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Yes, I would like to subscribe to stay connected to the latest Intel technologies and industry trends by email and telephone. 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.
Assume that the current compilation unit represents the whole program being compiled. This option should not be used in combination with -flto. Instead relying on a linker plugin should provide safer and more precise information. This option runs the standard link-time optimizer. When the object files are linked together, all the function bodies are read from these ELF sections and instantiated as if they had been part of the same translation unit.
To use the link-time optimizer, -flto and optimization options should be specified at compile time and during the final link. It is recommended that you compile all the files participating in the same link with the same options and also specify those options at link time. For example:.
The first two invocations to GCC save a bytecode representation of GIMPLE into special ELF sections inside foo. o and bar. The final invocation reads the GIMPLE bytecode from foo. o , merges the two files into a single internal image, and compiles the result as usual. Since both foo. o are merged into a single image, this causes all the interprocedural analyses and optimizations in GCC to work across the two files as if they were a single one.
This means, for example, that the inliner is able to inline functions in bar. o into functions in foo. o and vice-versa. The above generates bytecode for foo. c and bar. c , merges them together into a single GIMPLE representation and optimizes them as usual to produce myprog. The important thing to keep in mind is that to enable link-time optimizations you need to use the GCC driver to perform the link step.
GCC automatically performs link-time optimization if any of the objects involved were compiled with the -flto command-line option. You can always override the automatic decision to do link-time optimization by passing -fno-lto to the link command. To make whole program optimization effective, it is necessary to make certain whole program assumptions. The compiler needs to know what functions and variables can be accessed by libraries and runtime outside of the link-time optimized unit.
When supported by the linker, the linker plugin see -fuse-linker-plugin passes information to the compiler about used and externally visible symbols. When the linker plugin is not available, -fwhole-program should be used to allow the compiler to make these assumptions, which leads to more aggressive optimization decisions.
When a file is compiled with -flto without -fuse-linker-plugin , the generated object file is larger than a regular object file because it contains GIMPLE bytecodes and the usual final code see -ffat-lto-objects.
This means that object files with LTO information can be linked as normal object files; if -fno-lto is passed to the linker, no interprocedural optimizations are applied.
Note that when -fno-fat-lto-objects is enabled the compile stage is faster but you cannot perform a regular, non-LTO link on them. When producing the final binary, GCC only applies link-time optimizations to those files that contain bytecode. Therefore, you can mix and match object files and libraries with GIMPLE bytecodes and final object code.
GCC automatically selects which files to optimize in LTO mode and which files to link without further processing. Generally, options specified at link time override those specified at compile time, although in some cases GCC attempts to infer link-time options from the settings used to compile the input files. If you do not specify an optimization level option -O at link time, then GCC uses the highest optimization level used when compiling the object files.
Note that it is generally ineffective to specify an optimization level option only at link time and not at compile time, for two reasons.
First, compiling without optimization suppresses compiler passes that gather information needed for effective optimization at link time. Second, some early optimization passes can be performed only at compile time and not at link time. There are some code generation flags preserved by GCC when generating bytecodes, as they need to be used during the final link.
Currently, the following options and their settings are taken from the first object file that explicitly specifies them: -fcommon , -fexceptions , -fnon-call-exceptions , -fgnu-tm and all the -m target flags. The following options -fPIC , -fpic , -fpie and -fPIE are combined based on the following scheme:. Certain ABI-changing flags are required to match in all compilation units, and trying to override this at link time with a conflicting value is ignored.
This includes options such as -freg-struct-return and -fpcc-struct-return. Other options such as -ffp-contract , -fno-strict-overflow , -fwrapv , -fno-trapv or -fno-strict-aliasing are passed through to the link stage and merged conservatively for conflicting translation units. You can override them at link time. Diagnostic options such as -Wstringop-overflow are passed through to the link stage and their setting matches that of the compile-step at function granularity.
Note that this matters only for diagnostics emitted during optimization. Note that code transforms such as inlining can lead to warnings being enabled or disabled for regions if code not consistent with the setting at compile time.
When you need to pass options to the assembler via -Wa or -Xassembler make sure to either compile such translation units with -fno-lto or consistently use the same assembler options on all translation units. You can alternatively also specify assembler options at LTO link time. To enable debug info generation you need to supply -g at compile time.
