gh-119053: Implement the fast path for list.__getitem__

corona10 · corona10 · commit 5fb201de38d3 · 2024-05-17T02:01:17.000-04:00
diff --git a/Objects/listobject.c b/Objects/listobject.c
@@ -651,11 +651,18 @@ static PyObject *
 list_item(PyObject *aa, Py_ssize_t i)
 {
     PyListObject *a = (PyListObject *)aa;
+    PyObject *item;
     if (!valid_index(i, PyList_GET_SIZE(a))) {
         PyErr_SetObject(PyExc_IndexError, &_Py_STR(list_err));
         return NULL;
     }
-    PyObject *item;
+#ifdef Py_GIL_DISABLED
+    PyObject **ob_item = _Py_atomic_load_ptr(&a->ob_item);
+    item = _Py_atomic_load_ptr(&ob_item[i]);
+    if (item && _Py_TryIncrefCompare(&ob_item[i], item)) {
+        return item;
+    }
+#endif
     Py_BEGIN_CRITICAL_SECTION(a);
 #ifdef Py_GIL_DISABLED
     if (!_Py_IsOwnedByCurrentThread((PyObject *)a) && !_PyObject_GC_IS_SHARED(a)) {
