as fixed registers.
12446 Specify that the target processor is the V850E1. The preprocessor
12447 constants @samp{__v850e1__} and @samp{__v850e__} will be defined if
12448 this option is used.
12452 Specify that the target processor is the V850E@. The preprocessor
12453 constant @samp{__v850e__} will be defined if this option is used.
12455 If neither @option{-mv850} nor @option{-mv850e} nor @option{-mv850e1}
12456 are defined then a default target processor will be chosen and the
12457 relevant @samp{__v850*__} preprocessor constant will be defined.
12459 The preprocessor constants @samp{__v850} and @samp{__v851__} are always
12460 defined, regardless of which processor variant is the target.
12462 @item -mdisable-callt
12463 @opindex mdisable-callt
12464 This option will suppress generation of the CALLT instruction for the
12465 v850e and v850e1 flavors of the v850 architecture. The default is
12466 @option{-mno-disable-callt} which allows the CALLT instruction to be used.
12471 @subsection VAX Options
12472 @cindex VAX options
12474 These @samp{-m} options are defined for the VAX:
12479 Do not output certain jump instructions (@code{aobleq} and so on)
12480 that the Unix assembler for the VAX cannot handle across long
12485 Do output those jump instructions, on the assumption that you
12486 will assemble with the GNU assembler.
12490 Output code for g-format floating point numbers instead of d-format.
12493 @node x86-64 Options
12494 @subsection x86-64 Options
12495 @cindex x86-64 options
12497 These are listed under @xref{i386 and x86-64 Options}.
12499 @node Xstormy16 Options
12500 @subsection Xstormy16 Options
12501 @cindex Xstormy16 Options
12503 These options are defined for Xstormy16:
12508 Choose startup files and linker script suitable for the simulator.
12511 @node Xtensa Options
12512 @subsection Xtensa Options
12513 @cindex Xtensa Options
12515 These options are supported for Xtensa targets:
12519 @itemx -mno-const16
12521 @opindex mno-const16
12522 Enable or disable use of @code{CONST16} instructions for loading
12523 constant values. The @code{CONST16} instruction is currently not a
12524 standard option from Tensilica. When enabled, @code{CONST16}
12525 instructions are always used in place of the standard @code{L32R}
12526 instructions. The use of @code{CONST16} is enabled by default only if
12527 the @code{L32R} instruction is not available.
12530 @itemx -mno-fused-madd
12531 @opindex mfused-madd
12532 @opindex mno-fused-madd
12533 Enable or disable use of fused multiply/add and multiply/subtract
12534 instructions in the floating-point option. This has no effect if the
12535 floating-point option is not also enabled. Disabling fused multiply/add
12536 and multiply/subtract instructions forces the compiler to use separate
12537 instructions for the multiply and add/subtract operations. This may be
12538 desirable in some cases where strict IEEE 754-compliant results are
12539 required: the fused multiply add/subtract instructions do not round the
12540 intermediate result, thereby producing results with @emph{more} bits of
12541 precision than specified by the IEEE standard. Disabling fused multiply
12542 add/subtract instructions also ensures that the program output is not
12543 sensitive to the compiler's ability to combine multiply and add/subtract
12546 @item -mtext-section-literals
12547 @itemx -mno-text-section-literals
12548 @opindex mtext-section-literals
12549 @opindex mno-text-section-literals
12550 Control the treatment of literal pools. The default is
12551 @option{-mno-text-section-literals}, which places literals in a separate
12552 section in the output file. This allows the literal pool to be placed
12553 in a data RAM/ROM, and it also allows the linker to combine literal
12554 pools from separate object files to remove redundant literals and
12555 improve code size. With @option{-mtext-section-literals}, the literals
12556 are interspersed in the text section in order to keep them as close as
12557 possible to their references. This may be necessary for large assembly
12560 @item -mtarget-align
12561 @itemx -mno-target-align
12562 @opindex mtarget-align
12563 @opindex mno-target-align
12564 When this option is enabled, GCC instructs the assembler to
12565 automatically align instructions to reduce branch penalties at the
12566 expense of some code density. The assembler attempts to widen density
12567 instructions to align branch targets and the instructions following call
12568 instructions. If there are not enough preceding safe density
12569 instructions to align a target, no widening will be performed. The
12570 default is @option{-mtarget-align}. These options do not affect the
12571 treatment of auto-aligned instructions like @code{LOOP}, which the
12572 assembler will always align, either by widening density instructions or
12573 by inserting no-op instructions.
12576 @itemx -mno-longcalls
12577 @opindex mlongcalls
12578 @opindex mno-longcalls
12579 When this option is enabled, GCC instructs the assembler to translate
12580 direct calls to indirect calls unless it can determine that the target
12581 of a direct call is in the range allowed by the call instruction. This
12582 translation typically occurs for calls to functions in other source
12583 files. Specifically, the assembler translates a direct @code{CALL}
12584 instruction into an @code{L32R} followed by a @code{CALLX} instruction.
