3.10 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. 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- 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:
-fdefer-pop
-fmerge-constants
-fthread-jumps
-floop-optimize
-fif-conversion
-fif-conversion2
-fdelayed-branch
-fguess-branch-probability
-fcprop-registers
-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. The compiler does not
perform loop unrolling or function inlining when you specify -O2.
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:
-fforce-mem
-foptimize-sibling-calls
-fstrength-reduce
-fcse-follow-jumps -fcse-skip-blocks
-frerun-cse-after-loop -frerun-loop-opt
-fgcse -fgcse-lm -fgcse-sm -fgcse-las
-fdelete-null-pointer-checks
-fexpensive-optimizations
-fregmove
-fschedule-insns -fschedule-insns2
-fsched-interblock -fsched-spec
-fcaller-saves
-fpeephole2
-freorder-blocks -freorder-functions
-fstrict-aliasing
-funit-at-a-time
-falign-functions -falign-jumps
-falign-loops -falign-labels
-fcrossjumping
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,
-fweb and -fgcse-after-reload options.
-O0- Do not optimize. This is the default.
-Os- Optimize for size. -Os enables all -O2 optimizations that
do not typically increase code size. It also performs further
optimizations designed to reduce code size.
-Os disables the following optimization flags:
-falign-functions -falign-jumps -falign-loops
-falign-labels -freorder-blocks -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- 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.
-fforce-mem- Force memory operands to be copied into registers before doing
arithmetic on them. This produces better code by making all memory
references potential common subexpressions. When they are not common
subexpressions, instruction combination should eliminate the separate
register-load.
Enabled at levels -O2, -O3, -Os.
-fforce-addr- Force memory address constants to be copied into registers before
doing arithmetic on them. This may produce better code just as
-fforce-mem may.
-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. See Register Usage (GNU Compiler Collection (GCC) Internals).
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-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-limit=n- By default, GCC limits the size of functions that can be inlined. This flag
allows the control of this limit for functions that are explicitly marked as
inline (i.e., marked with the inline keyword or defined within the class
definition in c++). n is the size of functions that can be inlined in
number of pseudo instructions (not counting parameter handling). The default
value of n is 600.
Increasing this value can result in more inlined code at
the cost of compilation time and memory consumption. Decreasing usually makes
the compilation faster and less code will be inlined (which presumably
means slower programs). This option is particularly useful for programs that
use inlining heavily such as those based on recursive templates with C++.
Inlining is actually controlled by a number of 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- is set to n/2.
max-inline-insns-auto- is set to n/2.
min-inline-insns- is set to 130 or n/4, whichever is smaller.
max-inline-insns-rtl- is set to n.
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- 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 C. 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 non-automatic variable to
have distinct location, 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.
-fnew-ra- Use a graph coloring register allocator. Currently this option is meant
only for testing. Users should not specify this option, since it is not
yet ready for production use.
-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, enabled when
-fstrength-reduce is enabled.
-fno-function-cse- Do not put function addresses in registers; make each instruction that
calls a constant function contain the function's address explicitly.
This option results in less efficient code, but some strange hacks
that alter the assembler output may be confused by the optimizations
performed when this option is not used.
The default is -ffunction-cse
-fno-zero-initialized-in-bss- If the target supports a BSS section, GCC by default puts variables that
are initialized to zero into BSS. This can save space in the resulting
code.
This option turns off this behavior because some 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- For front-ends that support it, generate additional code to check that
indices used to access arrays are within the declared range. This is
currently only supported by the Java and Fortran front-ends, where
this option defaults to true and false respectively.
-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.
-fstrength-reduce- Perform the optimizations of loop strength reduction and
elimination of iteration variables.
Enabled at levels -O2, -O3, -Os.
-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 -O, -O2, -O3, -Os.
-fcse-follow-jumps- In common subexpression elimination, 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.
-frerun-loop-opt- Run the loop optimizer twice.
Enabled at levels -O2, -O3, -Os.
