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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 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 in this section.
Most optimizations are only enabled if an -O level is set on the command line. Otherwise they are disabled, even if individual optimization flags are specified.
Depending on the target and how GCC was configured, a slightly different set of optimizations may be enabled at each -O level than those listed here. You can invoke GCC with -Q --help=optimizers to find out the exact set of optimizations that are enabled at each level. See Overall Options, for examples.
-O-O1With -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
-fbranch-count-reg
-fcombine-stack-adjustments
-fcompare-elim
-fcprop-registers
-fdce
-fdefer-pop
-fdelayed-branch
-fdse
-fforward-propagate
-fguess-branch-probability
-fif-conversion2
-fif-conversion
-finline-functions-called-once
-fipa-pure-const
-fipa-profile
-fipa-reference
-fmerge-constants
-fmove-loop-invariants
-fshrink-wrap
-fsplit-wide-types
-ftree-bit-ccp
-ftree-ccp
-fssa-phiopt
-ftree-ch
-ftree-copy-prop
-ftree-copyrename
-ftree-dce
-ftree-dominator-opts
-ftree-dse
-ftree-forwprop
-ftree-fre
-ftree-phiprop
-ftree-sink
-ftree-slsr
-ftree-sra
-ftree-pta
-ftree-ter
-funit-at-a-time
-O also turns on -fomit-frame-pointer on machines
where doing so does not interfere with debugging.
-O2-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
-fdevirtualize -fdevirtualize-speculatively
-fexpensive-optimizations
-fgcse -fgcse-lm
-fhoist-adjacent-loads
-finline-small-functions
-findirect-inlining
-fipa-cp
-fipa-cp-alignment
-fipa-sra
-fipa-icf
-fisolate-erroneous-paths-dereference
-flra-remat
-foptimize-sibling-calls
-foptimize-strlen
-fpartial-inlining
-fpeephole2
-freorder-blocks -freorder-blocks-and-partition -freorder-functions
-frerun-cse-after-loop
-fsched-interblock -fsched-spec
-fschedule-insns -fschedule-insns2
-fstrict-aliasing -fstrict-overflow
-ftree-builtin-call-dce
-ftree-switch-conversion -ftree-tail-merge
-ftree-pre
-ftree-vrp
-fipa-ra
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
-Ofast-OgIf 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 is -fno-foo. In the table below, only one of the forms is listed—the one you typically 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-defer-popDisabled at levels -O, -O2, -O3, -Os.
-fforward-propagateThis option is enabled by default at optimization levels -O,
-O2, -O3, -Os.
-ffp-contract=styleThe default is -ffp-contract=fast.
-fomit-frame-pointerOn 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.
The default setting (when not optimizing for size) for 32-bit GNU/Linux x86 and 32-bit Darwin x86 targets is -fomit-frame-pointer. You can configure GCC with the --enable-frame-pointer configure option to change the default.
Enabled at levels -O, -O2, -O3, -Os.
-foptimize-sibling-callsEnabled at levels -O2, -O3, -Os.
-foptimize-strlenstrlen,
strchr or strcpy) and
their _FORTIFY_SOURCE counterparts into faster alternatives.
Enabled at levels -O2, -O3.
-fno-inlinealways_inline attribute. This is the default when not
optimizing.
Single functions can be exempted from inlining by marking them
with the noinline attribute.
-finline-small-functionsEnabled at level -O2.
-findirect-inliningEnabled at level -O2.
-finline-functionsIf 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-oncestatic 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-inliningalways_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.
-fipa-sraEnabled at levels -O2, -O3 and -Os.
-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-singlemax-inline-insns-autoSee 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.
-fno-keep-inline-dllexportdllexport
attribute or declspec (See Declaring Attributes of Functions.)
-fkeep-inline-functionsstatic 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 C90. In C++, emit any and all
inline functions into the object file.
-fkeep-static-constsstatic 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 a variable is referenced, regardless of whether or not
optimization is turned on, use the -fno-keep-static-consts option.
