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[fluid-dev] Floats and doubles, simd and interpolation

From: David Henningsson
Subject: [fluid-dev] Floats and doubles, simd and interpolation
Date: Mon, 22 Nov 2010 09:17:38 +0100
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Just a short dump of what's been on my mind the last few weeks...

Yesterday, I tried to chase down a bug I discovered - I rendered a song but a few notes were slightly out of tune, but only when using floats, not when using doubles! I never bought that doubles should sound better than floats - floats has 24 bits of mantissa precision, that's the best you can get out of a soundcard anyway, and since all 24 bits soundcards have a noise floor that make them no more than 20 bits anyway (right?), floats should be enough, even given some rounding errors. (Not to mention all the analog components after the DAC...) I also doubt that anyone would hear the difference between 440.000 Hz and 440.001 Hz, if the error is that way. So it must be a bug somewhere.

While looking at that I saw that the interpolation routines has conversions to and from double (not fluid_real_t), so I suspect that could be why some people claim that doubles are faster than floats in some cases.

So the reason I like floats is that with SSE, you can process 4 floats simultaneously, but only 2 doubles. From running a perf I know that 2/3 of the time (for my testcase) was spent in the interpolation routine. Can we SIMD:ize that, we might get 3-4x speed improvement, that's at least what I hope for.

There is a library called "ORC", anybody heard of it? You write some pseudo-assembly code, and on first run ORC translates it into SSE, MMX, Altivec, etc, or plain old C depending on your hardware. I think it sounds interesting, and was hoping to see if I could make a test soon, but then I got busy trying to find that bug instead.

// David

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