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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. Optimization levels -O2 and above, in particular, enable unit-at-a-time mode, which allows the compiler to consider information gained from later functions in the file when compiling a function. Compiling multiple files at once to a single output file in unit-at-a-time 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
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:
-fdefer-pop -fdelayed-branch -fguess-branch-probability -fcprop-registers -floop-optimize -fif-conversion -fif-conversion2 -ftree-ccp -ftree-dce -ftree-dominator-opts -ftree-dse -ftree-ter -ftree-lrs -ftree-sra -ftree-copyrename -ftree-fre -ftree-ch -fmerge-constants
-O also turns on -fomit-frame-pointer on machines where doing so does not interfere with debugging.
-O doesn't turn on -ftree-sra for the Ada compiler.
This option must be explicitly specified on the command line to be
enabled for the Ada compiler.
-O2
-O2 turns on all optimization flags specified by -O. It also turns on the following optimization flags:
-fthread-jumps -fcrossjumping -foptimize-sibling-calls -fcse-follow-jumps -fcse-skip-blocks -fgcse -fgcse-lm -fexpensive-optimizations -fstrength-reduce -frerun-cse-after-loop -frerun-loop-opt -fcaller-saves -fforce-mem -fpeephole2 -fschedule-insns -fschedule-insns2 -fsched-interblock -fsched-spec -fregmove -fstrict-aliasing -fdelete-null-pointer-checks -freorder-blocks -freorder-functions -funit-at-a-time -falign-functions -falign-jumps -falign-loops -falign-labels -ftree-pre
Please note the warning under -fgcse about
invoking -O2 on programs that use computed gotos.
-O3
-O0
-Os
-Os disables the following optimization flags:
-falign-functions -falign-jumps -falign-loops -falign-labels -freorder-blocks -freorder-blocks-and-partition -fprefetch-loop-arrays
If you use multiple -O options, with or without level numbers, the last such option is the one that is effective.
Options of the form -fflag specify machine-independent flags. Most flags have both positive and negative forms; the negative form of -ffoo would be -fno-foo. In the table below, only one of the forms is listed—the one you typically will use. You can figure out the other form by either removing `no-' or adding it.
The following options control specific optimizations. They are either activated by -O options or are related to ones that are. You can use the following flags in the rare cases when “fine-tuning” of optimizations to be performed is desired.
-fno-default-inline
-fno-defer-pop
Disabled at levels -O, -O2, -O3, -Os.
-fforce-mem
Enabled at levels -O2, -O3, -Os.
-fforce-addr
-fomit-frame-pointer
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. See Register Usage.
Enabled at levels -O, -O2, -O3, -Os.
-foptimize-sibling-calls
Enabled at levels -O2, -O3, -Os.
-fno-inline
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-functions
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
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 if -funit-at-a-time is enabled.
-finline-limit=
nInlining is actually controlled by a number of parameters, which may be specified individually by using --param name=value. The -finline-limit=n option sets some of these parameters as follows:
max-inline-insns-single
max-inline-insns-auto
min-inline-insns
max-inline-insns-rtl
See below for a documentation of the individual parameters controlling inlining.
Note: pseudo instruction represents, in this particular context, an
abstract measurement of function's size. In no way, it represents a count
of assembly instructions and as such its exact meaning might change from one
release to an another.
-fkeep-inline-functions
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 C. In C++, emit any and all
inline functions into the object file.
-fkeep-static-consts
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
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
This option implies -fmerge-constants. In addition to
-fmerge-constants this considers e.g. even constant initialized
arrays or initialized constant variables with integral or floating point
types. Languages like C or C++ require each non-automatic variable to
have distinct location, so using this option will result in non-conforming
behavior.
-fmodulo-sched
-fno-branch-count-reg
The default is -fbranch-count-reg, enabled when
-fstrength-reduce is enabled.
-fno-function-cse
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
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.
-fbounds-check
-fmudflap -fmudflapth -fmudflapir
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.
-fstrength-reduce
Enabled at levels -O2, -O3, -Os.
-fthread-jumps
Enabled at levels -O2, -O3, -Os.
