[Bug tree-optimization/103061] [12 Regression] 527.cam4_r miscompiled with -O2 -march=znver1 since r12-4790-g4b3a325f07acebf4

[aldyh at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103061

--- Comment #7 from Aldy Hernandez <aldyh at gcc dot gnu.org> ---
Simplified version without the noise:

  <bb 35> [local count: 56063504182]:
  _134 = M.10_120 + 1;
  if (_71 <= _134)
    goto <bb 19>; [11.00%]
  else
    goto <bb 41>; [89.00%]

...
...

  <bb 41> [local count: 49896518755]:

  <bb 20> [local count: 56063503181]:
  # lb_75 = PHI <_134(41), 1(18)>
  _117 = mstep_49 + lb_75;
  _118 = _117 + -1;
  _119 = mstep_49 + _118;
  M.10_120 = MIN_EXPR <_119, _71>;
  if (lb_75 > M.10_120)
    goto <bb 21>; [11.00%]
  else
    goto <bb 22>; [89.00%]

If _134 can be assumed to be greater than M.10_120, then we can thread
41->20->21.