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| <h2 class="chapter">11 Debugging Optimized Code</h2> |
| |
| <p><a name="index-optimized-code_002c-debugging-661"></a><a name="index-debugging-optimized-code-662"></a> |
| Almost all compilers support optimization. With optimization |
| disabled, the compiler generates assembly code that corresponds |
| directly to your source code, in a simplistic way. As the compiler |
| applies more powerful optimizations, the generated assembly code |
| diverges from your original source code. With help from debugging |
| information generated by the compiler, <span class="sc">gdb</span> can map from |
| the running program back to constructs from your original source. |
| |
| <p><span class="sc">gdb</span> is more accurate with optimization disabled. If you |
| can recompile without optimization, it is easier to follow the |
| progress of your program during debugging. But, there are many cases |
| where you may need to debug an optimized version. |
| |
| <p>When you debug a program compiled with ‘<samp><span class="samp">-g -O</span></samp>’, remember that the |
| optimizer has rearranged your code; the debugger shows you what is |
| really there. Do not be too surprised when the execution path does not |
| exactly match your source file! An extreme example: if you define a |
| variable, but never use it, <span class="sc">gdb</span> never sees that |
| variable—because the compiler optimizes it out of existence. |
| |
| <p>Some things do not work as well with ‘<samp><span class="samp">-g -O</span></samp>’ as with just |
| ‘<samp><span class="samp">-g</span></samp>’, particularly on machines with instruction scheduling. If in |
| doubt, recompile with ‘<samp><span class="samp">-g</span></samp>’ alone, and if this fixes the problem, |
| please report it to us as a bug (including a test case!). |
| See <a href="Variables.html#Variables">Variables</a>, for more information about debugging optimized code. |
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