Re: [FFmpeg-devel] [PATCH] lavu/arm: remove GCC 4.6- stuff

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

Le lauantaina 8. kesäkuuta 2024, 9.23.48 EEST Andreas Rheinhardt a écrit :
> Rémi Denis-Courmont:
> > Le lauantaina 8. kesäkuuta 2024, 0.26.21 EEST Andreas Rheinhardt a écrit :
> >> (Also: IIRC Clang actually parses inline assembler and optimize it;
> > 
> > That sounds sketchy. While FFmpeg only uses inline assembler as intrinsic
> > ersatz, other projects can and definitely do use them for weirder stuff.
> > That will fail miserably if the compiler modifies the sequence.
> 
> See https://godbolt.org/z/ozeGoWzK7: Clang is smart enough to know that
> the x86-inline asm version of av_bswap32 is self-inverse. (It can also
> evaluate it for constants at compile-time.)

It seems that they have some heuristics not to touch assembler that they are 
not sure that they "completely" understand. If you add just a dummy numeric 
label, the optimisation is not done.

My preference would be to use builtin_bswap (as riscv/ does) on all 
architectures on GCC and Clang, and keep the bespoke C for other compilers and 
external header inclusion.

-- 
雷米‧德尼-库尔蒙
http://www.remlab.net/



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