3.9 Options for Debugging Your Program or GCC

GCC has various special options that are used for debugging either your program or GCC:

-g

Produce debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF 2). GDB can work with this debugging information.

On most systems that use stabs format, -g enables use of extra debugging information that only GDB can use; this extra information makes debugging work better in GDB but probably makes other debuggers crash or refuse to read the program. If you want to control for certain whether to generate the extra information, use -gstabs+, -gstabs, -gxcoff+, -gxcoff, or -gvms (see below).

GCC allows you to use -g with -O. The shortcuts taken by optimized code may occasionally produce surprising results: some variables you declared may not exist at all; flow of control may briefly move where you did not expect it; some statements may not be executed because they compute constant results or their values are already at hand; some statements may execute in different places because they have been moved out of loops.

Nevertheless it proves possible to debug optimized output. This makes it reasonable to use the optimizer for programs that might have bugs.

The following options are useful when GCC is generated with the capability for more than one debugging format.

-gsplit-dwarf

Separate as much dwarf debugging information as possible into a separate output file with the extension .dwo. This option allows the build system to avoid linking files with debug information. To be useful, this option requires a debugger capable of reading .dwo files.

-ggdb

Produce debugging information for use by GDB. This means to use the most expressive format available (DWARF 2, stabs, or the native format if neither of those are supported), including GDB extensions if at all possible.

-gpubnames

Generate dwarf .debug_pubnames and .debug_pubtypes sections.

-ggnu-pubnames

Generate .debug_pubnames and .debug_pubtypes sections in a format suitable for conversion into a GDB index. This option is only useful with a linker that can produce GDB index version 7.

-gstabs

Produce debugging information in stabs format (if that is supported), without GDB extensions. This is the format used by DBX on most BSD systems. On MIPS, Alpha and System V Release 4 systems this option produces stabs debugging output that is not understood by DBX or SDB. On System V Release 4 systems this option requires the GNU assembler.

-feliminate-unused-debug-symbols

Produce debugging information in stabs format (if that is supported), for only symbols that are actually used.

-femit-class-debug-always

Instead of emitting debugging information for a C++ class in only one object file, emit it in all object files using the class. This option should be used only with debuggers that are unable to handle the way GCC normally emits debugging information for classes because using this option increases the size of debugging information by as much as a factor of two.

-fdebug-types-section

When using DWARF Version 4 or higher, type DIEs can be put into their own .debug_types section instead of making them part of the .debug_info section. It is more efficient to put them in a separate comdat sections since the linker can then remove duplicates. But not all DWARF consumers support .debug_types sections yet and on some objects .debug_types produces larger instead of smaller debugging information.

-gstabs+

Produce debugging information in stabs format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program.

-gcoff

Produce debugging information in COFF format (if that is supported). This is the format used by SDB on most System V systems prior to System V Release 4.

-gxcoff

Produce debugging information in XCOFF format (if that is supported). This is the format used by the DBX debugger on IBM RS/6000 systems.

-gxcoff+

Produce debugging information in XCOFF format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program, and may cause assemblers other than the GNU assembler (GAS) to fail with an error.

-gdwarf-version

Produce debugging information in DWARF format (if that is supported). The value of version may be either 2, 3 or 4; the default version for most targets is 4.

Note that with DWARF Version 2, some ports require and always use some non-conflicting DWARF 3 extensions in the unwind tables.

Version 4 may require GDB 7.0 and -fvar-tracking-assignments for maximum benefit.

-grecord-gcc-switches

This switch causes the command-line options used to invoke the compiler that may affect code generation to be appended to the DW_AT_producer attribute in DWARF debugging information. The options are concatenated with spaces separating them from each other and from the compiler version. See also -frecord-gcc-switches for another way of storing compiler options into the object file. This is the default.

-gno-record-gcc-switches

Disallow appending command-line options to the DW_AT_producer attribute in DWARF debugging information.

-gstrict-dwarf

Disallow using extensions of later DWARF standard version than selected with -gdwarf-version. On most targets using non-conflicting DWARF extensions from later standard versions is allowed.

