RE: [PATCH v4] build: reduce use of AVX compiler flags

["Varghese, Vipin" <Vipin.Varghese@amd.com>]

[Public]

Snipped

> >
> > In above log I get `2 instances of march`; logs `-march=native -mrtm
> > -DALLOW_EXPERIMENTAL_API -DALLOW_INTERNAL_API -Wno-format-
> truncation -Wno-address-of-packed-member -
> DRTE_LOG_DEFAULT_LOGTYPE=pmd.net.i40e -DCC_AVX512_SUPPORT -
> march=skylake-avx512`.
> >
> > Question-1: I think this is not expected right? The `-march=native` is populated
> from `cflags` and `-march= skylake-avx512` is populated from ` cc_avx512_flags`.
>
> The above command is correct. So long as the compiler supports AVX-512 we will
> always compile the AVX-512 code paths for runtime selection. In practice, all
> supported compilers have AVX-512 support, so in reality we have the two scenarios
> you tested:
>
> * The target architecture e.g. znver3 in your case, doesn't support avx512,
>   so the meson.build file adds on the necessary flags to add this support,
>   i.e. that file is compiled with -march=skylake=avx512, which is the
>   minimum ISA that gives you the necessary support.
> * The target architecture, e.g. znver4, does support AVX-512, then no
>   additional flags are added and the files are compiled "as normal"
>
> In both these cases, whether the target architecture is specified as "native" or
> explicitly makes no difference.

So, for files which needs avx512 support like acl_avx512, fib_tire_avx512 and others, we then only pass `cc_avx512_flags`.
All other cases this is ignored. Thank you that makes sense.

>
> > Question-2: if the target is meet minimal ISA why not we use `-march=x86-64-
> v4`?
> >
>
> Good point, that would indeed be better. I'm just not sure whether it is supported
> widely enough on our compilers. Do you know what gcc and clang versions support
> that target?

As I recollect we have been using `x86-64-v4` this from gcc 12.3 and clang-14.
I am not sure if the older versions supports `avx512 that is x86-64-v4`

>
> > Note: I am yet to check for cross build. Will update on cross build how this comes
> out.

Shall we skip the check in cross build?

> >
> >
> >
> > >
> > > Regards,
> > > /Bruce