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POnanoseconds to milliseconds - fast division by 1000000
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  1. POnanoseconds to milliseconds - fast division by 1000000
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    copied!<p>I'm wanting to convert the output from gethrtime to milliseconds.</p> <p>The obvious way to do this is to divide by 1000000. However, I'm doing this quite often and wonder if it could become a bottleneck.</p> <p>Is there an optimized divide operation when dealing with numbers like 1000000?</p> <p>Note: Any code must be portable. I'm using gcc and this is generally on Sparc hardware</p> <p>Some quick testing using the code below... hope that is right.</p> <pre><code>#include &lt;sys/time.h&gt; #include &lt;iostream&gt; using namespace std; const double NANOSECONDS_TO_MILLISECONDS = 1.0 / 1000000.0; int main() { hrtime_t start; hrtime_t tmp; hrtime_t fin; start = gethrtime(); tmp = (hrtime_t)(start * NANOSECONDS_TO_MILLISECONDS); fin = gethrtime(); cout &lt;&lt; "Method 1" cout &lt;&lt; "Original val: " &lt;&lt; start &lt;&lt; endl; cout &lt;&lt; "Computed: " &lt;&lt; tmp &lt;&lt; endl; cout &lt;&lt; "Time:" &lt;&lt; fin - start &lt;&lt; endl; start = gethrtime(); tmp = (start / 1000000); fin = gethrtime(); cout "Method 2" cout &lt;&lt; "Original val: " &lt;&lt; start &lt;&lt; endl; cout &lt;&lt; "Computed: " &lt;&lt; tmp &lt;&lt; endl; cout &lt;&lt; "Time:" &lt;&lt; fin - start &lt;&lt; endl; return 0; } </code></pre> <p>Example outputs:</p> <pre><code>Original val: 3048161553965997 Computed: 3048161553 Time:82082 Original val: 3048161556359586 Computed: 3048161556 Time:31230 Original val: 3048239663018915 Computed: 3048239663 Time:79381 Original val: 3048239665393873 Computed: 3048239665 Time:31321 Original val: 3048249874282285 Computed: 3048249874 Time:81812 Original val: 3048249876664084 Computed: 3048249876 Time:34830 </code></pre> <p>If this is correct, then the multiple by reciprocal is actually slower in this case. It's probably due to using floating point math instead of fixed point math. I will just stick to integer division then which still takes hardly any time at all.</p>
 

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