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    <p>This is a great question, but I think you've fallen victim to the compiler's dependency analysis.</p> <p>The compiler only has to clear the high bits of <code>eax</code> once, and they remain clear for the second version. The second version would have to pay the price to <code>xor eax, eax</code> except that the compiler analysis proved it's been left cleared by the first version.</p> <p><strong>The second version is able to "cheat" by taking advantage of work the compiler did in the first version.</strong></p> <p>How are you measuring times? Is it "(version one, followed by version two) in a loop", or "(version one in a loop) followed by (version two in a loop)"?</p> <p>Don't do both tests in the same program (instead recompile for each version), or if you do, test both "version A first" and "version B first" and see if whichever comes first is paying a penalty.</p> <hr> <p>Illustration of the cheating:</p> <pre><code>timer1.start(); double x1 = 2 * sqrt(n + 37 * y + exp(z)); timer1.stop(); timer2.start(); double x2 = 31 * sqrt(n + 37 * y + exp(z)); timer2.stop(); </code></pre> <p>If <code>timer2</code> duration is less than <code>timer1</code> duration, we don't conclude that multiplying by 31 is faster than multiplying by 2. Instead, we realize that the compiler performed common subexpression analysis, and the code became:</p> <pre><code>timer1.start(); double common = sqrt(n + 37 * y + exp(z)); double x1 = 2 * common; timer1.stop(); timer2.start(); double x2 = 31 * common; timer2.stop(); </code></pre> <p>And the only thing proved is that multiplying by 31 is faster than computing <code>common</code>. Which is hardly surprising at all -- multiplication is far far faster than <code>sqrt</code> and <code>exp</code>.</p>
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    1. POint operators != and == when comparing to zero
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    1. COIf only I could +2.. Well done on the second half.
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    2. COBenchmarking code added. I ran benchmark1 and benchmark2 separately, same results. The only difference is the first benchmark that runs, then it "warms up" and is a bit slower.
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    3. COThis is somewhat unrelated, but wouldn't a compiler optimize a multiplication by 31 to a `(common << 5) - common`?
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