neulab/tldr huggingface dataset:
for (int i = 0; i < N; i++) for (int k = 0; k < N; k++) for (int j = 0; j < N; j++) c[i][j] = c[i][j] + a[i][k]*b[k][j];
-floop-unroll-and-jam Apply unroll and jam transformations on feasible loops.  In a loop nest this unrolls the outer loop by some factor and fuses the resulting multiple inner loops.  This flag is enabled by default at -O3.  It is also enabled by -fprofile-use and -fauto-profile.
-ftree-loop-im Perform loop invariant motion on trees.  This pass moves only invariants that are 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 loops for which determining number of iterations requires complicated analysis. Later optimizations then may determine the number easily.  Useful especially in connection with unrolling.
-ftree-scev-cprop Perform final value replacement.  If a variable is modified in a loop in such a way that its value when exiting the loop can be determined using only its initial value and the number of loop iterations, replace uses of the final value by such a computation, provided it is sufficiently cheap.  This reduces data dependencies and may allow further simplifications.  Enabled by default at -O and higher.
-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-pta Perform function-local points-to analysis on trees.  This flag is enabled by default at -O1 and higher, except for -Og.
-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 -O1 and higher, except for -Og.
-fstore-merging Perform merging of narrow stores to consecutive memory addresses. This pass merges contiguous stores of immediate values narrower than a word into fewer wider stores to reduce the number of instructions. This is enabled by default at -O2 and higher as well as -Os.
-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-slsr Perform straight-line strength reduction on trees.  This recognizes related expressions involving multiplications and replaces them by less expensive calculations when possible.  This is enabled by default at -O and higher.
-ftree-vectorize Perform vectorization on trees. This flag enables -ftree-loop-vectorize and -ftree-slp-vectorize if not explicitly specified.
-ftree-loop-vectorize Perform loop vectorization on trees. This flag is enabled by default at -O3 and by -ftree-vectorize, -fprofile-use, and -fauto-profile.
-ftree-slp-vectorize Perform basic block vectorization on trees. This flag is enabled by default at -O3 and by -ftree-vectorize, -fprofile-use, and -fauto-profile.
-fvect-cost-model=model Alter the cost model used for vectorization.  The model argument should be one of unlimited, dynamic, cheap or very-cheap.  With the unlimited model the vectorized code-path is assumed to be profitable while with the dynamic model a runtime check guards the vectorized code-path to enable it only for iteration counts that will likely execute faster than when executing the original scalar loop.  The cheap model disables vectorization of loops where doing so would be cost prohibitive for example due to required runtime checks for data dependence or alignment but otherwise is equal to the dynamic model. The very-cheap model only allows vectorization if the vector code would entirely replace the scalar code that is being vectorized.  For example, if each iteration of a vectorized loop would only be able to handle exactly four iterations of the scalar loop, the very-cheap model would only allow vectorization if the scalar iteration count is known to be a multiple of four.
The default cost model depends on other optimization flags and is either dynamic or cheap.
-fsimd-cost-model=model Alter the cost model used for vectorization of loops marked with the OpenMP simd directive.  The model argument should be one of unlimited, dynamic, cheap.  All values of model have the same meaning as described in -fvect-cost-model and by default a cost model defined with -fvect-cost-model is used.
-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.
-fsplit-paths Split paths leading to loop backedges.  This can improve dead code elimination and common subexpression elimination.  This is enabled by default at -O3 and above.
-fsplit-ivs-in-unroller Enables expression 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.
A combination of -fweb and CSE is often sufficient to obtain the same effect.  However, that is not reliable in cases where the loop body is more complicated than a single basic block.  It also does not work at all on some architectures due to restrictions in the CSE pass.
This optimization is enabled by default.
-fvariable-expansion-in-unroller With this option, the compiler creates multiple copies of some local variables when unrolling a loop, which can result in superior code.
This optimization is enabled by default for PowerPC targets, but disabled by default otherwise.
-fpartial-inlining Inline parts of functions.  This option has any effect only when inlining itself is turned on by the -finline-functions or -finline-small-functions options.