-gno-strict-dwarf

Allow using extensions of later DWARF standard version than selected with -gdwarf-version.

-gvms

Produce debugging information in Alpha/VMS debug format (if that is supported). This is the format used by DEBUG on Alpha/VMS systems.

-glevel

-ggdblevel

-gstabslevel

-gcofflevel

-gxcofflevel

-gvmslevel

Request debugging information and also use level to specify how much information. The default level is 2.

Level 0 produces no debug information at all. Thus, -g0 negates -g.

Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug. This includes descriptions of functions and external variables, and line number tables, but no information about local variables.

Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use -g3.

-gdwarf-2 does not accept a concatenated debug level, because GCC used to support an option -gdwarf that meant to generate debug information in version 1 of the DWARF format (which is very different from version 2), and it would have been too confusing. That debug format is long obsolete, but the option cannot be changed now. Instead use an additional -glevel option to change the debug level for DWARF.

-gtoggle

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.

-fsanitize=address

Enable AddressSanitizer, a fast memory error detector. Memory access instructions will be instrumented to detect out-of-bounds and use-after-free bugs. See http://code.google.com/p/address-sanitizer/ for more details. The run-time behavior can be influenced using the ASAN_OPTIONS environment variable; see https://code.google.com/p/address-sanitizer/wiki/Flags#Run-time_flags for a list of supported options.

-fsanitize=kernel-address

Enable AddressSanitizer for Linux kernel. See http://code.google.com/p/address-sanitizer/wiki/AddressSanitizerForKernel for more details.

-fsanitize=thread

Enable ThreadSanitizer, a fast data race detector. Memory access instructions will be instrumented to detect data race bugs. See http://code.google.com/p/thread-sanitizer/ for more details. The run-time behavior can be influenced using the TSAN_OPTIONS environment variable; see https://code.google.com/p/thread-sanitizer/wiki/Flags for a list of supported options.

-fsanitize=leak

Enable LeakSanitizer, a memory leak detector. This option only matters for linking of executables and if neither -fsanitize=address nor -fsanitize=thread is used. In that case it will link the executable against a library that overrides malloc and other allocator functions. See https://code.google.com/p/address-sanitizer/wiki/LeakSanitizer for more details. The run-time behavior can be influenced using the LSAN_OPTIONS environment variable.

-fsanitize=undefined

Enable UndefinedBehaviorSanitizer, a fast undefined behavior detector. Various computations will be instrumented to detect undefined behavior at runtime. Current suboptions are:

-fsanitize=shift

This option enables checking that the result of a shift operation is not undefined. Note that what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc.

-fsanitize=integer-divide-by-zero

Detect integer division by zero as well as INT_MIN / -1 division.

-fsanitize=unreachable

With this option, the compiler will turn the __builtin_unreachable call into a diagnostics message call instead. When reaching the __builtin_unreachable call, the behavior is undefined.

-fsanitize=vla-bound

This option instructs the compiler to check that the size of a variable length array is positive. This option does not have any effect in -std=c++1y mode, as the standard requires the exception be thrown instead.

-fsanitize=null

This option enables pointer checking. Particularly, the application built with this option turned on will issue an error message when it tries to dereference a NULL pointer, or if a reference (possibly an rvalue reference) is bound to a NULL pointer.

-fsanitize=return

This option enables return statement checking. Programs built with this option turned on will issue an error message when the end of a non-void function is reached without actually returning a value. This option works in C++ only.

-fsanitize=signed-integer-overflow

This option enables signed integer overflow checking. We check that the result of +, *, and both unary and binary - does not overflow in the signed arithmetics. Note, integer promotion rules must be taken into account. That is, the following is not an overflow:

signed char a = SCHAR_MAX;
a++;

While -ftrapv causes traps for signed overflows to be emitted, -fsanitize=undefined gives a diagnostic message. This currently works only for the C family of languages.

-fdump-final-insns[=file]

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.

-fcompare-debug[=opts]

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 is used.

-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 warning, setting GCC_COMPARE_DEBUG to ‘-w%n-fcompare-debug not overridden’ will do.