12585 The default is @option{-mno-longcalls}. This option should be used in
12586 programs where the call target can potentially be out of range. This
12587 option is implemented in the assembler, not the compiler, so the
12588 assembly code generated by GCC will still show direct call
12589 instructions---look at the disassembled object code to see the actual
12590 instructions. Note that the assembler will use an indirect call for
12591 every cross-file call, not just those that really will be out of range.
12594 @node zSeries Options
12595 @subsection zSeries Options
12596 @cindex zSeries options
12598 These are listed under @xref{S/390 and zSeries Options}.
12600 @node Code Gen Options
12601 @section Options for Code Generation Conventions
12602 @cindex code generation conventions
12603 @cindex options, code generation
12604 @cindex run-time options
12606 These machine-independent options control the interface conventions
12607 used in code generation.
12609 Most of them have both positive and negative forms; the negative form
12610 of @option{-ffoo} would be @option{-fno-foo}. In the table below, only
12611 one of the forms is listed---the one which is not the default. You
12612 can figure out the other form by either removing @samp{no-} or adding
12616 @item -fbounds-check
12617 @opindex fbounds-check
12618 For front-ends that support it, generate additional code to check that
12619 indices used to access arrays are within the declared range. This is
12620 currently only supported by the Java and Fortran 77 front-ends, where
12621 this option defaults to true and false respectively.
12625 This option generates traps for signed overflow on addition, subtraction,
12626 multiplication operations.
12630 This option instructs the compiler to assume that signed arithmetic
12631 overflow of addition, subtraction and multiplication wraps around
12632 using twos-complement representation. This flag enables some optimizations
12633 and disables others. This option is enabled by default for the Java
12634 front-end, as required by the Java language specification.
12637 @opindex fexceptions
12638 Enable exception handling. Generates extra code needed to propagate
12639 exceptions. For some targets, this implies GCC will generate frame
12640 unwind information for all functions, which can produce significant data
12641 size overhead, although it does not affect execution. If you do not
12642 specify this option, GCC will enable it by default for languages like
12643 C++ which normally require exception handling, and disable it for
12644 languages like C that do not normally require it. However, you may need
12645 to enable this option when compiling C code that needs to interoperate
12646 properly with exception handlers written in C++. You may also wish to
12647 disable this option if you are compiling older C++ programs that don't
12648 use exception handling.
12650 @item -fnon-call-exceptions
12651 @opindex fnon-call-exceptions
12652 Generate code that allows trapping instructions to throw exceptions.
12653 Note that this requires platform-specific runtime support that does
12654 not exist everywhere. Moreover, it only allows @emph{trapping}
12655 instructions to throw exceptions, i.e.@: memory references or floating
12656 point instructions. It does not allow exceptions to be thrown from
12657 arbitrary signal handlers such as @code{SIGALRM}.
12659 @item -funwind-tables
12660 @opindex funwind-tables
12661 Similar to @option{-fexceptions}, except that it will just generate any needed
12662 static data, but will not affect the generated code in any other way.
12663 You will normally not enable this option; instead, a language processor
12664 that needs this handling would enable it on your behalf.
12666 @item -fasynchronous-unwind-tables
12667 @opindex fasynchronous-unwind-tables
12668 Generate unwind table in dwarf2 format, if supported by target machine. The
12669 table is exact at each instruction boundary, so it can be used for stack
12670 unwinding from asynchronous events (such as debugger or garbage collector).
12672 @item -fpcc-struct-return
12673 @opindex fpcc-struct-return
12674 Return ``short'' @code{struct} and @code{union} values in memory like
12675 longer ones, rather than in registers. This convention is less
12676 efficient, but it has the advantage of allowing intercallability between
12677 GCC-compiled files and files compiled with other compilers, particularly
12678 the Portable C Compiler (pcc).
12680 The precise convention for returning structures in memory depends
12681 on the target configuration macros.
12683 Short structures and unions are those whose size and alignment match
12684 that of some integer type.
12686 @strong{Warning:} code compiled with the @option{-fpcc-struct-return}
12687 switch is not binary compatible with code compiled with the
12688 @option{-freg-struct-return} switch.
12689 Use it to conform to a non-default application binary interface.
12691 @item -freg-struct-return
12692 @opindex freg-struct-return
12693 Return @code{struct} and @code{union} values in registers when possible.
12694 This is more efficient for small structures than
12695 @option{-fpcc-struct-return}.
12697 If you specify neither @option{-fpcc-struct-return} nor
12698 @option{-freg-struct-return}, GCC defaults to whichever convention is
12699 standard for the target. If there is no standard convention, GCC
12700 defaults to @option{-fpcc-struct-return}, except on targets where GCC is
12701 the principal compiler. In those cases, we can choose the standard, and
12702 we chose the more efficient register return alternative.
12704 @strong{Warning:} code compiled with the @option{-freg-struct-return}
12705 switch is not binary compatible with code compiled with the
12706 @option{-fpcc-struct-return} switch.
12707 Use it to conform to a non-default application binary interface.
12709 @item -fshort-enums
12710 @opindex fshort-enums
12711 Allocate to an @code{enum} type only as many bytes as it needs for the
12712 declared range of possible values. Specifically, the @code{enum} type
12713 will be equivalent to the smallest integer type which has enough room.
