[Bug tree-optimization/64322] More optimize opportunity for constant folding

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64322

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
For VRP I'm thinking of (completely untested):
--- gcc/tree-vrp.c.jj    2014-12-01 14:57:30.000000000 +0100
+++ gcc/tree-vrp.c    2014-12-16 10:17:27.543111649 +0100
@@ -2434,6 +2434,7 @@ extract_range_from_binary_expr_1 (value_
       && code != MAX_EXPR
       && code != PLUS_EXPR
       && code != MINUS_EXPR
+      && code != RSHIFT_EXPR
       && (vr0.type == VR_VARYING
       || vr1.type == VR_VARYING
       || vr0.type != vr1.type
@@ -2948,6 +2949,15 @@ extract_range_from_binary_expr_1 (value_
     {
       if (code == RSHIFT_EXPR)
         {
+          /* Even if vr0 is VARYING or otherwise not usable, we can derive
+         useful ranges just from the shift count.  E.g.
+         x >> 63 for signed 64-bit x is always [-1, 0].  */
+          if (vr0.type != VR_RANGE || symbolic_range_p (&vr0))
+        {
+          vr0.type = type = VR_RANGE;
+          vr0.min = vrp_val_min (expr_type);
+          vr0.max = vrp_val_max (expr_type);
+        }
           extract_range_from_multiplicative_op_1 (vr, code, &vr0, &vr1);
           return;
         }