RE: [PATCH v3 1/3] config/arm: avoid mcpu and march conflicts

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

> > > <wathsala.vithanage@arm.com> wrote:
> > > >
> > > > Hi Pavan,
> > > >
> > > >> The compiler options march and mtune are a subset of mcpu and will
> > > >> lead
> > > to
> > > >> conflicts if improper march is chosen for a given mcpu.
> > > >> To avoid conflicts, force part number march when mcpu is available
> > > >> and is supported by the compiler.
> > > >
> > > > Why would one force the march specified in the part number when mcpu
> > > > for that part number is also available and supported by the compiler?
> > > >
> > > It would be good to explain the use case or the problem being faced.
> > >
> >
> > The idea of this patchset is to avoid mcpu and march conflicts that can
> happen
> > with the current build flow.
> >
> > #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'
> >
> > In some versions of GCC mcpu=neoverse-n2 is supported but -
> march=armv9-
> > a is not so, current build flow will choose the next supported march which is
> > armv8.6-a and report a conflict.
> >
> If compiler support is available for a certain CPU, then it is safe to assume that
> the
> Compiler knows the best architecture to use.
> Therefore, in such cases the best practice is to not provide -march.
> 

Ok, this deviates a lot from the current build flow, I will rework and send a v4. 

> > > >>
> > > >> Example:
> > > >> march = armv9-a
> > > >> mcpu = neoverse-n2
> > > >>
> > > >> mcpu supported, march supported
> > > >> machine_args = ['-mcpu=neoverse-n2', '-march=armv9-a']
> > > >
> > > > -march restricts the compiler to baseline architecture of the -mcpu.
> > > > For instance, Neoverse-n1's baseline architecture is armv8.2-a, but
> > > > it has some extensions from armv8.3-a, armv8.4-a, and armv8.5-a.
> > > > By setting -march to armv8.2-a the compiler will strictly omit
> > > > extensions from 8.3, 8.4 and 8.5 resulting in a suboptimal outcome.
> >
> > What if compiler only supports armv8.2-a?
> > Are you suggesting we don’t use march at all when mcpu is supported?
> > If so how do you express extensions that the SoC supports?
> > Neoverse-n2 has optional support for crypto and can only be enabled by
> > expressing it through march='armv9-a+crypto'
> >
> March extensions also works with mcpu, use mcpu=neoverse-n2+crypto
> instead of march.  It's documented in "-march and -mcpu Feature Modifiers"
> section in gcc manual.
> 
> > > >
> > > >>
> > > >> mcpu supported, march not supported machine_args =
> > > >> ['-mcpu=neoverse-n2']
> > > >
> > > > This will result in the best outcome.
> >
> > Isn't -mcpu=neoverse-n2 -march=armv9-a+sve2+crypto also the best
> > outcome?
> >
> Here also we can append feature modifiers like sve2 and crypto to CPU in
> -mcpu and drop -march arg.
> If the compiler supports neoverse-n2 but not armv9-a it will pick the next
> best architecture.
> -mcpu=neoverse-n2+sve2+crypto can replace - march=armv9-a+sve2+crypto
> 
> > > >
> > > >>
> > > >> mcpu not supported, march supported machine_args =
> > > >> ['-march=armv9-a']
> > > >
> > > > This too may result in a suboptimal outcome as optimization space is
> > > > limited to the given march (not using extensions from later
> > > > architectures when available).
> > > >
> >
> > What if compiler doesn’t support mcpu=neoverse-n2 and only supports
> > march=armv9-a
> >
> I agree there can be such corner cases where CPU enablement isn't done.
> Such cases can be handled with a new meson build parameter like
> -Dplatform=generic-armv9 to build armv9-a binaries (similar to
> -Dplatform=generic that builds armv8-a binaries today).
> Having such parameter forces the user to make a conscious decision rather
> than build system doing it for them.
> It also comes with the added benefit of having a simpler build system.
> 
> > > >>
> > > >> mcpu not supported, march not supported machine_args =
> > > >> ['-march=armv8.6-a']
> > > >
> > > > Compiler knows nothing about the target CPU or the architecture.
> > > > I think it's better to exit the build process with an error.