-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.  It is also enabled by -fprofile-use and -fauto-profile.
-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-printf-return-value Do not substitute constants for known return value of formatted output functions such as "sprintf", "snprintf", "vsprintf", and "vsnprintf" (but not "printf" of "fprintf").  This transformation allows GCC to optimize or even eliminate branches based on the known return value of these functions called with arguments that are either constant, or whose values are known to be in a range that makes determining the exact return value possible.  For example, when -fprintf-return-value is in effect, both the branch and the body of the "if" statement (but not the call to "snprint") can be optimized away when "i" is a 32-bit or smaller integer because the return value is guaranteed to be at most 8.
char buf[9]; if (snprintf (buf, "%08x", i) >= sizeof buf) ...
The -fprintf-return-value option relies on other optimizations and yields best results with -O2 and above.  It works in tandem with the -Wformat-overflow and -Wformat-truncation options.  The -fprintf-return-value option is enabled by default.
-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 uses heuristics to guess branch probabilities if they are not provided by profiling feedback (-fprofile-arcs).  These heuristics are based on the control flow graph.  If some branch probabilities are specified by "__builtin_expect", then the heuristics are used to guess branch probabilities for the rest of the control flow graph, taking the "__builtin_expect" info into account.  The interactions between the heuristics and "__builtin_expect" can be complex, and in some cases, it may be useful to disable the heuristics so that the effects of "__builtin_expect" are easier to understand.
It is also possible to specify expected probability of the expression with "__builtin_expect_with_probability" built-in function.
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.
-freorder-blocks-algorithm=algorithm Use the specified algorithm for basic block reordering.  The algorithm argument can be simple, which does not increase code size (except sometimes due to secondary effects like alignment), or stc, the "software trace cache" algorithm, which tries to put all often executed code together, minimizing the number of branches executed by making extra copies of code.
The default is simple at levels -O, -Os, and stc 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 or unwind tables (on targets using setjump/longjump or target specific scheme), for linkonce sections, for functions with a user-defined section attribute and on any architecture that does not support named sections.  When -fsplit-stack is used this option is not enabled by default (to avoid linker errors), but may be enabled explicitly (if using a working linker).
Enabled for x86 at levels -O2, -O3, -Os.
-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.
This option isn't effective unless you either provide profile feedback (see -fprofile-arcs for details) or manually annotate functions with "hot" or "cold" attributes.
-fstrict-aliasing Allow 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() { union a_union t; t.d = 3.0; return t.i; }
The practice of reading from a different union member than the one most recently written to (called "type-punning") is common.  Even with -fstrict-aliasing, type-punning is allowed, provided the memory is accessed through the union type.  So, the code above works as expected.    However, this code might not:
int f() { union a_union t; int* ip; t.d = 3.0; ip = &t.i; return *ip; }
Similarly, access by taking the address, casting the resulting pointer and dereferencing the result has undefined behavior, even if the cast uses a union type, e.g.:
int f() { double d = 3.0; return ((union a_union *) &d)->i; }
The -fstrict-aliasing option is enabled at levels -O2, -O3, -Os.
-falign-functions -falign-functions=n -falign-functions=n:m -falign-functions=n:m:n2 -falign-functions=n:m:n2:m2 Align the start of functions to the next power-of-two greater than or equal to n, skipping up to m-1 bytes.  This ensures that at least the first m bytes of the function can be fetched by the CPU without crossing an n-byte alignment boundary.
If m is not specified, it defaults to n.
Examples: -falign-functions=32 aligns functions to the next 32-byte boundary, -falign-functions=24 aligns to the next 32-byte boundary only if this can be done by skipping 23 bytes or less, -falign-functions=32:7 aligns to the next 32-byte boundary only if this can be done by skipping 6 bytes or less.
The second pair of n2:m2 values allows you to specify a secondary alignment: -falign-functions=64:7:32:3 aligns to the next 64-byte boundary if this can be done by skipping 6 bytes or less, otherwise aligns to the next 32-byte boundary if this can be done by skipping 2 bytes or less.  If m2 is not specified, it defaults to n2.
