Verify that double unboxing is never performed on large objects.

src/heap/spaces.cc

 // Copyright 2011 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/bits.h"
 #include "src/base/platform/platform.h"
 #include "src/full-codegen.h"
 #include "src/heap/mark-compact.h"
@@ skipping 3028 matching lines @@
     HeapObject* object = chunk->GetObject();
     Page* page = Page::FromAddress(object->address());
     CHECK(object->address() == page->area_start());
 
     // The first word should be a map, and we expect all map pointers to be
     // in map space.
     Map* map = object->map();
     CHECK(map->IsMap());
     CHECK(heap()->map_space()->Contains(map));
 
+    // Double unboxing in LO space is not allowed. This would break the
+    // lookup mechanism for store and slot buffer entries which use the
+    // page header tag.
+    CHECK(object->ContentType() != HeapObjectContents::kMixedValues);
+
     // We have only code, sequential strings, external strings
     // (sequential strings that have been morphed into external
     // strings), fixed arrays, byte arrays, and constant pool arrays in the
     // large object space.
     CHECK(object->IsCode() || object->IsSeqString() ||
           object->IsExternalString() || object->IsFixedArray() ||
           object->IsFixedDoubleArray() || object->IsByteArray());
 
     // The object itself should look OK.
     object->ObjectVerify();
@@ skipping 75 matching lines @@
     object->ShortPrint();
     PrintF("\n");
   }
   printf(" --------------------------------------\n");
   printf(" Marked: %x, LiveCount: %x\n", mark_size, LiveBytes());
 }
 
 #endif  // DEBUG
 }  // namespace internal
 }  // namespace v8