Support string add in crankshaft:

src/x64/code-stubs-x64.cc

Index: src/x64/code-stubs-x64.cc
diff --git a/src/x64/code-stubs-x64.cc b/src/x64/code-stubs-x64.cc
index 7e8ca78a9fc4d2ac9b62061217d9f481f5ba9b26..af09c369b0182504a099cdb6661e557a2acbc326 100644
--- a/src/x64/code-stubs-x64.cc
+++ b/src/x64/code-stubs-x64.cc
@@ -372,6 +372,9 @@ void TypeRecordingBinaryOpStub::Generate(MacroAssembler* masm) {
     case TRBinaryOpIC::ODDBALL:
       GenerateOddballStub(masm);
       break;
+    case TRBinaryOpIC::BOTH_STRING:
+      GenerateBothStringStub(masm);
+      break;
     case TRBinaryOpIC::STRING:
       GenerateStringStub(masm);
       break;
@@ -771,6 +774,36 @@ void TypeRecordingBinaryOpStub::GenerateStringStub(MacroAssembler* masm) {
 }
 
 
+void TypeRecordingBinaryOpStub::GenerateBothStringStub(MacroAssembler* masm) {
+  Label call_runtime;
+  ASSERT(operands_type_ == TRBinaryOpIC::BOTH_STRING);
+  ASSERT(op_ == Token::ADD);
+  // If both arguments are strings, call the string add stub.
+  // Otherwise, do a transition.
+
+  // Registers containing left and right operands respectively.
+  Register left = rdx;
+  Register right = rax;
+
+  // Test if left operand is a string.
+  __ JumpIfSmi(left, &call_runtime);
+  __ CmpObjectType(left, FIRST_NONSTRING_TYPE, rcx);
+  __ j(above_equal, &call_runtime);
+
+  // Test if right operand is a string.
+  __ JumpIfSmi(right, &call_runtime);
+  __ CmpObjectType(right, FIRST_NONSTRING_TYPE, rcx);
+  __ j(above_equal, &call_runtime);
+
+  StringAddStub string_add_stub(NO_STRING_CHECK_IN_STUB);
+  GenerateRegisterArgsPush(masm);
+  __ TailCallStub(&string_add_stub);
+
+  __ bind(&call_runtime);
+  GenerateTypeTransition(masm);
+}
+
+
 void TypeRecordingBinaryOpStub::GenerateOddballStub(MacroAssembler* masm) {
   Label call_runtime;