Use mutable heapnumbers to store doubles in fields.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index b988d3bf2bfea9a8a990a1f5f6d1fb616224164a..b45e40be719dd045f86941579dbea3587fdd2d7d 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -286,6 +286,14 @@ bool Object::HasValidElements() {
   return IsFixedArray() || IsFixedDoubleArray() || IsExternalArray();
 }
 
+
+MaybeObject* Object::StorageFor(Heap* heap, Representation representation) {

[danno, 2013/05/07 13:04:47]

AllocateNewStorageFor

[Toon Verwaest, 2013/05/07 15:08:52]

Done.

+  if (!FLAG_track_double_fields) return this;
+  if (!representation.IsDouble()) return this;
+  return heap->AllocateHeapNumber(Number());
+}
+
+
 StringShape::StringShape(String* str)
   : type_(str->map()->instance_type()) {
   set_valid();
@@ -1718,7 +1726,7 @@ Object* JSObject::FastPropertyAt(int index) {
 }
 
 
-Object* JSObject::FastPropertyAtPut(int index, Object* value) {
+void JSObject::FastPropertyAtPut(int index, Object* value) {
   // Adjust for the number of properties stored in the object.
   index -= map()->inobject_properties();
   if (index < 0) {
@@ -1729,7 +1737,6 @@ Object* JSObject::FastPropertyAtPut(int index, Object* value) {
     ASSERT(index < properties()->length());
     properties()->set(index, value);
   }
-  return value;
 }