-fcse-follow-jumps
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
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
Enabled at levels -O2, -O3, -Os.
-frerun-loop-opt
Enabled at levels -O2, -O3, -Os.
-fgcse
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
Enabled by default when gcse is enabled.
-fgcse-sm
Not enabled at any optimization level.
-fgcse-las
Not enabled at any optimization level.
-fgcse-after-reload
-floop-optimize
Enabled at levels -O, -O2, -O3, -Os.
-floop-optimize2
-fcrossjumping
Enabled at levels -O2, -O3, -Os.
-fif-conversion
if-conversion2
.
Enabled at levels -O, -O2, -O3, -Os.
-fif-conversion2
Enabled at levels -O, -O2, -O3, -Os.
-fdelete-null-pointer-checks
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
Enabled at levels -O2, -O3, -Os.
-foptimize-register-move
-fregmove
Note -fregmove and -foptimize-register-move are the same optimization.
Enabled at levels -O2, -O3, -Os.
-fdelayed-branch
Enabled at levels -O, -O2, -O3, -Os.
-fschedule-insns
Enabled at levels -O2, -O3, -Os.
-fschedule-insns2
Enabled at levels -O2, -O3, -Os.
-fno-sched-interblock
-fno-sched-spec
-fsched-spec-load
-fsched-spec-load-dangerous
-fsched-stalled-insns=
n-fsched-stalled-insns-dep=
n-fsched2-use-superblocks
This only makes sense when scheduling after register allocation, i.e. with
-fschedule-insns2 or at -O2 or higher.
-fsched2-use-traces
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.
-freschedule-modulo-scheduled-loops
-fcaller-saves
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.
-ftree-pre
-ftree-fre
-ftree-ccp
-ftree-dce
-ftree-dominator-opts
-ftree-ch
-ftree-loop-optimize
-ftree-loop-linear
-ftree-loop-im
-ftree-loop-ivcanon
-fivopts
-ftree-sra
-ftree-copyrename
-ftree-ter
-ftree-lrs
-ftree-vectorize
-ftracer
-funroll-loops
-funroll-all-loops
-fsplit-ivs-in-unroller
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
-fprefetch-loop-arrays
These options may generate better or worse code; results are highly
dependent on the structure of loops within the source code.
-fno-peephole
-fno-peephole2
-fpeephole is enabled by default.
-fpeephole2 enabled at levels -O2, -O3, -Os.
-fno-guess-branch-probability
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
Enabled at levels -O2, -O3.
-freorder-blocks-and-partition
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
.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
unsigned int
can alias an int
, but not a
void*
or a double
. A character type may alias any other
type.
Pay special attention to code like this:
union a_union { int i; double d; }; int f() { a_union t; t.d = 3.0; return t.i; }
The practice of reading from a different union member than the one most recently written to (called “type-punning”) is common. Even with -fstrict-aliasing, type-punning is allowed, provided the memory is accessed through the union type. So, the code above will work as expected. However, this code might not:
int f() { a_union t; int* ip; t.d = 3.0; ip = &t.i; return *ip; }
Every language that wishes to perform language-specific alias analysis
should define a function that computes, given an tree
node, an alias set for the node. Nodes in different alias sets are not
allowed to alias. For an example, see the C front-end function
c_get_alias_set
.
Enabled at levels -O2, -O3, -Os.
-falign-functions
-falign-functions=
n-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-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-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-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
asm
statements
are emitted, and will likely break code relying on some particular
ordering. The majority of such top-level asm
statements,
though, can be replaced by section
attributes.
asm
statement refers directly to variables or functions
that are otherwise unused. In that case either the variable/function
shall be listed as an operand of the asm
statement operand or,
in the case of top-level asm
statements the attribute used
shall be used on the declaration.
asm
statements calling functions directly. Again,
attribute used
will prevent this behavior.
As a temporary workaround, -fno-unit-at-a-time can be used, but this scheme may not be supported by future releases of GCC.
Enabled at levels -O2, -O3.
-fweb
Enabled at levels -O2, -O3, -Os,
on targets where the default format for debugging information supports
variable tracking.