-fmerge-constantsThis 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-constantsThis 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 results in non-conforming
behavior.
-fmodulo-sched-fmodulo-sched-allow-regmoves-fno-branch-count-regEnabled by default at -O1 and higher.
The default is -fbranch-count-reg.
-fno-function-cseThis 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-bssThis 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.
-fthread-jumpsEnabled at levels -O2, -O3, -Os.
-fsplit-wide-typeslong
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-jumpsif statement with an
else clause, CSE follows the jump when the condition
tested is false.
Enabled at levels -O2, -O3, -Os.
-fcse-skip-blocksif 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-loopEnabled at levels -O2, -O3, -Os.
-fgcseNote: When compiling a program using computed gotos, a GCC extension, you may get better run-time 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-lmEnabled by default when -fgcse is enabled.
-fgcse-smNot enabled at any optimization level.
-fgcse-lasNot enabled at any optimization level.
-fgcse-after-reload-faggressive-loop-optimizations-funsafe-loop-optimizations-fcrossjumpingEnabled at levels -O2, -O3, -Os.
-fauto-inc-dec-fdce-fdse-fif-conversionEnabled at levels -O, -O2, -O3, -Os.
-fif-conversion2Enabled at levels -O, -O2, -O3, -Os.
-fdeclone-ctor-dtorEnabled by -Os.
-fdelete-null-pointer-checksNote however that in some environments this assumption is not true. Use -fno-delete-null-pointer-checks to disable this optimization for programs that depend on that behavior.
Some targets, especially embedded ones, disable this option at all levels.
Otherwise it is enabled at all levels: -O0, -O1,
-O2, -O3, -Os. Passes that use the information
are enabled independently at different optimization levels.
-fdevirtualize-fdevirtualize-speculatively-fdevirtualize-at-ltrans-fexpensive-optimizationsEnabled at levels -O2, -O3, -Os.
-freeEnabled for Alpha, AArch64 and x86 at levels -O2,
-O3, -Os.
-fno-lifetime-dse-flive-range-shrinkage-fira-algorithm=algorithm-fira-region=region-fira-hoist-pressureThis option is enabled at level -Os for all targets.
-fira-loop-pressureThis option is enabled at level -O3 for some targets.
-fno-ira-share-save-slots-fno-ira-share-spill-slots-fira-verbose=n-flra-rematEnabled at levels -O2, -O3, -Os.
-fdelayed-branchEnabled at levels -O, -O2, -O3, -Os.
-fschedule-insnsEnabled at levels -O2, -O3.
-fschedule-insns2Enabled at levels -O2, -O3, -Os.
-fno-sched-interblock-fno-sched-spec-fsched-pressure-fsched-spec-load-fsched-spec-load-dangerous-fsched-stalled-insns-fsched-stalled-insns=n-fsched-stalled-insns-dep-fsched-stalled-insns-dep=n-fsched2-use-superblocksThis only makes sense when scheduling after register allocation, i.e. with
-fschedule-insns2 or at -O2 or higher.
-fsched-group-heuristic-fsched-critical-path-heuristic-fsched-spec-insn-heuristic-fsched-rank-heuristic-fsched-last-insn-heuristic-fsched-dep-count-heuristic-freschedule-modulo-scheduled-loops-fselective-scheduling-fselective-scheduling2-fsel-sched-pipelining-fsel-sched-pipelining-outer-loops-fsemantic-interposition-fshrink-wrap-fcaller-savesThis 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.
-fcombine-stack-adjustmentsEnabled by default at -O1 and higher.
-fipa-raEnabled at levels -O2, -O3, -Os.
-fconserve-stack-ftree-reassoc-ftree-pre-ftree-partial-pre-ftree-forwprop-ftree-fre-ftree-phiprop-fhoist-adjacent-loads-ftree-copy-prop-fipa-pure-const-fipa-reference-fipa-pta-fipa-profilecold, noreturn, static constructors or destructors) are identified. Cold
functions and loop less parts of functions executed once are then optimized for
size.
