Skip stop-the-world in object_set_class for uniquely referenced objects

Author: colesbury

Objects/typeobject.c

@@ -7170,13 +7170,19 @@ object_set_class(PyObject *self, PyObject *value, void *closure)
     }
 
 #ifdef Py_GIL_DISABLED
-    PyInterpreterState *interp = _PyInterpreterState_GET();
-    _PyEval_StopTheWorld(interp);
+    int unique = _PyObject_IsUniquelyReferenced(self);
+    PyInterpreterState *interp = NULL;
+    if (!unique) {
+        interp = _PyInterpreterState_GET();
+        _PyEval_StopTheWorld(interp);
+    }
 #endif
     PyTypeObject *oldto = Py_TYPE(self);
     int res = object_set_class_world_stopped(self, newto);
 #ifdef Py_GIL_DISABLED
-    _PyEval_StartTheWorld(interp);
+    if (!unique) {
+        _PyEval_StartTheWorld(interp);
+    }
 #endif
     if (res == 0) {
         if (oldto->tp_flags & Py_TPFLAGS_HEAPTYPE) {