[Bug tree-optimization/83518] [8 Regression] Missing optimization: useless instructions should be dropped

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83518

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
Yes, it's not really sth new but a known issue with late value-numbering.  Note
that FRE wouldn't know how to simplify this either, we'd need store-merging
to effectively vectorize the earlier sets.  BB vectorization doesn't do this
because after unrolling we see

  vect_cst__46 = { 5, 4, 3, 2 };
  MEM[(int *)&arr] = vect_cst__46;
  arr[4] = 1;
  t_2 = arr[0];
  arr[0] = 5;
  arr[0] = t_2;
  t_32 = arr[0];
  _65 = arr[1];
  arr[0] = _65;
  arr[1] = t_32;
  t_68 = arr[0];
  _69 = arr[2];
  arr[0] = _69;
  arr[2] = t_68;
  t_72 = arr[0];
  _73 = arr[3];
  arr[0] = _73;
  arr[3] = t_72;
  t_76 = arr[0];
  _77 = arr[4];
  arr[0] = _77;
  arr[4] = t_76;
  i_80 = 1;
  ivtmp_81 = 4;
  pretmp_82 = arr[0];
  t_87 = arr[i_80];
  arr[i_80] = pretmp_82;
...

and BB vectorization is confused by the dead stores (and DSE would be
by the missed constant propagations).