gh-113688: Split up gcmodule.c (gh-113715)

This splits part of Modules/gcmodule.c of into Python/gc.c, which
now contains the core garbage collection implementation. The Python
module remain in the Modules/gcmodule.c file.

9 files changed, 2031 insertions, 1960 deletions

diff --git a/Include/internal/pycore_gc.h b/Include/internal/pycore_gc.h
index 2d33aa7..2a79c40 100644
--- a/Include/internal/pycore_gc.h
+++ b/Include/internal/pycore_gc.h
@@ -64,6 +64,26 @@ static inline int _PyObject_GC_MAY_BE_TRACKED(PyObject *obj) {
 #define _PyGC_PREV_SHIFT           (2)
 #define _PyGC_PREV_MASK            (((uintptr_t) -1) << _PyGC_PREV_SHIFT)
 
+/* set for debugging information */
+#define _PyGC_DEBUG_STATS             (1<<0) /* print collection statistics */
+#define _PyGC_DEBUG_COLLECTABLE       (1<<1) /* print collectable objects */
+#define _PyGC_DEBUG_UNCOLLECTABLE     (1<<2) /* print uncollectable objects */
+#define _PyGC_DEBUG_SAVEALL           (1<<5) /* save all garbage in gc.garbage */
+#define _PyGC_DEBUG_LEAK              _PyGC_DEBUG_COLLECTABLE | \
+                                      _PyGC_DEBUG_UNCOLLECTABLE | \
+                                      _PyGC_DEBUG_SAVEALL
+
+typedef enum {
+    // GC was triggered by heap allocation
+    _Py_GC_REASON_HEAP,
+
+    // GC was called during shutdown
+    _Py_GC_REASON_SHUTDOWN,
+
+    // GC was called by gc.collect() or PyGC_Collect()
+    _Py_GC_REASON_MANUAL
+} _PyGC_Reason;
+
 // Lowest bit of _gc_next is used for flags only in GC.
 // But it is always 0 for normal code.
 static inline PyGC_Head* _PyGCHead_NEXT(PyGC_Head *gc) {
@@ -203,8 +223,19 @@ struct _gc_runtime_state {
 
 extern void _PyGC_InitState(struct _gc_runtime_state *);
 
+extern Py_ssize_t _PyGC_Collect(PyThreadState *tstate, int generation,
+                                _PyGC_Reason reason);
 extern Py_ssize_t _PyGC_CollectNoFail(PyThreadState *tstate);
 
+/* Freeze objects tracked by the GC and ignore them in future collections. */
+extern void _PyGC_Freeze(PyInterpreterState *interp);
+/* Unfreezes objects placing them in the oldest generation */
+extern void _PyGC_Unfreeze(PyInterpreterState *interp);
+/* Number of frozen objects */
+extern Py_ssize_t _PyGC_GetFreezeCount(PyInterpreterState *interp);
+
+extern PyObject *_PyGC_GetObjects(PyInterpreterState *interp, Py_ssize_t generation);
+extern PyObject *_PyGC_GetReferrers(PyInterpreterState *interp, PyObject *objs);
 
 // Functions to clear types free lists
 extern void _PyTuple_ClearFreeList(PyInterpreterState *interp);
diff --git a/Lib/test/test_gc.py b/Lib/test/test_gc.py
index db7cb9ac..1d71dd9 100644
--- a/Lib/test/test_gc.py
+++ b/Lib/test/test_gc.py
@@ -1225,7 +1225,7 @@ class GCCallbackTests(unittest.TestCase):
         p.stderr.close()
         # Verify that stderr has a useful error message:
         self.assertRegex(stderr,
-            br'gcmodule\.c:[0-9]+: gc_decref: Assertion "gc_get_refs\(g\) > 0" failed.')
+            br'gc\.c:[0-9]+: gc_decref: Assertion "gc_get_refs\(g\) > 0" failed.')
         self.assertRegex(stderr,
             br'refcount is too small')
         # "address : 0x7fb5062efc18"
diff --git a/Makefile.pre.in b/Makefile.pre.in
index 6a64547..44e8e4b 100644
--- a/Makefile.pre.in
+++ b/Makefile.pre.in
@@ -417,6 +417,7 @@ PYTHON_OBJS=	\
 		Python/frame.o \
 		Python/frozenmain.o \
 		Python/future.o \
+		Python/gc.o \
 		Python/getargs.o \
 		Python/getcompiler.o \
 		Python/getcopyright.o \
diff --git a/Modules/gcmodule.c b/Modules/gcmodule.c
index 2d1f381..9a827cb 100644
--- a/Modules/gcmodule.c
+++ b/Modules/gcmodule.c
@@ -1,148 +1,16 @@
 /*
-
-  Reference Cycle Garbage Collection
-  ==================================
-
-  Neil Schemenauer <nas@arctrix.com>
-
-  Based on a post on the python-dev list.  Ideas from Guido van Rossum,
-  Eric Tiedemann, and various others.
-
-  http://www.arctrix.com/nas/python/gc/
-
-  The following mailing list threads provide a historical perspective on
-  the design of this module.  Note that a fair amount of refinement has
-  occurred since those discussions.
-
-  http://mail.python.org/pipermail/python-dev/2000-March/002385.html
-  http://mail.python.org/pipermail/python-dev/2000-March/002434.html
-  http://mail.python.org/pipermail/python-dev/2000-March/002497.html
-
-  For a highlevel view of the collection process, read the collect
-  function.
-
-*/
+ * Python interface to the garbage collector.
+ *
+ * See Python/gc.c for the implementation of the garbage collector.
+ */
 
 #include "Python.h"
-#include "pycore_ceval.h"         // _Py_set_eval_breaker_bit()
-#include "pycore_context.h"
-#include "pycore_dict.h"          // _PyDict_MaybeUntrack()
-#include "pycore_initconfig.h"
-#include "pycore_interp.h"        // PyInterpreterState.gc
-#include "pycore_object.h"
-#include "pycore_pyerrors.h"
-#include "pycore_pystate.h"       // _PyThreadState_GET()
-#include "pycore_weakref.h"       // _PyWeakref_ClearRef()
-#include "pydtrace.h"
+#include "pycore_gc.h"
+#include "pycore_object.h"      // _PyObject_IS_GC()
+#include "pycore_pystate.h"     // _PyInterpreterState_GET()
 
 typedef struct _gc_runtime_state GCState;
 
-/*[clinic input]
-module gc
-[clinic start generated code]*/
-/*[clinic end generated code: output=da39a3ee5e6b4b0d input=b5c9690ecc842d79]*/
-
-
-#ifdef Py_DEBUG
-#  define GC_DEBUG
-#endif
-
-#define GC_NEXT _PyGCHead_NEXT
-#define GC_PREV _PyGCHead_PREV
-
-// update_refs() set this bit for all objects in current generation.
-// subtract_refs() and move_unreachable() uses this to distinguish
-// visited object is in GCing or not.
-//
-// move_unreachable() removes this flag from reachable objects.
-// Only unreachable objects have this flag.
-//
-// No objects in interpreter have this flag after GC ends.
-#define PREV_MASK_COLLECTING   _PyGC_PREV_MASK_COLLECTING
-
-// Lowest bit of _gc_next is used for UNREACHABLE flag.
-//
-// This flag represents the object is in unreachable list in move_unreachable()
-//
-// Although this flag is used only in move_unreachable(), move_unreachable()
-// doesn't clear this flag to skip unnecessary iteration.
-// move_legacy_finalizers() removes this flag instead.
-// Between them, unreachable list is not normal list and we can not use
-// most gc_list_* functions for it.
-#define NEXT_MASK_UNREACHABLE  (1)
-
-#define AS_GC(op) _Py_AS_GC(op)
-#define FROM_GC(gc) _Py_FROM_GC(gc)
-
-// Automatically choose the generation that needs collecting.
-#define GENERATION_AUTO (-1)
-
-typedef enum {
-    // GC was triggered by heap allocation
-    _Py_GC_REASON_HEAP,
-
-    // GC was called during shutdown
-    _Py_GC_REASON_SHUTDOWN,
-
-    // GC was called by gc.collect() or PyGC_Collect()
-    _Py_GC_REASON_MANUAL
-} _PyGC_Reason;
-
-
-static inline int
-gc_is_collecting(PyGC_Head *g)
-{
-    return (g->_gc_prev & PREV_MASK_COLLECTING) != 0;
-}
-
-static inline void
-gc_clear_collecting(PyGC_Head *g)
-{
-    g->_gc_prev &= ~PREV_MASK_COLLECTING;
-}
-
-static inline Py_ssize_t
-gc_get_refs(PyGC_Head *g)
-{
-    return (Py_ssize_t)(g->_gc_prev >> _PyGC_PREV_SHIFT);
-}
-
-static inline void
-gc_set_refs(PyGC_Head *g, Py_ssize_t refs)
-{
-    g->_gc_prev = (g->_gc_prev & ~_PyGC_PREV_MASK)
-        | ((uintptr_t)(refs) << _PyGC_PREV_SHIFT);
-}
-
-static inline void
-gc_reset_refs(PyGC_Head *g, Py_ssize_t refs)
-{
-    g->_gc_prev = (g->_gc_prev & _PyGC_PREV_MASK_FINALIZED)
-        | PREV_MASK_COLLECTING
-        | ((uintptr_t)(refs) << _PyGC_PREV_SHIFT);
-}
-
-static inline void
-gc_decref(PyGC_Head *g)
-{
-    _PyObject_ASSERT_WITH_MSG(FROM_GC(g),
-                              gc_get_refs(g) > 0,
-                              "refcount is too small");
-    g->_gc_prev -= 1 << _PyGC_PREV_SHIFT;
-}
-
-/* set for debugging information */
-#define DEBUG_STATS             (1<<0) /* print collection statistics */
-#define DEBUG_COLLECTABLE       (1<<1) /* print collectable objects */
-#define DEBUG_UNCOLLECTABLE     (1<<2) /* print uncollectable objects */
-#define DEBUG_SAVEALL           (1<<5) /* save all garbage in gc.garbage */
-#define DEBUG_LEAK              DEBUG_COLLECTABLE | \
-                DEBUG_UNCOLLECTABLE | \
-                DEBUG_SAVEALL
-
-#define GEN_HEAD(gcstate, n) (&(gcstate)->generations[n].head)
-
-
 static GCState *
 get_gc_state(void)
 {
@@ -150,1381 +18,10 @@ get_gc_state(void)
     return &interp->gc;
 }
 