-fno-cprop-registers
Disabled at levels -O, -O2, -O3, -Os.
-fprofile-generate
The following options are enabled: -fprofile-arcs
, -fprofile-values
, -fvpt
.
-fprofile-use
The following options are enabled: -fbranch-probabilities
,
-fvpt
, -funroll-loops
, -fpeel-loops
, -ftracer
.
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
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
This option causes the preprocessor macro __FAST_MATH__
to be defined.
This option should never be turned on by any -O option since
it can result in incorrect output for programs which depend on
an exact implementation of IEEE or ISO rules/specifications for
math functions.
-fno-math-errno
This option should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.
The default is -fmath-errno.
-funsafe-math-optimizations
This option should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.
The default is -fno-unsafe-math-optimizations.
-ffinite-math-only
This option should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications.
The default is -fno-finite-math-only.
-fno-trapping-math
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
The default is -fno-rounding-math.
This option is experimental and does not currently guarantee to
disable all GCC optimizations that are affected by rounding mode.
Future versions of GCC may provide finer control of this setting
using C99's FENV_ACCESS
pragma. This command line option
will be used to specify the default state for FENV_ACCESS
.
-fsignaling-nans
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
-fcx-limited-range
-fno-cx-limited-range
This option controls the default setting of the ISO C99
CX_LIMITED_RANGE
pragma. Nevertheless, the option applies to
all languages.
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
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
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
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.
-fspeculative-prefetching
With -fbranch-probabilities, it reads back the data gathered and issues prefetch instructions according to them. In addition to the opportunities noticed by -fprefetch-loop-arrays, it also notices more complicated memory access patterns—for example accesses to the data stored in linked list whose elements are usually allocated sequentially.
In order to prevent issuing double prefetches, usage of -fspeculative-prefetching implies -fno-prefetch-loop-arrays.
Enabled with -fprofile-generate and -fprofile-use.
-frename-registers
Not enabled by default at any level because it has known bugs.
-ftracer
Enabled with -fprofile-use.
-funroll-loops
Enabled with -fprofile-use.
-funroll-all-loops
-fpeel-loops
Enabled with -fprofile-use.
-fmove-loop-invariants
-funswitch-loops
-fprefetch-loop-arrays
Disabled at level -Os.
-ffunction-sections
-fdata-sections
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
-fbranch-target-load-optimize2
-fbtr-bb-exclusive
--param
name=
valueThe 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
sra-field-structure-ratio
max-crossjump-edges
min-crossjump-insns
max-goto-duplication-insns
max-delay-slot-insn-search
max-delay-slot-live-search
max-gcse-memory
max-gcse-passes
max-pending-list-length
max-inline-insns-single
max-inline-insns-auto
large-function-insns
large-function-growth
inline-unit-growth
max-inline-insns-recursive
max-inline-insns-recursive-auto
For functions declared inline --param max-inline-insns-recursive is
taken into acount. 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
For functions declared inline --param max-inline-recursive-depth is
taken into acount. 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 450.
inline-call-cost
max-unrolled-insns
max-average-unrolled-insns
max-unroll-times
max-peeled-insns
max-peel-times
max-completely-peeled-insns
max-completely-peel-times
max-unswitch-insns
max-unswitch-level
lim-expensive
iv-consider-all-candidates-bound
iv-max-considered-uses
iv-always-prune-cand-set-bound
scev-max-expr-size
max-iterations-to-track
hot-bb-count-fraction
hot-bb-frequency-fraction
tracer-dynamic-coverage
tracer-dynamic-coverage-feedback
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
tracer-min-branch-ratio
tracer-min-branch-ratio
tracer-min-branch-ratio-feedback
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
global-var-threshold
int * size_t
on the host machine; beware overflow).
max-aliased-vops
ggc-min-expand
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
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
very large effectively disables garbage collection. Setting this
parameter and ggc-min-expand to zero causes a full collection
to occur at every opportunity.
max-reload-search-insns
max-cselib-memory-location
reorder-blocks-duplicate
reorder-blocks-duplicate-feedback
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-region-blocks
max-sched-region-insns
max-last-value-rtl
integer-share-limit