[PATCH v2] build/x86: fix support for older compilers

[Bruce Richardson]

On Wed, Jul 02, 2025 at 11:22:42AM -0700, Stephen Hemminger wrote:
> On Wed,  2 Jul 2025 17:00:45 +0100
> Bruce Richardson <bruce.richardson at intel.com> wrote:
> 
> > 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 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
> 
> RHEL 8.5 reached end of extended support cycle (EOL).
> Why are we still supporting it?

Folks working for Redhat can answer better than me, but according to [1]
it's not EOL yet. Also, we support it in DPDK because users are still using
it - we had two separate people report seeing the issue on RHEL 8 on RC2,
so likely a reasonable number will be using it with the actual release.

/Bruce

[1] https://en.wikipedia.org/wiki/Red_Hat_Enterprise_Linux#Version_history_and_timeline