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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/67196] [10/11/12/13 Regression] loop-induced
 false positive from -Wmaybe-uninitialized
Date: Tue, 30 Aug 2022 09:40:38 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D67196

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 #7 from Richard Biener <rguenth at gcc dot gnu.org> ---
The issue is that the code supposed to deal with this,
uninit_analysis::overlap,
does not handle the case of chained PHIs very well, that is,

<bb 5> [local count: 955630225]:
# num_captions_in_row_5 =3D PHI <num_captions_in_row_25(11),
num_captions_in_row_18(4)>
# first_caption_7 =3D PHI <first_caption_27(11), 1(4)>
# first_caption_idx_9 =3D PHI <first_caption_idx_29(11), i_31(4)>
i_19 =3D i_31 + 1;
if (n_16(D) !=3D i_19)
  goto <bb 12>; [89.00%]
else
  goto <bb 6>; [11.00%]

...

<bb 6> [local count: 105119324]:
_2 =3D first_caption_7 !=3D 0;
_3 =3D num_captions_in_row_5 =3D=3D 1;
_4 =3D _2 & _3;
if (_4 !=3D 0)
  goto <bb 7>; [38.20%]
else
  goto <bb 13>; [61.80%]

...

<bb 8> [local count: 118111600]:
# _12 =3D PHI <first_caption_idx_9(7), 0(13), 0(10)>
return _12;

the code searchess for a PHI definition of num_captions_in_row_5 in the
same block as the _12 =3D PHI <>, not realizing that uninit analysis
also includes edges from the _9 =3D PHI <> in BB5 where the definition
ultimatively resides.

Then there's the issue that use predicates are not yet normalized at
this point so there's still the _4 !=3D 0 test instead of the two ANDed
ones.

With both fixed the diagnostic is gone.=