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| <h3 class="section">10.3 Using <samp><span class="command">gcov</span></samp> with GCC Optimization</h3> |
| |
| <p>If you plan to use <samp><span class="command">gcov</span></samp> to help optimize your code, you must |
| first compile your program with two special GCC options: |
| ‘<samp><span class="samp">-fprofile-arcs -ftest-coverage</span></samp>’. 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 class="noindent">can be compiled into one instruction on some machines. In this case, |
| there is no way for <samp><span class="command">gcov</span></samp> to calculate separate execution counts |
| for each line because there isn't separate code for each line. Hence |
| the <samp><span class="command">gcov</span></samp> 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. |
| |
| <p>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. |
| |
| <p>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 |
| <samp><span class="file">fileA.o</span></samp> and <samp><span class="file">fileB.o</span></samp> both contain out of line bodies of a |
| particular inlineable function, they will also both contain coverage |
| counts for that function. When <samp><span class="file">fileA.o</span></samp> and <samp><span class="file">fileB.o</span></samp> 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. |
| |
| <p>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. |
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