[Webkit-unassigned] [Bug 74054] fast path to accelerate processing in AudioBus::processWithGainFromMonoStereo

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=74054





--- Comment #4 from Wei James <james.wei at intel.com>  2011-12-08 18:23:08 PST ---
(In reply to comment #3)
> A side note on a different optimization.  It's not necessary to implement this instead of the proposed patch.
> 
> With the current patch, we still do (slightly) extra work if needDezipper is true.  During the loop, we may get close enough to the target that we don't need to dezipper anymore.
> 
> Since the dezippering is just an exponential filter, we can figure exactly how many steps will be needed to get with epsilon of the target value.  So compute this value, nConv, and have two loops:
> 
> N = min(nConv, framesToProcess);
> for (k = 0; k < N; ++k) {
>    <loop with dezippering>
> }
> 
> for (k < framesToProcess; ++k) {
>    <loop without dezippering>
> }
> 
> Assuming I did the math correctly, 
> 
> nConv = log(eps/abs(g0 - totalDesiredGain))/log(1 - DezipperRate)
>       = log(eps)/log(1-DezipperRate) 
>          - log(abs(g0-totalDesiredGain))/log(1-DezipperRate)
> 
> where g0 is the starting gain value at the beginning of the routine.  All of the logs are constants except for log(abs(g0-totalDesiredGain)), so the computation of nConv is not too bad.  log, however, is not cheap, so it's hard to tell if we would get a net win or not since the processing in the loops is fairly simple.  A quick and dirty log approximation might be good enough.

Raymond, thanks for the review. I think your optimization solution is more graceful. I want to try it and test the performance to decide whether or not to use it. 

for the performance improvement of my changes, on my Ubuntu 32 bit/Core i7 dev machine, it will get about 2.5~2.9x speed up in different code pathes(sumToBus as true or false, stereo or not etc).

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