This section describes command-line options that are primarily of interest to GCC developers, including options to support compiler testing and investigation of compiler bugs and compile-time performance problems. This includes options that produce debug dumps at various points in the compilation; that print statistics such as memory use and execution time; and that print information about GCC’s configuration, such as where it searches for libraries. You should rarely need to use any of these options for ordinary compilation and linking tasks.
Many developer options that cause GCC to dump output to a file take an optional ‘=filename’ suffix. You can specify ‘stdout’ or ‘-’ to dump to standard output, and ‘stderr’ for standard error.
If ‘=filename’ is omitted, a default dump file name is constructed by concatenating the base dump file name, a pass number, phase letter, and pass name. The base dump file name is the name of output file produced by the compiler if explicitly specified and not an executable; otherwise it is the source file name. The pass number is determined by the order passes are registered with the compiler’s pass manager. This is generally the same as the order of execution, but passes registered by plugins, target-specific passes, or passes that are otherwise registered late are numbered higher than the pass named ‘final’, even if they are executed earlier. The phase letter is one of ‘i’ (inter-procedural analysis), ‘l’ (language-specific), ‘r’ (RTL), or ‘t’ (tree). The files are created in the directory of the output file.
Says to make debugging dumps during compilation at times specified by letters. This is used for debugging the RTL-based passes of the compiler.
Some -dletters switches have different meaning when -E is used for preprocessing. See Preprocessor Options, for information about preprocessor-specific dump options.
Debug dumps can be enabled with a -fdump-rtl switch or some -d option letters. Here are the possible letters for use in pass and letters, and their meanings:
Dump after branch alignments have been computed.
Dump after fixing rtl statements that have unsatisfied in/out constraints.
Dump after auto-inc-dec discovery. This pass is only run on architectures that have auto inc or auto dec instructions.
Dump after cleaning up the barrier instructions.
Dump after partitioning hot and cold basic blocks.
Dump after block reordering.
-fdump-rtl-btl1 and -fdump-rtl-btl2 enable dumping after the two branch target load optimization passes.
Dump after jump bypassing and control flow optimizations.
Dump after the RTL instruction combination pass.
Dump after duplicating the computed gotos.
-fdump-rtl-ce1, -fdump-rtl-ce2, and -fdump-rtl-ce3 enable dumping after the three if conversion passes.
Dump after hard register copy propagation.
Dump after combining stack adjustments.
-fdump-rtl-cse1 and -fdump-rtl-cse2 enable dumping after the two common subexpression elimination passes.
Dump after the standalone dead code elimination passes.
Dump after delayed branch scheduling.
-fdump-rtl-dce1 and -fdump-rtl-dce2 enable dumping after the two dead store elimination passes.
Dump after finalization of EH handling code.
Dump after conversion of EH handling range regions.
Dump after RTL generation.
-fdump-rtl-fwprop1 and -fdump-rtl-fwprop2 enable dumping after the two forward propagation passes.
-fdump-rtl-gcse1 and -fdump-rtl-gcse2 enable dumping after global common subexpression elimination.
Dump after the initialization of the registers.
Dump after the computation of the initial value sets.
Dump after converting to cfglayout mode.
Dump after iterated register allocation.
Dump after the second jump optimization.
-fdump-rtl-loop2 enables dumping after the rtl loop optimization passes.
Dump after performing the machine dependent reorganization pass, if that pass exists.
Dump after removing redundant mode switches.
Dump after register renumbering.
Dump after converting from cfglayout mode.
Dump after the peephole pass.
Dump after post-reload optimizations.
Dump after generating the function prologues and epilogues.
-fdump-rtl-sched1 and -fdump-rtl-sched2 enable dumping after the basic block scheduling passes.
Dump after sign/zero extension elimination.
Dump after common sequence discovery.
Dump after shortening branches.
Dump after sibling call optimizations.
These options enable dumping after five rounds of instruction splitting.
Dump after modulo scheduling. This pass is only run on some architectures.
Dump after conversion from GCC’s “flat register file” registers to the x87’s stack-like registers. This pass is only run on x86 variants.
-fdump-rtl-subreg1 and -fdump-rtl-subreg2 enable dumping after the two subreg expansion passes.
Dump after all rtl has been unshared.
Dump after variable tracking.
Dump after converting virtual registers to hard registers.
Dump after live range splitting.
These dumps are defined but always produce empty files.
Produce all the dumps listed above.
Annotate the assembler output with miscellaneous debugging information.
Dump all macro definitions, at the end of preprocessing, in addition to normal output.
Produce a core dump whenever an error occurs.