-
-void
-_PyGC_InitState(GCState *gcstate)
-{
-#define INIT_HEAD(GEN) \
-    do { \
-        GEN.head._gc_next = (uintptr_t)&GEN.head; \
-        GEN.head._gc_prev = (uintptr_t)&GEN.head; \
-    } while (0)
-
-    for (int i = 0; i < NUM_GENERATIONS; i++) {
-        assert(gcstate->generations[i].count == 0);
-        INIT_HEAD(gcstate->generations[i]);
-    };
-    gcstate->generation0 = GEN_HEAD(gcstate, 0);
-    INIT_HEAD(gcstate->permanent_generation);
-
-#undef INIT_HEAD
-}
-
-
-PyStatus
-_PyGC_Init(PyInterpreterState *interp)
-{
-    GCState *gcstate = &interp->gc;
-
-    gcstate->garbage = PyList_New(0);
-    if (gcstate->garbage == NULL) {
-        return _PyStatus_NO_MEMORY();
-    }
-
-    gcstate->callbacks = PyList_New(0);
-    if (gcstate->callbacks == NULL) {
-        return _PyStatus_NO_MEMORY();
-    }
-
-    return _PyStatus_OK();
-}
-
-
-/*
-_gc_prev values
----------------
-
-Between collections, _gc_prev is used for doubly linked list.
-
-Lowest two bits of _gc_prev are used for flags.
-PREV_MASK_COLLECTING is used only while collecting and cleared before GC ends
-or _PyObject_GC_UNTRACK() is called.
-
-During a collection, _gc_prev is temporary used for gc_refs, and the gc list
-is singly linked until _gc_prev is restored.
-
-gc_refs
-    At the start of a collection, update_refs() copies the true refcount
-    to gc_refs, for each object in the generation being collected.
-    subtract_refs() then adjusts gc_refs so that it equals the number of
-    times an object is referenced directly from outside the generation
-    being collected.
-
-PREV_MASK_COLLECTING
-    Objects in generation being collected are marked PREV_MASK_COLLECTING in
-    update_refs().
-
-
-_gc_next values
----------------
-
-_gc_next takes these values:
-
-0
-    The object is not tracked
-
-!= 0
-    Pointer to the next object in the GC list.
-    Additionally, lowest bit is used temporary for
-    NEXT_MASK_UNREACHABLE flag described below.
-
-NEXT_MASK_UNREACHABLE
-    move_unreachable() then moves objects not reachable (whether directly or
-    indirectly) from outside the generation into an "unreachable" set and
-    set this flag.
-
-    Objects that are found to be reachable have gc_refs set to 1.
-    When this flag is set for the reachable object, the object must be in
-    "unreachable" set.
-    The flag is unset and the object is moved back to "reachable" set.
-
-    move_legacy_finalizers() will remove this flag from "unreachable" set.
-*/
-
-/*** list functions ***/
-
-static inline void
-gc_list_init(PyGC_Head *list)
-{
-    // List header must not have flags.
-    // We can assign pointer by simple cast.
-    list->_gc_prev = (uintptr_t)list;
-    list->_gc_next = (uintptr_t)list;
-}
-
-static inline int
-gc_list_is_empty(PyGC_Head *list)
-{
-    return (list->_gc_next == (uintptr_t)list);
-}
-
-/* Append `node` to `list`. */
-static inline void
-gc_list_append(PyGC_Head *node, PyGC_Head *list)
-{
-    PyGC_Head *last = (PyGC_Head *)list->_gc_prev;
-
-    // last <-> node
-    _PyGCHead_SET_PREV(node, last);
-    _PyGCHead_SET_NEXT(last, node);
-
-    // node <-> list
-    _PyGCHead_SET_NEXT(node, list);
-    list->_gc_prev = (uintptr_t)node;
-}
-
-/* Remove `node` from the gc list it's currently in. */
-static inline void
-gc_list_remove(PyGC_Head *node)
-{
-    PyGC_Head *prev = GC_PREV(node);
-    PyGC_Head *next = GC_NEXT(node);
-
-    _PyGCHead_SET_NEXT(prev, next);
-    _PyGCHead_SET_PREV(next, prev);
-
-    node->_gc_next = 0; /* object is not currently tracked */
-}
-
-/* Move `node` from the gc list it's currently in (which is not explicitly
- * named here) to the end of `list`.  This is semantically the same as
- * gc_list_remove(node) followed by gc_list_append(node, list).
- */
-static void
-gc_list_move(PyGC_Head *node, PyGC_Head *list)
-{
-    /* Unlink from current list. */
-    PyGC_Head *from_prev = GC_PREV(node);
-    PyGC_Head *from_next = GC_NEXT(node);
-    _PyGCHead_SET_NEXT(from_prev, from_next);
-    _PyGCHead_SET_PREV(from_next, from_prev);
-
-    /* Relink at end of new list. */
-    // list must not have flags.  So we can skip macros.
-    PyGC_Head *to_prev = (PyGC_Head*)list->_gc_prev;
-    _PyGCHead_SET_PREV(node, to_prev);
-    _PyGCHead_SET_NEXT(to_prev, node);
-    list->_gc_prev = (uintptr_t)node;
-    _PyGCHead_SET_NEXT(node, list);
-}
-
-/* append list `from` onto list `to`; `from` becomes an empty list */
-static void
-gc_list_merge(PyGC_Head *from, PyGC_Head *to)
-{
-    assert(from != to);
-    if (!gc_list_is_empty(from)) {
-        PyGC_Head *to_tail = GC_PREV(to);
-        PyGC_Head *from_head = GC_NEXT(from);
-        PyGC_Head *from_tail = GC_PREV(from);
-        assert(from_head != from);
-        assert(from_tail != from);
-
-        _PyGCHead_SET_NEXT(to_tail, from_head);
-        _PyGCHead_SET_PREV(from_head, to_tail);
-
-        _PyGCHead_SET_NEXT(from_tail, to);
-        _PyGCHead_SET_PREV(to, from_tail);
-    }
-    gc_list_init(from);
-}
-
-static Py_ssize_t
-gc_list_size(PyGC_Head *list)
-{
-    PyGC_Head *gc;
-    Py_ssize_t n = 0;
-    for (gc = GC_NEXT(list); gc != list; gc = GC_NEXT(gc)) {
-        n++;
-    }
-    return n;
-}
-
-/* Walk the list and mark all objects as non-collecting */
-static inline void
-gc_list_clear_collecting(PyGC_Head *collectable)
-{
-    PyGC_Head *gc;
-    for (gc = GC_NEXT(collectable); gc != collectable; gc = GC_NEXT(gc)) {
-        gc_clear_collecting(gc);
-    }
-}
-
-/* Append objects in a GC list to a Python list.
- * Return 0 if all OK, < 0 if error (out of memory for list)
- */
-static int
-append_objects(PyObject *py_list, PyGC_Head *gc_list)
-{
-    PyGC_Head *gc;
-    for (gc = GC_NEXT(gc_list); gc != gc_list; gc = GC_NEXT(gc)) {
-        PyObject *op = FROM_GC(gc);
-        if (op != py_list) {
-            if (PyList_Append(py_list, op)) {
-                return -1; /* exception */
-            }
-        }
-    }
-    return 0;
-}
-
-// Constants for validate_list's flags argument.
-enum flagstates {collecting_clear_unreachable_clear,
-                 collecting_clear_unreachable_set,
-                 collecting_set_unreachable_clear,
-                 collecting_set_unreachable_set};
-
-#ifdef GC_DEBUG
-// validate_list checks list consistency.  And it works as document
-// describing when flags are expected to be set / unset.
-// `head` must be a doubly-linked gc list, although it's fine (expected!) if
-// the prev and next pointers are "polluted" with flags.
-// What's checked:
-// - The `head` pointers are not polluted.
-// - The objects' PREV_MASK_COLLECTING and NEXT_MASK_UNREACHABLE flags are all
-//   `set or clear, as specified by the 'flags' argument.
-// - The prev and next pointers are mutually consistent.
-static void
-validate_list(PyGC_Head *head, enum flagstates flags)
-{
-    assert((head->_gc_prev & PREV_MASK_COLLECTING) == 0);
-    assert((head->_gc_next & NEXT_MASK_UNREACHABLE) == 0);
-    uintptr_t prev_value = 0, next_value = 0;
-    switch (flags) {
-        case collecting_clear_unreachable_clear:
-            break;
-        case collecting_set_unreachable_clear:
-            prev_value = PREV_MASK_COLLECTING;
-            break;
-        case collecting_clear_unreachable_set:
-            next_value = NEXT_MASK_UNREACHABLE;
-            break;
-        case collecting_set_unreachable_set:
-            prev_value = PREV_MASK_COLLECTING;
-            next_value = NEXT_MASK_UNREACHABLE;
-            break;
-        default:
-            assert(! "bad internal flags argument");
-    }
-    PyGC_Head *prev = head;
-    PyGC_Head *gc = GC_NEXT(head);
-    while (gc != head) {
-        PyGC_Head *trueprev = GC_PREV(gc);
-        PyGC_Head *truenext = (PyGC_Head *)(gc->_gc_next  & ~NEXT_MASK_UNREACHABLE);
-        assert(truenext != NULL);
-        assert(trueprev == prev);
-        assert((gc->_gc_prev & PREV_MASK_COLLECTING) == prev_value);
-        assert((gc->_gc_next & NEXT_MASK_UNREACHABLE) == next_value);
-        prev = gc;
-        gc = truenext;
-    }
-    assert(prev == GC_PREV(head));
-}
-#else
-#define validate_list(x, y) do{}while(0)
-#endif
-
-/*** end of list stuff ***/
-
-
-/* Set all gc_refs = ob_refcnt.  After this, gc_refs is > 0 and
- * PREV_MASK_COLLECTING bit is set for all objects in containers.
- */
-static void
-update_refs(PyGC_Head *containers)
-{
-    PyGC_Head *next;
-    PyGC_Head *gc = GC_NEXT(containers);
-
-    while (gc != containers) {
-        next = GC_NEXT(gc);
-        /* Move any object that might have become immortal to the
-         * permanent generation as the reference count is not accurately
-         * reflecting the actual number of live references to this object
-         */
-        if (_Py_IsImmortal(FROM_GC(gc))) {
-           gc_list_move(gc, &get_gc_state()->permanent_generation.head);
-           gc = next;
-           continue;
-        }
-        gc_reset_refs(gc, Py_REFCNT(FROM_GC(gc)));
-        /* Python's cyclic gc should never see an incoming refcount
-         * of 0:  if something decref'ed to 0, it should have been
-         * deallocated immediately at that time.
-         * Possible cause (if the assert triggers):  a tp_dealloc
-         * routine left a gc-aware object tracked during its teardown
-         * phase, and did something-- or allowed something to happen --
-         * that called back into Python.  gc can trigger then, and may
-         * see the still-tracked dying object.  Before this assert
-         * was added, such mistakes went on to allow gc to try to
-         * delete the object again.  In a debug build, that caused
-         * a mysterious segfault, when _Py_ForgetReference tried
-         * to remove the object from the doubly-linked list of all
-         * objects a second time.  In a release build, an actual
-         * double deallocation occurred, which leads to corruption
-         * of the allocator's internal bookkeeping pointers.  That's
-         * so serious that maybe this should be a release-build
-         * check instead of an assert?
-         */
-        _PyObject_ASSERT(FROM_GC(gc), gc_get_refs(gc) != 0);
-        gc = next;
-    }
-}
-
-/* A traversal callback for subtract_refs. */
-static int
-visit_decref(PyObject *op, void *parent)
-{
-    OBJECT_STAT_INC(object_visits);
-    _PyObject_ASSERT(_PyObject_CAST(parent), !_PyObject_IsFreed(op));
-
