Re: [RFA] [PR rtl-optimization/90275] Handle nop reg->reg copies in cse

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

On Fri, Feb 07, 2020 at 09:00:40AM -0700, Jeff Law wrote:
> On Thu, 2020-02-06 at 07:56 -0600, Segher Boessenkool wrote:
> > On Wed, Feb 05, 2020 at 11:48:23AM -0700, Jeff Law wrote:
> > > Yea, it's closely related.  In your case you need to effectively ignore
> > > the nop insn to get the optimization you want.  In mine that nop insn
> > > causes an ICE.
> > > 
> > > I think we could take your cse bits + adding a !CALL_P separately from
> > > the simplify-rtx stuff which Segher objected to.  THat'd likely solve
> > > the ARM ICEs and take you a tiny step forward on optimizing that SVE
> > > case.  Thoughts?
> > 
> > CSE should consistently keep track of what insns are no-op moves (in its
> > definition, all passes have a slightly different definition of this),
> > and use that everywhere consistently.
> So does that mean you object to the cse.c portion of Richard's patch?

It's more a "what we need to do in the future" thing, it is stage 4 now,
it is too big a change to do now.

What patch?  The "34" patch?  https://gcc.gnu.org/r278411 .

I don't think each stanza of code should use it's own "noop-ness", no.

I don't know if this patch makes matters worse or not.  It doesn't seem
suitable for stage 4 though.  And Richard said the cse.c part breaks
rs6000, if that is true, yes I do object ;-)


Segher