> > > >
> >
> > Then we would need to mark all old GCC versions as not supported by a
> newer
> > SoC I don’t think that’s needed since the binaries still run but not optimally,
> > currently we have a warning in place for march mismatch.
> >
> We don't have to deprecate older versions of GCC.
> I'm suggesting two options to let the user have greater autonomy on
> the kind of the binary they want rather than ending up with a binary
> the build system forced on them.
> Today -Dplatform=generic does something similar to this with armv8,
> it simply directs the build system to output an armv8 without any extras.
> First suggestion is that we simply have a -Dplatform=generic-armv9 that
> does the same but for armv9.
> The second suggestion is to empower a sophisticated user even further
> to override everything in the build system including generics via two
> parameters -Dmarch and -Dmcpu to set an arbitrary architecture and a cpu.
> Second option works as a catch-all for every unorthodox request that may
> come our way. Both these features can be suggested when build exits due
> to compiler not knowing the target CPU or the architecture.
> I think these parameters keep user in charge with a simpler build system.
> 
> > > >>
> > > >> Signed-off-by: Pavan Nikhilesh <pbhagavatula@marvell.com>
> > > >> ---
> > > >> v2 Changes:
> > > >> - Cleanup march inconsistencies. (Juraj Linkes)
> > > >> - Unify fallback march selection. (Juraj Linkes)
> > > >> - Tag along ARM WFE patch.
> > > >> v3 Changes:
> > > >> - Fix missing 'fallback_march' key check.
> > > >>
> > > >> config/arm/meson.build | 108 +++++++++++++++++++++++++---------
> --
> > --
> > > --
> > > >> -
> > > >> 1 file changed, 66 insertions(+), 42 deletions(-)
> > > >>
> > > >> diff --git a/config/arm/meson.build b/config/arm/meson.build index
> > > >> 36f21d22599a..ba859bd060b5 100644
> > > >> --- a/config/arm/meson.build
> > > >> +++ b/config/arm/meson.build
> > > >> @@ -58,18 +58,18 @@ implementer_generic = {  }
> > > >>
> > > >> part_number_config_arm = {
> > > >> -    '0xd03': {'compiler_options':  ['-mcpu=cortex-a53']},
> > > >> -    '0xd04': {'compiler_options':  ['-mcpu=cortex-a35']},
> > > >> -    '0xd05': {'compiler_options':  ['-mcpu=cortex-a55']},
> > > >> -    '0xd07': {'compiler_options':  ['-mcpu=cortex-a57']},
> > > >> -    '0xd08': {'compiler_options':  ['-mcpu=cortex-a72']},
> > > >> -    '0xd09': {'compiler_options':  ['-mcpu=cortex-a73']},
> > > >> -    '0xd0a': {'compiler_options':  ['-mcpu=cortex-a75']},
> > > >> -    '0xd0b': {'compiler_options':  ['-mcpu=cortex-a76']},
> > > >> +    '0xd03': {'mcpu': 'cortex-a53'},
> > > >> +    '0xd04': {'mcpu': 'cortex-a35'},
> > > >> +    '0xd05': {'mcpu': 'cortex-a55'},
> > > >> +    '0xd07': {'mcpu': 'cortex-a57'},
> > > >> +    '0xd08': {'mcpu': 'cortex-a72'},
> > > >> +    '0xd09': {'mcpu': 'cortex-a73'},
> > > >> +    '0xd0a': {'mcpu': 'cortex-a75'},
> > > >> +    '0xd0b': {'mcpu': 'cortex-a76'},
> > > >>     '0xd0c': {
> > > >>         'march': 'armv8.2-a',
> > > >>         'march_features': ['crypto', 'rcpc'],
> > > >> -        'compiler_options':  ['-mcpu=neoverse-n1'],
> > > >> +        'mcpu': 'neoverse-n1',
> > > >>         'flags': [
> > > >>             ['RTE_MACHINE', '"neoverse-n1"'],
> > > >>             ['RTE_ARM_FEATURE_ATOMICS', true], @@ -81,7 +81,7 @@
> > > >> part_number_config_arm = {
> > > >>     '0xd40': {
> > > >>         'march': 'armv8.4-a',
> > > >>         'march_features': ['sve'],
> > > >> -        'compiler_options':  ['-mcpu=neoverse-v1'],
> > > >> +        'mcpu': 'neoverse-v1',
> > > >>         'flags': [
> > > >>             ['RTE_MACHINE', '"neoverse-v1"'],
> > > >>             ['RTE_ARM_FEATURE_ATOMICS', true], @@ -92,8 +92,9 @@
