[PATCH] build/x86: fix support for older compilers

[Bruce Richardson]

Some older compilers e.g. gcc 8.5, do not support overriding
-march=native with another architecture, leading to build warnings such
as reported in Bugzilla (link below). Add a check for that case, and
explicitly add the avx512 flags if necessary.

Note: it appears that it is only the "native" flag that isn't
overridden, which makes the issue hard to reproduce e.g. using
godbolt.org, or on a modern machine. For example, testing with gcc 8.5
on a haswell machine, using 'native' vs explicit 'haswell':

gcc -march=native -march=skylake-avx512 -dM -E - < /dev/null | grep AVX

gcc -march=haswell -march=skylake-avx512 -dM -E - < /dev/null | grep AVX
...

Bugzilla ID: 1736
Fixes: e361ae3f59d3 ("build: reduce use of AVX compiler flags")

Signed-off-by: Bruce Richardson <bruce.richardson at intel.com>
---
 config/x86/meson.build | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/config/x86/meson.build b/config/x86/meson.build
index e2ccfb6d12..0dcc5ddee4 100644
--- a/config/x86/meson.build
+++ b/config/x86/meson.build
@@ -71,8 +71,13 @@ avx512_march_flag = '-march=x86-64-v4'
 if not cc.has_argument(avx512_march_flag)
     avx512_march_flag = '-march=skylake-avx512'
 endif
+# workaround for older compilers, e.g. gcc 8.5 on RHEL 8.
+# if march flag overriding doesn't work, explicitly add flags for AVX512.
+if cc.get_define('__AVX512F__', args: [machine_args, avx512_march_flag]) == ''
+    avx512_march_flag = ['-mavx512f', '-mavx512bw', '-mavx512cd', '-mavx512dq', '-mavx512vl']
+endif
 cc_avx512_flags = []
-if (binutils_ok and cc.has_argument(avx512_march_flag)
+if (binutils_ok and cc.has_multi_arguments(avx512_march_flag)
         and '-mno-avx512f' not in get_option('c_args'))
     # check if compiler is working with _mm512_extracti64x4_epi64
     # Ref: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82887
-- 
2.48.1