CONTENTS

"SYNOPSIS"

"DESCRIPTION"

Profiling tools help you analyze your code’s performance. Using a profiler such as gcov or gprof, you can find out some basic performance statistics, such as:

Software developers also use coverage testing in concert with testsuites, to make sure software is actually good enough for a release. Testsuites can verify that a program works as expected; a coverage program tests to see how much of the program is exercised by the testsuite. Developers can then determine what kinds of test cases need to be added to the testsuites to create both better testing and a better final product.

You should compile your code without optimization if you plan to use gcov because the optimization, by combining some lines of code into one function, may not give you as much information as you need to look for ‘hot spots’ where the code is using a great deal of computer time. Likewise, because gcov accumulates statistics by line (at the lowest resolution), it works best with a programming style that places only one statement on each line. If you use complicated macros that expand to loops or to other control structures, the statistics are less helpful---they only report on the line where the macro call appears. If your complex macros behave like functions, you can replace them with inline functions to solve this problem.

gcov creates a logfile called sourcefile.gcov which indicates how many times each line of a source file sourcefile.c has executed. You can use these logfiles along with gprof to aid in fine-tuning the performance of your programs. gprof gives timing information you can use along with the information you get from gcov.

Running the program will cause profile output to be generated. For each source file compiled with -fprofile-arcs, an accompanying .da file will be placed in the source directory.

Running gcov with your program’s source file names as arguments will now produce a listing of the code along with frequency of execution for each line. For example, if your program is called tmp.c, this is what you see when you use the basic gcov facility:

$ gcc -fprofile-arcs -ftest-coverage tmp.c
$ a.out
$ gcov tmp.c
87.50% of 8 source lines executed in file tmp.c
Creating tmp.c.gcov.

The file tmp.c.gcov contains output from gcov. Here is a sample:

main()
{
1 int i, total;

1 total = 0;

11 for (i = 0; i < 10; i++)
10 total += i;

1 if (total != 45)
###### printf ("Failure\n");
else
1 printf ("Success\n");
1 }

When you use the -b option, your output looks like this:

$ gcov -b tmp.c
87.50% of 8 source lines executed in file tmp.c
80.00% of 5 branches executed in file tmp.c
80.00% of 5 branches taken at least once in file tmp.c
50.00% of 2 calls executed in file tmp.c
Creating tmp.c.gcov.

Here is a sample of a resulting tmp.c.gcov file:

main()
{
1 int i, total;

1 total = 0;

11 for (i = 0; i < 10; i++)
branch 0 taken = 91%
branch 1 taken = 100%
branch 2 taken = 100%
10 total += i;

1 if (total != 45)
branch 0 taken = 100%
###### printf ("Failure\n");
call 0 never executed
branch 1 never executed
else
1 printf ("Success\n");
call 0 returns = 100%
1 }

For each basic block, a line is printed after the last line of the basic block describing the branch or call that ends the basic block. There can be multiple branches and calls listed for a single source line if there are multiple basic blocks that end on that line. In this case, the branches and calls are each given a number. There is no simple way to map these branches and calls back to source constructs. In general, though, the lowest numbered branch or call will correspond to the leftmost construct on the source line.

For a branch, if it was executed at least once, then a percentage indicating the number of times the branch was taken divided by the number of times the branch was executed will be printed. Otherwise, the message ‘‘never executed’’ is printed.

For a call, if it was executed at least once, then a percentage indicating the number of times the call returned divided by the number of times the call was executed will be printed. This will usually be 100%, but may be less for functions call ""exit"" or ""longjmp"", and thus may not return every time they are called.

The execution counts are cumulative. If the example program were executed again without removing the .da file, the count for the number of times each line in the source was executed would be added to the results of the previous run(s). This is potentially useful in several ways. For example, it could be used to accumulate data over a number of program runs as part of a test verification suite, or to provide more accurate long-term information over a large number of program runs.

if (a != b)
c = 1;
else
c = 0;

can be compiled into one instruction on some machines. In this case, there is no way for gcov to calculate separate execution counts for each line because there isn’t separate code for each line. Hence the gcov output looks like this if you compiled the program with optimization:

100 if (a != b)
100 c = 1;
100 else
100 c = 0;

"COPYRIGHT"