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pythongh-117376: Partial implementation deferred reference counting
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This marks objects as using deferred refrence counting using the
`ob_gc_bits` field in the free-threaded build and collects those objects
during GC.
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colesbury committed Apr 11, 2024
1 parent 25f6ff5 commit 206571b
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Showing 9 changed files with 91 additions and 11 deletions.
5 changes: 3 additions & 2 deletions Include/internal/pycore_gc.h
Original file line number Diff line number Diff line change
Expand Up @@ -39,12 +39,13 @@ static inline PyObject* _Py_FROM_GC(PyGC_Head *gc) {

/* Bit flags for ob_gc_bits (in Py_GIL_DISABLED builds) */
#ifdef Py_GIL_DISABLED
# define _PyGC_BITS_TRACKED (1)
# define _PyGC_BITS_FINALIZED (2)
# define _PyGC_BITS_TRACKED (1) // Tracked by the GC
# define _PyGC_BITS_FINALIZED (2) // tp_finalize was called
# define _PyGC_BITS_UNREACHABLE (4)
# define _PyGC_BITS_FROZEN (8)
# define _PyGC_BITS_SHARED (16)
# define _PyGC_BITS_SHARED_INLINE (32)
# define _PyGC_BITS_DEFERRED (64) // Use deferred reference counting
#endif

/* True if the object is currently tracked by the GC. */
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15 changes: 15 additions & 0 deletions Include/internal/pycore_object.h
Original file line number Diff line number Diff line change
Expand Up @@ -158,6 +158,21 @@ static inline void _Py_ClearImmortal(PyObject *op)
op = NULL; \
} while (0)

// Mark an object as supporting deferred reference counting. This is a no-op
// in the default (with GIL) build. Objects that use deferred reference
// counting should be tracked by the GC so that they are eventually collected.
extern void _PyObject_SetDeferredRefcount(PyObject *op);

static inline int
_PyObject_HasDeferredRefcount(PyObject *op)
{
#ifdef Py_GIL_DISABLED
return (op->ob_gc_bits & _PyGC_BITS_DEFERRED) != 0;
#else
return 0;
#endif
}

#if !defined(Py_GIL_DISABLED)
static inline void
_Py_DECREF_SPECIALIZED(PyObject *op, const destructor destruct)
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2 changes: 2 additions & 0 deletions Lib/test/test_code.py
Original file line number Diff line number Diff line change
Expand Up @@ -834,6 +834,7 @@ def test_free_called(self):

SetExtra(f.__code__, FREE_INDEX, ctypes.c_voidp(100))
del f
gc_collect() # For free-threaded build
self.assertEqual(LAST_FREED, 100)

def test_get_set(self):
Expand Down Expand Up @@ -872,6 +873,7 @@ def run(self):
del f
tt.start()
tt.join()
gc_collect() # For free-threaded build
self.assertEqual(LAST_FREED, 500)


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1 change: 1 addition & 0 deletions Objects/descrobject.c
Original file line number Diff line number Diff line change
Expand Up @@ -909,6 +909,7 @@ descr_new(PyTypeObject *descrtype, PyTypeObject *type, const char *name)

descr = (PyDescrObject *)PyType_GenericAlloc(descrtype, 0);
if (descr != NULL) {
_PyObject_SetDeferredRefcount((PyObject *)descr);
descr->d_type = (PyTypeObject*)Py_XNewRef(type);
descr->d_name = PyUnicode_InternFromString(name);
if (descr->d_name == NULL) {
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9 changes: 9 additions & 0 deletions Objects/funcobject.c
Original file line number Diff line number Diff line change
Expand Up @@ -127,6 +127,9 @@ _PyFunction_FromConstructor(PyFrameConstructor *constr)
op->func_typeparams = NULL;
op->vectorcall = _PyFunction_Vectorcall;
op->func_version = 0;
// NOTE: functions created via FrameConstructor do not use deferred
// reference counting because they are typically not part of cycles
// nor accessed by multiple threads.
_PyObject_GC_TRACK(op);
handle_func_event(PyFunction_EVENT_CREATE, op, NULL);
return op;
Expand Down Expand Up @@ -202,6 +205,12 @@ PyFunction_NewWithQualName(PyObject *code, PyObject *globals, PyObject *qualname
op->func_typeparams = NULL;
op->vectorcall = _PyFunction_Vectorcall;
op->func_version = 0;
if ((code_obj->co_flags & CO_NESTED) == 0) {
// Use deferred reference counting for top-level functions, but not
// nested functions because they are more likely to capture variables,
// which makes prompt deallocation more important.
_PyObject_SetDeferredRefcount((PyObject *)op);
}
_PyObject_GC_TRACK(op);
handle_func_event(PyFunction_EVENT_CREATE, op, NULL);
return (PyObject *)op;
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26 changes: 18 additions & 8 deletions Objects/moduleobject.c
Original file line number Diff line number Diff line change
Expand Up @@ -88,21 +88,31 @@ new_module_notrack(PyTypeObject *mt)
m->md_weaklist = NULL;
m->md_name = NULL;
m->md_dict = PyDict_New();
if (m->md_dict != NULL) {
return m;
if (m->md_dict == NULL) {
Py_DECREF(m);
return NULL;
}
Py_DECREF(m);
return NULL;
return m;
}

static void
track_module(PyModuleObject *m)
{
_PyObject_SetDeferredRefcount(m->md_dict);
PyObject_GC_Track(m->md_dict);

