Re: [PATCH v3] rs6000: Adjust mov optabs for opaque modes [PR103353]

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

Hi!

On Fri, Jun 24, 2022 at 09:03:59AM +0800, Kewen.Lin wrote:
> on 2022/6/24 03:06, Segher Boessenkool wrote:
> > On Wed, May 18, 2022 at 10:07:48PM +0800, Kewen.Lin wrote:
> >> As PR103353 shows, we may want to continue to expand a MMA built-in
> >> function like a normal function, even if we have already emitted
> >> error messages about some missing required conditions.  As shown in
> >> that PR, without one explicit mov optab on OOmode provided, it would
> >> call emit_move_insn recursively.
> > 
> > First off: lxvp is a VSX insn, not an MMA insn.  So please don't call it
> > that -- this confusion is what presumably caused the problem here, so it
> > would be good to root it out :-)
> 
> I guess the "it" in "don't call it call" is for "MMA built-in function"?
> It comes from the current code:

Your proposed commit message says "MMA built-in function".  It is not
an MMA builtin, or rather, it should not be.

> >> +  /* Opaque modes are only expected to be available when MMA is supported,
> > 
> > Why do people expect that?  It is completely wrong.  The name "opaque"
> > itself already says this is not just for MMA, but perhaps more
> > importantly, it is a basic VSX insn, doesn't touch any MMA resources,
> > and is useful in other contexts as well.
> 
> ... The above statements are also based on current code, for now, the
> related things like built-in functions, mov optab, hard_regno_ok etc.
> for these two modes are guarded by TARGET_MMA.

Opaque modes are a generic thing, not an rs6000 thing.  It is important
not to conflate completely different things that just happened to
coincide some months ago (but not anymore right now even!)

> I think I get your points here, you want to separate these opaque
> modes from MMA since the underlying lxvp/stxvp are not MMA specific,
> so those related things (bifs, mov optabs etc.) are not necessarily
> guarded under MMA.

Yup.  This can take some time of course, but in the mean time we should
stop pretending the status quo is correct.

> > So this needs some bigger surgery.
> 
> Yes, Peter may have more comments on this.

Yes.  Can you do a patch that just fixes this PR103353, without adding
more misleading comments?  :-)


Segher