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

[Darius Rad <darius@bluespec.com>]

On Mon, Apr 15, 2024 at 02:36:59PM -0700, Palmer Dabbelt wrote:
> 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.
> 

Got it.  I thought that might be the case, but wasn't sure if the vector
calling convention leaked in with the arch switch only (and not the abi).

> > 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...
> 

Understood.  Though when this hits, won't it be the most likely target for
vector?  Meaning at that point, rv64gcbv/lp64d would be less useful or
important.

> > 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.
> 

Maybe make an explicit note that if/when the vector ABI is in gcc, the
target will be switched to that, rather than another target being added.
Then there's some notice for people to not rely on rv64gcbv/lp64d, or at
least put up support and/or evidence of a use case to retain the target.

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