Generate more compact XOR on 64-bit architecture when using xor to zero out r...

src/x64/assembler-x64.h

 // Copyright (c) 1994-2006 Sun Microsystems Inc.
 // 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.
 //
@@ skipping 902 matching lines @@
   void testb(Register reg, Immediate mask);
   void testb(const Operand& op, Immediate mask);
   void testl(Register dst, Register src);
   void testl(Register reg, Immediate mask);
   void testl(const Operand& op, Immediate mask);
   void testq(const Operand& op, Register reg);
   void testq(Register dst, Register src);
   void testq(Register dst, Immediate mask);
 
   void xor_(Register dst, Register src) {
-    arithmetic_op(0x33, dst, src);
+    if (dst.code() == src.code()) {
+      arithmetic_op_32(0x33, dst, src);
+    } else {
+      arithmetic_op(0x33, dst, src);
+    }
   }
 
   void xorl(Register dst, Register src) {
     arithmetic_op_32(0x33, dst, src);
   }
 
   void xor_(Register dst, const Operand& src) {
     arithmetic_op(0x33, dst, src);
   }
 
@@ skipping 431 matching lines @@
  private:
   Assembler* assembler_;
 #ifdef DEBUG
   int space_before_;
 #endif
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_X64_ASSEMBLER_X64_H_