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Options
for debugging your program or GNU CC
GNU CC has the following
special options that are used for debugging either your program or GCC.
-
-g
Produce debugging information
in the operating systems native format (stabs, COFF, XCOFF, or DWARF).
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 will probably
make 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,
-gdwarf+1,
or -gdwarf-1
(see the explanations for each option in the following discussion).
-
Unlike most other C compilers,
GNU CC 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 were already
at hand; some statements may execute in different places because they were
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 GNU CC is generated with the capability for more than one
debugging format.
-
-ggdb
Produce debugging information
in the native format (if that is supported), including GDB extensions if
at all possible.
-
-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 which is not understood by DBX or SDB. On System V Release
4 systems this option requires the GNU assembler.
-
-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
Produce debugging
information in DWARF version 1 format (if that is supported). This is the
format used by SDB on most System V Release 4 systems.
-
-gdwarf+
Produce debugging information
in DWARF version 1 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.
-
-gdwarf-2
Produce debugging information
in DWARF version 2 format (if that is supported). This is the format used
by DBX on IRIX 6.
-
-glevel
-
-ggdblevel
-
-gstabslevel
-
-gcofflevel
-
-gxcofflevel
-
-gdwarflevel
-
-gdwarf-2level
Request debugging information
and also use level
to specify how much information. The default level
is 2.
Level 1 produces minimal
information, enough for making backtraces in parts of the program that
you dont 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.
-
-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.
-
-a
Generate extra code to write
profile information for basic blocks, which will record the number of times
each basic block is executed, the basic block start address, and the function
name containing the basic block. If -g
is used, the line number and filename of the start of the basic block will
also be recorded. If not overridden by the machine description, the default
action is to append to the text file bb.out.
This data could be analyzed
by a program like tcov.
However, the format of the data is not what tcov
expects. Eventually GNU gprof
should be extended to process this data.
-
-Q
Makes the compiler
print out each function name as it is compiled, and print some statistics
about each pass when it finishes.
-
-ax
Generate extra code to profile
basic blocks. Your executable will produce output that is a superset of
that produced when -a
is used. Additional output is the source and target address of the basic
blocks where a jump takes place, the number of times a jump is executed,
and (optionally) the complete sequence of basic blocks being executed.
The output is appended to file bb.out.
-
You can examine different profiling
aspects without recompilation. Your executable will read a list of function
names from file bb.in.
Profiling starts when a function on the list is entered and stops when
that invocation is exited. To exclude a function from profiling, use a
hyphen (-),
to prefix its name.
-
If a function name is not unique,
specify its location, for example, /path/filename.d:functionname.
Your executable will write the available paths and filenames in file bb.out.
-
The following function names
have a special meaning.
-
__bb_jumps__
Write source, target and
frequency of jumps to file bb.out.
-
__bb_hidecall__
Exclude function calls from
frequency count.
-
__bb_showret__
Include function returns
in frequency count.
-
__bb_trace__
Write the sequence of basic
blocks to file bbtrace.gz.
The file will be compressed using the program gzip,
which must exist in your PATH.
On systems without the popen
function, the file will be named bbtrace
and will not be compressed.
-
Profiling for even a few
seconds on these systems will produce a very large file.
-
Note:
__bb_hidecall__
and __bb_showret__
will not affect the sequence written to bbtrace.gz.
-
What follows is a short example
using different profiling parameters in file bb.in.
Assume function foo
consists of basic blocks 1 and 2 and is called twice from block 3 of function
main.
After the calls, block 3 transfers control to block 4 of main.
-
With __bb_trace__
and main
contained in file bb.in,
the following sequence of blocks is written to file bbtrace.gz:
0 3 1 2 1 2 4. The return from block 2 to block 3 is not shown, because
the return is to a point inside the block and not to the top. The block
address 0 always indicates that control is transferred to the trace from
somewhere outside the observed functions. With -foo
added to bb.in,
the blocks of function foo
are removed from the trace, so only 0 3 4 remains.
-
With __bb_jumps__
and main
contained in file bb.in,
jump frequencies will be written to file b.out.
The frequencies are obtained by constructing a trace of blocks and incrementing
a counter for every neighboring pair of blocks in the trace. The trace
0 3 1 2 1 2 4 displays the following frequencies.
Jump from block 0x0 to block 0x3 executed 1 time(s)
Jump from block 0x3 to block 0x1 executed 1 time(s)
Jump from block 0x1 to block 0x2 executed 2 time(s)
Jump from block 0x2 to block 0x1 executed 1 time(s)
Jump from block 0x2 to block 0x4 executed 1 time(s)
With __bb_hidecall__,
due to call instructions, control transfer is removed from the trace; that
is, the trace is cut into
three parts: 0 3 4, 0 1 2, and
0 1 2. With __bb_showret__,
control transfer is added to the trace. The trace becomes: 0
3 1 2 3 1 2 3 4.
