code: make code object use deferred reference counting

Code objects are frequently accessed by many threads, so enable deferred
reference counting so that in the future we can skip ref count
operations on code objects during frame evaluation. This requires making
PyCode_Type support GC.

Author: colesbury

Lib/test/test_capi/test_watchers.py

@@ -1,7 +1,7 @@
 import unittest
 
 from contextlib import contextmanager, ExitStack
-from test.support import catch_unraisable_exception, import_helper
+from test.support import catch_unraisable_exception, import_helper, gc_collect
 
 
 # Skip this test if the _testcapi module isn't available.
@@ -347,6 +347,7 @@ def code_watcher(self, which_watcher):
 
     def assert_event_counts(self, exp_created_0, exp_destroyed_0,
                             exp_created_1, exp_destroyed_1):
+        gc_collect()
         self.assertEqual(
             exp_created_0, _testcapi.get_code_watcher_num_created_events(0))
         self.assertEqual(

Lib/test/test_gc.py

@@ -220,12 +220,12 @@ class B(object):
 
     def test_function(self):
         # Tricky: f -> d -> f, code should call d.clear() after the exec to
-        # break the cycle.
+        # break the cycle. May collect f.__code__ as well.
         d = {}
         exec("def f(): pass\n", d)
         gc.collect()
         del d
-        self.assertEqual(gc.collect(), 2)
+        self.assertTrue(2 <= gc.collect() <= 3)
 
     def test_function_tp_clear_leaves_consistent_state(self):
         # https://github.com/python/cpython/issues/91636

Objects/codeobject.c

@@ -6,6 +6,7 @@
 #include "pycore_code.h"          // _PyCodeConstructor
 #include "pycore_frame.h"         // FRAME_SPECIALS_SIZE
 #include "pycore_interp.h"        // PyInterpreterState.co_extra_freefuncs
+#include "pycore_object.h"        // _PyObject_SET_DEFERRED_REFCOUNT
 #include "pycore_opcode.h"        // _PyOpcode_Deopt
 #include "pycore_pystate.h"       // _PyInterpreterState_GET()
 #include "pycore_tuple.h"         // _PyTuple_ITEMS()
@@ -550,13 +551,15 @@ _PyCode_New(struct _PyCodeConstructor *con)
     }
 
     Py_ssize_t size = PyBytes_GET_SIZE(con->code) / sizeof(_Py_CODEUNIT);
-    PyCodeObject *co = PyObject_NewVar(PyCodeObject, &PyCode_Type, size);
+    PyCodeObject *co = PyObject_GC_NewVar(PyCodeObject, &PyCode_Type, size);
     if (co == NULL) {
         Py_XDECREF(replacement_locations);
         PyErr_NoMemory();
         return NULL;
     }
     init_code(co, con);
+    _PyObject_SET_DEFERRED_REFCOUNT(co);
+    _PyObject_GC_TRACK(co);
     Py_XDECREF(replacement_locations);
     return co;
 }
@@ -1668,6 +1671,7 @@ code_dealloc(PyCodeObject *co)
 {
     notify_code_watchers(PY_CODE_EVENT_DESTROY, co);
 
+    _PyObject_GC_UNTRACK(co);
     if (co->co_extra != NULL) {
         PyInterpreterState *interp = _PyInterpreterState_GET();
         _PyCodeObjectExtra *co_extra = co->co_extra;
@@ -1705,7 +1709,14 @@ code_dealloc(PyCodeObject *co)
     if (co->_co_linearray) {
         PyMem_Free(co->_co_linearray);
     }
-    PyObject_Free(co);
+    PyObject_GC_Del(co);
+}
+
+static int
+code_traverse(PyCodeObject *co, visitproc visit, void *arg)
+{
+    Py_VISIT(co->co_consts);
+    return 0;
 }
 
 static PyObject *
@@ -2114,9 +2125,9 @@ PyTypeObject PyCode_Type = {
     PyObject_GenericGetAttr,            /* tp_getattro */
     0,                                  /* tp_setattro */
     0,                                  /* tp_as_buffer */
-    Py_TPFLAGS_DEFAULT,                 /* tp_flags */
+    Py_TPFLAGS_DEFAULT|Py_TPFLAGS_HAVE_GC,      /* tp_flags */
     code_new__doc__,                    /* tp_doc */
-    0,                                  /* tp_traverse */
+    (traverseproc)code_traverse,        /* tp_traverse */
     0,                                  /* tp_clear */
     code_richcompare,                   /* tp_richcompare */
     offsetof(PyCodeObject, co_weakreflist),     /* tp_weaklistoffset */