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gh-134821: Automatically enable deferred reference counting on shared objects #134880

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gh-134821: Automatically enable deferred reference counting on shared objects #134880

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ff23f21
Thin POC.
ZeroIntensity May 28, 2025
091429e
Only check the shared field.
ZeroIntensity May 29, 2025
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25 changes: 25 additions & 0 deletions Python/gc_free_threading.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1170,6 +1170,26 @@ has_legacy_finalizer(PyObject *op)
return Py_TYPE(op)->tp_del != NULL;
}

static void
maybe_enable_deferred_refcount(PyObject *op)
{
assert(op != NULL);
if (has_legacy_finalizer(op)) {
// Objects with finalizers would be broken by
// deferred reference counting.
return;
}

uint8_t bits = op->ob_gc_bits;
if ((bits & _PyGC_BITS_DEFERRED) != 0) {
// This object already has deferred refcounting
return;
}

op->ob_gc_bits |= _PyGC_BITS_DEFERRED;
op->ob_ref_shared += _Py_REF_SHARED(_Py_REF_DEFERRED, 0);
}

static bool
scan_heap_visitor(const mi_heap_t *heap, const mi_heap_area_t *area,
void *block, size_t block_size, void *args)
Expand Down Expand Up @@ -1198,6 +1218,11 @@ scan_heap_visitor(const mi_heap_t *heap, const mi_heap_area_t *area,
worklist_push(&state->unreachable, op);
}
return true;
} else if (_Py_REF_SHARED(op->ob_ref_shared, 0) >= 3) {
// Objects with 10 or more shared references are candidates
// for deferred reference counting.
// This probably needs tuning.
maybe_enable_deferred_refcount(op);
}

if (state->reason == _Py_GC_REASON_SHUTDOWN) {
Expand Down
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