[EXT] Re: [PATCH 2/2] config/arm: add support for fallback march

[Pavan Nikhilesh Bhagavatula]

> On Sun, Jan 21, 2024 at 10:37 AM <pbhagavatula at marvell.com> wrote:
> >
> > From: Pavan Nikhilesh <pbhagavatula at marvell.com>
> >
> > Some ARM CPUs have specific march requirements and
> > are not compatible with the supported march list.
> > Add fallback march in case the mcpu and the march
> > advertised in the part_number_config are not supported
> > by the compiler.
> >
> 
> It's not clear to me what this patch adds. We already have a fallback
> mechanism and this basically does the same thing, but there's some
> extra logic that's not clear to me. Looks like there are some extra
> conditions around mcpu. In that case, all of the mcpu/march processing
> should be done first and then we should do a common fallback.
> 

The current fallback does a simple reverse compatibility check with the compiler
when force march is not enabled. 
But this is not true for neoverse-n2 case, as it is based on armv9-a which is a super set of 
armv8.5-a and other features[1]
In the current fallback path if both march neoverse-n2 and mcpu armv9-a are not supported
then it would fallback to armv8.6-a but this not correct as neoverse-n2 is not based on armv8.5-a

The fallback march armv8.5-a kicks in (if supported) when neoverse-n2 and armv9-a are not supported.


[1] https://github.com/gcc-mirror/gcc/blob/615e25c82de97acc17ab438f88d6788cf7ffe1d6/gcc/config/arm/arm-cpus.in#L306
 

> > Example
> >         mcpu = neoverse-n2
> >         march = armv9-a
> >         fallback_march = armv8.5-a
> >
> >         mcpu, march not supported
> >         machine_args = ['-march=armv8.5-a']
> >
> >         mcpu, march, fallback_march not supported
> >         least march supported = armv8-a
> >
> >         machine_args = ['-march=armv8-a']
> >
> > Signed-off-by: Pavan Nikhilesh <pbhagavatula at marvell.com>
> > ---
> >  config/arm/meson.build | 15 +++++++++++++--
> >  1 file changed, 13 insertions(+), 2 deletions(-)
> >
> > diff --git a/config/arm/meson.build b/config/arm/meson.build
> > index 8c8cfccca0..2aaf78a81a 100644
> > --- a/config/arm/meson.build
> > +++ b/config/arm/meson.build
> > @@ -94,6 +94,7 @@ part_number_config_arm = {
> >      '0xd49': {
> >          'march': 'armv9-a',
> >          'march_features': ['sve2'],
> > +        'fallback_march': 'armv8.5-a',
> >          'mcpu': 'neoverse-n2',
> >          'flags': [
> >              ['RTE_MACHINE', '"neoverse-n2"'],
> > @@ -709,14 +710,14 @@ if update_flags
> >
> >      # probe supported archs and their features
> >      candidate_march = ''
> > +    supported_marchs = ['armv9-a', 'armv8.6-a', 'armv8.5-a', 'armv8.4-a',
> > +                        'armv8.3-a', 'armv8.2-a', 'armv8.1-a', 'armv8-a']
> >      if part_number_config.has_key('march')
> >          if part_number_config.get('force_march', false) or support_mcpu
> >              if cc.has_argument('-march=' +  part_number_config['march'])
> >                  candidate_march = part_number_config['march']
> >              endif
> >          else
> > -            supported_marchs = ['armv8.6-a', 'armv8.5-a', 'armv8.4-a', 'armv8.3-
> a',
> > -                                'armv8.2-a', 'armv8.1-a', 'armv8-a']
> >              check_compiler_support = false
> >              foreach supported_march: supported_marchs
> >                  if supported_march == part_number_config['march']
> > @@ -733,6 +734,16 @@ if update_flags
> >          endif
> >
> >          if candidate_march != part_number_config['march']
> > +            if part_number_config.has_key('fallback_march') and not
> support_mcpu
> > +                fallback_march = part_number_config['fallback_march']
> > +                foreach supported_march: supported_marchs
> > +                    if (supported_march == fallback_march
> > +                        and cc.has_argument('-march=' + supported_march))
> > +                        candidate_march = supported_march
> > +                        break
> > +                    endif
> > +                endforeach
> > +            endif
> >              warning('Configuration march version is @0@, not supported.'
> >                      .format(part_number_config['march']))
> >              if candidate_march != ''
> > --
> > 2.25.1
> >