_PyObject_SetDeferredRefcount((PyObject *)m);
PyObject_GC_Track(m);
}

static PyObject *
new_module(PyTypeObject *mt, PyObject *args, PyObject *kws)
{
PyObject *m = (PyObject *)new_module_notrack(mt);
PyModuleObject *m = new_module_notrack(mt);
if (m != NULL) {
PyObject_GC_Track(m);
track_module(m);
}
return m;
return (PyObject *)m;
}

PyObject *
Expand All @@ -113,7 +123,7 @@ PyModule_NewObject(PyObject *name)
return NULL;
if (module_init_dict(m, m->md_dict, name, NULL) != 0)
goto fail;
PyObject_GC_Track(m);
track_module(m);
return (PyObject *)m;

fail:
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13 changes: 13 additions & 0 deletions Objects/object.c
Original file line number Diff line number Diff line change
Expand Up @@ -2414,6 +2414,19 @@ _Py_SetImmortal(PyObject *op)
_Py_SetImmortalUntracked(op);
}

void
_PyObject_SetDeferredRefcount(PyObject *op)
{
#ifdef Py_GIL_DISABLED
assert(PyType_IS_GC(Py_TYPE(op)));
assert(_Py_IsOwnedByCurrentThread(op));
assert( op->ob_ref_shared == 0);
op->ob_gc_bits |= _PyGC_BITS_DEFERRED;
op->ob_ref_local += 1;
op->ob_ref_shared = _Py_REF_QUEUED;
#endif
}

void
_Py_ResurrectReference(PyObject *op)
{
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2 changes: 2 additions & 0 deletions Objects/typeobject.c
Original file line number Diff line number Diff line change
Expand Up @@ -3581,6 +3581,8 @@ type_new_alloc(type_new_ctx *ctx)
et->ht_module = NULL;
et->_ht_tpname = NULL;

_PyObject_SetDeferredRefcount((PyObject *)et);

return type;
}

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29 changes: 28 additions & 1 deletion Python/gc_free_threading.c
Original file line number Diff line number Diff line change
Expand Up @@ -159,6 +159,25 @@ gc_decref(PyObject *op)
op->ob_tid -= 1;
}

static inline Py_ssize_t
gc_refcount(PyObject *op)
{
Py_ssize_t refcount = Py_REFCNT(op);
if (_PyObject_HasDeferredRefcount(op)) {
refcount -= 1;
}
return refcount;
}

static void
disable_deferred_refcounting(PyObject *op)
{
if (_PyObject_HasDeferredRefcount(op)) {
op->ob_gc_bits &= ~_PyGC_BITS_DEFERRED;
op->ob_ref_shared -= (1 << _Py_REF_SHARED_SHIFT);
}
}

static Py_ssize_t
merge_refcount(PyObject *op, Py_ssize_t extra)
{
Expand Down Expand Up @@ -375,9 +394,10 @@ update_refs(const mi_heap_t *heap, const mi_heap_area_t *area,
}

Py_ssize_t refcount = Py_REFCNT(op);
refcount -= _PyObject_HasDeferredRefcount(op);
_PyObject_ASSERT(op, refcount >= 0);

if (refcount > 0) {
if (refcount > 0 && !_PyObject_HasDeferredRefcount(op)) {
// Untrack tuples and dicts as necessary in this pass, but not objects
// with zero refcount, which we will want to collect.
if (PyTuple_CheckExact(op)) {
Expand Down Expand Up @@ -466,6 +486,9 @@ mark_heap_visitor(const mi_heap_t *heap, const mi_heap_area_t *area,
return true;
}

_PyObject_ASSERT_WITH_MSG(op, gc_get_refs(op) >= 0,
"refcount is too small");

if (gc_is_unreachable(op) && gc_get_refs(op) != 0) {
// Object is reachable but currently marked as unreachable.
// Mark it as reachable and traverse its pointers to find
Expand Down Expand Up @@ -499,6 +522,10 @@ scan_heap_visitor(const mi_heap_t *heap, const mi_heap_area_t *area,

struct collection_state *state = (struct collection_state *)args;
if (gc_is_unreachable(op)) {
// Disable deferred refcounting for unreachable objects so that they
// are collected immediately after finalization.
disable_deferred_refcounting(op);

// Merge and add one to the refcount to prevent deallocation while we
// are holding on to it in a worklist.
merge_refcount(op, 1);
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