x86_64: prevent GCC and Clang from generating FMAs in cscal/zscal.

If e.g. -march=haswell is set in CFLAGS, GCC generates FMAs by default, which
is inconsistent with the microkernels, none of which use FMAs. These
inconsistencies cause a few failures in the LAPACK testcases, where
eigenvalue results with/without eigenvectors are compared.

Moreover using FMAs for multiplication of complex numbers can give surprising
results, see 22aa81f for more information.

This uses the same syntax as used in 22aa81f for zarch (s390x).

kernel/x86_64/cscal.c

@@ -25,6 +25,19 @@ OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE
 USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *****************************************************************************/
 
+/*
+ * Avoid contraction of floating point operations, specifically fused
+ * multiply-add, because they can cause unexpected results in complex
+ * multiplication.
+ */
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC optimize ("fp-contract=off")
+#endif
+
+#if defined(__clang__)
+#pragma clang fp contract(off)
+#endif
+
 #include "common.h"
 
 

kernel/x86_64/zscal.c

@@ -25,6 +25,19 @@ OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE
 USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *****************************************************************************/
 
+/*
+ * Avoid contraction of floating point operations, specifically fused
+ * multiply-add, because they can cause unexpected results in complex
+ * multiplication.
+ */
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC optimize ("fp-contract=off")
+#endif
+
+#if defined(__clang__)
+#pragma clang fp contract(off)
+#endif
+
 #include "common.h"