Enabled by default at -O and higher.
-fipa-cp-fipa-cp-clone-fipa-cp-alignmentThis flag is enabled by default at -O2 and -Os. It
requires that -fipa-cp is enabled.
-fipa-icfNevertheless the behavior is similar to Gold Linker ICF optimization, GCC ICF works on different levels and thus the optimizations are not same - there are equivalences that are found only by GCC and equivalences found only by Gold.
This flag is enabled by default at -O2 and -Os.
-fisolate-erroneous-paths-dereference-fisolate-erroneous-paths-attributereturns_nonnull or nonnull
attribute. Isolate those paths from the main control flow and turn the
statement with erroneous or undefined behavior into a trap. This is not
currently enabled, but may be enabled by -O2 in the future.
-ftree-sink-ftree-bit-ccp-ftree-ccp-fssa-phiopt-ftree-switch-conversion-ftree-tail-merge-ftree-dce-ftree-builtin-call-dceerrno 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-ftree-dse-ftree-ch-ftree-loop-optimize-ftree-loop-linear-floop-interchange DO J = 1, M
DO I = 1, N
A(J, I) = A(J, I) * C
ENDDO
ENDDO
loop interchange transforms the loop as if it were 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-isl to enable the Graphite loop transformation
infrastructure.
-floop-strip-mine DO I = 1, N
A(I) = A(I) + C
ENDDO
loop strip mining transforms the loop as if it were written:
DO II = 1, N, 51
DO I = II, min (II + 50, 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-isl to enable the Graphite loop
transformation infrastructure.
-floop-block DO I = 1, N
DO J = 1, M
A(J, I) = B(I) + C(J)
ENDDO
ENDDO
loop blocking transforms the loop as if it were written:
DO II = 1, N, 51
DO JJ = 1, M, 51
DO I = II, min (II + 50, N)
DO J = JJ, min (JJ + 50, 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 iterates over a smaller amount of data
which 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-isl
to enable the Graphite loop transformation infrastructure.
-fgraphite-identity-floop-nest-optimize-floop-unroll-and-jam-floop-parallelize-all-fcheck-data-deps-ftree-loop-if-convert-ftree-loop-if-convert-stores for (i = 0; i < N; i++)
if (cond)
A[i] = expr;
is transformed to
for (i = 0; i < N; i++)
A[i] = cond ? expr : A[i];
potentially producing data races.
-ftree-loop-distribution 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-distribute-patternsThis pass distributes the initialization loops and generates a call to memset zero. For example, the loop
DO I = 1, N
A(I) = 0
B(I) = A(I) + I
ENDDO
is transformed to
DO I = 1, N
A(I) = 0
ENDDO
DO I = 1, N
B(I) = A(I) + I
ENDDO
and the initialization loop is transformed into a call to memset zero.
-ftree-loop-im-ftree-loop-ivcanon-fivopts-ftree-parallelize-loops=n-ftree-pta-ftree-sra-ftree-copyrename-ftree-coalesce-inlined-vars-ftree-coalesce-vars-ftree-ter-ftree-slsr-ftree-vectorize-ftree-loop-vectorize-ftree-slp-vectorize-fvect-cost-model=model-fsimd-cost-model=model-ftree-vrp-fsplit-ivs-in-unrollerA combination of -fweb and CSE is often sufficient to obtain the same effect. However, that is not reliable in cases where the loop body is more complicated than a single basic block. It also does not work at all on some architectures due to restrictions in the CSE pass.
This optimization is enabled by default.
-fvariable-expansion-in-unroller-fpartial-inliningEnabled at level -O2.
-fpredictive-commoningThis option is enabled at level -O3.
-fprefetch-loop-arraysThis 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-fpeephole is enabled by default.
-fpeephole2 enabled at levels -O2, -O3, -Os.
-fno-guess-branch-probabilityGCC uses 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 are
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-blocksEnabled at levels -O2, -O3.
