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GeezerVison is showing github.com — gh-159137: Skip redundant GC resurrection analysis by pablogsal · Pull Request #159138 · python/cpython

gh-159137: Skip redundant GC resurrection analysis #159138

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Merged

pablogsal merged 1 commit into python:main from pablogsal:investigate/gc-pass-fusion-20261010

Oct 10, 2026

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gh-159137: Skip redundant GC resurrection analysis #159138

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gh-159137: Skip redundant GC resurrection analysis

pablogsal Oct 10, 2026

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9 changes: 9 additions & 0 deletions 9 Include/internal/pycore_genobject.h

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Original file line number	Diff line number	Diff line change
Expand Up		@@ -8,6 +8,7 @@ extern "C" {
		# error "this header requires Py_BUILD_CORE define"
		#endif

		#include "pycore_frame.h" // FRAME_STATE_FINISHED()
		#include "pycore_interpframe_structs.h" // _PyGenObject

		#include <stddef.h> // offsetof()
Expand All		@@ -21,6 +22,14 @@ PyGenObject _PyGen_GetGeneratorFromFrame(_PyInterpreterFrame frame)
		return (PyGenObject )(((char )frame) - offset_in_gen);
		}

		// The generator finalizer is a no-op once its frame is cleared.
		static inline int
		_PyGen_FinalizerIsNoop(PyObject *op)
		{
		return Py_TYPE(op)->tp_finalize == PyGen_Type.tp_finalize &&
		FRAME_STATE_FINISHED(((PyGenObject *)op)->gi_frame_state);
		}

		PyAPI_FUNC(PyObject )_PyGen_yf(PyGenObject );
		extern int _PyGen_ClearFrame(PyGenObject *self);

Expand Down

4 changes: 4 additions & 0 deletions 4 Misc/NEWS.d/next/Core_and_Builtins/2026-10-10-00-00-00.gh-issue-159137.gcReuse.rst

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Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,4 @@
		Avoid a second reachability analysis in the garbage collector when no weakref
		callback, finalizer, or collectable-object debug output ran. Finished generators,
		coroutines, and async generators can skip this analysis even when finalized.
		Free-threaded builds also avoid a second stop-the-world pause in this case.

32 changes: 27 additions & 5 deletions 32 Python/gc.c

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Original file line number	Diff line number	Diff line change
Expand Up		@@ -5,6 +5,7 @@
		#include "Python.h"
		#include "pycore_ceval.h" // _Py_set_eval_breaker_bit()
		#include "pycore_dict.h" // _PyInlineValuesSize()
		#include "pycore_genobject.h" // _PyGen_FinalizerIsNoop()
		#include "pycore_initconfig.h" // _PyStatus_OK()
		#include "pycore_context.h"
		#include "pycore_interp.h" // PyInterpreterState.gc
Expand Down Expand Up		@@ -805,7 +806,8 @@ move_legacy_finalizer_reachable(PyGC_Head *finalizers)
		* Python-level code. See bpo-38006 as an example bug.
		*/
		static int
		handle_weakref_callbacks(PyGC_Head unreachable, PyGC_Head old)
		handle_weakref_callbacks(PyGC_Head unreachable, PyGC_Head old,
		bool *needs_resurrection_check)
		{
		PyGC_Head *gc;
		PyGC_Head wrcb_to_call; /* weakrefs with callbacks to call */
Expand Down Expand Up		@@ -906,6 +908,9 @@ handle_weakref_callbacks(PyGC_Head unreachable, PyGC_Head old)
		/* Invoke the callbacks we decided to honor. It's safe to invoke them
		* because they can't reference unreachable objects.
		*/
		if (!*needs_resurrection_check) {
		*needs_resurrection_check = !gc_list_is_empty(&wrcb_to_call);
		}
		while (! gc_list_is_empty(&wrcb_to_call)) {
		PyObject *temp;
		PyObject *callback;
Expand Down Expand Up		@@ -1043,7 +1048,8 @@ handle_legacy_finalizers(PyThreadState *tstate,
		* list, due to refcounts falling to 0.
		*/
		static void
		finalize_garbage(PyThreadState tstate, PyGC_Head collectable)
		finalize_garbage(PyThreadState tstate, PyGC_Head collectable,
		bool *needs_resurrection_check)
		{
		destructor finalize;
		PyGC_Head seen;
Expand All		@@ -1065,6 +1071,9 @@ finalize_garbage(PyThreadState tstate, PyGC_Head collectable)
		if (!_PyGC_FINALIZED(op) &&
		(finalize = Py_TYPE(op)->tp_finalize) != NULL)
		{
		if (!_PyGen_FinalizerIsNoop(op)) {
		*needs_resurrection_check = true;
		}
		_PyGC_SET_FINALIZED(op);
		Py_INCREF(op);
		finalize(op);
Expand Down Expand Up		@@ -1550,26 +1559,39 @@ gc_collect_main(PyThreadState *tstate, int generation, _PyGC_Reason reason)
		validate_list(&finalizers, collecting_clear_unreachable_clear);
		validate_list(&unreachable, collecting_set_unreachable_clear);

		bool needs_resurrection_check = false;