-fcompare-debug-second

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 side-effect compiler outputs to files or to the standard output. 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.

-feliminate-dwarf2-dups

Compress DWARF 2 debugging information by eliminating duplicated information about each symbol. This option only makes sense when generating DWARF 2 debugging information with -gdwarf-2.

-femit-struct-debug-baseonly

Emit debug information for struct-like types only when the base name of the compilation source file matches the base name of file in which the struct is defined.

This option substantially reduces the size of debugging information, but at significant potential loss in type information to the debugger. See -femit-struct-debug-reduced for a less aggressive option. See -femit-struct-debug-detailed for more detailed control.

This option works only with DWARF 2.

-femit-struct-debug-reduced

Emit debug information for struct-like types only when the base name of the compilation source file matches the base name of file in which the type is defined, unless the struct is a template or defined in a system header.

This option significantly reduces the size of debugging information, with some potential loss in type information to the debugger. See -femit-struct-debug-baseonly for a more aggressive option. See -femit-struct-debug-detailed for more detailed control.

This option works only with DWARF 2.

-femit-struct-debug-detailed[=spec-list]

Specify the struct-like types for which the compiler generates debug information. The intent is to reduce duplicate struct debug information between different object files within the same program.

This option is a detailed version of -femit-struct-debug-reduced and -femit-struct-debug-baseonly, which serves for most needs.

A specification has the syntax [‘dir:’|‘ind:’][‘ord:’|‘gen:’](‘any’|‘sys’|‘base’|‘none’)

The optional first word limits the specification to structs that are used directly (‘dir:’) or used indirectly (‘ind:’). A struct type is used directly when it is the type of a variable, member. Indirect uses arise through pointers to structs. That is, when use of an incomplete struct is valid, the use is indirect. An example is ‘struct one direct; struct two * indirect;’.

The optional second word limits the specification to ordinary structs (‘ord:’) or generic structs (‘gen:’). Generic structs are a bit complicated to explain. For C++, these are non-explicit specializations of template classes, or non-template classes within the above. Other programming languages have generics, but -femit-struct-debug-detailed does not yet implement them.

The third word specifies the source files for those structs for which the compiler should emit debug information. The values ‘none’ and ‘any’ have the normal meaning. The value ‘base’ means that the base of name of the file in which the type declaration appears must match the base of the name of the main compilation file. In practice, this means that when compiling foo.c, debug information is generated for types declared in that file and foo.h, but not other header files. The value ‘sys’ means those types satisfying ‘base’ or declared in system or compiler headers.

You may need to experiment to determine the best settings for your application.

The default is -femit-struct-debug-detailed=all.

This option works only with DWARF 2.

-fno-merge-debug-strings

Direct the linker to not merge together strings in the debugging information that are identical in different object files. Merging is not supported by all assemblers or linkers. Merging decreases the size of the debug information in the output file at the cost of increasing link processing time. Merging is enabled by default.

-fdebug-prefix-map=old=new

When compiling files in directory old, record debugging information describing them as in new instead.

-fno-dwarf2-cfi-asm

Emit DWARF 2 unwind info as compiler generated .eh_frame section instead of using GAS .cfi_* directives.

-p

Generate extra code to write profile information suitable for the analysis program prof. You must use this option when compiling the source files you want data about, and you must also use it when linking.

-pg

Generate extra code to write profile information suitable for the analysis program gprof. You must use this option when compiling the source files you want data about, and you must also use it when linking.

-Q

Makes the compiler print out each function name as it is compiled, and print some statistics about each pass when it finishes.

-ftime-report

Makes the compiler print some statistics about the time consumed by each pass when it finishes.

-fmem-report

Makes the compiler print some statistics about permanent memory allocation when it finishes.

-fmem-report-wpa

Makes the compiler print some statistics about permanent memory allocation for the WPA phase only.

-fpre-ipa-mem-report

-fpost-ipa-mem-report

Makes the compiler print some statistics about permanent memory allocation before or after interprocedural optimization.

-fprofile-report

Makes the compiler print some statistics about consistency of the (estimated) profile and effect of individual passes.

