Initialize pre-allocated fields of JSObject with undefined.

src/heap.cc

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 3325 matching lines @@
   // according to the object's initial map.  For example, if the map's
   // instance type is JS_ARRAY_TYPE, the length field should be initialized
   // to a number (eg, Smi::FromInt(0)) and the elements initialized to a
   // fixed array (eg, Heap::empty_fixed_array()).  Currently, the object
   // verification code has to cope with (temporarily) invalid objects.  See
   // for example, JSArray::JSArrayVerify).
   Object* filler;
   // We cannot always fill with one_pointer_filler_map because objects
   // created from API functions expect their internal fields to be initialized
   // with undefined_value.
+  // Pre-allocated fields need to be initialized with undefined_value as well
+  // so that object accesses before the constructor completes (e.g. in the
+  // debugger) will not cause a crash.
   if (map->constructor()->IsJSFunction() &&
       JSFunction::cast(map->constructor())->shared()->
           IsInobjectSlackTrackingInProgress()) {
     // We might want to shrink the object later.
     ASSERT(obj->GetInternalFieldCount() == 0);
     filler = Heap::one_pointer_filler_map();
   } else {
     filler = Heap::undefined_value();
   }
-  obj->InitializeBody(map->instance_size(), filler);
+  obj->InitializeBody(map, Heap::undefined_value(), filler);
 }
 
 
 MaybeObject* Heap::AllocateJSObjectFromMap(Map* map, PretenureFlag pretenure) {
   // JSFunctions should be allocated using AllocateFunction to be
   // properly initialized.
   ASSERT(map->instance_type() != JS_FUNCTION_TYPE);
 
   // Both types of global objects should be allocated using
   // AllocateGlobalObject to be properly initialized.
@@ skipping 2923 matching lines @@
 }
 
 
 void ExternalStringTable::TearDown() {
   new_space_strings_.Free();
   old_space_strings_.Free();
 }
 
 
 } }  // namespace v8::internal