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From: "linkw at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/94043] [9/10 Regression] ICE in
 superloop_at_depth, at cfgloop.c:78
Date: Tue, 24 Mar 2020 02:59:35 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94043

--- Comment #10 from Kewen Lin <linkw at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #9)
> OK, so it's indeed vectorizable_live_operation not paying attention to
> loop-closed SSA form.
>=20
> What it should do before building the lane extract is create a _new_
> loop-closed PHI node for the vectorized def (vec_lhs), and then
> demote the loop-closed PHI node for the scalar def (lhs) which should
> _always_ exist to a copy.  So from
>=20
>=20
>  loop;
>=20
> # lhs' =3D PHI <lhs>
>=20
>=20
> go to
>=20
>   loop;
>=20
> # vec_lhs' =3D PHI <vec_lhs>
> new_tree =3D BIT_FIELD_REF <vec_lhs', ...>;
> lhs' =3D new_tree;
>=20
> I think you can assert that the block of the loop-closed PHI
> (single_exit()->dest) always has a single predecessor, otherwise
> things will be more complicated.
>=20
> Can you try rework the code in this way?  If that's too much just tell
> me and I'll take care of this.

Thanks Richi, I'll give it a shot!
I'd like to ensure my understanding: with the proposed fix, we ensure the
single_exit()->dest should be the correct BB to be inserted, no chance like
gimple_find_edge_insert_loc to get one new BB to be inserted, is it right?=