If any of the input files at link time were built with debug info generation enabled the link will enable debug info generation as well. Any elaborate debug info settings like the dwarf level -gdwarf-5 need to be explicitly repeated at the linker command line and mixing different settings in different translation units is discouraged.
If LTO encounters objects with C linkage declared with incompatible types in separate translation units to be linked together undefined behavior according to ISO C99 6. The behavior is still undefined at run time. Similar diagnostics may be raised for other languages.
Another feature of LTO is that it is possible to apply interprocedural optimizations on files written in different languages:. In general, when mixing languages in LTO mode, you should use the same link command options as when mixing languages in a regular non-LTO compilation.
If object files containing GIMPLE bytecode are stored in a library archive, say libfoo. a , it is possible to extract and use them in an LTO link if you are using a linker with plugin support. To create static libraries suitable for LTO, use gcc-ar and gcc-ranlib instead of ar and ranlib ; to show the symbols of object files with GIMPLE bytecode, use gcc-nm.
Those commands require that ar , ranlib and nm have been compiled with plugin support. At link time, use the flag -fuse-linker-plugin to ensure that the library participates in the LTO optimization process:. With the linker plugin enabled, the linker extracts the needed GIMPLE files from libfoo. a and passes them on to the running GCC to make them part of the aggregated GIMPLE image to be optimized. a are extracted and linked as usual, but they do not participate in the LTO optimization process.
In order to make a static library suitable for both LTO optimization and usual linkage, compile its object files with -flto -ffat-lto-objects. Link-time optimizations do not require the presence of the whole program to operate. If the program does not require any symbols to be exported, it is possible to combine -flto and -fwhole-program to allow the interprocedural optimizers to use more aggressive assumptions which may lead to improved optimization opportunities.
Use of -fwhole-program is not needed when linker plugin is active see -fuse-linker-plugin. The current implementation of LTO makes no attempt to generate bytecode that is portable between different types of hosts. The bytecode files are versioned and there is a strict version check, so bytecode files generated in one version of GCC do not work with an older or newer version of GCC.
Link-time optimization does not work well with generation of debugging information on systems other than those using a combination of ELF and DWARF. If you specify the optional n , the optimization and code generation done at link time is executed in parallel using n parallel jobs by utilizing an installed make program.
The environment variable MAKE may be used to override the program used. This is useful when the Makefile calling GCC is already executing in parallel. This option likely only works if MAKE is GNU make. Specify the partitioning algorithm used by the link-time optimizer. This option specifies the level of compression used for intermediate language written to LTO object files, and is only meaningful in conjunction with LTO mode -flto. GCC currently supports two LTO compression algorithms. For zstd, valid values are 0 no compression to 19 maximum compression , while zlib supports values from 0 to 9.
Values outside this range are clamped to either minimum or maximum of the supported values. If the option is not given, a default balanced compression setting is used. Enables the use of a linker plugin during link-time optimization. This option relies on plugin support in the linker, which is available in gold or in GNU ld 2. This option enables the extraction of object files with GIMPLE bytecode out of library archives.
This improves the quality of optimization by exposing more code to the link-time optimizer. This information specifies what symbols can be accessed externally by non-LTO object or during dynamic linking. Resulting code quality improvements on binaries and shared libraries that use hidden visibility are similar to -fwhole-program. See -flto for a description of the effect of this flag and how to use it. This option is enabled by default when LTO support in GCC is enabled and GCC was configured for use with a linker supporting plugins GNU ld 2.
Fat LTO objects are object files that contain both the intermediate language and the object code. This makes them usable for both LTO linking and normal linking. This option is effective only when compiling with -flto and is ignored at link time.
It requires a linker with linker plugin support for basic functionality. Additionally, nm , ar and ranlib need to support linker plugins to allow a full-featured build environment capable of building static libraries etc. GCC provides the gcc-ar , gcc-nm , gcc-ranlib wrappers to pass the right options to these tools.
With non fat LTO makefiles need to be modified to use them. Note that modern binutils provide plugin auto-load mechanism.
After register allocation and post-register allocation instruction splitting, identify arithmetic instructions that compute processor flags similar to a comparison operation based on that arithmetic. If possible, eliminate the explicit comparison operation. This pass only applies to certain targets that cannot explicitly represent the comparison operation before register allocation is complete. After register allocation and post-register allocation instruction splitting, perform a copy-propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy.
