- Dump after fixing rtl statements that have unsatisfied in/out constraints.
- -fdump-rtl-auto_inc_dec
- Dump after auto-inc-dec discovery. This pass is only run on architectures that have auto inc or auto dec instructions.
- -fdump-rtl-barriers
- Dump after cleaning up the barrier instructions.
- -fdump-rtl-bbpart
- Dump after partitioning hot and cold basic blocks.
- -fdump-rtl-bbro
- Dump after block reordering.
- -fdump-rtl-btl1
- -fdump-rtl-btl2
- -fdump-rtl-btl1 and -fdump-rtl-btl2 enable dumping after the two branch target load optimization passes.
- -fdump-rtl-bypass
- Dump after jump bypassing and control flow optimizations.
- -fdump-rtl-combine
- Dump after the RTL instruction combination pass.
- -fdump-rtl-compgotos
- Dump after duplicating the computed gotos.
- -fdump-rtl-ce1
- -fdump-rtl-ce2
- -fdump-rtl-ce3
- -fdump-rtl-ce1, -fdump-rtl-ce2, and -fdump-rtl-ce3 enable dumping after the three if conversion passes.
- -fdump-rtl-cprop_hardreg
- Dump after hard register copy propagation.
- -fdump-rtl-csa
- Dump after combining stack adjustments.
- -fdump-rtl-cse1
- -fdump-rtl-cse2
- -fdump-rtl-cse1 and -fdump-rtl-cse2 enable dumping after the two common sub-expression elimination passes.
- -fdump-rtl-dce
- Dump after the standalone dead code elimination passes.
- -fdump-rtl-dbr
- Dump after delayed branch scheduling.
- -fdump-rtl-dce1
- -fdump-rtl-dce2
- -fdump-rtl-dce1 and -fdump-rtl-dce2 enable dumping after the two dead store elimination passes.
- -fdump-rtl-eh
- Dump after finalization of EH handling code.
- -fdump-rtl-eh_ranges
- Dump after conversion of EH handling range regions.
- -fdump-rtl-expand
- Dump after RTL generation.
- -fdump-rtl-fwprop1
- -fdump-rtl-fwprop2
- -fdump-rtl-fwprop1 and -fdump-rtl-fwprop2 enable dumping after the two forward propagation passes.
- -fdump-rtl-gcse1
- -fdump-rtl-gcse2
- -fdump-rtl-gcse1 and -fdump-rtl-gcse2 enable dumping after global common subexpression elimination.
- -fdump-rtl-init-regs
- Dump after the initialization of the registers.
- -fdump-rtl-initvals
- Dump after the computation of the initial value sets.
- -fdump-rtl-into_cfglayout
- Dump after converting to cfglayout mode.
- -fdump-rtl-ira
- Dump after iterated register allocation.
- -fdump-rtl-jump
- Dump after the second jump optimization.
- -fdump-rtl-loop2
- -fdump-rtl-loop2 enables dumping after the rtl loop optimization passes.
- -fdump-rtl-mach
- Dump after performing the machine dependent reorganization pass, if that pass exists.
- -fdump-rtl-mode_sw
- Dump after removing redundant mode switches.
- -fdump-rtl-rnreg
- Dump after register renumbering.
- -fdump-rtl-outof_cfglayout
- Dump after converting from cfglayout mode.
- -fdump-rtl-peephole2
- Dump after the peephole pass.
- -fdump-rtl-postreload
- Dump after post-reload optimizations.
- -fdump-rtl-pro_and_epilogue
- Dump after generating the function pro and epilogues.
- -fdump-rtl-regmove
- Dump after the register move pass.
- -fdump-rtl-sched1
- -fdump-rtl-sched2
- -fdump-rtl-sched1 and -fdump-rtl-sched2 enable dumping after the basic block scheduling passes.
- -fdump-rtl-see
- Dump after sign extension elimination.
- -fdump-rtl-seqabstr
- Dump after common sequence discovery.
- -fdump-rtl-shorten
- Dump after shortening branches.
- -fdump-rtl-sibling
- Dump after sibling call optimizations.
- -fdump-rtl-split1
- -fdump-rtl-split2
- -fdump-rtl-split3
- -fdump-rtl-split4
- -fdump-rtl-split5
- -fdump-rtl-split1, -fdump-rtl-split2, -fdump-rtl-split3, -fdump-rtl-split4 and -fdump-rtl-split5 enable dumping after five rounds of instruction splitting.
- -fdump-rtl-sms
- Dump after modulo scheduling. This pass is only run on some architectures.
- -fdump-rtl-stack
- Dump after conversion from GCC's ``flat register file'' registers to the x87's stack-like registers. This pass is only run on x86 variants.
- -fdump-rtl-subreg1
- -fdump-rtl-subreg2
- -fdump-rtl-subreg1 and -fdump-rtl-subreg2 enable dumping after the two subreg expansion passes.
- -fdump-rtl-unshare
- Dump after all rtl has been unshared.
- -fdump-rtl-vartrack
- Dump after variable tracking.
- -fdump-rtl-vregs
- Dump after converting virtual registers to hard registers.
- -fdump-rtl-web
- Dump after live range splitting.
- -fdump-rtl-regclass
- -fdump-rtl-subregs_of_mode_init
- -fdump-rtl-subregs_of_mode_finish
- -fdump-rtl-dfinit
- -fdump-rtl-dfinish
- These dumps are defined but always produce empty files.
- -fdump-rtl-all
- Produce all the dumps listed above.
- -dA
- Annotate the assembler output with miscellaneous debugging information.
- -dD
- Dump all macro definitions, at the end of preprocessing, in addition to normal output.
- -dH
- Produce a core dump whenever an error occurs.
- -dm
- Print statistics on memory usage, at the end of the run, to standard error.
- -dp
- Annotate the assembler output with a comment indicating which pattern and alternative was used. The length of each instruction is also printed.
- -dP
- Dump the RTL in the assembler output as a comment before each instruction. Also turns on -dp annotation.
- -dv
- For each of the other indicated dump files (-fdump-rtl-pass), dump a representation of the control flow graph suitable for viewing with VCG to file.pass.vcg.
- -dx
- Just generate RTL for a function instead of compiling it. Usually used with -fdump-rtl-expand.
- -dy
- Dump debugging information during parsing, to standard error.
-
- -fdump-noaddr
- When doing debugging dumps, suppress address output. This makes it more feasible to use diff on debugging dumps for compiler invocations with different compiler binaries and/or different text / bss / data / heap / stack / dso start locations.
- -fdump-unnumbered
- When doing debugging dumps, suppress instruction numbers and address output. This makes it more feasible to use diff on debugging dumps for compiler invocations with different options, in particular with and without -g.
- -fdump-unnumbered-links
- When doing debugging dumps (see -d option above), suppress instruction numbers for the links to the previous and next instructions in a sequence.
- -fdump-translation-unit (C++ only)
- -fdump-translation-unit-options (C++ only)
- Dump a representation of the tree structure for the entire translation unit to a file. The file name is made by appending .tu to the source file name. If the -options form is used, options controls the details of the dump as described for the -fdump-tree options.
- -fdump-class-hierarchy (C++ only)
- -fdump-class-hierarchy-options (C++ only)
- Dump a representation of each class's hierarchy and virtual function table layout to a file. The file name is made by appending .class to the source file name. If the -options form is used, options controls the details of the dump as described for the -fdump-tree options.
- -fdump-ipa-switch
-
Control the dumping at various stages of inter-procedural analysis
language tree to a file. The file name is generated by appending a switch
specific suffix to the source file name. The following dumps are possible:
- -fdump-statistics-option
- Enable and control dumping of pass statistics in a separate file. The file name is generated by appending a suffix ending in .statistics to the source file name. If the -option form is used, -stats will cause counters to be summed over the whole compilation unit while -details will dump every event as the passes generate them. The default with no option is to sum counters for each function compiled.
- -fdump-tree-switch
- -fdump-tree-switch-options
-
Control the dumping at various stages of processing the intermediate
language tree to a file. The file name is generated by appending a switch
specific suffix to the source file name. If the -options
form is used, options is a list of - separated options that
control the details of the dump. Not all options are applicable to all
dumps, those which are not meaningful will be ignored. The following
options are available
-
- address
- Print the address of each node. Usually this is not meaningful as it changes according to the environment and source file. Its primary use is for tying up a dump file with a debug environment.
- slim
- Inhibit dumping of members of a scope or body of a function merely because that scope has been reached. Only dump such items when they are directly reachable by some other path. When dumping pretty-printed trees, this option inhibits dumping the bodies of control structures.
- raw
- Print a raw representation of the tree. By default, trees are pretty-printed into a C-like representation.
- details
- Enable more detailed dumps (not honored by every dump option).
- stats
- Enable dumping various statistics about the pass (not honored by every dump option).
- blocks
- Enable showing basic block boundaries (disabled in raw dumps).
- vops
- Enable showing virtual operands for every statement.
- lineno
- Enable showing line numbers for statements.
- uid
- Enable showing the unique ID ("DECL_UID") for each variable.
- verbose
- Enable showing the tree dump for each statement.
- all
- Turn on all options, except raw, slim, verbose and lineno.
-
The following tree dumps are possible:
- original
- Dump before any tree based optimization, to file.original.
- optimized
- Dump after all tree based optimization, to file.optimized.
- gimple
- Dump each function before and after the gimplification pass to a file. The file name is made by appending .gimple to the source file name.
- cfg
- Dump the control flow graph of each function to a file. The file name is made by appending .cfg to the source file name.
- vcg
- Dump the control flow graph of each function to a file in VCG format. The file name is made by appending .vcg to the source file name. Note that if the file contains more than one function, the generated file cannot be used directly by VCG. You will need to cut and paste each function's graph into its own separate file first.
- ch
- Dump each function after copying loop headers. The file name is made by appending .ch to the source file name.
- ssa
- Dump SSA related information to a file. The file name is made by appending .ssa to the source file name.
- alias
- Dump aliasing information for each function. The file name is made by appending .alias to the source file name.
- ccp
- Dump each function after CCP. The file name is made by appending .ccp to the source file name.
- storeccp
- Dump each function after STORE-CCP. The file name is made by appending .storeccp to the source file name.
- pre
- Dump trees after partial redundancy elimination. The file name is made by appending .pre to the source file name.
- fre
- Dump trees after full redundancy elimination. The file name is made by appending .fre to the source file name.
- copyprop
- Dump trees after copy propagation. The file name is made by appending .copyprop to the source file name.
- store_copyprop
- Dump trees after store copy-propagation. The file name is made by appending .store_copyprop to the source file name.
- dce
- Dump each function after dead code elimination. The file name is made by appending .dce to the source file name.
- mudflap
- Dump each function after adding mudflap instrumentation. The file name is made by appending .mudflap to the source file name.
- sra
- Dump each function after performing scalar replacement of aggregates. The file name is made by appending .sra to the source file name.
- sink
- Dump each function after performing code sinking. The file name is made by appending .sink to the source file name.
- dom
- Dump each function after applying dominator tree optimizations. The file name is made by appending .dom to the source file name.
- dse
- Dump each function after applying dead store elimination. The file name is made by appending .dse to the source file name.
- phiopt
- Dump each function after optimizing PHI nodes into straightline code. The file name is made by appending .phiopt to the source file name.
- forwprop
- Dump each function after forward propagating single use variables. The file name is made by appending .forwprop to the source file name.
- copyrename
- Dump each function after applying the copy rename optimization. The file name is made by appending .copyrename to the source file name.
- nrv
- Dump each function after applying the named return value optimization on generic trees. The file name is made by appending .nrv to the source file name.
- vect
- Dump each function after applying vectorization of loops. The file name is made by appending .vect to the source file name.
- vrp
- Dump each function after Value Range Propagation (VRP). The file name is made by appending .vrp to the source file name.
- all
- Enable all the available tree dumps with the flags provided in this option.
-
- -ftree-vectorizer-verbose=n
- This option controls the amount of debugging output the vectorizer prints. This information is written to standard error, unless -fdump-tree-all or -fdump-tree-vect is specified, in which case it is output to the usual dump listing file, .vect. For n=0 no diagnostic information is reported. If n=1 the vectorizer reports each loop that got vectorized, and the total number of loops that got vectorized. If n=2 the vectorizer also reports non-vectorized loops that passed the first analysis phase (vect_analyze_loop_form) - i.e. countable, inner-most, single-bb, single-entry/exit loops. This is the same verbosity level that -fdump-tree-vect-stats uses. Higher verbosity levels mean either more information dumped for each reported loop, or same amount of information reported for more loops: If n=3, alignment related information is added to the reports. If n=4, data-references related information (e.g. memory dependences, memory access-patterns) is added to the reports. If n=5, the vectorizer reports also non-vectorized inner-most loops that did not pass the first analysis phase (i.e., may not be countable, or may have complicated control-flow). If n=6, the vectorizer reports also non-vectorized nested loops. For n=7, all the information the vectorizer generates during its analysis and transformation is reported. This is the same verbosity level that -fdump-tree-vect-details uses.
