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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/101150] null pointer dereference false
 positive disappears when compiling an additional function
Date: Tue, 22 Jun 2021 00:11:53 +0000
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X-Bugzilla-Component: tree-optimization
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101150

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Target Milestone|---                         |11.2
             Status|UNCONFIRMED                 |NEW
           Keywords|                            |missed-optimization
          Component|c++                         |tree-optimization
   Last reconfirmed|                            |2021-06-22

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
There seems to be some missing optimizations (jump threading) here.

First one:

  long int _12;

  _12 =3D _9 - _15;
  if (_12 !=3D 0)
    goto <bb 4>; [33.00%]
  else
    goto <bb 3>; [67.00%] // this should just jump to bb 15 because _12 =3D=
=3D 0
and there for _9 =3D=3D _15

  <bb 3> [local count: 79134772]:
  if (_9 !=3D _15) /// This should always be false coming from bb 2 because=
 _12
=3D=3D 0 there for _9 =3D=3D _15
    goto <bb 11>; [89.00%]
  else
    goto <bb 15>; [11.00%]

...

  <bb 11> [local count: 104641928]:
  # _143 =3D PHI <_18(8), _9(3)>
  # _118 =3D PHI <_19(8), _15(3)>
  # v$_M_start_84 =3D PHI <_104(8), 0B(3)>
  # prephitmp_123 =3D PHI <_12(8), _12(3)>
  _133 =3D (unsigned long) _143;
  _110 =3D (unsigned long) _118;
  _125 =3D _133 - _110;

....
  <bb 15> [local count: 117575200]:
  return;

Second one:
...

  <bb 4> [local count: 19488414]:
  _64 =3D (long unsigned int) _12;
  if (_64 > 9223372036854775804)
    goto <bb 5>; [0.04%]
  else
    goto <bb 6>; [99.96%]

....

  <bb 6> [local count: 9740309]:
  if (_12 !=3D 0) // This should always be true as _12 should always !=3D 0=
 on
coming into this bb because we can only come via the if statement in bb 4 on
the true edge of the conditional.
    goto <bb 7>; [100.00%]
  else
    goto <bb 16>; [0.00%] /// NOTE BB 16 is where the null pointer write it
located

--------- CUT -------
The reason why if you have both foo and bar defined, there is a heuristics =
of
the inlining of vector::_M_default_append which causes the difference, noth=
ing
shocking really.=