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

[enh <enh@google.com>]

On Mon, Apr 15, 2024 at 12:27 PM Palmer Dabbelt <palmer@rivosinc.com> 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.

for Android -- which is in a similar situation -- having the build
system enable these extensions been pretty useful for finding
toolchain and qemu bugs, which seems useful in its own right, if only
for CI purposes...

> ---
>  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=[{},
> --
> 2.44.0
>