-freorder-blocks-and-partitionThis 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.
Enabled for x86 at levels -O2, -O3.
-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 to make this option effective. See -fprofile-arcs for details.
Enabled at levels -O2, -O3, -Os.
-fstrict-aliasingunsigned 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 works as expected. See Structures unions enumerations and bit-fields implementation. 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-overflowi + 10 > i is always 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
overflows 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-fno-align-functions and -falign-functions=1 are equivalent and mean that functions are not 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 are not 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 are not 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 are not aligned.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-funit-at-a-timeEnabled by default.
-fno-toplevel-reorderasm
statements. Output them in the same order that they appear in the
input file. When this option is used, unreferenced static variables
are not removed. This option is intended to support existing code
that relies on a particular ordering. For new code, it is better to
use attributes when possible.
Enabled at level -O0. When disabled explicitly, it also implies
-fno-section-anchors, which is otherwise enabled at -O0 on some
targets.
-fwebEnabled by default with -funroll-loops.
-fwhole-programmain
and those merged by attribute externally_visible become static functions
and in effect are optimized more aggressively by interprocedural optimizers.
This option should not be used in combination with -flto.
Instead relying on a linker plugin should provide safer and more precise
information.
-flto[=n]To use the link-time optimizer, -flto and optimization options should be specified at compile time and during the final link. For example:
gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.o
The first two invocations to GCC save a bytecode representation of GIMPLE into special ELF sections inside foo.o and bar.o. The final invocation reads the GIMPLE bytecode from foo.o and bar.o, merges the two files into a single internal image, and compiles the result as usual. Since both foo.o and bar.o are merged into a single image, this causes all the interprocedural analyses and optimizations in GCC to work across the two files as if they were a single one. This means, for example, that the inliner is able to inline functions in bar.o into functions in foo.o and vice-versa.
Another (simpler) way to enable link-time optimization is:
gcc -o myprog -flto -O2 foo.c bar.c
The above generates bytecode for foo.c and bar.c, merges them together into a single GIMPLE representation and optimizes them as usual to produce myprog.
The only important thing to keep in mind is that to enable link-time optimizations you need to use the GCC driver to perform the link-step. GCC then automatically performs link-time optimization if any of the objects involved were compiled with the -flto command-line option. You generally should specify the optimization options to be used for link-time optimization though GCC tries to be clever at guessing an optimization level to use from the options used at compile-time if you fail to specify one at link-time. You can always override the automatic decision to do link-time optimization at link-time by passing -fno-lto to the link command.
To make whole program optimization effective, it is necessary to make certain whole program assumptions. The compiler needs to know what functions and variables can be accessed by libraries and runtime outside of the link-time optimized unit. When supported by the linker, the linker plugin (see -fuse-linker-plugin) passes information to the compiler about used and externally visible symbols. When the linker plugin is not available, -fwhole-program should be used to allow the compiler to make these assumptions, which leads to more aggressive optimization decisions.
When -fuse-linker-plugin is not enabled then, when a file is compiled with -flto, the generated object file is larger than a regular object file because it contains GIMPLE bytecodes and the usual final code (see -ffat-lto-objects. This means that object files with LTO information can be linked as normal object files; if -fno-lto is passed to the linker, no interprocedural optimizations are applied. Note that when -fno-fat-lto-objects is enabled the compile-stage is faster but you cannot perform a regular, non-LTO link on them.
Additionally, the optimization flags used to compile individual files are not necessarily related to those used at link time. For instance,
gcc -c -O0 -ffat-lto-objects -flto foo.c
gcc -c -O0 -ffat-lto-objects -flto bar.c
gcc -o myprog -O3 foo.o bar.o
This produces individual object files with unoptimized assembler code, but the resulting binary myprog is optimized at -O3. If, instead, the final binary is generated with -fno-lto, then myprog is not optimized.
