From e7e3aa29482281edba46a27fcd452d7ed630f46a Mon Sep 17 00:00:00 2001
From: Bart Oldeman <bartoldeman@users.noreply.github.com>
Date: Thu, 27 Oct 2022 17:20:44 -0400
Subject: [PATCH] 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 | 13 +++++++++++++
 kernel/x86_64/zscal.c | 13 +++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/kernel/x86_64/cscal.c b/kernel/x86_64/cscal.c
index dc3f688c6..6ae66d973 100644
--- a/kernel/x86_64/cscal.c
+++ b/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"
 
 
diff --git a/kernel/x86_64/zscal.c b/kernel/x86_64/zscal.c
index 3744c98bb..dfdb4230b 100644
--- a/kernel/x86_64/zscal.c
+++ b/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"