VRP: allow unsigned truncating conversions that will fit

[Aldy Hernandez]

I'm still thinking about this one...

Is there a subtle reason why we're avoiding unsigned truncating 
conversions of the form:

        [X, +INF]

If the X fits in the new type, why can't we just build [X, +INF] in the 
new type?  See attached patch.

If there isn't, OK for trunk?
commit b2e8c9baeeca3e321957d205b2c4c8b83b9bc754
Author: Aldy Hernandez <al...@redhat.com>
Date:   Fri Sep 14 12:02:42 2018 +0200

            * wide-int-range.cc (wide_int_range_convert): Allow unsigned
            truncating conversions that will fit in the new type.

diff --git a/gcc/wide-int-range.cc b/gcc/wide-int-range.cc
index a85fe9f9ad7..4e323a63206 100644
--- a/gcc/wide-int-range.cc
+++ b/gcc/wide-int-range.cc
@@ -768,6 +768,24 @@ wide_int_range_convert (wide_int &min, wide_int &max,
       return (!wi::eq_p (min, wi::min_value (outer_prec, outer_sign))
 	      || !wi::eq_p (max, wi::max_value (outer_prec, outer_sign)));
     }
+  /* Unsigned truncating conversions whose lower bound fits, but whose
+     upper bound is +INF can be handled straightforwardly.  This will
+     handle things like:
+
+     [1,+INF] (i.e. ~[0,0]) from long unsigned to unsigned short
+     [1000, +INF] from long unsigned to unsigned short.  */
+  if (inner_sign == UNSIGNED
+      && inner_sign == outer_sign
+      && inner_prec > outer_prec
+      && wi::le_p (vr0_min,
+		   wi::mask (outer_prec, false, inner_prec),
+		   UNSIGNED)
+      && vr0_max == wi::max_value (inner_prec, UNSIGNED))
+    {
+      min = wide_int::from (vr0_min, outer_prec, inner_sign);
+      max = wi::mask (outer_prec, false, outer_prec);
+      return true;
+    }
   return false;
 }