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Re: g++ optimizer flag
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From: |
Ulrich Windl |
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Subject: |
Re: g++ optimizer flag |
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Date: |
Wed, 08 Dec 2010 15:42:22 -0000 |
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User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/23.1 (gnu/linux) |
Yue Li <xyly781@gmail.com> writes:
> Hello,
>
> I'm trying to figure out what optimizations have actually been
> performed by g++
> on my matrix multiplication code, I was looking for such an flag of
> g++, but seems could not find one, may I ask is there really a flag in
> g++
> that can provide a list of optimization done on my code? I found xlC
> could provide a list of optimized code resulting from only loop
> transformations.
What about reading the info file named "Optimize Options" (of gcc-4.4):
(gcc-4.4)Top:: > *Note Invoking GCC:: > Optimize Options
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. Compiling multiple files at once to a single output file mode
allows the compiler to use information gained from all of the files
when compiling each of them.
Not all optimizations are controlled directly by a flag. Only
optimizations that have a flag are listed.
`-O'
`-O1'
Optimize. Optimizing compilation takes somewhat more time, and a
lot more memory for a large function.
With `-O', the compiler tries to reduce code size and execution
time, without performing any optimizations that take a great deal
of compilation time.
`-O' turns on the following optimization flags:
-fauto-inc-dec
-fcprop-registers
-fdce
-fdefer-pop
-fdelayed-branch
-fdse
-fguess-branch-probability
-fif-conversion2
-fif-conversion
-finline-small-functions
-fipa-pure-const
-fipa-reference
-fmerge-constants
-fsplit-wide-types
-ftree-builtin-call-dce
-ftree-ccp
-ftree-ch
-ftree-copyrename
-ftree-dce
-ftree-dominator-opts
-ftree-dse
-ftree-fre
-ftree-sra
-ftree-ter
-funit-at-a-time
`-O' also turns on `-fomit-frame-pointer' on machines where doing
so does not interfere with debugging.
`-O2'
Optimize even more. GCC performs nearly all supported
optimizations that do not involve a space-speed tradeoff. As
compared to `-O', this option increases both compilation time and
the performance of the generated code.
`-O2' turns on all optimization flags specified by `-O'. It also
turns on the following optimization flags:
-fthread-jumps
-falign-functions -falign-jumps
-falign-loops -falign-labels
-fcaller-saves
-fcrossjumping
-fcse-follow-jumps -fcse-skip-blocks
-fdelete-null-pointer-checks
-fexpensive-optimizations
-fgcse -fgcse-lm
-findirect-inlining
-foptimize-sibling-calls
-fpeephole2
-fregmove
-freorder-blocks -freorder-functions
-frerun-cse-after-loop
-fsched-interblock -fsched-spec
-fschedule-insns -fschedule-insns2
-fstrict-aliasing -fstrict-overflow
-ftree-switch-conversion
-ftree-pre
-ftree-vrp
Please note the warning under `-fgcse' about invoking `-O2' on
programs that use computed gotos.
`-O3'
Optimize yet more. `-O3' turns on all optimizations specified by
`-O2' and also turns on the `-finline-functions',
`-funswitch-loops', `-fpredictive-commoning',
`-fgcse-after-reload' and `-ftree-vectorize' options.
`-O0'
Reduce compilation time and make debugging produce the expected
results. This is the default.
`-Os'
Optimize for size. `-Os' enables all `-O2' optimizations that do
not typically increase code size. It also performs further
optimizations designed to reduce code size.
`-Os' disables the following optimization flags:
-falign-functions -falign-jumps -falign-loops
-falign-labels -freorder-blocks -freorder-blocks-and-partition
-fprefetch-loop-arrays -ftree-vect-loop-version
If you use multiple `-O' options, with or without level numbers,
the last such option is the one that is effective.
Options of the form `-fFLAG' specify machine-independent flags. Most
flags have both positive and negative forms; the negative form of
`-ffoo' would be `-fno-foo'. In the table below, only one of the forms
is listed--the one you typically will use. You can figure out the
other form by either removing `no-' or adding it.
The following options control specific optimizations. They are either
activated by `-O' options or are related to ones that are. You can use
the following flags in the rare cases when "fine-tuning" of
optimizations to be performed is desired.
`-fno-default-inline'
Do not make member functions inline by default merely because they
are defined inside the class scope (C++ only). Otherwise, when
you specify `-O', member functions defined inside class scope are
compiled inline by default; i.e., you don't need to add `inline'
in front of the member function name.
`-fno-defer-pop'
Always pop the arguments to each function call as soon as that
function returns. For machines which must pop arguments after a
function call, the compiler normally lets arguments accumulate on
the stack for several function calls and pops them all at once.
Disabled at levels `-O', `-O2', `-O3', `-Os'.
`-fforward-propagate'
Perform a forward propagation pass on RTL. The pass tries to
combine two instructions and checks if the result can be
simplified. If loop unrolling is active, two passes are performed
and the second is scheduled after loop unrolling.
This option is enabled by default at optimization levels `-O2',
`-O3', `-Os'.
`-fomit-frame-pointer'
Don't keep the frame pointer in a register for functions that
don't need one. This avoids the instructions to save, set up and
restore frame pointers; it also makes an extra register available
in many functions. *It also makes debugging impossible on some
machines.*
On some machines, such as the VAX, this flag has no effect, because
the standard calling sequence automatically handles the frame
pointer and nothing is saved by pretending it doesn't exist. The
machine-description macro `FRAME_POINTER_REQUIRED' controls
whether a target machine supports this flag. *Note Register
Usage: (gccint)Registers.
Enabled at levels `-O', `-O2', `-O3', `-Os'.
`-foptimize-sibling-calls'
Optimize sibling and tail recursive calls.
Enabled at levels `-O2', `-O3', `-Os'.
`-fno-inline'
Don't pay attention to the `inline' keyword. Normally this option
is used to keep the compiler from expanding any functions inline.
Note that if you are not optimizing, no functions can be expanded
inline.
`-finline-small-functions'
Integrate functions into their callers when their body is smaller
than expected function call code (so overall size of program gets
smaller). The compiler heuristically decides which functions are
simple enough to be worth integrating in this way.
Enabled at level `-O2'.
`-findirect-inlining'
Inline also indirect calls that are discovered to be known at
compile time thanks to previous inlining. This option has any
effect only when inlining itself is turned on by the
`-finline-functions' or `-finline-small-functions' options.
Enabled at level `-O2'.
`-finline-functions'
Integrate all simple functions into their callers. The compiler
heuristically decides which functions are simple enough to be worth
integrating in this way.
If all calls to a given function are integrated, and the function
is declared `static', then the function is normally not output as
assembler code in its own right.
Enabled at level `-O3'.
`-finline-functions-called-once'
Consider all `static' functions called once for inlining into their
caller even if they are not marked `inline'. If a call to a given
function is integrated, then the function is not output as
assembler code in its own right.
Enabled at levels `-O1', `-O2', `-O3' and `-Os'.
`-fearly-inlining'
Inline functions marked by `always_inline' and functions whose
body seems smaller than the function call overhead early before
doing `-fprofile-generate' instrumentation and real inlining pass.
Doing so makes profiling significantly cheaper and usually
inlining faster on programs having large chains of nested wrapper
functions.
Enabled by default.
`-finline-limit=N'
By default, GCC limits the size of functions that can be inlined.
This flag allows coarse control of this limit. N is the size of
functions that can be inlined in number of pseudo instructions.
Inlining is actually controlled by a number of parameters, which
may be specified individually by using `--param NAME=VALUE'. The
`-finline-limit=N' option sets some of these parameters as follows:
`max-inline-insns-single'
is set to N/2.
`max-inline-insns-auto'
is set to N/2.
See below for a documentation of the individual parameters
controlling inlining and for the defaults of these parameters.
_Note:_ there may be no value to `-finline-limit' that results in
default behavior.
_Note:_ pseudo instruction represents, in this particular context,
an abstract measurement of function's size. In no way does it
represent a count of assembly instructions and as such its exact
meaning might change from one release to an another.
`-fkeep-inline-functions'
In C, emit `static' functions that are declared `inline' into the
object file, even if the function has been inlined into all of its
callers. This switch does not affect functions using the `extern
inline' extension in GNU C89. In C++, emit any and all inline
functions into the object file.
`-fkeep-static-consts'
Emit variables declared `static const' when optimization isn't
turned on, even if the variables aren't referenced.
GCC enables this option by default. If you want to force the
compiler to check if the variable was referenced, regardless of
whether or not optimization is turned on, use the
`-fno-keep-static-consts' option.
`-fmerge-constants'
Attempt to merge identical constants (string constants and
floating point constants) across compilation units.
This option is the default for optimized compilation if the
assembler and linker support it. Use `-fno-merge-constants' to
inhibit this behavior.
Enabled at levels `-O', `-O2', `-O3', `-Os'.
`-fmerge-all-constants'
Attempt to merge identical constants and identical variables.
This option implies `-fmerge-constants'. In addition to
`-fmerge-constants' this considers e.g. even constant initialized
arrays or initialized constant variables with integral or floating
point types. Languages like C or C++ require each variable,
including multiple instances of the same variable in recursive
calls, to have distinct locations, so using this option will
result in non-conforming behavior.
`-fmodulo-sched'
Perform swing modulo scheduling immediately before the first
scheduling pass. This pass looks at innermost loops and reorders
their instructions by overlapping different iterations.
`-fmodulo-sched-allow-regmoves'
Perform more aggressive SMS based modulo scheduling with register
moves allowed. By setting this flag certain anti-dependences
edges will be deleted which will trigger the generation of
reg-moves based on the life-range analysis. This option is
effective only with `-fmodulo-sched' enabled.