When producing the final binary, GCC only applies link-time optimizations to those files that contain bytecode. Therefore, you can mix and match object files and libraries with GIMPLE bytecodes and final object code. GCC automatically selects which files to optimize in LTO mode and which files to link without further processing.
There are some code generation flags preserved by GCC when generating bytecodes, as they need to be used during the final link stage. Generally options specified at link-time override those specified at compile-time.
If you do not specify an optimization level option -O at link-time then GCC computes one based on the optimization levels used when compiling the object files. The highest optimization level wins here.
Currently, the following options and their setting are take from the first object file that explicitely specified it: -fPIC, -fpic, -fpie, -fcommon, -fexceptions, -fnon-call-exceptions, -fgnu-tm and all the -m target flags.
Certain ABI changing flags are required to match in all compilation-units and trying to override this at link-time with a conflicting value is ignored. This includes options such as -freg-struct-return and -fpcc-struct-return.
Other options such as -ffp-contract, -fno-strict-overflow, -fwrapv, -fno-trapv or -fno-strict-aliasing are passed through to the link stage and merged conservatively for conflicting translation units. Specifically -fno-strict-overflow, -fwrapv and -fno-trapv take precedence and for example -ffp-contract=off takes precedence over -ffp-contract=fast. You can override them at linke-time.
It is recommended that you compile all the files participating in the same link with the same options and also specify those options at link time.
If LTO encounters objects with C linkage declared with incompatible types in separate translation units to be linked together (undefined behavior according to ISO C99 6.2.7), a non-fatal diagnostic may be issued. The behavior is still undefined at run time. Similar diagnostics may be raised for other languages.
Another feature of LTO is that it is possible to apply interprocedural optimizations on files written in different languages:
gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran
Notice that the final link is done with g++ to get the C++ runtime libraries and -lgfortran is added to get the Fortran runtime libraries. In general, when mixing languages in LTO mode, you should use the same link command options as when mixing languages in a regular (non-LTO) compilation.
If object files containing GIMPLE bytecode are stored in a library archive, say libfoo.a, it is possible to extract and use them in an LTO link if you are using a linker with plugin support. To create static libraries suitable for LTO, use gcc-ar and gcc-ranlib instead of ar and ranlib; to show the symbols of object files with GIMPLE bytecode, use gcc-nm. Those commands require that ar, ranlib and nm have been compiled with plugin support. At link time, use the the flag -fuse-linker-plugin to ensure that the library participates in the LTO optimization process:
gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo
With the linker plugin enabled, the linker extracts the needed GIMPLE files from libfoo.a and passes them on to the running GCC to make them part of the aggregated GIMPLE image to be optimized.
If you are not using a linker with plugin support and/or do not enable the linker plugin, then the objects inside libfoo.a are extracted and linked as usual, but they do not participate in the LTO optimization process. In order to make a static library suitable for both LTO optimization and usual linkage, compile its object files with -flto -ffat-lto-objects.
Link-time optimizations do not require the presence of the whole program to operate. If the program does not require any symbols to be exported, it is possible to combine -flto and -fwhole-program to allow the interprocedural optimizers to use more aggressive assumptions which may lead to improved optimization opportunities. Use of -fwhole-program is not needed when linker plugin is active (see -fuse-linker-plugin).
The current implementation of LTO makes no attempt to generate bytecode that is portable between different types of hosts. The bytecode files are versioned and there is a strict version check, so bytecode files generated in one version of GCC do not work with an older or newer version of GCC.
Link-time optimization does not work well with generation of debugging information. Combining -flto with -g is currently experimental and expected to produce unexpected results.
If you specify the optional n, the optimization and code generation done at link time is executed in parallel using n parallel jobs by utilizing an installed make program. The environment variable MAKE may be used to override the program used. The default value for n is 1.
You can also specify -flto=jobserver to use GNU make's
job server mode to determine the number of parallel jobs. This
is useful when the Makefile calling GCC is already executing in parallel.
