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

["Ronald S. Bultje" <rsbultje@gmail.com>]

Hi,

On Thu, May 30, 2024 at 10:03 AM Tomas Härdin <git@haerdin.se> wrote:

> tor 2024-05-30 klockan 16:06 +0300 skrev Rémi Denis-Courmont:
> >
> >
> > Le 30 mai 2024 12:50:05 GMT+03:00, "Tomas Härdin" <git@haerdin.se> a
> > écrit :
> > > tor 2024-05-30 klockan 10:54 +0300 skrev Rémi Denis-Courmont:
> > > > 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.
> > >
> > > I think we still support MSVC, but maybe we shouldn't? It's
> > > possible to
> > > cross-compile for Windows either way.
> >
> > I don't get how that prevents using the GCC and Clang builtins (on
> > GCC and Clang).
>
> Does MSVC have builtins for these? Do all compilers we support?
>

I think what Remi is suggesting is that someone (maybe Remi himself, maybe
you, maybe me, maybe someone else) should send a patch to use the built-ins
when available. Where not, we would continue using the C versions.

All of this is unrelated to this patch, which would continue to be useful
for the C fallback.

Ronald
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