Gcov and Optimization (Using the GNU Compiler Collection (GCC))

### 11.3 Using `gcov` with GCC Optimization ¶

If you plan to use `gcov` to help optimize your code, you must first compile your program with a special GCC option ‘`--coverage`’. Aside from that, you can use any other GCC options; but if you want to prove that every single line in your program was executed, you should not compile with optimization at the same time. On some machines the optimizer can eliminate some simple code lines by combining them with other lines. For example, code like this:

    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:   12:if (a != b)
          100:   13:  c = 1;
          100:   14:else
          100:   15:  c = 0;

The output shows that this block of code, combined by optimization, executed 100 times. In one sense this result is correct, because there was only one instruction representing all four of these lines. However, the output does not indicate how many times the result was 0 and how many times the result was 1.

Inlineable functions can create unexpected line counts. Line counts are shown for the source code of the inlineable function, but what is shown depends on where the function is inlined, or if it is not inlined at all.

If the function is not inlined, the compiler must emit an out of line copy of the function, in any object file that needs it. If `fileA.o` and `fileB.o` both contain out of line bodies of a particular inlineable function, they will also both contain coverage counts for that function. When `fileA.o` and `fileB.o` are linked together, the linker will, on many systems, select one of those out of line bodies for all calls to that function, and remove or ignore the other. Unfortunately, it will not remove the coverage counters for the unused function body. Hence when instrumented, all but one use of that function will show zero counts.

If the function is inlined in several places, the block structure in each location might not be the same. For instance, a condition might now be calculable at compile time in some instances. Because the coverage of all the uses of the inline function will be shown for the same source lines, the line counts themselves might seem inconsistent.

Long-running applications can use the `__gcov_reset` and `__gcov_dump` facilities to restrict profile collection to the program region of interest. Calling `__gcov_reset(void)` will clear all run-time profile counters to zero, and calling `__gcov_dump(void)` will cause the profile information collected at that point to be dumped to `.gcda` output files. Instrumented applications use a static destructor with priority 99 to invoke the `__gcov_dump` function. Thus `__gcov_dump` is executed after all user defined static destructors, as well as handlers registered with `atexit`.

If an executable loads a dynamic shared object via dlopen functionality, `-Wl,--dynamic-list-data` is needed to dump all profile data.

Profiling run-time library reports various errors related to profile manipulation and profile saving. Errors are printed into standard error output or ‘`GCOV_ERROR_FILE`’ file, if environment variable is used. In order to terminate immediately after an errors occurs set ‘`GCOV_EXIT_AT_ERROR`’ environment variable. That can help users to find profile clashing which leads to a misleading profile.