FFmpeg has instances of DECLARE_ALIGNED(32, ...) in a lot of structs, which then end up heap-allocated. By declaring any variable in a struct, or tree of structs, to be 32 byte aligned, it allows the compiler to safely assume the entire struct itself is also 32 byte aligned.
This might make the compiler emit code which straight up crashes or misbehaves in other ways, and at least in one instances is now documented to actually do (see ticket 10549 on trac). The issue there is that an unrelated variable in SingleChannelElement is declared to have an alignment of 32 bytes. So if the compiler does a copy in decode_cpe() with avx instructions, but ffmpeg is built with --disable-avx, this results in a crash, since the memory is only 16 byte aligned. Mind you, even if the compiler does not emit avx instructions, the code is still invalid and could misbehave. It just happens not to. Declaring any variable in a struct with a 32 byte alignment promises 32 byte alignment of the whole struct to the compiler. Instead of now going through all instances of variables in structs being declared as 32 byte aligned, this patch bumps the minimum alignment to 32 bytes. --- libavutil/mem.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/libavutil/mem.c b/libavutil/mem.c index 36b8940a0c..26a9b9753b 100644 --- a/libavutil/mem.c +++ b/libavutil/mem.c @@ -62,7 +62,7 @@ void free(void *ptr); #endif /* MALLOC_PREFIX */ -#define ALIGN (HAVE_AVX512 ? 64 : (HAVE_AVX ? 32 : 16)) +#define ALIGN (HAVE_AVX512 ? 64 : 32) /* NOTE: if you want to override these functions with your own * implementations (not recommended) you have to link libav* as -- 2.34.1 _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-devel To unsubscribe, visit link above, or email ffmpeg-devel-requ...@ffmpeg.org with subject "unsubscribe".