Some assemblers only support this flag when n is a power of two; in that case, it is rounded up.
-fno-align-functions and -falign-functions=1 are equivalent and mean that functions are not aligned.
If n is not specified or is zero, use a machine-dependent default. The maximum allowed n option value is 65536.
-flimit-function-alignment If this option is enabled, the compiler tries to avoid unnecessarily overaligning functions. It attempts to instruct the assembler to align by the amount specified by -falign-functions, but not to skip more bytes than the size of the function.
-falign-labels -falign-labels=n -falign-labels=n:m -falign-labels=n:m:n2 -falign-labels=n:m:n2:m2 Align all branch targets to a power-of-two boundary.
Parameters of this option are analogous to the -falign-functions option.  -fno-align-labels and -falign-labels=1 are equivalent and mean that labels are not aligned.
If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values are used instead.
If n is not specified or is zero, use a machine-dependent default which is very likely to be 1, meaning no alignment.  The maximum allowed n option value is 65536.
-falign-loops -falign-loops=n -falign-loops=n:m -falign-loops=n:m:n2 -falign-loops=n:m:n2:m2 Align loops to a power-of-two boundary.  If the loops are executed many times, this makes up for any execution of the dummy padding instructions.
If -falign-labels is greater than this value, then its value is used instead.
Parameters of this option are analogous to the -falign-functions option.  -fno-align-loops and -falign-loops=1 are equivalent and mean that loops are not aligned.  The maximum allowed n option value is 65536.
If n is not specified or is zero, use a machine-dependent default.
-falign-jumps -falign-jumps=n -falign-jumps=n:m -falign-jumps=n:m:n2 -falign-jumps=n:m:n2:m2 Align branch targets to a power-of-two boundary, for branch targets where the targets can only be reached by jumping.  In this case, no dummy operations need be executed.
Parameters of this option are analogous to the -falign-functions option.  -fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops are not aligned.
-fno-allocation-dce Do not remove unused C++ allocations in dead code elimination.
-fallow-store-data-races Allow the compiler to perform optimizations that may introduce new data races on stores, without proving that the variable cannot be concurrently accessed by other threads.  Does not affect optimization of local data.  It is safe to use this option if it is known that global data will not be accessed by multiple threads.
Examples of optimizations enabled by -fallow-store-data-races include hoisting or if-conversions that may cause a value that was already in memory to be re-written with that same value.  Such re-writing is safe in a single threaded context but may be unsafe in a multi- threaded context.  Note that on some processors, if-conversions may be required in order to enable vectorization.
Enabled at level -Ofast.
-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.
-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 are not removed.  This option is intended to support existing code that relies on a particular ordering.  For new code, it is better to use attributes when possible.
-ftoplevel-reorder is the default at -O1 and higher, and also at -O0 if -fsection-anchors is explicitly requested.  Additionally -fno-toplevel-reorder implies -fno-section-anchors.
-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 no longer stay in a "home register".
Enabled by default with -funroll-loops.
-fwhole-program Assume that the current compilation unit represents the 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 effect are optimized more aggressively by interprocedural optimizers.
This option should not be used in combination with -flto.  Instead relying on a linker plugin should provide safer and more precise information.
-flto[=n] This option runs the standard link-time optimizer.  When invoked with source code, it generates GIMPLE (one of GCC's internal representations) and writes it to special ELF sections in the object file.  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:
gcc -c -O2 -flto foo.c gcc -c -O2 -flto bar.c gcc -o myprog -flto -O2 foo.o bar.o
The first two invocations to GCC save a bytecode representation of GIMPLE into special ELF sections inside foo.o and bar.o.  The final invocation reads the GIMPLE bytecode from foo.o and bar.o, merges the two files into a single internal image, and compiles the result as usual.  Since both foo.o and bar.o are merged into a single image, this causes all the interprocedural analyses and optimizations in GCC to work across the two files as if they were a single one.  This means, for example, that the inliner is able to inline functions in bar.o into functions in foo.o and vice-versa.
