Make sure that top bits are zero when storing untagged 32 bit values

src/x64/assembler-x64.cc

 // Copyright 2011 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 1622 matching lines @@
 void Assembler::movsxlq(Register dst, const Operand& src) {
   EnsureSpace ensure_space(this);
   emit_rex_64(dst, src);
   emit(0x63);
   emit_operand(dst, src);
 }
 
 
 void Assembler::movzxbq(Register dst, const Operand& src) {
   EnsureSpace ensure_space(this);
+  // 32 bit operations zero the top 32 bits of 64 bit registers.  Therefore
+  // there is no need to make this a 64 bit operation.
   emit_optional_rex_32(dst, src);
   emit(0x0F);
   emit(0xB6);
   emit_operand(dst, src);
 }
 
 
 void Assembler::movzxbl(Register dst, const Operand& src) {
   EnsureSpace ensure_space(this);
   emit_optional_rex_32(dst, src);
@@ skipping 1388 matching lines @@
   // specially coded on x64 means that it is a relative 32 bit address, as used
   // by branch instructions.
   return (1 << rmode_) & kApplyMask;
 }
 
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64