Make sure that top bits are zero when storing untagged 32 bit 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 931 matching lines @@
   // 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);
 
+  // Abort execution if a 64 bit register containing a 32 bit payload does not
+  // have zeros in the top 32 bits.
+  void AbortIfNotZeroExtended(Register reg);
+
   // Abort execution if argument is a string. Used in debug code.
   void AbortIfNotString(Register object);
 
   // Abort execution if argument is not the root value with the given index.
   void AbortIfNotRootValue(Register src,
                            Heap::RootListIndex root_value_index,
                            const char* message);
 
   // ---------------------------------------------------------------------------
   // Exception handling
@@ skipping 503 matching lines @@
     masm->popfd();                                                        \
   }                                                                       \
   masm->
 #else
 #define ACCESS_MASM(masm) masm->
 #endif
 
 } }  // namespace v8::internal
 
 #endif  // V8_X64_MACRO_ASSEMBLER_X64_H_