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

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index dcf7bdea0ac812972304bd382d52b0a1f4c5eff2..1f573f77c4e1ef313903a913d7452546033b6167 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -1452,21 +1452,68 @@ class Heap {
   void TraceObjectStat(const char* name, int count, int size, double time);
   void CheckpointObjectStats();
 
-  // We don't use a LockGuard here since we want to lock the heap
-  // only when FLAG_concurrent_recompilation is true.
+  struct StrongRootsList {
+    Object** start_;
+    Object** end_;
+    StrongRootsList* next_;
+  };
+
+  void RegisterStrongRoots(Object** start, Object** end) {
+    StrongRootsList* list = new StrongRootsList();
+    list->next_ = strong_roots_list_;
+    list->start_ = start;
+    list->end_ = end;
+    strong_roots_list_ = list;
+  }
+
+  void UnregisterStrongRoots(Object** start) {
+    StrongRootsList* prev = NULL;
+    for (StrongRootsList* list = strong_roots_list_; list; list = list->next_) {
+      if (list->start_ == start) {
+        if (prev) {
+          prev->next_ = list->next_;
+        } else {
+          strong_roots_list_ = list->next_;
+        }
+        delete list;
+      }
+      prev = list;
+    }
+  }
+
+  // Taking this lock prevents the GC from entering a phase that relocates
+  // object references.
   class RelocationLock {
    public:
     explicit RelocationLock(Heap* heap) : heap_(heap) {
       heap_->relocation_mutex_.Lock();
     }
 
-
     ~RelocationLock() { heap_->relocation_mutex_.Unlock(); }
 
    private:
     Heap* heap_;
   };
 
+  // An optional version of the above lock that can be used for some critical
+  // sections on the mutator thread; only safe since the GC currently does not
+  // do concurrent compaction.
+  class OptionalRelocationLock {
+   public:
+    explicit OptionalRelocationLock(Heap* heap, bool concurrent)
+        : heap_(heap), concurrent_(concurrent) {
+      if (concurrent_) heap_->relocation_mutex_.Lock();
+    }
+
+    ~OptionalRelocationLock() {
+      if (concurrent_) heap_->relocation_mutex_.Unlock();
+    }
+
+   private:
+    Heap* heap_;
+    bool concurrent_;
+  };
+
   void AddWeakObjectToCodeDependency(Handle<HeapObject> obj,
                                      Handle<DependentCode> dep);
 
@@ -2152,6 +2199,8 @@ class Heap {
 
   bool concurrent_sweeping_enabled_;
 
+  StrongRootsList* strong_roots_list_;
+
   friend class AlwaysAllocateScope;
   friend class Deserializer;
   friend class Factory;