-fstack-usage

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:

• The name of the function.
• A number of bytes.
• One or more qualifiers: static, dynamic, bounded.

The qualifier 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.

The qualifier 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 bounded part.

-fprofile-arcs

Add code so that program flow arcs are instrumented. During execution the program records how many times each branch and call is executed and how many times it is taken or returns. When the compiled program exits it saves this data to a file called auxname.gcda for each source file. The data may be used for profile-directed optimizations (-fbranch-probabilities), or for test coverage analysis (-ftest-coverage). Each object file's auxname is generated from the name of the output file, if explicitly specified and it is not the final executable, otherwise it is the basename of the source file. In both cases any suffix is removed (e.g. foo.gcda for input file dir/foo.c, or dir/foo.gcda for output file specified as -o dir/foo.o). See Cross-profiling.

--coverage

This option is used to compile and link code instrumented for coverage analysis. The option is a synonym for -fprofile-arcs -ftest-coverage (when compiling) and -lgcov (when linking). See the documentation for those options for more details.

• Compile the source files with -fprofile-arcs plus optimization and code generation options. For test coverage analysis, use the additional -ftest-coverage option. You do not need to profile every source file in a program.
• Link your object files with -lgcov or -fprofile-arcs (the latter implies the former).
• Run the program on a representative workload to generate the arc profile information. This may be repeated any number of times. You can run concurrent instances of your program, and provided that the file system supports locking, the data files will be correctly updated. Also fork calls are detected and correctly handled (double counting will not happen).
• For profile-directed optimizations, compile the source files again with the same optimization and code generation options plus -fbranch-probabilities (see Options that Control Optimization).
• For test coverage analysis, use gcov to produce human readable information from the .gcno and .gcda files. Refer to the gcov documentation for further information.

With -fprofile-arcs, for each function of your program GCC creates a program flow graph, then finds a spanning tree for the graph. Only arcs that are not on the spanning tree have to be instrumented: the compiler adds code to count the number of times that these arcs are executed. When an arc is the only exit or only entrance to a block, the instrumentation code can be added to the block; otherwise, a new basic block must be created to hold the instrumentation code.

-ftest-coverage

Produce a notes file that the gcov code-coverage utility (see gcov—a Test Coverage Program) can use to show program coverage. Each source file's note file is called auxname.gcno. Refer to the -fprofile-arcs option above for a description of auxname and instructions on how to generate test coverage data. Coverage data matches the source files more closely if you do not optimize.

-fdbg-cnt-list

Print the name and the counter upper bound for all debug counters.

-fdbg-cnt=counter-value-list

Set the internal debug counter upper bound. counter-value-list is a comma-separated list of name:value pairs which sets the upper bound of each debug counter name to value. All debug counters have the initial upper bound of UINT_MAX; thus dbg_cnt() returns true always unless the upper bound is set by this option. For example, with -fdbg-cnt=dce:10,tail_call:0, dbg_cnt(dce) returns true only for first 10 invocations.

-fenable-kind-pass

-fdisable-kind-pass=range-list

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.

-fdisable-ipa-pass

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.

-fdisable-rtl-pass

-fdisable-rtl-pass=range-list

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.

-fdisable-tree-pass

-fdisable-tree-pass=range-list

Disable tree pass pass. See -fdisable-rtl for the description of option arguments.

-fenable-ipa-pass

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.

-fenable-rtl-pass

-fenable-rtl-pass=range-list

Enable RTL pass pass. See -fdisable-rtl for option argument description and examples.

-fenable-tree-pass

-fenable-tree-pass=range-list

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

-dletters

-fdump-rtl-pass

-fdump-rtl-pass=filename

Says to make debugging dumps during compilation at times specified by letters. This is used for debugging the RTL-based passes of the compiler. The file names for most of the dumps are made by appending a pass number and a word to the dumpname, and the files are created in the directory of the output file. In case of =filename option, the dump is output on the given file instead of the pass numbered dump files. Note that the pass number is computed statically as passes get registered into the pass manager. Thus the numbering is not related to the dynamic order of execution of passes. In particular, a pass installed by a plugin could have a number over 200 even if it executed quite early. dumpname 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. These switches may have different effects when -E is used for preprocessing.

