Re: [RFC] build-many-glibcs: Add a rv64gcbv-on-rv64gc/lp64d sub-variant

[Palmer Dabbelt <palmer@rivosinc.com>]

On Mon, 15 Apr 2024 13:24:14 PDT (-0700), Darius Rad wrote:
> Is this really lp64d?

I guess that depend on what you mean by "ABI".  Passing `-march=rv64gv 
-mabi=lp64d` will result in binaries that require the V extension to 
run, but can be linked with any other binary that is compiled with 
`-mabi=lp64d`.  It's essentially the same as `-march=rv64g -mabi=lp64`, 
except for vector instead of float.

> What is the status of the vector ABI?

We don't have a global vector ABI switch for GCC (ie, `-mabi=lp64dv`) 
and we decided not to put on in for GCC-14.  We've essentially got all 
the machinery there for it, as we've got the vector calling convention 
attribute, but that came in pretty late and we decided it'd be better to 
wait.

I don't see any reason it'd miss GCC-15, but no promises as there's not 
even a patch on the lists yet...

> If we need to
> have (and test) more targets that are actually useful, so be it.  But we
> don't want to be stuck testing targets nobody uses or that provide no
> testing value, and removing them is usually harder than adding them.

Ya, that's kind of my worry here.  We'd be going out on a limb here by 
targeting something without hardware availability, we've already got 
some baggage floating around from lightly-used targets and it'd be best 
to avoid too much of that.

IMO it's really more of a question for users/distros -- and I'm 
definitely a crazy person on that front, I run Gentoo ;)

> On Mon, Apr 15, 2024 at 12:24:15PM -0700, Palmer Dabbelt wrote:
>> Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
>> ---
>> So this is very much an RFC.  I had written a commit message as
>>
>>     This will hopefully be a widely used configuration in the future, but
>>     it's not all that common right now.  That said, let's be proactive and
>>     at start testing it.
>>
>> but I even that's kind of a strong statement here so I'm just stashing
>> it off to the side to be sure.
>>
>> A few of us were talking about the GCC meeting last week, but I figured
>> I'd send a glibc patch to at least start some discussion on the lists as
>> something similar had come up in the TLSDESC thread and a while ago when
>> we talked about glibc-hwcaps.
>>
>> I'm not really sure what the right thing to do here is: we don't have
>> broadly available hardware that supports these new extensions, but
>> they're pretty much table stakes for hardware that's competitive with
>> arm64/x86.  Without B and V we're going to end up piling up a ton of
>> IFUNC-based routines to try and work around the base ISA, but those
>> extensions are so fundamental to good codegen that it doesn't feel like
>> we're going to get where we want with just IFUNCs.
>>
>> Unfortunately I don't think we can drop support for the other base ISAs:
>> the distros appear to be targeting rv64gc and that's the only common
>> base for most hardware that's out there.  I guess we could drop support
>> for the rv*ima-based base ISAs, but I'm not sure that buys us much:
>> we've got a lot of embedded users so we won't be able to drop the GCC
>> support for soft float, and dropping the glibc support puts us in a
>> clunky spot for testing.
>>
>> So we're probably stuck in a bit of a performance hole for a while.
>> That said, we could at least put a stake in the ground and start testing
>> some target with some newer extensions.  That way distros that want to
>> try a larger base than rv64gc have something that's getting tested and
>> is thus likely to work.  For GCC we ended up adding some test targets
>> with pretty much all the non-embedded, but at least B and V seem like
>> the bare minimum.
>>
>> I'm not sure if all that is worth the extra build/test resource burden
>> for everyone, though, particularly given the lack of hardware.
>> ---
>>  scripts/build-many-glibcs.py | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/scripts/build-many-glibcs.py b/scripts/build-many-glibcs.py
>> index ecc743e672..21da46dc4a 100755
>> --- a/scripts/build-many-glibcs.py
>> +++ b/scripts/build-many-glibcs.py
>> @@ -411,7 +411,9 @@ class Context(object):
>>                          os_name='linux-gnu',
>>                          variant='rv64imafdc-lp64d',
>>                          gcc_cfg=['--with-arch=rv64imafdc', '--with-abi=lp64d',
>> -                                 '--disable-multilib'])
>> +                                 '--disable-multilib']),
>> +                        extra_glibcs=[{'variant': 'rv64gcbv-rv64gc-lp64d',
>> +                                       'ccopts': '-march=rv64gcv_zba_zbb_zbs'}]
>>          self.add_config(arch='s390x',
>>                          os_name='linux-gnu',
>>                          glibcs=[{},
>> --
>> 2