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

src/x64/lithium-codegen-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 1267 matching lines @@
 }
 
 
 void LCodeGen::DoFixedArrayBaseLength(LFixedArrayBaseLength* instr) {
   Register result = ToRegister(instr->result());
   Register array = ToRegister(instr->InputAt(0));
   __ movq(result, FieldOperand(array, FixedArrayBase::kLengthOffset));
 }
 
 
+void LCodeGen::DoMapEnumLength(LMapEnumLength* instr) {
+  Register result = ToRegister(instr->result());
+  Register map = ToRegister(instr->InputAt(0));
+  __ EnumLength(result, map);
+}
+
+
 void LCodeGen::DoElementsKind(LElementsKind* instr) {
   Register result = ToRegister(instr->result());
   Register input = ToRegister(instr->InputAt(0));
 
   // Load map into |result|.
   __ movq(result, FieldOperand(input, HeapObject::kMapOffset));
   // Load the map's "bit field 2" into |result|. We only need the first byte.
   __ movzxbq(result, FieldOperand(result, Map::kBitField2Offset));
   // Retrieve elements_kind from bit field 2.
   __ and_(result, Immediate(Map::kElementsKindMask));
@@ skipping 3912 matching lines @@
   __ CompareRoot(FieldOperand(rax, HeapObject::kMapOffset),
                  Heap::kMetaMapRootIndex);
   DeoptimizeIf(not_equal, instr->environment());
   __ bind(&use_cache);
 }
 
 
 void LCodeGen::DoForInCacheArray(LForInCacheArray* instr) {
   Register map = ToRegister(instr->map());
   Register result = ToRegister(instr->result());
+  Label load_cache, done;
+  __ EnumLength(result, map);
+  __ Cmp(result, Smi::FromInt(0));
+  __ j(not_equal, &load_cache);
+  __ LoadRoot(result, Heap::kEmptyFixedArrayRootIndex);
+  __ jmp(&done);
+  __ bind(&load_cache);
   __ LoadInstanceDescriptors(map, result);
   __ movq(result,
           FieldOperand(result, DescriptorArray::kEnumCacheOffset));
   __ movq(result,
           FieldOperand(result, FixedArray::SizeFor(instr->idx())));
+  __ bind(&done);
   Condition cc = masm()->CheckSmi(result);
   DeoptimizeIf(cc, instr->environment());
 }
 
 
 void LCodeGen::DoCheckMapValue(LCheckMapValue* instr) {
   Register object = ToRegister(instr->value());
   __ cmpq(ToRegister(instr->map()),
           FieldOperand(object, HeapObject::kMapOffset));
   DeoptimizeIf(not_equal, instr->environment());
@@ skipping 24 matching lines @@
                                FixedArray::kHeaderSize - kPointerSize));
   __ bind(&done);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64