Narrows predicate for type bounds

src/types.h

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 542 matching lines @@
     // Lower bounds are considered approximate, correct as necessary.
     t = Type::Intersect(t, b.upper, region);
     TypeHandle lower = Type::Union(b.lower, t, region);
     return BoundsImpl(lower, b.upper);
   }
   static BoundsImpl NarrowUpper(BoundsImpl b, TypeHandle t, Region* region) {
     TypeHandle lower = Type::Intersect(b.lower, t, region);
     TypeHandle upper = Type::Intersect(b.upper, t, region);
     return BoundsImpl(lower, upper);
   }
+
+  bool Narrows(BoundsImpl that) {
+    return that.lower->Is(this->lower) && this->upper->Is(that.upper);
+  }
 };
 
 typedef BoundsImpl<ZoneTypeConfig> Bounds;
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TYPES_H_