`-fno-branch-count-reg'
Do not use "decrement and branch" instructions on a count register,
but instead generate a sequence of instructions that decrement a
register, compare it against zero, then branch based upon the
result. This option is only meaningful on architectures that
support such instructions, which include x86, PowerPC, IA-64 and
S/390.
The default is `-fbranch-count-reg'.
`-fno-function-cse'
Do not put function addresses in registers; make each instruction
that calls a constant function contain the function's address
explicitly.
This option results in less efficient code, but some strange hacks
that alter the assembler output may be confused by the
optimizations performed when this option is not used.
The default is `-ffunction-cse'
`-fno-zero-initialized-in-bss'
If the target supports a BSS section, GCC by default puts
variables that are initialized to zero into BSS. This can save
space in the resulting code.
This option turns off this behavior because some programs
explicitly rely on variables going to the data section. E.g., so
that the resulting executable can find the beginning of that
section and/or make assumptions based on that.
The default is `-fzero-initialized-in-bss'.
`-fmudflap -fmudflapth -fmudflapir'
For front-ends that support it (C and C++), instrument all risky
pointer/array dereferencing operations, some standard library
string/heap functions, and some other associated constructs with
range/validity tests. Modules so instrumented should be immune to
buffer overflows, invalid heap use, and some other classes of C/C++
programming errors. The instrumentation relies on a separate
runtime library (`libmudflap'), which will be linked into a
program if `-fmudflap' is given at link time. Run-time behavior
of the instrumented program is controlled by the `MUDFLAP_OPTIONS'
environment variable. See `env MUDFLAP_OPTIONS=-help a.out' for
its options.
Use `-fmudflapth' instead of `-fmudflap' to compile and to link if
your program is multi-threaded. Use `-fmudflapir', in addition to
`-fmudflap' or `-fmudflapth', if instrumentation should ignore
pointer reads. This produces less instrumentation (and therefore
faster execution) and still provides some protection against
outright memory corrupting writes, but allows erroneously read
data to propagate within a program.
`-fthread-jumps'
Perform optimizations where we check to see if a jump branches to a
location where another comparison subsumed by the first is found.
If so, the first branch is redirected to either the destination of
the second branch or a point immediately following it, depending
on whether the condition is known to be true or false.
Enabled at levels `-O2', `-O3', `-Os'.
`-fsplit-wide-types'
When using a type that occupies multiple registers, such as `long
long' on a 32-bit system, split the registers apart and allocate
them independently. This normally generates better code for those
types, but may make debugging more difficult.
Enabled at levels `-O', `-O2', `-O3', `-Os'.
`-fcse-follow-jumps'
In common subexpression elimination (CSE), scan through jump
instructions when the target of the jump is not reached by any
other path. For example, when CSE encounters an `if' statement
with an `else' clause, CSE will follow the jump when the condition
tested is false.
Enabled at levels `-O2', `-O3', `-Os'.
`-fcse-skip-blocks'
This is similar to `-fcse-follow-jumps', but causes CSE to follow
jumps which conditionally skip over blocks. When CSE encounters a
simple `if' statement with no else clause, `-fcse-skip-blocks'
causes CSE to follow the jump around the body of the `if'.
Enabled at levels `-O2', `-O3', `-Os'.
`-frerun-cse-after-loop'
Re-run common subexpression elimination after loop optimizations
has been performed.
Enabled at levels `-O2', `-O3', `-Os'.
`-fgcse'
Perform a global common subexpression elimination pass. This pass
also performs global constant and copy propagation.
_Note:_ When compiling a program using computed gotos, a GCC
extension, you may get better runtime performance if you disable
the global common subexpression elimination pass by adding
`-fno-gcse' to the command line.
Enabled at levels `-O2', `-O3', `-Os'.
`-fgcse-lm'
When `-fgcse-lm' is enabled, global common subexpression
elimination will attempt to move loads which are only killed by
stores into themselves. This allows a loop containing a
load/store sequence to be changed to a load outside the loop, and
a copy/store within the loop.
Enabled by default when gcse is enabled.
`-fgcse-sm'
When `-fgcse-sm' is enabled, a store motion pass is run after
global common subexpression elimination. This pass will attempt
to move stores out of loops. When used in conjunction with
`-fgcse-lm', loops containing a load/store sequence can be changed
to a load before the loop and a store after the loop.
Not enabled at any optimization level.
`-fgcse-las'
When `-fgcse-las' is enabled, the global common subexpression
elimination pass eliminates redundant loads that come after stores
to the same memory location (both partial and full redundancies).
Not enabled at any optimization level.
`-fgcse-after-reload'
When `-fgcse-after-reload' is enabled, a redundant load elimination
pass is performed after reload. The purpose of this pass is to
cleanup redundant spilling.
`-funsafe-loop-optimizations'
If given, the loop optimizer will assume that loop indices do not
overflow, and that the loops with nontrivial exit condition are not
infinite. This enables a wider range of loop optimizations even if
the loop optimizer itself cannot prove that these assumptions are
valid. Using `-Wunsafe-loop-optimizations', the compiler will
warn you if it finds this kind of loop.
`-fcrossjumping'
Perform cross-jumping transformation. This transformation unifies
equivalent code and save code size. The resulting code may or may
not perform better than without cross-jumping.
Enabled at levels `-O2', `-O3', `-Os'.
`-fauto-inc-dec'
Combine increments or decrements of addresses with memory accesses.
This pass is always skipped on architectures that do not have
instructions to support this. Enabled by default at `-O' and
higher on architectures that support this.
`-fdce'
Perform dead code elimination (DCE) on RTL. Enabled by default at
`-O' and higher.
`-fdse'
Perform dead store elimination (DSE) on RTL. Enabled by default
at `-O' and higher.
`-fif-conversion'
Attempt to transform conditional jumps into branch-less
equivalents. This include use of conditional moves, min, max, set
flags and abs instructions, and some tricks doable by standard
arithmetics. The use of conditional execution on chips where it
is available is controlled by `if-conversion2'.
Enabled at levels `-O', `-O2', `-O3', `-Os'.
`-fif-conversion2'
Use conditional execution (where available) to transform
conditional jumps into branch-less equivalents.
Enabled at levels `-O', `-O2', `-O3', `-Os'.
`-fdelete-null-pointer-checks'
Use global dataflow analysis to identify and eliminate useless
checks for null pointers. The compiler assumes that dereferencing
a null pointer would have halted the program. If a pointer is
checked after it has already been dereferenced, it cannot be null.
In some environments, this assumption is not true, and programs can
safely dereference null pointers. Use
`-fno-delete-null-pointer-checks' to disable this optimization for
programs which depend on that behavior.
Enabled at levels `-O2', `-O3', `-Os'.
`-fexpensive-optimizations'
Perform a number of minor optimizations that are relatively
expensive.
Enabled at levels `-O2', `-O3', `-Os'.
`-foptimize-register-move'
`-fregmove'
Attempt to reassign register numbers in move instructions and as
operands of other simple instructions in order to maximize the
amount of register tying. This is especially helpful on machines
with two-operand instructions.
Note `-fregmove' and `-foptimize-register-move' are the same
optimization.
Enabled at levels `-O2', `-O3', `-Os'.
`-fira-algorithm=ALGORITHM'
Use specified coloring algorithm for the integrated register
allocator. The ALGORITHM argument should be `priority' or `CB'.
The first algorithm specifies Chow's priority coloring, the second
one specifies Chaitin-Briggs coloring. The second algorithm can
be unimplemented for some architectures. If it is implemented, it
is the default because Chaitin-Briggs coloring as a rule generates
a better code.
`-fira-region=REGION'
Use specified regions for the integrated register allocator. The
REGION argument should be one of `all', `mixed', or `one'. The
first value means using all loops as register allocation regions,
the second value which is the default means using all loops except
for loops with small register pressure as the regions, and third
one means using all function as a single region. The first value
can give best result for machines with small size and irregular
register set, the third one results in faster and generates decent
code and the smallest size code, and the default value usually
give the best results in most cases and for most architectures.
`-fira-coalesce'
Do optimistic register coalescing. This option might be
profitable for architectures with big regular register files.
`-fno-ira-share-save-slots'
Switch off sharing stack slots used for saving call used hard
registers living through a call. Each hard register will get a
separate stack slot and as a result function stack frame will be
bigger.
`-fno-ira-share-spill-slots'
Switch off sharing stack slots allocated for pseudo-registers.
Each pseudo-register which did not get a hard register will get a
separate stack slot and as a result function stack frame will be
bigger.
`-fira-verbose=N'
Set up how verbose dump file for the integrated register allocator
will be. Default value is 5. If the value is greater or equal to
10, the dump file will be stderr as if the value were N minus 10.
`-fdelayed-branch'
If supported for the target machine, attempt to reorder
instructions to exploit instruction slots available after delayed
branch instructions.
Enabled at levels `-O', `-O2', `-O3', `-Os'.
`-fschedule-insns'
If supported for the target machine, attempt to reorder
instructions to eliminate execution stalls due to required data
being unavailable. This helps machines that have slow floating
point or memory load instructions by allowing other instructions
to be issued until the result of the load or floating point
instruction is required.
Enabled at levels `-O2', `-O3', `-Os'.
`-fschedule-insns2'
Similar to `-fschedule-insns', but requests an additional pass of
instruction scheduling after register allocation has been done.
This is especially useful on machines with a relatively small
number of registers and where memory load instructions take more
than one cycle.
Enabled at levels `-O2', `-O3', `-Os'.
`-fno-sched-interblock'
Don't schedule instructions across basic blocks. This is normally
enabled by default when scheduling before register allocation, i.e.
with `-fschedule-insns' or at `-O2' or higher.
`-fno-sched-spec'
Don't allow speculative motion of non-load instructions. This is
normally enabled by default when scheduling before register
allocation, i.e. with `-fschedule-insns' or at `-O2' or higher.
