Use the macro assembler Set instead of explicit xor for clearing registers.

src/x64/assembler-x64.cc

 // Copyright 2010 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 56 matching lines @@
   __ pop(rax);
   __ movq(rdx, rax);
   __ xor_(rax, Immediate(0x200000));  // Flip bit 21.
   __ push(rax);
   __ popfq();
   __ pushfq();
   __ pop(rax);
   __ xor_(rax, rdx);  // Different if CPUID is supported.
   __ j(not_zero, &cpuid);
 
-  // CPUID not supported. Clear the supported features in edx:eax.
+  // CPUID not supported. Clear the supported features in rax.
   __ xor_(rax, rax);
   __ jmp(&done);
 
   // Invoke CPUID with 1 in eax to get feature information in
   // ecx:edx. Temporarily enable CPUID support because we know it's
   // safe here.
   __ bind(&cpuid);
   __ movq(rax, Immediate(1));
   supported_ = kDefaultCpuFeatures | (1 << CPUID);
   { Scope fscope(CPUID);
@@ skipping 2899 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