Annotate the assembler output with a comment indicating which pattern and alternative is used. The length and cost of each instruction are also printed.
Dump the RTL in the assembler output as a comment before each instruction. Also turns on -dp annotation.
Just generate RTL for a function instead of compiling it. Usually used with -fdump-rtl-expand.
When doing debugging dumps, suppress address output. This makes it more feasible to use diff on debugging dumps for compiler invocations with different compiler binaries and/or different text / bss / data / heap / stack / dso start locations.
Collect and dump debug information into a temporary file if an internal compiler error (ICE) occurs.
When doing debugging dumps, suppress instruction numbers and address output. This makes it more feasible to use diff on debugging dumps for compiler invocations with different options, in particular with and without -g.
When doing debugging dumps (see -d option above), suppress instruction numbers for the links to the previous and next instructions in a sequence.
Control the dumping at various stages of inter-procedural analysis language tree to a file. The file name is generated by appending a switch specific suffix to the source file name, and the file is created in the same directory as the output file. The following dumps are possible:
Enables all inter-procedural analysis dumps.
Dumps information about call-graph optimization, unused function removal, and inlining decisions.
Dump after function inlining.
Control the dumping of language-specific information. The options and filename portions behave as described in the -fdump-tree option. The following switch values are accepted:
Enable all language-specific dumps.
Dump class hierarchy information. Virtual table information is emitted unless ’slim’ is specified. This option is applicable to C++ only.
Dump the raw internal tree data. This option is applicable to C++ only.
Print on stderr the list of optimization passes that are turned on and off by the current command-line options.
Enable and control dumping of pass statistics in a separate file. The file name is generated by appending a suffix ending in ‘.statistics’ to the source file name, and the file is created in the same directory as the output file. If the ‘-option’ form is used, ‘-stats’ causes counters to be summed over the whole compilation unit while ‘-details’ dumps every event as the passes generate them. The default with no option is to sum counters for each function compiled.
Control the dumping at various stages of processing the intermediate language tree to a file. If the ‘-options’ form is used, options is a list of ‘-’ separated options which control the details of the dump. Not all options are applicable to all dumps; those that are not meaningful are ignored. The following options are available
Print the address of each node. Usually this is not meaningful as it changes according to the environment and source file. Its primary use is for tying up a dump file with a debug environment.
DECL_ASSEMBLER_NAME has been set for a given decl, use that
in the dump instead of
DECL_NAME. Its primary use is ease of
use working backward from mangled names in the assembly file.
When dumping front-end intermediate representations, inhibit dumping of members of a scope or body of a function merely because that scope has been reached. Only dump such items when they are directly reachable by some other path.
When dumping pretty-printed trees, this option inhibits dumping the bodies of control structures.
When dumping RTL, print the RTL in slim (condensed) form instead of the default LISP-like representation.
Print a raw representation of the tree. By default, trees are pretty-printed into a C-like representation.
Enable more detailed dumps (not honored by every dump option). Also include information from the optimization passes.
Enable dumping various statistics about the pass (not honored by every dump option).
Enable showing basic block boundaries (disabled in raw dumps).
For each of the other indicated dump files (-fdump-rtl-pass), dump a representation of the control flow graph suitable for viewing with GraphViz to file.passid.pass.dot. Each function in the file is pretty-printed as a subgraph, so that GraphViz can render them all in a single plot.
This option currently only works for RTL dumps, and the RTL is always dumped in slim form.
Enable showing virtual operands for every statement.
Enable showing line numbers for statements.
Enable showing the unique ID (
DECL_UID) for each variable.
Enable showing the tree dump for each statement.
Enable showing the EH region number holding each statement.
Enable showing scalar evolution analysis details.
Enable showing optimization information (only available in certain passes).
Enable showing missed optimization information (only available in certain passes).
Enable other detailed optimization information (only available in certain passes).
Turn on all options, except raw, slim, verbose and lineno.
Turn on all optimization options, i.e., optimized, missed, and note.
To determine what tree dumps are available or find the dump for a pass of interest follow the steps below.
tree-vrp2correspond to the three Value Range Propagation passes. The number at the end distinguishes distinct invocations of the same pass.
Controls optimization dumps from various optimization passes. If the ‘-options’ form is used, options is a list of ‘-’ separated option keywords to select the dump details and optimizations.
The options can be divided into two groups: options describing the verbosity of the dump, and options describing which optimizations should be included. The options from both the groups can be freely mixed as they are non-overlapping. However, in case of any conflicts, the later options override the earlier options on the command line.