`-fsched-spec-load'
Allow speculative motion of some load instructions. This only
makes sense when scheduling before register allocation, i.e. with
`-fschedule-insns' or at `-O2' or higher.
`-fsched-spec-load-dangerous'
Allow speculative motion of more load instructions. This only
makes sense when scheduling before register allocation, i.e. with
`-fschedule-insns' or at `-O2' or higher.
`-fsched-stalled-insns'
`-fsched-stalled-insns=N'
Define how many insns (if any) can be moved prematurely from the
queue of stalled insns into the ready list, during the second
scheduling pass. `-fno-sched-stalled-insns' means that no insns
will be moved prematurely, `-fsched-stalled-insns=0' means there
is no limit on how many queued insns can be moved prematurely.
`-fsched-stalled-insns' without a value is equivalent to
`-fsched-stalled-insns=1'.
`-fsched-stalled-insns-dep'
`-fsched-stalled-insns-dep=N'
Define how many insn groups (cycles) will be examined for a
dependency on a stalled insn that is candidate for premature
removal from the queue of stalled insns. This has an effect only
during the second scheduling pass, and only if
`-fsched-stalled-insns' is used. `-fno-sched-stalled-insns-dep'
is equivalent to `-fsched-stalled-insns-dep=0'.
`-fsched-stalled-insns-dep' without a value is equivalent to
`-fsched-stalled-insns-dep=1'.
`-fsched2-use-superblocks'
When scheduling after register allocation, do use superblock
scheduling algorithm. Superblock scheduling allows motion across
basic block boundaries resulting on faster schedules. This option
is experimental, as not all machine descriptions used by GCC model
the CPU closely enough to avoid unreliable results from the
algorithm.
This only makes sense when scheduling after register allocation,
i.e. with `-fschedule-insns2' or at `-O2' or higher.
`-fsched2-use-traces'
Use `-fsched2-use-superblocks' algorithm when scheduling after
register allocation and additionally perform code duplication in
order to increase the size of superblocks using tracer pass. See
`-ftracer' for details on trace formation.
This mode should produce faster but significantly longer programs.
Also without `-fbranch-probabilities' the traces constructed may
not match the reality and hurt the performance. This only makes
sense when scheduling after register allocation, i.e. with
`-fschedule-insns2' or at `-O2' or higher.
`-fsee'
Eliminate redundant sign extension instructions and move the
non-redundant ones to optimal placement using lazy code motion
(LCM).
`-freschedule-modulo-scheduled-loops'
The modulo scheduling comes before the traditional scheduling, if
a loop was modulo scheduled we may want to prevent the later
scheduling passes from changing its schedule, we use this option
to control that.
`-fselective-scheduling'
Schedule instructions using selective scheduling algorithm.
Selective scheduling runs instead of the first scheduler pass.
`-fselective-scheduling2'
Schedule instructions using selective scheduling algorithm.
Selective scheduling runs instead of the second scheduler pass.
`-fsel-sched-pipelining'
Enable software pipelining of innermost loops during selective
scheduling. This option has no effect until one of
`-fselective-scheduling' or `-fselective-scheduling2' is turned on.
`-fsel-sched-pipelining-outer-loops'
When pipelining loops during selective scheduling, also pipeline
outer loops. This option has no effect until
`-fsel-sched-pipelining' is turned on.
`-fcaller-saves'
Enable values to be allocated in registers that will be clobbered
by function calls, by emitting extra instructions to save and
restore the registers around such calls. Such allocation is done
only when it seems to result in better code than would otherwise
be produced.
This option is always enabled by default on certain machines,
usually those which have no call-preserved registers to use
instead.
Enabled at levels `-O2', `-O3', `-Os'.
`-fconserve-stack'
Attempt to minimize stack usage. The compiler will attempt to use
less stack space, even if that makes the program slower. This
option implies setting the `large-stack-frame' parameter to 100
and the `large-stack-frame-growth' parameter to 400.
`-ftree-reassoc'
Perform reassociation on trees. This flag is enabled by default
at `-O' and higher.
`-ftree-pre'
Perform partial redundancy elimination (PRE) on trees. This flag
is enabled by default at `-O2' and `-O3'.
`-ftree-fre'
Perform full redundancy elimination (FRE) on trees. The difference
between FRE and PRE is that FRE only considers expressions that
are computed on all paths leading to the redundant computation.
This analysis is faster than PRE, though it exposes fewer
redundancies. This flag is enabled by default at `-O' and higher.
`-ftree-copy-prop'
Perform copy propagation on trees. This pass eliminates
unnecessary copy operations. This flag is enabled by default at
`-O' and higher.
`-fipa-pure-const'
Discover which functions are pure or constant. Enabled by default
at `-O' and higher.
`-fipa-reference'
Discover which static variables do not escape cannot escape the
compilation unit. Enabled by default at `-O' and higher.
`-fipa-struct-reorg'
Perform structure reorganization optimization, that change C-like
structures layout in order to better utilize spatial locality.
This transformation is affective for programs containing arrays of
structures. Available in two compilation modes: profile-based
(enabled with `-fprofile-generate') or static (which uses built-in
heuristics). Require `-fipa-type-escape' to provide the safety of
this transformation. It works only in whole program mode, so it
requires `-fwhole-program' and `-combine' to be enabled.
Structures considered `cold' by this transformation are not
affected (see `--param struct-reorg-cold-struct-ratio=VALUE').
With this flag, the program debug info reflects a new structure
layout.
`-fipa-pta'
Perform interprocedural pointer analysis. This option is
experimental and does not affect generated code.
`-fipa-cp'
Perform interprocedural constant propagation. This optimization
analyzes the program to determine when values passed to functions
are constants and then optimizes accordingly. This optimization
can substantially increase performance if the application has
constants passed to functions. This flag is enabled by default at
`-O2', `-Os' and `-O3'.
`-fipa-cp-clone'
Perform function cloning to make interprocedural constant
propagation stronger. When enabled, interprocedural constant
propagation will perform function cloning when externally visible
function can be called with constant arguments. Because this
optimization can create multiple copies of functions, it may
significantly increase code size (see `--param
ipcp-unit-growth=VALUE'). This flag is enabled by default at
`-O3'.
`-fipa-matrix-reorg'
Perform matrix flattening and transposing. Matrix flattening
tries to replace a m-dimensional matrix with its equivalent
n-dimensional matrix, where n < m. This reduces the level of
indirection needed for accessing the elements of the matrix. The
second optimization is matrix transposing that attempts to change
the order of the matrix's dimensions in order to improve cache
locality. Both optimizations need the `-fwhole-program' flag.
Transposing is enabled only if profiling information is available.
`-ftree-sink'
Perform forward store motion on trees. This flag is enabled by
default at `-O' and higher.
`-ftree-ccp'
Perform sparse conditional constant propagation (CCP) on trees.
This pass only operates on local scalar variables and is enabled
by default at `-O' and higher.
`-ftree-switch-conversion'
Perform conversion of simple initializations in a switch to
initializations from a scalar array. This flag is enabled by
default at `-O2' and higher.
`-ftree-dce'
Perform dead code elimination (DCE) on trees. This flag is
enabled by default at `-O' and higher.
`-ftree-builtin-call-dce'
Perform conditional dead code elimination (DCE) for calls to
builtin functions that may set `errno' but are otherwise
side-effect free. This flag is enabled by default at `-O2' and
higher if `-Os' is not also specified.
`-ftree-dominator-opts'
Perform a variety of simple scalar cleanups (constant/copy
propagation, redundancy elimination, range propagation and
expression simplification) based on a dominator tree traversal.
This also performs jump threading (to reduce jumps to jumps). This
flag is enabled by default at `-O' and higher.
`-ftree-dse'
Perform dead store elimination (DSE) on trees. A dead store is a
store into a memory location which will later be overwritten by
another store without any intervening loads. In this case the
earlier store can be deleted. This flag is enabled by default at
`-O' and higher.
`-ftree-ch'
Perform loop header copying on trees. This is beneficial since it
increases effectiveness of code motion optimizations. It also
saves one jump. This flag is enabled by default at `-O' and
higher. It is not enabled for `-Os', since it usually increases
code size.
`-ftree-loop-optimize'
Perform loop optimizations on trees. This flag is enabled by
default at `-O' and higher.
`-ftree-loop-linear'
Perform linear loop transformations on tree. This flag can
improve cache performance and allow further loop optimizations to
take place.
`-floop-interchange'
Perform loop interchange transformations on loops. Interchanging
two nested loops switches the inner and outer loops. For example,
given a loop like:
DO J = 1, M
DO I = 1, N
A(J, I) = A(J, I) * C
ENDDO
ENDDO
loop interchange will transform the loop as if the user had
written:
DO I = 1, N
DO J = 1, M
A(J, I) = A(J, I) * C
ENDDO
ENDDO
which can be beneficial when `N' is larger than the caches,
because in Fortran, the elements of an array are stored in memory
contiguously by column, and the original loop iterates over rows,
potentially creating at each access a cache miss. This
optimization applies to all the languages supported by GCC and is
not limited to Fortran. To use this code transformation, GCC has
to be configured with `--with-ppl' and `--with-cloog' to enable the
Graphite loop transformation infrastructure.
`-floop-strip-mine'
Perform loop strip mining transformations on loops. Strip mining
splits a loop into two nested loops. The outer loop has strides
equal to the strip size and the inner loop has strides of the
original loop within a strip. For example, given a loop like:
DO I = 1, N
A(I) = A(I) + C
ENDDO
loop strip mining will transform the loop as if the user had
written:
DO II = 1, N, 4
DO I = II, min (II + 3, N)
A(I) = A(I) + C
ENDDO
ENDDO
This optimization applies to all the languages supported by GCC
and is not limited to Fortran. To use this code transformation,
GCC has to be configured with `--with-ppl' and `--with-cloog' to
enable the Graphite loop transformation infrastructure.