Profiles collected using an instrumented binary for multi-threaded programs may be inconsistent due to missed counter updates. When this option is specified, GCC uses heuristics to correct or smooth out such inconsistencies. By default, GCC emits an error message when an inconsistent profile is detected.
With -fprofile-use all portions of programs not executed during train run are optimized agressively for size rather than speed. In some cases it is not practical to train all possible hot paths in the program. For example, program may contain functions specific for a given hardware and trianing may not cover all hardware configurations program is run on. With -fprofile-partial-training profile feedback will be ignored for all functions not executed during the train run leading them to be optimized as if they were compiled without profile feedback.
This leads to better performance when train run is not representative but also leads to significantly bigger code. Enable profile feedback-directed optimizations, and the following optimizations, many of which are generally profitable only with profile feedback available:. Before you can use this option, you must first generate profiling information.
See Instrumentation Options , for information about the -fprofile-generate option. By default, GCC emits an error message if the feedback profiles do not match the source code. Note this may result in poorly optimized code. Additionally, by default, GCC also emits a warning message if the feedback profiles do not exist see -Wmissing-profile. If path is specified, GCC looks at the path to find the profile feedback data files.
See -fprofile-dir. Enable sampling-based feedback-directed optimizations, and the following optimizations, many of which are generally profitable only with profile feedback available:. path is the name of a file containing AutoFDO profile information. If omitted, it defaults to fbdata. afdo in the current directory. You must also supply the unstripped binary for your program to this tool. The following options control compiler behavior regarding floating-point arithmetic.
These options trade off between speed and correctness. All must be specifically enabled. 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 where the floating registers of the 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. This option allows further control over excess precision on machines where floating-point operations occur in a format with more precision or range than the IEEE standard and interchange floating-point types.
It may, however, yield faster code for programs that do not require the guarantees of these specifications. Do not set errno after calling math functions that are executed with a single instruction, e. A program that relies on IEEE exceptions for math error handling may want to use this flag for speed while maintaining IEEE arithmetic compatibility.
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. 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.
Enables -fno-signed-zeros , -fno-trapping-math , -fassociative-math and -freciprocal-math. Allow re-association of operands in series of floating-point operations. 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.
For Fortran the option is automatically enabled when both -fno-signed-zeros and -fno-trapping-math are in effect. Allow the reciprocal of a value to be used instead of dividing by the value if this enables optimizations. Note that this loses precision and increases the number of flops operating on the value. Allow optimizations for floating-point arithmetic that ignore the signedness of zero. 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. 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. This option is experimental and does not currently guarantee to disable all GCC optimizations that are affected by rounding mode. 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 is experimental and does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior. The default is -ffp-int-builtin-inexact , allowing the exception to be raised, unless C2X or a later C standard is selected.
This option does nothing unless -ftrapping-math is in effect. Treat floating-point constants as single precision instead of implicitly converting them to double-precision constants. When enabled, this option states that a range reduction step is not needed when performing complex division. The default is -fno-cx-limited-range , but is enabled by -ffast-math. Nevertheless, the option applies to all languages.
Complex multiplication and division follow 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. After running a program compiled with -fprofile-arcs see Instrumentation 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 a 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.
See details about the file naming in -fprofile-arcs. These can be used to improve optimization. Currently, they are only used in one place: in reorg. 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 for usage in optimizations.
Enabled by -fprofile-generate , -fprofile-use , and -fauto-profile. Function reordering based on profile instrumentation collects first time of execution of a function and orders these functions in ascending order. If combined with -fprofile-arcs , this option instructs the compiler to add 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 operations using the knowledge about the value of the denominator.
Attempt to avoid false dependencies in scheduled code by making use of registers left over after register allocation. This optimization most benefits processors with lots of registers. Performs a target dependent pass over the instruction stream to schedule instructions of same type together because target machine can execute them more efficiently if they are adjacent to each other in the instruction flow. Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function allowing other optimizations to do a better job.
Unroll loops whose number of iterations can be determined at compile time or upon entry to the loop. It also turns on complete loop peeling i. complete removal of loops with a small constant number of iterations.
This option makes code larger, and may or may not make it run faster. Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This usually makes programs run more slowly.
