From: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
To: ffmpeg-devel@ffmpeg.org
Subject: Re: [FFmpeg-devel] [PATCH] lavu/arm: remove GCC 4.6- stuff
Date: Sat, 8 Jun 2024 08:23:48 +0200
Message-ID: <AS8P250MB0744A51D65CDB8C28B5CF31A8FC42@AS8P250MB0744.EURP250.PROD.OUTLOOK.COM> (raw)
In-Reply-To: <2053795.EyUWCETuIM@basile.remlab.net>
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.)
>
> The Linux kernel for instance pushes/pops sections or patches the byte
> sequence at load time or even run time. Some code also assumes exact byte
> offsets between instructions (not sure Linux specifically). So, AFAICT, the only
> thing the compiler can sanely do is move, and maybe eliminate/replicate, the
> entire assembler block.
>
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next prev parent reply other threads:[~2024-06-08 6:24 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-06-07 16:49 Rémi Denis-Courmont
2024-06-07 21:16 ` Sean McGovern
2024-06-07 21:26 ` Andreas Rheinhardt
2024-06-08 6:15 ` Rémi Denis-Courmont
2024-06-08 6:23 ` Andreas Rheinhardt [this message]
2024-06-08 6:43 ` Rémi Denis-Courmont
2024-06-08 13:41 ` Andreas Rheinhardt
2024-06-08 14:38 ` Rémi Denis-Courmont
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