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From: "msebor at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/100137] [10/11 Regression] -Warray-bounds
 false positive on varying offset plus negative
Date: Mon, 19 Apr 2021 15:39:26 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100137

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |msebor at gcc dot gnu.org
     Ever confirmed|0                           |1
      Known to work|                            |10.2.0
          Component|c++                         |tree-optimization
             Status|UNCONFIRMED                 |NEW
            Summary|-Werror=3Darray-bounds false  |[10/11 Regression]
                   |positive:"subscript -1 is   |-Warray-bounds false
                   |outside array bounds"       |positive on varying offset
                   |                            |plus negative
   Last reconfirmed|                            |2021-04-19
      Known to fail|                            |10.3.0, 11.0

--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
Confirmed.  The bug is caused by -Warray-bounds ignoring varying offsets
instead of setting the offset range to that of the referenced object.=20
Bisection points to g:e7fd3b783238d034018443e43a58ff87908b4db6 but that just
exposed the underlying bug in the warning.  The test case triggers the false
positive in 10.3.0 and 11.0 but not in 10.2.0 or prior.=20

int main ()
{
  ...
  <bb 5> [local count: 114863531]:
  # len_11 =3D PHI <18446744073709551615(3), len_23(4)>
  s =3D{v} {CLOBBER};
  s.first_ =3D &hello;
  iftmp.0_13 =3D &hello + len_11;                          <<< len_11:
VR_VARYING, iftmp.0_13 taken to point to &hello
  s.last_ =3D iftmp.0_13;
  _15 =3D len_11 !=3D 0;
  MEM[(char &)iftmp.0_13 + 18446744073709551615] =3D 50;   <<< iftmp.0_13[-=
1] =3D
'2'; <<< -Warray-bounds
  hello =3D{v} {CLOBBER};
  s =3D{v} {CLOBBER};
  return 0;

}=