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

Objects/listobject.c

@@ -647,6 +647,17 @@ list_contains(PyObject *aa, PyObject *el)
     return 0;
 }
 
+static inline PyObject *
+list_item_try_lock_free(PyListObject *a, Py_ssize_t i)
+{
+    PyObject **ob_item = FT_ATOMIC_LOAD_PTR(a->ob_item);
+    PyObject *item = FT_ATOMIC_LOAD_PTR(ob_item[i]);
+    if (!item || !_Py_TryIncrefCompare(&ob_item[i], item)) {
+        return NULL;
+    }
+    return item;
+}
+
 static PyObject *
 list_item(PyObject *aa, Py_ssize_t i)
 {
@@ -655,15 +666,21 @@ list_item(PyObject *aa, Py_ssize_t i)
         PyErr_SetObject(PyExc_IndexError, &_Py_STR(list_err));
         return NULL;
     }
-    PyObject *item;
+#ifdef Py_GIL_DISABLED
+    PyObject *item = list_item_try_lock_free(a, i);
+    if (item != NULL) {
+        goto end;
+    }
+#endif
     Py_BEGIN_CRITICAL_SECTION(a);
+    item = Py_NewRef(a->ob_item[i]);
+    Py_END_CRITICAL_SECTION();
 #ifdef Py_GIL_DISABLED
+end:
     if (!_Py_IsOwnedByCurrentThread((PyObject *)a) && !_PyObject_GC_IS_SHARED(a)) {
         _PyObject_GC_SET_SHARED(a);
     }
 #endif
-    item = Py_NewRef(a->ob_item[i]);
-    Py_END_CRITICAL_SECTION();
     return item;
 }