The following options control the dump verbosity:
Print information when an optimization is successfully applied. It is up to a pass to decide which information is relevant. For example, the vectorizer passes print the source location of loops which are successfully vectorized.
Print information about missed optimizations. Individual passes control which information to include in the output.
Print verbose information about optimizations, such as certain transformations, more detailed messages about decisions etc.
Print detailed optimization information. This includes ‘optimized’, ‘missed’, and ‘note’.
One or more of the following option keywords can be used to describe a group of optimizations:
Enable dumps from all interprocedural optimizations.
Enable dumps from all loop optimizations.
Enable dumps from all inlining optimizations.
Enable dumps from all OMP (Offloading and Multi Processing) optimizations.
Enable dumps from all vectorization optimizations.
Enable dumps from all optimizations. This is a superset of the optimization groups listed above.
If options is omitted, it defaults to ‘optimized-optall’, which means to dump all info about successful optimizations from all the passes.
If the filename is provided, then the dumps from all the applicable optimizations are concatenated into the filename. Otherwise the dump is output onto stderr. Though multiple -fopt-info options are accepted, only one of them can include a filename. If other filenames are provided then all but the first such option are ignored.
Note that the output filename is overwritten in case of multiple translation units. If a combined output from multiple translation units is desired, stderr should be used instead.
In the following example, the optimization info is output to stderr:
gcc -O3 -fopt-info
gcc -O3 -fopt-info-missed=missed.all
outputs missed optimization report from all the passes into missed.all, and this one:
gcc -O2 -ftree-vectorize -fopt-info-vec-missed
prints information about missed optimization opportunities from vectorization passes on stderr. Note that -fopt-info-vec-missed is equivalent to -fopt-info-missed-vec. The order of the optimization group names and message types listed after -fopt-info does not matter.
As another example,
gcc -O3 -fopt-info-inline-optimized-missed=inline.txt
outputs information about missed optimizations as well as optimized locations from all the inlining passes into inline.txt.
gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt
Here the two output filenames vec.miss and loop.opt are in conflict since only one output file is allowed. In this case, only the first option takes effect and the subsequent options are ignored. Thus only vec.miss is produced which contains dumps from the vectorizer about missed opportunities.
On targets that use instruction scheduling, this option controls the amount of debugging output the scheduler prints to the dump files.
For n greater than zero, -fsched-verbose outputs the same information as -fdump-rtl-sched1 and -fdump-rtl-sched2. For n greater than one, it also output basic block probabilities, detailed ready list information and unit/insn info. For n greater than two, it includes RTL at abort point, control-flow and regions info. And for n over four, -fsched-verbose also includes dependence info.
This is a set of options that are used to explicitly disable/enable optimization passes. These options are intended for use for debugging GCC. Compiler users should use regular options for enabling/disabling passes instead.
Disable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.
Disable RTL pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1. range-list is a comma-separated list of function ranges or assembler names. Each range is a number pair separated by a colon. The range is inclusive in both ends. If the range is trivial, the number pair can be simplified as a single number. If the function’s call graph node’s uid falls within one of the specified ranges, the pass is disabled for that function. The uid is shown in the function header of a dump file, and the pass names can be dumped by using option -fdump-passes.
Disable tree pass pass. See -fdisable-rtl for the description of option arguments.
Enable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.
Enable RTL pass pass. See -fdisable-rtl for option argument description and examples.
Enable tree pass pass. See -fdisable-rtl for the description of option arguments.
Here are some examples showing uses of these options.
# disable ccp1 for all functions -fdisable-tree-ccp1 # disable complete unroll for function whose cgraph node uid is 1 -fenable-tree-cunroll=1 # disable gcse2 for functions at the following ranges [1,1], # [300,400], and [400,1000] # disable gcse2 for functions foo and foo2 -fdisable-rtl-gcse2=foo,foo2 # disable early inlining -fdisable-tree-einline # disable ipa inlining -fdisable-ipa-inline # enable tree full unroll -fenable-tree-unroll
Enable internal consistency checking. The default depends on the compiler configuration. -fchecking=2 enables further internal consistency checking that might affect code generation.
This option provides a seed that GCC uses in place of random numbers in generating certain symbol names that have to be different in every compiled file. It is also used to place unique stamps in coverage data files and the object files that produce them. You can use the -frandom-seed option to produce reproducibly identical object files.
The string can either be a number (decimal, octal or hex) or an arbitrary string (in which case it’s converted to a number by computing CRC32).
The string should be different for every file you compile.
