[heap] Fix live object iterator bail out case.

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 6792f432a3707ea32c124d6b27b28f12fdf2ef37..402e3bf0af8571c1101cb48537fb814353634921 100644
--- a/src/heap/mark-compact-inl.h
+++ b/src/heap/mark-compact-inl.h
@@ -174,16 +174,13 @@ HeapObject* LiveObjectIterator<T>::Next() {
         // However, if there is a black area at the end of the page, and the
         // last word is a one word filler, we are not allowed to advance. In
         // that case we can return immediately.

[Michael Lippautz, 2017/05/03 07:41:35]

I have a troubles following the iterator and the statement above. Is it possible
that what we actually want is 

DCHECK(!it.Done());
if (!it.Advance()) { ... return nullptr; }
// Access the iterator

it.Done() checks the current iterator position, right? So in this branch here we
should have a valid current position but we are actually not sure whether we can
advance?

What am I missing?

-        if (it_.Done()) {
+        if (it_.Done() || !it_.Advance()) {
           DCHECK(HeapObject::FromAddress(addr)->map() ==
                  HeapObject::FromAddress(addr)
                      ->GetHeap()
                      ->one_pointer_filler_map());
           return nullptr;
         }
-        bool not_done = it_.Advance();
-        USE(not_done);
-        DCHECK(not_done);
         cell_base_ = it_.CurrentCellBase();
         current_cell_ = *it_.CurrentCell();
       }