		/* Print debugging information. */
		if (gcstate->debug & _PyGC_DEBUG_COLLECTABLE) {
		/* sys.stderr.write() can resurrect objects. */
		needs_resurrection_check = true;
		for (gc = GC_NEXT(&unreachable); gc != &unreachable; gc = GC_NEXT(gc)) {
		debug_cycle("collectable", FROM_GC(gc));
		}
		}

		/* Clear weakrefs and invoke callbacks as necessary. */
		stats.collected += handle_weakref_callbacks(&unreachable, old);
		stats.collected += handle_weakref_callbacks(&unreachable, old,
		&needs_resurrection_check);
		validate_list(old, collecting_clear_unreachable_clear);
		validate_list(&unreachable, collecting_set_unreachable_clear);

		/* Call tp_finalize on objects which have one. */
		finalize_garbage(tstate, &unreachable);
		finalize_garbage(tstate, &unreachable, &needs_resurrection_check);

		/* Handle any objects that may have resurrected after the call
		* to 'finalize_garbage' and continue the collection with the
		* objects that are still unreachable */
		PyGC_Head final_unreachable;
		handle_resurrected_objects(&unreachable, &final_unreachable, old);
		if (needs_resurrection_check) {
		handle_resurrected_objects(&unreachable, &final_unreachable, old);
		}
		else {
		/* No Python code ran during finalization, so the original
		* unreachable set is still valid. */
		gc_list_init(&final_unreachable);
		gc_list_merge(&unreachable, &final_unreachable);
		}

		/* Clear weakrefs to objects in the unreachable set. No Python-level
		* code must be allowed to access those unreachable objects. During
Expand Down

51 changes: 36 additions & 15 deletions 51 Python/gc_free_threading.c

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Original file line number	Diff line number	Diff line change
Expand Up		@@ -1542,11 +1542,19 @@ move_legacy_finalizer_reachable(struct collection_state *state)
		// or missing `tp_traverse` methods. When that object goes away, the callback
		// for weakref can be executed and that could reveal unreachable objects to
		// Python-level code. See bpo-38006 as an example bug.
		static void
		find_weakref_callbacks(struct collection_state *state)
		static bool
		prepare_finalization(struct collection_state *state,
		bool *needs_resurrection_check)
		{
		bool has_finalizers = false;
		PyObject *op;
		WORKSTACK_FOR_EACH(&state->unreachable, op) {
		if (!_PyGC_FINALIZED(op) && Py_TYPE(op)->tp_finalize != NULL) {
		has_finalizers = true;
		if (!_PyGen_FinalizerIsNoop(op)) {
		*needs_resurrection_check = true;
		}
		}
		if (!_PyType_SUPPORTS_WEAKREFS(Py_TYPE(op))) {
		continue;
		}
Expand Down Expand Up		@@ -1590,12 +1598,14 @@ find_weakref_callbacks(struct collection_state *state)

		// Enqueue weakref to be called later.
		worklist_push(&state->wrcb_to_call, (PyObject *)wr);
		*needs_resurrection_check = true;
		}
		}
		return has_finalizers;
		}

		// Clear weakrefs to objects in the unreachable set. See comments
		// above find_weakref_callbacks() for why this clearing is required.
		// above prepare_finalization() for why this clearing is required.
		static void
		clear_weakrefs(struct collection_state *state)
		{
Expand Down Expand Up		@@ -2132,29 +2142,40 @@ gc_collect_internal(PyInterpreterState interp, struct collection_state state,
		gc_visit_heaps(interp, &validate_alive_bits, &state->base);
		#endif

		// Deallocations after the refcount merge may run Python code.
		bool needs_resurrection_check = state->objs_to_decref.head != 0;

		// Print debugging information.
		if (interp->gc.debug & _PyGC_DEBUG_COLLECTABLE) {
		needs_resurrection_check = true;
		PyObject *op;
		WORKSTACK_FOR_EACH(&state->unreachable, op) {
		debug_cycle("collectable", op);
		}
		}

		// Find weakref callbacks we will honor (but do not call them).
		find_weakref_callbacks(state);
		_PyEval_StartTheWorld(interp);
		// Find weakref callbacks and pending finalizers (but do not call them).
		bool has_finalizers = prepare_finalization(state, &needs_resurrection_check);
		if (needs_resurrection_check) {
		_PyEval_StartTheWorld(interp);

		// Deallocate any object from the refcount merge step
		cleanup_worklist(&state->objs_to_decref);
		// Deallocate any object from the refcount merge step
		cleanup_worklist(&state->objs_to_decref);

		// Call weakref callbacks and finalizers after unpausing other threads to
		// avoid potential deadlocks.
		call_weakref_callbacks(state);
		finalize_garbage(state);
		// Call weakref callbacks and finalizers after unpausing other threads to
		// avoid potential deadlocks.
		call_weakref_callbacks(state);
		finalize_garbage(state);

		_PyEval_StopTheWorld(interp);
		// Handle any objects that may have resurrected after the finalization.
		err = handle_resurrected_objects(state);
		_PyEval_StopTheWorld(interp);
		// Handle any objects that may have resurrected after the finalization.
		err = handle_resurrected_objects(state);
		}
		else if (has_finalizers) {
		// Only no-op generator finalizers remain; keep other threads paused.
		finalize_garbage(state);
		}
		// Otherwise, keep other threads paused so the unreachable set stays valid.
		// Clear free lists in all threads
		_PyGC_ClearAllFreeLists(interp);
		if (err == 0) {
Expand Down

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