`-floop-block'
Perform loop blocking transformations on loops. Blocking strip
mines each loop in the loop nest such that the memory accesses of
the element loops fit inside caches. For example, given a loop
like:
DO I = 1, N
DO J = 1, M
A(J, I) = B(I) + C(J)
ENDDO
ENDDO
loop blocking will transform the loop as if the user had written:
DO II = 1, N, 64
DO JJ = 1, M, 64
DO I = II, min (II + 63, N)
DO J = JJ, min (JJ + 63, M)
A(J, I) = B(I) + C(J)
ENDDO
ENDDO
ENDDO
ENDDO
which can be beneficial when `M' is larger than the caches,
because the innermost loop will iterate over a smaller amount of
data that can be kept in the caches. This optimization applies to
all the languages supported by GCC and is not limited to Fortran.
To use this code transformation, GCC has to be configured with
`--with-ppl' and `--with-cloog' to enable the Graphite loop
transformation infrastructure.
`-fcheck-data-deps'
Compare the results of several data dependence analyzers. This
option is used for debugging the data dependence analyzers.
`-ftree-loop-distribution'
Perform loop distribution. This flag can improve cache
performance on big loop bodies and allow further loop
optimizations, like parallelization or vectorization, to take
place. For example, the loop
DO I = 1, N
A(I) = B(I) + C
D(I) = E(I) * F
ENDDO
is transformed to
DO I = 1, N
A(I) = B(I) + C
ENDDO
DO I = 1, N
D(I) = E(I) * F
ENDDO
`-ftree-loop-im'
Perform loop invariant motion on trees. This pass moves only
invariants that would be hard to handle at RTL level (function
calls, operations that expand to nontrivial sequences of insns).
With `-funswitch-loops' it also moves operands of conditions that
are invariant out of the loop, so that we can use just trivial
invariantness analysis in loop unswitching. The pass also includes
store motion.
`-ftree-loop-ivcanon'
Create a canonical counter for number of iterations in the loop
for that determining number of iterations requires complicated
analysis. Later optimizations then may determine the number
easily. Useful especially in connection with unrolling.
`-fivopts'
Perform induction variable optimizations (strength reduction,
induction variable merging and induction variable elimination) on
trees.
`-ftree-parallelize-loops=n'
Parallelize loops, i.e., split their iteration space to run in n
threads. This is only possible for loops whose iterations are
independent and can be arbitrarily reordered. The optimization is
only profitable on multiprocessor machines, for loops that are
CPU-intensive, rather than constrained e.g. by memory bandwidth.
This option implies `-pthread', and thus is only supported on
targets that have support for `-pthread'.
`-ftree-sra'
Perform scalar replacement of aggregates. This pass replaces
structure references with scalars to prevent committing structures
to memory too early. This flag is enabled by default at `-O' and
higher.
`-ftree-copyrename'
Perform copy renaming on trees. This pass attempts to rename
compiler temporaries to other variables at copy locations, usually
resulting in variable names which more closely resemble the
original variables. This flag is enabled by default at `-O' and
higher.
`-ftree-ter'
Perform temporary expression replacement during the SSA->normal
phase. Single use/single def temporaries are replaced at their
use location with their defining expression. This results in
non-GIMPLE code, but gives the expanders much more complex trees
to work on resulting in better RTL generation. This is enabled by
default at `-O' and higher.
`-ftree-vectorize'
Perform loop vectorization on trees. This flag is enabled by
default at `-O3'.
`-ftree-vect-loop-version'
Perform loop versioning when doing loop vectorization on trees.
When a loop appears to be vectorizable except that data alignment
or data dependence cannot be determined at compile time then
vectorized and non-vectorized versions of the loop are generated
along with runtime checks for alignment or dependence to control
which version is executed. This option is enabled by default
except at level `-Os' where it is disabled.
`-fvect-cost-model'
Enable cost model for vectorization.
`-ftree-vrp'
Perform Value Range Propagation on trees. This is similar to the
constant propagation pass, but instead of values, ranges of values
are propagated. This allows the optimizers to remove unnecessary
range checks like array bound checks and null pointer checks.
This is enabled by default at `-O2' and higher. Null pointer check
elimination is only done if `-fdelete-null-pointer-checks' is
enabled.
`-ftracer'
Perform tail duplication to enlarge superblock size. This
transformation simplifies the control flow of the function
allowing other optimizations to do better job.
`-funroll-loops'
Unroll loops whose number of iterations can be determined at
compile time or upon entry to the loop. `-funroll-loops' implies
`-frerun-cse-after-loop'. This option makes code larger, and may
or may not make it run faster.
`-funroll-all-loops'
Unroll all loops, even if their number of iterations is uncertain
when the loop is entered. This usually makes programs run more
slowly. `-funroll-all-loops' implies the same options as
`-funroll-loops',
`-fsplit-ivs-in-unroller'
Enables expressing of values of induction variables in later
iterations of the unrolled loop using the value in the first
iteration. This breaks long dependency chains, thus improving
efficiency of the scheduling passes.
Combination of `-fweb' and CSE is often sufficient to obtain the
same effect. However in cases the loop body is more complicated
than a single basic block, this is not reliable. It also does not
work at all on some of the architectures due to restrictions in
the CSE pass.
This optimization is enabled by default.
`-fvariable-expansion-in-unroller'
With this option, the compiler will create multiple copies of some
local variables when unrolling a loop which can result in superior
code.
`-fpredictive-commoning'
Perform predictive commoning optimization, i.e., reusing
computations (especially memory loads and stores) performed in
previous iterations of loops.
This option is enabled at level `-O3'.
`-fprefetch-loop-arrays'
If supported by the target machine, generate instructions to
prefetch memory to improve the performance of loops that access
large arrays.
This option may generate better or worse code; results are highly
dependent on the structure of loops within the source code.
Disabled at level `-Os'.
`-fno-peephole'
`-fno-peephole2'
Disable any machine-specific peephole optimizations. The
difference between `-fno-peephole' and `-fno-peephole2' is in how
they are implemented in the compiler; some targets use one, some
use the other, a few use both.
`-fpeephole' is enabled by default. `-fpeephole2' enabled at
levels `-O2', `-O3', `-Os'.
`-fno-guess-branch-probability'
Do not guess branch probabilities using heuristics.
GCC will use heuristics to guess branch probabilities if they are
not provided by profiling feedback (`-fprofile-arcs'). These
heuristics are based on the control flow graph. If some branch
probabilities are specified by `__builtin_expect', then the
heuristics will be used to guess branch probabilities for the rest
of the control flow graph, taking the `__builtin_expect' info into
account. The interactions between the heuristics and
`__builtin_expect' can be complex, and in some cases, it may be
useful to disable the heuristics so that the effects of
`__builtin_expect' are easier to understand.
The default is `-fguess-branch-probability' at levels `-O', `-O2',
`-O3', `-Os'.
`-freorder-blocks'
Reorder basic blocks in the compiled function in order to reduce
number of taken branches and improve code locality.
Enabled at levels `-O2', `-O3'.
`-freorder-blocks-and-partition'
In addition to reordering basic blocks in the compiled function,
in order to reduce number of taken branches, partitions hot and
cold basic blocks into separate sections of the assembly and .o
files, to improve paging and cache locality performance.
This optimization is automatically turned off in the presence of
exception handling, for linkonce sections, for functions with a
user-defined section attribute and on any architecture that does
not support named sections.
`-freorder-functions'
Reorder functions in the object file in order to improve code
locality. This is implemented by using special subsections
`.text.hot' for most frequently executed functions and
`.text.unlikely' for unlikely executed functions. Reordering is
done by the linker so object file format must support named
sections and linker must place them in a reasonable way.
Also profile feedback must be available in to make this option
effective. See `-fprofile-arcs' for details.
Enabled at levels `-O2', `-O3', `-Os'.
`-fstrict-aliasing'
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. *Note Structures unions enumerations and
bit-fields implementation::. However, this code might not:
int f() {
a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
Similarly, access by taking the address, casting the resulting
pointer and dereferencing the result has undefined behavior, even
if the cast uses a union type, e.g.:
int f() {
double d = 3.0;
return ((union a_union *) &d)->i;
}
The `-fstrict-aliasing' option is enabled at levels `-O2', `-O3',
`-Os'.
`-fstrict-overflow'
Allow the compiler to assume strict signed overflow rules,
depending on the language being compiled. For C (and C++) this
means that overflow when doing arithmetic with signed numbers is
undefined, which means that the compiler may assume that it will
not happen. This permits various optimizations. For example, the
compiler will assume that an expression like `i + 10 > i' will
always be true for signed `i'. This assumption is only valid if
signed overflow is undefined, as the expression is false if `i +
10' overflows when using twos complement arithmetic. When this
option is in effect any attempt to determine whether an operation
on signed numbers will overflow must be written carefully to not
actually involve overflow.
This option also allows the compiler to assume strict pointer
semantics: given a pointer to an object, if adding an offset to
that pointer does not produce a pointer to the same object, the
addition is undefined. This permits the compiler to conclude that
`p + u > p' is always true for a pointer `p' and unsigned integer
`u'. This assumption is only valid because pointer wraparound is
undefined, as the expression is false if `p + u' overflows using
twos complement arithmetic.
See also the `-fwrapv' option. Using `-fwrapv' means that integer
signed overflow is fully defined: it wraps. When `-fwrapv' is
used, there is no difference between `-fstrict-overflow' and
`-fno-strict-overflow' for integers. With `-fwrapv' certain types
of overflow are permitted. For example, if the compiler gets an
overflow when doing arithmetic on constants, the overflowed value
can still be used with `-fwrapv', but not otherwise.