Store the usual “temporary” intermediate files permanently; place them in the current directory and name them based on the source file. Thus, compiling foo.c with -c -save-temps produces files foo.i and foo.s, as well as foo.o. This creates a preprocessed foo.i output file even though the compiler now normally uses an integrated preprocessor.
When used in combination with the -x command-line option, -save-temps is sensible enough to avoid over writing an input source file with the same extension as an intermediate file. The corresponding intermediate file may be obtained by renaming the source file before using -save-temps.
If you invoke GCC in parallel, compiling several different source files that share a common base name in different subdirectories or the same source file compiled for multiple output destinations, it is likely that the different parallel compilers will interfere with each other, and overwrite the temporary files. For instance:
gcc -save-temps -o outdir1/foo.o indir1/foo.c& gcc -save-temps -o outdir2/foo.o indir2/foo.c&
may result in foo.i and foo.o being written to simultaneously by both compilers.
Store the usual “temporary” intermediate files permanently. If the -o option is used, the temporary files are based on the object file. If the -o option is not used, the -save-temps=obj switch behaves like -save-temps.
gcc -save-temps=obj -c foo.c gcc -save-temps=obj -c bar.c -o dir/xbar.o gcc -save-temps=obj foobar.c -o dir2/yfoobar
creates foo.i, foo.s, dir/xbar.i, dir/xbar.s, dir2/yfoobar.i, dir2/yfoobar.s, and dir2/yfoobar.o.
Report the CPU time taken by each subprocess in the compilation sequence. For C source files, this is the compiler proper and assembler (plus the linker if linking is done).
Without the specification of an output file, the output looks like this:
# cc1 0.12 0.01 # as 0.00 0.01
The first number on each line is the “user time”, that is time spent executing the program itself. The second number is “system time”, time spent executing operating system routines on behalf of the program. Both numbers are in seconds.
With the specification of an output file, the output is appended to the named file, and it looks like this:
0.12 0.01 cc1 options 0.00 0.01 as options
The “user time” and the “system time” are moved before the program name, and the options passed to the program are displayed, so that one can later tell what file was being compiled, and with which options.
Dump the final internal representation (RTL) to file. If the
optional argument is omitted (or if file is
.), the name
of the dump file is determined by appending
.gkd to the
compilation output file name.
If no error occurs during compilation, run the compiler a second time, adding opts and -fcompare-debug-second to the arguments passed to the second compilation. Dump the final internal representation in both compilations, and print an error if they differ.
If the equal sign is omitted, the default -gtoggle is used.
The environment variable
GCC_COMPARE_DEBUG, if defined, non-empty
and nonzero, implicitly enables -fcompare-debug. If
GCC_COMPARE_DEBUG is defined to a string starting with a dash,
then it is used for opts, otherwise the default -gtoggle
-fcompare-debug=, with the equal sign but without opts,
is equivalent to -fno-compare-debug, which disables the dumping
of the final representation and the second compilation, preventing even
GCC_COMPARE_DEBUG from taking effect.
To verify full coverage during -fcompare-debug testing, set
GCC_COMPARE_DEBUG to say -fcompare-debug-not-overridden,
which GCC rejects as an invalid option in any actual compilation
(rather than preprocessing, assembly or linking). To get just a
GCC_COMPARE_DEBUG to ‘-w%n-fcompare-debug
not overridden’ will do.
This option is implicitly passed to the compiler for the second
compilation requested by -fcompare-debug, along with options to
silence warnings, and omitting other options that would cause the compiler
to produce output to files or to standard output as a side effect. Dump
files and preserved temporary files are renamed so as to contain the
.gk additional extension during the second compilation, to avoid
overwriting those generated by the first.
When this option is passed to the compiler driver, it causes the first compilation to be skipped, which makes it useful for little other than debugging the compiler proper.
Turn off generation of debug info, if leaving out this option generates it, or turn it on at level 2 otherwise. The position of this argument in the command line does not matter; it takes effect after all other options are processed, and it does so only once, no matter how many times it is given. This is mainly intended to be used with -fcompare-debug.
Toggle -fvar-tracking-assignments, in the same way that -gtoggle toggles -g.
Makes the compiler print out each function name as it is compiled, and print some statistics about each pass when it finishes.
Makes the compiler print some statistics about the time consumed by each pass when it finishes.
Record the time consumed by infrastructure parts separately for each pass.
Control the verbosity of the dump file for the integrated register allocator. The default value is 5. If the value n is greater or equal to 10, the dump output is sent to stderr using the same format as n minus 10.
Prints a report with internal details on the workings of the link-time optimizer. The contents of this report vary from version to version. It is meant to be useful to GCC developers when processing object files in LTO mode (via -flto).
