bpo-34303: Micro-optimizations in functools.reduce() (GH-8598)

Misc/NEWS.d/next/Library/2018-08-03-09-47-20.bpo-34303.tOE2HP.rst

@@ -0,0 +1,2 @@
+Performance of :func:`functools.reduce` is slightly improved. Patch by
+Sergey Fedoseev.

Modules/_functoolsmodule.c

@@ -626,10 +626,13 @@ functools_reduce(PyObject *self, PyObject *args)
         if (result == NULL)
             result = op2;
         else {
-            PyTuple_SetItem(args, 0, result);
-            PyTuple_SetItem(args, 1, op2);
-            if ((result = PyEval_CallObject(func, args)) == NULL)
+            /* Update the args tuple in-place */
+            assert(args->ob_refcnt == 1);
+            Py_XSETREF(_PyTuple_ITEMS(args)[0], result);
+            Py_XSETREF(_PyTuple_ITEMS(args)[1], op2);
+            if ((result = PyObject_Call(func, args, NULL)) == NULL) {
                 goto Fail;
+            }
         }
     }