The `-fstrict-overflow' option is enabled at levels `-O2', `-O3',
`-Os'.
`-falign-functions'
`-falign-functions=N'
Align the start of functions to the next power-of-two greater than
N, skipping up to N bytes. For instance, `-falign-functions=32'
aligns functions to the next 32-byte boundary, but
`-falign-functions=24' would align to the next 32-byte boundary
only if this can be done by skipping 23 bytes or less.
`-fno-align-functions' and `-falign-functions=1' are equivalent
and mean that functions will not be aligned.
Some assemblers only support this flag when N is a power of two;
in that case, it is rounded up.
If N is not specified or is zero, use a machine-dependent default.
Enabled at levels `-O2', `-O3'.
`-falign-labels'
`-falign-labels=N'
Align all branch targets to a power-of-two boundary, skipping up to
N bytes like `-falign-functions'. This option can easily make
code slower, because it must insert dummy operations for when the
branch target is reached in the usual flow of the code.
`-fno-align-labels' and `-falign-labels=1' are equivalent and mean
that labels will not be aligned.
If `-falign-loops' or `-falign-jumps' are applicable and are
greater than this value, then their values are used instead.
If N is not specified or is zero, use a machine-dependent default
which is very likely to be `1', meaning no alignment.
Enabled at levels `-O2', `-O3'.
`-falign-loops'
`-falign-loops=N'
Align loops to a power-of-two boundary, skipping up to N bytes
like `-falign-functions'. The hope is that the loop will be
executed many times, which will make up for any execution of the
dummy operations.
`-fno-align-loops' and `-falign-loops=1' are equivalent and mean
that loops will not be aligned.
If N is not specified or is zero, use a machine-dependent default.
Enabled at levels `-O2', `-O3'.
`-falign-jumps'
`-falign-jumps=N'
Align branch targets to a power-of-two boundary, for branch targets
where the targets can only be reached by jumping, skipping up to N
bytes like `-falign-functions'. In this case, no dummy operations
need be executed.
`-fno-align-jumps' and `-falign-jumps=1' are equivalent and mean
that loops will not be aligned.
If N is not specified or is zero, use a machine-dependent default.
Enabled at levels `-O2', `-O3'.
`-funit-at-a-time'
This option is left for compatibility reasons. `-funit-at-a-time'
has no effect, while `-fno-unit-at-a-time' implies
`-fno-toplevel-reorder' and `-fno-section-anchors'.
Enabled by default.
`-fno-toplevel-reorder'
Do not reorder top-level functions, variables, and `asm'
statements. Output them in the same order that they appear in the
input file. When this option is used, unreferenced static
variables will not be removed. This option is intended to support
existing code which relies on a particular ordering. For new
code, it is better to use attributes.
Enabled at level `-O0'. When disabled explicitly, it also imply
`-fno-section-anchors' that is otherwise enabled at `-O0' on some
targets.
`-fweb'
Constructs webs as commonly used for register allocation purposes
and assign each web individual pseudo register. This allows the
register allocation pass to operate on pseudos directly, but also
strengthens several other optimization passes, such as CSE, loop
optimizer and trivial dead code remover. It can, however, make
debugging impossible, since variables will no longer stay in a
"home register".
Enabled by default with `-funroll-loops'.
`-fwhole-program'
Assume that the current compilation unit represents whole program
being compiled. All public functions and variables with the
exception of `main' and those merged by attribute
`externally_visible' become static functions and in a affect gets
more aggressively optimized by interprocedural optimizers. While
this option is equivalent to proper use of `static' keyword for
programs consisting of single file, in combination with option
`--combine' this flag can be used to compile most of smaller scale
C programs since the functions and variables become local for the
whole combined compilation unit, not for the single source file
itself.
This option is not supported for Fortran programs.
`-fcprop-registers'
After register allocation and post-register allocation instruction
splitting, we perform a copy-propagation pass to try to reduce
scheduling dependencies and occasionally eliminate the copy.
Enabled at levels `-O', `-O2', `-O3', `-Os'.
`-fprofile-correction'
Profiles collected using an instrumented binary for multi-threaded
programs may be inconsistent due to missed counter updates. When
this option is specified, GCC will use heuristics to correct or
smooth out such inconsistencies. By default, GCC will emit an
error message when an inconsistent profile is detected.
`-fprofile-dir=PATH'
Set the directory to search the profile data files in to PATH.
This option affects only the profile data generated by
`-fprofile-generate', `-ftest-coverage', `-fprofile-arcs' and used
by `-fprofile-use' and `-fbranch-probabilities' and its related
options. By default, GCC will use the current directory as PATH
thus the profile data file will appear in the same directory as
the object file.
`-fprofile-generate'
`-fprofile-generate=PATH'
Enable options usually used for instrumenting application to
produce profile useful for later recompilation with profile
feedback based optimization. You must use `-fprofile-generate'
both when compiling and when linking your program.
The following options are enabled: `-fprofile-arcs',
`-fprofile-values', `-fvpt'.
If PATH is specified, GCC will look at the PATH to find the
profile feedback data files. See `-fprofile-dir'.
`-fprofile-use'
`-fprofile-use=PATH'
Enable profile feedback directed optimizations, and optimizations
generally profitable only with profile feedback available.
The following options are enabled: `-fbranch-probabilities',
`-fvpt', `-funroll-loops', `-fpeel-loops', `-ftracer'
By default, GCC emits an error message if the feedback profiles do
not match the source code. This error can be turned into a
warning by using `-Wcoverage-mismatch'. Note this may result in
poorly optimized code.
If PATH is specified, GCC will look at the PATH to find the
profile feedback data files. See `-fprofile-dir'.
The following options control compiler behavior regarding floating
point arithmetic. These options trade off between speed and
correctness. All must be specifically enabled.
`-ffloat-store'
Do not store floating point variables in registers, and inhibit
other options that might change whether a floating point value is
taken from a register or memory.
This option prevents undesirable excess precision on machines such
as the 68000 where the floating registers (of the 68881) keep more
precision than a `double' is supposed to have. Similarly for the
x86 architecture. For most programs, the excess precision does
only good, but a few programs rely on the precise definition of
IEEE floating point. Use `-ffloat-store' for such programs, after
modifying them to store all pertinent intermediate computations
into variables.
`-ffast-math'
Sets `-fno-math-errno', `-funsafe-math-optimizations',
`-ffinite-math-only', `-fno-rounding-math', `-fno-signaling-nans'
and `-fcx-limited-range'.
This option causes the preprocessor macro `__FAST_MATH__' to be
defined.
This option is not turned on by any `-O' option since it can
result in incorrect output for programs which depend on an exact
implementation of IEEE or ISO rules/specifications for math
functions. It may, however, yield faster code for programs that do
not require the guarantees of these specifications.
`-fno-math-errno'
Do not set ERRNO after calling math functions that are executed
with a single instruction, e.g., sqrt. A program that relies on
IEEE exceptions for math error handling may want to use this flag
for speed while maintaining IEEE arithmetic compatibility.
This option is not turned on by any `-O' option since it can
result in incorrect output for programs which depend on an exact
implementation of IEEE or ISO rules/specifications for math
functions. It may, however, yield faster code for programs that do
not require the guarantees of these specifications.
The default is `-fmath-errno'.
On Darwin systems, the math library never sets `errno'. There is
therefore no reason for the compiler to consider the possibility
that it might, and `-fno-math-errno' is the default.
`-funsafe-math-optimizations'
Allow optimizations for floating-point arithmetic that (a) assume
that arguments and results are valid and (b) may violate IEEE or
ANSI standards. When used at link-time, it may include libraries
or startup files that change the default FPU control word or other
similar optimizations.
This option is not turned on by any `-O' option since it can
result in incorrect output for programs which depend on an exact
implementation of IEEE or ISO rules/specifications for math
functions. It may, however, yield faster code for programs that do
not require the guarantees of these specifications. Enables
`-fno-signed-zeros', `-fno-trapping-math', `-fassociative-math'
and `-freciprocal-math'.
The default is `-fno-unsafe-math-optimizations'.
`-fassociative-math'
Allow re-association of operands in series of floating-point
operations. This violates the ISO C and C++ language standard by
possibly changing computation result. NOTE: re-ordering may
change the sign of zero as well as ignore NaNs and inhibit or
create underflow or overflow (and thus cannot be used on a code
which relies on rounding behavior like `(x + 2**52) - 2**52)'.
May also reorder floating-point comparisons and thus may not be
used when ordered comparisons are required. This option requires
that both `-fno-signed-zeros' and `-fno-trapping-math' be in
effect. Moreover, it doesn't make much sense with
`-frounding-math'.
The default is `-fno-associative-math'.
`-freciprocal-math'
Allow the reciprocal of a value to be used instead of dividing by
the value if this enables optimizations. For example `x / y' can
be replaced with `x * (1/y)' which is useful if `(1/y)' is subject
to common subexpression elimination. Note that this loses
precision and increases the number of flops operating on the value.
The default is `-fno-reciprocal-math'.
`-ffinite-math-only'
Allow optimizations for floating-point arithmetic that assume that
arguments and results are not NaNs or +-Infs.
This option is not turned on by any `-O' option since it can
result in incorrect output for programs which depend on an exact
implementation of IEEE or ISO rules/specifications for math
functions. It may, however, yield faster code for programs that do
not require the guarantees of these specifications.
The default is `-fno-finite-math-only'.
`-fno-signed-zeros'
Allow optimizations for floating point arithmetic that ignore the
signedness of zero. IEEE arithmetic specifies the behavior of
distinct +0.0 and -0.0 values, which then prohibits simplification
of expressions such as x+0.0 or 0.0*x (even with
`-ffinite-math-only'). This option implies that the sign of a
zero result isn't significant.
The default is `-fsigned-zeros'.