- -frandom-seed=string
-
This option provides a seed that GCC uses when it would otherwise use
random numbers. It is used to generate certain symbol names
that have to be different in every compiled file. It is also used to
place unique stamps in coverage data files and the object files that
produce them. You can use the -frandom-seed option to produce
reproducibly identical object files.
The string should be different for every file you compile.
- -fsched-verbose=n
-
On targets that use instruction scheduling, this option controls the
amount of debugging output the scheduler prints. This information is
written to standard error, unless -fdump-rtl-sched1 or
-fdump-rtl-sched2 is specified, in which case it is output
to the usual dump listing file, .sched or .sched2
respectively. However for n greater than nine, the output is
always printed to standard error.
For n greater than zero, -fsched-verbose outputs the same information as -fdump-rtl-sched1 and -fdump-rtl-sched2. For n greater than one, it also output basic block probabilities, detailed ready list information and unit/insn info. For n greater than two, it includes RTL at abort point, control-flow and regions info. And for n over four, -fsched-verbose also includes dependence info.
- -save-temps
-
Store the usual ``temporary'' intermediate files permanently; place them
in the current directory and name them based on the source file. Thus,
compiling foo.c with -c -save-temps would produce files
foo.i and foo.s, as well as foo.o. This creates a
preprocessed foo.i output file even though the compiler now
normally uses an integrated preprocessor.
When used in combination with the -x command line option, -save-temps is sensible enough to avoid over writing an input source file with the same extension as an intermediate file. The corresponding intermediate file may be obtained by renaming the source file before using -save-temps.
- -time[=file]
-
Report the CPU time taken by each subprocess in the compilation
sequence. For C source files, this is the compiler proper and assembler
(plus the linker if linking is done).
Without the specification of an output file, the output looks like this:
# cc1 0.12 0.01 # as 0.00 0.01
The first number on each line is the ``user time'', that is time spent executing the program itself. The second number is ``system time'', time spent executing operating system routines on behalf of the program. Both numbers are in seconds.
With the specification of an output file, the output is appended to the named file, and it looks like this:
0.12 0.01 cc1 <options> 0.00 0.01 as <options>
The ``user time'' and the ``system time'' are moved before the program name, and the options passed to the program are displayed, so that one can later tell what file was being compiled, and with which options.
- -fvar-tracking
-
Run variable tracking pass. It computes where variables are stored at each
position in code. Better debugging information is then generated
(if the debugging information format supports this information).
It is enabled by default when compiling with optimization (-Os, -O, -O2, ...), debugging information (-g) and the debug info format supports it.
- -fvar-tracking-assignments
-
Annotate assignments to user variables early in the compilation and
attempt to carry the annotations over throughout the compilation all the
way to the end, in an attempt to improve debug information while
optimizing.
It can be enabled even if var-tracking is disabled, in which case annotations will be created and maintained, but discarded at the end.
- -fvar-tracking-assignments-toggle
- Toggle -fvar-tracking-assignments, in the same way that -gtoggle toggles -g.
- -print-file-name=library
- Print the full absolute name of the library file library that would be used when linking---and don't do anything else. With this option, GCC does not compile or link anything; it just prints the file name.
- -print-multi-directory
- Print the directory name corresponding to the multilib selected by any other switches present in the command line. This directory is supposed to exist in GCC_EXEC_PREFIX.
- -print-multi-lib
- Print the mapping from multilib directory names to compiler switches that enable them. The directory name is separated from the switches by ;, and each switch starts with an @} instead of the @samp{-, without spaces between multiple switches. This is supposed to ease shell-processing.
- -print-multi-os-directory
- Print the path to OS libraries for the selected multilib, relative to some lib subdirectory. If OS libraries are present in the lib subdirectory and no multilibs are used, this is usually just ., if OS libraries are present in libsuffix sibling directories this prints e.g. ../lib64, ../lib or ../lib32, or if OS libraries are present in lib/subdir subdirectories it prints e.g. amd64, sparcv9 or ev6.
- -print-prog-name=program
- Like -print-file-name, but searches for a program such as cpp.
- -print-libgcc-file-name
-
Same as -print-file-name=libgcc.a.
This is useful when you use -nostdlib or -nodefaultlibs but you do want to link with libgcc.a. You can do
gcc -nostdlib <files>... `gcc -print-libgcc-file-name`
- -print-search-dirs
-
Print the name of the configured installation directory and a list of
program and library directories gcc will search---and don't do anything else.
This is useful when gcc prints the error message installation problem, cannot exec cpp0: No such file or directory. To resolve this you either need to put cpp0 and the other compiler components where gcc expects to find them, or you can set the environment variable GCC_EXEC_PREFIX to the directory where you installed them. Don't forget the trailing /.
- -print-sysroot
- Print the target sysroot directory that will be used during compilation. This is the target sysroot specified either at configure time or using the --sysroot option, possibly with an extra suffix that depends on compilation options. If no target sysroot is specified, the option prints nothing.
- -print-sysroot-headers-suffix
- Print the suffix added to the target sysroot when searching for headers, or give an error if the compiler is not configured with such a suffix---and don't do anything else.
- -dumpmachine
- Print the compiler's target machine (for example, i686-pc-linux-gnu)---and don't do anything else.
- -dumpversion
- Print the compiler version (for example, 3.0)---and don't do anything else.
- -dumpspecs
- Print the compiler's built-in specs---and don't do anything else. (This is used when GCC itself is being built.)
- -feliminate-unused-debug-types
- Normally, when producing DWARF2 output, GCC will emit debugging information for all types declared in a compilation unit, regardless of whether or not they are actually used in that compilation unit. Sometimes this is useful, such as if, in the debugger, you want to cast a value to a type that is not actually used in your program (but is declared). More often, however, this results in a significant amount of wasted space. With this option, GCC will avoid producing debug symbol output for types that are nowhere used in the source file being compiled.
Options That Control Optimization
These options control various sorts of optimizations.Without any optimization option, the compiler's goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you would expect from the source code.
Turning on optimization flags makes the compiler attempt to improve the performance and/or code size at the expense of compilation time and possibly the ability to debug the program.
The compiler performs optimization based on the knowledge it has of the program. Compiling multiple files at once to a single output file mode allows the compiler to use information gained from all of the files when compiling each of them.
Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed.
- -O
- -O1
-
Optimize. Optimizing compilation takes somewhat more time, and a lot
more memory for a large function.
With -O, the compiler tries to reduce code size and execution time, without performing any optimizations that take a great deal of compilation time.
-O turns on the following optimization flags:
-fauto-inc-dec -fcprop-registers -fdce -fdefer-pop -fdelayed-branch -fdse -fguess-branch-probability -fif-conversion2 -fif-conversion -finline-small-functions -fipa-pure-const -fipa-reference -fmerge-constants -fsplit-wide-types -ftree-builtin-call-dce -ftree-ccp -ftree-ch -ftree-copyrename -ftree-dce -ftree-dominator-opts -ftree-dse -ftree-fre -ftree-sra -ftree-ter -funit-at-a-time
-O also turns on -fomit-frame-pointer on machines where doing so does not interfere with debugging.
- -O2
-
Optimize even more. GCC performs nearly all supported optimizations
that do not involve a space-speed tradeoff.
As compared to -O, this option increases both compilation time
and the performance of the generated code.
-O2 turns on all optimization flags specified by -O. It also turns on the following optimization flags: -fthread-jumps -falign-functions -falign-jumps -falign-loops -falign-labels -fcaller-saves -fcrossjumping -fcse-follow-jumps -fcse-skip-blocks -fdelete-null-pointer-checks -fexpensive-optimizations -fgcse -fgcse-lm -findirect-inlining -foptimize-sibling-calls -fpeephole2 -fregmove -freorder-blocks -freorder-functions -frerun-cse-after-loop -fsched-interblock -fsched-spec -fschedule-insns -fschedule-insns2 -fstrict-aliasing -fstrict-overflow -ftree-switch-conversion -ftree-pre -ftree-vrp
Please note the warning under -fgcse about invoking -O2 on programs that use computed gotos.
- -O3
- Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the -finline-functions, -funswitch-loops, -fpredictive-commoning, -fgcse-after-reload, -ftree-vectorize and -fipa-cp-clone options.
- -O0
- Reduce compilation time and make debugging produce the expected results. This is the default.
- -Os
-
Optimize for size. -Os enables all -O2 optimizations that
do not typically increase code size. It also performs further
optimizations designed to reduce code size.
-Os disables the following optimization flags: -falign-functions -falign-jumps -falign-loops -falign-labels -freorder-blocks -freorder-blocks-and-partition -fprefetch-loop-arrays -ftree-vect-loop-version
If you use multiple -O options, with or without level numbers, the last such option is the one that is effective.
Options of the form -fflag specify machine-independent flags. Most flags have both positive and negative forms; the negative form of -ffoo would be -fno-foo. In the table below, only one of the forms is listed---the one you typically will use. You can figure out the other form by either removing no- or adding it.
The following options control specific optimizations. They are either activated by -O options or are related to ones that are. You can use the following flags in the rare cases when ``fine-tuning'' of optimizations to be performed is desired.
- -fno-default-inline
- Do not make member functions inline by default merely because they are defined inside the class scope (C++ only). Otherwise, when you specify -O, member functions defined inside class scope are compiled inline by default; i.e., you don't need to add inline in front of the member function name.
- -fno-defer-pop
-
Always pop the arguments to each function call as soon as that function
returns. For machines which must pop arguments after a function call,
the compiler normally lets arguments accumulate on the stack for several
function calls and pops them all at once.
Disabled at levels -O, -O2, -O3, -Os.
- -fforward-propagate
-
Perform a forward propagation pass on RTL. The pass tries to combine two
instructions and checks if the result can be simplified. If loop unrolling
is active, two passes are performed and the second is scheduled after
loop unrolling.
This option is enabled by default at optimization levels -O2, -O3, -Os.
- -fomit-frame-pointer
-
Don't keep the frame pointer in a register for functions that
don't need one. This avoids the instructions to save, set up and
restore frame pointers; it also makes an extra register available
in many functions. It also makes debugging impossible on
some machines.
On some machines, such as the VAX, this flag has no effect, because the standard calling sequence automatically handles the frame pointer and nothing is saved by pretending it doesn't exist. The machine-description macro "FRAME_POINTER_REQUIRED" controls whether a target machine supports this flag.
Enabled at levels -O, -O2, -O3, -Os.
- -foptimize-sibling-calls
-
Optimize sibling and tail recursive calls.
Enabled at levels -O2, -O3, -Os.
- -fno-inline
- Don't pay attention to the "inline" keyword. Normally this option is used to keep the compiler from expanding any functions inline. Note that if you are not optimizing, no functions can be expanded inline.
- -finline-small-functions
-
Integrate functions into their callers when their body is smaller than expected
function call code (so overall size of program gets smaller). The compiler
heuristically decides which functions are simple enough to be worth integrating
in this way.
Enabled at level -O2.
- -findirect-inlining
-
Inline also indirect calls that are discovered to be known at compile
time thanks to previous inlining. This option has any effect only
when inlining itself is turned on by the -finline-functions
or -finline-small-functions options.
Enabled at level -O2.
- -finline-functions
-
Integrate all simple functions into their callers. The compiler
heuristically decides which functions are simple enough to be worth
integrating in this way.
If all calls to a given function are integrated, and the function is declared "static", then the function is normally not output as assembler code in its own right.
Enabled at level -O3.
- -finline-functions-called-once
-
Consider all "static" functions called once for inlining into their
caller even if they are not marked "inline". If a call to a given
function is integrated, then the function is not output as assembler code
in its own right.
Enabled at levels -O1, -O2, -O3 and -Os.
- -fearly-inlining
-
Inline functions marked by "always_inline" and functions whose body seems
smaller than the function call overhead early before doing
-fprofile-generate instrumentation and real inlining pass. Doing so
makes profiling significantly cheaper and usually inlining faster on programs
having large chains of nested wrapper functions.
Enabled by default.
- -finline-limit=n
-
By default, GCC limits the size of functions that can be inlined. This flag
allows coarse control of this limit. n is the size of functions that
can be inlined in number of pseudo instructions.
Inlining is actually controlled by a number of parameters, which may be specified individually by using --param name=value. The -finline-limit=n option sets some of these parameters as follows:
-
See below for a documentation of the individual parameters controlling inlining and for the defaults of these parameters.
Note: there may be no value to -finline-limit that results in default behavior.
Note: pseudo instruction represents, in this particular context, an abstract measurement of function's size. In no way does it represent a count of assembly instructions and as such its exact meaning might change from one release to an another.
-
- -fkeep-inline-functions
- In C, emit "static" functions that are declared "inline" into the object file, even if the function has been inlined into all of its callers. This switch does not affect functions using the "extern inline" extension in GNU C89. In C++, emit any and all inline functions into the object file.
- -fkeep-static-consts
-
Emit variables declared "static const" when optimization isn't turned
on, even if the variables aren't referenced.