12715 @strong{Warning:} the @option{-fshort-enums} switch causes GCC to generate
12716 code that is not binary compatible with code generated without that switch.
12717 Use it to conform to a non-default application binary interface.
12719 @item -fshort-double
12720 @opindex fshort-double
12721 Use the same size for @code{double} as for @code{float}.
12723 @strong{Warning:} the @option{-fshort-double} switch causes GCC to generate
12724 code that is not binary compatible with code generated without that switch.
12725 Use it to conform to a non-default application binary interface.
12727 @item -fshort-wchar
12728 @opindex fshort-wchar
12729 Override the underlying type for @samp{wchar_t} to be @samp{short
12730 unsigned int} instead of the default for the target. This option is
12731 useful for building programs to run under WINE@.
12733 @strong{Warning:} the @option{-fshort-wchar} switch causes GCC to generate
12734 code that is not binary compatible with code generated without that switch.
12735 Use it to conform to a non-default application binary interface.
12737 @item -fshared-data
12738 @opindex fshared-data
12739 Requests that the data and non-@code{const} variables of this
12740 compilation be shared data rather than private data. The distinction
12741 makes sense only on certain operating systems, where shared data is
12742 shared between processes running the same program, while private data
12743 exists in one copy per process.
12746 @opindex fno-common
12747 In C, allocate even uninitialized global variables in the data section of the
12748 object file, rather than generating them as common blocks. This has the
12749 effect that if the same variable is declared (without @code{extern}) in
12750 two different compilations, you will get an error when you link them.
12751 The only reason this might be useful is if you wish to verify that the
12752 program will work on other systems which always work this way.
12756 Ignore the @samp{#ident} directive.
12758 @item -finhibit-size-directive
12759 @opindex finhibit-size-directive
12760 Don't output a @code{.size} assembler directive, or anything else that
12761 would cause trouble if the function is split in the middle, and the
12762 two halves are placed at locations far apart in memory. This option is
12763 used when compiling @file{crtstuff.c}; you should not need to use it
12766 @item -fverbose-asm
12767 @opindex fverbose-asm
12768 Put extra commentary information in the generated assembly code to
12769 make it more readable. This option is generally only of use to those
12770 who actually need to read the generated assembly code (perhaps while
12771 debugging the compiler itself).
12773 @option{-fno-verbose-asm}, the default, causes the
12774 extra information to be omitted and is useful when comparing two assembler
12779 @cindex global offset table
12781 Generate position-independent code (PIC) suitable for use in a shared
12782 library, if supported for the target machine. Such code accesses all
12783 constant addresses through a global offset table (GOT)@. The dynamic
12784 loader resolves the GOT entries when the program starts (the dynamic
12785 loader is not part of GCC; it is part of the operating system). If
12786 the GOT size for the linked executable exceeds a machine-specific
12787 maximum size, you get an error message from the linker indicating that
12788 @option{-fpic} does not work; in that case, recompile with @option{-fPIC}
12789 instead. (These maximums are 8k on the SPARC and 32k
12790 on the m68k and RS/6000. The 386 has no such limit.)
12792 Position-independent code requires special support, and therefore works
12793 only on certain machines. For the 386, GCC supports PIC for System V
12794 but not for the Sun 386i. Code generated for the IBM RS/6000 is always
12795 position-independent.
12799 If supported for the target machine, emit position-independent code,
12800 suitable for dynamic linking and avoiding any limit on the size of the
12801 global offset table. This option makes a difference on the m68k,
12802 PowerPC and SPARC@.
12804 Position-independent code requires special support, and therefore works
12805 only on certain machines.
12811 These options are similar to @option{-fpic} and @option{-fPIC}, but
12812 generated position independent code can be only linked into executables.
12813 Usually these options are used when @option{-pie} GCC option will be
12814 used during linking.
12816 @item -fno-jump-tables
12817 @opindex fno-jump-tables
12818 Do not use jump tables for switch statements even where it would be
12819 more efficient than other code generation strategies. This option is
12820 of use in conjunction with @option{-fpic} or @option{-fPIC} for
12821 building code which forms part of a dynamic linker and cannot
12822 reference the address of a jump table. On some targets, jump tables
12823 do not require a GOT and this option is not needed.
12825 @item -ffixed-@var{reg}
12827 Treat the register named @var{reg} as a fixed register; generated code
12828 should never refer to it (except perhaps as a stack pointer, frame
12829 pointer or in some other fixed role).
12831 @var{reg} must be the name of a register. The register names accepted
12832 are machine-specific and are defined in the @code{REGISTER_NAMES}
12833 macro in the machine description macro file.
12835 This flag does not have a negative form, because it specifies a
12838 @item -fcall-used-@var{reg}
12839 @opindex fcall-used
12840 Treat the register named @var{reg} as an allocable register that is
12841 clobbered by function calls. It may be allocated for temporaries or
12842 variables that do not live across a call. Functions compiled this way
12843 will not save and restore the register @var{reg}.