`-fno-trapping-math'
Compile code assuming that floating-point operations cannot
generate user-visible traps. These traps include division by
zero, overflow, underflow, inexact result and invalid operation.
This option requires that `-fno-signaling-nans' be in effect.
Setting this option may allow faster code if one relies on
"non-stop" IEEE arithmetic, for example.
This option should never be turned on by any `-O' option since it
can result in incorrect output for programs which depend on an
exact implementation of IEEE or ISO rules/specifications for math
functions.
The default is `-ftrapping-math'.
`-frounding-math'
Disable transformations and optimizations that assume default
floating point rounding behavior. This is round-to-zero for all
floating point to integer conversions, and round-to-nearest for
all other arithmetic truncations. This option should be specified
for programs that change the FP rounding mode dynamically, or that
may be executed with a non-default rounding mode. This option
disables constant folding of floating point expressions at
compile-time (which may be affected by rounding mode) and
arithmetic transformations that are unsafe in the presence of
sign-dependent rounding modes.
The default is `-fno-rounding-math'.
This option is experimental and does not currently guarantee to
disable all GCC optimizations that are affected by rounding mode.
Future versions of GCC may provide finer control of this setting
using C99's `FENV_ACCESS' pragma. This command line option will
be used to specify the default state for `FENV_ACCESS'.
`-frtl-abstract-sequences'
It is a size optimization method. This option is to find identical
sequences of code, which can be turned into pseudo-procedures and
then replace all occurrences with calls to the newly created
subroutine. It is kind of an opposite of `-finline-functions'.
This optimization runs at RTL level.
`-fsignaling-nans'
Compile code assuming that IEEE signaling NaNs may generate
user-visible traps during floating-point operations. Setting this
option disables optimizations that may change the number of
exceptions visible with signaling NaNs. This option implies
`-ftrapping-math'.
This option causes the preprocessor macro `__SUPPORT_SNAN__' to be
defined.
The default is `-fno-signaling-nans'.
This option is experimental and does not currently guarantee to
disable all GCC optimizations that affect signaling NaN behavior.
`-fsingle-precision-constant'
Treat floating point constant as single precision constant instead
of implicitly converting it to double precision constant.
`-fcx-limited-range'
When enabled, this option states that a range reduction step is not
needed when performing complex division. Also, there is no
checking whether the result of a complex multiplication or
division is `NaN + I*NaN', with an attempt to rescue the situation
in that case. The default is `-fno-cx-limited-range', but is
enabled by `-ffast-math'.
This option controls the default setting of the ISO C99
`CX_LIMITED_RANGE' pragma. Nevertheless, the option applies to
all languages.
`-fcx-fortran-rules'
Complex multiplication and division follow Fortran rules. Range
reduction is done as part of complex division, but there is no
checking whether the result of a complex multiplication or
division is `NaN + I*NaN', with an attempt to rescue the situation
in that case.
The default is `-fno-cx-fortran-rules'.
The following options control optimizations that may improve
performance, but are not enabled by any `-O' options. This section
includes experimental options that may produce broken code.
`-fbranch-probabilities'
After running a program compiled with `-fprofile-arcs' (*note
Options for Debugging Your Program or `gcc': Debugging Options.),
you can compile it a second time using `-fbranch-probabilities',
to improve optimizations based on the number of times each branch
was taken. When the program compiled with `-fprofile-arcs' exits
it saves arc execution counts to a file called `SOURCENAME.gcda'
for each source file. The information in this data file is very
dependent on the structure of the generated code, so you must use
the same source code and the same optimization options for both
compilations.
With `-fbranch-probabilities', GCC puts a `REG_BR_PROB' note on
each `JUMP_INSN' and `CALL_INSN'. These can be used to improve
optimization. Currently, they are only used in one place: in
`reorg.c', instead of guessing which path a branch is mostly to
take, the `REG_BR_PROB' values are used to exactly determine which
path is taken more often.
`-fprofile-values'
If combined with `-fprofile-arcs', it adds code so that some data
about values of expressions in the program is gathered.
With `-fbranch-probabilities', it reads back the data gathered
from profiling values of expressions and adds `REG_VALUE_PROFILE'
notes to instructions for their later usage in optimizations.
Enabled with `-fprofile-generate' and `-fprofile-use'.
`-fvpt'
If combined with `-fprofile-arcs', it instructs the compiler to add
a code to gather information about values of expressions.
With `-fbranch-probabilities', it reads back the data gathered and
actually performs the optimizations based on them. Currently the
optimizations include specialization of division operation using
the knowledge about the value of the denominator.
`-frename-registers'
Attempt to avoid false dependencies in scheduled code by making use
of registers left over after register allocation. This
optimization will most benefit processors with lots of registers.
Depending on the debug information format adopted by the target,
however, it can make debugging impossible, since variables will no
longer stay in a "home register".
Enabled by default with `-funroll-loops'.
`-ftracer'
Perform tail duplication to enlarge superblock size. This
transformation simplifies the control flow of the function
allowing other optimizations to do better job.
Enabled with `-fprofile-use'.
`-funroll-loops'
Unroll loops whose number of iterations can be determined at
compile time or upon entry to the loop. `-funroll-loops' implies
`-frerun-cse-after-loop', `-fweb' and `-frename-registers'. It
also turns on complete loop peeling (i.e. complete removal of
loops with small constant number of iterations). This option
makes code larger, and may or may not make it run faster.
Enabled with `-fprofile-use'.
`-funroll-all-loops'
Unroll all loops, even if their number of iterations is uncertain
when the loop is entered. This usually makes programs run more
slowly. `-funroll-all-loops' implies the same options as
`-funroll-loops'.
`-fpeel-loops'
Peels the loops for that there is enough information that they do
not roll much (from profile feedback). It also turns on complete
loop peeling (i.e. complete removal of loops with small constant
number of iterations).
Enabled with `-fprofile-use'.
`-fmove-loop-invariants'
Enables the loop invariant motion pass in the RTL loop optimizer.
Enabled at level `-O1'
`-funswitch-loops'
Move branches with loop invariant conditions out of the loop, with
duplicates of the loop on both branches (modified according to
result of the condition).
`-ffunction-sections'
`-fdata-sections'
Place each function or data item into its own section in the output
file if the target supports arbitrary sections. The name of the
function or the name of the data item determines the section's name
in the output file.
Use these options on systems where the linker can perform
optimizations to improve locality of reference in the instruction
space. Most systems using the ELF object format and SPARC
processors running Solaris 2 have linkers with such optimizations.
AIX may have these optimizations in the future.
Only use these options when there are significant benefits from
doing so. When you specify these options, the assembler and
linker will create larger object and executable files and will
also be slower. You will not be able to use `gprof' on all
systems if you specify this option and you may have problems with
debugging if you specify both this option and `-g'.
`-fbranch-target-load-optimize'
Perform branch target register load optimization before prologue /
epilogue threading. The use of target registers can typically be
exposed only during reload, thus hoisting loads out of loops and
doing inter-block scheduling needs a separate optimization pass.
`-fbranch-target-load-optimize2'
Perform branch target register load optimization after prologue /
epilogue threading.
`-fbtr-bb-exclusive'
When performing branch target register load optimization, don't
reuse branch target registers in within any basic block.
`-fstack-protector'
Emit extra code to check for buffer overflows, such as stack
smashing attacks. This is done by adding a guard variable to
functions with vulnerable objects. This includes functions that
call alloca, and functions with buffers larger than 8 bytes. The
guards are initialized when a function is entered and then checked
when the function exits. If a guard check fails, an error message
is printed and the program exits.
`-fstack-protector-all'
Like `-fstack-protector' except that all functions are protected.
`-fsection-anchors'
Try to reduce the number of symbolic address calculations by using
shared "anchor" symbols to address nearby objects. This
transformation can help to reduce the number of GOT entries and
GOT accesses on some targets.
For example, the implementation of the following function `foo':
static int a, b, c;
int foo (void) { return a + b + c; }
would usually calculate the addresses of all three variables, but
if you compile it with `-fsection-anchors', it will access the
variables from a common anchor point instead. The effect is
similar to the following pseudocode (which isn't valid C):
int foo (void)
{
register int *xr = &x;
return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}
Not all targets support this option.
`--param NAME=VALUE'
In some places, GCC uses various constants to control the amount of
optimization that is done. For example, GCC will not inline
functions that contain more that a certain number of instructions.
You can control some of these constants on the command-line using
the `--param' option.
The names of specific parameters, and the meaning of the values,
are tied to the internals of the compiler, and are subject to
change without notice in future releases.
In each case, the VALUE is an integer. The allowable choices for
NAME are given in the following table:
`sra-max-structure-size'
The maximum structure size, in bytes, at which the scalar
replacement of aggregates (SRA) optimization will perform
block copies. The default value, 0, implies that GCC will
select the most appropriate size itself.
`sra-field-structure-ratio'
The threshold ratio (as a percentage) between instantiated
fields and the complete structure size. We say that if the
ratio of the number of bytes in instantiated fields to the
number of bytes in the complete structure exceeds this
parameter, then block copies are not used. The default is 75.
`struct-reorg-cold-struct-ratio'
The threshold ratio (as a percentage) between a structure
frequency and the frequency of the hottest structure in the
program. This parameter is used by struct-reorg optimization
enabled by `-fipa-struct-reorg'. We say that if the ratio of
a structure frequency, calculated by profiling, to the
hottest structure frequency in the program is less than this
parameter, then structure reorganization is not applied to
this structure. The default is 10.
`predictable-branch-cost-outcome'
When branch is predicted to be taken with probability lower
than this threshold (in percent), then it is considered well
predictable. The default is 10.
`max-crossjump-edges'
The maximum number of incoming edges to consider for
crossjumping. The algorithm used by `-fcrossjumping' is
O(N^2) in the number of edges incoming to each block.