GCC enables this option by default. If you want to force the compiler to check if the variable was referenced, regardless of whether or not optimization is turned on, use the -fno-keep-static-consts option.
- -fmerge-constants
-
Attempt to merge identical constants (string constants and floating point
constants) across compilation units.
This option is the default for optimized compilation if the assembler and linker support it. Use -fno-merge-constants to inhibit this behavior.
Enabled at levels -O, -O2, -O3, -Os.
- -fmerge-all-constants
-
Attempt to merge identical constants and identical variables.
This option implies -fmerge-constants. In addition to -fmerge-constants this considers e.g. even constant initialized arrays or initialized constant variables with integral or floating point types. Languages like C or C++ require each variable, including multiple instances of the same variable in recursive calls, to have distinct locations, so using this option will result in non-conforming behavior.
- -fmodulo-sched
- Perform swing modulo scheduling immediately before the first scheduling pass. This pass looks at innermost loops and reorders their instructions by overlapping different iterations.
- -fmodulo-sched-allow-regmoves
- Perform more aggressive SMS based modulo scheduling with register moves allowed. By setting this flag certain anti-dependences edges will be deleted which will trigger the generation of reg-moves based on the life-range analysis. This option is effective only with -fmodulo-sched enabled.
- -fno-branch-count-reg
-
Do not use ``decrement and branch'' instructions on a count register,
but instead generate a sequence of instructions that decrement a
register, compare it against zero, then branch based upon the result.
This option is only meaningful on architectures that support such
instructions, which include x86, PowerPC, IA-64 and S/390.
The default is -fbranch-count-reg.
- -fno-function-cse
-
Do not put function addresses in registers; make each instruction that
calls a constant function contain the function's address explicitly.
This option results in less efficient code, but some strange hacks that alter the assembler output may be confused by the optimizations performed when this option is not used.
The default is -ffunction-cse
- -fno-zero-initialized-in-bss
-
If the target supports a BSS section, GCC by default puts variables that
are initialized to zero into BSS. This can save space in the resulting
code.
This option turns off this behavior because some programs explicitly rely on variables going to the data section. E.g., so that the resulting executable can find the beginning of that section and/or make assumptions based on that.
The default is -fzero-initialized-in-bss.
- -fmudflap -fmudflapth -fmudflapir
-
For front-ends that support it (C and C++), instrument all risky
pointer/array dereferencing operations, some standard library
string/heap functions, and some other associated constructs with
range/validity tests. Modules so instrumented should be immune to
buffer overflows, invalid heap use, and some other classes of C/C++
programming errors. The instrumentation relies on a separate runtime
library (libmudflap), which will be linked into a program if
-fmudflap is given at link time. Run-time behavior of the
instrumented program is controlled by the MUDFLAP_OPTIONS
environment variable. See "env MUDFLAP_OPTIONS=-help a.out"
for its options.
Use -fmudflapth instead of -fmudflap to compile and to link if your program is multi-threaded. Use -fmudflapir, in addition to -fmudflap or -fmudflapth, if instrumentation should ignore pointer reads. This produces less instrumentation (and therefore faster execution) and still provides some protection against outright memory corrupting writes, but allows erroneously read data to propagate within a program.
- -fthread-jumps
-
Perform optimizations where we check to see if a jump branches to a
location where another comparison subsumed by the first is found. If
so, the first branch is redirected to either the destination of the
second branch or a point immediately following it, depending on whether
the condition is known to be true or false.
Enabled at levels -O2, -O3, -Os.
- -fsplit-wide-types
-
When using a type that occupies multiple registers, such as "long
long" on a 32-bit system, split the registers apart and allocate them
independently. This normally generates better code for those types,
but may make debugging more difficult.
Enabled at levels -O, -O2, -O3, -Os.
- -fcse-follow-jumps
-
In common subexpression elimination (CSE), scan through jump instructions
when the target of the jump is not reached by any other path. For
example, when CSE encounters an "if" statement with an
"else" clause, CSE will follow the jump when the condition
tested is false.
Enabled at levels -O2, -O3, -Os.
- -fcse-skip-blocks
-
This is similar to -fcse-follow-jumps, but causes CSE to
follow jumps which conditionally skip over blocks. When CSE
encounters a simple "if" statement with no else clause,
-fcse-skip-blocks causes CSE to follow the jump around the
body of the "if".
Enabled at levels -O2, -O3, -Os.
- -frerun-cse-after-loop
-
Re-run common subexpression elimination after loop optimizations has been
performed.
Enabled at levels -O2, -O3, -Os.
- -fgcse
-
Perform a global common subexpression elimination pass.
This pass also performs global constant and copy propagation.
Note: When compiling a program using computed gotos, a GCC extension, you may get better runtime performance if you disable the global common subexpression elimination pass by adding -fno-gcse to the command line.
Enabled at levels -O2, -O3, -Os.
- -fgcse-lm
-
When -fgcse-lm is enabled, global common subexpression elimination will
attempt to move loads which are only killed by stores into themselves. This
allows a loop containing a load/store sequence to be changed to a load outside
the loop, and a copy/store within the loop.
Enabled by default when gcse is enabled.
- -fgcse-sm
-
When -fgcse-sm is enabled, a store motion pass is run after
global common subexpression elimination. This pass will attempt to move
stores out of loops. When used in conjunction with -fgcse-lm,
loops containing a load/store sequence can be changed to a load before
the loop and a store after the loop.
Not enabled at any optimization level.
- -fgcse-las
-
When -fgcse-las is enabled, the global common subexpression
elimination pass eliminates redundant loads that come after stores to the
same memory location (both partial and full redundancies).
Not enabled at any optimization level.
- -fgcse-after-reload
- When -fgcse-after-reload is enabled, a redundant load elimination pass is performed after reload. The purpose of this pass is to cleanup redundant spilling.
- -funsafe-loop-optimizations
- If given, the loop optimizer will assume that loop indices do not overflow, and that the loops with nontrivial exit condition are not infinite. This enables a wider range of loop optimizations even if the loop optimizer itself cannot prove that these assumptions are valid. Using -Wunsafe-loop-optimizations, the compiler will warn you if it finds this kind of loop.
- -fcrossjumping
-
Perform cross-jumping transformation. This transformation unifies equivalent code and save code size. The
resulting code may or may not perform better than without cross-jumping.
Enabled at levels -O2, -O3, -Os.
- -fauto-inc-dec
- Combine increments or decrements of addresses with memory accesses. This pass is always skipped on architectures that do not have instructions to support this. Enabled by default at -O and higher on architectures that support this.
- -fdce
- Perform dead code elimination (DCE) on RTL. Enabled by default at -O and higher.
- -fdse
- Perform dead store elimination (DSE) on RTL. Enabled by default at -O and higher.
- -fif-conversion
-
Attempt to transform conditional jumps into branch-less equivalents. This
include use of conditional moves, min, max, set flags and abs instructions, and
some tricks doable by standard arithmetics. The use of conditional execution
on chips where it is available is controlled by "if-conversion2".
Enabled at levels -O, -O2, -O3, -Os.
- -fif-conversion2
-
Use conditional execution (where available) to transform conditional jumps into
branch-less equivalents.
Enabled at levels -O, -O2, -O3, -Os.
- -fdelete-null-pointer-checks
-
Use global dataflow analysis to identify and eliminate useless checks
for null pointers. The compiler assumes that dereferencing a null
pointer would have halted the program. If a pointer is checked after
it has already been dereferenced, it cannot be null.
In some environments, this assumption is not true, and programs can safely dereference null pointers. Use -fno-delete-null-pointer-checks to disable this optimization for programs which depend on that behavior.
Enabled at levels -O2, -O3, -Os.
- -fexpensive-optimizations
-
Perform a number of minor optimizations that are relatively expensive.
Enabled at levels -O2, -O3, -Os.
- -foptimize-register-move
- -fregmove
-
Attempt to reassign register numbers in move instructions and as
operands of other simple instructions in order to maximize the amount of
register tying. This is especially helpful on machines with two-operand
instructions.
Note -fregmove and -foptimize-register-move are the same optimization.
Enabled at levels -O2, -O3, -Os.
- -fira-algorithm=algorithm
- Use specified coloring algorithm for the integrated register allocator. The algorithm argument should be "priority" or "CB". The first algorithm specifies Chow's priority coloring, the second one specifies Chaitin-Briggs coloring. The second algorithm can be unimplemented for some architectures. If it is implemented, it is the default because Chaitin-Briggs coloring as a rule generates a better code.
- -fira-region=region
- Use specified regions for the integrated register allocator. The region argument should be one of "all", "mixed", or "one". The first value means using all loops as register allocation regions, the second value which is the default means using all loops except for loops with small register pressure as the regions, and third one means using all function as a single region. The first value can give best result for machines with small size and irregular register set, the third one results in faster and generates decent code and the smallest size code, and the default value usually give the best results in most cases and for most architectures.
- -fira-coalesce
- Do optimistic register coalescing. This option might be profitable for architectures with big regular register files.
- -fno-ira-share-save-slots
- Switch off sharing stack slots used for saving call used hard registers living through a call. Each hard register will get a separate stack slot and as a result function stack frame will be bigger.
- -fno-ira-share-spill-slots
- Switch off sharing stack slots allocated for pseudo-registers. Each pseudo-register which did not get a hard register will get a separate stack slot and as a result function stack frame will be bigger.
- -fira-verbose=n
- Set up how verbose dump file for the integrated register allocator will be. Default value is 5. If the value is greater or equal to 10, the dump file will be stderr as if the value were n minus 10.
- -fdelayed-branch
-
If supported for the target machine, attempt to reorder instructions
to exploit instruction slots available after delayed branch
instructions.
Enabled at levels -O, -O2, -O3, -Os.
- -fschedule-insns
-
If supported for the target machine, attempt to reorder instructions to
eliminate execution stalls due to required data being unavailable. This
helps machines that have slow floating point or memory load instructions
by allowing other instructions to be issued until the result of the load
or floating point instruction is required.
Enabled at levels -O2, -O3, -Os.
- -fschedule-insns2
-
Similar to -fschedule-insns, but requests an additional pass of
instruction scheduling after register allocation has been done. This is
especially useful on machines with a relatively small number of
registers and where memory load instructions take more than one cycle.
Enabled at levels -O2, -O3, -Os.
- -fno-sched-interblock
- Don't schedule instructions across basic blocks. This is normally enabled by default when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
- -fno-sched-spec
- Don't allow speculative motion of non-load instructions. This is normally enabled by default when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
- -fsched-spec-load
- Allow speculative motion of some load instructions. This only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
- -fsched-spec-load-dangerous
- Allow speculative motion of more load instructions. This only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
- -fsched-stalled-insns
- -fsched-stalled-insns=n
- Define how many insns (if any) can be moved prematurely from the queue of stalled insns into the ready list, during the second scheduling pass. -fno-sched-stalled-insns means that no insns will be moved prematurely, -fsched-stalled-insns=0 means there is no limit on how many queued insns can be moved prematurely. -fsched-stalled-insns without a value is equivalent to -fsched-stalled-insns=1.
- -fsched-stalled-insns-dep
- -fsched-stalled-insns-dep=n
- Define how many insn groups (cycles) will be examined for a dependency on a stalled insn that is candidate for premature removal from the queue of stalled insns. This has an effect only during the second scheduling pass, and only if -fsched-stalled-insns is used. -fno-sched-stalled-insns-dep is equivalent to -fsched-stalled-insns-dep=0. -fsched-stalled-insns-dep without a value is equivalent to -fsched-stalled-insns-dep=1.
- -fsched2-use-superblocks
-
When scheduling after register allocation, do use superblock scheduling
algorithm. Superblock scheduling allows motion across basic block boundaries
resulting on faster schedules. This option is experimental, as not all machine
descriptions used by GCC model the CPU closely enough to avoid unreliable
results from the algorithm.
This only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher.
- -fsched2-use-traces
-
Use -fsched2-use-superblocks algorithm when scheduling after register
allocation and additionally perform code duplication in order to increase the
size of superblocks using tracer pass. See -ftracer for details on
trace formation.
This mode should produce faster but significantly longer programs. Also without -fbranch-probabilities the traces constructed may not match the reality and hurt the performance. This only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher.
- -fsee
- Eliminate redundant sign extension instructions and move the non-redundant ones to optimal placement using lazy code motion (LCM).
- -freschedule-modulo-scheduled-loops
- The modulo scheduling comes before the traditional scheduling, if a loop was modulo scheduled we may want to prevent the later scheduling passes from changing its schedule, we use this option to control that.
- -fselective-scheduling
- Schedule instructions using selective scheduling algorithm. Selective scheduling runs instead of the first scheduler pass.
- -fselective-scheduling2
- Schedule instructions using selective scheduling algorithm. Selective scheduling runs instead of the second scheduler pass.
- -fsel-sched-pipelining
- Enable software pipelining of innermost loops during selective scheduling. This option has no effect until one of -fselective-scheduling or -fselective-scheduling2 is turned on.
