[opus] [Opus][RFC][FFT][Fixed-Point][NEON] NEON-Optimize Fixed-Point FFT?

[Phil Wang]

Thanks Timothy and Jean-Marc,

I will start NEON optimizing fixed-point FFT. Is it int32 good enough? Benchmark data shows that FFT using int16 is much faster than FFT using int32.

> -----Original Message-----
> From: Timothy B. Terriberry [mailto:tterribe at xiph.org]
> Sent: Friday, December 26, 2014 6:52 AM
> To: Phil Wang; opus at xiph.org
> Cc: Zhongwei Yao; Yang Zhang; Zhou (Joe) Yu; Steve Bannister
> Subject: Re: [opus] [Opus][RFC][FFT][Fixed-Point][NEON] NEON-Optimize
> Fixed-Point FFT?
>
> Jean-Marc Valin wrote:
> > There is definitely some use for a Neon fixed-point FFT. How much
> > exactly I'm not sure. Fixed-point is a bit more than just a fall-back
>
> Well, we use fixed-point mode by default in Firefox for both Firefox OS and
> Fennec (Firefox on Android). The reason is that, although there is some NEON-
> class hardware where float does finally appear to be a little bit faster (e.g.,
> recent A9's), there are still plenty where it is _significantly_ slower. So if
> you're going to pick one version to run on many devices, fixed-point has much
> better worst-case performance.


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