[Bug tree-optimization/101150] null pointer dereference false positive disappears when compiling an additional function

["pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101150

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

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

...

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

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

Second one:
...

  <bb 4> [local count: 19488414]:
  _64 = (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 != 0) // This should always be true as _12 should always != 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, nothing
shocking really.