- -fsel-sched-pipelining-outer-loops
- When pipelining loops during selective scheduling, also pipeline outer loops. This option has no effect until -fsel-sched-pipelining is turned on.
- -fcaller-saves
-
Enable values to be allocated in registers that will be clobbered by
function calls, by emitting extra instructions to save and restore the
registers around such calls. Such allocation is done only when it
seems to result in better code than would otherwise be produced.
This option is always enabled by default on certain machines, usually those which have no call-preserved registers to use instead.
Enabled at levels -O2, -O3, -Os.
- -fconserve-stack
- Attempt to minimize stack usage. The compiler will attempt to use less stack space, even if that makes the program slower. This option implies setting the large-stack-frame parameter to 100 and the large-stack-frame-growth parameter to 400.
- -ftree-reassoc
- Perform reassociation on trees. This flag is enabled by default at -O and higher.
- -ftree-pre
- Perform partial redundancy elimination (PRE) on trees. This flag is enabled by default at -O2 and -O3.
- -ftree-fre
- Perform full redundancy elimination (FRE) on trees. The difference between FRE and PRE is that FRE only considers expressions that are computed on all paths leading to the redundant computation. This analysis is faster than PRE, though it exposes fewer redundancies. This flag is enabled by default at -O and higher.
- -ftree-copy-prop
- Perform copy propagation on trees. This pass eliminates unnecessary copy operations. This flag is enabled by default at -O and higher.
- -fipa-pure-const
- Discover which functions are pure or constant. Enabled by default at -O and higher.
- -fipa-reference
- Discover which static variables do not escape cannot escape the compilation unit. Enabled by default at -O and higher.
- -fipa-struct-reorg
-
Perform structure reorganization optimization, that change C-like structures
layout in order to better utilize spatial locality. This transformation is
effective for programs containing arrays of structures. Available in two
compilation modes: profile-based (enabled with -fprofile-generate)
or static (which uses built-in heuristics). Require -fipa-type-escape
to provide the safety of this transformation. It works only in whole program
mode, so it requires -fwhole-program and -combine to be
enabled. Structures considered cold by this transformation are not
affected (see --param struct-reorg-cold-struct-ratio=value).
With this flag, the program debug info reflects a new structure layout.
- -fipa-pta
- Perform interprocedural pointer analysis. This option is experimental and does not affect generated code.
- -fipa-cp
- Perform interprocedural constant propagation. This optimization analyzes the program to determine when values passed to functions are constants and then optimizes accordingly. This optimization can substantially increase performance if the application has constants passed to functions. This flag is enabled by default at -O2, -Os and -O3.
- -fipa-cp-clone
- Perform function cloning to make interprocedural constant propagation stronger. When enabled, interprocedural constant propagation will perform function cloning when externally visible function can be called with constant arguments. Because this optimization can create multiple copies of functions, it may significantly increase code size (see --param ipcp-unit-growth=value). This flag is enabled by default at -O3.
- -fipa-matrix-reorg
- Perform matrix flattening and transposing. Matrix flattening tries to replace a m-dimensional matrix with its equivalent n-dimensional matrix, where n < m. This reduces the level of indirection needed for accessing the elements of the matrix. The second optimization is matrix transposing that attempts to change the order of the matrix's dimensions in order to improve cache locality. Both optimizations need the -fwhole-program flag. Transposing is enabled only if profiling information is available.
- -ftree-sink
- Perform forward store motion on trees. This flag is enabled by default at -O and higher.
- -ftree-ccp
- Perform sparse conditional constant propagation (CCP) on trees. This pass only operates on local scalar variables and is enabled by default at -O and higher.
- -ftree-switch-conversion
- Perform conversion of simple initializations in a switch to initializations from a scalar array. This flag is enabled by default at -O2 and higher.
- -ftree-dce
- Perform dead code elimination (DCE) on trees. This flag is enabled by default at -O and higher.
- -ftree-builtin-call-dce
- Perform conditional dead code elimination (DCE) for calls to builtin functions that may set "errno" but are otherwise side-effect free. This flag is enabled by default at -O2 and higher if -Os is not also specified.
- -ftree-dominator-opts
- Perform a variety of simple scalar cleanups (constant/copy propagation, redundancy elimination, range propagation and expression simplification) based on a dominator tree traversal. This also performs jump threading (to reduce jumps to jumps). This flag is enabled by default at -O and higher.
- -ftree-dse
- Perform dead store elimination (DSE) on trees. A dead store is a store into a memory location which will later be overwritten by another store without any intervening loads. In this case the earlier store can be deleted. This flag is enabled by default at -O and higher.
- -ftree-ch
- Perform loop header copying on trees. This is beneficial since it increases effectiveness of code motion optimizations. It also saves one jump. This flag is enabled by default at -O and higher. It is not enabled for -Os, since it usually increases code size.
- -ftree-loop-optimize
- Perform loop optimizations on trees. This flag is enabled by default at -O and higher.
- -ftree-loop-linear
- Perform linear loop transformations on tree. This flag can improve cache performance and allow further loop optimizations to take place.
- -floop-interchange
-
Perform loop interchange transformations on loops. Interchanging two
nested loops switches the inner and outer loops. For example, given a
loop like:
DO J = 1, M DO I = 1, N A(J, I) = A(J, I) * C ENDDO ENDDO
loop interchange will transform the loop as if the user had written:
DO I = 1, N DO J = 1, M A(J, I) = A(J, I) * C ENDDO ENDDO
which can be beneficial when "N" is larger than the caches, because in Fortran, the elements of an array are stored in memory contiguously by column, and the original loop iterates over rows, potentially creating at each access a cache miss. This optimization applies to all the languages supported by GCC and is not limited to Fortran. To use this code transformation, GCC has to be configured with --with-ppl and --with-cloog to enable the Graphite loop transformation infrastructure.
- -floop-strip-mine
-
Perform loop strip mining transformations on loops. Strip mining
splits a loop into two nested loops. The outer loop has strides
equal to the strip size and the inner loop has strides of the
original loop within a strip. For example, given a loop like:
DO I = 1, N A(I) = A(I) + C ENDDO
loop strip mining will transform the loop as if the user had written:
DO II = 1, N, 4 DO I = II, min (II + 3, N) A(I) = A(I) + C ENDDO ENDDO
This optimization applies to all the languages supported by GCC and is not limited to Fortran. To use this code transformation, GCC has to be configured with --with-ppl and --with-cloog to enable the Graphite loop transformation infrastructure.
- -floop-block
-
Perform loop blocking transformations on loops. Blocking strip mines
each loop in the loop nest such that the memory accesses of the
element loops fit inside caches. For example, given a loop like:
DO I = 1, N DO J = 1, M A(J, I) = B(I) + C(J) ENDDO ENDDO
loop blocking will transform the loop as if the user had written:
DO II = 1, N, 64 DO JJ = 1, M, 64 DO I = II, min (II + 63, N) DO J = JJ, min (JJ + 63, M) A(J, I) = B(I) + C(J) ENDDO ENDDO ENDDO ENDDO
which can be beneficial when "M" is larger than the caches, because the innermost loop will iterate over a smaller amount of data that can be kept in the caches. This optimization applies to all the languages supported by GCC and is not limited to Fortran. To use this code transformation, GCC has to be configured with --with-ppl and --with-cloog to enable the Graphite loop transformation infrastructure.
- -fcheck-data-deps
- Compare the results of several data dependence analyzers. This option is used for debugging the data dependence analyzers.
- -ftree-loop-distribution
-
Perform loop distribution. This flag can improve cache performance on
big loop bodies and allow further loop optimizations, like
parallelization or vectorization, to take place. For example, the loop
DO I = 1, N A(I) = B(I) + C D(I) = E(I) * F ENDDO
is transformed to
DO I = 1, N A(I) = B(I) + C ENDDO DO I = 1, N D(I) = E(I) * F ENDDO
- -ftree-loop-im
- Perform loop invariant motion on trees. This pass moves only invariants that would be hard to handle at RTL level (function calls, operations that expand to nontrivial sequences of insns). With -funswitch-loops it also moves operands of conditions that are invariant out of the loop, so that we can use just trivial invariantness analysis in loop unswitching. The pass also includes store motion.
- -ftree-loop-ivcanon
- Create a canonical counter for number of iterations in the loop for that determining number of iterations requires complicated analysis. Later optimizations then may determine the number easily. Useful especially in connection with unrolling.
- -fivopts
- Perform induction variable optimizations (strength reduction, induction variable merging and induction variable elimination) on trees.
- -ftree-parallelize-loops=n
- Parallelize loops, i.e., split their iteration space to run in n threads. This is only possible for loops whose iterations are independent and can be arbitrarily reordered. The optimization is only profitable on multiprocessor machines, for loops that are CPU-intensive, rather than constrained e.g. by memory bandwidth. This option implies -pthread, and thus is only supported on targets that have support for -pthread.
- -ftree-sra
- Perform scalar replacement of aggregates. This pass replaces structure references with scalars to prevent committing structures to memory too early. This flag is enabled by default at -O and higher.
- -ftree-copyrename
- Perform copy renaming on trees. This pass attempts to rename compiler temporaries to other variables at copy locations, usually resulting in variable names which more closely resemble the original variables. This flag is enabled by default at -O and higher.
- -ftree-coalesce-inlined-vars
- Permit the copyrename pass to subject inlined variables to coalescing into other variables. This may harm debug information of such inlined variables, but it will keep variables of the main function apart from each other, such that they are more likely to contain the expected values in a debugging session.
- -ftree-coalesce-vars
- Permit the copyrename pass to subject all variables to SSA coalescing. This may severely limit the ability to debug an optimized program compiled without -fvar-tracking-assignments. In the negated form, this flag prevents SSA coalescing of user variables, including inlined ones.
- -ftree-ter
- Perform temporary expression replacement during the SSA->normal phase. Single use/single def temporaries are replaced at their use location with their defining expression. This results in non-GIMPLE code, but gives the expanders much more complex trees to work on resulting in better RTL generation. This is enabled by default at -O and higher.
- -ftree-vectorize
- Perform loop vectorization on trees. This flag is enabled by default at -O3.
- -ftree-vect-loop-version
- Perform loop versioning when doing loop vectorization on trees. When a loop appears to be vectorizable except that data alignment or data dependence cannot be determined at compile time then vectorized and non-vectorized versions of the loop are generated along with runtime checks for alignment or dependence to control which version is executed. This option is enabled by default except at level -Os where it is disabled.
- -fvect-cost-model
- Enable cost model for vectorization.
- -ftree-vrp
- Perform Value Range Propagation on trees. This is similar to the constant propagation pass, but instead of values, ranges of values are propagated. This allows the optimizers to remove unnecessary range checks like array bound checks and null pointer checks. This is enabled by default at -O2 and higher. Null pointer check elimination is only done if -fdelete-null-pointer-checks is enabled.
- -ftracer
- Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function allowing other optimizations to do better job.
- -funroll-loops
- Unroll loops whose number of iterations can be determined at compile time or upon entry to the loop. -funroll-loops implies -frerun-cse-after-loop. This option makes code larger, and may or may not make it run faster.
- -funroll-all-loops
- Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This usually makes programs run more slowly. -funroll-all-loops implies the same options as -funroll-loops,
- -fsplit-ivs-in-unroller
-
Enables expressing of values of induction variables in later iterations
of the unrolled loop using the value in the first iteration. This breaks
long dependency chains, thus improving efficiency of the scheduling passes.
Combination of -fweb and CSE is often sufficient to obtain the same effect. However in cases the loop body is more complicated than a single basic block, this is not reliable. It also does not work at all on some of the architectures due to restrictions in the CSE pass.
This optimization is enabled by default.
- -fvariable-expansion-in-unroller
- With this option, the compiler will create multiple copies of some local variables when unrolling a loop which can result in superior code.
- -fpredictive-commoning
-
Perform predictive commoning optimization, i.e., reusing computations
(especially memory loads and stores) performed in previous
iterations of loops.
This option is enabled at level -O3.
- -fprefetch-loop-arrays
-
If supported by the target machine, generate instructions to prefetch
memory to improve the performance of loops that access large arrays.
This option may generate better or worse code; results are highly dependent on the structure of loops within the source code.
Disabled at level -Os.
- -fno-peephole
- -fno-peephole2
-
Disable any machine-specific peephole optimizations. The difference
between -fno-peephole and -fno-peephole2 is in how they
are implemented in the compiler; some targets use one, some use the
other, a few use both.
-fpeephole is enabled by default. -fpeephole2 enabled at levels -O2, -O3, -Os.
- -fno-guess-branch-probability
-
Do not guess branch probabilities using heuristics.
GCC will use heuristics to guess branch probabilities if they are not provided by profiling feedback (-fprofile-arcs). These heuristics are based on the control flow graph. If some branch probabilities are specified by __builtin_expect, then the heuristics will be used to guess branch probabilities for the rest of the control flow graph, taking the __builtin_expect info into account. The interactions between the heuristics and __builtin_expect can be complex, and in some cases, it may be useful to disable the heuristics so that the effects of __builtin_expect are easier to understand.