-    if (_PyObject_IS_GC(op)) {
-        PyGC_Head *gc = AS_GC(op);
-        /* We're only interested in gc_refs for objects in the
-         * generation being collected, which can be recognized
-         * because only they have positive gc_refs.
-         */
-        if (gc_is_collecting(gc)) {
-            gc_decref(gc);
-        }
-    }
-    return 0;
-}
-
-/* Subtract internal references from gc_refs.  After this, gc_refs is >= 0
- * for all objects in containers, and is GC_REACHABLE for all tracked gc
- * objects not in containers.  The ones with gc_refs > 0 are directly
- * reachable from outside containers, and so can't be collected.
- */
-static void
-subtract_refs(PyGC_Head *containers)
-{
-    traverseproc traverse;
-    PyGC_Head *gc = GC_NEXT(containers);
-    for (; gc != containers; gc = GC_NEXT(gc)) {
-        PyObject *op = FROM_GC(gc);
-        traverse = Py_TYPE(op)->tp_traverse;
-        (void) traverse(op,
-                        visit_decref,
-                        op);
-    }
-}
-
-/* A traversal callback for move_unreachable. */
-static int
-visit_reachable(PyObject *op, void *arg)
-{
-    PyGC_Head *reachable = arg;
-    OBJECT_STAT_INC(object_visits);
-    if (!_PyObject_IS_GC(op)) {
-        return 0;
-    }
-
-    PyGC_Head *gc = AS_GC(op);
-    const Py_ssize_t gc_refs = gc_get_refs(gc);
-
-    // Ignore objects in other generation.
-    // This also skips objects "to the left" of the current position in
-    // move_unreachable's scan of the 'young' list - they've already been
-    // traversed, and no longer have the PREV_MASK_COLLECTING flag.
-    if (! gc_is_collecting(gc)) {
-        return 0;
-    }
-    // It would be a logic error elsewhere if the collecting flag were set on
-    // an untracked object.
-    assert(gc->_gc_next != 0);
-
-    if (gc->_gc_next & NEXT_MASK_UNREACHABLE) {
-        /* This had gc_refs = 0 when move_unreachable got
-         * to it, but turns out it's reachable after all.
-         * Move it back to move_unreachable's 'young' list,
-         * and move_unreachable will eventually get to it
-         * again.
-         */
-        // Manually unlink gc from unreachable list because the list functions
-        // don't work right in the presence of NEXT_MASK_UNREACHABLE flags.
-        PyGC_Head *prev = GC_PREV(gc);
-        PyGC_Head *next = (PyGC_Head*)(gc->_gc_next & ~NEXT_MASK_UNREACHABLE);
-        _PyObject_ASSERT(FROM_GC(prev),
-                         prev->_gc_next & NEXT_MASK_UNREACHABLE);
-        _PyObject_ASSERT(FROM_GC(next),
-                         next->_gc_next & NEXT_MASK_UNREACHABLE);
-        prev->_gc_next = gc->_gc_next;  // copy NEXT_MASK_UNREACHABLE
-        _PyGCHead_SET_PREV(next, prev);
-
-        gc_list_append(gc, reachable);
-        gc_set_refs(gc, 1);
-    }
-    else if (gc_refs == 0) {
-        /* This is in move_unreachable's 'young' list, but
-         * the traversal hasn't yet gotten to it.  All
-         * we need to do is tell move_unreachable that it's
-         * reachable.
-         */
-        gc_set_refs(gc, 1);
-    }
-    /* Else there's nothing to do.
-     * If gc_refs > 0, it must be in move_unreachable's 'young'
-     * list, and move_unreachable will eventually get to it.
-     */
-    else {
-        _PyObject_ASSERT_WITH_MSG(op, gc_refs > 0, "refcount is too small");
-    }
-    return 0;
-}
-
-/* Move the unreachable objects from young to unreachable.  After this,
- * all objects in young don't have PREV_MASK_COLLECTING flag and
- * unreachable have the flag.
- * All objects in young after this are directly or indirectly reachable
- * from outside the original young; and all objects in unreachable are
- * not.
- *
- * This function restores _gc_prev pointer.  young and unreachable are
- * doubly linked list after this function.
- * But _gc_next in unreachable list has NEXT_MASK_UNREACHABLE flag.
- * So we can not gc_list_* functions for unreachable until we remove the flag.
- */
-static void
-move_unreachable(PyGC_Head *young, PyGC_Head *unreachable)
-{
-    // previous elem in the young list, used for restore gc_prev.
-    PyGC_Head *prev = young;
-    PyGC_Head *gc = GC_NEXT(young);
-
-    /* Invariants:  all objects "to the left" of us in young are reachable
-     * (directly or indirectly) from outside the young list as it was at entry.
-     *
-     * All other objects from the original young "to the left" of us are in
-     * unreachable now, and have NEXT_MASK_UNREACHABLE.  All objects to the
-     * left of us in 'young' now have been scanned, and no objects here
-     * or to the right have been scanned yet.
-     */
-
-    while (gc != young) {
-        if (gc_get_refs(gc)) {
-            /* gc is definitely reachable from outside the
-             * original 'young'.  Mark it as such, and traverse
-             * its pointers to find any other objects that may
-             * be directly reachable from it.  Note that the
-             * call to tp_traverse may append objects to young,
-             * so we have to wait until it returns to determine
-             * the next object to visit.
-             */
-            PyObject *op = FROM_GC(gc);
-            traverseproc traverse = Py_TYPE(op)->tp_traverse;
-            _PyObject_ASSERT_WITH_MSG(op, gc_get_refs(gc) > 0,
-                                      "refcount is too small");
-            // NOTE: visit_reachable may change gc->_gc_next when
-            // young->_gc_prev == gc.  Don't do gc = GC_NEXT(gc) before!
-            (void) traverse(op,
-                    visit_reachable,
-                    (void *)young);
-            // relink gc_prev to prev element.
-            _PyGCHead_SET_PREV(gc, prev);
-            // gc is not COLLECTING state after here.
-            gc_clear_collecting(gc);
-            prev = gc;
-        }
-        else {
-            /* This *may* be unreachable.  To make progress,
-             * assume it is.  gc isn't directly reachable from
-             * any object we've already traversed, but may be
-             * reachable from an object we haven't gotten to yet.
-             * visit_reachable will eventually move gc back into
-             * young if that's so, and we'll see it again.
-             */
-            // Move gc to unreachable.
-            // No need to gc->next->prev = prev because it is single linked.
-            prev->_gc_next = gc->_gc_next;
-
-            // We can't use gc_list_append() here because we use
-            // NEXT_MASK_UNREACHABLE here.
-            PyGC_Head *last = GC_PREV(unreachable);
-            // NOTE: Since all objects in unreachable set has
-            // NEXT_MASK_UNREACHABLE flag, we set it unconditionally.
-            // But this may pollute the unreachable list head's 'next' pointer
-            // too. That's semantically senseless but expedient here - the
-            // damage is repaired when this function ends.
-            last->_gc_next = (NEXT_MASK_UNREACHABLE | (uintptr_t)gc);
-            _PyGCHead_SET_PREV(gc, last);
-            gc->_gc_next = (NEXT_MASK_UNREACHABLE | (uintptr_t)unreachable);
-            unreachable->_gc_prev = (uintptr_t)gc;
-        }
-        gc = (PyGC_Head*)prev->_gc_next;
-    }
-    // young->_gc_prev must be last element remained in the list.
-    young->_gc_prev = (uintptr_t)prev;
-    // don't let the pollution of the list head's next pointer leak
-    unreachable->_gc_next &= ~NEXT_MASK_UNREACHABLE;
-}
-
-static void
-untrack_tuples(PyGC_Head *head)
-{
-    PyGC_Head *next, *gc = GC_NEXT(head);
-    while (gc != head) {
-        PyObject *op = FROM_GC(gc);
-        next = GC_NEXT(gc);
-        if (PyTuple_CheckExact(op)) {
-            _PyTuple_MaybeUntrack(op);
-        }
-        gc = next;
-    }
-}
-
-/* Try to untrack all currently tracked dictionaries */
-static void
-untrack_dicts(PyGC_Head *head)
-{
-    PyGC_Head *next, *gc = GC_NEXT(head);
-    while (gc != head) {
-        PyObject *op = FROM_GC(gc);
-        next = GC_NEXT(gc);
-        if (PyDict_CheckExact(op)) {
-            _PyDict_MaybeUntrack(op);
-        }
-        gc = next;
-    }
-}
-
-/* Return true if object has a pre-PEP 442 finalization method. */
-static int
-has_legacy_finalizer(PyObject *op)
-{
-    return Py_TYPE(op)->tp_del != NULL;
-}
-
-/* Move the objects in unreachable with tp_del slots into `finalizers`.
- *
- * This function also removes NEXT_MASK_UNREACHABLE flag
- * from _gc_next in unreachable.
- */
-static void
-move_legacy_finalizers(PyGC_Head *unreachable, PyGC_Head *finalizers)
-{
-    PyGC_Head *gc, *next;
-    assert((unreachable->_gc_next & NEXT_MASK_UNREACHABLE) == 0);
-
-    /* March over unreachable.  Move objects with finalizers into
-     * `finalizers`.
-     */
-    for (gc = GC_NEXT(unreachable); gc != unreachable; gc = next) {
-        PyObject *op = FROM_GC(gc);
-
-        _PyObject_ASSERT(op, gc->_gc_next & NEXT_MASK_UNREACHABLE);
-        gc->_gc_next &= ~NEXT_MASK_UNREACHABLE;
-        next = (PyGC_Head*)gc->_gc_next;
-
-        if (has_legacy_finalizer(op)) {
-            gc_clear_collecting(gc);
-            gc_list_move(gc, finalizers);
-        }
-    }
-}
-
-static inline void
-clear_unreachable_mask(PyGC_Head *unreachable)
-{
-    /* Check that the list head does not have the unreachable bit set */
-    assert(((uintptr_t)unreachable & NEXT_MASK_UNREACHABLE) == 0);
-
-    PyGC_Head *gc, *next;
-    assert((unreachable->_gc_next & NEXT_MASK_UNREACHABLE) == 0);
-    for (gc = GC_NEXT(unreachable); gc != unreachable; gc = next) {
-        _PyObject_ASSERT((PyObject*)FROM_GC(gc), gc->_gc_next & NEXT_MASK_UNREACHABLE);
-        gc->_gc_next &= ~NEXT_MASK_UNREACHABLE;
-        next = (PyGC_Head*)gc->_gc_next;
-    }
-    validate_list(unreachable, collecting_set_unreachable_clear);
-}
-
-/* A traversal callback for move_legacy_finalizer_reachable. */
-static int
-visit_move(PyObject *op, void *arg)
-{
-    PyGC_Head *tolist = arg;
-    OBJECT_STAT_INC(object_visits);
-    if (_PyObject_IS_GC(op)) {
-        PyGC_Head *gc = AS_GC(op);
-        if (gc_is_collecting(gc)) {
-            gc_list_move(gc, tolist);
-            gc_clear_collecting(gc);
-        }
-    }
-    return 0;
-}
-
-/* Move objects that are reachable from finalizers, from the unreachable set
- * into finalizers set.
- */
-static void
-move_legacy_finalizer_reachable(PyGC_Head *finalizers)
