Use a special EnumLength field to indicate number of valid enum cache values.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 3349921d9fc08ef9d12046186807cee4e3303bb5..650729f1c53097689485465102ff7785dd10f966 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -4644,6 +4644,7 @@ class Map: public HeapObject {
   class IsShared:              public BitField<bool, 0, 1> {};
   class FunctionWithPrototype: public BitField<bool, 1, 1> {};
   class LastAddedBits:         public BitField<int, 2, 11> {};
+  class EnumLengthBits:        public BitField<int, 13, 11> {};
 
   // Tells whether the object in the prototype property will be used
   // for instances created from this function.  If the prototype
@@ -4876,6 +4877,14 @@ class Map: public HeapObject {
     set_bit_field3(LastAddedBits::update(bit_field3(), index));
   }
 
+  int EnumLength() {
+    return EnumLengthBits::decode(bit_field3());
+  }
+
+  void SetEnumLength(int index) {
+    set_bit_field3(EnumLengthBits::update(bit_field3(), index));
+  }
+
   int LastAdded() {

[Michael Starzinger, 2012/08/06 15:06:22]

Can we move LastAdded() up just above SetLastAdded()?

[Toon Verwaest, 2012/08/07 10:49:47]

Done.

     return LastAddedBits::decode(bit_field3());
   }
@@ -5028,6 +5037,7 @@ class Map: public HeapObject {
 
   // Constant for denoting that the LastAdded field was not yet set.
   static const int kNoneAdded = LastAddedBits::kMax;
+  static const int kInvalidEnumCache = EnumLengthBits::kMax;
 
   // Layout description.
   static const int kInstanceSizesOffset = HeapObject::kHeaderSize;