use stackrefs in _PyObject_GetMethod and calling APIs

[kumaraditya303]

Feature or enhancement

Currently the calling APIs such as PyObject_VectorcallMethod use _PyObject_GetMethod to avoid creating a bound method object, however _PyObject_GetMethod increfs and decrefs the object even when the underlying object supports deferred reference counting. This leads to reference counting contention on the object and it doesn't scale well in free threading. This API is heavily used by modules such as asyncio so it is important that _PyObject_GetMethod should scale well with threads.

cpython/Objects/call.c

Lines 829 to 859 in 54a6875

	PyObject_VectorcallMethod(PyObject *name, PyObject *const *args,
	size_t nargsf, PyObject *kwnames)
	{
	assert(name != NULL);
	assert(args != NULL);
	assert(PyVectorcall_NARGS(nargsf) >= 1);
	PyThreadState *tstate = _PyThreadState_GET();
	PyObject *callable = NULL;
	/* Use args[0] as "self" argument */
	int unbound = _PyObject_GetMethod(args[0], name, &callable);
	if (callable == NULL) {
	return NULL;
	}
	if (unbound) {
	/* We must remove PY_VECTORCALL_ARGUMENTS_OFFSET since
	* that would be interpreted as allowing to change args[-1] */
	nargsf &= ~PY_VECTORCALL_ARGUMENTS_OFFSET;
	}
	else {
	/* Skip "self". We can keep PY_VECTORCALL_ARGUMENTS_OFFSET since
	* args[-1] in the onward call is args[0] here. */
	args++;
	nargsf--;
	}
	EVAL_CALL_STAT_INC_IF_FUNCTION(EVAL_CALL_METHOD, callable);
	PyObject *result = _PyObject_VectorcallTstate(tstate, callable,
	args, nargsf, kwnames);
	Py_DECREF(callable);
	return result;

Proposal:

To take advantage of deferred reference counting, we should add stack ref variant of _PyObject_GetMethod and use it in all calling APIs to avoid reference counting contention on the function object.

Implementation:

• The new stack ref variant _PyObject_GetMethodStackRef will use _PyType_LookupStackRefAndVersion when looking up method from type cache.
• A new stack ref variant of _PyObject_TryGetInstanceAttributeStackRef will be added to to be used with _PyObject_GetMethodStackRef.
• Possibly we can avoid incref and decref when looking up the attribute on instance dict by delaying freeing of object's dictionary and using _Py_dict_lookup_threadsafe_stackref.

  cpython/Objects/object.c

  Lines 1632 to 1640 in 54a6875

  	Py_INCREF(dict);
  	if (PyDict_GetItemRef(dict, name, method) != 0) {
  	// found or error
  	Py_DECREF(dict);
  	Py_XDECREF(descr);
  	return 0;
  	}
  	// not found
  	Py_DECREF(dict);

Linked PRs

• gh-134043: add _PyObject_GetMethodStackRef #134044