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:

-fdump-rtl-alignments

Dump after branch alignments have been computed.

-fdump-rtl-asmcons

Dump after fixing rtl statements that have unsatisfied in/out constraints.

-fdump-rtl-auto_inc_dec

Dump after auto-inc-dec discovery. This pass is only run on architectures that have auto inc or auto dec instructions.

-fdump-rtl-barriers

Dump after cleaning up the barrier instructions.

-fdump-rtl-bbpart

Dump after partitioning hot and cold basic blocks.

-fdump-rtl-bbro

Dump after block reordering.

-fdump-rtl-btl1

-fdump-rtl-btl2

-fdump-rtl-btl1 and -fdump-rtl-btl2 enable dumping after the two branch target load optimization passes.

-fdump-rtl-bypass

Dump after jump bypassing and control flow optimizations.

-fdump-rtl-combine

Dump after the RTL instruction combination pass.

-fdump-rtl-compgotos

Dump after duplicating the computed gotos.

-fdump-rtl-ce1

-fdump-rtl-ce2

-fdump-rtl-ce3

-fdump-rtl-ce1, -fdump-rtl-ce2, and -fdump-rtl-ce3 enable dumping after the three if conversion passes.

-fdump-rtl-cprop_hardreg

Dump after hard register copy propagation.

-fdump-rtl-csa

Dump after combining stack adjustments.

-fdump-rtl-cse1

-fdump-rtl-cse2

-fdump-rtl-cse1 and -fdump-rtl-cse2 enable dumping after the two common subexpression elimination passes.

-fdump-rtl-dce

Dump after the standalone dead code elimination passes.

-fdump-rtl-dbr

Dump after delayed branch scheduling.

-fdump-rtl-dce1

-fdump-rtl-dce2

-fdump-rtl-dce1 and -fdump-rtl-dce2 enable dumping after the two dead store elimination passes.

-fdump-rtl-eh

Dump after finalization of EH handling code.

-fdump-rtl-eh_ranges

Dump after conversion of EH handling range regions.

-fdump-rtl-expand

Dump after RTL generation.

-fdump-rtl-fwprop1

-fdump-rtl-fwprop2

-fdump-rtl-fwprop1 and -fdump-rtl-fwprop2 enable dumping after the two forward propagation passes.

-fdump-rtl-gcse1

-fdump-rtl-gcse2

-fdump-rtl-gcse1 and -fdump-rtl-gcse2 enable dumping after global common subexpression elimination.

-fdump-rtl-init-regs

Dump after the initialization of the registers.

-fdump-rtl-initvals

Dump after the computation of the initial value sets.

-fdump-rtl-into_cfglayout

Dump after converting to cfglayout mode.

-fdump-rtl-ira

Dump after iterated register allocation.

-fdump-rtl-jump

Dump after the second jump optimization.

-fdump-rtl-loop2

-fdump-rtl-loop2 enables dumping after the rtl loop optimization passes.

-fdump-rtl-mach

Dump after performing the machine dependent reorganization pass, if that pass exists.

-fdump-rtl-mode_sw

Dump after removing redundant mode switches.

-fdump-rtl-rnreg

Dump after register renumbering.

-fdump-rtl-outof_cfglayout

Dump after converting from cfglayout mode.

-fdump-rtl-peephole2

Dump after the peephole pass.

-fdump-rtl-postreload

Dump after post-reload optimizations.

-fdump-rtl-pro_and_epilogue

Dump after generating the function prologues and epilogues.

-fdump-rtl-sched1

-fdump-rtl-sched2

-fdump-rtl-sched1 and -fdump-rtl-sched2 enable dumping after the basic block scheduling passes.

-fdump-rtl-ree

Dump after sign/zero extension elimination.

-fdump-rtl-seqabstr

Dump after common sequence discovery.

-fdump-rtl-shorten

Dump after shortening branches.

-fdump-rtl-sibling

Dump after sibling call optimizations.

