Using gcov with GCC Optimization

If you plan to use gcov to help optimize your code, you must first compile your program with two special GCC options: -fprofile-arcs -ftest-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.