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

["Martin Storsjö" <martin@martin.st>]

On Sun, 12 Jun 2022, Soft Works 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.

Can you qualify this statement? This does not match my understanding.

(I'm not opposed to the patch itself though.)

// Martin

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