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POHow does GCC optimize out an unused variable incremented inside a loop?
 

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  1. POHow does GCC optimize out an unused variable incremented inside a loop?
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    <p>I wrote this simple C program:</p> <pre class="lang-c prettyprint-override"><code>int main() { int i; int count = 0; for(i = 0; i &lt; 2000000000; i++){ count = count + 1; } } </code></pre> <p>I wanted to see how the gcc compiler optimizes this loop (clearly add <em>1</em> 2000000000 times should be "add <em>2000000000</em> one time"). So:</p> <p><strong>gcc test.c</strong> and then <code>time</code> on <code>a.out</code> gives:</p> <pre><code>real 0m7.717s user 0m7.710s sys 0m0.000s </code></pre> <p><strong>$ gcc -O2 test.c</strong> and then <code>time on</code>a.out` gives: </p> <pre><code>real 0m0.003s user 0m0.000s sys 0m0.000s </code></pre> <p>Then I disassembled both with <code>gcc -S</code>. First one seems quite clear:</p> <pre><code> .file "test.c" .text .globl main .type main, @function main: .LFB0: .cfi_startproc pushq %rbp .cfi_def_cfa_offset 16 movq %rsp, %rbp .cfi_offset 6, -16 .cfi_def_cfa_register 6 movl $0, -8(%rbp) movl $0, -4(%rbp) jmp .L2 .L3: addl $1, -8(%rbp) addl $1, -4(%rbp) .L2: cmpl $1999999999, -4(%rbp) jle .L3 leave .cfi_def_cfa 7, 8 ret .cfi_endproc .LFE0: .size main, .-main .ident "GCC: (Ubuntu/Linaro 4.5.2-8ubuntu4) 4.5.2" .section .note.GNU-stack,"",@progbits </code></pre> <p>L3 adds, L2 compare <code>-4(%rbp)</code> with <code>1999999999</code> and loops to L3 if <code>i &lt; 2000000000</code>.</p> <p><strong>Now the optimized one:</strong></p> <pre><code> .file "test.c" .text .p2align 4,,15 .globl main .type main, @function main: .LFB0: .cfi_startproc rep ret .cfi_endproc .LFE0: .size main, .-main .ident "GCC: (Ubuntu/Linaro 4.5.2-8ubuntu4) 4.5.2" .section .note.GNU-stack,"",@progbits </code></pre> <p>I can't understand at all what's going on there! I've got little knowledge of assembly, but I expected something like</p> <pre><code>addl $2000000000, -8(%rbp) </code></pre> <p>I even tried with <strong>gcc -c -g -Wa,-a,-ad -O2 test.c</strong> to see the C code together with the assembly it was converted to, but the result was no more clear that the previous one. </p> <p><strong>Can someone briefly explain:</strong></p> <ol> <li>The <strong>gcc -S -O2</strong> output.</li> <li>If the loop is optimized as I expected (one sum instead of many sums)?</li> </ol>
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    1. USCiro Santilli 新疆改造中心 六四事件 法轮功
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    1. CONice question btw, and welcome to Stackoverflow! This, is a fine example of an excellent first question to ask. :)
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