Increasing values mean more aggressive optimization, making
the compile time increase with probably small improvement in
executable size.
`min-crossjump-insns'
The minimum number of instructions which must be matched at
the end of two blocks before crossjumping will be performed
on them. This value is ignored in the case where all
instructions in the block being crossjumped from are matched.
The default value is 5.
`max-grow-copy-bb-insns'
The maximum code size expansion factor when copying basic
blocks instead of jumping. The expansion is relative to a
jump instruction. The default value is 8.
`max-goto-duplication-insns'
The maximum number of instructions to duplicate to a block
that jumps to a computed goto. To avoid O(N^2) behavior in a
number of passes, GCC factors computed gotos early in the
compilation process, and unfactors them as late as possible.
Only computed jumps at the end of a basic blocks with no more
than max-goto-duplication-insns are unfactored. The default
value is 8.
`max-delay-slot-insn-search'
The maximum number of instructions to consider when looking
for an instruction to fill a delay slot. If more than this
arbitrary number of instructions is searched, the time
savings from filling the delay slot will be minimal so stop
searching. Increasing values mean more aggressive
optimization, making the compile time increase with probably
small improvement in executable run time.
`max-delay-slot-live-search'
When trying to fill delay slots, the maximum number of
instructions to consider when searching for a block with
valid live register information. Increasing this arbitrarily
chosen value means more aggressive optimization, increasing
the compile time. This parameter should be removed when the
delay slot code is rewritten to maintain the control-flow
graph.
`max-gcse-memory'
The approximate maximum amount of memory that will be
allocated in order to perform the global common subexpression
elimination optimization. If more memory than specified is
required, the optimization will not be done.
`max-gcse-passes'
The maximum number of passes of GCSE to run. The default is
1.
`max-pending-list-length'
The maximum number of pending dependencies scheduling will
allow before flushing the current state and starting over.
Large functions with few branches or calls can create
excessively large lists which needlessly consume memory and
resources.
`max-inline-insns-single'
Several parameters control the tree inliner used in gcc.
This number sets the maximum number of instructions (counted
in GCC's internal representation) in a single function that
the tree inliner will consider for inlining. This only
affects functions declared inline and methods implemented in
a class declaration (C++). The default value is 450.
`max-inline-insns-auto'
When you use `-finline-functions' (included in `-O3'), a lot
of functions that would otherwise not be considered for
inlining by the compiler will be investigated. To those
functions, a different (more restrictive) limit compared to
functions declared inline can be applied. The default value
is 90.
`large-function-insns'
The limit specifying really large functions. For functions
larger than this limit after inlining, inlining is
constrained by `--param large-function-growth'. This
parameter is useful primarily to avoid extreme compilation
time caused by non-linear algorithms used by the backend.
The default value is 2700.
`large-function-growth'
Specifies maximal growth of large function caused by inlining
in percents. The default value is 100 which limits large
function growth to 2.0 times the original size.
`large-unit-insns'
The limit specifying large translation unit. Growth caused
by inlining of units larger than this limit is limited by
`--param inline-unit-growth'. For small units this might be
too tight (consider unit consisting of function A that is
inline and B that just calls A three time. If B is small
relative to A, the growth of unit is 300\% and yet such
inlining is very sane. For very large units consisting of
small inlineable functions however the overall unit growth
limit is needed to avoid exponential explosion of code size.
Thus for smaller units, the size is increased to `--param
large-unit-insns' before applying `--param
inline-unit-growth'. The default is 10000
`inline-unit-growth'
Specifies maximal overall growth of the compilation unit
caused by inlining. The default value is 30 which limits
unit growth to 1.3 times the original size.
`ipcp-unit-growth'
Specifies maximal overall growth of the compilation unit
caused by interprocedural constant propagation. The default
value is 10 which limits unit growth to 1.1 times the
original size.
`large-stack-frame'
The limit specifying large stack frames. While inlining the
algorithm is trying to not grow past this limit too much.
Default value is 256 bytes.
`large-stack-frame-growth'
Specifies maximal growth of large stack frames caused by
inlining in percents. The default value is 1000 which limits
large stack frame growth to 11 times the original size.
`max-inline-insns-recursive'
`max-inline-insns-recursive-auto'
Specifies maximum number of instructions out-of-line copy of
self recursive inline function can grow into by performing
recursive inlining.
For functions declared inline `--param
max-inline-insns-recursive' is taken into account. For
function not declared inline, recursive inlining happens only
when `-finline-functions' (included in `-O3') is enabled and
`--param max-inline-insns-recursive-auto' is used. The
default value is 450.
`max-inline-recursive-depth'
`max-inline-recursive-depth-auto'
Specifies maximum recursion depth used by the recursive
inlining.
For functions declared inline `--param
max-inline-recursive-depth' is taken into account. For
function not declared inline, recursive inlining happens only
when `-finline-functions' (included in `-O3') is enabled and
`--param max-inline-recursive-depth-auto' is used. The
default value is 8.
`min-inline-recursive-probability'
Recursive inlining is profitable only for function having
deep recursion in average and can hurt for function having
little recursion depth by increasing the prologue size or
complexity of function body to other optimizers.
When profile feedback is available (see `-fprofile-generate')
the actual recursion depth can be guessed from probability
that function will recurse via given call expression. This
parameter limits inlining only to call expression whose
probability exceeds given threshold (in percents). The
default value is 10.
`inline-call-cost'
Specify cost of call instruction relative to simple
arithmetics operations (having cost of 1). Increasing this
cost disqualifies inlining of non-leaf functions and at the
same time increases size of leaf function that is believed to
reduce function size by being inlined. In effect it
increases amount of inlining for code having large
abstraction penalty (many functions that just pass the
arguments to other functions) and decrease inlining for code
with low abstraction penalty. The default value is 12.
`min-vect-loop-bound'
The minimum number of iterations under which a loop will not
get vectorized when `-ftree-vectorize' is used. The number
of iterations after vectorization needs to be greater than
the value specified by this option to allow vectorization.
The default value is 0.
`max-unrolled-insns'
The maximum number of instructions that a loop should have if
that loop is unrolled, and if the loop is unrolled, it
determines how many times the loop code is unrolled.
`max-average-unrolled-insns'
The maximum number of instructions biased by probabilities of
their execution that a loop should have if that loop is
unrolled, and if the loop is unrolled, it determines how many
times the loop code is unrolled.
`max-unroll-times'
The maximum number of unrollings of a single loop.
`max-peeled-insns'
The maximum number of instructions that a loop should have if
that loop is peeled, and if the loop is peeled, it determines
how many times the loop code is peeled.
`max-peel-times'
The maximum number of peelings of a single loop.
`max-completely-peeled-insns'
The maximum number of insns of a completely peeled loop.
`max-completely-peel-times'
The maximum number of iterations of a loop to be suitable for
complete peeling.
`max-unswitch-insns'
The maximum number of insns of an unswitched loop.
`max-unswitch-level'
The maximum number of branches unswitched in a single loop.
`lim-expensive'
The minimum cost of an expensive expression in the loop
invariant motion.
`iv-consider-all-candidates-bound'
Bound on number of candidates for induction variables below
that all candidates are considered for each use in induction
variable optimizations. Only the most relevant candidates
are considered if there are more candidates, to avoid
quadratic time complexity.
`iv-max-considered-uses'
The induction variable optimizations give up on loops that
contain more induction variable uses.
`iv-always-prune-cand-set-bound'
If number of candidates in the set is smaller than this value,
we always try to remove unnecessary ivs from the set during
its optimization when a new iv is added to the set.
`scev-max-expr-size'
Bound on size of expressions used in the scalar evolutions
analyzer. Large expressions slow the analyzer.
`omega-max-vars'
The maximum number of variables in an Omega constraint system.
The default value is 128.
`omega-max-geqs'
The maximum number of inequalities in an Omega constraint
system. The default value is 256.
`omega-max-eqs'
The maximum number of equalities in an Omega constraint
system. The default value is 128.
`omega-max-wild-cards'
The maximum number of wildcard variables that the Omega
solver will be able to insert. The default value is 18.
`omega-hash-table-size'
The size of the hash table in the Omega solver. The default
value is 550.
`omega-max-keys'
The maximal number of keys used by the Omega solver. The
default value is 500.
`omega-eliminate-redundant-constraints'
When set to 1, use expensive methods to eliminate all
redundant constraints. The default value is 0.
`vect-max-version-for-alignment-checks'
The maximum number of runtime checks that can be performed
when doing loop versioning for alignment in the vectorizer.
See option ftree-vect-loop-version for more information.
`vect-max-version-for-alias-checks'
The maximum number of runtime checks that can be performed
when doing loop versioning for alias in the vectorizer. See
option ftree-vect-loop-version for more information.
`max-iterations-to-track'
The maximum number of iterations of a loop the brute force
algorithm for analysis of # of iterations of the loop tries
to evaluate.
`hot-bb-count-fraction'
Select fraction of the maximal count of repetitions of basic
block in program given basic block needs to have to be
considered hot.
`hot-bb-frequency-fraction'
Select fraction of the maximal frequency of executions of
basic block in function given basic block needs to have to be
considered hot
`max-predicted-iterations'
The maximum number of loop iterations we predict statically.
This is useful in cases where function contain single loop
with known bound and other loop with unknown. We predict the
known number of iterations correctly, while the unknown
number of iterations average to roughly 10. This means that
the loop without bounds would appear artificially cold
relative to the other one.
`align-threshold'
Select fraction of the maximal frequency of executions of
basic block in function given basic block will get aligned.
`align-loop-iterations'
A loop expected to iterate at lest the selected number of
iterations will get aligned.
`tracer-dynamic-coverage'
`tracer-dynamic-coverage-feedback'
This value is used to limit superblock formation once the
given percentage of executed instructions is covered. This
limits unnecessary code size expansion.
