[ovs-dev] [PATCH 2/2] configure: explicitly disable avx512 if bintuils check fails

[Harry van Haaren]

This commit explicitly disables avx512f if the binutils assembler
check fails to correctly assemble its input.

Without this fix, there is a possibility that users can see undefined
behaviour when compiling with -march=native on a CPU which supports
avx512 and with a buggy binutils version (v2.30 and 2.31), without a
backported fix, if the compiler's vectorizing optimizations convert
scalar code to avx512 instructions.

Signed-off-by: Harry van Haaren <harry.van.haaren at intel.com>

---

Note that the compiler autovectorization with binutils bug could be
present in previous releases, and was not introduced with the 2.14
dpcls avx512 optimizations. The adding of avx512 to dpcls required
the binutils checks, resulting in a shorter fix to this issue.

---
 m4/openvswitch.m4 | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/m4/openvswitch.m4 b/m4/openvswitch.m4
index 7c9a507e5..6fe79297e 100644
--- a/m4/openvswitch.m4
+++ b/m4/openvswitch.m4
@@ -426,8 +426,12 @@ AC_DEFUN([OVS_CHECK_BINUTILS_AVX512],
          CFLAGS="$CFLAGS -DHAVE_LD_AVX512_GOOD"
        else
          ovs_cv_binutils_avx512_good=no
+         dnl Explicitly disallow avx512f to stop compiler auto-vectorizing
+         dnl and causing zmm usage with buggy binutils versions.
+         CFLAGS="$CFLAGS -mno-avx512f"
        fi
      else
+       dnl non x86_64 architectures don't have avx512, so not affected
        ovs_cv_binutils_avx512_good=no
      fi])
      rm $OBJFILE
-- 
2.17.1