Re: [FFmpeg-devel] [PATCH 3/4] aarch64: Add linux runtime cpu feature detection using getauxval(AT_HWCAP)

["Rémi Denis-Courmont" <remi@remlab.net>]

Le lauantaina 27. toukokuuta 2023, 23.35.14 EEST Martin Storsjö a écrit :
> > NEON detection could be added here, though I've yet to see an Armv8
> > implementation without AdvSIMD.
> 
> I guess we could, but as it's part of the require baseline for armv8-a I
> don't think there's much need for it?

I mean that if you have 95% of the code needed, you might as well add the last 
5%. I don't think it's used by much anything.

> If configured with --disable-neon we
> don't return that cpuflag though.
> 
> > FWIW, DotProd is exposed as HWCAP_ASIMDDP and I8MM is exposed via
> > HWCAP2_I8MM, using trapped ID registers is not (yet) necessary.
> 
> Ah, I see. I guess using those would be more straightforward.
> 
> OTOH, HWCAP_CPUID is available much earlier than HWCAP_ASIMDDP or
> HWCAP2_I8MM (I do some amount of cross building with a fairly old
> sysroot).

If the sysroot shares the not-too-old kernel of the host, then ID registers 
will work anyway. The age of the userspace toolchain affects availability of 
HWCAP constant definitions, not that of ID registers, which is a purely in-
kernel feature.

> I'll think about it, whether it's worth complicating things to
> try both approaches, or if we should just go with the plain HWCAPs here.

I don't really mind either way.

-- 
雷米‧德尼-库尔蒙
http://www.remlab.net/



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