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

[Palmer Dabbelt <palmer@rivosinc.com>]

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=[{},
-- 
2.44.0