Decoupling MarkDescriptorArray as much as possible from the ContentArray.

Review URL: https://chromiumcodereview.appspot.com/10417030

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11683 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/mark-compact.cc

@@ -1872,17 +1872,16 @@ void Marker<T>::MarkDescriptorArray(DescriptorArray* descriptors) {
   if (!base_marker()->MarkObjectAndPush(descriptors)) return;
   FixedArray* contents = FixedArray::cast(
       descriptors->get(DescriptorArray::kContentArrayIndex));
-  ASSERT(contents->length() >= 2);
   ASSERT(Marking::IsWhite(Marking::MarkBitFrom(contents)));
   base_marker()->MarkObjectWithoutPush(contents);
 
-  // Contents contains (value, details) pairs.  If the descriptor contains a
-  // transition (value is a Map), we don't mark the value as live.  It might
-  // be set to the NULL_DESCRIPTOR in ClearNonLiveTransitions later.
-  for (int i = 0; i < contents->length(); i += 2) {
-    PropertyDetails details(Smi::cast(contents->get(i + 1)));
+  // If the descriptor contains a transition (value is a Map), we don't mark the
+  // value as live. It might be set to the NULL_DESCRIPTOR in
+  // ClearNonLiveTransitions later.
+  for (int i = 0; i < descriptors->number_of_descriptors(); ++i) {
+    PropertyDetails details(descriptors->GetDetails(i));
+    Object** slot = descriptors->GetValueSlot(i);
 
-    Object** slot = contents->data_start() + i;
     if (!(*slot)->IsHeapObject()) continue;
     HeapObject* value = HeapObject::cast(*slot);
 

src/objects-inl.h

@@ -1939,6 +1939,12 @@ String* DescriptorArray::GetKey(int descriptor_number) {
 }
 
 
+Object** DescriptorArray::GetValueSlot(int descriptor_number) {
+  ASSERT(descriptor_number < number_of_descriptors());
+  return GetContentArray()->data_start() + ToValueIndex(descriptor_number);
+}
+
+
 Object* DescriptorArray::GetValue(int descriptor_number) {
   ASSERT(descriptor_number < number_of_descriptors());
   return GetContentArray()->get(ToValueIndex(descriptor_number));

src/objects.cc

@@ -6069,7 +6069,10 @@ MaybeObject* DescriptorArray::RemoveTransitions() {
   return new_descriptors;
 }
 
-
+// We need the whiteness witness since sort will reshuffle the entries in the
+// descriptor array. If the descriptor array were to be black, the shuffling
+// would move a slot that was already recorded as pointing into an evacuation
+// candidate. This would result in missing updates upon evacuation.
 void DescriptorArray::SortUnchecked(const WhitenessWitness& witness) {
   // In-place heap sort.
   int len = number_of_descriptors();

src/objects.h

@@ -2453,6 +2453,7 @@ class DescriptorArray: public FixedArray {
   // Accessors for fetching instance descriptor at descriptor number.
   inline String* GetKey(int descriptor_number);
   inline Object* GetValue(int descriptor_number);
+  inline Object** GetValueSlot(int descriptor_number);
   inline PropertyDetails GetDetails(int descriptor_number);
   inline PropertyType GetType(int descriptor_number);
   inline int GetFieldIndex(int descriptor_number);
@@ -2463,6 +2464,14 @@ class DescriptorArray: public FixedArray {
   inline bool IsTransitionOnly(int descriptor_number);
   inline bool IsNullDescriptor(int descriptor_number);
 
+  // WhitenessWitness is used to prove that a specific descriptor array is white
+  // (unmarked), so incremental write barriers can be skipped because the
+  // marking invariant cannot be broken and slots pointing into evacuation
+  // candidates will be discovered when the object is scanned. A witness is
+  // always stack-allocated right after creating a descriptor array. By
+  // allocating a witness, incremental marking is globally disabled. The witness
+  // is then passed along wherever needed to statically prove that the
+  // descriptor array is known to be white.
   class WhitenessWitness {
    public:
     inline explicit WhitenessWitness(DescriptorArray* array);