Use a special EnumLength field to indicate number of valid enum cache values.

src/hydrogen.cc

 // Copyright 2012 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 4302 matching lines @@
   HInstruction* map = AddInstruction(new(zone()) HForInPrepareMap(
       environment()->LookupContext(), enumerable));
   AddSimulate(stmt->PrepareId());
 
   HInstruction* array = AddInstruction(
       new(zone()) HForInCacheArray(
           enumerable,
           map,
           DescriptorArray::kEnumCacheBridgeCacheIndex));
 
-  HInstruction* array_length = AddInstruction(
-      new(zone()) HFixedArrayBaseLength(array));
+  HInstruction* enum_length = AddInstruction(new(zone()) HMapEnumLength(map));
 
   HInstruction* start_index = AddInstruction(new(zone()) HConstant(
       Handle<Object>(Smi::FromInt(0)), Representation::Integer32()));
 
   Push(map);
   Push(array);
-  Push(array_length);
+  Push(enum_length);
   Push(start_index);
 
   HInstruction* index_cache = AddInstruction(
       new(zone()) HForInCacheArray(
           enumerable,
           map,
           DescriptorArray::kEnumCacheBridgeIndicesCacheIndex));
   HForInCacheArray::cast(array)->set_index_cache(
       HForInCacheArray::cast(index_cache));
 
@@ skipping 5301 matching lines @@
     }
   }
 
 #ifdef DEBUG
   if (graph_ != NULL) graph_->Verify(false);  // No full verify.
   if (allocator_ != NULL) allocator_->Verify();
 #endif
 }
 
 } }  // namespace v8::internal