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

[Juraj Linkeš]

On Mon, Jan 22, 2024 at 1:16 PM Pavan Nikhilesh Bhagavatula
<pbhagavatula at marvell.com> wrote:
>
> > 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.
>

Can the two fallback mechanisms be combined? They seem very similar.

>
> [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
> > >