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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/110228] [14 Regression] llvm-16 miscompilation on
 small case switch, minimized
Date: Mon, 12 Jun 2023 17:39:34 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110228

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Phiopt does this:
```
        v_13 =3D=3D 1 ? 1 : LookupFlags_6
Matching expression match.pd:1990, gimple-match-5.cc:23
Matching expression match.pd:1990, gimple-match-5.cc:23
Matching expression match.pd:2479, gimple-match-4.cc:35
Matching expression match.pd:2482, gimple-match-3.cc:66
Matching expression match.pd:2489, gimple-match-2.cc:58
Matching expression match.pd:1947, gimple-match-7.cc:20
Applying pattern match.pd:4742, gimple-match-7.cc:15326
Folded into the sequence:
_17 =3D v_13 =3D=3D 1;
_18 =3D LookupFlags_6 | _17;
Removing basic block 5
;; basic block 5, loop depth 1
;;  pred:       4
;;  succ:       6
```
As zero_one_valued_p returns true for LookupFlags_6 because
tree_nonzero_bits/get_nonzero_bits returns 1 for it.

So it is ranger in the end that returns that it has a nonzero bits of 1.
Which is completely wrong as LookupFlags_6 is defined by:
  # LookupFlags_6 =3D PHI <LookupFlags_9(D)(2), LookupFlags_5(5)>


Which has an uninitialized variable in it which is not defined at what its
value would be ....=