-fdump-rtl-split1

-fdump-rtl-split2

-fdump-rtl-split3

-fdump-rtl-split4

-fdump-rtl-split5

-fdump-rtl-split1, -fdump-rtl-split2, -fdump-rtl-split3, -fdump-rtl-split4 and -fdump-rtl-split5 enable dumping after five rounds of instruction splitting.

-fdump-rtl-sms

Dump after modulo scheduling. This pass is only run on some architectures.

-fdump-rtl-stack

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

-fdump-rtl-subreg2

-fdump-rtl-subreg1 and -fdump-rtl-subreg2 enable dumping after the two subreg expansion passes.

-fdump-rtl-unshare

Dump after all rtl has been unshared.

-fdump-rtl-vartrack

Dump after variable tracking.

-fdump-rtl-vregs

Dump after converting virtual registers to hard registers.

-fdump-rtl-web

Dump after live range splitting.

-fdump-rtl-regclass

-fdump-rtl-subregs_of_mode_init

-fdump-rtl-subregs_of_mode_finish

-fdump-rtl-dfinit

-fdump-rtl-dfinish

These dumps are defined but always produce empty files.

-da

-fdump-rtl-all

Produce all the dumps listed above.

-dA

Annotate the assembler output with miscellaneous debugging information.

-dD

Dump all macro definitions, at the end of preprocessing, in addition to normal output.

-dH

Produce a core dump whenever an error occurs.

-dp

Annotate the assembler output with a comment indicating which pattern and alternative is used. The length of each instruction is also printed.

-dP

Dump the RTL in the assembler output as a comment before each instruction. Also turns on -dp annotation.

-dx

Just generate RTL for a function instead of compiling it. Usually used with -fdump-rtl-expand.

-fdump-noaddr

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.

-fdump-unnumbered

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.

-fdump-unnumbered-links

When doing debugging dumps (see -d option above), suppress instruction numbers for the links to the previous and next instructions in a sequence.

-fdump-translation-unit (C++ only)

-fdump-translation-unit-options (C++ only)

Dump a representation of the tree structure for the entire translation unit to a file. The file name is made by appending .tu to the source file name, and the file is created in the same directory as the output file. If the ‘-options’ form is used, options controls the details of the dump as described for the -fdump-tree options.

-fdump-class-hierarchy (C++ only)

-fdump-class-hierarchy-options (C++ only)

Dump a representation of each class's hierarchy and virtual function table layout to a file. The file name is made by appending .class to the source file name, and the file is created in the same directory as the output file. If the ‘-options’ form is used, options controls the details of the dump as described for the -fdump-tree options.

-fdump-ipa-switch

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:

‘all’

Enables all inter-procedural analysis dumps.

‘cgraph’

Dumps information about call-graph optimization, unused function removal, and inlining decisions.

‘inline’

Dump after function inlining.

-fdump-passes

Dump the list of optimization passes that are turned on and off by the current command-line options.

-fdump-statistics-option

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.

-fdump-tree-switch

-fdump-tree-switch-options

-fdump-tree-switch-options=filename

Control the dumping at various stages of processing the intermediate 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. In case of =filename option, the dump is output on the given file instead of the auto named dump files. 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

‘address’

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.

‘asmname’

If 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.

‘slim’

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.

‘raw’

Print a raw representation of the tree. By default, trees are pretty-printed into a C-like representation.

‘details’

Enable more detailed dumps (not honored by every dump option). Also include information from the optimization passes.

‘stats’

Enable dumping various statistics about the pass (not honored by every dump option).

‘blocks’

Enable showing basic block boundaries (disabled in raw dumps).

‘graph’

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.

‘vops’

Enable showing virtual operands for every statement.

‘lineno’

Enable showing line numbers for statements.

‘uid’

Enable showing the unique ID (DECL_UID) for each variable.

‘verbose’

Enable showing the tree dump for each statement.

‘eh’

Enable showing the EH region number holding each statement.

‘scev’

Enable showing scalar evolution analysis details.

‘optimized’

Enable showing optimization information (only available in certain passes).