Another (simpler) way to enable link-time optimization is:
gcc -o myprog -flto -O2 foo.c bar.c
The above generates bytecode for foo.c and bar.c, merges them together into a single GIMPLE representation and optimizes them as usual to produce myprog.
The 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:
B<-fPIC> + B<-fpic> = B<-fpic> B<-fPIC> + B<-fno-pic> = B<-fno-pic> B<-fpic/-fPIC> + (no option) = (no option) B<-fPIC> + B<-fPIE> = B<-fPIE> B<-fpic> + B<-fPIE> = B<-fpie> B<-fPIC/-fpic> + B<-fpie> = B<-fpie>
Certain ABI-changing flags are required to match in all compilation units, and trying to override this at link time with a conflicting value is ignored.  This includes options such as -freg-struct-return and -fpcc-struct-return.
Other options such as -ffp-contract, -fno-strict-overflow, -fwrapv, -fno-trapv or -fno-strict-aliasing are passed through to the link stage and merged conservatively for conflicting translation units. Specifically -fno-strict-overflow, -fwrapv and -fno-trapv take precedence; and for example -ffp-contract=off takes precedence over -ffp-contract=fast.  You can override them at 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 6.2.7), a non-fatal diagnostic may be issued.  The behavior is still undefined at run time.  Similar diagnostics may be raised for other languages.
Another feature of LTO is that it is possible to apply interprocedural optimizations on files written in different languages:
gcc -c -flto foo.c g++ -c -flto bar.cc gfortran -c -flto baz.f90 g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran
Notice that the final link is done with g++ to get the C++ runtime libraries and -lgfortran is added to get the Fortran runtime libraries.  In general, when mixing languages in LTO mode, you should use the same link command options as when mixing languages in a regular (non-LTO) compilation.
If object files containing GIMPLE bytecode are stored in a library archive, say libfoo.a, it is possible to extract and use them in an LTO link if you are using a linker with plugin support.  To create static libraries suitable for LTO, use gcc-ar and gcc-ranlib instead of ar and ranlib; to show the symbols of object files with GIMPLE bytecode, use gcc-nm.  Those commands require that ar, ranlib and nm have been compiled with plugin support.  At link time, use the flag -fuse-linker-plugin to ensure that the library participates in the LTO optimization process:
gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo
With the linker plugin enabled, the linker extracts the needed GIMPLE files from libfoo.a and passes them on to the running GCC to make them part of the aggregated GIMPLE image to be optimized.
If you are not using a linker with plugin support and/or do not enable the linker plugin, then the objects inside libfoo.a are extracted and linked as usual, but they do not participate in the LTO optimization process.  In order to make a static library suitable for both LTO optimization and usual linkage, compile its object files with -flto -ffat-lto-objects.
Link-time optimizations do not require the presence of the whole program to operate.  If the program does not require any symbols to be exported, it is possible to combine -flto and -fwhole-program to allow the interprocedural optimizers to use more aggressive assumptions which may lead to improved optimization opportunities. Use of -fwhole-program is not needed when linker plugin is active (see -fuse-linker-plugin).
The current implementation of LTO makes no attempt to generate bytecode that is portable between different types of hosts.  The bytecode files are versioned and there is a strict version check, so bytecode files generated in one version of GCC do not work with an older or newer version of GCC.
Link-time optimization does not work well with generation of debugging information 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.
You can also specify -flto=jobserver to use GNU make's job server mode to determine the number of parallel jobs. This is useful when the Makefile calling GCC is already executing in parallel.  You must prepend a + to the command recipe in the parent Makefile for this to work.  This option likely only works if MAKE is GNU make.  Even without the option value, GCC tries to automatically detect a running GNU make's job server.
Use -flto=auto to use GNU make's job server, if available, or otherwise fall back to autodetection of the number of CPU threads present in your system.
