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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/108355] [13 Regression] Dead Code Elimination
 Regression at -O2 since r13-2772-g9baee6181b4e42
Date: Tue, 10 Jan 2023 16:20:26 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108355

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |rguenth at gcc dot =
gnu.org

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.  We're doing

Value numbering stmt =3D _2 =3D e[5][a.0_1];
Skipping possible redundant definition e[5][0] =3D 1;
Setting value number of _2 to _2 (changed)

now, by skipping the def we're no longer then optimistically assuming
a.0_1 =3D=3D 0 when formerly using the assignment-from-constant _3 trick.

That wasn't done by design - if we'd handle this optimistically by
design we'd have more canonical reference ops for e[5][a.0_1].

Not sure if worth fixing.=