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

[Timothy B. Terriberry]

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.