[x64] Use xorl to materialize smi zero.

src/x64/macro-assembler-x64.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #if V8_TARGET_ARCH_X64
 
 #include "src/base/bits.h"
 #include "src/base/division-by-constant.h"
 #include "src/bootstrapper.h"
 #include "src/codegen.h"
@@ skipping 1082 matching lines @@
       Push(Immediate(value ^ jit_cookie()));
       xorp(Operand(rsp, 0), Immediate(jit_cookie()));
     }
   } else {
     Push(src);
   }
 }
 
 
 Register MacroAssembler::GetSmiConstant(Smi* source) {
+  STATIC_ASSERT(kSmiTag == 0);
   int value = source->value();
   if (value == 0) {
     xorl(kScratchRegister, kScratchRegister);
     return kScratchRegister;
   }
   LoadSmiConstant(kScratchRegister, source);
   return kScratchRegister;
 }
 
 
 void MacroAssembler::LoadSmiConstant(Register dst, Smi* source) {
-  // Special-casing 0 here to use xorl seems to make things slower, so we don't
-  // do it.
-  Move(dst, source, Assembler::RelocInfoNone());
+  STATIC_ASSERT(kSmiTag == 0);
+  int value = source->value();
+  if (value == 0) {
+    xorl(dst, dst);
+  } else {
+    Move(dst, source, Assembler::RelocInfoNone());
+  }
 }
 
 
 void MacroAssembler::Integer32ToSmi(Register dst, Register src) {
   STATIC_ASSERT(kSmiTag == 0);
   if (!dst.is(src)) {
     movl(dst, src);
   }
   shlp(dst, Immediate(kSmiShift));
 }
@@ skipping 4425 matching lines @@
   movl(rax, dividend);
   shrl(rax, Immediate(31));
   addl(rdx, rax);
 }
 
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_TARGET_ARCH_X64