[Bug tree-optimization/37242] missed FRE opportunity because of signedness of addition

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=37242

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |rguenth at gcc dot gnu.org

--- Comment #25 from Richard Biener <rguenth at gcc dot gnu.org> ---
I think parts of this was fixed with the fix for PR45397 (r245752).  I can't
really figure if the issue in the benchmark is fixed though.  I still see
in PRE

  <bb 3> [local count: 1014686024]:
  # top_52 = PHI <top_36(D)(2), maxIdx_31(6)>
  # maxIdx_54 = PHI <maxIdx_47(2), maxIdx_40(6)>
  _4 = (long unsigned int) maxIdx_54;
  _5 = _4 * 4;
  _6 = numbers_37(D) + _5;
  _7 = *_6;
  _8 = _4 + 1;
  _9 = _8 * 4;
  _10 = numbers_37(D) + _9;
  _11 = *_10;
  if (_7 < _11)
    goto <bb 4>; [50.00%]
  else
    goto <bb 5>; [50.00%]

  <bb 4> [local count: 507343012]:
  maxIdx_42 = maxIdx_54 + 1;
  _68 = (long unsigned int) maxIdx_42;
  _70 = _68 * 4;
  _72 = numbers_37(D) + _70;
  pretmp_74 = *_72;

suggesting it is not fixed in GCC 8 at least.  Same with GCC 9 and trunk.

Testcase:

unsigned long a, b;
void foo (int m, int f)
{
  unsigned long tem = (unsigned long)m;
  a = tem + 1;
  if (f)
    {
      int tem2 = m + 1;
      b = (unsigned long)tem2;
    }
}

note to value-number the expressions the same you need to apply knowledge
that in if (f), m + 1 cannot overflow.