[heap] Make LiveObjectIterator concurrency safe

src/heap/mark-compact-inl.h

Index: src/heap/mark-compact-inl.h
diff --git a/src/heap/mark-compact-inl.h b/src/heap/mark-compact-inl.h
index fe71fb1177efa7bad7e78c8831c2ceba84778587..7ce7dbd699b6cf911cc76aa880dff2846e39ad4e 100644
--- a/src/heap/mark-compact-inl.h
+++ b/src/heap/mark-compact-inl.h
@@ -163,12 +163,14 @@ HeapObject* LiveObjectIterator<T>::Next() {
         current_cell_ = *it_.CurrentCell();
       }
 
+      Map* map = nullptr;
       if (current_cell_ & second_bit_index) {
         // We found a black object. If the black object is within a black area,
         // make sure that we skip all set bits in the black area until the
         // object ends.
         HeapObject* black_object = HeapObject::FromAddress(addr);
-        Address end = addr + black_object->Size() - kPointerSize;
+        map = black_object->synchronized_map();

[Hannes Payer (out of office), 2016/11/07 09:26:55]

Size() uses no barrier accessor. No need to synchronize here, however we can if
it is not a performance issue.

[Michael Lippautz, 2016/11/07 09:36:50]

Changed to NoBarrier read.

+        Address end = addr + black_object->SizeFromMap(map) - kPointerSize;
         // One word filler objects do not borrow the second mark bit. We have
         // to jump over the advancing and clearing part.
         // Note that we know that we are at a one word filler when
@@ -198,9 +200,9 @@ HeapObject* LiveObjectIterator<T>::Next() {
 
       // We found a live object.
       if (object != nullptr) {
-        if (object->IsFiller()) {
-          // Black areas together with slack tracking may result in black filler
-          // objects. We filter these objects out in the iterator.
+        if (map == nullptr || map == heap()->one_pointer_filler_map()) {

[Hannes Payer (out of office), 2016/11/07 09:26:55]

How can map become nullptr?

[Michael Lippautz, 2016/11/07 09:36:50]

As discussed offline: This happens when we take the branch for gray objects.

+          // Black areas together with slack tracking may result in black one
+          // word filler objects. We filter these objects out in the iterator.
           object = nullptr;
         } else {
           break;