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3.9 Options for Debugging Your Program or GCC
GCC has various special options that are used for debugging either your program or GCC:
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.
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.
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.
Generate dwarf .debug_pubnames and .debug_pubtypes sections.
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.
Produce debugging information in stabs format (if that is supported), for only symbols that are actually used.
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.
When using DWARF Version 4 or higher, type DIEs can be put into their own
.debug_typessection instead of making them part of the
.debug_infosection. 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_typessections yet and on some objects
.debug_typesproduces larger instead of smaller debugging information.
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.
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.
Produce debugging information in XCOFF format (if that is supported). This is the format used by the DBX debugger on IBM RS/6000 systems.
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.
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.
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.
Disallow appending command-line options to the DW_AT_producer attribute in DWARF debugging information.
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.
Allow using extensions of later DWARF standard version than selected with ‘-gdwarf-version’.
Produce debugging information in Alpha/VMS debug format (if that is supported). This is the format used by DEBUG on Alpha/VMS systems.
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, but no information about local variables and no line numbers.
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.
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’.
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.
Enable ThreadSanitizer, a fast data race detector. Memory access instructions will be instrumented to detect data race bugs. See http://code.google.com/p/data-race-test/wiki/ThreadSanitizer for more details.
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
.gkdto 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_DEBUGis 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_DEBUGfrom taking effect.
To verify full coverage during ‘-fcompare-debug’ testing, set
GCC_COMPARE_DEBUGto 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_DEBUGto ‘-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 side-effect compiler outputs to files or to the standard output. Dump files and preserved temporary files are renamed so as to contain the
.gkadditional 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.
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’.
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.
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.
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
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.
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.
When compiling files in directory ‘old’, record debugging information describing them as in ‘new’ instead.
Emit DWARF 2 unwind info as compiler generated
.eh_framesection instead of using GAS
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.
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.
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.
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:
- The name of the function.
- A number of bytes.
One or more qualifiers:
staticmeans 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.
dynamicmeans 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
boundedis 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.
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 section Data file relocation to support cross-profiling.
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
forkcalls 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 section Options that Control Optimization).
For test coverage analysis, use
gcovto produce human readable information from the ‘.gcno’ and ‘.gcda’ files. Refer to the
gcovdocumentation 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.
Produce a notes file that the
gcovcode-coverage utility (see section
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.
Print the name and the counter upper bound for all debug counters.
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
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.
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
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:
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.
Dump after the register move pass.
‘-fdump-rtl-sched1’ and ‘-fdump-rtl-sched2’ enable dumping after the basic block scheduling passes.
Dump after sign extension elimination.
Dump after common sequence discovery.
Dump after shortening branches.
Dump after sibling call optimizations.
‘-fdump-rtl-split1’, ‘-fdump-rtl-split2’, ‘-fdump-rtl-split3’, ‘-fdump-rtl-split4’ and ‘-fdump-rtl-split5’ 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 of each instruction is 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.
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.
-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.
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.
Dump 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. 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
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_NAMEhas 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).
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.
Turn on all options, except ‘raw’, ‘slim’, ‘verbose’ and ‘lineno’.
Turn on all optimization options, i.e., ‘optimized’, ‘missed’, and ‘note’.
The following tree dumps are possible:
Dump before any tree based optimization, to ‘file.original’.
Dump after all tree based optimization, to ‘file.optimized’.
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.
Dump the control flow graph of each function to a file. The file name is made by appending ‘.cfg’ to the source file name.
Dump each function after copying loop headers. The file name is made by appending ‘.ch’ to the source file name.
Dump SSA related information to a file. The file name is made by appending ‘.ssa’ to the source file name.
Dump aliasing information for each function. The file name is made by appending ‘.alias’ to the source file name.
Dump each function after CCP. The file name is made by appending ‘.ccp’ to the source file name.
Dump each function after STORE-CCP. The file name is made by appending ‘.storeccp’ to the source file name.
Dump trees after partial redundancy elimination. The file name is made by appending ‘.pre’ to the source file name.
Dump trees after full redundancy elimination. The file name is made by appending ‘.fre’ to the source file name.
Dump trees after copy propagation. The file name is made by appending ‘.copyprop’ to the source file name.
Dump trees after store copy-propagation. The file name is made by appending ‘.store_copyprop’ to the source file name.
Dump each function after dead code elimination. The file name is made by appending ‘.dce’ to the source file name.
Dump each function after adding mudflap instrumentation. The file name is made by appending ‘.mudflap’ to the source file name.
Dump each function after performing scalar replacement of aggregates. The file name is made by appending ‘.sra’ to the source file name.
Dump each function after performing code sinking. The file name is made by appending ‘.sink’ to the source file name.
Dump each function after applying dominator tree optimizations. The file name is made by appending ‘.dom’ to the source file name.
Dump each function after applying dead store elimination. The file name is made by appending ‘.dse’ to the source file name.
Dump each function after optimizing PHI nodes into straightline code. The file name is made by appending ‘.phiopt’ to the source file name.
Dump each function after forward propagating single use variables. The file name is made by appending ‘.forwprop’ to the source file name.
Dump each function after applying the copy rename optimization. The file name is made by appending ‘.copyrename’ to the source file name.
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.
Dump each function after applying vectorization of loops. The file name is made by appending ‘.vect’ to the source file name.
Dump each function after applying vectorization of basic blocks. The file name is made by appending ‘.slp’ to the source file name.
Dump each function after Value Range Propagation (VRP). The file name is made by appending ‘.vrp’ to the source file name.
Enable all the available tree dumps with the flags provided in this option.
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 ‘all’ 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
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.
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.
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.
The second set of options describes a group of optimizations and may include one or more of the following.
Enable dumps from all interprocedural optimizations.
Enable dumps from all loop optimizations.
Enable dumps from all inlining optimizations.
Enable dumps from all vectorization optimizations.
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 cotaints dumps from the vectorizer about missed opportunities.
This option is deprecated and is implemented in terms of ‘-fopt-info’. Please use ‘-fopt-info-kind’ form instead, where kind is one of the valid opt-info options. It prints additional optimization information. For n=0 no diagnostic information is reported. If n=1 the vectorizer reports each loop that got vectorized, and the total number of loops that got vectorized. If n=2 the vectorizer reports locations which could not be vectorized and the reasons for those. For any higher verbosity levels all the analysis and transformation information from the vectorizer is reported.
Note that the information output by ‘-ftree-vectorizer-verbose’ option is sent to ‘stderr’. If the equivalent form ‘-fopt-info-options=filename’ is used then the output is sent into filename instead.
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.
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.
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.
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.
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.
Toggle ‘-fvar-tracking-assignments’, in the same way that ‘-gtoggle’ toggles ‘-g’.
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 ‘cpp’.
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
gccsearches—and don’t do anything else.
This is useful when
gccprints 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
gccexpects to find them, or you can set the environment variable
GCC_EXEC_PREFIXto the directory where you installed them. Don’t forget the trailing ‘/’. See section Environment Variables Affecting GCC.
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, ‘3.0’)—and don’t do anything else.
Print the compiler’s built-in specs—and don’t do anything else. (This is used when GCC itself is being built.) See section Specifying subprocesses and the switches to pass to them.
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.
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