[heap] Use size-based live object iterator.

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 ceca4c4d8ce7c1ccd5b3c04486aca391b157dfc9..7ead42150b698f7b44fb198738fbe854066c56d0 100644
--- a/src/heap/mark-compact-inl.h
+++ b/src/heap/mark-compact-inl.h
@@ -148,25 +148,66 @@ HeapObject* LiveObjectIterator<T>::Next() {
         second_bit_index = 0x1;
         // The overlapping case; there has to exist a cell after the current
         // cell.
-        DCHECK(!it_.Done());
+        // 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.
+        if (it_.Done()) {
+          DCHECK(HeapObject::FromAddress(addr)->map() ==
+                 HeapObject::FromAddress(addr)
+                     ->GetHeap()
+                     ->one_pointer_filler_map());
+          return nullptr;
+        }
         it_.Advance();
         cell_base_ = it_.CurrentCellBase();
         current_cell_ = *it_.CurrentCell();
       }
-      if (T == kBlackObjects && (current_cell_ & second_bit_index)) {
-        object = HeapObject::FromAddress(addr);
-      } else if (T == kGreyObjects && !(current_cell_ & second_bit_index)) {
-        object = HeapObject::FromAddress(addr);
-      } else if (T == kAllLiveObjects) {
+
+      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;
+        // 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
+        // object_start + object_size - kPointerSize == object_start.
+        if (addr != end) {
+          DCHECK_EQ(chunk_, MemoryChunk::FromAddress(end));
+          uint32_t end_mark_bit_index = chunk_->AddressToMarkbitIndex(end);
+          unsigned int end_cell_index =
+              end_mark_bit_index >> Bitmap::kBitsPerCellLog2;
+          MarkBit::CellType end_index_mask =
+              1u << Bitmap::IndexInCell(end_mark_bit_index);
+          if (it_.Advance(end_cell_index)) {
+            cell_base_ = it_.CurrentCellBase();
+            current_cell_ = *it_.CurrentCell();
+          }
+
+          // Clear all bits in current_cell, including the end index.
+          current_cell_ &= ~(end_index_mask + end_index_mask - 1);
+        }
+
+        if (T == kBlackObjects || T == kAllLiveObjects) {
+          object = black_object;
+        }
+      } else if ((T == kGreyObjects || T == kAllLiveObjects)) {
         object = HeapObject::FromAddress(addr);
       }
 
-      // Clear the second bit of the found object.
-      current_cell_ &= ~second_bit_index;
-
       // We found a live object.
-      if (object != nullptr) break;
+      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.
+          object = nullptr;
+        } else {
+          break;
+        }
+      }
     }
+
     if (current_cell_ == 0) {
       if (!it_.Done()) {
         it_.Advance();