[FFmpeg-devel] [PATCH] avutil: Fix linking x86 asm constants with Clang in MSVC mode

[Martin Storsjö]

On Thu, 12 Jun 2025, Martin Storsjö wrote:

> This fixes building with Clang in MSVC mode, for x86, which was
> broken in 6e49b8699657b808b7dc80033f2c3f2d0e029fa3 (in Nov 2024);
> previously it failed with undefined symbols for the constants
> defined with DECLARE_ASM_CONST, accessed via inline assembly.
>
> Before 57861911a34e1c33796be97f2b2f44e05fffd647, there was an
>    #elif defined(__GNUC__) || defined(__clang__)
> case before the
>    #elif defined(_MSC_VER)
> case for defining DECLARE_ASM_CONST, which included av_used.
> (This case included the explicit "defined(__clang__)" since
> f637046d3134a331e4b5a7243ac3dfb92735b8a5.)
>
> After 57861911a34e1c33796be97f2b2f44e05fffd647, it used the
> generic definition of DECLARE_ASM_CONST that also included
> av_used - which also worked for Clang in MSVC mode. But after
> 6e49b8699657b808b7dc80033f2c3f2d0e029fa3, Clang in MSVC mode
> ended up using the MSVC specific variant which lacked the
> av_used declaration, causing linker errors due to undefined
> symbols.
> ---
> libavutil/mem_internal.h | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/libavutil/mem_internal.h b/libavutil/mem_internal.h
> index d58881d09c..78adc4f407 100644
> --- a/libavutil/mem_internal.h
> +++ b/libavutil/mem_internal.h
> @@ -82,8 +82,8 @@
>     #define DECLARE_ASM_CONST(n,t,v)    alignas(FFMIN(n, 16)) static const t av_used v
> #elif defined(_MSC_VER)
>     #define DECLARE_ALIGNED_T(n,t,v)    __declspec(align(n)) t v
> -    #define DECLARE_ASM_ALIGNED(n,t,v)  __declspec(align(n)) t v
> -    #define DECLARE_ASM_CONST(n,t,v)    __declspec(align(n)) static const t v
> +    #define DECLARE_ASM_ALIGNED(n,t,v)  __declspec(align(n)) t av_used v
> +    #define DECLARE_ASM_CONST(n,t,v)    __declspec(align(n)) static const t av_used v
> #else
>     #define DECLARE_ALIGNED_T(n,t,v)    alignas(n) t v
>     #define DECLARE_ASM_ALIGNED(n,t,v)  alignas(n) t av_used v
> -- 
> 2.43.0

OK'd by Hendrik on irc, as I've tested it with both clang and actual MSVC 
- will push.

// Martin