[PATCH v3] build/x86: fix support for older compilers

[Bruce Richardson <bruce.richardson@intel.com>]

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 AVX512 | wc -l
0

gcc -march=haswell -march=skylake-avx512 -dM -E - < /dev/null | grep AVX512 | wc -l
5

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

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
---
V3: fix build when using meson 0.57 which doesn't flatten the sub-arrays
V2: fix commit log
---
 config/x86/meson.build | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/config/x86/meson.build b/config/x86/meson.build
index e2ccfb6d12..bd135eb02d 100644
--- a/config/x86/meson.build
+++ b/config/x86/meson.build
@@ -67,26 +67,31 @@ if is_linux or cc.get_id() == 'gcc'
     endif
 endif
 
-avx512_march_flag = '-march=x86-64-v4'
+avx512_march_flag = ['-march=x86-64-v4']
 if not cc.has_argument(avx512_march_flag)
-    avx512_march_flag = '-march=skylake-avx512'
+    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
     code = '''#include <immintrin.h>
     void test(__m512i zmm){
         __m256i ymm = _mm512_extracti64x4_epi64(zmm, 0);}'''
-    result = cc.compiles(code, args : [avx512_march_flag], name : 'AVX512 checking')
+    result = cc.compiles(code, args : avx512_march_flag, name : 'AVX512 checking')
     if result == false
         machine_args += '-mno-avx512f'
         warning('Broken _mm512_extracti64x4_epi64, disabling AVX512 support')
     else
         cc_has_avx512 = true
         if cc.get_define('__AVX512F__', args: machine_args) == ''
-            cc_avx512_flags = [avx512_march_flag]
+            cc_avx512_flags = avx512_march_flag
             if cc.has_argument('-Wno-overriding-option')
                 cc_avx512_flags += '-Wno-overriding-option'
             endif
-- 
2.48.1