[FFmpeg-cvslog] intreadwrite: Indicate potential aliasing in AV_RN/AV_WN for Clang/MSVC mode

Martin Storsjö git at videolan.org
Fri Aug 4 21:51:48 EEST 2023

ffmpeg | branch: master | Martin Storsjö <martin at martin.st> | Wed Aug  2 00:30:29 2023 +0300| [62fac040c83bea33d43bbc98d6f11189b058d097] | committer: Martin Storsjö

intreadwrite: Indicate potential aliasing in AV_RN/AV_WN for Clang/MSVC mode

Use the GCC specific codepath for Clang in MSVC mode too.
This matches the condition used in a number of other places.

MSVC doesn't have a way to signal potential aliasing, while GCC
(and Clang) can use __attribute__((may_alias)) for this purpose.

When building with Clang in MSVC mode, __GNUC__ isn't defined but
_MSC_VER is as Clang primarily impersonates MSVC - but even then it
does support the GCC style attributes.

The GCC specific codepath uses av_alias, which expands to
the may_alias attribute if supported. The MSVC specific codepath
doesn't use av_alias so far (as MSVC doesn't support any
corresponding attribute).

This fixes a couple HEVC decoder tests when built with Clang 14 or
newer in MSVC mode (with issues observed on all of x86_64, armv7
and aarch64).

Signed-off-by: Martin Storsjö <martin at martin.st>

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=62fac040c83bea33d43bbc98d6f11189b058d097

 libavutil/intreadwrite.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libavutil/intreadwrite.h b/libavutil/intreadwrite.h
index c4679fdba4..21df7887f3 100644
--- a/libavutil/intreadwrite.h
+++ b/libavutil/intreadwrite.h
@@ -213,7 +213,7 @@ typedef union {
  * by per-arch headers.
-#if defined(__GNUC__)
+#if defined(__GNUC__) || defined(__clang__)
 union unaligned_64 { uint64_t l; } __attribute__((packed)) av_alias;
 union unaligned_32 { uint32_t l; } __attribute__((packed)) av_alias;

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