The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os.
- -freorder-blocks
-
Reorder basic blocks in the compiled function in order to reduce number of
taken branches and improve code locality.
Enabled at levels -O2, -O3.
- -freorder-blocks-and-partition
-
In addition to reordering basic blocks in the compiled function, in order
to reduce number of taken branches, partitions hot and cold basic blocks
into separate sections of the assembly and .o files, to improve
paging and cache locality performance.
This optimization is automatically turned off in the presence of exception handling, for linkonce sections, for functions with a user-defined section attribute and on any architecture that does not support named sections.
- -freorder-functions
-
Reorder functions in the object file in order to
improve code locality. This is implemented by using special
subsections ".text.hot" for most frequently executed functions and
".text.unlikely" for unlikely executed functions. Reordering is done by
the linker so object file format must support named sections and linker must
place them in a reasonable way.
Also profile feedback must be available in to make this option effective. See -fprofile-arcs for details.
Enabled at levels -O2, -O3, -Os.
- -fstrict-aliasing
-
Allow the compiler to assume the strictest aliasing rules applicable to
the language being compiled. For C (and C++), this activates
optimizations based on the type of expressions. In particular, an
object of one type is assumed never to reside at the same address as an
object of a different type, unless the types are almost the same. For
example, an "unsigned int" can alias an "int", but not a
"void*" or a "double". A character type may alias any other
type.
Pay special attention to code like this:
union a_union { int i; double d; }; int f() { union a_union t; t.d = 3.0; return t.i; }
The practice of reading from a different union member than the one most recently written to (called ``type-punning'') is common. Even with -fstrict-aliasing, type-punning is allowed, provided the memory is accessed through the union type. So, the code above will work as expected. However, this code might not:
int f() { union a_union t; int* ip; t.d = 3.0; ip = &t.i; return *ip; }
Similarly, access by taking the address, casting the resulting pointer and dereferencing the result has undefined behavior, even if the cast uses a union type, e.g.:
int f() { double d = 3.0; return ((union a_union *) &d)->i; }
The -fstrict-aliasing option is enabled at levels -O2, -O3, -Os.
- -fstrict-overflow
-
Allow the compiler to assume strict signed overflow rules, depending
on the language being compiled. For C (and C++) this means that
overflow when doing arithmetic with signed numbers is undefined, which
means that the compiler may assume that it will not happen. This
permits various optimizations. For example, the compiler will assume
that an expression like "i + 10 > i" will always be true for
signed "i". This assumption is only valid if signed overflow is
undefined, as the expression is false if "i + 10" overflows when
using twos complement arithmetic. When this option is in effect any
attempt to determine whether an operation on signed numbers will
overflow must be written carefully to not actually involve overflow.
This option also allows the compiler to assume strict pointer semantics: given a pointer to an object, if adding an offset to that pointer does not produce a pointer to the same object, the addition is undefined. This permits the compiler to conclude that "p + u > p" is always true for a pointer "p" and unsigned integer "u". This assumption is only valid because pointer wraparound is undefined, as the expression is false if "p + u" overflows using twos complement arithmetic.
See also the -fwrapv option. Using -fwrapv means that integer signed overflow is fully defined: it wraps. When -fwrapv is used, there is no difference between -fstrict-overflow and -fno-strict-overflow for integers. With -fwrapv certain types of overflow are permitted. For example, if the compiler gets an overflow when doing arithmetic on constants, the overflowed value can still be used with -fwrapv, but not otherwise.
The -fstrict-overflow option is enabled at levels -O2, -O3, -Os.
- -falign-functions
- -falign-functions=n
-
Align the start of functions to the next power-of-two greater than
n, skipping up to n bytes. For instance,
-falign-functions=32 aligns functions to the next 32-byte
boundary, but -falign-functions=24 would align to the next
32-byte boundary only if this can be done by skipping 23 bytes or less.
-fno-align-functions and -falign-functions=1 are equivalent and mean that functions will not be aligned.
Some assemblers only support this flag when n is a power of two; in that case, it is rounded up.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
- -falign-labels
- -falign-labels=n
-
Align all branch targets to a power-of-two boundary, skipping up to
n bytes like -falign-functions. This option can easily
make code slower, because it must insert dummy operations for when the
branch target is reached in the usual flow of the code.
-fno-align-labels and -falign-labels=1 are equivalent and mean that labels will not be aligned.
If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values are used instead.
If n is not specified or is zero, use a machine-dependent default which is very likely to be 1, meaning no alignment.
Enabled at levels -O2, -O3.
- -falign-loops
- -falign-loops=n
-
Align loops to a power-of-two boundary, skipping up to n bytes
like -falign-functions. The hope is that the loop will be
executed many times, which will make up for any execution of the dummy
operations.
-fno-align-loops and -falign-loops=1 are equivalent and mean that loops will not be aligned.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
- -falign-jumps
- -falign-jumps=n
-
Align branch targets to a power-of-two boundary, for branch targets
where the targets can only be reached by jumping, skipping up to n
bytes like -falign-functions. In this case, no dummy operations
need be executed.
-fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops will not be aligned.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
- -funit-at-a-time
-
This option is left for compatibility reasons. -funit-at-a-time
has no effect, while -fno-unit-at-a-time implies
-fno-toplevel-reorder and -fno-section-anchors.
Enabled by default.
- -fno-toplevel-reorder
-
Do not reorder top-level functions, variables, and "asm"
statements. Output them in the same order that they appear in the
input file. When this option is used, unreferenced static variables
will not be removed. This option is intended to support existing code
which relies on a particular ordering. For new code, it is better to
use attributes.
Enabled at level -O0. When disabled explicitly, it also imply -fno-section-anchors that is otherwise enabled at -O0 on some targets.
- -fweb
-
Constructs webs as commonly used for register allocation purposes and assign
each web individual pseudo register. This allows the register allocation pass
to operate on pseudos directly, but also strengthens several other optimization
passes, such as CSE, loop optimizer and trivial dead code remover. It can,
however, make debugging impossible, since variables will no longer stay in a
``home register''.
Enabled by default with -funroll-loops.
- -fwhole-program
-
Assume that the current compilation unit represents whole program being
compiled. All public functions and variables with the exception of "main"
and those merged by attribute "externally_visible" become static functions
and in a affect gets more aggressively optimized by interprocedural optimizers.
While this option is equivalent to proper use of "static" keyword for
programs consisting of single file, in combination with option
--combine this flag can be used to compile most of smaller scale C
programs since the functions and variables become local for the whole combined
compilation unit, not for the single source file itself.
This option is not supported for Fortran programs.
- -fcprop-registers
-
After register allocation and post-register allocation instruction splitting,
we perform a copy-propagation pass to try to reduce scheduling dependencies
and occasionally eliminate the copy.
Enabled at levels -O, -O2, -O3, -Os.
- -fprofile-correction
- Profiles collected using an instrumented binary for multi-threaded programs may be inconsistent due to missed counter updates. When this option is specified, GCC will use heuristics to correct or smooth out such inconsistencies. By default, GCC will emit an error message when an inconsistent profile is detected.
- -fprofile-dir=path
- Set the directory to search the profile data files in to path. This option affects only the profile data generated by -fprofile-generate, -ftest-coverage, -fprofile-arcs and used by -fprofile-use and -fbranch-probabilities and its related options. By default, GCC will use the current directory as path thus the profile data file will appear in the same directory as the object file.
- -fprofile-generate
- -fprofile-generate=path
-
Enable options usually used for instrumenting application to produce
profile useful for later recompilation with profile feedback based
optimization. You must use -fprofile-generate both when
compiling and when linking your program.
The following options are enabled: "-fprofile-arcs", "-fprofile-values", "-fvpt".
If path is specified, GCC will look at the path to find the profile feedback data files. See -fprofile-dir.
- -fprofile-use
- -fprofile-use=path
-
Enable profile feedback directed optimizations, and optimizations
generally profitable only with profile feedback available.
The following options are enabled: "-fbranch-probabilities", "-fvpt", "-funroll-loops", "-fpeel-loops", "-ftracer"
By default, GCC emits an error message if the feedback profiles do not match the source code. This error can be turned into a warning by using -Wcoverage-mismatch. Note this may result in poorly optimized code.
If path is specified, GCC will look at the path to find the profile feedback data files. See -fprofile-dir.
The following options control compiler behavior regarding floating point arithmetic. These options trade off between speed and correctness. All must be specifically enabled.
- -ffloat-store
-
Do not store floating point variables in registers, and inhibit other
options that might change whether a floating point value is taken from a
register or memory.
This option prevents undesirable excess precision on machines such as the 68000 where the floating registers (of the 68881) keep more precision than a "double" is supposed to have. Similarly for the x86 architecture. For most programs, the excess precision does only good, but a few programs rely on the precise definition of IEEE floating point. Use -ffloat-store for such programs, after modifying them to store all pertinent intermediate computations into variables.
- -ffast-math
-
Sets -fno-math-errno, -funsafe-math-optimizations,
-ffinite-math-only, -fno-rounding-math,
-fno-signaling-nans and -fcx-limited-range.
This option causes the preprocessor macro "__FAST_MATH__" to be defined.
This option is not turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.
- -fno-math-errno
-
Do not set ERRNO after calling math functions that are executed
with a single instruction, e.g., sqrt. A program that relies on
IEEE exceptions for math error handling may want to use this flag
for speed while maintaining IEEE arithmetic compatibility.
This option is not turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.
The default is -fmath-errno.
On Darwin systems, the math library never sets "errno". There is therefore no reason for the compiler to consider the possibility that it might, and -fno-math-errno is the default.
- -funsafe-math-optimizations
-
Allow optimizations for floating-point arithmetic that (a) assume
that arguments and results are valid and (b) may violate IEEE or
ANSI standards. When used at link-time, it may include libraries
or startup files that change the default FPU control word or other
similar optimizations.
This option is not turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications. Enables -fno-signed-zeros, -fno-trapping-math, -fassociative-math and -freciprocal-math.
The default is -fno-unsafe-math-optimizations.
- -fassociative-math
-
Allow re-association of operands in series of floating-point operations.
This violates the ISO C and C++ language standard by possibly changing
computation result. NOTE: re-ordering may change the sign of zero as
well as ignore NaNs and inhibit or create underflow or overflow (and
thus cannot be used on a code which relies on rounding behavior like
"(x + 2**52) - 2**52)". May also reorder floating-point comparisons
and thus may not be used when ordered comparisons are required.
This option requires that both -fno-signed-zeros and
-fno-trapping-math be in effect. Moreover, it doesn't make
much sense with -frounding-math.
The default is -fno-associative-math.
- -freciprocal-math
-
Allow the reciprocal of a value to be used instead of dividing by
the value if this enables optimizations. For example "x / y"
can be replaced with "x * (1/y)" which is useful if "(1/y)"
is subject to common subexpression elimination. Note that this loses
precision and increases the number of flops operating on the value.
The default is -fno-reciprocal-math.
- -ffinite-math-only
-
Allow optimizations for floating-point arithmetic that assume
that arguments and results are not NaNs or +-Infs.
This option is not turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.
The default is -fno-finite-math-only.
- -fno-signed-zeros
-
Allow optimizations for floating point arithmetic that ignore the
signedness of zero. IEEE arithmetic specifies the behavior of
distinct +0.0 and -0.0 values, which then prohibits simplification
of expressions such as x+0.0 or 0.0*x (even with -ffinite-math-only).
This option implies that the sign of a zero result isn't significant.
The default is -fsigned-zeros.
- -fno-trapping-math
-
Compile code assuming that floating-point operations cannot generate
user-visible traps. These traps include division by zero, overflow,
underflow, inexact result and invalid operation. This option requires
that -fno-signaling-nans be in effect. Setting this option may
allow faster code if one relies on ``non-stop'' IEEE arithmetic, for example.
This option should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.
The default is -ftrapping-math.
- -frounding-math
-
Disable transformations and optimizations that assume default floating
point rounding behavior. This is round-to-zero for all floating point
to integer conversions, and round-to-nearest for all other arithmetic
truncations. This option should be specified for programs that change
the FP rounding mode dynamically, or that may be executed with a
non-default rounding mode. This option disables constant folding of
floating point expressions at compile-time (which may be affected by
rounding mode) and arithmetic transformations that are unsafe in the
presence of sign-dependent rounding modes.
The default is -fno-rounding-math.
This option is experimental and does not currently guarantee to disable all GCC optimizations that are affected by rounding mode. Future versions of GCC may provide finer control of this setting using C99's "FENV_ACCESS" pragma. This command line option will be used to specify the default state for "FENV_ACCESS".
- -frtl-abstract-sequences
- It is a size optimization method. This option is to find identical sequences of code, which can be turned into pseudo-procedures and then replace all occurrences with calls to the newly created subroutine. It is kind of an opposite of -finline-functions. This optimization runs at RTL level.
