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).
