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gh-159137: Skip unnecessary free-threaded GC finalizer and weakref scans #160464

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gh-159137: Skip unnecessary free-threaded GC finalizer and weakref scans #160464

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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2026-10-23-19-45-02.gh-issue-159137.gcFlags.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Skip finalizer preparation and weakref cleanup in the free-threaded garbage
collector when unreachable objects require neither phase.
15 changes: 13 additions & 2 deletions Python/gc_free_threading.c
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Original file line number Diff line number Diff line change
Expand Up @@ -80,6 +80,8 @@ struct collection_state {
Py_ssize_t uncollectable;
Py_ssize_t candidates;
Py_ssize_t long_lived_total;
bool has_finalizer_work;
bool has_weakref_work;
struct worklist unreachable;
struct worklist legacy_finalizers;
struct worklist wrcb_to_call;
Expand Down Expand Up @@ -1215,6 +1217,12 @@ scan_heap_visitor(const mi_heap_t *heap, const mi_heap_area_t *area,
worklist_push(&state->legacy_finalizers, op);
}
else {
if (Py_TYPE(op)->tp_finalize != NULL) {
state->has_finalizer_work = true;
}
if (PyWeakref_Check(op) || _PyType_SUPPORTS_WEAKREFS(Py_TYPE(op))) {
state->has_weakref_work = true;
}
worklist_push(&state->unreachable, op);
}
// It is possible this object will be resurrected but
Expand Down Expand Up @@ -2163,7 +2171,10 @@ gc_collect_internal(PyInterpreterState *interp, struct collection_state *state,
}

// Find weakref callbacks and pending finalizers (but do not call them).
bool has_finalizers = prepare_finalization(state, &needs_resurrection_check);
bool has_finalizers = false;
if (state->has_finalizer_work || state->has_weakref_work) {
has_finalizers = prepare_finalization(state, &needs_resurrection_check);
}
if (needs_resurrection_check) {
_PyEval_StartTheWorld(interp);

Expand All @@ -2186,7 +2197,7 @@ gc_collect_internal(PyInterpreterState *interp, struct collection_state *state,
// Otherwise, keep other threads paused so the unreachable set stays valid.
// Clear free lists in all threads
_PyGC_ClearAllFreeLists(interp);
if (err == 0) {
if (err == 0 && state->has_weakref_work) {
clear_weakrefs(state);
}
// Record the number of live GC objects
Expand Down
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