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POFloating Point Math Execution Time
 

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    <p>What accounts for the added execution time of the first data set? The assembly instructions are the same. </p> <p>With DN_FLUSH flag not on, the first data set takes 63 milliseconds, the second set takes 15 milliseconds.<br> With DN_FLUSH flag on, the first data set takes 15 milliseconds, the second set takes ~0 milliseconds.</p> <p>Therefore, in both cases the execution time of the first data set is much greater.</p> <p>Is there any way to decrease the execution time to be closer in line with the second data set?</p> <p>I am using C++ Visual Studio 2005, /arch:SSE2 /fp:fast running on Intel Core 2 Duo T7700 @ 2.4Ghz Windows XP Pro. </p> <pre><code>#define NUMLOOPS 1000000 // Denormal values flushed to zero by hardware on ALPHA and x86 // processors with SSE2 support. Ignored on other x86 platforms // Setting this decreases execution time from 63 milliseconds to 16 millisecond // _controlfp(_DN_FLUSH, _MCW_DN); float denormal = 1.0e-38; float denormalTwo = 1.0e-39; float denormalThree = 1; tickStart = GetTickCount(); // Run First Calculation Loop for (loops=0; loops &lt; NUMLOOPS; loops++) { denormalThree = denormal - denormalTwo; } // Get execution time duration = GetTickCount()-tickStart; printf("Duration = %dms\n", duration); float normal = 1.0e-10; float normalTwo = 1.0e-2; float normalThree = 1; tickStart = GetTickCount(); // Run Second Calculation Loop for (loops=0; loops &lt; NUMLOOPS; loops++) { normalThree = normal - normalTwo; } // Get execution time duration = GetTickCount()-tickStart; printf("Duration = %dms\n", duration); </code></pre>
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    1. COYou should be aware that GetTickCount() is nearly worthless for timing code. It's granularity tends to be very large, perhaps as large as 100 ms, varying from system to system. Use QueryPerformanceCounter() instead.
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    2. COQueryPerformanceCounter() is a pain to use... try timeGetTime(). Also, GetTickCount() isn't that bad for things that run several seconds or more - just be aware of the accuracy.
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    3. COBTW, don't assume that using /arch:SSE2 /fp:fast actually make your code faster. For my codes, I have found that /fp:precise and using the FP stack is actually faster. Similarly, don't assume floats are faster than doubles. Test all options.
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