optimise unicode.join()

Cython/Compiler/Optimize.py

@@ -1444,6 +1444,24 @@ class OptimizeBuiltinCalls(Visitor.EnvTransform):
             node, "PyUnicode_Splitlines", self.PyUnicode_Splitlines_func_type,
             'splitlines', is_unbound_method, args)
 
+    PyUnicode_Join_func_type = PyrexTypes.CFuncType(
+        Builtin.unicode_type, [
+            PyrexTypes.CFuncTypeArg("sep", Builtin.unicode_type, None),
+            PyrexTypes.CFuncTypeArg("iterable", PyrexTypes.py_object_type, None),
+            ])
+
+    def _handle_simple_method_unicode_join(self, node, args, is_unbound_method):
+        """Replace unicode.join(...) by a direct call to the
+        corresponding C-API function.
+        """
+        if len(args) != 2:
+            self._error_wrong_arg_count('unicode.join', node, args, 2)
+            return node
+
+        return self._substitute_method_call(
+            node, "PyUnicode_Join", self.PyUnicode_Join_func_type,
+            'join', is_unbound_method, args)
+
     PyUnicode_Split_func_type = PyrexTypes.CFuncType(
         Builtin.list_type, [
             PyrexTypes.CFuncTypeArg("str", Builtin.unicode_type, None),