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<h3 class="section">Using <code>gcov</code> with GCC Optimization</h3>
<p>If you plan to use <code>gcov</code> to help optimize your code, you must
first compile your program with two special GCC options:
<code>-fprofile-arcs -ftest-coverage</code>. 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:
<pre class="smallexample"> if (a != b)
c = 1;
else
c = 0;
</pre>
<p>can be compiled into one instruction on some machines. In this case,
there is no way for <code>gcov</code> to calculate separate execution counts
for each line because there isn't separate code for each line. Hence
the <code>gcov</code> output looks like this if you compiled the program with
optimization:
<pre class="smallexample"> 100: 12:if (a != b)
100: 13: c = 1;
100: 14:else
100: 15: c = 0;
</pre>
<p>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.
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