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From: "rsandifo at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/94261] [10 Regression] ICE in
 vect_get_vec_def_for_operand_1 for 3-element condition reduction
Date: Mon, 23 Mar 2020 11:35:37 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94261

--- Comment #7 from rsandifo at gcc dot gnu.org <rsandifo at gcc dot gnu.or=
g> ---
(In reply to Richard Biener from comment #6)
> (In reply to rsandifo@gcc.gnu.org from comment #5)
> > (In reply to Richard Biener from comment #3)
> > > (In reply to Richard Biener from comment #2)
> > > > when placing gcc_unreachable () at the swapping place and most test=
cases
> > > > still pass when removing the IL operand swapping, only vect-cselim-=
1.c
> > > > runfails (investigating).
> > >=20
> > > Ah, SLP ultimately fails here so the scalar IL is broken by the opera=
tion
> > > code adjustment.  In the end we should be able to avoid doing tree co=
de
> > > adjustments as well since we're swapping to make the code the same
> > > as the first stmts code and we only ever look at the first scalar stmt
> > > during SLP code-gen/analysis.=20
> >=20
> > Yeah, started wondering about that later too...
>=20
> So there's probably a single path where this is not true which is when we
> go through vect_attempt_slp_rearrange_stmts thus when we have a SLPed
> set of cond-reductions.  I'm trying to come up with a testcase ...
>=20
> But I think the cure here should be to punt on vect_slp_rearrange_stmts,
> do you agree?

You know SLP much better than me :-)  But yeah, that sounds good.
Would we need to punt even for swap=3D=3D1, or just swap=3D=3D2?=