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

src/x64/macro-assembler-x64.h

 // 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 927 matching lines @@
   void FCmp();
 
   void ClampUint8(Register reg);
 
   void ClampDoubleToUint8(XMMRegister input_reg,
                           XMMRegister temp_xmm_reg,
                           Register result_reg,
                           Register temp_reg);
 
   void LoadInstanceDescriptors(Register map, Register descriptors);
+  void EnumLength(Register dst, Register map);
+
+  template<typename Field>
+  void DecodeField(Register reg) {
+    static const int full_shift = Field::kShift + kSmiShift;
+    static const int low_mask = Field::kMask >> Field::kShift;
+    shr(reg, Immediate(full_shift));
+    and_(reg, Immediate(low_mask));
+  }
 
   // Abort execution if argument is not a number. Used in debug code.
   void AbortIfNotNumber(Register object);
 
   // Abort execution if argument is a smi. Used in debug code.
   void AbortIfSmi(Register object);
 
   // Abort execution if argument is not a smi. Used in debug code.
   void AbortIfNotSmi(Register object);
   void AbortIfNotSmi(const Operand& object);
@@ skipping 522 matching lines @@
     masm->popfd();                                                        \
   }                                                                       \
   masm->
 #else
 #define ACCESS_MASM(masm) masm->
 #endif
 
 } }  // namespace v8::internal
 
 #endif  // V8_X64_MACRO_ASSEMBLER_X64_H_