Re: [FFmpeg-devel] [PATCH 6/6] avcodec/vc1: Introduce fast path for unescaping bitstream buffer

[Ben Avison <bavison@riscosopen.org>]

On 18/03/2022 19:10, Andreas Rheinhardt wrote:
> Ben Avison:
>> +static int vc1_unescape_buffer_neon(const uint8_t *src, int size, uint8_t *dst)
>> +{
>> +    /* Dealing with starting and stopping, and removing escape bytes, are
>> +     * comparatively less time-sensitive, so are more clearly expressed using
>> +     * a C wrapper around the assembly inner loop. Note that we assume a
>> +     * little-endian machine that supports unaligned loads. */
> 
> You should nevertheless use AV_RL32 for your unaligned LE loads

Thanks - I wasn't aware of that. I'll add it in.

> 1. You should add some benchmarks to the commit message.

Do you mean for each commit, or this one in particular? Are there any 
particular standard files you'd expect to see benchmarked, or will the 
ones I used in the cover-letter do? (Those were just snippets from 
problematic BluRay rips, but that does mean I don't have the rights to 
redistribute them.) I believe there should be conformance bitstreams for 
VC-1 somewhere, but I wasn't able to locate them.

During development, I wrote a simple benchmarker for this particular 
patch, which measures the throughput of processing random data (which 
doesn't contain the escape sequence at any point). I've just pushed it 
here if anyone's interested:

https://github.com/bavison/test-unescape

The compile-time define VERSION there takes a few different values:
1: the original C implementation of vc1_unescape_buffer()
2: an early prototype version I wrote that uses unaligned 32-bit loads, 
again in pure C
3: the NEON assembly versions

The sort of speeds this measures are:
             AArch32    AArch64
version 1   210 MB/s   292 MB/s
version 2   461 MB/s   435 MB/s
version 3  1294 MB/s  1554 MB/s

> 2. The unescaping process for VC1 is basically the same as for H.264 and
> HEVC* and for those we already have better optimized code in
> libavcodec/h2645_parse.c. Can you check the performance of this code
> here against (re)using the code from h2645_parse.c?

I've hacked that around a bit to match the calling conditions of 
vc1_unescape_buffer(), though not adapted it for the slightly different 
rules you noted for VC-1 as opposed to H.264/265. Hopefully it should 
still give some indication of the approximate performance that could be 
expected, but I didn't take time to fully understand everything it was 
doing, so do please say if I've messed something up.

This can be selected by #defining VERSION 4:

             AArch32    AArch64
version 4   737 MB/s  1286 MB/s

This suggests it's much better than the original C, but my NEON versions 
still have the edge, especially on AArch32. The NEON code is very much a 
brute force check, but it's effectively able to do the testing in 
parallel with the memcpy - each byte only gets loaded once.

Ben
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