Re: [FFmpeg-devel] [PATCH 03/10] checkasm: Add idctdsp add/put-pixels-clamped tests

["Martin Storsjö" <martin@martin.st>]

On Tue, 29 Mar 2022, Ben Avison wrote:

>> Thirdly - the added test also occasionally fails for the other existing 
>> functions (armv6, neon) and the newly added aarch64 neon version. If you 
>> have e.g. src[] = 32767, dst[] = 255, then the widening 8->16 addition 
>> will overflow, as there's no operation that both widens and clamps at 
>> the same time.
>
> So it does. I obviously just didn't hit those cases in my test runs!
>
> I can't easily test all codecs that use this function, but I just tried 
> instrumenting the VC-1 case and it doesn't appear to actually use this 
> particular function, so I'm none the wiser!
>
> Should I just limit the 16-bit values to +/-0x100 and re-enable the 
> armv4 fast path then?

Yes, I think that'd be the safest path forward. Worst case, the test would 
be slightly too narrow and could miss some valid case - but that's at 
least better than having the test give false positives for perfectly 
correct assembly, that would work just fine for actual decoder use.

// Martin

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