-{
-    traverseproc traverse;
-    PyGC_Head *gc = GC_NEXT(finalizers);
-    for (; gc != finalizers; gc = GC_NEXT(gc)) {
-        /* Note that the finalizers list may grow during this. */
-        traverse = Py_TYPE(FROM_GC(gc))->tp_traverse;
-        (void) traverse(FROM_GC(gc),
-                        visit_move,
-                        (void *)finalizers);
-    }
-}
-
-/* Clear all weakrefs to unreachable objects, and if such a weakref has a
- * callback, invoke it if necessary.  Note that it's possible for such
- * weakrefs to be outside the unreachable set -- indeed, those are precisely
- * the weakrefs whose callbacks must be invoked.  See gc_weakref.txt for
- * overview & some details.  Some weakrefs with callbacks may be reclaimed
- * directly by this routine; the number reclaimed is the return value.  Other
- * weakrefs with callbacks may be moved into the `old` generation.  Objects
- * moved into `old` have gc_refs set to GC_REACHABLE; the objects remaining in
- * unreachable are left at GC_TENTATIVELY_UNREACHABLE.  When this returns,
- * no object in `unreachable` is weakly referenced anymore.
- */
-static int
-handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)
-{
-    PyGC_Head *gc;
-    PyObject *op;               /* generally FROM_GC(gc) */
-    PyWeakReference *wr;        /* generally a cast of op */
-    PyGC_Head wrcb_to_call;     /* weakrefs with callbacks to call */
-    PyGC_Head *next;
-    int num_freed = 0;
-
-    gc_list_init(&wrcb_to_call);
-
-    /* Clear all weakrefs to the objects in unreachable.  If such a weakref
-     * also has a callback, move it into `wrcb_to_call` if the callback
-     * needs to be invoked.  Note that we cannot invoke any callbacks until
-     * all weakrefs to unreachable objects are cleared, lest the callback
-     * resurrect an unreachable object via a still-active weakref.  We
-     * make another pass over wrcb_to_call, invoking callbacks, after this
-     * pass completes.
-     */
-    for (gc = GC_NEXT(unreachable); gc != unreachable; gc = next) {
-        PyWeakReference **wrlist;
-
-        op = FROM_GC(gc);
-        next = GC_NEXT(gc);
-
-        if (PyWeakref_Check(op)) {
-            /* A weakref inside the unreachable set must be cleared.  If we
-             * allow its callback to execute inside delete_garbage(), it
-             * could expose objects that have tp_clear already called on
-             * them.  Or, it could resurrect unreachable objects.  One way
-             * this can happen is if some container objects do not implement
-             * tp_traverse.  Then, wr_object can be outside the unreachable
-             * set but can be deallocated as a result of breaking the
-             * reference cycle.  If we don't clear the weakref, the callback
-             * will run and potentially cause a crash.  See bpo-38006 for
-             * one example.
-             */
-            _PyWeakref_ClearRef((PyWeakReference *)op);
-        }
-
-        if (! _PyType_SUPPORTS_WEAKREFS(Py_TYPE(op)))
-            continue;
-
-        /* It supports weakrefs.  Does it have any?
-         *
-         * This is never triggered for static types so we can avoid the
-         * (slightly) more costly _PyObject_GET_WEAKREFS_LISTPTR().
-         */
-        wrlist = _PyObject_GET_WEAKREFS_LISTPTR_FROM_OFFSET(op);
-
-        /* `op` may have some weakrefs.  March over the list, clear
-         * all the weakrefs, and move the weakrefs with callbacks
-         * that must be called into wrcb_to_call.
-         */
-        for (wr = *wrlist; wr != NULL; wr = *wrlist) {
-            PyGC_Head *wrasgc;                  /* AS_GC(wr) */
-
-            /* _PyWeakref_ClearRef clears the weakref but leaves
-             * the callback pointer intact.  Obscure:  it also
-             * changes *wrlist.
-             */
-            _PyObject_ASSERT((PyObject *)wr, wr->wr_object == op);
-            _PyWeakref_ClearRef(wr);
-            _PyObject_ASSERT((PyObject *)wr, wr->wr_object == Py_None);
-            if (wr->wr_callback == NULL) {
-                /* no callback */
-                continue;
-            }
-
-            /* Headache time.  `op` is going away, and is weakly referenced by
-             * `wr`, which has a callback.  Should the callback be invoked?  If wr
-             * is also trash, no:
-             *
-             * 1. There's no need to call it.  The object and the weakref are
-             *    both going away, so it's legitimate to pretend the weakref is
-             *    going away first.  The user has to ensure a weakref outlives its
-             *    referent if they want a guarantee that the wr callback will get
-             *    invoked.
-             *
-             * 2. It may be catastrophic to call it.  If the callback is also in
-             *    cyclic trash (CT), then although the CT is unreachable from
-             *    outside the current generation, CT may be reachable from the
-             *    callback.  Then the callback could resurrect insane objects.
-             *
-             * Since the callback is never needed and may be unsafe in this case,
-             * wr is simply left in the unreachable set.  Note that because we
-             * already called _PyWeakref_ClearRef(wr), its callback will never
-             * trigger.
-             *
-             * OTOH, if wr isn't part of CT, we should invoke the callback:  the
-             * weakref outlived the trash.  Note that since wr isn't CT in this
-             * case, its callback can't be CT either -- wr acted as an external
-             * root to this generation, and therefore its callback did too.  So
-             * nothing in CT is reachable from the callback either, so it's hard
-             * to imagine how calling it later could create a problem for us.  wr
-             * is moved to wrcb_to_call in this case.
-             */
-            if (gc_is_collecting(AS_GC((PyObject *)wr))) {
-                /* it should already have been cleared above */
-                assert(wr->wr_object == Py_None);
-                continue;
-            }
-
-            /* Create a new reference so that wr can't go away
-             * before we can process it again.
-             */
-            Py_INCREF(wr);
-
-            /* Move wr to wrcb_to_call, for the next pass. */
-            wrasgc = AS_GC((PyObject *)wr);
-            assert(wrasgc != next); /* wrasgc is reachable, but
-                                       next isn't, so they can't
-                                       be the same */
-            gc_list_move(wrasgc, &wrcb_to_call);
-        }
-    }
-
-    /* Invoke the callbacks we decided to honor.  It's safe to invoke them
-     * because they can't reference unreachable objects.
-     */
-    while (! gc_list_is_empty(&wrcb_to_call)) {
-        PyObject *temp;
-        PyObject *callback;
-
-        gc = (PyGC_Head*)wrcb_to_call._gc_next;
-        op = FROM_GC(gc);
-        _PyObject_ASSERT(op, PyWeakref_Check(op));
-        wr = (PyWeakReference *)op;
-        callback = wr->wr_callback;
-        _PyObject_ASSERT(op, callback != NULL);
-
-        /* copy-paste of weakrefobject.c's handle_callback() */
-        temp = PyObject_CallOneArg(callback, (PyObject *)wr);
-        if (temp == NULL)
-            PyErr_WriteUnraisable(callback);
-        else
-            Py_DECREF(temp);
-
-        /* Give up the reference we created in the first pass.  When
-         * op's refcount hits 0 (which it may or may not do right now),
-         * op's tp_dealloc will decref op->wr_callback too.  Note
-         * that the refcount probably will hit 0 now, and because this
-         * weakref was reachable to begin with, gc didn't already
-         * add it to its count of freed objects.  Example:  a reachable
-         * weak value dict maps some key to this reachable weakref.
-         * The callback removes this key->weakref mapping from the
-         * dict, leaving no other references to the weakref (excepting
-         * ours).
-         */
-        Py_DECREF(op);
-        if (wrcb_to_call._gc_next == (uintptr_t)gc) {
-            /* object is still alive -- move it */
-            gc_list_move(gc, old);
-        }
-        else {
-            ++num_freed;
-        }
-    }
-
-    return num_freed;
-}
-
-static void
-debug_cycle(const char *msg, PyObject *op)
-{
-    PySys_FormatStderr("gc: %s <%s %p>\n",
-                       msg, Py_TYPE(op)->tp_name, op);
-}
-
-/* Handle uncollectable garbage (cycles with tp_del slots, and stuff reachable
- * only from such cycles).
- * If DEBUG_SAVEALL, all objects in finalizers are appended to the module
- * garbage list (a Python list), else only the objects in finalizers with
- * __del__ methods are appended to garbage.  All objects in finalizers are
- * merged into the old list regardless.
- */
-static void
-handle_legacy_finalizers(PyThreadState *tstate,
-                         GCState *gcstate,
-                         PyGC_Head *finalizers, PyGC_Head *old)
-{
-    assert(!_PyErr_Occurred(tstate));
-    assert(gcstate->garbage != NULL);
-
-    PyGC_Head *gc = GC_NEXT(finalizers);
-    for (; gc != finalizers; gc = GC_NEXT(gc)) {
-        PyObject *op = FROM_GC(gc);
-
-        if ((gcstate->debug & DEBUG_SAVEALL) || has_legacy_finalizer(op)) {
-            if (PyList_Append(gcstate->garbage, op) < 0) {
-                _PyErr_Clear(tstate);
-                break;
-            }
-        }
-    }
-
-    gc_list_merge(finalizers, old);
-}
-
-/* Run first-time finalizers (if any) on all the objects in collectable.
- * Note that this may remove some (or even all) of the objects from the
- * list, due to refcounts falling to 0.
- */
-static void
-finalize_garbage(PyThreadState *tstate, PyGC_Head *collectable)
-{
-    destructor finalize;
-    PyGC_Head seen;
-
-    /* While we're going through the loop, `finalize(op)` may cause op, or
-     * other objects, to be reclaimed via refcounts falling to zero.  So
-     * there's little we can rely on about the structure of the input
-     * `collectable` list across iterations.  For safety, we always take the
-     * first object in that list and move it to a temporary `seen` list.
-     * If objects vanish from the `collectable` and `seen` lists we don't
-     * care.
-     */
-    gc_list_init(&seen);