-fgcse- Perform a global common subexpression elimination pass.
This pass also performs global constant and copy propagation.
Note: When compiling a program using computed gotos, a GCC
extension, you may get better runtime performance if you disable
the global common subexpression 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.
Enabled by default when gcse is enabled.
-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).
Enabled by default when gcse is enabled.
-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.
-floop-optimize- Perform loop optimizations: move constant expressions out of loops, simplify
exit test conditions and optionally do strength-reduction as well.
Enabled at levels -O, -O2, -O3, -Os.
-floop-optimize2- Perform loop optimizations using the new loop optimizer. The optimizations
(loop unrolling, peeling and unswitching, loop invariant motion) are enabled
by separate flags.
-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.
-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.
-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=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.
-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. Has an effect only during the second scheduling pass,
and only if -fsched-stalled-insns is used and its value is not zero.
-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.
-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.
-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.
-ftree-pre- Perform Partial Redundancy Elimination (PRE) on trees. This flag is
enabled by default at -O and higher.
-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 faster than PRE, though it exposes fewer redundancies.
This flag is enabled by default at -O and higher.
-ftree-ccp- Perform sparse conditional constant propagation (CCP) on trees. This flag
is enabled by default at -O and higher.
-ftree-dce- Perform dead code elimination (DCE) on trees. This flag is enabled by
default at -O and higher.
-ftree-dominator-opts- Perform dead code elimination (DCE) on trees. 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.
-ftree-lim- Perform loop invariant motion on trees. This pass moves only invartiants that
would be hard to handle on 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.
-fivcanon- 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-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-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-lrs- Perform live range splitting during the SSA->normal phase. Distinct live
ranges of a variable are split into unique variables, allowing for better
optimization later. This is enabled by default at -O and higher.
-ftree-vectorize- Perform loop vectorization on trees.
-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 both
-fstrength-reduce and -frerun-cse-after-loop. This
option makes code larger, and may or may not make it run faster.
-funroll-all-loops- Unroll all loops, even if their number of iterations is uncertain when
the loop is entered. This usually makes programs run more slowly.
-funroll-all-loops implies the same options as
-funroll-loops,
-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
(for best results, -fweb should be used as well).
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.
-fprefetch-loop-arrays- If supported by the target machine, generate instructions to prefetch
memory to improve the performance of loops that access large 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- 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 basic blocks in the compiled function in order to reduce number of
taken branches and 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- Allows the compiler to assume the strictest aliasing rules applicable to
the language being compiled. For C (and C++), this activates
optimizations based on the type of expressions. In particular, an
object of one type is assumed never to reside at the same address as an
object of a different type, unless the types are almost the same. For
example, an
unsigned int can alias an int, but not a
void* or a double. A character type may alias any other
type.
Pay special attention to code like this:
union a_union {
int i;
double d;
};
int f() {
a_union t;
t.d = 3.0;
return t.i;
}
The practice of reading from a different union member than the one most
recently written to (called “type-punning”) is common. Even with
-fstrict-aliasing, type-punning is allowed, provided the memory
is accessed through the union type. So, the code above will work as
expected. However, this code might not:
int f() {
a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
Every language that wishes to perform language-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- 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- Parse the whole compilation unit before starting to produce code.
This allows some extra optimizations to take place but consumes
more memory (in general). There are some compatibility issues
with unit-at-at-time mode:
- enabling unit-at-a-time mode may change the order
in which functions, variables, and top-level
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.
- unit-at-a-time mode removes unreferenced static variables
and functions are removed. This may result in undefined references
when an
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.
- Static functions now can use non-standard passing conventions that
may break
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- 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 at levels -O2, -O3, -Os,
on targets where the default format for debugging information supports
variable tracking.
-fno-cprop-registers- After register allocation and post-register allocation instruction splitting,
we perform a copy-propagation pass to try to reduce scheduling dependencies
and occasionally eliminate the copy.
Disabled at levels -O, -O2, -O3, -Os.
