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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/105337] [12 Regression] wrong code at -Os and
 above on x86_64-linux-gnu
Date: Thu, 21 Apr 2022 16:58:52 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105337

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

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

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
I'd say that ivopts is at fault here.  Before that we have:
  <bb 2> [local count: 312727306]:

  <bb 3> [local count: 805306369]:
  # b_14 =3D PHI <b_9(6), 0(2)>
  # c_15 =3D PHI <c_7(6), c_5(D)(2)>
  # ivtmp_3 =3D PHI <ivtmp_2(6), 3(2)>
  a.0_1 =3D a;
  if (a.0_1 =3D=3D 0)
    goto <bb 7>; [5.50%]
  else
    goto <bb 4>; [94.50%]
...
  <bb 7> [local count: 44291850]:
  # b_4 =3D PHI <b_14(3)>

  <bb 5> [local count: 312727306]:
  # b_10 =3D PHI <b_4(7), 3(8)>
  return b_10;
as the path actually taken at runtime, yes, we use uninitialized c in the P=
HI,
but that doesn't correspond to UB at runtime.
But ivopts changes that to:
  <bb 2> [local count: 312727306]:
  _19 =3D (unsigned int) c_5(D);
  _20 =3D _19 + 4294967293;
  _21 =3D (int) _20;

  <bb 3> [local count: 805306369]:
  # c_15 =3D PHI <c_7(6), c_5(D)(2)>
  _13 =3D (unsigned int) c_5(D);
  _12 =3D (unsigned int) c_15;
  _11 =3D -_12;
  _18 =3D _11 + _13;
  b_14 =3D (int) _18;
  a.0_1 =3D a;
  if (a.0_1 =3D=3D 0)
    goto <bb 7>; [5.50%]
  else
    goto <bb 4>; [94.50%]
...
  <bb 7> [local count: 44291850]:
  # b_4 =3D PHI <b_14(3)>

  <bb 5> [local count: 312727306]:
  # b_10 =3D PHI <b_4(7), 3(8)>
  return b_10;
and now it uses the uninitialized value in arithmetics in the actually exec=
uted
code.  And, even the return value which gets a copy of b_14 now depends on =
that
uninitialized value.  I think we had some other PR about this...
Then comes ccp4 and figures out that many values are UNDEFINED and optimize=
s it
into return 3;=