-
-    while (!gc_list_is_empty(collectable)) {
-        PyGC_Head *gc = GC_NEXT(collectable);
-        PyObject *op = FROM_GC(gc);
-        gc_list_move(gc, &seen);
-        if (!_PyGCHead_FINALIZED(gc) &&
-                (finalize = Py_TYPE(op)->tp_finalize) != NULL) {
-            _PyGCHead_SET_FINALIZED(gc);
-            Py_INCREF(op);
-            finalize(op);
-            assert(!_PyErr_Occurred(tstate));
-            Py_DECREF(op);
-        }
-    }
-    gc_list_merge(&seen, collectable);
-}
-
-/* Break reference cycles by clearing the containers involved.  This is
- * tricky business as the lists can be changing and we don't know which
- * objects may be freed.  It is possible I screwed something up here.
- */
-static void
-delete_garbage(PyThreadState *tstate, GCState *gcstate,
-               PyGC_Head *collectable, PyGC_Head *old)
-{
-    assert(!_PyErr_Occurred(tstate));
-
-    while (!gc_list_is_empty(collectable)) {
-        PyGC_Head *gc = GC_NEXT(collectable);
-        PyObject *op = FROM_GC(gc);
-
-        _PyObject_ASSERT_WITH_MSG(op, Py_REFCNT(op) > 0,
-                                  "refcount is too small");
-
-        if (gcstate->debug & DEBUG_SAVEALL) {
-            assert(gcstate->garbage != NULL);
-            if (PyList_Append(gcstate->garbage, op) < 0) {
-                _PyErr_Clear(tstate);
-            }
-        }
-        else {
-            inquiry clear;
-            if ((clear = Py_TYPE(op)->tp_clear) != NULL) {
-                Py_INCREF(op);
-                (void) clear(op);
-                if (_PyErr_Occurred(tstate)) {
-                    PyErr_FormatUnraisable("Exception ignored in tp_clear of %s",
-                                           Py_TYPE(op)->tp_name);
-                }
-                Py_DECREF(op);
-            }
-        }
-        if (GC_NEXT(collectable) == gc) {
-            /* object is still alive, move it, it may die later */
-            gc_clear_collecting(gc);
-            gc_list_move(gc, old);
-        }
-    }
-}
-
-/* Clear all free lists
- * All free lists are cleared during the collection of the highest generation.
- * Allocated items in the free list may keep a pymalloc arena occupied.
- * Clearing the free lists may give back memory to the OS earlier.
- */
-static void
-clear_freelists(PyInterpreterState *interp)
-{
-    _PyTuple_ClearFreeList(interp);
-    _PyFloat_ClearFreeList(interp);
-    _PyList_ClearFreeList(interp);
-    _PyDict_ClearFreeList(interp);
-    _PyAsyncGen_ClearFreeLists(interp);
-    _PyContext_ClearFreeList(interp);
-}
-
-// Show stats for objects in each generations
-static void
-show_stats_each_generations(GCState *gcstate)
-{
-    char buf[100];
-    size_t pos = 0;
-
-    for (int i = 0; i < NUM_GENERATIONS && pos < sizeof(buf); i++) {
-        pos += PyOS_snprintf(buf+pos, sizeof(buf)-pos,
-                             " %zd",
-                             gc_list_size(GEN_HEAD(gcstate, i)));
-    }
-
-    PySys_FormatStderr(
-        "gc: objects in each generation:%s\n"
-        "gc: objects in permanent generation: %zd\n",
-        buf, gc_list_size(&gcstate->permanent_generation.head));
-}
-
-/* Deduce which objects among "base" are unreachable from outside the list
-   and move them to 'unreachable'. The process consist in the following steps:
-
-1. Copy all reference counts to a different field (gc_prev is used to hold
-   this copy to save memory).
-2. Traverse all objects in "base" and visit all referred objects using
-   "tp_traverse" and for every visited object, subtract 1 to the reference
-   count (the one that we copied in the previous step). After this step, all
-   objects that can be reached directly from outside must have strictly positive
-   reference count, while all unreachable objects must have a count of exactly 0.
-3. Identify all unreachable objects (the ones with 0 reference count) and move
-   them to the "unreachable" list. This step also needs to move back to "base" all
-   objects that were initially marked as unreachable but are referred transitively
-   by the reachable objects (the ones with strictly positive reference count).
-
-Contracts:
-
-    * The "base" has to be a valid list with no mask set.
-
-    * The "unreachable" list must be uninitialized (this function calls
-      gc_list_init over 'unreachable').
-
-IMPORTANT: This function leaves 'unreachable' with the NEXT_MASK_UNREACHABLE
-flag set but it does not clear it to skip unnecessary iteration. Before the
-flag is cleared (for example, by using 'clear_unreachable_mask' function or
-by a call to 'move_legacy_finalizers'), the 'unreachable' list is not a normal
-list and we can not use most gc_list_* functions for it. */
-static inline void
-deduce_unreachable(PyGC_Head *base, PyGC_Head *unreachable) {
-    validate_list(base, collecting_clear_unreachable_clear);
-    /* Using ob_refcnt and gc_refs, calculate which objects in the
-     * container set are reachable from outside the set (i.e., have a
-     * refcount greater than 0 when all the references within the
-     * set are taken into account).
-     */
-    update_refs(base);  // gc_prev is used for gc_refs
-    subtract_refs(base);
-
-    /* Leave everything reachable from outside base in base, and move
-     * everything else (in base) to unreachable.
-     *
-     * NOTE:  This used to move the reachable objects into a reachable
-     * set instead.  But most things usually turn out to be reachable,
-     * so it's more efficient to move the unreachable things.  It "sounds slick"
-     * to move the unreachable objects, until you think about it - the reason it
-     * pays isn't actually obvious.
-     *
-     * Suppose we create objects A, B, C in that order.  They appear in the young
-     * generation in the same order.  If B points to A, and C to B, and C is
-     * reachable from outside, then the adjusted refcounts will be 0, 0, and 1
-     * respectively.
-     *
-     * When move_unreachable finds A, A is moved to the unreachable list.  The
-     * same for B when it's first encountered.  Then C is traversed, B is moved
-     * _back_ to the reachable list.  B is eventually traversed, and then A is
-     * moved back to the reachable list.
-     *
-     * So instead of not moving at all, the reachable objects B and A are moved
-     * twice each.  Why is this a win?  A straightforward algorithm to move the
-     * reachable objects instead would move A, B, and C once each.
-     *
-     * The key is that this dance leaves the objects in order C, B, A - it's
-     * reversed from the original order.  On all _subsequent_ scans, none of
-     * them will move.  Since most objects aren't in cycles, this can save an
-     * unbounded number of moves across an unbounded number of later collections.
-     * It can cost more only the first time the chain is scanned.
-     *
-     * Drawback:  move_unreachable is also used to find out what's still trash
-     * after finalizers may resurrect objects.  In _that_ case most unreachable
-     * objects will remain unreachable, so it would be more efficient to move
-     * the reachable objects instead.  But this is a one-time cost, probably not
-     * worth complicating the code to speed just a little.
-     */
-    gc_list_init(unreachable);
-    move_unreachable(base, unreachable);  // gc_prev is pointer again
-    validate_list(base, collecting_clear_unreachable_clear);
-    validate_list(unreachable, collecting_set_unreachable_set);
-}
-
-/* Handle objects that may have resurrected after a call to 'finalize_garbage', moving
-   them to 'old_generation' and placing the rest on 'still_unreachable'.
-
-   Contracts:
-       * After this function 'unreachable' must not be used anymore and 'still_unreachable'
-         will contain the objects that did not resurrect.
-
-       * The "still_unreachable" list must be uninitialized (this function calls
-         gc_list_init over 'still_unreachable').
-
-IMPORTANT: After a call to this function, the 'still_unreachable' set will have the
-PREV_MARK_COLLECTING set, but the objects in this set are going to be removed so
-we can skip the expense of clearing the flag to avoid extra iteration. */
-static inline void
-handle_resurrected_objects(PyGC_Head *unreachable, PyGC_Head* still_unreachable,
-                           PyGC_Head *old_generation)
-{
-    // Remove the PREV_MASK_COLLECTING from unreachable
-    // to prepare it for a new call to 'deduce_unreachable'
-    gc_list_clear_collecting(unreachable);
-
-    // After the call to deduce_unreachable, the 'still_unreachable' set will
-    // have the PREV_MARK_COLLECTING set, but the objects are going to be
-    // removed so we can skip the expense of clearing the flag.
-    PyGC_Head* resurrected = unreachable;
-    deduce_unreachable(resurrected, still_unreachable);
-    clear_unreachable_mask(still_unreachable);
-
-    // Move the resurrected objects to the old generation for future collection.
-    gc_list_merge(resurrected, old_generation);
-}
-
-
-/* Invoke progress callbacks to notify clients that garbage collection
- * is starting or stopping
- */
-static void
-invoke_gc_callback(PyThreadState *tstate, const char *phase,
-                   int generation, Py_ssize_t collected,
-                   Py_ssize_t uncollectable)
-{
-    assert(!_PyErr_Occurred(tstate));
-
-    /* we may get called very early */
-    GCState *gcstate = &tstate->interp->gc;
-    if (gcstate->callbacks == NULL) {
-        return;
-    }
-
-    /* The local variable cannot be rebound, check it for sanity */
-    assert(PyList_CheckExact(gcstate->callbacks));
-    PyObject *info = NULL;
-    if (PyList_GET_SIZE(gcstate->callbacks) != 0) {
-        info = Py_BuildValue("{sisnsn}",
-            "generation", generation,
-            "collected", collected,
-            "uncollectable", uncollectable);
-        if (info == NULL) {
-            PyErr_FormatUnraisable("Exception ignored on invoking gc callbacks");
-            return;
-        }
-    }
-
-    PyObject *phase_obj = PyUnicode_FromString(phase);
-    if (phase_obj == NULL) {
-        Py_XDECREF(info);
-        PyErr_FormatUnraisable("Exception ignored on invoking gc callbacks");
-        return;
-    }
-
-    PyObject *stack[] = {phase_obj, info};
-    for (Py_ssize_t i=0; i<PyList_GET_SIZE(gcstate->callbacks); i++) {