Peels loops for which there is enough information that they do not roll much from profile feedback or static analysis. complete removal of loops with small constant number of iterations.
Enables the loop invariant motion pass in the RTL loop optimizer. Enabled at level -O1 and higher, except for -Og. Enables the loop store motion pass in the GIMPLE loop optimizer. This moves invariant stores to after the end of the loop in exchange for carrying the stored value in a register across the iteration. Note for this option to have an effect -ftree-loop-im has to be enabled as well.
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. If a loop iterates over an array with a variable stride, create another version of the loop that assumes the stride is always one.
This is particularly useful for assumed-shape arrays in Fortran where for example it allows better vectorization assuming contiguous accesses. Place each function or data item into its own section in the output file if the target supports arbitrary sections. Use these options on systems where the linker can perform optimizations to improve locality of reference in the instruction space. Most systems using the ELF object format have linkers with such optimizations. On AIX, the linker rearranges sections CSECTs based on the call graph.
The performance impact varies. Together with a linker garbage collection linker --gc-sections option these options may lead to smaller statically-linked executables after stripping. Only use these options when there are significant benefits from doing so. When you specify these options, the assembler and linker create larger object and executable files and are also slower.
These options affect code generation. They prevent optimizations by the compiler and assembler using relative locations inside a translation unit since the locations are unknown until link time. An example of such an optimization is relaxing calls to short call instructions. This transformation can help to reduce the number of GOT entries and GOT accesses on some targets. usually calculates the addresses of all three variables, but if you compile it with -fsection-anchors , it accesses the variables from a common anchor point instead.
Zero call-used registers at function return to increase program security by either mitigating Return-Oriented Programming ROP attacks or preventing information leakage through registers. In some places, GCC uses various constants to control the amount of optimization that is done.
For example, GCC does not inline functions that contain more than 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 following choices of name are recognized for all targets:. When branch is predicted to be taken with probability lower than this threshold in percent , then it is considered well predictable. RTL if-conversion tries to remove conditional branches around a block and replace them with conditionally executed instructions.
This parameter gives the maximum number of instructions in a block which should be considered for if-conversion. The compiler will also use other heuristics to decide whether if-conversion is likely to be profitable. RTL if-conversion will try to remove conditional branches around a block and replace them with conditionally executed instructions. These parameters give the maximum permissible cost for the sequence that would be generated by if-conversion depending on whether the branch is statically determined to be predictable or not.
The maximum number of incoming edges to consider for cross-jumping. Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in executable size. The minimum number of instructions that must be matched at the end of two blocks before cross-jumping is performed on them.
This value is ignored in the case where all instructions in the block being cross-jumped from are matched. The maximum code size expansion factor when copying basic blocks instead of jumping. The expansion is relative to a jump instruction. The maximum number of instructions to duplicate to a block that jumps to a computed goto. Only computed jumps at the end of a basic blocks with no more than max-goto-duplication-insns are unfactored. 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 are searched, the time savings from filling the delay slot are minimal, so stop searching. Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in execution time. 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 compilation time. This parameter should be removed when the delay slot code is rewritten to maintain the control-flow graph. The approximate maximum amount of memory in kB that can be allocated in order to perform the global common subexpression elimination optimization. If more memory than specified is required, the optimization is not done.
If the ratio of expression insertions to deletions is larger than this value for any expression, then RTL PRE inserts or removes the expression and thus leaves partially redundant computations in the instruction stream.
The maximum number of pending dependencies scheduling allows 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. The maximum number of backtrack attempts the scheduler should make when modulo scheduling a loop. Larger values can exponentially increase compilation time. Maximal loop depth of a call considered by inline heuristics that tries to inline all functions called once.
Several parameters control the tree inliner used in GCC. When you use -finline-functions included in -O3 , a lot of functions that would otherwise not be considered for inlining by the compiler are investigated. To those functions, a different more restrictive limit compared to functions declared inline can be applied --param max-inline-insns-auto.
This is bound applied to calls which are considered relevant with -finline-small-functions. This is bound applied to calls which are optimized for size. Small growth may be desirable to anticipate optimization oppurtunities exposed by inlining. Number of instructions accounted by inliner for function overhead such as function prologue and epilogue.
