[runtime] Allocate space for computed property names

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 45f5ba0c0a3c7f3d1cb5250abbfd7711bd3c760c..2d358048957998409514bcab8214aaf59879ab48 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -1062,6 +1062,7 @@ template <class C> inline bool Is(Object* obj);
   V(DependentCode)               \
   V(HandlerTable)                \
   V(FixedArray)                  \
+  V(ConstantProperties)          \
   V(FixedDoubleArray)            \
   V(WeakFixedArray)              \
   V(ArrayList)                   \
@@ -2914,6 +2915,28 @@ class FixedArray: public FixedArrayBase {
   DISALLOW_IMPLICIT_CONSTRUCTORS(FixedArray);
 };
 
+// ConstantProperties is a special FixedArray of constant properties that
+// provides access to the total number of properties (constant and
+// non-constant).
+// Usually, the number of properties is half the length of the
+// FixedArray (half because it consists of key-value pairs), but there could
+// have been non-constant property names for which we need to allocate space in
+// the property backing store as well.
+// The parser stores the total number after the last object in
+// the FixedArray. ConstantProperties simplifies access to this number.

[Toon Verwaest, 2017/01/18 08:49:53]

What about also adding name(i) and value(i) which just does get(i*2) and
get(i*2+1)?

Perhaps ConstantProperties isn't exactly the best name since their values aren't
constant... What about just BoilerplateDescription or so?

[Franzi, 2017/01/18 14:23:10]

name() and value() looks nice. Thanks! Changed the name as well. 

+class ConstantProperties : public FixedArray {
+ public:
+  // Number of constant and non-constant properties.
+  int const number_of_properties();
+  // Actual size of the FixedArray, i.e., twice the number of constant
+  // properties without the field that contains the total number.
+  int const length();
+
+  DECLARE_CAST(ConstantProperties)
+
+ private:
+  bool const has_number_of_properties();
+};
 
 // FixedDoubleArray describes fixed-sized arrays with element type double.
 class FixedDoubleArray: public FixedArrayBase {
@@ -6397,7 +6420,6 @@ class Map: public HeapObject {
   static const int kInstanceTypeAndBitFieldOffset =
       kInstanceAttributesOffset + 0;
   static const int kBitField2Offset = kInstanceAttributesOffset + 2;
-  static const int kUnusedPropertyFieldsByte = 3;

[Toon Verwaest, 2017/01/18 08:49:53]

why remove this?

[Franzi, 2017/01/18 14:23:10]

Noticed that it's unused. Any reason we need to keep it?

   static const int kUnusedPropertyFieldsOffset = kInstanceAttributesOffset + 3;
 
   STATIC_ASSERT(kInstanceTypeAndBitFieldOffset ==