configura.ac: fix test for SSE2 & SSSE3 assembler support

This patch modifies the SSE2 & SSSE3 tests in configure.ac to use a
global variable to initialize vector variables. In addition, we now
return the value of the computation instead of 0.

This is done so gcc 4.9 (and lower) won't optimize the SSE assembly
instructions (when using -O1 and higher), because then the configure test
might incorrectly pass even though the assembler doesn't support the
SSE instructions (the test will pass because the compiler does support
the intrinsics).

v2: instead of using volatile, use a global variable as input

Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>

diff --git a/configure.ac b/configure.ac
index 3a669092df59b03098b5d95e9dbed4c131f0675f..ae4d3c65bef34381f1f1eb65433282539274449e 100644 (file)
--- a/configure.ac
+++ b/configure.ac
@@ -429,10 +429,11 @@ AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
 #include <mmintrin.h>
 #include <xmmintrin.h>
 #include <emmintrin.h>
+int param;
 int main () {
-    __m128i a = _mm_set1_epi32 (0), b = _mm_set1_epi32 (0), c;
+    __m128i a = _mm_set1_epi32 (param), b = _mm_set1_epi32 (param + 1), c;
        c = _mm_xor_si128 (a, b);
-    return 0;
+    return _mm_cvtsi128_si32(c);
 }]])], have_sse2_intrinsics=yes)
 CFLAGS=$xserver_save_CFLAGS
 
@@ -473,10 +474,11 @@ AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
 #include <xmmintrin.h>
 #include <emmintrin.h>
 #include <tmmintrin.h>
+int param;
 int main () {
-    __m128i a = _mm_set1_epi32 (0), b = _mm_set1_epi32 (0), c;
+    __m128i a = _mm_set1_epi32 (param), b = _mm_set1_epi32 (param + 1), c;
     c = _mm_maddubs_epi16 (a, b);
-    return 0;
+    return _mm_cvtsi128_si32(c);
 }]])], have_ssse3_intrinsics=yes)
 CFLAGS=$xserver_save_CFLAGS