Disabled by default.
Like -flto-report, but only print for the WPA phase of Link Time Optimization.
Makes the compiler print some statistics about permanent memory allocation when it finishes.
Makes the compiler print some statistics about permanent memory allocation for the WPA phase only.
Makes the compiler print some statistics about permanent memory allocation before or after interprocedural optimization.
Makes the compiler print some statistics about consistency of the (estimated) profile and effect of individual passes.
Makes the compiler output stack usage information for the program, on a per-function basis. The filename for the dump is made by appending .su to the auxname. auxname is generated from the name of the output file, if explicitly specified and it is not an executable, otherwise it is the basename of the source file. An entry is made up of three fields:
static means that the function manipulates the stack
statically: a fixed number of bytes are allocated for the frame on function
entry and released on function exit; no stack adjustments are otherwise made
in the function. The second field is this fixed number of bytes.
dynamic means that the function manipulates the stack
dynamically: in addition to the static allocation described above, stack
adjustments are made in the body of the function, for example to push/pop
arguments around function calls. If the qualifier
bounded is also
present, the amount of these adjustments is bounded at compile time and
the second field is an upper bound of the total amount of stack used by
the function. If it is not present, the amount of these adjustments is
not bounded at compile time and the second field only represents the
Emit statistics about front-end processing at the end of the compilation. This option is supported only by the C++ front end, and the information is generally only useful to the G++ development team.
Print the name and the counter upper bound for all debug counters.
Set the internal debug counter lower and upper bound. counter-value-list
is a comma-separated list of name:lower_bound:upper_bound
tuples which sets the lower and the upper bound of each debug
counter name. The lower_bound is optional and is zero
initialized if not set.
All debug counters have the initial upper bound of
dbg_cnt returns true always unless the upper bound
is set by this option.
For example, with -fdbg-cnt=dce:2:4,tail_call:10,
dbg_cnt(dce) returns true only for third and fourth invocation.
dbg_cnt(tail_call) true is returned for first 10 invocations.
Print the full absolute name of the library file library that would be used when linking—and don’t do anything else. With this option, GCC does not compile or link anything; it just prints the file name.
Print the directory name corresponding to the multilib selected by any
other switches present in the command line. This directory is supposed
to exist in
Print the mapping from multilib directory names to compiler switches that enable them. The directory name is separated from the switches by ‘;’, and each switch starts with an ‘@’ instead of the ‘-’, without spaces between multiple switches. This is supposed to ease shell processing.
Print the path to OS libraries for the selected multilib, relative to some lib subdirectory. If OS libraries are present in the lib subdirectory and no multilibs are used, this is usually just ., if OS libraries are present in libsuffix sibling directories this prints e.g. ../lib64, ../lib or ../lib32, or if OS libraries are present in lib/subdir subdirectories it prints e.g. amd64, sparcv9 or ev6.
Print the path to OS libraries for the selected multiarch, relative to some lib subdirectory.
Like -print-file-name, but searches for a program such as
Same as -print-file-name=libgcc.a.
This is useful when you use -nostdlib or -nodefaultlibs but you do want to link with libgcc.a. You can do:
gcc -nostdlib files… `gcc -print-libgcc-file-name`
Print the name of the configured installation directory and a list of
program and library directories
gcc searches—and don’t do anything else.
This is useful when
gcc prints the error message
‘installation problem, cannot exec cpp0: No such file or directory’.
To resolve this you either need to put cpp0 and the other compiler
gcc expects to find them, or you can set the environment
GCC_EXEC_PREFIX to the directory where you installed them.
Don’t forget the trailing ‘/’.
See Environment Variables.
Print the target sysroot directory that is used during compilation. This is the target sysroot specified either at configure time or using the --sysroot option, possibly with an extra suffix that depends on compilation options. If no target sysroot is specified, the option prints nothing.
Print the suffix added to the target sysroot when searching for headers, or give an error if the compiler is not configured with such a suffix—and don’t do anything else.
Print the compiler’s target machine (for example, ‘i686-pc-linux-gnu’)—and don’t do anything else.
Print the compiler version (for example,
7)—and don’t do
anything else. This is the compiler version used in filesystem paths,
specs, can be depending on how the compiler has been configured just
a single number (major version), two numbers separated by dot (major and
minor version) or three numbers separated by dots (major, minor and patchlevel
Print the full compiler version, always 3 numbers separated by dots, major, minor and patchlevel version.
Print the compiler’s built-in specs—and don’t do anything else. (This is used when GCC itself is being built.) See Spec Files.