Re: [FFmpeg-devel] [PATCH 4/5] lavu/intmath.h: Fix UB in ff_ctz_c() and ff_ctzll_c()

From: Michael Niedermayer <michael@niedermayer.cc>
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
Subject: Re: [FFmpeg-devel] [PATCH 4/5] lavu/intmath.h: Fix UB in ff_ctz_c() and ff_ctzll_c()
Date: Fri, 31 May 2024 02:41:40 +0200
Message-ID: <20240531004140.GL2821752@pb2> (raw)
In-Reply-To: <14560CCC-A959-44E7-A2E8-764DBE6478C0@remlab.net>

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On Thu, May 30, 2024 at 10:54:46AM +0300, Rémi Denis-Courmont wrote:
> Can't we just use the compiler built-ins here? AFAIK, they (GCC, LLVM) use the same algorithm if the CPU doesn't support native CTZ. And they will pick the right instruction if CPU does have CTZ.
> 
> I get it that maybe it wasn't working so well 20 years ago, but we've increased compiler version requirements since then.

ffmpeg is written in C not GNU-C nor LLVM-C
so we need to have non buggy C code

A modern compiler should turn a built-in into a efficient piece of code
but so should it recognize that efficient piece of code and turn it into
a single instruction if such exist.

In the end with a modern compiler it shouldnt matter how you write this
a loop, some optimized standard implementation or a built in
the disavantage of a builtin is that it requires #ifs and checks
for each compiler.
So we should go with the simplest/cleanest that reliably produces optimal code
across all supported platforms (and also consider potential future platforms
so we dont have a maintaince nightmare but something that we can write once
and expect it will be close to optimal forever)

Above though is off topic in this thread, which was a bugfix that we need anyway
(also for backporting)

thx

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Does the universe only have a finite lifespan? No, its going to go on
forever, its just that you wont like living in it. -- Hiranya Peiri

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