[FFmpeg-devel] [PATCH]levc/hevc_cabac Optimise ff_hevc_hls_residual_coding (especially ARM)

[Christophe Gisquet]

Hi,

2016-01-20 15:27 GMT+01:00 John Cox <jc at kynesim.co.uk>:
> The by22 code gained me an overall factor of two in the abs level decode
> - the gains do depend a lot on the quantity of residual - you gain a lot
> more on I-frames than you do otherwise as they tend to have much longer
> residuals.  The higher the bitrate the more useful this code is.  But as
> you note it didn't use vast amounts of time relative to everything else
> anyway.
>
> The reworking / simplification of the loop(s) around the abs level
> decode and the scaling gave me the biggest single improvement.

The thing is, it provided no gain on no Win64 system I had at hand. Or
very minor, once I switched off things. The amount of new/changed code
would make it worth discussing, were it not for actual gains on arm.

> After that the reworking of get_sig_ceoff_flag_idxs was a useful gain

Yes, this is the most agreeable part of the non-applied parts.

> Special caseing the single coeff path gave a similar gain

This is a big slowdown on Win64 and UHD-bluray like sequences, but
that can be switched off in that case.

> After that the scale rework - now probably 75% faster than it was
> previously but it wasn't taking a huge amount of time.

The work is done, I don't mind.

> And after that all the other bits - my experience with optimising this
> sort of code (I did a lot of work on a TI H.264 implementation in the
> past) is that no single change is going to do everything, you just have
> to polish everything until it goes fast enough.

Sure. There may be positive interactions, but my own figures showed
the sigmap/greater than flags were the only ones worth optimizing on
Win64.

> Sorry - I don't quite understand what you've said here.

Doesn't matter anymore, I think I have just laid out the parts
actually mattering, and for haswell/Win64 (ie x86_64).

I'll reply more in depth to the new patchset, but not until you're on
holidays. Which should leave me more time for reviewing it, so all the
better.

Best regards,
-- 
Christophe