Extra time accounted by inliner for function overhead such as time needed to execute function prologue and epilogue. The scale in percents applied to inline-insns-single , inline-insns-single-O2 , inline-insns-auto when inline heuristics hints that inlining is very profitable will enable later optimizations.
Same as --param uninlined-function-insns and --param uninlined-function-time but applied to function thunks. 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 back end. Specifies maximal growth of large function caused by inlining in percents.
For example, parameter value limits large function growth to 2. 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. For example, consider a unit consisting of function A that is inline and B that just calls A three times.
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. Specifies maximal overall growth of the compilation unit caused by inlining. For example, parameter value 20 limits unit growth to 1.
Cold functions either marked cold via an attribute or by profile feedback are not accounted into the unit size. Specifies maximal overall growth of the compilation unit caused by interprocedural constant propagation. For example, parameter value 10 limits unit growth to 1. The limit specifying large stack frames. While inlining the algorithm is trying to not grow past this limit too much. Specifies maximal growth of large stack frames caused by inlining in percents.
For example, parameter value limits large stack frame growth to 11 times the original size. Specifies the maximum number of instructions an out-of-line copy of a self-recursive inline function can grow into by performing recursive inlining. For functions not declared inline, recursive inlining happens only when -finline-functions included in -O3 is enabled; --param max-inline-insns-recursive-auto applies instead.
For functions not declared inline, recursive inlining happens only when -finline-functions included in -O3 is enabled; --param max-inline-recursive-depth-auto applies instead. 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 the probability that function recurses via a given call expression. This parameter limits inlining only to call expressions whose probability exceeds the given threshold in percents. Specify growth that the early inliner can make. In effect it increases the amount of inlining for code having a large abstraction penalty.
Limit of iterations of the early inliner. This basically bounds the number of nested indirect calls the early inliner can resolve. Deeper chains are still handled by late inlining. This parameter ought to be bigger than --param modref-max-bases and --param modref-max-refs.
Specifies the maximum depth of DFS walk used by modref escape analysis. Setting to 0 disables the analysis completely. A parameter to control whether to use function internal id in profile database lookup. If the value is 0, the compiler uses an id that is based on function assembler name and filename, which makes old profile data more tolerant to source changes such as function reordering etc. The minimum number of iterations under which loops are not 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. Scaling factor in calculation of maximum distance an expression can be moved by GCSE optimizations. This is currently supported only in the code hoisting pass. The bigger the ratio, the more aggressive code hoisting is with simple expressions, i.
Specifying 0 disables hoisting of simple expressions. Cost, roughly measured as the cost of a single typical machine instruction, at which GCSE optimizations do not constrain the distance an expression can travel. The lesser the cost, the more aggressive code hoisting is. Specifying 0 allows all expressions to travel unrestricted distances. The depth of search in the dominator tree for expressions to hoist. This is used to avoid quadratic behavior in hoisting algorithm.
The value of 0 does not limit on the search, but may slow down compilation of huge functions. The maximum amount of similar bbs to compare a bb with. This is used to avoid quadratic behavior in tree tail merging. The maximum amount of iterations of the pass over the function. This is used to limit compilation time in tree tail merging.
The maximum number of store chains to track at the same time in the attempt to merge them into wider stores in the store merging pass.
The maximum number of stores to track at the same time in the attemt to to merge them into wider stores in the store merging pass. The maximum number of instructions that a loop may have to be unrolled. If a loop is unrolled, this parameter also determines how many times the loop code is unrolled. The maximum number of instructions biased by probabilities of their execution that a loop may have to be unrolled.
The maximum number of instructions that a loop may have to be peeled. If a loop is peeled, this parameter also determines how many times the loop code is peeled.
When FDO profile information is available, min-loop-cond-split-prob specifies minimum threshold for probability of semi-invariant condition statement to trigger loop split. Bound on number of candidates for induction variables, below which all candidates are considered for each use in induction variable optimizations. If there are more candidates than this, only the most relevant ones are considered to avoid quadratic time complexity.
If the number of candidates in the set is smaller than this value, always try to remove unnecessary ivs from the set when adding a new one. Maximum size in bytes of objects tracked bytewise by dead store elimination. Larger values may result in larger compilation times. Maximum number of queries into the alias oracle per store. Larger values result in larger compilation times and may result in more removed dead stores. Bound on size of expressions used in the scalar evolutions analyzer.
