Stay in fast enum case if the empty_slow_element_dictionary is used.

src/x64/macro-assembler-x64.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 4914 matching lines @@
 
   // For all objects but the receiver, check that the cache is empty.
   EnumLength(rdx, rbx);
   Cmp(rdx, Smi::FromInt(0));
   j(not_equal, call_runtime);
 
   bind(&start);
 
   // Check that there are no elements. Register rcx contains the current JS
   // object we've reached through the prototype chain.
+  Label no_elements;
   cmpq(empty_fixed_array_value,
        FieldOperand(rcx, JSObject::kElementsOffset));
+  j(equal, &no_elements);
+
+  // Second chance, the object may be using the empty slow element dictionary.
+  LoadRoot(kScratchRegister, Heap::kEmptySlowElementDictionaryRootIndex);
+  cmpq(kScratchRegister, FieldOperand(rcx, JSObject::kElementsOffset));
   j(not_equal, call_runtime);
 
+  bind(&no_elements);
   movp(rcx, FieldOperand(rbx, Map::kPrototypeOffset));
   cmpq(rcx, null_value);
   j(not_equal, &next);
 }
 
 void MacroAssembler::TestJSArrayForAllocationMemento(
     Register receiver_reg,
     Register scratch_reg,
     Label* no_memento_found) {
   ExternalReference new_space_start =
@@ skipping 35 matching lines @@
   j(equal, found);
   movp(current, FieldOperand(current, Map::kPrototypeOffset));
   CompareRoot(current, Heap::kNullValueRootIndex);
   j(not_equal, &loop_again);
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64