You must prepend a ‘+’ to the command recipe in the parent Makefile
for this to work. This option likely only works if MAKE is
GNU make.
-flto-partition=alg-flto-odr-type-merging-flto-compression-level=n-flto-reportDisabled by default.
-flto-report-wpa-fuse-linker-pluginThis option enables the extraction of object files with GIMPLE bytecode out of library archives. This improves the quality of optimization by exposing more code to the link-time optimizer. This information specifies what symbols can be accessed externally (by non-LTO object or during dynamic linking). Resulting code quality improvements on binaries (and shared libraries that use hidden visibility) are similar to -fwhole-program. See -flto for a description of the effect of this flag and how to use it.
This option is enabled by default when LTO support in GCC is enabled
and GCC was configured for use with
a linker supporting plugins (GNU ld 2.21 or newer or gold).
-ffat-lto-objects-fno-fat-lto-objects improves compilation time over plain LTO, but requires the complete toolchain to be aware of LTO. It requires a linker with linker plugin support for basic functionality. Additionally, nm, ar and ranlib need to support linker plugins to allow a full-featured build environment (capable of building static libraries etc). GCC provides the gcc-ar, gcc-nm, gcc-ranlib wrappers to pass the right options to these tools. With non fat LTO makefiles need to be modified to use them.
The default is -fno-fat-lto-objects on targets with linker plugin
support.
-fcompare-elimThis pass only applies to certain targets that cannot explicitly represent the comparison operation before register allocation is complete.
Enabled at levels -O, -O2, -O3, -Os.
-fcprop-registersEnabled at levels -O, -O2, -O3, -Os.
-fprofile-correction-fprofile-dir=path-fprofile-generate-fprofile-generate=pathThe following options are enabled: -fprofile-arcs, -fprofile-values, -fvpt.
If path is specified, GCC looks at the path to find
the profile feedback data files. See -fprofile-dir.
-fprofile-use-fprofile-use=pathBy 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 looks at the path to find
the profile feedback data files. See -fprofile-dir.
-fauto-profile-fauto-profile=pathpath is the name of a file containing AutoFDO profile information. If omitted, it defaults to fbdata.afdo in the current directory.
Producing an AutoFDO profile data file requires running your program with the perf utility on a supported GNU/Linux target system. For more information, see https://perf.wiki.kernel.org/.
E.g.
perf record -e br_inst_retired:near_taken -b -o perf.data \
-- your_program
Then use the create_gcov tool to convert the raw profile data to a format that can be used by GCC. You must also supply the unstripped binary for your program to this tool. See https://github.com/google/autofdo.
E.g.
create_gcov --binary=your_program.unstripped --profile=perf.data \
--gcov=profile.afdo
The following options control compiler behavior regarding floating-point arithmetic. These options trade off between speed and correctness. All must be specifically enabled.
-ffloat-storeThis 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.
-fexcess-precision=stylefloat and double types and the processor does not
support operations rounding to those types. By default,
-fexcess-precision=fast is in effect; this means that
operations are carried out in the precision of the registers and that
it is unpredictable when rounding to the types specified in the source
code takes place. When compiling C, if
-fexcess-precision=standard is specified then excess
precision follows the rules specified in ISO C99; in particular,
both casts and assignments cause values to be rounded to their
semantic types (whereas -ffloat-store only affects
assignments). This option is enabled by default for C if a strict
conformance option such as -std=c99 is used.
-fexcess-precision=standard is not implemented for languages
other than C, and has no effect if
-funsafe-math-optimizations or -ffast-math is
specified. On the x86, it also has no effect if -mfpmath=sse
or -mfpmath=sse+387 is specified; in the former case, IEEE
semantics apply without excess precision, and in the latter, rounding
is unpredictable.
-ffast-mathThis option causes the preprocessor macro __FAST_MATH__ to be defined.
This option is not turned on by any -O option besides
-Ofast since it can result in incorrect output for programs
that 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-errnoerrno 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 that 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-optimizationsThis option is not turned on by any -O option since it can result in incorrect output for programs that 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(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. For Fortran the option
is automatically enabled when both -fno-signed-zeros and
-fno-trapping-math are in effect.