12845 It is an error to used this flag with the frame pointer or stack pointer.
12846 Use of this flag for other registers that have fixed pervasive roles in
12847 the machine's execution model will produce disastrous results.
12849 This flag does not have a negative form, because it specifies a
12852 @item -fcall-saved-@var{reg}
12853 @opindex fcall-saved
12854 Treat the register named @var{reg} as an allocable register saved by
12855 functions. It may be allocated even for temporaries or variables that
12856 live across a call. Functions compiled this way will save and restore
12857 the register @var{reg} if they use it.
12859 It is an error to used this flag with the frame pointer or stack pointer.
12860 Use of this flag for other registers that have fixed pervasive roles in
12861 the machine's execution model will produce disastrous results.
12863 A different sort of disaster will result from the use of this flag for
12864 a register in which function values may be returned.
12866 This flag does not have a negative form, because it specifies a
12869 @item -fpack-struct[=@var{n}]
12870 @opindex fpack-struct
12871 Without a value specified, pack all structure members together without
12872 holes. When a value is specified (which must be a small power of two), pack
12873 structure members according to this value, representing the maximum
12874 alignment (that is, objects with default alignment requirements larger than
12875 this will be output potentially unaligned at the next fitting location.
12877 @strong{Warning:} the @option{-fpack-struct} switch causes GCC to generate
12878 code that is not binary compatible with code generated without that switch.
12879 Additionally, it makes the code suboptimal.
12880 Use it to conform to a non-default application binary interface.
12882 @item -finstrument-functions
12883 @opindex finstrument-functions
12884 Generate instrumentation calls for entry and exit to functions. Just
12885 after function entry and just before function exit, the following
12886 profiling functions will be called with the address of the current
12887 function and its call site. (On some platforms,
12888 @code{__builtin_return_address} does not work beyond the current
12889 function, so the call site information may not be available to the
12890 profiling functions otherwise.)
12893 void __cyg_profile_func_enter (void *this_fn,
12895 void __cyg_profile_func_exit (void *this_fn,
12899 The first argument is the address of the start of the current function,
12900 which may be looked up exactly in the symbol table.
12902 This instrumentation is also done for functions expanded inline in other
12903 functions. The profiling calls will indicate where, conceptually, the
12904 inline function is entered and exited. This means that addressable
12905 versions of such functions must be available. If all your uses of a
12906 function are expanded inline, this may mean an additional expansion of
12907 code size. If you use @samp{extern inline} in your C code, an
12908 addressable version of such functions must be provided. (This is
12909 normally the case anyways, but if you get lucky and the optimizer always
12910 expands the functions inline, you might have gotten away without
12911 providing static copies.)
12913 A function may be given the attribute @code{no_instrument_function}, in
12914 which case this instrumentation will not be done. This can be used, for
12915 example, for the profiling functions listed above, high-priority
12916 interrupt routines, and any functions from which the profiling functions
12917 cannot safely be called (perhaps signal handlers, if the profiling
12918 routines generate output or allocate memory).
12920 @item -fstack-check
12921 @opindex fstack-check
12922 Generate code to verify that you do not go beyond the boundary of the
12923 stack. You should specify this flag if you are running in an
12924 environment with multiple threads, but only rarely need to specify it in
12925 a single-threaded environment since stack overflow is automatically
12926 detected on nearly all systems if there is only one stack.
12928 Note that this switch does not actually cause checking to be done; the
12929 operating system must do that. The switch causes generation of code
12930 to ensure that the operating system sees the stack being extended.
12932 @item -fstack-limit-register=@var{reg}
12933 @itemx -fstack-limit-symbol=@var{sym}
12934 @itemx -fno-stack-limit
12935 @opindex fstack-limit-register
12936 @opindex fstack-limit-symbol
12937 @opindex fno-stack-limit
12938 Generate code to ensure that the stack does not grow beyond a certain value,
12939 either the value of a register or the address of a symbol. If the stack
12940 would grow beyond the value, a signal is raised. For most targets,
12941 the signal is raised before the stack overruns the boundary, so
12942 it is possible to catch the signal without taking special precautions.
12944 For instance, if the stack starts at absolute address @samp{0x80000000}
12945 and grows downwards, you can use the flags
12946 @option{-fstack-limit-symbol=__stack_limit} and
12947 @option{-Wl,--defsym,__stack_limit=0x7ffe0000} to enforce a stack limit
12948 of 128KB@. Note that this may only work with the GNU linker.
12950 @cindex aliasing of parameters
12951 @cindex parameters, aliased
12952 @item -fargument-alias
12953 @itemx -fargument-noalias
12954 @itemx -fargument-noalias-global
12955 @opindex fargument-alias
12956 @opindex fargument-noalias
12957 @opindex fargument-noalias-global
12958 Specify the possible relationships among parameters and between
12959 parameters and global data.
12961 @option{-fargument-alias} specifies that arguments (parameters) may
12962 alias each other and may alias global storage.@*
12963 @option{-fargument-noalias} specifies that arguments do not alias
12964 each other, but may alias global storage.@*
12965 @option{-fargument-noalias-global} specifies that arguments do not
12966 alias each other and do not alias global storage.
