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

src/field-index.h

Index: src/field-index.h
diff --git a/src/field-index.h b/src/field-index.h
index 096b33509f7d40dd16d7f046df897b70d511fdf5..e542623ad899d69f2644ab9d1561a1d378cd58f3 100644
--- a/src/field-index.h
+++ b/src/field-index.h
@@ -34,6 +34,8 @@ class FieldIndex V8_FINAL {
     return IsInObjectBits::decode(bit_field_);
   }
 
+  bool is_hidden_field() const { return IsHiddenField::decode(bit_field_); }
+
   bool is_double() const {
     return IsDoubleBits::decode(bit_field_);
   }
@@ -55,7 +57,7 @@ class FieldIndex V8_FINAL {
   // Zero-based from the first inobject property. Overflows to out-of-object
   // properties.
   int property_index() const {
-    ASSERT(!IsHiddenField::decode(bit_field_));
+    ASSERT(!is_hidden_field());
     int result = index() - first_inobject_property_offset() / kPointerSize;
     if (!is_inobject()) {
       result += InObjectPropertyBits::decode(bit_field_);
@@ -84,7 +86,7 @@ class FieldIndex V8_FINAL {
   }
 
   int first_inobject_property_offset() const {
-    ASSERT(!IsHiddenField::decode(bit_field_));
+    ASSERT(!is_hidden_field());
     return FirstInobjectPropertyOffsetBits::decode(bit_field_);
   }