Re: [FFmpeg-devel] [PATCH] all: Replace if (ARCH_FOO) checks by #if ARCH_FOO

[Hendrik Leppkes <h.leppkes@gmail.com>]

On Sun, Jun 12, 2022 at 10:17 AM Soft Works <softworkz@hotmail.com> wrote:
>
>
>
> > -----Original Message-----
> > From: ffmpeg-devel <ffmpeg-devel-bounces@ffmpeg.org> On Behalf Of
> > Andreas Rheinhardt
> > Sent: Sunday, June 12, 2022 7:28 AM
> > To: ffmpeg-devel@ffmpeg.org
> > Cc: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
> > Subject: [FFmpeg-devel] [PATCH] all: Replace if (ARCH_FOO) checks by
> > #if ARCH_FOO
> >
> > This is more spec-compliant because it does not rely
> > on dead-code elimination by the compiler. Especially
> > MSVC has problems with this, as can be seen in
> > https://ffmpeg.org/pipermail/ffmpeg-devel/2022-May/296373.html
> > or
> > https://ffmpeg.org/pipermail/ffmpeg-devel/2022-May/297022.html
> >
> > This commit does not eliminate every instance where we rely
> > on the dead code elimination: It only tackles branching to
> > the initialization of arch-specific dsp code, not e.g. all
> > uses of CONFIG_ and HAVE_ checks. But maybe it is already
> > enough to compile FFmpeg with MSVC with whole-programm-optimizations
> > enabled (if one does not disable too many components).
> >
> > Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
> > ---
>
> LGTM.
>
> It's not really a story as simple as "poor MSVC is unable
> to perform dead-code-elimination". It is actually capable to do that,
> but the ffmpeg code was not only requiring the compiler to eliminate
> dead code, it actually required a compiler to ignore dead code blocks
> even when those would contain invalid code that cannot be compiled
> at all.

This is inaccurate. This code can compile just fine. It is linking
that fails, because the functions are not generated, which would not
be a problem if it is removed as part of DCE before the linking stage.

- Hendrik
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