-
Note:
The previous trace is not
the same as the sequence written to bbtrace.gz.
It is solely used for counting jump frequencies.
-fprofile-arcs
Instrument arcs during
compilation. For each function of your program, GNU CC 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.
-
Since not every arc in the program
must be instrumented, programs compiled with this option run faster than
programs compiled with -a,
which adds instrumentation code to every basic block in the program. The
tradeoff: since gcov
does not have execution counts for all branches, it must start with the
execution counts for the instrumented branches, and then iterate over the
program flow graph until the entire graph has been solved. Hence, gcov
runs a little more slowly than a program which uses information from -a.
-fprofile-arcs
also makes it possible to estimate branch probabilities, and to calculate
basic block execution counts. In general, basic block execution counts
do not give enough information to estimate all branch probabilities. When
the compiled program exits, it saves the arc execution counts to a file
called sourcename.da.
Use the compiler option, -fbranch-probabilities,
when recompiling, to optimize using estimated branch probabilities (see
Options that control
optimization).
-ftest-coverage
Create data files for the
gcov
code-coverage utility (see gcov:
a Test Coverage Program).
The data filenames begin with the name of your source file:
-
sourcename.bb
A mapping from basic blocks
to line numbers, which gcov
uses to associate basic block execution counts with line numbers.
-
sourcename.bbg
A list of all arcs in the
program flow graph. This allows gcov
to reconstruct the program flow graph, so that it can compute all basic
block and arc execution counts from the information in the sourcename.da
file (this last file is the output from -fprofile-arcs).
-Q
Makes the compiler
print out each function name as it is compiled, and print some statistics
about each pass when it finishes.
-dletters
This option says to
make debugging dumps during compilation at times specified by letters.
This is used for debugging the compiler. The filenames for most of the
dumps are made by appending a word to the source filename (e.g., foo.c.rtl
or foo.c.jump).
What follows are the possible letters for use in letters,
and their meanings.
-
a
Produce all the dumps previously
listed.
-
A
Annotate the assembler output
with miscellaneous debugging information.
-
b
Dump after computing branch
probabilities, to file.bp.
-
c
Dump after instruction combination,
to file.combine.
-
d
Dump after delayed branch
scheduling, to file.dbr.
-
D
Dump all macro definitions,
at the end of preprocessing, in addition to normal output.
-
F
Dump after purging ADDRESSOF,
to file.addressof.
-
f
Dump after flow analysis,
to file.flow.
-
g
Dump after global register
allocation, to file.greg.
-
G
Dump after GCSE, to file.gcse.
-
j
Dump after first jump optimization,
to file.jump.
-
J
Dump after last jump optimization,
to file.jump2.
-
k
Dump after conversion from
registers to stack, to file.stack.
-
l
Dump after local register
allocation, to file.lreg.
-
L
Dump after loop optimization,
to file.loop.
-
m
Print statistics on memory
usage, at the end of the run, to standard error.
-
M
Dump all macro definitions,
at the end of preprocessing, and write no output.
-
N
Dump all macro names, at
the end of preprocessing.
-
p
Annotate the assembler output
with a comment indicating which pattern and alternative was used.
-
r
Dump after RTL generation,
to file.rtl.
-
R
Dump after the second instruction
scheduling pass, to file.sched2.
-
s
Dump after CSE (including
the jump optimization that sometimes follows CSE), to file.cse.
-
S
Dump after the first instruction
scheduling pass, to file.sched.
-
t
Dump after the second CSE
pass (including the jump optimization that sometimes follows CSE), to file.cse2.
-
x
Just generate RTL for a
function instead of compiling it. Usually used with r.
-
y
Dump debugging information
during parsing, to standard error.
-fpretend-float
When running a cross-compiler,
pretend that the target machine uses the same floating point format as
the host machine. This causes incorrect output of the actual floating constants,
but the actual instruction sequence will probably be the same as GNU CC
would make when running on the target machine.
-save-temps
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
would produce files foo.i
and foo.s,
as well as foo.o.
-print-file-name=library
Print the full absolute
name of the library file, library,
that would be used when linkingand dont do anything else. With this option,
GNU CC does not compile or link anything; it just prints the filename.
-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.
Use the following input.
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
will searchand dont do anything else.
-
This is useful when gcc
prints the following error message:
installation problem, cannot exec cpp: No such file or directory
To resolve this you either need
to put cpp
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. (Dont forget the trailing slash).
See Environment variables
affecting GNU CC.
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