GNU Compiler Collection (GCC) Internals: gcov Testing

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### 7.6 Support for testing `gcov`

Language-independent support for testing `gcov`, and for checking that branch profiling produces expected values, is provided by the expect file `lib/gcov.exp`. `gcov` tests also rely on procedures in `lib/gcc-dg.exp` to compile and run the test program. A typical `gcov` test contains the following DejaGnu commands within comments:

    { dg-options "--coverage" }
    { dg-do run { target native } }
    { dg-final { run-gcov sourcefile } }

Checks of `gcov` output can include line counts, branch percentages, and call return percentages. All of these checks are requested via commands that appear in comments in the test’s source file. Commands to check line counts are processed by default. Commands to check branch percentages and call return percentages are processed if the `run-gcov` command has arguments `branches` or `calls`, respectively. For example, the following specifies checking both, as well as passing `-b` to `gcov`:

    { dg-final { run-gcov branches calls { -b sourcefile } } }

A line count command appears within a comment on the source line that is expected to get the specified count and has the form `count(cnt)`. A test should only check line counts for lines that will get the same count for any architecture.

Commands to check branch percentages (`branch`) and call return percentages (`returns`) are very similar to each other. A beginning command appears on or before the first of a range of lines that will report the percentage, and the ending command follows that range of lines. The beginning command can include a list of percentages, all of which are expected to be found within the range. A range is terminated by the next command of the same kind. A command `branch(end)` or `returns(end)` marks the end of a range without starting a new one. For example:

    if (i > 10 && j > i && j < 20)  /* branch(27 50 75) */
                                    /* branch(end) */
      foo (i, j);

For a call return percentage, the value specified is the percentage of calls reported to return. For a branch percentage, the value is either the expected percentage or 100 minus that value, since the direction of a branch can differ depending on the target or the optimization level.

Not all branches and calls need to be checked. A test should not check for branches that might be optimized away or replaced with predicated instructions. Don’t check for calls inserted by the compiler or ones that might be inlined or optimized away.

A single test can check for combinations of line counts, branch percentages, and call return percentages. The command to check a line count must appear on the line that will report that count, but commands to check branch percentages and call return percentages can bracket the lines that report them.

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