12968 Each language will automatically use whatever option is required by
12969 the language standard. You should not need to use these options yourself.
12971 @item -fleading-underscore
12972 @opindex fleading-underscore
12973 This option and its counterpart, @option{-fno-leading-underscore}, forcibly
12974 change the way C symbols are represented in the object file. One use
12975 is to help link with legacy assembly code.
12977 @strong{Warning:} the @option{-fleading-underscore} switch causes GCC to
12978 generate code that is not binary compatible with code generated without that
12979 switch. Use it to conform to a non-default application binary interface.
12980 Not all targets provide complete support for this switch.
12982 @item -ftls-model=@var{model}
12983 Alter the thread-local storage model to be used (@pxref{Thread-Local}).
12984 The @var{model} argument should be one of @code{global-dynamic},
12985 @code{local-dynamic}, @code{initial-exec} or @code{local-exec}.
12987 The default without @option{-fpic} is @code{initial-exec}; with
12988 @option{-fpic} the default is @code{global-dynamic}.
12990 @item -fvisibility=@var{default|internal|hidden|protected}
12991 @opindex fvisibility
12992 Set the default ELF image symbol visibility to the specified option---all
12993 symbols will be marked with this unless overridden within the code.
12994 Using this feature can very substantially improve linking and
12995 load times of shared object libraries, produce more optimized
12996 code, provide near-perfect API export and prevent symbol clashes.
12997 It is @strong{strongly} recommended that you use this in any shared objects
13000 Despite the nomenclature, @code{default} always means public ie;
13001 available to be linked against from outside the shared object.
13002 @code{protected} and @code{internal} are pretty useless in real-world
13003 usage so the only other commonly used option will be @code{hidden}.
13004 The default if @option{-fvisibility} isn't specified is
13005 @code{default}, i.e., make every
13006 symbol public---this causes the same behavior as previous versions of
13009 A good explanation of the benefits offered by ensuring ELF
13010 symbols have the correct visibility is given by ``How To Write
13011 Shared Libraries'' by Ulrich Drepper (which can be found at
13012 @w{@uref{http://people.redhat.com/~drepper/}})---however a superior
13013 solution made possible by this option to marking things hidden when
13014 the default is public is to make the default hidden and mark things
13015 public. This is the norm with DLL's on Windows and with @option{-fvisibility=hidden}
13016 and @code{__attribute__ ((visibility("default")))} instead of
13017 @code{__declspec(dllexport)} you get almost identical semantics with
13018 identical syntax. This is a great boon to those working with
13019 cross-platform projects.
13021 For those adding visibility support to existing code, you may find
13022 @samp{#pragma GCC visibility} of use. This works by you enclosing
13023 the declarations you wish to set visibility for with (for example)
13024 @samp{#pragma GCC visibility push(hidden)} and
13025 @samp{#pragma GCC visibility pop}.
13026 Bear in mind that symbol visibility should be viewed @strong{as
13027 part of the API interface contract} and thus all new code should
13028 always specify visibility when it is not the default ie; declarations
13029 only for use within the local DSO should @strong{always} be marked explicitly
13030 as hidden as so to avoid PLT indirection overheads---making this
13031 abundantly clear also aids readability and self-documentation of the code.
13032 Note that due to ISO C++ specification requirements, operator new and
13033 operator delete must always be of default visibility.
13035 An overview of these techniques, their benefits and how to use them
13036 is at @w{@uref{http://gcc.gnu.org/wiki/Visibility}}.
13042 @node Environment Variables
13043 @section Environment Variables Affecting GCC
13044 @cindex environment variables
13046 @c man begin ENVIRONMENT
13047 This section describes several environment variables that affect how GCC
13048 operates. Some of them work by specifying directories or prefixes to use
13049 when searching for various kinds of files. Some are used to specify other
13050 aspects of the compilation environment.
13052 Note that you can also specify places to search using options such as
13053 @option{-B}, @option{-I} and @option{-L} (@pxref{Directory Options}). These
13054 take precedence over places specified using environment variables, which
13055 in turn take precedence over those specified by the configuration of GCC@.
13056 @xref{Driver,, Controlling the Compilation Driver @file{gcc}, gccint,
13057 GNU Compiler Collection (GCC) Internals}.
13062 @c @itemx LC_COLLATE
13064 @c @itemx LC_MONETARY
13065 @c @itemx LC_NUMERIC
13070 @c @findex LC_COLLATE
13071 @findex LC_MESSAGES
13072 @c @findex LC_MONETARY
13073 @c @findex LC_NUMERIC
13077 These environment variables control the way that GCC uses
13078 localization information that allow GCC to work with different
13079 national conventions. GCC inspects the locale categories
13080 @env{LC_CTYPE} and @env{LC_MESSAGES} if it has been configured to do
13081 so. These locale categories can be set to any value supported by your
13082 installation. A typical value is @samp{en_GB.UTF-8} for English in the United
13083 Kingdom encoded in UTF-8.
13085 The @env{LC_CTYPE} environment variable specifies character
13086 classification. GCC uses it to determine the character boundaries in
13087 a string; this is needed for some multibyte encodings that contain quote
13088 and escape characters that would otherwise be interpreted as a string
13091 The @env{LC_MESSAGES} environment variable specifies the language to
13092 use in diagnostic messages.
