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POWhy in this example using floats makes me go 2x slower than with doubles?
 

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  1. POWhy in this example using floats makes me go 2x slower than with doubles?
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    <p>I've been doing some profiling lately and I've encountered one case which is driving me nuts. The following is a piece of unsafe C# code which basically copies a source sample buffer to a target buffer with a different sample rate. As it is now, it takes up ~0.17% of the total processing time per frame. What I don't get is that if I use floats instead of doubles, the processing time will raise to 0.38%. Could someone please explain what's going on here?</p> <p><b>Fast version (~17%)</b></p> <pre><code>double rateIncr = ... double readOffset = ... double offsetIncr = ... float v = ... // volume // Source and target buffers. float* src = ... float* tgt = ... for( var c = 0; c &lt; chunkCount; ++c) { for( var s = 0; s &lt; chunkSampleSize; ++s ) { // Source sample var iReadOffset = (int)readOffset; // Interpolate factor var k = (float)readOffset - iReadOffset; // Linearly interpolate 2 contiguous samples and write result to target. *tgt++ += (src[ iReadOffset ] * (1f - k) + src[ iReadOffset + 1 ] * k) * v; // Increment source offset. readOffset += offsetIncr; } // Increment sample rate offsetIncr += rateIncr; } </code></pre> <p><b>Slow version (~38%)</b></p> <pre><code>float rateIncr = ... float readOffset = ... float offsetIncr = ... float v = ... // volume // Source and target buffers. float* src = ... float* tgt = ... for( var c = 0; c &lt; chunkCount; ++c) { for( var s = 0; s &lt; chunkSampleSize; ++s ) { var iReadOffset = (int)readOffset; // The cast to float is removed var k = readOffset - iReadOffset; *tgt++ += (src[ iReadOffset ] * (1f - k) + src[ iReadOffset + 1 ] * k) * v; readOffset += offsetIncr; } offsetIncr += rateIncr; } </code></pre> <p><b>Odd version(~22%)</b></p> <pre><code>float rateIncr = ... float readOffset = ... float offsetIncr = ... float v = ... // volume // Source and target buffers. float* src = ... float* tgt = ... for( var c = 0; c &lt; chunkCount; ++c) { for( var s = 0; s &lt; chunkSampleSize; ++s ) { var iReadOffset = (int)readOffset; var k = readOffset - iReadOffset; // By just placing this test it goes down from 38% to 22%, // and the condition is NEVER met. if( (k != 0) &amp;&amp; Math.Abs( k ) &lt; 1e-38 ) { Console.WriteLine( "Denormalized float?" ); } *tgt++ += (src[ iReadOffset ] * (1f - k) + src[ iReadOffset + 1 ] * k) * v; readOffset += offsetIncr; } offsetIncr += rateIncr; } </code></pre> <p>All I know by now is that I know nothing</p>
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      1. POWhy in this example using floats makes me go 2x slower than with doubles?
      1. USAdam Ralph
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      1. UScyberconte
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    1. COIs it unsafe C# or C++? please specify and retag your question accordingly.
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      1. USem70
    2. COIt's unsafe C#, (second line of question), but I thought I'd tag it as C++ also so I could benefit from both groups' experience.
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    3. COJust to appease my curiosity what happens in the second case if you add a line after the declaration of k that casts k to a float (k = (float)k). I know that it is already a float, but based on the second link I supplied below it may not be a float internally unless you explicitly cast it - "if you want narrowing, you will have to insert (float) and (double) casts by hand". The extra "do nothing" code in the third example may be forcing this narrowing to happen and speed up the rest of the method.
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      1. USMartin Harris
 

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