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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/110896] [12/13/14 Regression]
 gcc.dg/ubsan/pr81981.c is xfailed
Date: Fri, 04 Aug 2023 07:07:13 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110896

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2023-08-04

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
We simplify this to t[0] * 2 (by luck, so it could also be u[0] * 2) which
means we lose track of the use of the other variable.

Value numbering stmt =3D t$0_4 =3D PHI <t$0_13(D)(2), t$0_15(3)>
Setting value number of t$0_4 to t$0_4 (changed)
Making available beyond BB4 t$0_4 for value t$0_4
Value numbering stmt =3D u$0_12 =3D PHI <u$0_14(D)(2), u$0_16(3)>
Marking CSEd to PHI node t$0_4 =3D PHI <t$0_13(D)(2), t$0_15(3)>
Setting value number of u$0_12 to t$0_4 (changed)
...
Replaced redundant PHI node defining u$0_12 with t$0_4
gimple_simplified to _9 =3D t$0_4 * 2;

early uninit sees conditional init of the memory and so refrains from
diagnosing this.

I suppose this is kind-of a duplicate of the many missed uninit diagnostics
because of CCP optimistic propagation (and only because we don't do optimis=
tic
copyprop we do not have even more such cases).=