Re: [PATCH for-8.0 15/29] include/qemu/int128: Add vector type to Int128Alias

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

On 18/11/22 10:47, Richard Henderson wrote:
> Adding a vector type will make it easier to handle i386
> have_atomic16 via AVX.
> 
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>   include/qemu/int128.h | 10 +++++-----
>   1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/include/qemu/int128.h b/include/qemu/int128.h
> index f62a46b48c..f29f90e6f4 100644
> --- a/include/qemu/int128.h
> +++ b/include/qemu/int128.h
> @@ -479,16 +479,16 @@ static inline void bswap128s(Int128 *s)
>   /*
>    * When compiler supports a 128-bit type, define a combination of
>    * a possible structure and the native types.  Ease parameter passing
> - * via use of the transparent union extension.
> + * via use of the transparent union extension.  Provide a vector type
> + * for use in atomicity on some hosts.
>    */
> -#ifdef CONFIG_INT128
>   typedef union {
>       Int128 s;
> +    uint64_t v __attribute__((vector_size(16)));
> +#ifdef CONFIG_INT128
>       __int128_t i;
>       __uint128_t u;
> -} Int128Alias __attribute__((transparent_union));
> -#else
> -typedef Int128 Int128Alias;
>   #endif /* CONFIG_INT128 */
> +} Int128Alias __attribute__((transparent_union));
>   
>   #endif /* INT128_H */

This triggers a warning with GCC:

include/qemu/int128.h:487:14: warning: alignment of field 'v' (128 bits) 
does not match the alignment of the first field in transparent union; 
transparent_union attribute ignored [-Wignored-attributes]
     uint64_t v __attribute__((vector_size(16)));
              ^
include/qemu/int128.h:486:12: note: alignment of first field is 64 bits
     Int128 s;
            ^
Meson:

Project version: 7.1.91
C compiler for the host machine: gcc-12 (gcc 12.2.0 "gcc-12 (Homebrew 
GCC 12.2.0) 12.2.0")
C linker for the host machine: gcc-12 ld64 819.6
Host machine cpu family: aarch64
Host machine cpu: arm64