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POWhat is faster on division? doubles / floats / UInt32 / UInt64 ? in C++/C
 

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  1. POWhat is faster on division? doubles / floats / UInt32 / UInt64 ? in C++/C
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    <p>I did some speed testing to figure out what is the fastest, when doing multiplication or division on numbers. I had to really work hard to defeat the optimiser. I got nonsensical results such as a massive loop operating in 2 microseconds, or that multiplication was the same speed as division (if only that were true).</p> <p>After I finally worked hard enough to defeat enough of the compiler optimisations, while still letting it optimise for speed, I got these speed results. They maybe of interest to someone else?</p> <p>If my test is STILL FLAWED, let me know, but be kind seeing as I just spend two hours writing this crap :P</p> <pre><code>64 time: 3826718 us 32 time: 2476484 us D(mul) time: 936524 us D(div) time: 3614857 us S time: 1506020 us </code></pre> <p>"Multiplying to divide" using doubles seems the fastest way to do a division, followed by integer division. I did not test the accuracy of division. Could it be that "proper division" is more accurate? I have no desire to find out after these speed test results as I'll just be using integer division on a base 10 constant and letting my compiler optimise it for me ;) (and not defeating it's optimisations either).</p> <p>Here's the code I used to get the results:</p> <pre><code>#include &lt;iostream&gt; int Run(int bla, int div, int add, int minus) { // these parameters are to force the compiler to not be able to optimise away the // multiplications and divides :) long LoopMax = 100000000; uint32_t Origbla32 = 1000000000; long i = 0; uint32_t bla32 = Origbla32; uint32_t div32 = div; clock_t Time32 = clock(); for (i = 0; i &lt; LoopMax; i++) { div32 += add; div32 -= minus; bla32 = bla32 / div32; bla32 += bla; bla32 = bla32 * div32; } Time32 = clock() - Time32; uint64_t bla64 = bla32; clock_t Time64 = clock(); uint64_t div64 = div; for (long i = 0; i &lt; LoopMax; i++) { div64 += add; div64 -= minus; bla64 = bla64 / div64; bla64 += bla; bla64 = bla64 * div64; } Time64 = clock() - Time64; double blaDMul = Origbla32; double multodiv = 1.0 / (double)div; double multomul = div; clock_t TimeDMul = clock(); for (i = 0; i &lt; LoopMax; i++) { multodiv += add; multomul -= minus; blaDMul = blaDMul * multodiv; blaDMul += bla; blaDMul = blaDMul * multomul; } TimeDMul = clock() - TimeDMul; double blaDDiv = Origbla32; clock_t TimeDDiv = clock(); for (i = 0; i &lt; LoopMax; i++) { multodiv += add; multomul -= minus; blaDDiv = blaDDiv / multomul; blaDDiv += bla; blaDDiv = blaDDiv / multodiv; } TimeDDiv = clock() - TimeDDiv; float blaS = Origbla32; float divS = div; clock_t TimeS = clock(); for (i = 0; i &lt; LoopMax; i++) { divS += add; divS -= minus; blaS = blaS / divS; blaS += bla; blaS = blaS * divS; } TimeS = clock() - TimeS; printf("64 time: %i us (%i)\n", (int)Time64, (int)bla64); printf("32 time: %i us (%i)\n", (int)Time32, bla32); printf("D(mul) time: %i us (%f)\n", (int)TimeDMul, blaDMul); printf("D(div) time: %i us (%f)\n", (int)TimeDDiv, blaDDiv); printf("S time: %i us (%f)\n", (int)TimeS, blaS); return 0; } int main(int argc, char* const argv[]) { Run(0, 10, 0, 0); // adds and minuses 0 so it doesn't affect the math, only kills the opts return 0; } </code></pre>
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    1. USJames McNellis
    1. USboytheo
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      1. POWhat is faster on division? doubles / floats / UInt32 / UInt64 ? in C++/C
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      1. POWhat is faster on division? doubles / floats / UInt32 / UInt64 ? in C++/C
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      1. POWhat is faster on division? doubles / floats / UInt32 / UInt64 ? in C++/C
      1. USChris Vest
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    1. COHighly dependant on platform. What processor is this on?
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      1. POWhat is faster on division? doubles / floats / UInt32 / UInt64 ? in C++/C
      1. USJoe Koberg
    2. COApologies, I meant to add my platform but I was so tired after correcting so many mistakes. Intel Core Duo, 2.1Ghz, Mac OSX 10.6
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      1. USboytheo
    3. COWhy do you think divison is longer the multiplication. On integer it will just be a lookup in a table.
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      1. USMartin York
 

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