-        PyObject *r, *cb = PyList_GET_ITEM(gcstate->callbacks, i);
-        Py_INCREF(cb); /* make sure cb doesn't go away */
-        r = PyObject_Vectorcall(cb, stack, 2, NULL);
-        if (r == NULL) {
-            PyErr_WriteUnraisable(cb);
-        }
-        else {
-            Py_DECREF(r);
-        }
-        Py_DECREF(cb);
-    }
-    Py_DECREF(phase_obj);
-    Py_XDECREF(info);
-    assert(!_PyErr_Occurred(tstate));
-}
-
-
-/* Find the oldest generation (highest numbered) where the count
- * exceeds the threshold.  Objects in the that generation and
- * generations younger than it will be collected. */
-static int
-gc_select_generation(GCState *gcstate)
-{
-    for (int i = NUM_GENERATIONS-1; i >= 0; i--) {
-        if (gcstate->generations[i].count > gcstate->generations[i].threshold) {
-            /* Avoid quadratic performance degradation in number
-               of tracked objects (see also issue #4074):
-
-               To limit the cost of garbage collection, there are two strategies;
-                 - make each collection faster, e.g. by scanning fewer objects
-                 - do less collections
-               This heuristic is about the latter strategy.
-
-               In addition to the various configurable thresholds, we only trigger a
-               full collection if the ratio
-
-                long_lived_pending / long_lived_total
-
-               is above a given value (hardwired to 25%).
-
-               The reason is that, while "non-full" collections (i.e., collections of
-               the young and middle generations) will always examine roughly the same
-               number of objects -- determined by the aforementioned thresholds --,
-               the cost of a full collection is proportional to the total number of
-               long-lived objects, which is virtually unbounded.
-
-               Indeed, it has been remarked that doing a full collection every
-               <constant number> of object creations entails a dramatic performance
-               degradation in workloads which consist in creating and storing lots of
-               long-lived objects (e.g. building a large list of GC-tracked objects would
-               show quadratic performance, instead of linear as expected: see issue #4074).
-
-               Using the above ratio, instead, yields amortized linear performance in
-               the total number of objects (the effect of which can be summarized
-               thusly: "each full garbage collection is more and more costly as the
-               number of objects grows, but we do fewer and fewer of them").
-
-               This heuristic was suggested by Martin von Löwis on python-dev in
-               June 2008. His original analysis and proposal can be found at:
-               http://mail.python.org/pipermail/python-dev/2008-June/080579.html
-            */
-            if (i == NUM_GENERATIONS - 1
-                && gcstate->long_lived_pending < gcstate->long_lived_total / 4)
-                continue;
-            return i;
-        }
-    }
-    return -1;
-}
-
-
-/* This is the main function.  Read this to understand how the
- * collection process works. */
-static Py_ssize_t
-gc_collect_main(PyThreadState *tstate, int generation, _PyGC_Reason reason)
-{
-    int i;
-    Py_ssize_t m = 0; /* # objects collected */
-    Py_ssize_t n = 0; /* # unreachable objects that couldn't be collected */
-    PyGC_Head *young; /* the generation we are examining */
-    PyGC_Head *old; /* next older generation */
-    PyGC_Head unreachable; /* non-problematic unreachable trash */
-    PyGC_Head finalizers;  /* objects with, & reachable from, __del__ */
-    PyGC_Head *gc;
-    _PyTime_t t1 = 0;   /* initialize to prevent a compiler warning */
-    GCState *gcstate = &tstate->interp->gc;
-
-    // gc_collect_main() must not be called before _PyGC_Init
-    // or after _PyGC_Fini()
-    assert(gcstate->garbage != NULL);
-    assert(!_PyErr_Occurred(tstate));
-
-    int expected = 0;
-    if (!_Py_atomic_compare_exchange_int(&gcstate->collecting, &expected, 1)) {
-        // Don't start a garbage collection if one is already in progress.
-        return 0;
-    }
-
-    if (generation == GENERATION_AUTO) {
-        // Select the oldest generation that needs collecting. We will collect
-        // objects from that generation and all generations younger than it.
-        generation = gc_select_generation(gcstate);
-        if (generation < 0) {
-            // No generation needs to be collected.
-            _Py_atomic_store_int(&gcstate->collecting, 0);
-            return 0;
-        }
-    }
-
-    assert(generation >= 0 && generation < NUM_GENERATIONS);
-
-#ifdef Py_STATS
-    if (_Py_stats) {
-        _Py_stats->object_stats.object_visits = 0;
-    }
-#endif
-    GC_STAT_ADD(generation, collections, 1);
-
-    if (reason != _Py_GC_REASON_SHUTDOWN) {
-        invoke_gc_callback(tstate, "start", generation, 0, 0);
-    }
-
-    if (gcstate->debug & DEBUG_STATS) {
-        PySys_WriteStderr("gc: collecting generation %d...\n", generation);
-        show_stats_each_generations(gcstate);
-        t1 = _PyTime_GetPerfCounter();
-    }
-
-    if (PyDTrace_GC_START_ENABLED())
-        PyDTrace_GC_START(generation);
-
-    /* update collection and allocation counters */
-    if (generation+1 < NUM_GENERATIONS)
-        gcstate->generations[generation+1].count += 1;
-    for (i = 0; i <= generation; i++)
-        gcstate->generations[i].count = 0;
-
-    /* merge younger generations with one we are currently collecting */
-    for (i = 0; i < generation; i++) {
-        gc_list_merge(GEN_HEAD(gcstate, i), GEN_HEAD(gcstate, generation));
-    }
-
-    /* handy references */
-    young = GEN_HEAD(gcstate, generation);
-    if (generation < NUM_GENERATIONS-1)
-        old = GEN_HEAD(gcstate, generation+1);
-    else
-        old = young;
-    validate_list(old, collecting_clear_unreachable_clear);
-
-    deduce_unreachable(young, &unreachable);
-
-    untrack_tuples(young);
-    /* Move reachable objects to next generation. */
-    if (young != old) {
-        if (generation == NUM_GENERATIONS - 2) {
-            gcstate->long_lived_pending += gc_list_size(young);
-        }
-        gc_list_merge(young, old);
-    }
-    else {
-        /* We only un-track dicts in full collections, to avoid quadratic
-           dict build-up. See issue #14775. */
-        untrack_dicts(young);
-        gcstate->long_lived_pending = 0;
-        gcstate->long_lived_total = gc_list_size(young);
-    }
-
-    /* All objects in unreachable are trash, but objects reachable from
-     * legacy finalizers (e.g. tp_del) can't safely be deleted.
-     */
-    gc_list_init(&finalizers);
-    // NEXT_MASK_UNREACHABLE is cleared here.
-    // After move_legacy_finalizers(), unreachable is normal list.
-    move_legacy_finalizers(&unreachable, &finalizers);
-    /* finalizers contains the unreachable objects with a legacy finalizer;
-     * unreachable objects reachable *from* those are also uncollectable,
-     * and we move those into the finalizers list too.
-     */
-    move_legacy_finalizer_reachable(&finalizers);
-
-    validate_list(&finalizers, collecting_clear_unreachable_clear);
-    validate_list(&unreachable, collecting_set_unreachable_clear);
-
-    /* Print debugging information. */
-    if (gcstate->debug & DEBUG_COLLECTABLE) {
-        for (gc = GC_NEXT(&unreachable); gc != &unreachable; gc = GC_NEXT(gc)) {
-            debug_cycle("collectable", FROM_GC(gc));
-        }
-    }
-
-    /* Clear weakrefs and invoke callbacks as necessary. */
-    m += handle_weakrefs(&unreachable, old);
-
-    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);
-
-    /* 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);
-
-    /* Call tp_clear on objects in the final_unreachable set.  This will cause
-    * the reference cycles to be broken.  It may also cause some objects
-    * in finalizers to be freed.
-    */
-    m += gc_list_size(&final_unreachable);
-    delete_garbage(tstate, gcstate, &final_unreachable, old);
-
-    /* Collect statistics on uncollectable objects found and print
-     * debugging information. */
-    for (gc = GC_NEXT(&finalizers); gc != &finalizers; gc = GC_NEXT(gc)) {
-        n++;
-        if (gcstate->debug & DEBUG_UNCOLLECTABLE)
-            debug_cycle("uncollectable", FROM_GC(gc));
-    }
-    if (gcstate->debug & DEBUG_STATS) {
-        double d = _PyTime_AsSecondsDouble(_PyTime_GetPerfCounter() - t1);
-        PySys_WriteStderr(
-            "gc: done, %zd unreachable, %zd uncollectable, %.4fs elapsed\n",
-            n+m, n, d);
-    }
-
-    /* Append instances in the uncollectable set to a Python
-     * reachable list of garbage.  The programmer has to deal with
-     * this if they insist on creating this type of structure.
-     */
-    handle_legacy_finalizers(tstate, gcstate, &finalizers, old);
-    validate_list(old, collecting_clear_unreachable_clear);
-
-    /* Clear free list only during the collection of the highest
-     * generation */
-    if (generation == NUM_GENERATIONS-1) {
-        clear_freelists(tstate->interp);
-    }
-
-    if (_PyErr_Occurred(tstate)) {
-        if (reason == _Py_GC_REASON_SHUTDOWN) {
-            _PyErr_Clear(tstate);
-        }
-        else {
-            PyErr_FormatUnraisable("Exception ignored in garbage collection");
-        }
-    }
-
-    /* Update stats */
-    struct gc_generation_stats *stats = &gcstate->generation_stats[generation];
-    stats->collections++;
-    stats->collected += m;
-    stats->uncollectable += n;
-
-    GC_STAT_ADD(generation, objects_collected, m);
-#ifdef Py_STATS
-    if (_Py_stats) {
-        GC_STAT_ADD(generation, object_visits,
-            _Py_stats->object_stats.object_visits);
-        _Py_stats->object_stats.object_visits = 0;
-    }
-#endif
-
-    if (PyDTrace_GC_DONE_ENABLED()) {
-        PyDTrace_GC_DONE(n + m);
-    }
-
-    if (reason != _Py_GC_REASON_SHUTDOWN) {
-        invoke_gc_callback(tstate, "stop", generation, m, n);
-    }
-
-    assert(!_PyErr_Occurred(tstate));
-    _Py_atomic_store_int(&gcstate->collecting, 0);
-    return n + m;
-}
-
+/*[clinic input]
+module gc
+[clinic start generated code]*/
+/*[clinic end generated code: output=da39a3ee5e6b4b0d input=b5c9690ecc842d79]*/
 #include "clinic/gcmodule.c.h"
 