-flto-partition=alg Specify the partitioning algorithm used by the link-time optimizer. The value is either 1to1 to specify a partitioning mirroring the original source files or balanced to specify partitioning into equally sized chunks (whenever possible) or max to create new partition for every symbol where possible.  Specifying none as an algorithm disables partitioning and streaming completely.  The default value is balanced. While 1to1 can be used as an workaround for various code ordering issues, the max partitioning is intended for internal testing only.  The value one specifies that exactly one partition should be used while the value none bypasses partitioning and executes the link-time optimization step directly from the WPA phase.
-flto-compression-level=n 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.
-fuse-linker-plugin 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 2.21 or newer.
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 2.21 or newer or gold).
-ffat-lto-objects 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.
-fno-fat-lto-objects improves compilation time over plain LTO, but requires the complete toolchain to be aware of LTO. It requires a linker with linker plugin support for basic functionality. Additionally, nm, ar and ranlib need to support linker plugins to allow a full-featured build environment (capable of building static libraries etc).  GCC provides the gcc-ar, gcc-nm, gcc-ranlib wrappers to pass the right options to these tools. With non fat LTO makefiles need to be modified to use them.
Note that modern binutils provide plugin auto-load mechanism. Installing the linker plugin into $libdir/bfd-plugins has the same effect as usage of the command wrappers (gcc-ar, gcc-nm and gcc- ranlib).
The default is -fno-fat-lto-objects on targets with linker plugin support.
-fcompare-elim 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.
-fcprop-registers 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.
-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 uses heuristics to correct or smooth out such inconsistencies. By default, GCC emits an error message when an inconsistent profile is detected.
This option is enabled by -fauto-profile.
-fprofile-partial-training 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.
-fprofile-use -fprofile-use=path Enable profile feedback-directed optimizations, and the following optimizations, many of which are generally profitable only with profile feedback available:
-fbranch-probabilities  -fprofile-values -funroll-loops  -fpeel-loops -ftracer  -fvpt -finline-functions  -fipa-cp  -fipa-cp-clone -fipa-bit-cp -fpredictive-commoning  -fsplit-loops  -funswitch-loops -fgcse-after-reload  -ftree-loop-vectorize  -ftree-slp-vectorize -fvect-cost-model=dynamic  -ftree-loop-distribute-patterns -fprofile-reorder-functions
Before you can use this option, you must first generate profiling information.
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 -Wno-error=coverage-mismatch.  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.
-fauto-profile -fauto-profile=path Enable sampling-based feedback-directed optimizations, and the following optimizations, many of which are generally profitable only with profile feedback available:
-fbranch-probabilities  -fprofile-values -funroll-loops  -fpeel-loops -ftracer  -fvpt -finline-functions  -fipa-cp  -fipa-cp-clone -fipa-bit-cp -fpredictive-commoning  -fsplit-loops  -funswitch-loops -fgcse-after-reload  -ftree-loop-vectorize  -ftree-slp-vectorize -fvect-cost-model=dynamic  -ftree-loop-distribute-patterns -fprofile-correction
path is the name of a file containing AutoFDO profile information. If omitted, it defaults to fbdata.afdo in the current directory.
Producing an AutoFDO profile data file requires running your program with the perf utility on a supported GNU/Linux target system.  For more information, see <https://perf.wiki.kernel.org/>.
perf record -e br_inst_retired:near_taken -b -o perf.data \ -- your_program
Then use the create_gcov tool to convert the raw profile data to a format that can be used by GCC.  You must also supply the unstripped binary for your program to this tool.  See <https://github.com/google/autofdo>.
create_gcov --binary=your_program.unstripped --profile=perf.data \ --gcov=profile.afdo
The following options control compiler behavior regarding floating-point arithmetic.  These options trade off between speed and correctness.  All must be specifically enabled.
-ffloat-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.
-fexcess-precision=style 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. By default, -fexcess-precision=fast is in effect; this means that operations may be carried out in a wider precision than the types specified in the source if that would result in faster code, and it is unpredictable when rounding to the types specified in the source code takes place.  When compiling C, if -fexcess-precision=standard is specified then excess precision follows the rules specified in ISO C99; in particular, both casts and assignments cause values to be rounded to their semantic types (whereas -ffloat-store only affects assignments).  This option is enabled by default for C if a strict conformance option such as -std=c99 is used.  -ffast-math enables -fexcess-precision=fast by default regardless of whether a strict conformance option is used.
