Re: [FFmpeg-devel] [PATCH v2 3/5] lavc/vp9dsp: R-V V mc tap h v - Rémi Denis-Courmont

From: "Rémi Denis-Courmont" <remi@remlab.net>
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
Subject: Re: [FFmpeg-devel] [PATCH v2 3/5] lavc/vp9dsp: R-V V mc tap h v
Date: Sat, 25 May 2024 20:36:03 +0300
Message-ID: <2679398.BddDVKsqQX@basile.remlab.net> (raw)
In-Reply-To: <CAEa-L+tKWWS8=VBjRY89Jjf5N8cvq3HYxGwW0BskJkg_zvnjFQ@mail.gmail.com>

Le lauantaina 25. toukokuuta 2024, 13.38.39 EEST flow gg a écrit :
> > Is there a reason that you cannot use the tables from C code?
> 
> Similar to VP8, to adjust the positive and negative data and prevent small
> probability overflow during calculations.
> 
> > AFAICT, regular and sharp are identical, except for the base address of
> > the filter table, so it should be possible to share the byte code
> 
> Initially, they used the same code, but after testing hundreds of times,
> there were always a few failures...

AFAICT, the C reference and the AArch64 assembler are exactly the same for all 
3 filters, except for the offset in the filter table.  So logically, it ought toe 
be possible to merge regular and sharp with almost no changes, and merge 
smooth with a few fixes.

As for the tables themselves, it seems the "problem" is that C uses 16-bit 
values for no apparent reasons. We should probably change the C code to use 8-
bit on all platforms - except AArch64, because the NEON code probably relies 
on 16-bit format.

-- 
Rémi Denis-Courmont
http://www.remlab.net/

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