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    <p>The compiler is even smarter than that. :)</p> <p>In fact, it realizes that you aren't using the result of the loop. So it took out the entire loop completely!</p> <p>This is called <a href="http://en.wikipedia.org/wiki/Dead_code_elimination" rel="noreferrer">Dead Code Elimination</a>.</p> <p>A better test is to print the result:</p> <pre><code>#include &lt;stdio.h&gt; int main(void) { int i; int count = 0; for(i = 0; i &lt; 2000000000; i++){ count = count + 1; } // Print result to prevent Dead Code Elimination printf("%d\n", count); } </code></pre> <p><strong>EDIT :</strong> I've added the required <code>#include &lt;stdio.h&gt;</code>; the MSVC assembly listing corresponds to a version without the <code>#include</code>, but it should be the same.</p> <hr> <p>I don't have GCC in front of me at the moment, since I'm booted into Windows. But here's the disassembly of the version with the <code>printf()</code> on MSVC:</p> <p><strong>EDIT : I had the wrong assembly output. Here's the correct one.</strong></p> <pre><code>; 57 : int main(){ $LN8: sub rsp, 40 ; 00000028H ; 58 : ; 59 : ; 60 : int i; int count = 0; ; 61 : for(i = 0; i &lt; 2000000000; i++){ ; 62 : count = count + 1; ; 63 : } ; 64 : ; 65 : // Print result to prevent Dead Code Elimination ; 66 : printf("%d\n",count); lea rcx, OFFSET FLAT:??_C@_03PMGGPEJJ@?$CFd?6?$AA@ mov edx, 2000000000 ; 77359400H call QWORD PTR __imp_printf ; 67 : ; 68 : ; 69 : ; 70 : ; 71 : return 0; xor eax, eax ; 72 : } add rsp, 40 ; 00000028H ret 0 </code></pre> <p>So yes, Visual Studio does this optimization. I'd assume GCC probably does too.</p> <p>And yes, GCC performs a similar optimization. Here's an assembly listing for the same program with <code>gcc -S -O2 test.c</code> (gcc 4.5.2, Ubuntu 11.10, x86):</p> <pre><code> .file "test.c" .section .rodata.str1.1,"aMS",@progbits,1 .LC0: .string "%d\n" .text .p2align 4,,15 .globl main .type main, @function main: pushl %ebp movl %esp, %ebp andl $-16, %esp subl $16, %esp movl $2000000000, 8(%esp) movl $.LC0, 4(%esp) movl $1, (%esp) call __printf_chk leave ret .size main, .-main .ident "GCC: (Ubuntu/Linaro 4.5.2-8ubuntu4) 4.5.2" .section .note.GNU-stack,"",@progbits </code></pre>
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    1. POHow does GCC optimize out an unused variable incremented inside a loop?
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    1. USKeith Thompson
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    1. POHow does GCC optimize out an unused variable incremented inside a loop?
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    1. COWell, I feel really dumb right now. Didn't think (eew.. didn't know) about Dead code elimination. I tried with printf() and gcc, and it produces quite the same optimized code. Thank you for you answer!
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    2. CODon't feel dumb. This kind of thing is not obvious at all if you're just getting into micro-benchmarking. It's just part of the learning process.
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    3. COIt would be interesting to know how the compiler makes this kind of decisions. What if that loop was really needed for some reason?
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