- -fsignaling-nans
-
Compile code assuming that IEEE signaling NaNs may generate user-visible
traps during floating-point operations. Setting this option disables
optimizations that may change the number of exceptions visible with
signaling NaNs. This option implies -ftrapping-math.
This option causes the preprocessor macro "__SUPPORT_SNAN__" to be defined.
The default is -fno-signaling-nans.
This option is experimental and does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior.
- -fsingle-precision-constant
- Treat floating point constant as single precision constant instead of implicitly converting it to double precision constant.
- -fcx-limited-range
-
When enabled, this option states that a range reduction step is not
needed when performing complex division. Also, there is no checking
whether the result of a complex multiplication or division is "NaN
+ I*NaN", with an attempt to rescue the situation in that case. The
default is -fno-cx-limited-range, but is enabled by
-ffast-math.
This option controls the default setting of the ISO C99 "CX_LIMITED_RANGE" pragma. Nevertheless, the option applies to all languages.
- -fcx-fortran-rules
-
Complex multiplication and division follow Fortran rules. Range
reduction is done as part of complex division, but there is no checking
whether the result of a complex multiplication or division is "NaN
+ I*NaN", with an attempt to rescue the situation in that case.
The default is -fno-cx-fortran-rules.
The following options control optimizations that may improve performance, but are not enabled by any -O options. This section includes experimental options that may produce broken code.
- -fbranch-probabilities
-
After running a program compiled with -fprofile-arcs, you can compile it a second time using
-fbranch-probabilities, to improve optimizations based on
the number of times each branch was taken. When the program
compiled with -fprofile-arcs exits it saves arc execution
counts to a file called sourcename.gcda for each source
file. The information in this data file is very dependent on the
structure of the generated code, so you must use the same source code
and the same optimization options for both compilations.
With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each JUMP_INSN and CALL_INSN. These can be used to improve optimization. Currently, they are only used in one place: in reorg.c, instead of guessing which path a branch is mostly to take, the REG_BR_PROB values are used to exactly determine which path is taken more often.
- -fprofile-values
-
If combined with -fprofile-arcs, it adds code so that some
data about values of expressions in the program is gathered.
With -fbranch-probabilities, it reads back the data gathered from profiling values of expressions and adds REG_VALUE_PROFILE notes to instructions for their later usage in optimizations.
Enabled with -fprofile-generate and -fprofile-use.
- -fvpt
-
If combined with -fprofile-arcs, it instructs the compiler to add
a code to gather information about values of expressions.
With -fbranch-probabilities, it reads back the data gathered and actually performs the optimizations based on them. Currently the optimizations include specialization of division operation using the knowledge about the value of the denominator.
- -frename-registers
-
Attempt to avoid false dependencies in scheduled code by making use
of registers left over after register allocation. This optimization
will most benefit processors with lots of registers. Depending on the
debug information format adopted by the target, however, it can
make debugging impossible, since variables will no longer stay in
a ``home register''.
Enabled by default with -funroll-loops.
- -ftracer
-
Perform tail duplication to enlarge superblock size. This transformation
simplifies the control flow of the function allowing other optimizations to do
better job.
Enabled with -fprofile-use.
- -funroll-loops
-
Unroll loops whose number of iterations can be determined at compile time or
upon entry to the loop. -funroll-loops implies
-frerun-cse-after-loop, -fweb and -frename-registers.
It also turns on complete loop peeling (i.e. complete removal of loops with
small constant number of iterations). This option makes code larger, and may
or may not make it run faster.
Enabled with -fprofile-use.
- -funroll-all-loops
- Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This usually makes programs run more slowly. -funroll-all-loops implies the same options as -funroll-loops.
- -fpeel-loops
-
Peels the loops for that there is enough information that they do not
roll much (from profile feedback). It also turns on complete loop peeling
(i.e. complete removal of loops with small constant number of iterations).
Enabled with -fprofile-use.
- -fmove-loop-invariants
- Enables the loop invariant motion pass in the RTL loop optimizer. Enabled at level -O1
- -funswitch-loops
- Move branches with loop invariant conditions out of the loop, with duplicates of the loop on both branches (modified according to result of the condition).
- -ffunction-sections
- -fdata-sections
-
Place each function or data item into its own section in the output
file if the target supports arbitrary sections. The name of the
function or the name of the data item determines the section's name
in the output file.
Use these options on systems where the linker can perform optimizations to improve locality of reference in the instruction space. Most systems using the ELF object format and SPARC processors running Solaris 2 have linkers with such optimizations. AIX may have these optimizations in the future.
Only use these options when there are significant benefits from doing so. When you specify these options, the assembler and linker will create larger object and executable files and will also be slower. You will not be able to use "gprof" on all systems if you specify this option and you may have problems with debugging if you specify both this option and -g.
- -fbranch-target-load-optimize
- Perform branch target register load optimization before prologue / epilogue threading. The use of target registers can typically be exposed only during reload, thus hoisting loads out of loops and doing inter-block scheduling needs a separate optimization pass.
- -fbranch-target-load-optimize2
- Perform branch target register load optimization after prologue / epilogue threading.
- -fbtr-bb-exclusive
- When performing branch target register load optimization, don't reuse branch target registers in within any basic block.
- -fstack-protector
- Emit extra code to check for buffer overflows, such as stack smashing attacks. This is done by adding a guard variable to functions with vulnerable objects. This includes functions that call alloca, and functions with buffers larger than 8 bytes. The guards are initialized when a function is entered and then checked when the function exits. If a guard check fails, an error message is printed and the program exits.
- -fstack-protector-all
- Like -fstack-protector except that all functions are protected.
- -fsection-anchors
-
Try to reduce the number of symbolic address calculations by using
shared ``anchor'' symbols to address nearby objects. This transformation
can help to reduce the number of GOT entries and GOT accesses on some
targets.
For example, the implementation of the following function "foo":
static int a, b, c; int foo (void) { return a + b + c; }
would usually calculate the addresses of all three variables, but if you compile it with -fsection-anchors, it will access the variables from a common anchor point instead. The effect is similar to the following pseudocode (which isn't valid C):
int foo (void) { register int *xr = &x; return xr[&a - &x] + xr[&b - &x] + xr[&c - &x]; }
Not all targets support this option.
- --param name=value
-
In some places, GCC uses various constants to control the amount of
optimization that is done. For example, GCC will not inline functions
that contain more that a certain number of instructions. You can
control some of these constants on the command-line using the
--param option.
The names of specific parameters, and the meaning of the values, are tied to the internals of the compiler, and are subject to change without notice in future releases.
In each case, the value is an integer. The allowable choices for name are given in the following table:
-
- sra-max-structure-size
- The maximum structure size, in bytes, at which the scalar replacement of aggregates (SRA) optimization will perform block copies. The default value, 0, implies that GCC will select the most appropriate size itself.
- sra-field-structure-ratio
- The threshold ratio (as a percentage) between instantiated fields and the complete structure size. We say that if the ratio of the number of bytes in instantiated fields to the number of bytes in the complete structure exceeds this parameter, then block copies are not used. The default is 75.
- struct-reorg-cold-struct-ratio
- The threshold ratio (as a percentage) between a structure frequency and the frequency of the hottest structure in the program. This parameter is used by struct-reorg optimization enabled by -fipa-struct-reorg. We say that if the ratio of a structure frequency, calculated by profiling, to the hottest structure frequency in the program is less than this parameter, then structure reorganization is not applied to this structure. The default is 10.
- predictable-branch-cost-outcome
- When branch is predicted to be taken with probability lower than this threshold (in percent), then it is considered well predictable. The default is 10.
- max-crossjump-edges
- The maximum number of incoming edges to consider for crossjumping. The algorithm used by -fcrossjumping is O(N^2) in the number of edges incoming to each block. Increasing values mean more aggressive optimization, making the compile time increase with probably small improvement in executable size.
- min-crossjump-insns
- The minimum number of instructions which must be matched at the end of two blocks before crossjumping will be performed on them. This value is ignored in the case where all instructions in the block being crossjumped from are matched. The default value is 5.
- max-grow-copy-bb-insns
- The maximum code size expansion factor when copying basic blocks instead of jumping. The expansion is relative to a jump instruction. The default value is 8.
- max-goto-duplication-insns
- The maximum number of instructions to duplicate to a block that jumps to a computed goto. To avoid O(N^2) behavior in a number of passes, GCC factors computed gotos early in the compilation process, and unfactors them as late as possible. Only computed jumps at the end of a basic blocks with no more than max-goto-duplication-insns are unfactored. The default value is 8.
- max-delay-slot-insn-search
- The maximum number of instructions to consider when looking for an instruction to fill a delay slot. If more than this arbitrary number of instructions is searched, the time savings from filling the delay slot will be minimal so stop searching. Increasing values mean more aggressive optimization, making the compile time increase with probably small improvement in executable run time.
- max-delay-slot-live-search
- When trying to fill delay slots, the maximum number of instructions to consider when searching for a block with valid live register information. Increasing this arbitrarily chosen value means more aggressive optimization, increasing the compile time. This parameter should be removed when the delay slot code is rewritten to maintain the control-flow graph.
- max-gcse-memory
- The approximate maximum amount of memory that will be allocated in order to perform the global common subexpression elimination optimization. If more memory than specified is required, the optimization will not be done.
- max-gcse-passes
- The maximum number of passes of GCSE to run. The default is 1.
- max-pending-list-length
- The maximum number of pending dependencies scheduling will allow before flushing the current state and starting over. Large functions with few branches or calls can create excessively large lists which needlessly consume memory and resources.
- max-inline-insns-single
- Several parameters control the tree inliner used in gcc. This number sets the maximum number of instructions (counted in GCC's internal representation) in a single function that the tree inliner will consider for inlining. This only affects functions declared inline and methods implemented in a class declaration (C++). The default value is 450.
- max-inline-insns-auto
- When you use -finline-functions (included in -O3), a lot of functions that would otherwise not be considered for inlining by the compiler will be investigated. To those functions, a different (more restrictive) limit compared to functions declared inline can be applied. The default value is 90.
- large-function-insns
- The limit specifying really large functions. For functions larger than this limit after inlining, inlining is constrained by --param large-function-growth. This parameter is useful primarily to avoid extreme compilation time caused by non-linear algorithms used by the backend. The default value is 2700.
- large-function-growth
- Specifies maximal growth of large function caused by inlining in percents. The default value is 100 which limits large function growth to 2.0 times the original size.
- large-unit-insns
- The limit specifying large translation unit. Growth caused by inlining of units larger than this limit is limited by --param inline-unit-growth. For small units this might be too tight (consider unit consisting of function A that is inline and B that just calls A three time. If B is small relative to A, the growth of unit is 300\% and yet such inlining is very sane. For very large units consisting of small inlineable functions however the overall unit growth limit is needed to avoid exponential explosion of code size. Thus for smaller units, the size is increased to --param large-unit-insns before applying --param inline-unit-growth. The default is 10000
- inline-unit-growth
- Specifies maximal overall growth of the compilation unit caused by inlining. The default value is 30 which limits unit growth to 1.3 times the original size.
- ipcp-unit-growth
- Specifies maximal overall growth of the compilation unit caused by interprocedural constant propagation. The default value is 10 which limits unit growth to 1.1 times the original size.
- large-stack-frame
- The limit specifying large stack frames. While inlining the algorithm is trying to not grow past this limit too much. Default value is 256 bytes.
- large-stack-frame-growth
- Specifies maximal growth of large stack frames caused by inlining in percents. The default value is 1000 which limits large stack frame growth to 11 times the original size.
- max-inline-insns-recursive
- max-inline-insns-recursive-auto
-
Specifies maximum number of instructions out-of-line copy of self recursive inline
function can grow into by performing recursive inlining.
For functions declared inline --param max-inline-insns-recursive is taken into account. For function not declared inline, recursive inlining happens only when -finline-functions (included in -O3) is enabled and --param max-inline-insns-recursive-auto is used. The default value is 450.
- max-inline-recursive-depth
- max-inline-recursive-depth-auto
-
Specifies maximum recursion depth used by the recursive inlining.
For functions declared inline --param max-inline-recursive-depth is taken into account. For function not declared inline, recursive inlining happens only when -finline-functions (included in -O3) is enabled and --param max-inline-recursive-depth-auto is used. The default value is 8.
- min-inline-recursive-probability
-
Recursive inlining is profitable only for function having deep recursion
in average and can hurt for function having little recursion depth by
increasing the prologue size or complexity of function body to other
optimizers.
When profile feedback is available (see -fprofile-generate) the actual recursion depth can be guessed from probability that function will recurse via given call expression. This parameter limits inlining only to call expression whose probability exceeds given threshold (in percents). The default value is 10.
- inline-call-cost
- Specify cost of call instruction relative to simple arithmetics operations (having cost of 1). Increasing this cost disqualifies inlining of non-leaf functions and at the same time increases size of leaf function that is believed to reduce function size by being inlined. In effect it increases amount of inlining for code having large abstraction penalty (many functions that just pass the arguments to other functions) and decrease inlining for code with low abstraction penalty. The default value is 12.
