[Bug tree-optimization/103721] [12 regression] wrong code generated for loop with conditional since r12-4790-g4b3a325f07acebf4

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103721

--- Comment #7 from Aldy Hernandez <aldyh at gcc dot gnu.org> ---
After chatting with Andrew about this, it seems the problem is we are starting
a path mid-loop and crossing a backedge.  This causes us to use relations we
had on one iteration in another iteration.

  <bb 8> [local count: 955630225]:
  # searchVolume_11 = PHI <1(4), 0(3)>
  # currentVolume_8 = PHI <searchVolume_5(4), searchVolume_5(3)>
  # **BACKEDGE**

  <bb 9> [local count: 1073741824]:
  # searchVolume_5 = PHI <searchVolume_11(8), world_7(D)(2)>
  # currentVolume_6 = PHI <currentVolume_8(8), 0(2)>
  _2 = searchVolume_5 != currentVolume_6;
  _3 = searchVolume_5 != 0;
  _4 = _2 & _3;
  if (_4 != 0)
    goto <bb 3>; [89.00%]
  else
    goto <bb 7>; [11.00%]
...
...
 <bb3>
 <bb4>
 <bb8>

The _8 == _5 relation in BB8 is using _5 from a previous iteration (that came
through BB9).  Basically 8->9 has a use before def of _5.

Once we hit a back edge, we should probably kill all current relations and
disable looking outside the path altogether for relations.

I will experiment.