 /*[clinic input]
@@ -1593,7 +90,7 @@ gc_collect_impl(PyObject *module, int generation)
         return -1;
     }
 
-    return gc_collect_main(tstate, generation, _Py_GC_REASON_MANUAL);
+    return _PyGC_Collect(tstate, generation, _Py_GC_REASON_MANUAL);
 }
 
 /*[clinic input]
@@ -1693,36 +190,6 @@ gc_get_count_impl(PyObject *module)
                          gcstate->generations[2].count);
 }
 
-static int
-referrersvisit(PyObject* obj, void *arg)
-{
-    PyObject *objs = arg;
-    Py_ssize_t i;
-    for (i = 0; i < PyTuple_GET_SIZE(objs); i++)
-        if (PyTuple_GET_ITEM(objs, i) == obj)
-            return 1;
-    return 0;
-}
-
-static int
-gc_referrers_for(PyObject *objs, PyGC_Head *list, PyObject *resultlist)
-{
-    PyGC_Head *gc;
-    PyObject *obj;
-    traverseproc traverse;
-    for (gc = GC_NEXT(list); gc != list; gc = GC_NEXT(gc)) {
-        obj = FROM_GC(gc);
-        traverse = Py_TYPE(obj)->tp_traverse;
-        if (obj == objs || obj == resultlist)
-            continue;
-        if (traverse(obj, referrersvisit, objs)) {
-            if (PyList_Append(resultlist, obj) < 0)
-                return 0; /* error */
-        }
-    }
-    return 1; /* no error */
-}
-
 PyDoc_STRVAR(gc_get_referrers__doc__,
 "get_referrers(*objs) -> list\n\
 Return the list of objects that directly refer to any of objs.");
@@ -1734,19 +201,8 @@ gc_get_referrers(PyObject *self, PyObject *args)
         return NULL;
     }
 
