gcc.info: Gcov and Optimization

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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 GNU CC options: `-fprofile-arcs
-ftest-coverage'.  Aside from that, you can use any other GNU CC
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  if (a != b)
           100    c = 1;
           100  else
           100    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.