Re: [PATCH 5/6] acl: make compatible with instruction set updates for MSVC

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

On Tue, Feb 25, 2025 at 08:37:27AM -0800, Andre Muezerie wrote:
> On Tue, Feb 25, 2025 at 09:03:37AM +0000, Bruce Richardson wrote:
> > On Mon, Feb 24, 2025 at 01:01:18PM -0800, Andre Muezerie wrote:
> > > Top level 'cc_avx2_flags' was created and holds the correct flags
> > > depending on the compiler used.
> > > 
> > > File meson.build was updated to handle the correct AVX512 flags
> > > depending on compiler used.
> > > 
> > > Signed-off-by: Andre Muezerie <andremue@linux.microsoft.com>
> > > ---
> > >  lib/acl/meson.build | 16 +++++++++-------
> > >  1 file changed, 9 insertions(+), 7 deletions(-)
> > > 
> > > diff --git a/lib/acl/meson.build b/lib/acl/meson.build
> > > index fefe131a48..24e47b6cc1 100644
> > > --- a/lib/acl/meson.build
> > > +++ b/lib/acl/meson.build
> > > @@ -19,7 +19,7 @@ if dpdk_conf.has('RTE_ARCH_X86')
> > >      avx2_tmplib = static_library('avx2_tmp',
> > >              'acl_run_avx2.c',
> > >              dependencies: static_rte_eal,
> > > -            c_args: cflags + ['-mavx2'])
> > > +            c_args: [cflags, cc_avx2_flags])
> > >      objs += avx2_tmplib.extract_objects('acl_run_avx2.c')
> > >  
> > >      # compile AVX512 version if:
> > > @@ -38,6 +38,12 @@ if dpdk_conf.has('RTE_ARCH_X86')
> > >          # compiler flags, and then have the .o file from static lib
> > >          # linked into main lib.
> > >  
> > > +        if is_ms_compiler
> > > +            acl_avx512_args = cc_avx512_flags
> > > +        else
> > > +            acl_avx512_args = ['-mavx512f', '-mavx512vl', '-mavx512cd', '-mavx512bw']
> > > +        endif
> > > +
> > 
> > in the non-msvc case are these flags not the same as cc_avx512_flags too?
> > If so, let's just get rid of the acl_avx512_args variable generally.
> > 
> > /Bruce
> 
> It's not an exact match. I did not look further to see if this was intentional or result
> of a typo. TBH I'm not even sure if it would be possible to deduct this from the code.
> Also, all the CPUs I have looked at bring all these 5 instruction sets together, but we
> know this might not hold true in the future as each one of them has an independent CPUID flag.
> 

Yes, they are independent flags. However, given that AVX-512 has been
around a long time without any CPUs being released with only partial
support of the initial 5 sets introduced with the first AVX-512 CPUs, I'd
take the view that we are probably ok just mandating all 5 for AVX-512
support. That way, if it does happen that a CPU with partial support is
released, we just end up without AVX-512 support on it, rather than a
broken build.  We can then fix that later if such a situation occurs. Until
such time, we get nice simplicity in our code of having a standard AVX-512
flag-set.

>     cc_avx512_flags = ['-mavx512f', '-mavx512vl', '-mavx512dq', '-mavx512bw']
> 
> My choice was to keep the flags that were used initially, but I can change that if the
> maintainers tell me this was a mistake.

I'd add in "avx512cd" into the basic avx512 flags and then reuse the
variable. I suspect I just missed it when creating the list of flags.

/Bruce