- min-vect-loop-bound
- The minimum number of iterations under which a loop will not get vectorized when -ftree-vectorize is used. The number of iterations after vectorization needs to be greater than the value specified by this option to allow vectorization. The default value is 0.
- max-unrolled-insns
- The maximum number of instructions that a loop should have if that loop is unrolled, and if the loop is unrolled, it determines how many times the loop code is unrolled.
- max-average-unrolled-insns
- The maximum number of instructions biased by probabilities of their execution that a loop should have if that loop is unrolled, and if the loop is unrolled, it determines how many times the loop code is unrolled.
- max-unroll-times
- The maximum number of unrollings of a single loop.
- max-peeled-insns
- The maximum number of instructions that a loop should have if that loop is peeled, and if the loop is peeled, it determines how many times the loop code is peeled.
- max-peel-times
- The maximum number of peelings of a single loop.
- max-completely-peeled-insns
- The maximum number of insns of a completely peeled loop.
- max-completely-peel-times
- The maximum number of iterations of a loop to be suitable for complete peeling.
- max-completely-peel-loop-nest-depth
- The maximum depth of a loop nest suitable for complete peeling.
- max-unswitch-insns
- The maximum number of insns of an unswitched loop.
- max-unswitch-level
- The maximum number of branches unswitched in a single loop.
- lim-expensive
- The minimum cost of an expensive expression in the loop invariant motion.
- iv-consider-all-candidates-bound
- Bound on number of candidates for induction variables below that all candidates are considered for each use in induction variable optimizations. Only the most relevant candidates are considered if there are more candidates, to avoid quadratic time complexity.
- iv-max-considered-uses
- The induction variable optimizations give up on loops that contain more induction variable uses.
- iv-always-prune-cand-set-bound
- If number of candidates in the set is smaller than this value, we always try to remove unnecessary ivs from the set during its optimization when a new iv is added to the set.
- scev-max-expr-size
- Bound on size of expressions used in the scalar evolutions analyzer. Large expressions slow the analyzer.
- omega-max-vars
- The maximum number of variables in an Omega constraint system. The default value is 128.
- omega-max-geqs
- The maximum number of inequalities in an Omega constraint system. The default value is 256.
- omega-max-eqs
- The maximum number of equalities in an Omega constraint system. The default value is 128.
- omega-max-wild-cards
- The maximum number of wildcard variables that the Omega solver will be able to insert. The default value is 18.
- omega-hash-table-size
- The size of the hash table in the Omega solver. The default value is 550.
- omega-max-keys
- The maximal number of keys used by the Omega solver. The default value is 500.
- omega-eliminate-redundant-constraints
- When set to 1, use expensive methods to eliminate all redundant constraints. The default value is 0.
- vect-max-version-for-alignment-checks
- The maximum number of runtime checks that can be performed when doing loop versioning for alignment in the vectorizer. See option ftree-vect-loop-version for more information.
- vect-max-version-for-alias-checks
- The maximum number of runtime checks that can be performed when doing loop versioning for alias in the vectorizer. See option ftree-vect-loop-version for more information.
- max-iterations-to-track
- The maximum number of iterations of a loop the brute force algorithm for analysis of # of iterations of the loop tries to evaluate.
- hot-bb-count-fraction
- Select fraction of the maximal count of repetitions of basic block in program given basic block needs to have to be considered hot.
- hot-bb-frequency-fraction
- Select fraction of the maximal frequency of executions of basic block in function given basic block needs to have to be considered hot
- max-predicted-iterations
- The maximum number of loop iterations we predict statically. This is useful in cases where function contain single loop with known bound and other loop with unknown. We predict the known number of iterations correctly, while the unknown number of iterations average to roughly 10. This means that the loop without bounds would appear artificially cold relative to the other one.
- align-threshold
- Select fraction of the maximal frequency of executions of basic block in function given basic block will get aligned.
- align-loop-iterations
- A loop expected to iterate at lest the selected number of iterations will get aligned.
- tracer-dynamic-coverage
- tracer-dynamic-coverage-feedback
-
This value is used to limit superblock formation once the given percentage of
executed instructions is covered. This limits unnecessary code size
expansion.
The tracer-dynamic-coverage-feedback is used only when profile feedback is available. The real profiles (as opposed to statically estimated ones) are much less balanced allowing the threshold to be larger value.
- tracer-max-code-growth
- Stop tail duplication once code growth has reached given percentage. This is rather hokey argument, as most of the duplicates will be eliminated later in cross jumping, so it may be set to much higher values than is the desired code growth.
- tracer-min-branch-ratio
- Stop reverse growth when the reverse probability of best edge is less than this threshold (in percent).
- tracer-min-branch-ratio
- tracer-min-branch-ratio-feedback
-
Stop forward growth if the best edge do have probability lower than this
threshold.
Similarly to tracer-dynamic-coverage two values are present, one for compilation for profile feedback and one for compilation without. The value for compilation with profile feedback needs to be more conservative (higher) in order to make tracer effective.
- max-cse-path-length
- Maximum number of basic blocks on path that cse considers. The default is 10.
- max-cse-insns
- The maximum instructions CSE process before flushing. The default is 1000.
- max-aliased-vops
-
Maximum number of virtual operands per function allowed to represent
aliases before triggering the alias partitioning heuristic. Alias
partitioning reduces compile times and memory consumption needed for
aliasing at the expense of precision loss in alias information. The
default value for this parameter is 100 for -O1, 500 for -O2 and 1000
for -O3.
Notice that if a function contains more memory statements than the value of this parameter, it is not really possible to achieve this reduction. In this case, the compiler will use the number of memory statements as the value for max-aliased-vops.
- avg-aliased-vops
- Average number of virtual operands per statement allowed to represent aliases before triggering the alias partitioning heuristic. This works in conjunction with max-aliased-vops. If a function contains more than max-aliased-vops virtual operators, then memory symbols will be grouped into memory partitions until either the total number of virtual operators is below max-aliased-vops or the average number of virtual operators per memory statement is below avg-aliased-vops. The default value for this parameter is 1 for -O1 and -O2, and 3 for -O3.
- ggc-min-expand
-
GCC uses a garbage collector to manage its own memory allocation. This
parameter specifies the minimum percentage by which the garbage
collector's heap should be allowed to expand between collections.
Tuning this may improve compilation speed; it has no effect on code
generation.
The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM >= 1GB. If "getrlimit" is available, the notion of ``RAM'' is the smallest of actual RAM and "RLIMIT_DATA" or "RLIMIT_AS". If GCC is not able to calculate RAM on a particular platform, the lower bound of 30% is used. Setting this parameter and ggc-min-heapsize to zero causes a full collection to occur at every opportunity. This is extremely slow, but can be useful for debugging.
- ggc-min-heapsize
-
Minimum size of the garbage collector's heap before it begins bothering
to collect garbage. The first collection occurs after the heap expands
by ggc-min-expand% beyond ggc-min-heapsize. Again,
tuning this may improve compilation speed, and has no effect on code
generation.
The default is the smaller of RAM/8, RLIMIT_RSS, or a limit which tries to ensure that RLIMIT_DATA or RLIMIT_AS are not exceeded, but with a lower bound of 4096 (four megabytes) and an upper bound of 131072 (128 megabytes). If GCC is not able to calculate RAM on a particular platform, the lower bound is used.  Setting this parameter and ggc-min-expand to zero causes a full collection to occur at every opportunity.
- max-reload-search-insns
- The maximum number of instruction reload should look backward for equivalent register. Increasing values mean more aggressive optimization, making the compile time increase with probably slightly better performance. The default value is 100.
- max-cselib-memory-locations
- The maximum number of memory locations cselib should take into account. Increasing values mean more aggressive optimization, making the compile time increase with probably slightly better performance. The default value is 500.
- reorder-blocks-duplicate
- reorder-blocks-duplicate-feedback
-
Used by basic block reordering pass to decide whether to use unconditional
branch or duplicate the code on its destination. Code is duplicated when its
estimated size is smaller than this value multiplied by the estimated size of
unconditional jump in the hot spots of the program.
The reorder-block-duplicate-feedback is used only when profile feedback is available and may be set to higher values than reorder-block-duplicate since information about the hot spots is more accurate.
- max-sched-ready-insns
- The maximum number of instructions ready to be issued the scheduler should consider at any given time during the first scheduling pass. Increasing values mean more thorough searches, making the compilation time increase with probably little benefit. The default value is 100.
- max-sched-region-blocks
- The maximum number of blocks in a region to be considered for interblock scheduling. The default value is 10.
- max-pipeline-region-blocks
- The maximum number of blocks in a region to be considered for pipelining in the selective scheduler. The default value is 15.
- max-sched-region-insns
- The maximum number of insns in a region to be considered for interblock scheduling. The default value is 100.
- max-pipeline-region-insns
- The maximum number of insns in a region to be considered for pipelining in the selective scheduler. The default value is 200.
- min-spec-prob
- The minimum probability (in percents) of reaching a source block for interblock speculative scheduling. The default value is 40.
- max-sched-extend-regions-iters
- The maximum number of iterations through CFG to extend regions. 0 - disable region extension, N - do at most N iterations. The default value is 0.
- max-sched-insn-conflict-delay
- The maximum conflict delay for an insn to be considered for speculative motion. The default value is 3.
- sched-spec-prob-cutoff
- The minimal probability of speculation success (in percents), so that speculative insn will be scheduled. The default value is 40.
- sched-mem-true-dep-cost
- Minimal distance (in CPU cycles) between store and load targeting same memory locations. The default value is 1.
- selsched-max-lookahead
- The maximum size of the lookahead window of selective scheduling. It is a depth of search for available instructions. The default value is 50.
- selsched-max-sched-times
- The maximum number of times that an instruction will be scheduled during selective scheduling. This is the limit on the number of iterations through which the instruction may be pipelined. The default value is 2.
- selsched-max-insns-to-rename
- The maximum number of best instructions in the ready list that are considered for renaming in the selective scheduler. The default value is 2.
- max-last-value-rtl
- The maximum size measured as number of RTLs that can be recorded in an expression in combiner for a pseudo register as last known value of that register. The default is 10000.
- integer-share-limit
- Small integer constants can use a shared data structure, reducing the compiler's memory usage and increasing its speed. This sets the maximum value of a shared integer constant. The default value is 256.
- min-virtual-mappings
- Specifies the minimum number of virtual mappings in the incremental SSA updater that should be registered to trigger the virtual mappings heuristic defined by virtual-mappings-ratio. The default value is 100.
- virtual-mappings-ratio
- If the number of virtual mappings is virtual-mappings-ratio bigger than the number of virtual symbols to be updated, then the incremental SSA updater switches to a full update for those symbols. The default ratio is 3.
- ssp-buffer-size
- The minimum size of buffers (i.e. arrays) that will receive stack smashing protection when -fstack-protection is used.
- max-jump-thread-duplication-stmts
- Maximum number of statements allowed in a block that needs to be duplicated when threading jumps.
- max-fields-for-field-sensitive
- Maximum number of fields in a structure we will treat in a field sensitive manner during pointer analysis. The default is zero for -O0, and -O1 and 100 for -Os, -O2, and -O3.
- prefetch-latency
- Estimate on average number of instructions that are executed before prefetch finishes. The distance we prefetch ahead is proportional to this constant. Increasing this number may also lead to less streams being prefetched (see simultaneous-prefetches).
- simultaneous-prefetches
- Maximum number of prefetches that can run at the same time.
- l1-cache-line-size
- The size of cache line in L1 cache, in bytes.
- l1-cache-size
- The size of L1 cache, in kilobytes.
- l2-cache-size
- The size of L2 cache, in kilobytes.
- use-canonical-types
- Whether the compiler should use the ``canonical'' type system. By default, this should always be 1, which uses a more efficient internal mechanism for comparing types in C++ and Objective-C++. However, if bugs in the canonical type system are causing compilation failures, set this value to 0 to disable canonical types.
- switch-conversion-max-branch-ratio
- Switch initialization conversion will refuse to create arrays that are bigger than switch-conversion-max-branch-ratio times the number of branches in the switch.
- max-partial-antic-length
- Maximum length of the partial antic set computed during the tree partial redundancy elimination optimization (-ftree-pre) when optimizing at -O3 and above. For some sorts of source code the enhanced partial redundancy elimination optimization can run away, consuming all of the memory available on the host machine. This parameter sets a limit on the length of the sets that are computed, which prevents the runaway behavior. Setting a value of 0 for this parameter will allow an unlimited set length.
- sccvn-max-scc-size
- Maximum size of a strongly connected component (SCC) during SCCVN processing. If this limit is hit, SCCVN processing for the whole function will not be done and optimizations depending on it will be disabled. The default maximum SCC size is 10000.
- ira-max-loops-num
- IRA uses a regional register allocation by default. If a function contains loops more than number given by the parameter, only at most given number of the most frequently executed loops will form regions for the regional register allocation. The default value of the parameter is 100.
