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

src/x64/builtins-x64.cc

Index: src/x64/builtins-x64.cc
===================================================================
--- src/x64/builtins-x64.cc	(revision 6129)
+++ src/x64/builtins-x64.cc	(working copy)
@@ -422,7 +422,7 @@
   // [rsp+0x20] : argv
 
   // Clear the context before we push it when entering the JS frame.
-  __ xor_(rsi, rsi);
+  __ Set(rsi, 0);
   __ EnterInternalFrame();
 
   // Load the function context into rsi.
@@ -451,7 +451,7 @@
   // rdi : function
 
   // Clear the context before we push it when entering the JS frame.
-  __ xor_(rsi, rsi);
+  __ Set(rsi, 0);
   // Enter an internal frame.
   __ EnterInternalFrame();
 
@@ -479,7 +479,7 @@
   // Register rbx points to array of pointers to handle locations.
   // Push the values of these handles.
   Label loop, entry;
-  __ xor_(rcx, rcx);  // Set loop variable to 0.
+  __ Set(rcx, 0);  // Set loop variable to 0.
   __ jmp(&entry);
   __ bind(&loop);
   __ movq(kScratchRegister, Operand(rbx, rcx, times_pointer_size, 0));
@@ -668,7 +668,7 @@
   //     become the receiver.
   __ bind(&non_function);
   __ movq(Operand(rsp, rax, times_pointer_size, 0), rdi);
-  __ xor_(rdi, rdi);
+  __ Set(rdi, 0);
 
   // 4. Shift arguments and return address one slot down on the stack
   //    (overwriting the original receiver).  Adjust argument count to make
@@ -689,7 +689,7 @@
   { Label function;
     __ testq(rdi, rdi);
     __ j(not_zero, &function);
-    __ xor_(rbx, rbx);
+    __ Set(rbx, 0);
     __ GetBuiltinEntry(rdx, Builtins::CALL_NON_FUNCTION);
     __ Jump(Handle<Code>(builtin(ArgumentsAdaptorTrampoline)),
             RelocInfo::CODE_TARGET);