Re: [PATCH] Move xx* builtins to vsx.md.

[Segher Boessenkool <segher@kernel.crashing.org>]

On Thu, Aug 19, 2021 at 06:10:46PM -0400, Michael Meissner wrote:
> On Wed, Aug 18, 2021 at 06:11:03PM -0500, Segher Boessenkool wrote:
> > I think the current vector.md / altivec.md / vsx.md / rs6000.md
> > division is artificial at best.  Most of the basic (movement etc.)
> > things are in rs6000.md (and all should be), but nothing else is clear.
> > 
> > The name "altivec.md" suggests it is only for the very old things, but
> > it is not used that way, and that it untenable anyway: we have more
> > recent insns to plug holes in that (for example 64-bit integer support),
> > so it arguably is not just for that.
> > 
> > Using it for instructions that only work on the high 32 VSRs (i.e. the
> > VRs) is quite artificial as well -- sometimes there are equivalent insns
> > for the other 32 VSRs already, sometimes it is just because of opcode
> > scarcity, sometimes it is because it is for the slow vector unit only
> > (but those seem to live in rs6000.md and crypto.md anyway).
> > 
> > Maybe we should give up on dividing these things, and put both in one
> > file, say vector.md?
> 
> Yes but that is more ambitious.

Absolutely.  But setting a destination before starting to walk is
sometimes helpful ;-)

> Basically I have 2 patches coming that use and
> update the xxsplti instructions.  I can avoid putting in this specific change
> and reformulate them for altivec.md instead of vsx.md.  Or I can check in these
> changes.  Which do you want?  I don't want to do both insn movement and new
> patches at the same time.

I am fine with this patch, it is a clear improvement already.

> The original design of vector.md was to allow for alternate vector units, and
> vector.md was just the define_expands.  But the likely hood of new vector units
> is probably low.

Right, history has caught up with us.

> When I wrote vsx.md in the power7 days, we were toying with the notion of doing
> VSX and not Altivec instructions.  But I quickly realized you always need
> Altivec for VSX.

There also were 8-byte vectors back then.  Or was that completely
separate code?

> In general, I would prefer not to have a flag day where everything gets moved
> all at once.

Yup, it is too easy to make mistakes here, ordering in machine
descriptions is significant.  Although comparing the generated insn-*
files before and after might help.


Segher