gh-81381: Reduce allcoated size of PyType_GenericAlloc if possible

corona10 · corona10 · commit e6091f8a356f · 2023-01-09T23:53:55.000+09:00
diff --git a/Misc/NEWS.d/next/Core and Builtins/2023-01-09-00-55-03.gh-issue-81381.21bU3z.rst b/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'.
diff --git a/Objects/typeobject.c b/Objects/typeobject.c
@@ -1289,9 +1289,12 @@ PyObject *
 _PyType_AllocNoTrack(PyTypeObject *type, Py_ssize_t nitems)
 {
     PyObject *obj;
-    const size_t size = _PyObject_VAR_SIZE(type, nitems+1);
-    /* note that we need to add one, for the sentinel */
-
+    size_t extra = 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;
+    }
+    const size_t size = _PyObject_VAR_SIZE(type, nitems + extra);
     const size_t presize = _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);
     }
     return obj;
 }

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,2 @@`
	`1`	+Reduce the extra allocation size of :c:func:`PyType_GenericAlloc` except the
	`2`	`+type is if a subtype of 'type'.`
Original file line number	Diff line number	Diff line change
`@@ -1289,9 +1289,12 @@ PyObject *`
`1289`	`1289`	`_PyType_AllocNoTrack(PyTypeObject *type, Py_ssize_t nitems)`
`1290`	`1290`	`{`
`1291`	`1291`	`PyObject *obj;`
`1292`		`- const size_t size = _PyObject_VAR_SIZE(type, nitems+1);`
`1293`		`- /* note that we need to add one, for the sentinel */`
`1294`		`-`
	`1292`	`+ size_t extra = 0;`
	`1293`	`+ if (type->tp_flags & Py_TPFLAGS_TYPE_SUBCLASS \|\| type->tp_flags & Py_TPFLAGS_HAVE_VECTORCALL) {`
	`1294`	`+ /* note that we need to add one, for the sentinel */`
	`1295`	`+ extra = 1;`
	`1296`	`+ }`
	`1297`	`+ const size_t size = _PyObject_VAR_SIZE(type, nitems + extra);`
`1295`	`1298`	`const size_t presize = _PyType_PreHeaderSize(type);`
`1296`	`1299`	`char *alloc = PyObject_Malloc(size + presize);`
`1297`	`1300`	`if (alloc == NULL) {`
`@@ -1309,7 +1312,7 @@ _PyType_AllocNoTrack(PyTypeObject *type, Py_ssize_t nitems)`
`1309`	`1312`	`_PyObject_Init(obj, type);`
`1310`	`1313`	`}`
`1311`	`1314`	`else {`
`1312`		`- _PyObject_InitVar((PyVarObject *)obj, type, nitems);`
	`1315`	`+ _PyObject_InitVar((PyVarObject *)obj, type, nitems + extra);`
`1313`	`1316`	`}`
`1314`	`1317`	`return obj;`
`1315`	`1318`	`}`