13094 If the @env{LC_ALL} environment variable is set, it overrides the value
13095 of @env{LC_CTYPE} and @env{LC_MESSAGES}; otherwise, @env{LC_CTYPE}
13096 and @env{LC_MESSAGES} default to the value of the @env{LANG}
13097 environment variable. If none of these variables are set, GCC
13098 defaults to traditional C English behavior.
13102 If @env{TMPDIR} is set, it specifies the directory to use for temporary
13103 files. GCC uses temporary files to hold the output of one stage of
13104 compilation which is to be used as input to the next stage: for example,
13105 the output of the preprocessor, which is the input to the compiler
13108 @item GCC_EXEC_PREFIX
13109 @findex GCC_EXEC_PREFIX
13110 If @env{GCC_EXEC_PREFIX} is set, it specifies a prefix to use in the
13111 names of the subprograms executed by the compiler. No slash is added
13112 when this prefix is combined with the name of a subprogram, but you can
13113 specify a prefix that ends with a slash if you wish.
13115 If @env{GCC_EXEC_PREFIX} is not set, GCC will attempt to figure out
13116 an appropriate prefix to use based on the pathname it was invoked with.
13118 If GCC cannot find the subprogram using the specified prefix, it
13119 tries looking in the usual places for the subprogram.
13121 The default value of @env{GCC_EXEC_PREFIX} is
13122 @file{@var{prefix}/lib/gcc/} where @var{prefix} is the value
13123 of @code{prefix} when you ran the @file{configure} script.
13125 Other prefixes specified with @option{-B} take precedence over this prefix.
13127 This prefix is also used for finding files such as @file{crt0.o} that are
13130 In addition, the prefix is used in an unusual way in finding the
13131 directories to search for header files. For each of the standard
13132 directories whose name normally begins with @samp{/usr/local/lib/gcc}
13133 (more precisely, with the value of @env{GCC_INCLUDE_DIR}), GCC tries
13134 replacing that beginning with the specified prefix to produce an
13135 alternate directory name. Thus, with @option{-Bfoo/}, GCC will search
13136 @file{foo/bar} where it would normally search @file{/usr/local/lib/bar}.
13137 These alternate directories are searched first; the standard directories
13140 @item COMPILER_PATH
13141 @findex COMPILER_PATH
13142 The value of @env{COMPILER_PATH} is a colon-separated list of
13143 directories, much like @env{PATH}. GCC tries the directories thus
13144 specified when searching for subprograms, if it can't find the
13145 subprograms using @env{GCC_EXEC_PREFIX}.
13148 @findex LIBRARY_PATH
13149 The value of @env{LIBRARY_PATH} is a colon-separated list of
13150 directories, much like @env{PATH}. When configured as a native compiler,
13151 GCC tries the directories thus specified when searching for special
13152 linker files, if it can't find them using @env{GCC_EXEC_PREFIX}. Linking
13153 using GCC also uses these directories when searching for ordinary
13154 libraries for the @option{-l} option (but directories specified with
13155 @option{-L} come first).
13159 @cindex locale definition
13160 This variable is used to pass locale information to the compiler. One way in
13161 which this information is used is to determine the character set to be used
13162 when character literals, string literals and comments are parsed in C and C++.
13163 When the compiler is configured to allow multibyte characters,
13164 the following values for @env{LANG} are recognized:
13168 Recognize JIS characters.
13170 Recognize SJIS characters.
13172 Recognize EUCJP characters.
13175 If @env{LANG} is not defined, or if it has some other value, then the
13176 compiler will use mblen and mbtowc as defined by the default locale to
13177 recognize and translate multibyte characters.
13181 Some additional environments variables affect the behavior of the
13184 @include cppenv.texi
13188 @node Precompiled Headers
13189 @section Using Precompiled Headers
13190 @cindex precompiled headers
13191 @cindex speed of compilation
13193 Often large projects have many header files that are included in every
13194 source file. The time the compiler takes to process these header files
13195 over and over again can account for nearly all of the time required to
13196 build the project. To make builds faster, GCC allows users to
13197 `precompile' a header file; then, if builds can use the precompiled
13198 header file they will be much faster.
13200 To create a precompiled header file, simply compile it as you would any
13201 other file, if necessary using the @option{-x} option to make the driver
13202 treat it as a C or C++ header file. You will probably want to use a
13203 tool like @command{make} to keep the precompiled header up-to-date when
13204 the headers it contains change.
13206 A precompiled header file will be searched for when @code{#include} is
13207 seen in the compilation. As it searches for the included file
13208 (@pxref{Search Path,,Search Path,cpp,The C Preprocessor}) the
13209 compiler looks for a precompiled header in each directory just before it
13210 looks for the include file in that directory. The name searched for is
13211 the name specified in the @code{#include} with @samp{.gch} appended. If
13212 the precompiled header file can't be used, it is ignored.
