Re: [PATCH] Fix mixed input kind permute optimization

[Richard Biener <rguenther@suse.de>]

On Wed, 22 May 2024, Richard Sandiford wrote:

> Richard Sandiford <richard.sandiford@arm.com> writes:
> > Richard Biener <rguenther@suse.de> writes:
> >> When change_vec_perm_layout runs into a permute combining two
> >> nodes where one is invariant and one internal the partition of
> >> one input can be -1 but the other might not be.  The following
> >> supports this case by simply ignoring inputs with input partiton -1.
> >>
> >> I'm not sure this is correct but it avoids ICEing when accessing
> >> that partitions layout for gcc.target/i386/pr98928.c with the
> >> change to avoid splitting store dataref groups during SLP discovery.
> >>
> >> Bootstrap and regtest running on x86_64-unknown-linux-gnu (ontop of
> >> the SLP series).  The change can't break anything that's already
> >> broken but I'm not sure this does the right thing - the testcase
> >> has an uniform constant.  I'll try to come up with a better runtime
> >> testcase tomorrow.  Hints as to where to correctly fix such case
> >> appreciated.
> >
> > Famous last words, but yeah, it looks correct to me.  I think the
> > routine in principle should have a free choice of which layout to
> > choose for invariants (as long as it's consistent for all queries
> > about the same node).  So it should just be a question of whether
> > keeping the original layout is more likely to give a valid
> > permutation, or whether going with out_layout_i would be better.
> > I don't have a strong intuition either way.
> 
> BTW, I should have said that using a different layout from 0
> would require compensating code in the materialize function.
> So this is definitely the simplest and most direct fix.

Yeah, I guess we can improve on that later.  I'm going to push the
change after lunch together with the other two fixes - the ARM CI
discovered its share of testsuite fallout for the actual change
I'm going to look at.

Richard.