Removed static type inference and add a dynamic test for string addition.

src/x64/codegen-x64.cc

 // Copyright 2009 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 2320 matching lines @@
       flags = ((loop_nesting() > 0) && type->IsLikelySmi())
               ? SMI_CODE_INLINED
               : SMI_CODE_IN_STUB;
       break;
   }
 
   Result right = frame_->Pop();
   Result left = frame_->Pop();
 
   if (op == Token::ADD) {
-    bool left_is_string = left.static_type().is_jsstring();
-    bool right_is_string = right.static_type().is_jsstring();
+    bool left_is_string = left.is_constant() && left.handle()->IsString();
+    bool right_is_string = right.is_constant() && right.handle()->IsString();
     if (left_is_string || right_is_string) {
       frame_->Push(&left);
       frame_->Push(&right);
       Result answer;
       if (left_is_string) {
         if (right_is_string) {
-          // TODO(lrn): if (left.is_constant() && right.is_constant())
+          // TODO(lrn): if both are constant strings
           // -- do a compile time cons, if allocation during codegen is allowed.
           answer = frame_->CallRuntime(Runtime::kStringAdd, 2);
         } else {
           answer =
             frame_->InvokeBuiltin(Builtins::STRING_ADD_LEFT, CALL_FUNCTION, 2);
         }
       } else if (right_is_string) {
         answer =
           frame_->InvokeBuiltin(Builtins::STRING_ADD_RIGHT, CALL_FUNCTION, 2);
       }
-      answer.set_static_type(StaticType::jsstring());
       frame_->Push(&answer);
       return;
     }
     // Neither operand is known to be a string.
   }
 
   bool left_is_smi = left.is_constant() && left.handle()->IsSmi();
   bool left_is_non_smi = left.is_constant() && !left.handle()->IsSmi();
   bool right_is_smi = right.is_constant() && right.handle()->IsSmi();
   bool right_is_non_smi = right.is_constant() && !right.handle()->IsSmi();
@@ skipping 1619 matching lines @@
       break;
     default:
       UNREACHABLE();
   }
 }
 
 
 #undef __
 
 } }  // namespace v8::internal