- ira-max-conflict-table-size
- Although IRA uses a sophisticated algorithm of compression conflict table, the table can be still big for huge functions. If the conflict table for a function could be more than size in MB given by the parameter, the conflict table is not built and faster, simpler, and lower quality register allocation algorithm will be used. The algorithm do not use pseudo-register conflicts. The default value of the parameter is 2000.
- loop-invariant-max-bbs-in-loop
- Loop invariant motion can be very expensive, both in compile time and in amount of needed compile time memory, with very large loops. Loops with more basic blocks than this parameter won't have loop invariant motion optimization performed on them. The default value of the parameter is 1000 for -O1 and 10000 for -O2 and above.
- max-vartrack-size
- Sets a maximum number of hash table slots to use during variable tracking dataflow analysis of any function. If this limit is exceeded with variable tracking at assignments enabled, analysis for that function is retried without it, after removing all debug insns from the function. If the limit is exceeded even without debug insns, var tracking analysis is completely disabled for the function. Setting the parameter to zero makes it unlimited.
- min-nondebug-insn-uid
- Use uids starting at this parameter for nondebug insns. The range below the parameter is reserved exclusively for debug insns created by -fvar-tracking-assignments, but debug insns may get (non-overlapping) uids above it if the reserved range is exhausted.
-
Options Controlling the Preprocessor
These options control the C preprocessor, which is run on each C source file before actual compilation.If you use the -E option, nothing is done except preprocessing. Some of these options make sense only together with -E because they cause the preprocessor output to be unsuitable for actual compilation.
- -Wp,option
- You can use -Wp,option to bypass the compiler driver and pass option directly through to the preprocessor. If option contains commas, it is split into multiple options at the commas. However, many options are modified, translated or interpreted by the compiler driver before being passed to the preprocessor, and -Wp forcibly bypasses this phase. The preprocessor's direct interface is undocumented and subject to change, so whenever possible you should avoid using -Wp and let the driver handle the options instead.
- -Xpreprocessor option
-
Pass option as an option to the preprocessor. You can use this to
supply system-specific preprocessor options which GCC does not know how to
recognize.
If you want to pass an option that takes an argument, you must use -Xpreprocessor twice, once for the option and once for the argument.
- -D name
- Predefine name as a macro, with definition 1.
- -D name=definition
-
The contents of definition are tokenized and processed as if
they appeared during translation phase three in a #define
directive. In particular, the definition will be truncated by
embedded newline characters.
If you are invoking the preprocessor from a shell or shell-like program you may need to use the shell's quoting syntax to protect characters such as spaces that have a meaning in the shell syntax.
If you wish to define a function-like macro on the command line, write its argument list with surrounding parentheses before the equals sign (if any). Parentheses are meaningful to most shells, so you will need to quote the option. With sh and csh, -D'name(args...)=definition' works.
-D and -U options are processed in the order they are given on the command line. All -imacros file and -include file options are processed after all -D and -U options.
- -U name
- Cancel any previous definition of name, either built in or provided with a -D option.
- -undef
- Do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined.
- -I dir
- Add the directory dir to the list of directories to be searched for header files. Directories named by -I are searched before the standard system include directories. If the directory dir is a standard system include directory, the option is ignored to ensure that the default search order for system directories and the special treatment of system headers are not defeated . If dir begins with "=", then the "=" will be replaced by the sysroot prefix; see --sysroot and -isysroot.
- -o file
- Write output to file. This is the same as specifying file as the second non-option argument to cpp. gcc has a different interpretation of a second non-option argument, so you must use -o to specify the output file.
- -Wall
- Turns on all optional warnings which are desirable for normal code. At present this is -Wcomment, -Wtrigraphs, -Wmultichar and a warning about integer promotion causing a change of sign in "#if" expressions. Note that many of the preprocessor's warnings are on by default and have no options to control them.
- -Wcomment
- -Wcomments
- Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a backslash-newline appears in a // comment. (Both forms have the same effect.)
- -Wtrigraphs
-
Most trigraphs in comments cannot affect the meaning of the program.
However, a trigraph that would form an escaped newline (??/ at
the end of a line) can, by changing where the comment begins or ends.
Therefore, only trigraphs that would form escaped newlines produce
warnings inside a comment.
This option is implied by -Wall. If -Wall is not given, this option is still enabled unless trigraphs are enabled. To get trigraph conversion without warnings, but get the other -Wall warnings, use -trigraphs -Wall -Wno-trigraphs.
- -Wtraditional
- Warn about certain constructs that behave differently in traditional and ISO C. Also warn about ISO C constructs that have no traditional C equivalent, and problematic constructs which should be avoided.
- -Wundef
- Warn whenever an identifier which is not a macro is encountered in an #if directive, outside of defined. Such identifiers are replaced with zero.
- -Wunused-macros
-
Warn about macros defined in the main file that are unused. A macro
is used if it is expanded or tested for existence at least once.
The preprocessor will also warn if the macro has not been used at the
time it is redefined or undefined.
Built-in macros, macros defined on the command line, and macros defined in include files are not warned about.
Note: If a macro is actually used, but only used in skipped conditional blocks, then CPP will report it as unused. To avoid the warning in such a case, you might improve the scope of the macro's definition by, for example, moving it into the first skipped block. Alternatively, you could provide a dummy use with something like:
#if defined the_macro_causing_the_warning #endif
- -Wendif-labels
-
Warn whenever an #else or an #endif are followed by text.
This usually happens in code of the form
#if FOO ... #else FOO ... #endif FOO
The second and third "FOO" should be in comments, but often are not in older programs. This warning is on by default.
- -Werror
- Make all warnings into hard errors. Source code which triggers warnings will be rejected.
- -Wsystem-headers
- Issue warnings for code in system headers. These are normally unhelpful in finding bugs in your own code, therefore suppressed. If you are responsible for the system library, you may want to see them.
- -w
- Suppress all warnings, including those which GNU CPP issues by default.
- -pedantic
- Issue all the mandatory diagnostics listed in the C standard. Some of them are left out by default, since they trigger frequently on harmless code.
- -pedantic-errors
- Issue all the mandatory diagnostics, and make all mandatory diagnostics into errors. This includes mandatory diagnostics that GCC issues without -pedantic but treats as warnings.
- -M
-
Instead of outputting the result of preprocessing, output a rule
suitable for make describing the dependencies of the main
source file. The preprocessor outputs one make rule containing
the object file name for that source file, a colon, and the names of all
the included files, including those coming from -include or
-imacros command line options.
Unless specified explicitly (with -MT or -MQ), the object file name consists of the name of the source file with any suffix replaced with object file suffix and with any leading directory parts removed. If there are many included files then the rule is split into several lines using \-newline. The rule has no commands.
This option does not suppress the preprocessor's debug output, such as -dM. To avoid mixing such debug output with the dependency rules you should explicitly specify the dependency output file with -MF, or use an environment variable like DEPENDENCIES_OUTPUT. Debug output will still be sent to the regular output stream as normal.
Passing -M to the driver implies -E, and suppresses warnings with an implicit -w.
- -MM
-
Like -M but do not mention header files that are found in
system header directories, nor header files that are included,
directly or indirectly, from such a header.
This implies that the choice of angle brackets or double quotes in an #include directive does not in itself determine whether that header will appear in -MM dependency output. This is a slight change in semantics from GCC versions 3.0 and earlier.
- -MF file
-
When used with -M or -MM, specifies a
file to write the dependencies to. If no -MF switch is given
the preprocessor sends the rules to the same place it would have sent
preprocessed output.
When used with the driver options -MD or -MMD, -MF overrides the default dependency output file.
- -MG
-
In conjunction with an option such as -M requesting
dependency generation, -MG assumes missing header files are
generated files and adds them to the dependency list without raising
an error. The dependency filename is taken directly from the
"#include" directive without prepending any path. -MG
also suppresses preprocessed output, as a missing header file renders
this useless.
This feature is used in automatic updating of makefiles.
- -MP
-
This option instructs CPP to add a phony target for each dependency
other than the main file, causing each to depend on nothing. These
dummy rules work around errors make gives if you remove header
files without updating the Makefile to match.
This is typical output:
test.o: test.c test.h test.h:
- -MT target
-
Change the target of the rule emitted by dependency generation. By
default CPP takes the name of the main input file, deletes any
directory components and any file suffix such as .c, and
appends the platform's usual object suffix. The result is the target.
An -MT option will set the target to be exactly the string you specify. If you want multiple targets, you can specify them as a single argument to -MT, or use multiple -MT options.
For example, -MT '$(objpfx)foo.o' might give
$(objpfx)foo.o: foo.c
- -MQ target
-
Same as -MT, but it quotes any characters which are special to
Make. -MQ '$(objpfx)foo.o' gives
$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it were given with -MQ.
- -MD
-
-MD is equivalent to -M -MF file, except that
-E is not implied. The driver determines file based on
whether an -o option is given. If it is, the driver uses its
argument but with a suffix of .d, otherwise it takes the name
of the input file, removes any directory components and suffix, and
applies a .d suffix.
If -MD is used in conjunction with -E, any -o switch is understood to specify the dependency output file, but if used without -E, each -o is understood to specify a target object file.
Since -E is not implied, -MD can be used to generate a dependency output file as a side-effect of the compilation process.
- -MMD
- Like -MD except mention only user header files, not system header files.
- -fpch-deps
- When using precompiled headers, this flag will cause the dependency-output flags to also list the files from the precompiled header's dependencies. If not specified only the precompiled header would be listed and not the files that were used to create it because those files are not consulted when a precompiled header is used.
- -fpch-preprocess
-
This option allows use of a precompiled header together with -E. It inserts a special "#pragma",
"#pragma GCC pch_preprocess "<filename>"" in the output to mark
the place where the precompiled header was found, and its filename. When
-fpreprocessed is in use, GCC recognizes this "#pragma" and
loads the PCH.
This option is off by default, because the resulting preprocessed output is only really suitable as input to GCC. It is switched on by -save-temps.
You should not write this "#pragma" in your own code, but it is safe to edit the filename if the PCH file is available in a different location. The filename may be absolute or it may be relative to GCC's current directory.
- -x c
- -x c++
- -x objective-c
- -x assembler-with-cpp
-
Specify the source language: C, C++, Objective-C, or assembly. This has
nothing to do with standards conformance or extensions; it merely
selects which base syntax to expect. If you give none of these options,
cpp will deduce the language from the extension of the source file:
.c, .cc, .m, or .S. Some other common
extensions for C++ and assembly are also recognized. If cpp does not
recognize the extension, it will treat the file as C; this is the most
generic mode.
Note: Previous versions of cpp accepted a -lang option which selected both the language and the standards conformance level. This option has been removed, because it conflicts with the -l option.
- -std=standard
- -ansi
-
Specify the standard to which the code should conform. Currently CPP
knows about C and C++ standards; others may be added in the future.
standard may be one of:
-
- iso9899:1990
- c89
-
The ISO C standard from 1990. c89 is the customary shorthand for
this version of the standard.
The -ansi option is equivalent to -std=c89.
- iso9899:199409
- The 1990 C standard, as amended in 1994.
- iso9899:1999
- c99
- iso9899:199x
- c9x
- The revised ISO C standard, published in December 1999. Before publication, this was known as C9X.
- gnu89
- The 1990 C standard plus GNU extensions. This is the default.
- gnu99
- gnu9x
- The 1999 C standard plus GNU extensions.
- c++98
- The 1998 ISO C++ standard plus amendments.
- gnu++98
- The same as -std=c++98 plus GNU extensions. This is the default for C++ code.
-
- -I-
-
Split the include path. Any directories specified with -I
options before -I- are searched only for headers requested with
"#include "file""; they are not searched for
"#include <file>". If additional directories are
specified with -I options after the -I-, those
directories are searched for all #include directives.
In addition, -I- inhibits the use of the directory of the current file directory as the first search directory for "#include "file"". This option has been deprecated.
- -nostdinc
- Do not search the standard system directories for header files. Only the directories you have specified with -I options (and the directory of the current file, if appropriate) are searched.
- -nostdinc++
- Do not search for header files in the C++-specific standard directories, but do still search the other standard directories. (This option is used when building the C++ library.)
- -include file
-
Process file as if "#include "file"" appeared as the first
line of the primary source file. However, the first directory searched
for file is the preprocessor's working directory instead of
the directory containing the main source file. If not found there, it
is searched for in the remainder of the "#include "..."" search
chain as normal.
If multiple -include options are given, the files are included in the order they appear on the command line.
- -imacros file
-
Exactly like -include, except that any output produced by
scanning file is thrown away. Macros it defines remain defined.
This allows you to acquire all the macros from a header without also
processing its declarations.
All files specified by -imacros are processed before all files specified by -include.
- -idirafter dir
- Search dir for header files, but do it after all directories specified with -I and the standard system directories have been exhausted. dir is treated as a system include directory. If dir begins with "=", then the "=" will be replaced by the sysroot prefix; see --sysroot and -isysroot.