‘missed’

Enable showing missed optimization information (only available in certain passes).

‘notes’

Enable other detailed optimization information (only available in certain passes).

‘=filename’

Instead of an auto named dump file, output into the given file name. The file names stdout and stderr are treated specially and are considered already open standard streams. For example,

gcc -O2 -ftree-vectorize -fdump-tree-vect-blocks=foo.dump
     -fdump-tree-pre=stderr file.c

outputs vectorizer dump into foo.dump, while the PRE dump is output on to stderr. If two conflicting dump filenames are given for the same pass, then the latter option overrides the earlier one.

‘all’

Turn on all options, except raw, slim, verbose and lineno.

‘optall’

Turn on all optimization options, i.e., optimized, missed, and note.

The following tree dumps are possible:

‘original’

Dump before any tree based optimization, to file.original.

‘optimized’

Dump after all tree based optimization, to file.optimized.

‘gimple’

Dump each function before and after the gimplification pass to a file. The file name is made by appending .gimple to the source file name.

‘cfg’

Dump the control flow graph of each function to a file. The file name is made by appending .cfg to the source file name.

‘ch’

Dump each function after copying loop headers. The file name is made by appending .ch to the source file name.

‘ssa’

Dump SSA related information to a file. The file name is made by appending .ssa to the source file name.

‘alias’

Dump aliasing information for each function. The file name is made by appending .alias to the source file name.

‘ccp’

Dump each function after CCP. The file name is made by appending .ccp to the source file name.

‘storeccp’

Dump each function after STORE-CCP. The file name is made by appending .storeccp to the source file name.

‘pre’

Dump trees after partial redundancy elimination. The file name is made by appending .pre to the source file name.

‘fre’

Dump trees after full redundancy elimination. The file name is made by appending .fre to the source file name.

‘copyprop’

Dump trees after copy propagation. The file name is made by appending .copyprop to the source file name.

‘store_copyprop’

Dump trees after store copy-propagation. The file name is made by appending .store_copyprop to the source file name.

‘dce’

Dump each function after dead code elimination. The file name is made by appending .dce to the source file name.

‘sra’

Dump each function after performing scalar replacement of aggregates. The file name is made by appending .sra to the source file name.

‘sink’

Dump each function after performing code sinking. The file name is made by appending .sink to the source file name.

‘dom’

Dump each function after applying dominator tree optimizations. The file name is made by appending .dom to the source file name.

‘dse’

Dump each function after applying dead store elimination. The file name is made by appending .dse to the source file name.

‘phiopt’

Dump each function after optimizing PHI nodes into straightline code. The file name is made by appending .phiopt to the source file name.

‘forwprop’

Dump each function after forward propagating single use variables. The file name is made by appending .forwprop to the source file name.

‘copyrename’

Dump each function after applying the copy rename optimization. The file name is made by appending .copyrename to the source file name.

‘nrv’

Dump each function after applying the named return value optimization on generic trees. The file name is made by appending .nrv to the source file name.

‘vect’

Dump each function after applying vectorization of loops. The file name is made by appending .vect to the source file name.

‘slp’

Dump each function after applying vectorization of basic blocks. The file name is made by appending .slp to the source file name.

‘vrp’

Dump each function after Value Range Propagation (VRP). The file name is made by appending .vrp to the source file name.

‘all’

Enable all the available tree dumps with the flags provided in this option.

-fopt-info

-fopt-info-options

-fopt-info-options=filename

Controls optimization dumps from various optimization passes. If the ‘-options’ form is used, options is a list of ‘-’ separated options to select the dump details and optimizations. If options is not specified, it defaults to optimized for details and optall for optimization groups. If the filename is not specified, it defaults to stderr. Note that the output filename will be overwritten in case of multiple translation units. If a combined output from multiple translation units is desired, stderr should be used instead.

The options can be divided into two groups, 1) options describing the verbosity of the dump, and 2) 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 latter options override the earlier options on the command line. Though multiple -fopt-info options are accepted, only one of them can have =filename. If other filenames are provided then all but the first one are ignored.

