Introduce instance type for transition arrays.

src/transitions.cc

Index: src/transitions.cc
diff --git a/src/transitions.cc b/src/transitions.cc
index 0d1b7d65c02c2d3af29f0d1093d276616c150f69..9b52d849b4341e156c5111b6bf9a7e4cb1dfbedb 100644
--- a/src/transitions.cc
+++ b/src/transitions.cc
@@ -391,7 +391,7 @@ int TransitionArray::Capacity(Object* raw_transitions) {
 Handle<TransitionArray> TransitionArray::Allocate(Isolate* isolate,
                                                   int number_of_transitions,
                                                   int slack) {
-  Handle<FixedArray> array = isolate->factory()->NewFixedArray(
+  Handle<FixedArray> array = isolate->factory()->NewTransitionArray(
       LengthFor(number_of_transitions + slack));
   array->set(kPrototypeTransitionsIndex, Smi::FromInt(0));

[Michael Lippautz, 2015/11/27 14:27:28]

Do we need to properly initialize the next field?

[ulan, 2015/11/27 14:43:45]

Done.

NewTransitionArray() initializes with undefined. But let's explicitly set it to
undefined here too to be robust.

   array->set(kTransitionLengthIndex, Smi::FromInt(number_of_transitions));
@@ -399,10 +399,13 @@ Handle<TransitionArray> TransitionArray::Allocate(Isolate* isolate,
 }
 
 
-static void ZapTransitionArray(TransitionArray* transitions) {
-  MemsetPointer(transitions->data_start(),
+// static
+void TransitionArray::ZapTransitionArray(TransitionArray* transitions) {
+  // Do not zap the next link that is used by GC.
+  STATIC_ASSERT(kNextLinkIndex + 1 == kPrototypeTransitionsIndex);
+  MemsetPointer(transitions->data_start() + kPrototypeTransitionsIndex,
                 transitions->GetHeap()->the_hole_value(),
-                transitions->length());
+                transitions->length() - kPrototypeTransitionsIndex);
 }