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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/106155] [12/13 Regression] spurious "may be
 used uninitialized" warning
Date: Wed, 31 Aug 2022 14:04:11 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106155

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

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

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
The issue here is that we see

<bb 3> [local count: 805306369]:
# emax_41 =3D PHI <emax_9(9), emax_22(D)(2)>
# ivtmp.7_44 =3D PHI <ivtmp.7_45(9), 0(2)>
i_46 =3D (int) ivtmp.7_44;
if (i_46 =3D=3D 2)
  goto <bb 4>; [12.49%] // init
else
  goto <bb 10>; [87.51%]

...


<bb 7> [local count: 805306369]:
fh.1_6 =3D fh;
f4 (fh.1_6);
e.2_7 =3D e;
*e.2_7 =3D 0;
f1 ();
if (ivtmp.7_44 =3D=3D 2)
  goto <bb 8>; [12.49%] // use
else
  goto <bb 9>; [87.51%]

and the predicates i_46 =3D=3D 2 and ivtmp.7_44 =3D=3D 2 do not match up.

Oh, and my r13-2149-g200baf7698a100 change made us run into

  if (dominated_by_p (CDI_POST_DOMINATORS, def_bb, use_bb))
    /* The use is not guarded.  */
    return false;

for this use on the loop exit because post-dominance doesn't cover
loop exit conditions.

I will think about this a bit more.

I'll also note it is a missed optimization to not replace the i_46 =3D=3D 2
check.  -fno-ivopts makes the warning disappear (when the above is fixed).
For some reason we do not optimize the

  i_46 =3D (int) ivtmp.7_44;
  if (i_46 =3D=3D 2)

stmt, likely because ivtmp is 64bit while i is 32bit and we don't check
ivtmps value range here.=