Avoid handle dereference during graph optimization.

src/handles-inl.h

Index: src/handles-inl.h
diff --git a/src/handles-inl.h b/src/handles-inl.h
index 130798647ba2d6be2c68a1c810fd57a3bf39d1e0..c084fb901417108f0c78bda099a794e987a358b7 100644
--- a/src/handles-inl.h
+++ b/src/handles-inl.h
@@ -63,9 +63,18 @@ template <typename T>
 inline T* Handle<T>::operator*() const {
   ASSERT(location_ != NULL);
   ASSERT(reinterpret_cast<Address>(*location_) != kHandleZapValue);
+  SLOW_ASSERT(ISOLATE->allow_handle_deref());
   return *BitCast<T**>(location_);
 }
 
+template <typename T>
+inline T** Handle<T>::location() const {
+  ASSERT(location_ == NULL ||
+         reinterpret_cast<Address>(*location_) != kZapValue);
+  SLOW_ASSERT(ISOLATE->allow_handle_deref());
+  return location_;
+}
+
 
 HandleScope::HandleScope() {
   Isolate* isolate = Isolate::Current();
@@ -175,6 +184,38 @@ inline NoHandleAllocation::~NoHandleAllocation() {
     data->level = level_;
   }
 }
+
+
+NoHandleDereference::NoHandleDereference() {
+  // The guard is set on a per-isolate basis, so it affects all threads.
+  // That's why we can only use it when running without parallel recompilation.
+  if (FLAG_parallel_recompilation) return;
+  Isolate* isolate = Isolate::Current();
+  old_state_ = isolate->allow_handle_deref();
+  isolate->set_allow_handle_deref(false);
+}
+
+
+NoHandleDereference::~NoHandleDereference() {
+  if (FLAG_parallel_recompilation) return;
+  Isolate::Current()->set_allow_handle_deref(old_state_);
+}
+
+
+AllowHandleDereference::AllowHandleDereference() {
+  // The guard is set on a per-isolate basis, so it affects all threads.
+  // That's why we can only use it when running without parallel recompilation.
+  if (FLAG_parallel_recompilation) return;
+  Isolate* isolate = Isolate::Current();
+  old_state_ = isolate->allow_handle_deref();
+  isolate->set_allow_handle_deref(true);
+}
+
+
+AllowHandleDereference::~AllowHandleDereference() {
+  if (FLAG_parallel_recompilation) return;
+  Isolate::Current()->set_allow_handle_deref(old_state_);
+}
 #endif