mirror of git://gcc.gnu.org/git/gcc.git
Doug Evans
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@c Copyright (C) 1996, 1997 Free Software Foundation, Inc.
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@c This is part of the GCC manual.
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@c For copying conditions, see the file gcc.texi.
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@node Gcov
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@chapter @code{gcov}: a Test Coverage Program
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@code{gcov} is a tool you can use in conjunction with @sc{gnu} CC to
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test code coverage in your programs.
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This chapter describes version 1.5 of @code{gcov}.
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@menu
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* Gcov Intro:: Introduction to gcov.
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* Invoking Gcov:: How to use gcov.
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* Gcov and Optimization:: Using gcov with GCC optimization.
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* Gcov Data Files:: The files used by gcov.
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@end menu
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@node Gcov Intro
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@section Introduction to @code{gcov}
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@code{gcov} is a test coverage program. Use it in concert with @sc{gnu}
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CC to analyze your programs to help create more efficient, faster
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running code. You can use @code{gcov} as a profiling tool to help
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discover where your optimization efforts will best affect your code. You
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can also use @code{gcov} along with the other profiling tool,
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@code{gprof}, to assess which parts of your code use the greatest amount
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of computing time.
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Profiling tools help you analyze your code's performance. Using a
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profiler such as @code{gcov} or @code{gprof}, you can find out some
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basic performance statistics, such as:
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@itemize @bullet
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@item
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how often each line of code executes
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@item
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what lines of code are actually executed
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@item
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how much computing time each section of code uses
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@end itemize
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Once you know these things about how your code works when compiled, you
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can look at each module to see which modules should be optimized.
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@code{gcov} helps you determine where to work on optimization.
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Software developers also use coverage testing in concert with
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testsuites, to make sure software is actually good enough for a release.
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Testsuites can verify that a program works as expected; a coverage
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program tests to see how much of the program is exercised by the
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testsuite. Developers can then determine what kinds of test cases need
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to be added to the testsuites to create both better testing and a better
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final product.
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You should compile your code without optimization if you plan to use
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@code{gcov} because the optimization, by combining some lines of code
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into one function, may not give you as much information as you need to
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look for `hot spots' where the code is using a great deal of computer
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time. Likewise, because @code{gcov} accumulates statistics by line (at
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the lowest resolution), it works best with a programming style that
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places only one statement on each line. If you use complicated macros
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that expand to loops or to other control structures, the statistics are
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less helpful---they only report on the line where the macro call
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appears. If your complex macros behave like functions, you can replace
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them with inline functions to solve this problem.
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@code{gcov} creates a logfile called @file{@var{sourcefile}.gcov} which
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indicates how many times each line of a source file @file{@var{sourcefile}.c}
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has executed. You can use these logfiles along with @code{gprof} to aid
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in fine-tuning the performance of your programs. @code{gprof} gives
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timing information you can use along with the information you get from
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@code{gcov}.
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@code{gcov} works only on code compiled with @sc{gnu} CC. It is not
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compatible with any other profiling or test coverage mechanism.
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@node Invoking Gcov
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@section Invoking gcov
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@smallexample
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gcov [-b] [-v] [-n] [-l] [-f] [-o directory] @var{sourcefile}
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@end smallexample
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@table @code
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@item -b
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Write branch frequencies to the output file, and write branch summary
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info to the standard output. This option allows you to see how often
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each branch in your program was taken.
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@item -v
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Display the @code{gcov} version number (on the standard error stream).
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@item -n
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Do not create the @code{gcov} output file.
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@item -l
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Create long file names for included source files. For example, if the
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header file @samp{x.h} contains code, and was included in the file
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@samp{a.c}, then running @code{gcov} on the file @samp{a.c} will produce
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an output file called @samp{a.c.x.h.gcov} instead of @samp{x.h.gcov}.
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This can be useful if @samp{x.h} is included in multiple source files.
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@item -f
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Output summaries for each function in addition to the file level summary.
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@item -o
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The directory where the object files live. Gcov will search for @code{.bb},
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@code{.bbg}, and @code{.da} files in this directory.
