RE: [EXT] Re: [PATCH 2/2] config/arm: add armv9-a march

[Pavan Nikhilesh Bhagavatula <pbhagavatula@marvell.com>]

> On 2024/1/4 5:18 PM, pbhagavatula@marvell.com wrote:
> > From: Pavan Nikhilesh <pbhagavatula@marvell.com>
> >
> > Now that major versions of GCC recognize armv9-a march option,
> > add it to the list of supported march.
> > Update neoverse-n2 part number to include march as armv9-a.
> >
> > Signed-off-by: Pavan Nikhilesh <pbhagavatula@marvell.com>
> > ---
> >   config/arm/meson.build | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/config/arm/meson.build b/config/arm/meson.build
> > index 36f21d2259..0804877b57 100644
> > --- a/config/arm/meson.build
> > +++ b/config/arm/meson.build
> > @@ -92,6 +92,7 @@ part_number_config_arm = {
> >           'march': 'armv8.4-a',
> >       },
> >       '0xd49': {
> > +        'march': 'armv9-a',
> >           'march_features': ['sve2'],
> >           'compiler_options': ['-mcpu=neoverse-n2'],
> >           'flags': [
> > @@ -701,7 +702,7 @@ if update_flags
> >           if part_number_config.get('force_march', false)
> >               candidate_march = part_number_config['march']
> >           else
> > -            supported_marchs = ['armv8.6-a', 'armv8.5-a', 'armv8.4-a', 'armv8.3-
> a',
> > +            supported_marchs = ['armv9-a', 'armv8.6-a', 'armv8.5-a', 'armv8.4-
> a', 'armv8.3-a',
> 
> armv9.x-a have some overlap with armv8.x-a.
> For armv9.0, the next candidate fallback should be armv8.4-a instead of
> armv8.6-a.
> armv9 should be treated differently for fallback support.


From [1] looks like base arch is v8.5a, also GCC(13) still complains incompatibility with neoverse-n2 [2]

I believe we have to express mcpu separately instead of having it in compiler flags and switch to fallback march only when 
mcpu is not supported. If mcpu is supported and march is not then we shouldn’t pick from supported_marchs.

Example:
march=armv9-a
mcpu= neoverse-n2
fallback_march=armv8.5-a

If compiler supports mcpu as neoverse-n2 and march as armv9-a
machine_args = ['-march=armv9-a', '-mcpu=neoverse-n2']

If compiler supports mcpu as neoverse-n2 and doesn’t support armv9-a
machine_args = ['-mcpu=neoverse-n2']

If compiler supports doesn’t support mcpu as neoverse-n2 and supports armv9-a
machine_args = ['-march=armv9-a']

If compiler supports doesn’t support mcpu as neoverse-n2 and doesn’t support armv9-a
machine_args = ['-march=armv8.5-a']

I will spin up a new patch with the above change.

Thanks,
Pavan. 

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

[2]
#aarch64-linux-gnu-gcc --version                              
aarch64-linux-gnu-gcc (GCC) 13.2.0

#aarch64-linux-gnu-gcc -mcpu=neoverse-n2 -march=armv8-a shrn.c 
cc1: warning: switch '-mcpu=neoverse-n2' conflicts with '-march=armv8-a' switch
#aarch64-linux-gnu-gcc -mcpu=neoverse-n2 -march=armv8.1-a shrn.c
cc1: warning: switch '-mcpu=neoverse-n2' conflicts with '-march=armv8.1-a' switch
#aarch64-linux-gnu-gcc -mcpu=neoverse-n2 -march=armv8.2-a shrn.c
cc1: warning: switch '-mcpu=neoverse-n2' conflicts with '-march=armv8.2-a' switch
#aarch64-linux-gnu-gcc -mcpu=neoverse-n2 -march=armv8.3-a shrn.c
cc1: warning: switch '-mcpu=neoverse-n2' conflicts with '-march=armv8.3-a' switch
#aarch64-linux-gnu-gcc -mcpu=neoverse-n2 -march=armv8.4-a shrn.c     
cc1: warning: switch '-mcpu=neoverse-n2' conflicts with '-march=armv8.4-a' switch
#aarch64-linux-gnu-gcc -mcpu=neoverse-n2 -march=armv8.5-a shrn.c
cc1: warning: switch '-mcpu=neoverse-n2' conflicts with '-march=armv8.5-a' switch
#aarch64-linux-gnu-gcc -mcpu=neoverse-n2 -march=armv8.6-a shrn.c
cc1: warning: switch '-mcpu=neoverse-n2' conflicts with '-march=armv8.6-a' switch
#aarch64-linux-gnu-gcc -mcpu=neoverse-n2 -march=armv8.7-a shrn.c
cc1: warning: switch '-mcpu=neoverse-n2' conflicts with '-march=armv8.7-a' switch

> 
> >                                   'armv8.2-a', 'armv8.1-a', 'armv8-a']
> >               check_compiler_support = false
> >               foreach supported_march: supported_marchs