Handle store buffer slot overwrite during object promotion.

src/heap/heap.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/accessors.h"
 #include "src/api.h"
 #include "src/base/bits.h"
 #include "src/base/once.h"
@@ skipping 2020 matching lines @@
 
     HeapObject* target = NULL;  // Initialization to please compiler.
     if (allocation.To(&target)) {
       if (alignment != kObjectAlignment) {
         target = EnsureDoubleAligned(heap, target, allocation_size);
       }
 
       // Order is important: slot might be inside of the target if target
       // was allocated over a dead object and slot comes from the store
       // buffer.
-      *slot = target;
+
+      // Unfortunately, the allocation can also write over the slot if the slot
+      // was in free space and the allocation wrote free list data (such as the
+      // free list map or entry size) over the slot.  We guard against this by
+      // checking that the slot still points to the object being moved.  This
+      // should be sufficient because neither the free list map nor the free
+      // list entry size should look like a new space pointer (the former is an
+      // old space pointer, the latter is word-aligned).
+      if (*slot == object) {
+        *slot = target;
+      }
       MigrateObject(heap, object, target, object_size);
 
       if (object_contents == POINTER_OBJECT) {
         if (map->instance_type() == JS_FUNCTION_TYPE) {
           heap->promotion_queue()->insert(target,
                                           JSFunction::kNonWeakFieldsEndOffset);
         } else {
           heap->promotion_queue()->insert(target, object_size);
         }
       }
@@ skipping 4200 matching lines @@
       static_cast<int>(object_sizes_last_time_[index]));
   CODE_AGE_LIST_COMPLETE(ADJUST_LAST_TIME_OBJECT_COUNT)
 #undef ADJUST_LAST_TIME_OBJECT_COUNT
 
   MemCopy(object_counts_last_time_, object_counts_, sizeof(object_counts_));
   MemCopy(object_sizes_last_time_, object_sizes_, sizeof(object_sizes_));
   ClearObjectStats();
 }
 }
 }  // namespace v8::internal