riscv: Implement Zbb based strlen and prefer it over the RVV based strlen implementation when Zbb is available

[Peter Bergner]

On 6/11/26 2:51 PM, Adhemerval Zanella Netto wrote:
> And there is another question if adding a zbb sysdep is really a good move here.
> My understanding is chip produces and distros are moving to RVA23U64/RVA23S64,
> which implies in zbb and thus you can select it without the need to add an 
> specific implementation.

So there is an question here, which hopefully Adhemerval can correct me if
I'm wrong or confirm my memory is correct.  For non-multiarch builds, it's
the sysdeps/riscv/preconfigure fragment that detects which sysdeps/riscv/*
subdirectory to start our subdirectory search priority with by setting the
"machine" variable.  The Implies file in that subdirectory then points to
the next subdirectory to search in, etc. etc.  I believe (Adhemerval correct
me if I'm wrong), is that if preconfigure says start at subdirectory "a" and
its Implies file points to "b" and "b"'s Implies points to "c", etc, then if
"a" is enabled via compiler flags, that implies "b", "c", etc. are all also
enabled via the compiler flags, correct?  Meaning if "a" is enabled, then
it cannot be that case that any of "b", "c", etc. are disabled, correct?

If that is the case, then we have a slight problem, in that rvv and zbb are
orthogonal extensions, so one doesn't "imply" the other, so it wouldn't really
be "legal" to have one in the Implies file of the other.

However, from a practical standpoint, there are cores that have implemented zbb,
but have not implemented rvv, but I don't know of any cores that have rvv
without also implementing zbb, so from a purely practical standpoint, rvv
seems to "imply" zbb.  That said, I'm not sure whether we should rely on that?

It's also opposite the order we want the strlen implementation to be picked up,
which is zbb first and then fallback to rvv.

I suppose one option is if we decide that rvv does (from a practical standpoint)
imply zbb, then we could create the zbb subdir, have it in rvv's Implies file
and then remove the strlen-vector.S and place your strlen-zbb.S in the zbb
subdir.  That would make all zbb only and rvv+zbb cores use the zbb strlen.

Thoughts?

Peter