On Sun, 12 Jun 2022, Martin Storsjö wrote:
On Sun, 12 Jun 2022, Soft Works wrote:
-----Original Message-----
From: ffmpeg-devel <ffmpeg-devel-boun...@ffmpeg.org> On Behalf Of
Andreas Rheinhardt
Sent: Sunday, June 12, 2022 7:28 AM
To: ffmpeg-devel@ffmpeg.org
Cc: Andreas Rheinhardt <andreas.rheinha...@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.rheinha...@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.
... because eliminating unused code (for e.g. debug printouts) using DCE
instead of ifdefs has got the advantage that the skipped code still is
syntax checked (so it doesn't bitrot), compared to in ifdefs where it can
easily become stale.
Still not an argument against this patch, only against seemingly incorrect
statements.
// Martin
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