The `tracer-dynamic-coverage-feedback' is used only when
profile feedback is available. The real profiles (as opposed
to statically estimated ones) are much less balanced allowing
the threshold to be larger value.
`tracer-max-code-growth'
Stop tail duplication once code growth has reached given
percentage. This is rather hokey argument, as most of the
duplicates will be eliminated later in cross jumping, so it
may be set to much higher values than is the desired code
growth.
`tracer-min-branch-ratio'
Stop reverse growth when the reverse probability of best edge
is less than this threshold (in percent).
`tracer-min-branch-ratio'
`tracer-min-branch-ratio-feedback'
Stop forward growth if the best edge do have probability
lower than this threshold.
Similarly to `tracer-dynamic-coverage' two values are
present, one for compilation for profile feedback and one for
compilation without. The value for compilation with profile
feedback needs to be more conservative (higher) in order to
make tracer effective.
`max-cse-path-length'
Maximum number of basic blocks on path that cse considers.
The default is 10.
`max-cse-insns'
The maximum instructions CSE process before flushing. The
default is 1000.
`max-aliased-vops'
Maximum number of virtual operands per function allowed to
represent aliases before triggering the alias partitioning
heuristic. Alias partitioning reduces compile times and
memory consumption needed for aliasing at the expense of
precision loss in alias information. The default value for
this parameter is 100 for -O1, 500 for -O2 and 1000 for -O3.
Notice that if a function contains more memory statements
than the value of this parameter, it is not really possible
to achieve this reduction. In this case, the compiler will
use the number of memory statements as the value for
`max-aliased-vops'.
`avg-aliased-vops'
Average number of virtual operands per statement allowed to
represent aliases before triggering the alias partitioning
heuristic. This works in conjunction with
`max-aliased-vops'. If a function contains more than
`max-aliased-vops' virtual operators, then memory symbols
will be grouped into memory partitions until either the total
number of virtual operators is below `max-aliased-vops' or
the average number of virtual operators per memory statement
is below `avg-aliased-vops'. The default value for this
parameter is 1 for -O1 and -O2, and 3 for -O3.
`ggc-min-expand'
GCC uses a garbage collector to manage its own memory
allocation. This parameter specifies the minimum percentage
by which the garbage collector's heap should be allowed to
expand between collections. Tuning this may improve
compilation speed; it has no effect on code generation.
The default is 30% + 70% * (RAM/1GB) with an upper bound of
100% when RAM >= 1GB. If `getrlimit' is available, the
notion of "RAM" is the smallest of actual RAM and
`RLIMIT_DATA' or `RLIMIT_AS'. If GCC is not able to
calculate RAM on a particular platform, the lower bound of
30% is used. Setting this parameter and `ggc-min-heapsize'
to zero causes a full collection to occur at every
opportunity. This is extremely slow, but can be useful for
debugging.
`ggc-min-heapsize'
Minimum size of the garbage collector's heap before it begins
bothering to collect garbage. The first collection occurs
after the heap expands by `ggc-min-expand'% beyond
`ggc-min-heapsize'. Again, tuning this may improve
compilation speed, and has no effect on code generation.
The default is the smaller of RAM/8, RLIMIT_RSS, or a limit
which tries to ensure that RLIMIT_DATA or RLIMIT_AS are not
exceeded, but with a lower bound of 4096 (four megabytes) and
an upper bound of 131072 (128 megabytes). If GCC is not able
to calculate RAM on a particular platform, the lower bound is
used. Setting this parameter 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-locations'
The maximum number of memory locations cselib should take
into account. Increasing values mean more aggressive
optimization, making the compile time increase with probably
slightly better performance. The default value is 500.
`reorder-blocks-duplicate'
`reorder-blocks-duplicate-feedback'
Used by basic block reordering pass to decide whether to use
unconditional branch or duplicate the code on its
destination. Code is duplicated when its estimated size is
smaller than this value multiplied by the estimated size of
unconditional jump in the hot spots of the program.
The `reorder-block-duplicate-feedback' is used only when
profile feedback is available and may be set to higher values
than `reorder-block-duplicate' since information about the
hot spots is more accurate.
`max-sched-ready-insns'
The maximum number of instructions ready to be issued the
scheduler should consider at any given time during the first
scheduling pass. Increasing values mean more thorough
searches, making the compilation time increase with probably
little benefit. The default value is 100.
`max-sched-region-blocks'
The maximum number of blocks in a region to be considered for
interblock scheduling. The default value is 10.
`max-pipeline-region-blocks'
The maximum number of blocks in a region to be considered for
pipelining in the selective scheduler. The default value is
15.
`max-sched-region-insns'
The maximum number of insns in a region to be considered for
interblock scheduling. The default value is 100.
`max-pipeline-region-insns'
The maximum number of insns in a region to be considered for
pipelining in the selective scheduler. The default value is
200.
`min-spec-prob'
The minimum probability (in percents) of reaching a source
block for interblock speculative scheduling. The default
value is 40.
`max-sched-extend-regions-iters'
The maximum number of iterations through CFG to extend
regions. 0 - disable region extension, N - do at most N
iterations. The default value is 0.
`max-sched-insn-conflict-delay'
The maximum conflict delay for an insn to be considered for
speculative motion. The default value is 3.
`sched-spec-prob-cutoff'
The minimal probability of speculation success (in percents),
so that speculative insn will be scheduled. The default
value is 40.
`sched-mem-true-dep-cost'
Minimal distance (in CPU cycles) between store and load
targeting same memory locations. The default value is 1.
`selsched-max-lookahead'
The maximum size of the lookahead window of selective
scheduling. It is a depth of search for available
instructions. The default value is 50.
`selsched-max-sched-times'
The maximum number of times that an instruction will be
scheduled during selective scheduling. This is the limit on
the number of iterations through which the instruction may be
pipelined. The default value is 2.
`selsched-max-insns-to-rename'
The maximum number of best instructions in the ready list
that are considered for renaming in the selective scheduler.
The default value is 2.
`max-last-value-rtl'
The maximum size measured as number of RTLs that can be
recorded in an expression in combiner for a pseudo register
as last known value of that register. The default is 10000.
`integer-share-limit'
Small integer constants can use a shared data structure,
reducing the compiler's memory usage and increasing its
speed. This sets the maximum value of a shared integer
constant. The default value is 256.
`min-virtual-mappings'
Specifies the minimum number of virtual mappings in the
incremental SSA updater that should be registered to trigger
the virtual mappings heuristic defined by
virtual-mappings-ratio. The default value is 100.
`virtual-mappings-ratio'
If the number of virtual mappings is virtual-mappings-ratio
bigger than the number of virtual symbols to be updated, then
the incremental SSA updater switches to a full update for
those symbols. The default ratio is 3.
`ssp-buffer-size'
The minimum size of buffers (i.e. arrays) that will receive
stack smashing protection when `-fstack-protection' is used.
`max-jump-thread-duplication-stmts'
Maximum number of statements allowed in a block that needs to
be duplicated when threading jumps.
`max-fields-for-field-sensitive'
Maximum number of fields in a structure we will treat in a
field sensitive manner during pointer analysis. The default
is zero for -O0, and -O1 and 100 for -Os, -O2, and -O3.
`prefetch-latency'
Estimate on average number of instructions that are executed
before prefetch finishes. The distance we prefetch ahead is
proportional to this constant. Increasing this number may
also lead to less streams being prefetched (see
`simultaneous-prefetches').
`simultaneous-prefetches'
Maximum number of prefetches that can run at the same time.
`l1-cache-line-size'
The size of cache line in L1 cache, in bytes.
`l1-cache-size'
The size of L1 cache, in kilobytes.
`l2-cache-size'
The size of L2 cache, in kilobytes.
`use-canonical-types'
Whether the compiler should use the "canonical" type system.
By default, this should always be 1, which uses a more
efficient internal mechanism for comparing types in C++ and
Objective-C++. However, if bugs in the canonical type system
are causing compilation failures, set this value to 0 to
disable canonical types.
`switch-conversion-max-branch-ratio'
Switch initialization conversion will refuse to create arrays
that are bigger than `switch-conversion-max-branch-ratio'
times the number of branches in the switch.
`max-partial-antic-length'
Maximum length of the partial antic set computed during the
tree partial redundancy elimination optimization
(`-ftree-pre') when optimizing at `-O3' and above. For some
sorts of source code the enhanced partial redundancy
elimination optimization can run away, consuming all of the
memory available on the host machine. This parameter sets a
limit on the length of the sets that are computed, which
prevents the runaway behavior. Setting a value of 0 for this
parameter will allow an unlimited set length.
`sccvn-max-scc-size'
Maximum size of a strongly connected component (SCC) during
SCCVN processing. If this limit is hit, SCCVN processing for
the whole function will not be done and optimizations
depending on it will be disabled. The default maximum SCC
size is 10000.
`ira-max-loops-num'
IRA uses a regional register allocation by default. If a
function contains loops more than number given by the
parameter, only at most given number of the most frequently
executed loops will form regions for the regional register
allocation. The default value of the parameter is 100.
`ira-max-conflict-table-size'
Although IRA uses a sophisticated algorithm of compression
conflict table, the table can be still big for huge
functions. If the conflict table for a function could be
more than size in MB given by the parameter, the conflict
table is not built and faster, simpler, and lower quality
register allocation algorithm will be used. The algorithm do
not use pseudo-register conflicts. The default value of the
parameter is 2000.
`loop-invariant-max-bbs-in-loop'
Loop invariant motion can be very expensive, both in compile
time and in amount of needed compile time memory, with very
large loops. Loops with more basic blocks than this
parameter won't have loop invariant motion optimization
performed on them. The default value of the parameter is
1000 for -O1 and 10000 for -O2 and above.
Regards,
Ulrich
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