[Bug tree-optimization/69882] [6 regression] Excessive reduction statements generated by SLP

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69882

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
The IL you refer to makes sense to me, the vectorizer sees

t.f90:13:0: note: Final SLP tree for instance:
t.f90:13:0: note: node
t.f90:13:0: note:       stmt 0 M.0_140 = MAX_EXPR <_139, c_I_lsm.10_129>;
t.f90:13:0: note:       stmt 1 M.0_143 = MAX_EXPR <_142, c_I_lsm.11_130>;
t.f90:13:0: note: node
t.f90:13:0: note:       stmt 0 _139 = *a_22(D)[_138];
t.f90:13:0: note:       stmt 1 _142 = *a_22(D)[_141];
t.f90:13:0: note: === vect_make_slp_decision ===
t.f90:13:0: note: Decided to SLP 1 instances. Unrolling factor 2

and thus it is a SLP reduction of two scalars.  Not sure how it became that,
the simple testcase became a whack-a-mole due to earlier opts.

  <bb 17>:
  # k_127 = PHI <k.4_11(16), k_154(19)>
  # c_I_lsm.10_129 = PHI <c_I_lsm.10_48(16), M.0_140(19)>
  # c_I_lsm.11_130 = PHI <c_I_lsm.11_3(16), M.0_143(19)>
  _136 = (integer(kind=8)) k_127;
  _137 = _136 * 4;
  _138 = _137 + -4;
  _139 = *a_22(D)[_138];
  M.0_140 = MAX_EXPR <c_I_lsm.10_129, _139>;
  _141 = _137 + -3;
  _142 = *a_22(D)[_141];
  M.0_143 = MAX_EXPR <c_I_lsm.11_130, _142>;
  k_154 = k_127 + 1;
  if (k_127 == 26)
    goto <bb 18>;
  else
    goto <bb 19>;

  <bb 18>:
  # c_I_lsm.13_234 = PHI <c_I_lsm.13_84(17)>
  # c_I_lsm.12_219 = PHI <c_I_lsm.12_13(17)>
  # M.0_218 = PHI <M.0_143(17)>
  # M.0_217 = PHI <M.0_140(17)>
  goto <bb 42>;

so we produce vectorized reductions for _218 and _217.

loop unswitching produces all those cases.  Simplifying the testcase to
a constant 'y' makes it still fail but very much easier to analyze.