> > > >> part_number_config_arm = {
> > > >>         'march': 'armv8.4-a',
> > > >>     },
> > > >>     '0xd49': {
> > > >> +        'march': 'armv9-a',
> > > >>         'march_features': ['sve2'],
> > > >> -        'compiler_options': ['-mcpu=neoverse-n2'],
> > > >> +        'mcpu': 'neoverse-n2',
> > > >>         'flags': [
> > > >>             ['RTE_MACHINE', '"neoverse-n2"'],
> > > >>             ['RTE_ARM_FEATURE_ATOMICS', true], @@ -127,21 +128,23
> > > >> @@ implementer_cavium = {
> > > >>     ],
> > > >>     'part_number_config': {
> > > >>         '0xa1': {
> > > >> -            'compiler_options': ['-mcpu=thunderxt88'],
> > > >> +            'mcpu': 'thunderxt88',
> > > >>             'flags': flags_part_number_thunderx
> > > >>         },
> > > >>         '0xa2': {
> > > >> -            'compiler_options': ['-mcpu=thunderxt81'],
> > > >> +            'mcpu': 'thunderxt81',
> > > >>             'flags': flags_part_number_thunderx
> > > >>         },
> > > >>         '0xa3': {
> > > >> -            'compiler_options': ['-march=armv8-a+crc', '-
> mcpu=thunderxt83'],
> > > >> +            'march': 'armv8-a',
> > > >> +            'march_features': ['crc'],
> > > >> +            'mcpu': 'thunderxt83',
> > > >>             'flags': flags_part_number_thunderx
> > > >>         },
> > > >>         '0xaf': {
> > > >>             'march': 'armv8.1-a',
> > > >>             'march_features': ['crc', 'crypto'],
> > > >> -            'compiler_options': ['-mcpu=thunderx2t99'],
> > > >> +            'mcpu': 'thunderx2t99',
> > > >>             'flags': [
> > > >>                 ['RTE_MACHINE', '"thunderx2"'],
> > > >>                 ['RTE_ARM_FEATURE_ATOMICS', true], @@ -153,7 +156,7
> > > >> @@ implementer_cavium = {
> > > >>         '0xb2': {
> > > >>             'march': 'armv8.2-a',
> > > >>             'march_features': ['crc', 'crypto', 'lse'],
> > > >> -            'compiler_options': ['-mcpu=octeontx2'],
> > > >> +            'mcpu': 'octeontx2',
> > > >>             'flags': [
> > > >>                 ['RTE_MACHINE', '"cn9k"'],
> > > >>                 ['RTE_ARM_FEATURE_ATOMICS', true], @@ -176,7 +179,7
> > > >> @@ implementer_ampere = {
> > > >>         '0x0': {
> > > >>             'march': 'armv8-a',
> > > >>             'march_features': ['crc', 'crypto'],
> > > >> -            'compiler_options':  ['-mtune=emag'],
> > > >> +            'mcpu': 'emag',
> > > >>             'flags': [
> > > >>                 ['RTE_MACHINE', '"eMAG"'],
> > > >>                 ['RTE_MAX_LCORE', 32], @@ -186,7 +189,7 @@
> > > >> implementer_ampere = {
> > > >>         '0xac3': {
> > > >>             'march': 'armv8.6-a',
> > > >>             'march_features': ['crc', 'crypto'],
> > > >> -            'compiler_options':  ['-mcpu=ampere1'],
> > > >> +            'mcpu': 'ampere1',
> > > >>             'flags': [
> > > >>                 ['RTE_MACHINE', '"AmpereOne"'],
> > > >>                 ['RTE_MAX_LCORE', 320], @@ -206,7 +209,7 @@
> > > >> implementer_hisilicon = {
> > > >>         '0xd01': {
> > > >>             'march': 'armv8.2-a',
> > > >>             'march_features': ['crypto'],
> > > >> -            'compiler_options': ['-mtune=tsv110'],
> > > >> +            'mcpu': 'tsv110',
> > > >>             'flags': [
> > > >>                 ['RTE_MACHINE', '"Kunpeng 920"'],
> > > >>                 ['RTE_ARM_FEATURE_ATOMICS', true], @@ -695,11
> > > >> +698,21 @@
> > > if
> > > >> update_flags
> > > >>
> > > >>     machine_args = [] # Clear previous machine args
> > > >>
> > > >> +    candidate_mcpu = ''
> > > >> +    if part_number_config.has_key('mcpu')
> > > >> +        mcpu = part_number_config['mcpu']
> > > >> +        if (cc.has_argument('-mcpu=' + mcpu))
> > > >> +            candidate_mcpu = mcpu