The default is -fno-associative-math.
-freciprocal-mathx / 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-onlyThis option is not turned on by any -O option since it can result in incorrect output for programs that 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-zerosThe default is -fsigned-zeros.
-fno-trapping-mathThis option should never be turned on by any -O option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions.
The default is -ftrapping-math.
-frounding-mathThe 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-nansThis 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-rangeNaN
+ 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-rulesNaN
+ 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-probabilitiesWith -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 most likely to take, the ‘REG_BR_PROB’ values are used to
exactly determine which path is taken more often.
-fprofile-valuesWith -fbranch-probabilities, it reads back the data gathered from profiling values of expressions for usage in optimizations.
Enabled with -fprofile-generate and -fprofile-use.
-fprofile-reorder-functionsEnabled with -fprofile-use.
-fvptWith -fbranch-probabilities, it reads back the data gathered
and actually performs the optimizations based on them.
Currently the optimizations include specialization of division operations
using the knowledge about the value of the denominator.
-frename-registersEnabled by default with -funroll-loops and -fpeel-loops.
-fschedule-fusionEnabled at levels -O2, -O3, -Os.
-ftracerEnabled with -fprofile-use.
-funroll-loopsEnabled with -fprofile-use.
-funroll-all-loops-fpeel-loopsEnabled with -fprofile-use.
-fmove-loop-invariants-funswitch-loops-ffunction-sections-fdata-sectionsUse 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
create larger object and executable files and are also slower.
You cannot 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-fstack-protectoralloca, 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-fstack-protector-strong-fstack-protector-explicitstack_protect attribute
-fstdarg-opt-fsection-anchorsFor example, the implementation of the following function foo:
static int a, b, c;
int foo (void) { return a + b + c; }
usually calculates the addresses of all three variables, but if you compile it with -fsection-anchors, it accesses 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=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:
predictable-branch-outcomemax-crossjump-edgesmin-crossjump-insnsmax-grow-copy-bb-insnsmax-goto-duplication-insnsmax-delay-slot-insn-searchmax-delay-slot-live-searchmax-gcse-memorymax-gcse-insertion-ratiomax-pending-list-lengthmax-modulo-backtrack-attemptsmax-inline-insns-singlemax-inline-insns-autoinline-min-speeduplarge-function-insnslarge-function-growthlarge-unit-insnsinline-unit-growthipcp-unit-growthlarge-stack-framelarge-stack-frame-growthmax-inline-insns-recursivemax-inline-insns-recursive-auto--param max-inline-insns-recursive applies to functions
declared inline.
For functions not declared inline, recursive inlining
happens only when -finline-functions (included in -O3) is
enabled; --param max-inline-insns-recursive-auto applies instead. The
default value is 450.
max-inline-recursive-depthmax-inline-recursive-depth-auto--param max-inline-recursive-depth applies to functions
declared inline. For functions not declared inline, recursive inlining
happens only when -finline-functions (included in -O3) is
enabled; --param max-inline-recursive-depth-auto applies instead. The
default value is 8.
min-inline-recursive-probabilityWhen profile feedback is available (see -fprofile-generate) the actual
recursion depth can be guessed from probability that function recurses via a
given call expression. This parameter limits inlining only to call expressions
whose probability exceeds the given threshold (in percents).
