Re: [FFmpeg-devel] [PATCH] avutil/mem: always align by at least 32 bytes

[James Almer <jamrial@gmail.com>]

On 12/3/2023 5:10 PM, Timo Rothenpieler wrote:
> 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.

Wont we run into similar issues with avx512 eventually?

> 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
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