[runtime] We don't need an actual instance type for JSIteratorResult.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 209226322c1e87c29aeb4f25653d33b3d534e334..2d910e75a0d7eab1699c74e7cfc57b5aa1c5fe91 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -429,7 +429,6 @@ const int kStubMinorKeyBits = kSmiValueSize - kStubMajorKeyBits - 1;
   V(JS_MAP_TYPE)                                                \
   V(JS_SET_ITERATOR_TYPE)                                       \
   V(JS_MAP_ITERATOR_TYPE)                                       \
-  V(JS_ITERATOR_RESULT_TYPE)                                    \
   V(JS_WEAK_MAP_TYPE)                                           \
   V(JS_WEAK_SET_TYPE)                                           \
   V(JS_PROMISE_TYPE)                                            \
@@ -723,7 +722,6 @@ enum InstanceType {
   JS_MAP_TYPE,
   JS_SET_ITERATOR_TYPE,
   JS_MAP_ITERATOR_TYPE,
-  JS_ITERATOR_RESULT_TYPE,
   JS_WEAK_MAP_TYPE,
   JS_WEAK_SET_TYPE,
   JS_PROMISE_TYPE,
@@ -963,7 +961,6 @@ template <class C> inline bool Is(Object* obj);
   V(JSMap)                         \
   V(JSSetIterator)                 \
   V(JSMapIterator)                 \
-  V(JSIteratorResult)              \
   V(JSWeakCollection)              \
   V(JSWeakMap)                     \
   V(JSWeakSet)                     \
@@ -2600,6 +2597,24 @@ class JSDataPropertyDescriptor: public JSObject {
 };
 
 
+// JSIteratorResult is just a JSObject with a specific initial map.
+// This initial map adds in-object properties for "done" and "value,
+// as specified by ES6 section 25.1.1.3 The IteratorResult Interface
+class JSIteratorResult: public JSObject {

[Yang, 2016/02/08 06:27:13]

do we want to keep the "final" keyword here?

[Benedikt Meurer, 2016/02/08 06:28:22]

Style fix :-)

+ public:
+  // Offsets of object fields.
+  static const int kValueOffset = JSObject::kHeaderSize;
+  static const int kDoneOffset = kValueOffset + kPointerSize;
+  static const int kSize = kDoneOffset + kPointerSize;
+  // Indices of in-object properties.
+  static const int kValueIndex = 0;
+  static const int kDoneIndex = 1;
+
+ private:
+  DISALLOW_IMPLICIT_CONSTRUCTORS(JSIteratorResult);
+};
+
+
 // Common superclass for FixedArrays that allow implementations to share
 // common accessors and some code paths.
 class FixedArrayBase: public HeapObject {
@@ -9880,40 +9895,6 @@ class JSMapIterator: public OrderedHashTableIterator<JSMapIterator,
 };
 
 
-// ES6 section 25.1.1.3 The IteratorResult Interface
-class JSIteratorResult final : public JSObject {
- public:
-  // [done]: This is the result status of an iterator next method call.  If the
-  // end of the iterator was reached done is true.  If the end was not reached
-  // done is false and a [value] is available.
-  DECL_ACCESSORS(done, Object)
-
-  // [value]: If [done] is false, this is the current iteration element value.
-  // If [done] is true, this is the return value of the iterator, if it supplied
-  // one.  If the iterator does not have a return value, value is undefined.
-  // In that case, the value property may be absent from the conforming object
-  // if it does not inherit an explicit value property.
-  DECL_ACCESSORS(value, Object)
-
-  // Dispatched behavior.
-  DECLARE_PRINTER(JSIteratorResult)
-  DECLARE_VERIFIER(JSIteratorResult)
-
-  DECLARE_CAST(JSIteratorResult)
-
-  static const int kValueOffset = JSObject::kHeaderSize;
-  static const int kDoneOffset = kValueOffset + kPointerSize;
-  static const int kSize = kDoneOffset + kPointerSize;
-
-  // Indices of in-object properties.
-  static const int kValueIndex = 0;
-  static const int kDoneIndex = 1;
-
- private:
-  DISALLOW_IMPLICIT_CONSTRUCTORS(JSIteratorResult);
-};
-
-
 // Base class for both JSWeakMap and JSWeakSet
 class JSWeakCollection: public JSObject {
  public: