Check mark bit of the found object in MarkCompactCollector::IsSlotInBlackObject.

src/heap/mark-compact.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #include "src/base/atomicops.h"
 #include "src/base/bits.h"
 #include "src/code-stubs.h"
 #include "src/compilation-cache.h"
@@ skipping 3120 matching lines @@
 
   // Find the last live object in the cell.
   unsigned int leading_zeros =
       base::bits::CountLeadingZeros32(current_cell & slot_mask);
   CHECK(leading_zeros != 32);
   unsigned int offset = Bitmap::kBitIndexMask - leading_zeros;
 
   cell_base += (start_index - cell_base_start_index) * 32 * kPointerSize;
   Address address = cell_base + offset * kPointerSize;
   HeapObject* object = HeapObject::FromAddress(address);
+  CHECK(Marking::IsBlack(Marking::MarkBitFrom(object)));
   CHECK(object->address() < reinterpret_cast<Address>(slot));
   if (object->address() <= slot &&
       (object->address() + object->Size()) > slot) {
     // If the slot is within the last found object in the cell, the slot is
     // in a live object.
     *out_object = object;
     return true;
   }
   return false;
 }
@@ skipping 1599 matching lines @@
   SlotsBuffer* buffer = *buffer_address;
   while (buffer != NULL) {
     SlotsBuffer* next_buffer = buffer->next();
     DeallocateBuffer(buffer);
     buffer = next_buffer;
   }
   *buffer_address = NULL;
 }
 }
 }  // namespace v8::internal