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From: "aldyh at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/101912] -Wmaybe-uninitialized false alarm in
 tzdb localtime.c
Date: Tue, 30 Nov 2021 15:41:07 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101912

Aldy Hernandez <aldyh at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |aldyh at gcc dot gnu.org,
                   |                            |rguenth at gcc dot gnu.org

--- Comment #2 from Aldy Hernandez <aldyh at gcc dot gnu.org> ---
ISTM that the warning is correct, given the IL the uninit pass gets.

On the 7->3 entry to the loop prevcorr_23 can be undefined, and it is read =
in
BB4 with no guard whatsoever:

  <bb 7> [local count: 85931698]:

  <bb 3> [local count: 1034442873]:
  # prevcorr_23 =3D PHI <corr_14(8), prevcorr_12(D)(7)>
  # leapcnt_24 =3D PHI <leapcnt_15(8), 0(7)>
  corr_14 =3D getint ();
  if (corr_14 <=3D 0)
    goto <bb 10>; [3.66%]
  else
    goto <bb 4>; [96.34%]

  <bb 10> [local count: 37860610]:
  goto <bb 6>; [100.00%]

  <bb 4> [local count: 996582262]:
  _1 =3D corr_14 =3D=3D 1;
  _2 =3D leapcnt_24 !=3D 0;
  _7 =3D (unsigned int) prevcorr_23;
  _5 =3D _7 + 4294967295;
  _4 =3D _5 > 1;
  _22 =3D _1 & _2;
  _18 =3D _4 & _22;
  if (_18 !=3D 0)
    goto <bb 11>; [1.21%]
  else
    goto <bb 5>; [98.79%]

However, in the original source we shouldn't read from prevcorr if leapcnt =
=3D=3D
0, which is the case on entry to the loop:

              && !(leapcnt =3D=3D 0
                   || (prevcorr < corr
                       ? corr =3D=3D prevcorr + 1
                       : (corr =3D=3D prevcorr
                          || corr =3D=3D prevcorr - 1)))))

It looks like we lost the leapcnt gate protecting the prevcorr_23 read.

Unless I'm missing some subtle language thinggie, we loose the gate in the
ifcombine pass.  Before it, the read from prevcorr was protected by:

  <bb 4> [local count: 996582262]:
  _1 =3D corr_14 =3D=3D 1;
  _2 =3D leapcnt_5 !=3D 0;
  _3 =3D _1 & _2;
  if (_3 !=3D 0)
    goto <bb 5>; [50.00%]
  else
    goto <bb 7>; [50.00%]

and after ifcombine we have:

  <bb 4> [local count: 996582262]:
  _1 =3D corr_14 =3D=3D 1;
  _2 =3D leapcnt_5 !=3D 0;
  _3 =3D _1 & _2;
  _19 =3D prevcorr_4 !=3D 1;
  _16 =3D _3 & _19;
  _21 =3D prevcorr_4 !=3D 2;
  _18 =3D _16 & _21;
  if (_18 !=3D 0)
    goto <bb 7>; [1.21%]
  else
    goto <bb 5>; [98.79%]

Unfortunately, adding -fdisable-tree-ifcombine doesn't make the warning go
away, but at least the IL by the uninit pass seems correct.=