Do not dereference handles during relocation.

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index 7976cf8aa7f1a4965dc844c21d4254f07580a8df..f2f0b9bac7421f2441a0aa8787d0babeffea83ac 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -1300,6 +1300,8 @@ class ScavengeWeakObjectRetainer : public WeakObjectRetainer {
 
 
 void Heap::Scavenge() {
+  RelocationLock relocation_lock(this);
+
 #ifdef VERIFY_HEAP
   if (FLAG_verify_heap) VerifyNonPointerSpacePointers();
 #endif
@@ -6635,6 +6637,11 @@ bool Heap::SetUp() {
 
   store_buffer()->SetUp();
 
+  if (FLAG_parallel_recompilation) relocation_mutex_ = OS::CreateMutex();
+#ifdef DEBUG
+  relocation_mutex_locked_by_optimizer_thread_ = false;
+#endif  // DEBUG
+
   return true;
 }
 
@@ -6737,6 +6744,8 @@ void Heap::TearDown() {
   incremental_marking()->TearDown();
 
   isolate_->memory_allocator()->TearDown();
+
+  delete relocation_mutex_;
 }
 
 
@@ -7866,4 +7875,15 @@ void Heap::CheckpointObjectStats() {
   ClearObjectStats();
 }
 
+
+Heap::RelocationLock::RelocationLock(Heap* heap) : heap_(heap) {
+  if (FLAG_parallel_recompilation) {
+    heap_->relocation_mutex_->Lock();
+#ifdef DEBUG
+    heap_->relocation_mutex_locked_by_optimizer_thread_ =
+        heap_->isolate()->optimizing_compiler_thread()->IsOptimizerThread();
+#endif  // DEBUG
+  }
+}
+
 } }  // namespace v8::internal