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@end table
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@need 3000
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When using @code{gcov}, you must first compile your program with two
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special @sc{gnu} CC options: @samp{-fprofile-arcs -ftest-coverage}.
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This tells the compiler to generate additional information needed by
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gcov (basically a flow graph of the program) and also includes
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additional code in the object files for generating the extra profiling
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information needed by gcov. These additional files are placed in the
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directory where the source code is located.
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Running the program will cause profile output to be generated. For each
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source file compiled with -fprofile-arcs, an accompanying @code{.da}
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file will be placed in the source directory.
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Running @code{gcov} with your program's source file names as arguments
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will now produce a listing of the code along with frequency of execution
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for each line. For example, if your program is called @samp{tmp.c}, this
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is what you see when you use the basic @code{gcov} facility:
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@smallexample
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$ gcc -fprofile-arcs -ftest-coverage tmp.c
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$ a.out
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$ gcov tmp.c
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87.50% of 8 source lines executed in file tmp.c
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Creating tmp.c.gcov.
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@end smallexample
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The file @file{tmp.c.gcov} contains output from @code{gcov}.
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Here is a sample:
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@smallexample
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main()
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@{
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1 int i, total;
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1 total = 0;
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11 for (i = 0; i < 10; i++)
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10 total += i;
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1 if (total != 45)
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###### printf ("Failure\n");
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else
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1 printf ("Success\n");
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1 @}
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@end smallexample
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@need 450
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When you use the @samp{-b} option, your output looks like this:
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@smallexample
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$ gcov -b tmp.c
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87.50% of 8 source lines executed in file tmp.c
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80.00% of 5 branches executed in file tmp.c
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80.00% of 5 branches taken at least once in file tmp.c
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50.00% of 2 calls executed in file tmp.c
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Creating tmp.c.gcov.
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@end smallexample
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Here is a sample of a resulting @file{tmp.c.gcov} file:
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@smallexample
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main()
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@{
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1 int i, total;
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1 total = 0;
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11 for (i = 0; i < 10; i++)
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branch 0 taken = 91%
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branch 1 taken = 100%
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branch 2 taken = 100%
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10 total += i;
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1 if (total != 45)
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branch 0 taken = 100%
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###### printf ("Failure\n");
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call 0 never executed
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branch 1 never executed
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else
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1 printf ("Success\n");
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call 0 returns = 100%
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1 @}
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@end smallexample
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For each basic block, a line is printed after the last line of the basic
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block describing the branch or call that ends the basic block. There can
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be multiple branches and calls listed for a single source line if there
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are multiple basic blocks that end on that line. In this case, the
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branches and calls are each given a number. There is no simple way to map
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these branches and calls back to source constructs. In general, though,
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the lowest numbered branch or call will correspond to the leftmost construct
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on the source line.
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For a branch, if it was executed at least once, then a percentage
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indicating the number of times the branch was taken divided by the
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number of times the branch was executed will be printed. Otherwise, the
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message ``never executed'' is printed.
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For a call, if it was executed at least once, then a percentage
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indicating the number of times the call returned divided by the number
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of times the call was executed will be printed. This will usually be
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100%, but may be less for functions call @code{exit} or @code{longjmp},
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and thus may not return everytime they are called.
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The execution counts are cumulative. If the example program were
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executed again without removing the @code{.da} file, the count for the
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number of times each line in the source was executed would be added to
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the results of the previous run(s). This is potentially useful in
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several ways. For example, it could be used to accumulate data over a
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number of program runs as part of a test verification suite, or to
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provide more accurate long-term information over a large number of
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program runs.
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The data in the @code{.da} files is saved immediately before the program
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exits. For each source file compiled with -fprofile-arcs, the profiling
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code first attempts to read in an existing @code{.da} file; if the file
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doesn't match the executable (differing number of basic block counts) it
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will ignore the contents of the file. It then adds in the new execution
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counts and finally writes the data to the file.
