Re: [FFmpeg-devel] [PATCH 1/5] lavu/common.h: Fix UB in av_clipl_int32_c() - Rémi Denis-Courmont

From: "Rémi Denis-Courmont" <remi@remlab.net>
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
Subject: Re: [FFmpeg-devel] [PATCH 1/5] lavu/common.h: Fix UB in av_clipl_int32_c()
Date: Thu, 30 May 2024 14:50:29 +0300
Message-ID: <F1FFB120-235B-4A3A-824F-AD34C2916C22@remlab.net> (raw)
In-Reply-To: <efc216aa13735024377e2efacfffa090da1e7e9b.camel@haerdin.se>

Le 30 mai 2024 12:40:20 GMT+03:00, "Tomas Härdin" <git@haerdin.se> a écrit :
>tor 2024-05-30 klockan 09:41 +0300 skrev Rémi Denis-Courmont:
>> Hi,
>> 
>> Le 30 mai 2024 01:13:14 GMT+03:00, "Tomas Härdin" <git@haerdin.se> a
>> écrit :
>> > The entire patchset passes FATE
>> 
>> Is the version in riscv/intmath.h safe? It looks to me that the GCC
>> codegen for not only RV64 but also AArch{32,64} and x86-64 is better
>> than this.
>
>I haven't checked. It seems weird to me to have two different C
>versions.

The common one ends up horrendously bad on RV, and presumably on MIPS and some other RISC ISA.

> We shouldn't rely on type punning.

Because?

We should depend on punning as long as it conforms to the standard.

> The standard compliant way
>is to use memcpy()

That's way worse than union in terms of how proactively the compiler needs to optimise, and both approaches are as confirming.
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