[Bug tree-optimization/100366] spurious warning - std::vector::clear followed by std::vector::insert(vec.end(), ...) with -O2

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100366

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
The issue is that FRE sees

  <bb 2> [local count: 1073741824]:
  _6 = vec_2(D)->D.33436._M_impl.D.32749._M_start;
  _7 = vec_2(D)->D.33436._M_impl.D.32749._M_finish;
  if (_6 != _7)
    goto <bb 3>; [70.00%]
  else
    goto <bb 4>; [30.00%]

  <bb 3> [local count: 751619281]:
  vec_2(D)->D.33436._M_impl.D.32749._M_finish = _6;

  <bb 4> [local count: 1073741824]:
  _5 = MEM[(char * const &)vec_2(D) + 8];

so clearly _5 is _not_ equal to _7 but on the path 2 -> 3 -> 4 it is equal to
_6.
Only PRE transforms the load in _5 to a select of both (a PHI node):

  <bb 2> [local count: 1073741824]:
  _6 = vec_2(D)->D.33436._M_impl.D.32749._M_start;
  _7 = vec_2(D)->D.33436._M_impl.D.32749._M_finish;
  if (_6 != _7)
    goto <bb 4>; [70.00%]
  else
    goto <bb 3>; [30.00%]

  <bb 3> [local count: 322122544]:
  _86 = _7 - _6;
  _125 = (sizetype) _86;
  goto <bb 5>; [100.00%]

  <bb 4> [local count: 751619281]:
  vec_2(D)->D.33436._M_impl.D.32749._M_finish = _6;

  <bb 5> [local count: 1073741824]:
  # prephitmp_141 = PHI <_7(3), _6(4)>
  # prephitmp_83 = PHI <_86(3), 0(4)>
  # prephitmp_65 = PHI <_125(3), 0(4)>
  _21 = vec_2(D)->D.33436._M_impl.D.32749._M_end_of_storage;

PRE also removes some partial redundancies here (the other two PHIs).