Large expressions slow the analyzer. Bound on the complexity of the expressions in the scalar evolutions analyzer. Complex expressions slow the analyzer. Maximum number of arguments in a PHI supported by TREE if conversion unless the loop is marked with simd pragma. The maximum number of possible vector layouts such as permutations to consider when optimizing to-be-vectorized code.
The maximum number of run-time checks that can be performed when doing loop versioning for alignment in the vectorizer. The maximum number of run-time checks that can be performed when doing loop versioning for alias in the vectorizer. The maximum number of loop peels to enhance access alignment for vectorizer. Value -1 means no limit. The maximum number of iterations of a loop the brute-force algorithm for analysis of the number of iterations of the loop tries to evaluate.
Used in non-LTO mode. The number of most executed permilles, ranging from 0 to , of the profiled execution of the entire program to which the execution count of a basic block must be part of in order to be considered hot. The default is , which means that a basic block is considered hot if its execution count contributes to the upper permilles, or Used in LTO mode.
The maximum number of loop iterations we predict statically. This is useful in cases where a function contains a single loop with known bound and another loop with unknown bound. The known number of iterations is predicted correctly, while the unknown number of iterations average to roughly This means that the loop without bounds appears artificially cold relative to the other one.
Control the probability of the expression having the specified value. This parameter takes a percentage i. Select fraction of the maximal frequency of executions of a basic block in a function to align the basic block.
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 parameter is used only when profile feedback is available.
The real profiles as opposed to statically estimated ones are much less balanced allowing the threshold to be larger value. Stop tail duplication once code growth has reached given percentage. This is a rather artificial limit, as most of the duplicates are eliminated later in cross jumping, so it may be set to much higher values than is the desired code growth.
Stop reverse growth when the reverse probability of best edge is less than this threshold in percent. Similarly to tracer-dynamic-coverage two parameters are provided. tracer-min-branch-probability-feedback is used for compilation with profile feedback and tracer-min-branch-probability compilation without.
The value for compilation with profile feedback needs to be more conservative higher in order to make tracer effective. Specify the size of the operating system provided stack guard as 2 raised to num bytes. Higher values may reduce the number of explicit probes, but a value larger than the operating system provided guard will leave code vulnerable to stack clash style attacks.
Stack clash protection involves probing stack space as it is allocated. This param controls the maximum distance between probes into the stack as 2 raised to num bytes. GCC uses a garbage collector to manage its own memory allocation. Tuning this may improve compilation speed; it has no effect on code generation. 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.
Again, tuning this may improve compilation speed, and has no effect on code generation. 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. The maximum number of instruction reload should look backward for equivalent register. Increasing values mean more aggressive optimization, making the compilation time increase with probably slightly better performance.
The maximum number of memory locations cselib should take into account. 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 maximum number of blocks in a region to be considered for pipelining in the selective scheduler. The maximum number of insns in a region to be considered for pipelining in the selective scheduler. The minimum probability in percents of reaching a source block for interblock speculative scheduling.
The maximum number of iterations through CFG to extend regions. A value of 0 disables region extensions. The minimal probability of speculation success in percents , so that speculative insns are scheduled. The maximum size of the lookahead window of selective scheduling.
It is a depth of search for available instructions. The maximum number of times that an instruction is scheduled during selective scheduling.
This is the limit on the number of iterations through which the instruction may be pipelined. The maximum number of best instructions in the ready list that are considered for renaming in the selective scheduler. 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.
This sets the maximum value of a shared integer constant. The minimum size of buffers i. arrays that receive stack smashing protection when -fstack-protector is used. Maximum number of statements allowed in a block that needs to be duplicated when threading jumps. The maximum number of paths to consider when searching for jump threading opportunities. When arriving at a block, incoming edges are only considered if the number of paths to be searched so far multiplied by the number of incoming edges does not exhaust the specified maximum number of paths to consider.
Maximum number of fields in a structure treated in a field sensitive manner during pointer analysis. Estimate on average number of instructions that are executed before prefetch finishes. The distance prefetched ahead is proportional to this constant.
Increasing this number may also lead to less streams being prefetched see simultaneous-prefetches. Whether the loop array prefetch pass should issue software prefetch hints for strides that are non-constant. In some cases this may be beneficial, though the fact the stride is non-constant may make it hard to predict when there is clear benefit to issuing these hints.
