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POSlow XOR operator
 

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    <p>EDIT: Indeed, I had a weird error in my timing code leading to these results. When I fixed my error, the smart version ended up faster as expected. My timing code looked like this:</p> <pre><code>bool x = false; before = now(); for (int i=0; i&lt;N; ++i) { x ^= smart_xor(A[i],B[i]); } after = now(); </code></pre> <p>I had done the <code>^=</code> to discourage my compiler from optimizing the for-loop away. But I think that the <code>^=</code> somehow interacts strangely with the two xor functions. I changed my timing code to simply fill out an array of the xor results, and then do computation with that array outside of the timed code. And that fixed things.</p> <p>Should I delete this question?</p> <p>END EDIT</p> <p>I defined two C++ functions as follows:</p> <pre><code>bool smart_xor(bool a, bool b) { return a^b; } bool dumb_xor(bool a, bool b) { return a?!b:b; } </code></pre> <p>My timing tests indicate that <code>dumb_xor()</code> is slightly faster (1.31ns vs 1.90ns when inlined, 1.92ns vs 2.21ns when not inlined). This puzzles me, as the <code>^</code> operator should be a single machine operation. I'm wondering if anyone has an explanation.</p> <p>The assembly looks like this (when not inlined):</p> <pre><code> .file "xor.cpp" .text .p2align 4,,15 .globl _Z9smart_xorbb .type _Z9smart_xorbb, @function _Z9smart_xorbb: .LFB0: .cfi_startproc .cfi_personality 0x3,__gxx_personality_v0 movl %esi, %eax xorl %edi, %eax ret .cfi_endproc .LFE0: .size _Z9smart_xorbb, .-_Z9smart_xorbb .p2align 4,,15 .globl _Z8dumb_xorbb .type _Z8dumb_xorbb, @function _Z8dumb_xorbb: .LFB1: .cfi_startproc .cfi_personality 0x3,__gxx_personality_v0 movl %esi, %edx movl %esi, %eax xorl $1, %edx testb %dil, %dil cmovne %edx, %eax ret .cfi_endproc .LFE1: .size _Z8dumb_xorbb, .-_Z8dumb_xorbb .ident "GCC: (Ubuntu 4.4.3-4ubuntu5) 4.4.3" .section .note.GNU-stack,"",@progbits </code></pre> <p>I'm using g++ 4.4.3-4ubuntu5 on an Intel Xeon X5570. I compiled with -O3.</p>
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    1. COnanosecond is likely too small to make any meaningful comparison.
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    2. COYou may need to show your timing code. That's easy to get wrong.
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      1. USDrew Dormann
    3. COI'd prefer `a != b` over `a?!b:b`.
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      1. USPubby
 

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