Store the index in tp_subclasses.

Author: ericsnowcurrently

Include/cpython/object.h

@@ -218,7 +218,7 @@ struct _typeobject {
     PyObject *tp_bases;
     PyObject *tp_mro; /* method resolution order */
     PyObject *tp_cache; /* no longer used */
-    PyObject *tp_subclasses;  /* not used for static types (see gh-94673) */
+    PyObject *tp_subclasses;  /* for static builtin types this is an index */
     PyObject *tp_weaklist;
     destructor tp_del;
 
@@ -227,7 +227,6 @@ struct _typeobject {
 
     destructor tp_finalize;
     vectorcallfunc tp_vectorcall;
-    size_t tp_static_builtin_index;  /* 0 means "not initialized" */
 };
 
 /* This struct is used by the specializer

Objects/typeobject.c

@@ -73,29 +73,29 @@ static inline PyTypeObject * subclass_from_ref(PyObject *ref);
 static inline int
 static_builtin_index_is_set(PyTypeObject *self)
 {
-    return self->tp_static_builtin_index > 0;
+    return self->tp_subclasses != NULL;
 }
 
 static inline size_t
 static_builtin_index_get(PyTypeObject *self)
 {
     assert(static_builtin_index_is_set(self));
     /* We store a 1-based index so 0 can mean "not initialized". */
-    return self->tp_static_builtin_index - 1;
+    return (size_t)self->tp_subclasses - 1;
 }
 
 static inline void
 static_builtin_index_set(PyTypeObject *self, size_t index)
 {
     assert(index < _Py_MAX_STATIC_BUILTIN_TYPES);
     /* We store a 1-based index so 0 can mean "not initialized". */
-    self->tp_static_builtin_index = index + 1;
+    self->tp_subclasses = (PyObject *)(index + 1);
 }
 
 static inline void
 static_builtin_index_clear(PyTypeObject *self)
 {
-    self->tp_static_builtin_index = 0;
+    self->tp_subclasses = NULL;
 }
 
 static inline static_builtin_state *