Set to 1 if the prefetch hints should be issued for non-constant strides. Set to 0 if prefetch hints should be issued only for strides that are known to be constant and below prefetch-minimum-stride. Minimum constant stride, in bytes, to start using prefetch hints for.
If the stride is less than this threshold, prefetch hints will not be issued. This setting is useful for processors that have hardware prefetchers, in which case there may be conflicts between the hardware prefetchers and the software prefetchers. If the hardware prefetchers have a maximum stride they can handle, it should be used here to improve the use of software prefetchers. The destructive interference size is the minimum recommended offset between two independent concurrently-accessed objects; the constructive interference size is the maximum recommended size of contiguous memory accessed together.
Typically both will be the size of an L1 cache line for the target, in bytes. For a generic target covering a range of L1 cache line sizes, typically the constructive interference size will be the small end of the range and the destructive size will be the large end. The destructive interference size is intended to be used for layout, and thus has ABI impact. The default value is not expected to be stable, and on some targets varies with -mtune , so use of this variable in a context where ABI stability is important, such as the public interface of a library, is strongly discouraged; if it is used in that context, users can stabilize the value using this option.
The minimum ratio between the number of instructions and the number of prefetches to enable prefetching in a loop. The minimum ratio between the number of instructions and the number of memory references to enable prefetching in a loop.
However, if bugs in the canonical type system are causing compilation failures, set this value to 0 to disable canonical types. Switch initialization conversion refuses to create arrays that are bigger than switch-conversion-max-branch-ratio times the number of branches in the switch. 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 allows an unlimited set length.
Maximum loop depth that is value-numbered optimistically. When the limit hits the innermost rpo-vn-max-loop-depth loops and the outermost loop in the loop nest are value-numbered optimistically and the remaining ones not. Maximum number of alias-oracle queries we perform when looking for redundancies for loads and stores.
If this limit is hit the search is aborted and the load or store is not considered redundant. The number of queries is algorithmically limited to the number of stores on all paths from the load to the function entry. IRA uses regional register allocation by default. If a function contains more loops than the number given by this parameter, only at most the given number of the most frequently-executed loops form regions for regional register allocation.
Although IRA uses a sophisticated algorithm to compress the conflict table, the table can still require excessive amounts of memory for huge functions. If the conflict table for a function could be more than the size in MB given by this parameter, the register allocator instead uses a faster, simpler, and lower-quality algorithm that does not require building a pseudo-register conflict table.
IRA can be used to evaluate more accurate register pressure in loops for decisions to move loop invariants see -O3. The number of available registers reserved for some other purposes is given by this parameter.
Default of the parameter is the best found from numerous experiments. Make IRA to consider matching constraint duplicated operand number heavily in all available alternatives for preferred register class. Otherwise, it means IRA will check all available alternatives for preferred register class even if it has found some choice with an appropriate register class and respect the found qualified matching constraint.
LRA tries to reuse values reloaded in registers in subsequent insns. This optimization is called inheritance. EBB is used as a region to do this optimization. The parameter defines a minimal fall-through edge probability in percentage used to add BB to inheritance EBB in LRA.
The default value was chosen from numerous runs of SPEC on x Loop invariant motion can be very expensive, both in compilation time and in amount of needed compile-time memory, with very large loops. Building data dependencies is expensive for very large loops. This parameter limits the number of data references in loops that are considered for data dependence analysis.
These large loops are no handled by the optimizations using loop data dependencies. Sets a maximum number of hash table slots to use during variable tracking dataflow analysis of any function.
If this limit is exceeded with variable tracking at assignments enabled, analysis for that function is retried without it, after removing all debug insns from the function.
If the limit is exceeded even without debug insns, var tracking analysis is completely disabled for the function. Setting the parameter to zero makes it unlimited. Sets a maximum number of recursion levels when attempting to map variable names or debug temporaries to value expressions. This trades compilation time for more complete debug information.
If this is set too low, value expressions that are available and could be represented in debug information may end up not being used; setting this higher may enable the compiler to find more complex debug expressions, but compile time and memory use may grow. Sets a threshold on the number of debug markers e. begin stmt markers to avoid complexity explosion at inlining or expanding to RTL.
If a function has more such gimple stmts than the set limit, such stmts will be dropped from the inlined copy of a function, and from its RTL expansion.
