[Mesa-dev] [PATCH] configure.ac: fix test for SSE4.1 assembler support

[Matt Turner]

On Tue, Dec 8, 2015 at 9:37 PM, Jonathan Gray <jsg at jsg.id.au> wrote:
> Change the __m128i variables to be volatile so gcc 4.9 won't optimise
> all of them out with -O1 or greater.  The _mm_set1_epi32/pinsrd calls
> still get optimised out but now there is at least one SSE4.1 instruction
> generated via _mm_max_epu32/pmaxud.  When all of the sse4.1 instructions
> got optimised out the configure test would incorrectly pass when the
> compiler supported the intrinsics and the assembler didn't support the
> instructions.
>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91806
> Signed-off-by: Jonathan Gray <jsg at jsg.id.au>
> Cc: "11.0 11.1" <mesa-stable at lists.freedesktop.org>
> ---
>  configure.ac | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/configure.ac b/configure.ac
> index 260934d..1d82e47 100644
> --- a/configure.ac
> +++ b/configure.ac
> @@ -384,7 +384,7 @@ CFLAGS="$SSE41_CFLAGS $CFLAGS"
>  AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
>  #include <smmintrin.h>
>  int main () {
> -    __m128i a = _mm_set1_epi32 (0), b = _mm_set1_epi32 (0), c;
> +    volatile __m128i a = _mm_set1_epi32 (0), b = _mm_set1_epi32 (0), c;
>      c = _mm_max_epu32(a, b);
>      return 0;

I would have extracted an int from the result of _mm_max_epu32 and
returned that instead of 0.