The dump verbosity has the following options

‘optimized’

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 got successfully vectorized.

‘missed’

Print information about missed optimizations. Individual passes control which information to include in the output. For example,

gcc -O2 -ftree-vectorize -fopt-info-vec-missed

will print information about missed optimization opportunities from vectorization passes on stderr.

‘note’

Print verbose information about optimizations, such as certain transformations, more detailed messages about decisions etc.

‘all’

Print detailed optimization information. This includes optimized, missed, and note.

The second set of options describes a group of optimizations and may include one or more of the following.

‘ipa’

Enable dumps from all interprocedural optimizations.

‘loop’

Enable dumps from all loop optimizations.

‘inline’

Enable dumps from all inlining optimizations.

‘vec’

Enable dumps from all vectorization optimizations.

‘optall’

Enable dumps from all optimizations. This is a superset of the optimization groups listed above.

For example,

gcc -O3 -fopt-info-missed=missed.all

outputs missed optimization report from all the passes into missed.all.

As another example,

gcc -O3 -fopt-info-inline-optimized-missed=inline.txt

will output information about missed optimizations as well as optimized locations from all the inlining passes into inline.txt.

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. If options is omitted, it defaults to all-optall, which means dump all available optimization info from all the passes. In the following example, all optimization info is output on to stderr.

gcc -O3 -fopt-info

Note that -fopt-info-vec-missed behaves the same as -fopt-info-missed-vec.

As another example, consider

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 the vec.miss is produced which contains dumps from the vectorizer about missed opportunities.

-frandom-seed=string

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 should be different for every file you compile.

-fsched-verbose=n

On targets that use instruction scheduling, this option controls the amount of debugging output the scheduler prints. This information is written to standard error, unless -fdump-rtl-sched1 or -fdump-rtl-sched2 is specified, in which case it is output to the usual dump listing file, .sched1 or .sched2 respectively. However for n greater than nine, the output is always printed to standard error.

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.

-save-temps

-save-temps=cwd

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.

-save-temps=obj

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.

For example:

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.

-time[=file]

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.

-fvar-tracking

Run variable tracking pass. It computes where variables are stored at each position in code. Better debugging information is then generated (if the debugging information format supports this information).

It is enabled by default when compiling with optimization (-Os, -O, -O2, ...), debugging information (-g) and the debug info format supports it.

-fvar-tracking-assignments

Annotate assignments to user variables early in the compilation and attempt to carry the annotations over throughout the compilation all the way to the end, in an attempt to improve debug information while optimizing. Use of -gdwarf-4 is recommended along with it.

It can be enabled even if var-tracking is disabled, in which case annotations are created and maintained, but discarded at the end.

-fvar-tracking-assignments-toggle

Toggle -fvar-tracking-assignments, in the same way that -gtoggle toggles -g.

-print-file-name=library

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-multi-directory

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 GCC_EXEC_PREFIX.

-print-multi-lib

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-multi-os-directory

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-multiarch

Print the path to OS libraries for the selected multiarch, relative to some lib subdirectory.

-print-prog-name=program

Like -print-file-name, but searches for a program such as ‘cpp’.

-print-libgcc-file-name

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-search-dirs

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 components where gcc expects to find them, or you can set the environment variable GCC_EXEC_PREFIX to the directory where you installed them. Don't forget the trailing ‘/’. See Environment Variables.

-print-sysroot

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-sysroot-headers-suffix

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.

-dumpmachine

Print the compiler's target machine (for example, ‘i686-pc-linux-gnu’)—and don't do anything else.

-dumpversion

Print the compiler version (for example, ‘3.0’)—and don't do anything else.

-dumpspecs

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.

-fno-eliminate-unused-debug-types

Normally, when producing DWARF 2 output, GCC avoids producing debug symbol output for types that are nowhere used in the source file being compiled. Sometimes it is useful to have GCC emit debugging information for all types declared in a compilation unit, regardless of whether or not they are actually used in that compilation unit, for example if, in the debugger, you want to cast a value to a type that is not actually used in your program (but is declared). More often, however, this results in a significant amount of wasted space.