Use uids starting at this parameter for nondebug insns. The range below the parameter is reserved exclusively for debug insns created by -fvar-tracking-assignments , but debug insns may get non-overlapping uids above it if the reserved range is exhausted. IPA-SRA replaces a pointer which is known not be NULL with one or more new parameters only when the probability in percent, relative to function entry of it being dereferenced is higher than this parameter.
IPA-SRA replaces a pointer to an aggregate with one or more new parameters only when their cumulative size is less or equal to ipa-sra-ptr-growth-factor times the size of the original pointer parameter. Additional maximum allowed growth of total size of new parameters that ipa-sra replaces a pointer to an aggregate with, if it points to a local variable that the caller only writes to and passes it as an argument to other functions.
Maximum pieces of an aggregate that IPA-SRA tracks. As a consequence, it is also the maximum number of replacements of a formal parameter. The two Scalar Reduction of Aggregates passes SRA and IPA-SRA aim to replace scalar parts of aggregates with uses of independent scalar variables.
These parameters control the maximum size, in storage units, of aggregate which is considered for replacement when compiling for speed sra-max-scalarization-size-Ospeed or size sra-max-scalarization-size-Osize respectively.
The maximum number of artificial accesses that Scalar Replacement of Aggregates SRA will track, per one local variable, in order to facilitate copy propagation. This option only applies when using -fgnu-tm. To avoid exponential effects in the Graphite loop transforms, the number of parameters in a Static Control Part SCoP is bounded.
A value of zero can be used to lift the bound. A variable whose value is unknown at compilation time and defined outside a SCoP is a parameter of the SCoP. Loop blocking or strip mining transforms, enabled with -floop-block or -floop-strip-mine , strip mine each loop in the loop nest by a given number of iterations.
The strip length can be changed using the loop-block-tile-size parameter. ipa-cp-value-list-size is the maximum number of values and types it stores per one formal parameter of a function. IPA-CP calculates its own score of cloning profitability heuristics and performs those cloning opportunities with scores that exceed ipa-cp-eval-threshold. When using -fprofile-use option, IPA-CP will consider the measured execution count of a call graph edge at this percentage position in their histogram as the basis for its heuristics calculation.
The number of times interprocedural copy propagation expects recursive functions to call themselves. Percentage penalty functions containing a single call to another function will receive when they are evaluated for cloning. IPA-CP is also capable to propagate a number of scalar values passed in an aggregate. ipa-max-agg-items controls the maximum number of such values per one parameter. When IPA-CP determines that a cloning candidate would make the number of iterations of a loop known, it adds a bonus of ipa-cp-loop-hint-bonus to the profitability score of the candidate.
The maximum number of different predicates IPA will use to describe when loops in a function have known properties. During its analysis of function bodies, IPA-CP employs alias analysis in order to track values pointed to by function parameters.
In order not spend too much time analyzing huge functions, it gives up and consider all memory clobbered after examining ipa-max-aa-steps statements modifying memory. Maximal number of boundary endpoints of case ranges of switch statement.
For switch exceeding this limit, IPA-CP will not construct cloning cost predicate, which is used to estimate cloning benefit, for default case of the switch statement. IPA-CP will analyze conditional statement that references some function parameter to estimate benefit for cloning upon certain constant value.
But if number of operations in a parameter expression exceeds ipa-max-param-expr-ops , the expression is treated as complicated one, and is not handled by IPA analysis. Specify desired number of partitions produced during WHOPR compilation. The number of partitions should exceed the number of CPUs used for compilation. Size of minimal partition for WHOPR in estimated instructions. This prevents expenses of splitting very small programs into too many partitions.
Size of max partition for WHOPR in estimated instructions. to provide an upper bound for individual size of partition. Meant to be used only with balanced partitioning.
Larger numbers result in more aggressive statement sinking. A small positive adjustment is applied for statements with memory operands as those are even more profitable so sink. The maximum number of conditional store pairs that can be sunk.
Set to 0 if either vectorization -ftree-vectorize or if-conversion -ftree-loop-if-convert is disabled. The smallest number of different values for which it is best to use a jump-table instead of a tree of conditional branches.
If the value is 0, use the default for the machine. The maximum code size growth ratio when expanding into a jump table in percent.
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