13214 For instance, if you have @code{#include "all.h"}, and you have
13215 @file{all.h.gch} in the same directory as @file{all.h}, then the
13216 precompiled header file will be used if possible, and the original
13217 header will be used otherwise.
13219 Alternatively, you might decide to put the precompiled header file in a
13220 directory and use @option{-I} to ensure that directory is searched
13221 before (or instead of) the directory containing the original header.
13222 Then, if you want to check that the precompiled header file is always
13223 used, you can put a file of the same name as the original header in this
13224 directory containing an @code{#error} command.
13226 This also works with @option{-include}. So yet another way to use
13227 precompiled headers, good for projects not designed with precompiled
13228 header files in mind, is to simply take most of the header files used by
13229 a project, include them from another header file, precompile that header
13230 file, and @option{-include} the precompiled header. If the header files
13231 have guards against multiple inclusion, they will be skipped because
13232 they've already been included (in the precompiled header).
13234 If you need to precompile the same header file for different
13235 languages, targets, or compiler options, you can instead make a
13236 @emph{directory} named like @file{all.h.gch}, and put each precompiled
13237 header in the directory, perhaps using @option{-o}. It doesn't matter
13238 what you call the files in the directory, every precompiled header in
13239 the directory will be considered. The first precompiled header
13240 encountered in the directory that is valid for this compilation will
13241 be used; they're searched in no particular order.
13243 There are many other possibilities, limited only by your imagination,
13244 good sense, and the constraints of your build system.
13246 A precompiled header file can be used only when these conditions apply:
13250 Only one precompiled header can be used in a particular compilation.
13253 A precompiled header can't be used once the first C token is seen. You
13254 can have preprocessor directives before a precompiled header; you can
13255 even include a precompiled header from inside another header, so long as
13256 there are no C tokens before the @code{#include}.
13259 The precompiled header file must be produced for the same language as
13260 the current compilation. You can't use a C precompiled header for a C++
13264 The precompiled header file must have been produced by the same compiler
13265 binary as the current compilation is using.
13268 Any macros defined before the precompiled header is included must
13269 either be defined in the same way as when the precompiled header was
13270 generated, or must not affect the precompiled header, which usually
13271 means that they don't appear in the precompiled header at all.
13273 The @option{-D} option is one way to define a macro before a
13274 precompiled header is included; using a @code{#define} can also do it.
13275 There are also some options that define macros implicitly, like
13276 @option{-O} and @option{-Wdeprecated}; the same rule applies to macros
13279 @item If debugging information is output when using the precompiled
13280 header, using @option{-g} or similar, the same kind of debugging information
13281 must have been output when building the precompiled header. However,
13282 a precompiled header built using @option{-g} can be used in a compilation
13283 when no debugging information is being output.
13285 @item The same @option{-m} options must generally be used when building
13286 and using the precompiled header. @xref{Submodel Options},
13287 for any cases where this rule is relaxed.
13289 @item Each of the following options must be the same when building and using
13290 the precompiled header:
13292 @gccoptlist{-fexceptions -funit-at-a-time}
13295 Some other command-line options starting with @option{-f},
13296 @option{-p}, or @option{-O} must be defined in the same way as when
13297 the precompiled header was generated. At present, it's not clear
13298 which options are safe to change and which are not; the safest choice
13299 is to use exactly the same options when generating and using the
13300 precompiled header. The following are known to be safe:
13302 @gccoptlist{-fmessage-length= -fpreprocessed
13303 -fsched-interblock -fsched-spec -fsched-spec-load -fsched-spec-load-dangerous
13304 -fsched-verbose=<number> -fschedule-insns -fvisibility=
13309 For all of these except the last, the compiler will automatically
13310 ignore the precompiled header if the conditions aren't met. If you
13311 find an option combination that doesn't work and doesn't cause the
13312 precompiled header to be ignored, please consider filing a bug report,
13315 If you do use differing options when generating and using the
13316 precompiled header, the actual behavior will be a mixture of the
13317 behavior for the options. For instance, if you use @option{-g} to
13318 generate the precompiled header but not when using it, you may or may
13319 not get debugging information for routines in the precompiled header.
13321 @node Running Protoize
13322 @section Running Protoize
13324 The program @code{protoize} is an optional part of GCC@. You can use
13325 it to add prototypes to a program, thus converting the program to ISO
13326 C in one respect. The companion program @code{unprotoize} does the
13327 reverse: it removes argument types from any prototypes that are found.
13329 When you run these programs, you must specify a set of source files as
13330 command line arguments. The conversion programs start out by compiling
13331 these files to see what functions they define. The information gathered
13332 about a file @var{foo} is saved in a file named @file{@var{foo}.X}.
13334 After scanning comes actual conversion. The specified files are all
13335 eligible to be converted; any files they include (whether sources or
13336 just headers) are eligible as well.