> > > >> +        endif
> > > >> +    endif
> > > >> +
> > > >>     # probe supported archs and their features
> > > >>     candidate_march = ''
> > > >>     if part_number_config.has_key('march')
> > > >> -        if part_number_config.get('force_march', false)
> > > >> -            candidate_march = part_number_config['march']
> > > >> +        if part_number_config.get('force_march', false) or
> > > >> + candidate_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'] @@ -717,32 +730,43 @@ if update_flags
> > > >>                 endif
> > > >>             endforeach
> > > >>         endif
> > > >> -        if candidate_march == ''
> > > >> -            error('No suitable armv8 march version found.')
> > > >> -        endif
> > > >> +
> > > >>         if candidate_march != part_number_config['march']
> > > >> -            warning('Configuration march version is ' +
> > > >> -                    '@0@, but the compiler supports only @1@.'
> > > >> -                    .format(part_number_config['march'], candidate_march))
> > > >> +            warning('Configuration march version is @0@, not supported.'
> > > >> +                    .format(part_number_config['march']))
> > > >> +            if candidate_march != ''
> > > >> +                warning('Using march version
> @0@.'.format(candidate_march))
> > > >> +            endif
> > > >>         endif
> > > >> -        candidate_march = '-march=' + candidate_march
> > > >>
> > > >> -        march_features = []
> > > >> -        if part_number_config.has_key('march_features')
> > > >> -            march_features += part_number_config['march_features']
> > > >> -        endif
> > > >> -        if soc_config.has_key('extra_march_features')
> > > >> -            march_features += soc_config['extra_march_features']
> > > >> +        if candidate_march == '' and candidate_mcpu == ''
> > > >> +            error('No suitable ARM march/mcpu version found.')
> > > >>         endif
> > > >> -        foreach feature: march_features
> > > >> -            if cc.has_argument('+'.join([candidate_march, feature]))
> > > >> -                candidate_march = '+'.join([candidate_march, feature])
> > > >> -            else
> > > >> -                warning('The compiler does not support feature @0@'
> > > >> -                    .format(feature))
> > > >> +
> > > >> +        if candidate_march != ''
> > > >> +            candidate_march = '-march=' + candidate_march
> > > >> +            march_features = []
> > > >> +            if part_number_config.has_key('march_features')
> > > >> +                march_features +=
> > > >> + part_number_config['march_features']
> > > >>             endif
> > > >> -        endforeach
> > > >> -        machine_args += candidate_march
> > > >> +            if soc_config.has_key('extra_march_features')
> > > >> +                march_features += soc_config['extra_march_features']
> > > >> +            endif
> > > >> +            foreach feature: march_features
> > > >> +                if cc.has_argument('+'.join([candidate_march, feature]))
> > > >> +                    candidate_march = '+'.join([candidate_march, feature])
> > > >> +                else
> > > >> +                    warning('The compiler does not support feature @0@'
> > > >> +                        .format(feature))
> > > >> +                endif
> > > >> +            endforeach
> > > >> +            machine_args += candidate_march
> > > >> +        endif
> > > >> +    endif
> > > >> +
> > > >> +    if candidate_mcpu != ''
> > > >> +        candidate_mcpu = '-mcpu=' + candidate_mcpu
> > > >> +        machine_args += candidate_mcpu
> > > >>     endif
> > > >>
> > > >>     # apply supported compiler options
> > > >> --
> > > >> 2.43.0
> > > >