[Webkit-unassigned] [Bug 75528] Optimize the multiply-add in Biquad.cpp::process

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=75528





--- Comment #12 from xingnan.wang at intel.com  2012-01-05 17:59:50 PST ---
(In reply to comment #8)
> (From update of attachment 121082 [details])
> View in context: https://bugs.webkit.org/attachment.cgi?id=121082&action=review
> 
> > Source/WebCore/platform/audio/Biquad.cpp:90
> > +#ifdef __SSE2__
> 
> Did you test this on windows?  I think the asm syntax below only works for gcc, so this conditionalization should test for gcc too.  (But I guess on windows, __SSE2__ is probably never defined.)
Yes, there is another macro in windows. I just started with linux support only, in case of any potential issues. I can add the windows support and do the test.
> 
> > Source/WebCore/platform/audio/Biquad.cpp:93
> > +    __asm__(
> 
> I am far from an expert in sse2, but this seems rather complex.  Could we do something like this?
> 
> Create array yy of length 2 (or maybe use the destination array directly?):
> yy[0] = y
> yy[1] = y1
> 
> load xmm0 with (b0 b1 b2 0)
> load xmm1 with (-a0 -a1 0 0)
> load xmm2 from *source to get (x[0] x[1] x[2] x[3])
> xmm2 = xmm2 * xmm0 to get (b0*x0 b1*x1 b2*x2 0)
> load xmm3 from yy to get (y0 y1 junk junk)
> xmm3 = xmm3 * xmm1 to get (-a0*y0 a1*y1 0 0)
> xmm3 = xmm3 + xmm2 to get (b0*x0-a0*y0, b1*x1-a1*y1, b2*x2, 0)
> 
> Extract each part of xmm3 and add them together.  (We could gain something here if we had SSE3 to do the add, I think.)
> 
> yy[0] = yy[1]
> yy[1] = result of sum.
> 
> Don't know if this is faster or slower.  This will change results slightly because we do everything in single precision.

Your solution may work in single precision and I didn`t try.
I have tried this way in double precision but no too much improvement. Then I changed to use inline asm so that I can pipeline the instructions, also get the benefit from SSE2.

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