The default value is 10.
early-inlining-insnsmax-early-inliner-iterationscomdat-sharing-probabilityprofile-func-internal-idmin-vect-loop-boundgcse-cost-distance-ratiogcse-unrestricted-costmax-hoist-depthmax-tail-merge-comparisonsmax-tail-merge-iterationsmax-unrolled-insnsmax-average-unrolled-insnsmax-unroll-timesmax-peeled-insnsmax-peel-timesmax-peel-branchesmax-completely-peeled-insnsmax-completely-peel-timesmax-completely-peel-loop-nest-depthmax-unswitch-insnsmax-unswitch-levellim-expensiveiv-consider-all-candidates-boundiv-max-considered-usesiv-always-prune-cand-set-boundscev-max-expr-sizescev-max-expr-complexityomega-max-varsomega-max-geqsomega-max-eqsomega-max-wild-cardsomega-hash-table-sizeomega-max-keysomega-eliminate-redundant-constraintsvect-max-version-for-alignment-checksvect-max-version-for-alias-checksvect-max-peeling-for-alignmentmax-iterations-to-trackhot-bb-count-ws-permillehot-bb-frequency-fractionmax-predicted-iterationsbuiltin-expect-probabilityalign-thresholdalign-loop-iterationstracer-dynamic-coveragetracer-dynamic-coverage-feedbackThe tracer-dynamic-coverage-feedback parameter
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-growthtracer-min-branch-ratiotracer-min-branch-ratiotracer-min-branch-ratio-feedbackSimilarly 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-lengthmax-cse-insnsggc-min-expandThe 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-heapsizeThe default is the smaller of RAM/8, RLIMIT_RSS, or a limit that
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-insnsmax-cselib-memory-locationsreorder-blocks-duplicatereorder-blocks-duplicate-feedbackThe reorder-block-duplicate-feedback parameter
is used only when profile
feedback is available. It may be set to higher values than
reorder-block-duplicate since information about the hot spots is more
accurate.
max-sched-ready-insnsmax-sched-region-blocksmax-pipeline-region-blocksmax-sched-region-insnsmax-pipeline-region-insnsmin-spec-probmax-sched-extend-regions-itersmax-sched-insn-conflict-delaysched-spec-prob-cutoffsched-spec-state-edge-prob-cutoffsched-mem-true-dep-costselsched-max-lookaheadselsched-max-sched-timesselsched-max-insns-to-renamesms-min-scmax-last-value-rtlmax-combine-insnsinteger-share-limitssp-buffer-sizemin-size-for-stack-sharingmax-jump-thread-duplication-stmtsmax-fields-for-field-sensitiveprefetch-latencysimultaneous-prefetchesl1-cache-line-sizel1-cache-sizel2-cache-sizemin-insn-to-prefetch-ratioprefetch-min-insn-to-mem-ratiouse-canonical-typesswitch-conversion-max-branch-ratiomax-partial-antic-lengthsccvn-max-scc-sizesccvn-max-alias-queries-per-accessira-max-loops-numira-max-conflict-table-sizeira-loop-reserved-regslra-inheritance-ebb-probability-cutoffloop-invariant-max-bbs-in-looploop-max-datarefs-for-datadepsmax-vartrack-sizemax-vartrack-expr-depthmin-nondebug-insn-uidipa-sra-ptr-growth-factorsra-max-scalarization-size-Ospeedsra-max-scalarization-size-Osizetm-max-aggregate-sizegraphite-max-nb-scop-paramsgraphite-max-bbs-per-functionloop-block-tile-sizeloop-unroll-jam-sizeloop-unroll-jam-depthipa-cp-value-list-sizeipa-cp-eval-thresholdipa-cp-recursion-penaltyipa-cp-single-call-penaltyipa-max-agg-itemsipa-cp-loop-hint-bonusipa-cp-array-index-hint-bonusipa-max-aa-stepslto-partitionslto-minpartitioncxx-max-namespaces-for-diagnostic-helpsink-frequency-thresholdmax-stores-to-sinkallow-store-data-racescase-values-thresholdtree-reassoc-widthsched-pressure-algorithmThe default choice depends on the target.
max-slsr-cand-scanasan-globalsasan-stackasan-instrument-readsasan-instrument-writesasan-memintrinasan-use-after-returnasan-instrumentation-with-call-thresholdchkp-max-ctor-sizemax-fsm-thread-path-insnsmax-fsm-thread-lengthmax-fsm-thread-paths