[Bug tree-optimization/128410] Optimize std::bit_repeat with constant second argument

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=128410

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Actually, maybe I was just misreading the dumps.
Cunroll turns bit_repeat (x, 3) into
  __x_6 = x_2(D) & 7;
  _14 = __x_6 << 3;
  __x_16 = __x_6 | _14;
  _22 = __x_16 << 6;
  __x_23 = __x_16 | _22;
  _29 = __x_23 << 12;
  __x_30 = __x_23 | _29;
  _36 = __x_30 << 24;
  __x_37 = __x_30 | _36;
  _9 = __x_37 << 48;
  __x_10 = _9 | __x_37;
  return __x_10;
vrp2 turns those iors into multiplications
   __x_6 = x_2(D) & 7;
   _14 = __x_6 << 3;
-  __x_16 = __x_6 | _14;
+  __x_16 = __x_6 * 9;
   _22 = __x_16 << 6;
-  __x_23 = __x_16 | _22;
+  __x_23 = __x_16 * 65;
   _29 = __x_23 << 12;
-  __x_30 = __x_23 | _29;
+  __x_30 = __x_23 * 4097;
   _36 = __x_30 << 24;
-  __x_37 = __x_30 | _36;
+  __x_37 = __x_30 * 16777217;
   _9 = __x_37 << 48;
-  __x_10 = _9 | __x_37;
+  __x_10 = __x_37 * 281474976710657;
   return __x_10;
and forwprop4 optimizes that into
  __x_6 = x_2(D) & 7;
  __x_10 = __x_6 * 10540996613548315209;
  return __x_10;