-fexcess-precision=standard is not implemented for languages other than C.  On the x86, it has no effect if -mfpmath=sse or -mfpmath=sse+387 is specified; in the former case, IEEE semantics apply without excess precision, and in the latter, rounding is unpredictable.
-ffast-math Sets the options -fno-math-errno, -funsafe-math-optimizations, -ffinite-math-only, -fno-rounding-math, -fno-signaling-nans, -fcx-limited-range and -fexcess-precision=fast.
This option causes the preprocessor macro "__FAST_MATH__" to be defined.
This option is not turned on by any -O option besides -Ofast since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.
-fno-math-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 that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.
The default is -fmath-errno.
On Darwin systems, the math library never sets "errno".  There is therefore no reason for the compiler to consider the possibility that it might, and -fno-math-errno is the default.
-funsafe-math-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 that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.  Enables -fno-signed-zeros, -fno-trapping-math, -fassociative-math and -freciprocal-math.
The default is -fno-unsafe-math-optimizations.
-fassociative-math 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 code that 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. For Fortran the option is automatically enabled when both -fno-signed-zeros and -fno-trapping-math are in effect.
The default is -fno-associative-math.
-freciprocal-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.
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 that depend on an exact implementation of IEEE or ISO rules/specifications for math functions.
The default is -ftrapping-math.
-frounding-math Disable transformations and optimizations that assume default floating-point rounding behavior.  This is round-to-zero for all floating point to integer conversions, and round-to-nearest for all other arithmetic truncations.  This option should be specified for programs that change the FP rounding mode dynamically, or that may be executed with a non-default rounding mode.  This option disables constant folding of floating-point expressions at compile time (which may be affected by rounding mode) and arithmetic transformations that are unsafe in the presence of sign-dependent rounding modes.
The default is -fno-rounding-math.
This option is experimental and does not currently guarantee to disable all GCC optimizations that are affected by rounding mode. Future versions of GCC may provide finer control of this setting using C99's "FENV_ACCESS" pragma.  This command-line option will be used to specify the default state for "FENV_ACCESS".
-fsignaling-nans Compile code assuming that IEEE signaling NaNs may generate user- visible traps during floating-point operations.  Setting this option disables optimizations that may change the number of exceptions visible with signaling NaNs.  This option implies -ftrapping-math.
This option causes the preprocessor macro "__SUPPORT_SNAN__" to be defined.
The default is -fno-signaling-nans.
This option is experimental and does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior.
-fno-fp-int-builtin-inexact Do not allow the built-in functions "ceil", "floor", "round" and "trunc", and their "float" and "long double" variants, to generate code that raises the "inexact" floating-point exception for noninteger arguments.  ISO C99 and C11 allow these functions to raise the "inexact" exception, but ISO/IEC TS 18661-1:2014, the C bindings to IEEE 754-2008, as integrated into ISO C2X, does not allow these functions to do so.
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.
Even if -fno-fp-int-builtin-inexact is used, if the functions generate a call to a library function then the "inexact" exception may be raised if the library implementation does not follow TS 18661.
-fsingle-precision-constant Treat floating-point constants as single precision instead of implicitly converting them to double-precision constants.
-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, 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.
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 most likely 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 for usage in optimizations.
Enabled by -fprofile-generate, -fprofile-use, and -fauto-profile.
-fprofile-reorder-functions Function reordering based on profile instrumentation collects first time of execution of a function and orders these functions in ascending order.
Enabled with -fprofile-use.
-fvpt 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.
Enabled with -fprofile-use and -fauto-profile.
-frename-registers 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.  Depending on the debug information format adopted by the target, however, it can make debugging impossible, since variables no longer stay in a "home register".
-fschedule-fusion 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.
-ftracer Perform tail duplication to enlarge superblock size.  This transformation simplifies the control flow of the function allowing other optimizations to do a 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, -fweb and -frename-registers.  It also turns on complete loop peeling (i.e. 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.