-fprofile-generate-
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.
-fprofile-use- 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.
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,
-fno-trapping-math, -ffinite-math-only,
-fno-rounding-math and -fno-signaling-nans.
This option causes the preprocessor macro __FAST_MATH__ to be defined.
This option should never be turned on by any -O option since
it can result in incorrect output for programs which depend on
an exact implementation of IEEE or ISO rules/specifications for
math functions.
-fno-math-errno- Do not set ERRNO after calling math functions that are executed
with a single instruction, e.g., sqrt. A program that relies on
IEEE exceptions for math error handling may want to use this flag
for speed while maintaining IEEE arithmetic compatibility.
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- 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 should never be turned on by any -O option since
it can result in incorrect output for programs which depend on
an exact implementation of IEEE or ISO rules/specifications for
math functions.
The default is -fno-unsafe-math-optimizations.
-ffinite-math-only- Allow optimizations for floating-point arithmetic that assume
that arguments and results are not NaNs or +-Infs.
This option should never be turned on by any -O option since
it can result in incorrect output for programs which depend on
an exact implementation of IEEE or ISO rules/specifications.
The default is -fno-finite-math-only.
-fno-trapping-math- Compile code assuming that floating-point operations cannot generate
user-visible traps. These traps include division by zero, overflow,
underflow, inexact result and invalid operation. This option implies
-fno-signaling-nans. Setting this option may allow faster
code if one relies on “non-stop” IEEE 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.
-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.
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
(see Options for Debugging Your Program or gcc), 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.
-fspeculative-prefetching- If combined with -fprofile-arcs, it instructs the compiler to add
a code to gather information about addresses of memory references in the
program.
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- 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”.
Not enabled by default at any level because it has known bugs.
-fnew-ra- Use a graph coloring register allocator. Currently this option is meant
for testing, so we are interested to hear about miscompilations with
-fnew-ra.
-ftracer- Perform tail duplication to enlarge superblock size. This transformation
simplifies the control flow of the function allowing other optimizations to do
better job.
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. 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 new 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).
-fprefetch-loop-arrays- If supported by the target machine, generate instructions to prefetch
memory to improve the performance of loops that access large arrays.
Disabled at level -Os.
-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.
--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.
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-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 500.
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 120.
large-function-insns- The limit specifying really large functions. For functions greater than this
limit 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.
This parameter is ignored when -funit-at-a-time is not used.
The default value is 3000.
large-function-growth- Specifies maximal growth of large function caused by inlining in percents.
This parameter is ignored when -funit-at-a-time is not used.
The default value is 200.
inline-unit-growth- Specifies maximal overall growth of the compilation unit caused by inlining.
This parameter is ignored when -funit-at-a-time is not used.
The default value is 150.
max-inline-insns-recursivemax-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 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 500.
max-inline-recursive-depthmax-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 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 500.
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-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.
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
tracer-dynamic-coveragetracer-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-ratiotracer-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.
global-var-threshold-
Counts the number of function calls (n) and the number of
call-clobbered variables (v). If nxv is larger than this limit, a
single artificial variable will be created to represent all the
call-clobbered variables at function call sites. This artificial
variable will then be made to alias every call-clobbered variable.
(done as
int * size_t on the host machine; beware overflow).
max-aliased-vops-
Maximum number of virtual operands allowed to represent aliases
before triggering the alias grouping heuristic. Alias grouping
reduces compile times and memory consumption needed for aliasing at
the expense of precision loss in alias information.
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
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- 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-location- The maximum number of memory locations cselib should take into acount.
Increasing values mean more aggressive optimization, making the compile time
increase with probably slightly better performance. The default value is 500.
reorder-blocks-duplicatereorder-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-region-blocks- The maximum number of blocks in a region to be considered for
interblock scheduling. The default value is 10.
max-sched-region-insns- The maximum number of insns in a region to be considered for
interblock scheduling. The default value is 100.
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's. The default value is 256.
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