In-object double fields unboxing (for 64-bit only).

src/property-details.h

Index: src/property-details.h
diff --git a/src/property-details.h b/src/property-details.h
index 61fecedd1bf265115cd36d9f8132e76425f979a3..39f633a49968156413e1f7a8c0b7229e70646163 100644
--- a/src/property-details.h
+++ b/src/property-details.h
@@ -248,6 +248,8 @@ class PropertyDetails BASE_EMBEDDED {
     return FieldIndexField::decode(value_);
   }
 
+  inline int field_width_in_words() const;
+
   inline PropertyDetails AsDeleted() const;
 
   static bool IsValidIndex(int index) {