gh-81381: Reduce allcoated size of PyType_GenericAlloc if possible

Author: corona10

Misc/NEWS.d/next/Core and Builtins/2023-01-09-00-55-03.gh-issue-81381.21bU3z.rst

@@ -0,0 +1,2 @@
+Reduce the extra allocation size of :c:func:`PyType_GenericAlloc` except the
+type is if a subtype of 'type'.

Objects/typeobject.c

@@ -1289,9 +1289,12 @@ PyObject *
 _PyType_AllocNoTrack(PyTypeObject*type, Py_ssize_tnitems)
 {
 PyObject*obj;
-constsize_tsize=_PyObject_VAR_SIZE(type, nitems+1);
-/* note that we need to add one, for the sentinel */
-
+size_textra=0;
+if (type->tp_flags&Py_TPFLAGS_TYPE_SUBCLASS||type->tp_flags&Py_TPFLAGS_HAVE_VECTORCALL) {
+/* note that we need to add one, for the sentinel */
+extra=1;
+ }
+constsize_tsize=_PyObject_VAR_SIZE(type, nitems+extra);
 constsize_tpresize=_PyType_PreHeaderSize(type);
 char*alloc=PyObject_Malloc(size+presize);
 if (alloc==NULL) {
@@ -1309,7 +1312,7 @@ _PyType_AllocNoTrack(PyTypeObject *type, Py_ssize_t nitems)
 _PyObject_Init(obj, type);
  }
 else {
-_PyObject_InitVar((PyVarObject*)obj, type, nitems);
+_PyObject_InitVar((PyVarObject*)obj, type, nitems+extra);
  }
 returnobj;
 }