Implement IdentityMap<V>, a robust, GC-safe object-identity HashMap.

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index e9379ac5658c9653cb21023f180d567a2364c068..255bab9deeb36d23d364592cd4bdfcb18fcde579 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -141,7 +141,8 @@ Heap::Heap()
       chunks_queued_for_free_(NULL),
       gc_callbacks_depth_(0),
       deserialization_complete_(false),
-      concurrent_sweeping_enabled_(false) {
+      concurrent_sweeping_enabled_(false),
+      strong_roots_list_(NULL) {
 // Allow build-time customization of the max semispace size. Building
 // V8 with snapshots and a non-default max semispace size is much
 // easier if you can define it as part of the build environment.
@@ -4999,6 +5000,9 @@ void Heap::IterateStrongRoots(ObjectVisitor* v, VisitMode mode) {
   // output a flag to the snapshot.  However at this point the serializer and
   // deserializer are deliberately a little unsynchronized (see above) so the
   // checking of the sync flag in the snapshot would fail.
+  for (StrongRootsList* list = strong_roots_list_; list; list = list->next_) {
+    v->VisitPointers(list->start_, list->end_);
+  }
 }
 
 
@@ -5489,6 +5493,13 @@ void Heap::TearDown() {
   store_buffer()->TearDown();
 
   isolate_->memory_allocator()->TearDown();
+
+  StrongRootsList* next = NULL;
+  for (StrongRootsList* list = strong_roots_list_; list; list = next) {
+    next = list->next_;
+    delete list;
+  }
+  strong_roots_list_ = NULL;
 }