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@node Gcov and Optimization
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@section Using @code{gcov} with GCC Optimization
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If you plan to use @code{gcov} to help optimize your code, you must
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first compile your program with two special @sc{gnu} CC options:
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@samp{-fprofile-arcs -ftest-coverage}. Aside from that, you can use any
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other @sc{gnu} CC options; but if you want to prove that every single line
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in your program was executed, you should not compile with optimization
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at the same time. On some machines the optimizer can eliminate some
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simple code lines by combining them with other lines. For example, code
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like this:
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@smallexample
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if (a != b)
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c = 1;
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else
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c = 0;
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@end smallexample
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@noindent
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can be compiled into one instruction on some machines. In this case,
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there is no way for @code{gcov} to calculate separate execution counts
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for each line because there isn't separate code for each line. Hence
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the @code{gcov} output looks like this if you compiled the program with
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optimization:
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@smallexample
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100 if (a != b)
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100 c = 1;
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100 else
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100 c = 0;
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@end smallexample
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The output shows that this block of code, combined by optimization,
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executed 100 times. In one sense this result is correct, because there
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was only one instruction representing all four of these lines. However,
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the output does not indicate how many times the result was 0 and how
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many times the result was 1.
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@node Gcov Data Files
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@section Brief description of @code{gcov} data files
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@code{gcov} uses three files for doing profiling. The names of these
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files are derived from the original @emph{source} file by substituting
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the file suffix with either @code{.bb}, @code{.bbg}, or @code{.da}. All
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of these files are placed in the same directory as the source file, and
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contain data stored in a platform-independent method.
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The @code{.bb} and @code{.bbg} files are generated when the source file
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is compiled with the @sc{gnu} CC @samp{-ftest-coverage} option. The
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@code{.bb} file contains a list of source files (including headers),
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functions within those files, and line numbers corresponding to each
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basic block in the source file.
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The @code{.bb} file format consists of several lists of 4-byte integers
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which correspond to the line numbers of each basic block in the
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file. Each list is terminated by a line number of 0. A line number of -1
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is used to designate that the source file name (padded to a 4-byte
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boundary and followed by another -1) follows. In addition, a line number
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of -2 is used to designate that the name of a function (also padded to a
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4-byte boundary and followed by a -2) follows.
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The @code{.bbg} file is used to reconstruct the program flow graph for
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the source file. It contains a list of the program flow arcs (possible
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branches taken from one basic block to another) for each function which,
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in combination with the @code{.bb} file, enables gcov to reconstruct the
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program flow.
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In the @code{.bbg} file, the format is:
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@smallexample
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number of basic blocks for function #0 (4-byte number)
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total number of arcs for function #0 (4-byte number)
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count of arcs in basic block #0 (4-byte number)
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destination basic block of arc #0 (4-byte number)
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flag bits (4-byte number)
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destination basic block of arc #1 (4-byte number)
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flag bits (4-byte number)
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...
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destination basic block of arc #N (4-byte number)
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flag bits (4-byte number)
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count of arcs in basic block #1 (4-byte number)
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destination basic block of arc #0 (4-byte number)
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flag bits (4-byte number)
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...
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@end smallexample
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A -1 (stored as a 4-byte number) is used to separate each function's
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list of basic blocks, and to verify that the file has been read
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correctly.
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The @code{.da} file is generated when a program containing object files
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built with the @sc{gnu} CC @samp{-fprofile-arcs} option is executed. A
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separate @code{.da} file is created for each source file compiled with
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this option, and the name of the @code{.da} file is stored as an
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absolute pathname in the resulting object file. This path name is
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derived from the source file name by substituting a @code{.da} suffix.
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The format of the @code{.da} file is fairly simple. The first 8-byte
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number is the number of counts in the file, followed by the counts
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(stored as 8-byte numbers). Each count corresponds to the number of
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times each arc in the program is executed. The counts are cumulative;
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each time the program is executed, it attemps to combine the existing
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@code{.da} files with the new counts for this invocation of the
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program. It ignores the contents of any @code{.da} files whose number of
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arcs doesn't correspond to the current program, and merely overwrites
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them instead.
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All three of these files use the functions in @code{gcov-io.h} to store
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integers; the functions in this header provide a machine-independent
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mechanism for storing and retrieving data from a stream.
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