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POHow to enable optimization in G++ with #pragma
 

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  1. POHow to enable optimization in G++ with #pragma
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    <p>I want to enable optimization in g++ without command line parameter. I know GCC can do it by writing <code>#pragma GCC optimize (2)</code> in my code. But it seems won't work in G++.</p> <p>This page may help: <a href="http://gcc.gnu.org/onlinedocs/gcc/Function-Specific-Option-Pragmas.html">http://gcc.gnu.org/onlinedocs/gcc/Function-Specific-Option-Pragmas.html</a></p> <p>My compiler version:</p> <pre><code>$ g++ --version g++ (Ubuntu/Linaro 4.6.1-9ubuntu3) 4.6.1 &lt;suppressed copyright message&gt; $ gcc --version gcc (Ubuntu/Linaro 4.6.1-9ubuntu3) 4.6.1 &lt;suppressed copyright message&gt; </code></pre> <p>I worte some code like this:</p> <pre><code>#pragma GCC optimize (2) int main(){ long x; x=11; x+=12; x*=13; x/=14; return 0; } </code></pre> <p>And compiled it with GCC <strong>Not G++</strong>. Then I used objdump, which output</p> <pre><code>08048300 &lt;main&gt;: 8048300: 55 push %ebp 8048301: 31 c0 xor %eax,%eax 8048303: 89 e5 mov %esp,%ebp 8048305: 5d pop %ebp 8048306: c3 ret 8048307: 90 nop </code></pre> <p>When I removed <code>#param GCC optimize(2)</code> . objdump output:</p> <pre><code>080483b4 &lt;main&gt;: 80483b4: 55 push %ebp 80483b5: 89 e5 mov %esp,%ebp 80483b7: 83 ec 10 sub $0x10,%esp 80483ba: c7 45 fc 0b 00 00 00 movl $0xb,-0x4(%ebp) 80483c1: 83 45 fc 0c addl $0xc,-0x4(%ebp) 80483c5: 8b 55 fc mov -0x4(%ebp),%edx 80483c8: 89 d0 mov %edx,%eax 80483ca: 01 c0 add %eax,%eax 80483cc: 01 d0 add %edx,%eax 80483ce: c1 e0 02 shl $0x2,%eax 80483d1: 01 d0 add %edx,%eax 80483d3: 89 45 fc mov %eax,-0x4(%ebp) 80483d6: 8b 4d fc mov -0x4(%ebp),%ecx 80483d9: ba 93 24 49 92 mov $0x92492493,%edx 80483de: 89 c8 mov %ecx,%eax 80483e0: f7 ea imul %edx 80483e2: 8d 04 0a lea (%edx,%ecx,1),%eax 80483e5: 89 c2 mov %eax,%edx 80483e7: c1 fa 03 sar $0x3,%edx 80483ea: 89 c8 mov %ecx,%eax 80483ec: c1 f8 1f sar $0x1f,%eax 80483ef: 89 d1 mov %edx,%ecx 80483f1: 29 c1 sub %eax,%ecx 80483f3: 89 c8 mov %ecx,%eax 80483f5: 89 45 fc mov %eax,-0x4(%ebp) 80483f8: b8 00 00 00 00 mov $0x0,%eax 80483fd: c9 leave 80483fe: c3 ret 80483ff: 90 nop </code></pre> <p>However, it won't work with G++!</p>
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    1. COWhat _version_ of GCC are you using?
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    2. COWhat are you seeing that makes you think it isn't working? Is the binary produced the same with and without the #pragma?
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    3. COI use objdump to check whether the binary code is simplified.
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