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POint operators != and == when comparing to zero
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  1. POint operators != and == when comparing to zero
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    copied!<p>I've found that != and == are not the fastest ways for testing for zero or non-zero.</p> <pre><code>bool nonZero1 = integer != 0; xor eax, eax test ecx, ecx setne al bool nonZero2 = integer &lt; 0 || integer &gt; 0; test ecx, ecx setne al bool zero1 = integer == 0; xor eax, eax test ecx, ecx sete al bool zero2 = !(integer &lt; 0 || integer &gt; 0); test ecx, ecx sete al </code></pre> <p>Compiler: VC++ 11 Optimization flags: /O2 /GL /LTCG</p> <p>This is the assembly output for x86-32. The second versions of both comparisons were ~12% faster on both x86-32 and x86-64. However, on x86-64 the instructions were identical (first versions looked exactly like the second versions), but the second versions were still faster.</p> <ol> <li>Why doesn't the compiler generate the faster version on x86-32?</li> <li>Why are the second versions still faster on x86-64 when the assembly output is identical?</li> </ol> <p><strong>EDIT: I've added benchmarking code. ZERO: 1544ms, 1358ms NON_ZERO: 1544ms, 1358ms</strong> <a href="http://pastebin.com/m7ZSUrcP" rel="noreferrer">http://pastebin.com/m7ZSUrcP</a> or <a href="http://anonymouse.org/cgi-bin/anon-www.cgi/http://pastebin.com/m7ZSUrcP" rel="noreferrer">http://anonymouse.org/cgi-bin/anon-www.cgi/http://pastebin.com/m7ZSUrcP</a></p> <p>Note: It's probably inconvenient to locate these functions when compiled in a single source file, because main.asm goes quite big. I had zero1, zero2, nonZero1, nonZero2 in a separate source file.</p> <p><strong>EDIT2: Could someone with both VC++11 and VC++2010 installed run the benchmarking code and post the timings? It might indeed be a bug in VC++11.</strong></p>
 

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