13338 But not all the eligible files are converted. By default,
13339 @code{protoize} and @code{unprotoize} convert only source and header
13340 files in the current directory. You can specify additional directories
13341 whose files should be converted with the @option{-d @var{directory}}
13342 option. You can also specify particular files to exclude with the
13343 @option{-x @var{file}} option. A file is converted if it is eligible, its
13344 directory name matches one of the specified directory names, and its
13345 name within the directory has not been excluded.
13347 Basic conversion with @code{protoize} consists of rewriting most
13348 function definitions and function declarations to specify the types of
13349 the arguments. The only ones not rewritten are those for varargs
13352 @code{protoize} optionally inserts prototype declarations at the
13353 beginning of the source file, to make them available for any calls that
13354 precede the function's definition. Or it can insert prototype
13355 declarations with block scope in the blocks where undeclared functions
13358 Basic conversion with @code{unprotoize} consists of rewriting most
13359 function declarations to remove any argument types, and rewriting
13360 function definitions to the old-style pre-ISO form.
13362 Both conversion programs print a warning for any function declaration or
13363 definition that they can't convert. You can suppress these warnings
13366 The output from @code{protoize} or @code{unprotoize} replaces the
13367 original source file. The original file is renamed to a name ending
13368 with @samp{.save} (for DOS, the saved filename ends in @samp{.sav}
13369 without the original @samp{.c} suffix). If the @samp{.save} (@samp{.sav}
13370 for DOS) file already exists, then the source file is simply discarded.
13372 @code{protoize} and @code{unprotoize} both depend on GCC itself to
13373 scan the program and collect information about the functions it uses.
13374 So neither of these programs will work until GCC is installed.
13376 Here is a table of the options you can use with @code{protoize} and
13377 @code{unprotoize}. Each option works with both programs unless
13381 @item -B @var{directory}
13382 Look for the file @file{SYSCALLS.c.X} in @var{directory}, instead of the
13383 usual directory (normally @file{/usr/local/lib}). This file contains
13384 prototype information about standard system functions. This option
13385 applies only to @code{protoize}.
13387 @item -c @var{compilation-options}
13388 Use @var{compilation-options} as the options when running @command{gcc} to
13389 produce the @samp{.X} files. The special option @option{-aux-info} is
13390 always passed in addition, to tell @command{gcc} to write a @samp{.X} file.
13392 Note that the compilation options must be given as a single argument to
13393 @code{protoize} or @code{unprotoize}. If you want to specify several
13394 @command{gcc} options, you must quote the entire set of compilation options
13395 to make them a single word in the shell.
13397 There are certain @command{gcc} arguments that you cannot use, because they
13398 would produce the wrong kind of output. These include @option{-g},
13399 @option{-O}, @option{-c}, @option{-S}, and @option{-o} If you include these in
13400 the @var{compilation-options}, they are ignored.
13403 Rename files to end in @samp{.C} (@samp{.cc} for DOS-based file
13404 systems) instead of @samp{.c}. This is convenient if you are converting
13405 a C program to C++. This option applies only to @code{protoize}.
13408 Add explicit global declarations. This means inserting explicit
13409 declarations at the beginning of each source file for each function
13410 that is called in the file and was not declared. These declarations
13411 precede the first function definition that contains a call to an
13412 undeclared function. This option applies only to @code{protoize}.
13414 @item -i @var{string}
13415 Indent old-style parameter declarations with the string @var{string}.
13416 This option applies only to @code{protoize}.
13418 @code{unprotoize} converts prototyped function definitions to old-style
13419 function definitions, where the arguments are declared between the
13420 argument list and the initial @samp{@{}. By default, @code{unprotoize}
13421 uses five spaces as the indentation. If you want to indent with just
13422 one space instead, use @option{-i " "}.
13425 Keep the @samp{.X} files. Normally, they are deleted after conversion
13429 Add explicit local declarations. @code{protoize} with @option{-l} inserts
13430 a prototype declaration for each function in each block which calls the
13431 function without any declaration. This option applies only to
13435 Make no real changes. This mode just prints information about the conversions
13436 that would have been done without @option{-n}.
13439 Make no @samp{.save} files. The original files are simply deleted.
13440 Use this option with caution.
13442 @item -p @var{program}
13443 Use the program @var{program} as the compiler. Normally, the name
13444 @file{gcc} is used.
13447 Work quietly. Most warnings are suppressed.
13450 Print the version number, just like @option{-v} for @command{gcc}.
13453 If you need special compiler options to compile one of your program's
13454 source files, then you should generate that file's @samp{.X} file
13455 specially, by running @command{gcc} on that source file with the
13456 appropriate options and the option @option{-aux-info}. Then run
13457 @code{protoize} on the entire set of files. @code{protoize} will use
13458 the existing @samp{.X} file because it is newer than the source file.
13462 gcc -Dfoo=bar file1.c -aux-info file1.X
13467 You need to include the special files along with the rest in the
13468 @code{protoize} command, even though their @samp{.X} files already
13469 exist, because otherwise they won't get converted.
13471 @xref{Protoize Caveats}, for more information on how to use
13472 @code{protoize} successfully.