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From: "tnfchris at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/111950] [14 Regression] ICE in
 compute_live_loop_exits, at tree-ssa-loop-manip.cc:250 since
 r14-4786-gd118738e71c
Date: Fri, 27 Oct 2023 16:02:53 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D111950

--- Comment #4 from Tamar Christina <tnfchris at gcc dot gnu.org> ---
> turning c_I_lsm.18_38 into a fully invariant reduction def which likely i=
sn't
supported - we had bugs here in the past with not relevant but live stmts.
But if-conversion also performs the (now valid) hoisting, this is maybe
why it was triggered by that rev.

Ah yeah this is something different from what I just fixed.

Indeed, this causes find_guard_arg to no longer find the tie to the original
PHI.

It's trying to match=20

  # c_I_lsm.18_60 =3D PHI <a.3_23(33), 1(36)>

and

  # c_I_lsm.18_79 =3D PHI <c_I_lsm.18_60(35)>

after it adds the edge. Normally loop invariant values are left in the the
guard block for this.  In this case we've left

 # c_I_lsm.18_65 =3D PHI <c_I_lsm.18_38(11)>

Normally instead of=20

  # c_I_lsm.18_60 =3D PHI <a.3_23(33), 1(36)>

we'd find c_I_lsm.18_65 here.  The value is as you mentioned loop invariant.

but since c_I_lsm.18_38 is no longer a PHI node the link was broken.

I don't think we can really recover this in the vectorizer can we?

Would the proper fix perhaps be to have ifconvert fully convert things?

It seems to have missed that

  # c_I_lsm.18_60 =3D PHI <a.3_23(33), 1(36)>

is just

  c_I_lsm.18_60 =3D _58 ? 1 : a.3_23;

that would prevent the PHI node confusion=