-    PyObject *result = PyList_New(0);
-    if (!result) {
-        return NULL;
-    }
-
-    GCState *gcstate = get_gc_state();
-    for (int i = 0; i < NUM_GENERATIONS; i++) {
-        if (!(gc_referrers_for(args, GEN_HEAD(gcstate, i), result))) {
-            Py_DECREF(result);
-            return NULL;
-        }
-    }
-    return result;
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    return _PyGC_GetReferrers(interp, args);
 }
 
 /* Append obj to list; return true if error (out of memory), false if OK. */
@@ -1805,54 +261,25 @@ static PyObject *
 gc_get_objects_impl(PyObject *module, Py_ssize_t generation)
 /*[clinic end generated code: output=48b35fea4ba6cb0e input=ef7da9df9806754c]*/
 {
-    PyThreadState *tstate = _PyThreadState_GET();
-    int i;
-    PyObject* result;
-    GCState *gcstate = &tstate->interp->gc;
-
     if (PySys_Audit("gc.get_objects", "n", generation) < 0) {
         return NULL;
     }
 
-    result = PyList_New(0);
-    if (result == NULL) {
-        return NULL;
-    }
-
-    /* If generation is passed, we extract only that generation */
-    if (generation != -1) {
-        if (generation >= NUM_GENERATIONS) {
-            _PyErr_Format(tstate, PyExc_ValueError,
-                          "generation parameter must be less than the number of "
-                          "available generations (%i)",
-                           NUM_GENERATIONS);
-            goto error;
-        }
-
-        if (generation < 0) {
-            _PyErr_SetString(tstate, PyExc_ValueError,
-                             "generation parameter cannot be negative");
-            goto error;
-        }
-
-        if (append_objects(result, GEN_HEAD(gcstate, generation))) {
-            goto error;
-        }
-
-        return result;
+    if (generation >= NUM_GENERATIONS) {
+        return PyErr_Format(PyExc_ValueError,
+                            "generation parameter must be less than the number of "
+                            "available generations (%i)",
+                            NUM_GENERATIONS);
     }
 
-    /* If generation is not passed or None, get all objects from all generations */
-    for (i = 0; i < NUM_GENERATIONS; i++) {
-        if (append_objects(result, GEN_HEAD(gcstate, i))) {
-            goto error;
-        }
+    if (generation < -1) {
+        PyErr_SetString(PyExc_ValueError,
+                        "generation parameter cannot be negative");
+        return NULL;
     }
-    return result;
 
-error:
-    Py_DECREF(result);
-    return NULL;
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    return _PyGC_GetObjects(interp, generation);
 }
 
 /*[clinic input]
@@ -1960,11 +387,8 @@ static PyObject *
 gc_freeze_impl(PyObject *module)
 /*[clinic end generated code: output=502159d9cdc4c139 input=b602b16ac5febbe5]*/
 {
-    GCState *gcstate = get_gc_state();
-    for (int i = 0; i < NUM_GENERATIONS; ++i) {
-        gc_list_merge(GEN_HEAD(gcstate, i), &gcstate->permanent_generation.head);
-        gcstate->generations[i].count = 0;
-    }
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    _PyGC_Freeze(interp);
     Py_RETURN_NONE;
 }
 
@@ -1980,9 +404,8 @@ static PyObject *
 gc_unfreeze_impl(PyObject *module)
 /*[clinic end generated code: output=1c15f2043b25e169 input=2dd52b170f4cef6c]*/
 {
-    GCState *gcstate = get_gc_state();
-    gc_list_merge(&gcstate->permanent_generation.head,
-                  GEN_HEAD(gcstate, NUM_GENERATIONS-1));
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    _PyGC_Unfreeze(interp);
     Py_RETURN_NONE;
 }
 
@@ -1996,8 +419,8 @@ static Py_ssize_t
 gc_get_freeze_count_impl(PyObject *module)
 /*[clinic end generated code: output=61cbd9f43aa032e1 input=45ffbc65cfe2a6ed]*/
 {
-    GCState *gcstate = get_gc_state();
-    return gc_list_size(&gcstate->permanent_generation.head);
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    return _PyGC_GetFreezeCount(interp);
 }
 
 
@@ -2063,7 +486,7 @@ gcmodule_exec(PyObject *module)
         return -1;
     }
 
-#define ADD_INT(NAME) if (PyModule_AddIntConstant(module, #NAME, NAME) < 0) { return -1; }
+#define ADD_INT(NAME) if (PyModule_AddIntConstant(module, #NAME, _PyGC_ ## NAME) < 0) { return -1; }
     ADD_INT(DEBUG_STATS);
     ADD_INT(DEBUG_COLLECTABLE);
     ADD_INT(DEBUG_UNCOLLECTABLE);
@@ -2093,353 +516,3 @@ PyInit_gc(void)
 {
     return PyModuleDef_Init(&gcmodule);
 }
-
-/* C API for controlling the state of the garbage collector */
-int
-PyGC_Enable(void)
-{
-    GCState *gcstate = get_gc_state();
-    int old_state = gcstate->enabled;
-    gcstate->enabled = 1;
-    return old_state;
-}
-
-int
-PyGC_Disable(void)
-{
-    GCState *gcstate = get_gc_state();
-    int old_state = gcstate->enabled;
-    gcstate->enabled = 0;
-    return old_state;
-}
-
-int
-PyGC_IsEnabled(void)
-{
-    GCState *gcstate = get_gc_state();
-    return gcstate->enabled;
-}
-
-/* Public API to invoke gc.collect() from C */
-Py_ssize_t
-PyGC_Collect(void)
-{
-    PyThreadState *tstate = _PyThreadState_GET();
-    GCState *gcstate = &tstate->interp->gc;
-
-    if (!gcstate->enabled) {
-        return 0;
-    }
-
-    Py_ssize_t n;
-    PyObject *exc = _PyErr_GetRaisedException(tstate);
-    n = gc_collect_main(tstate, NUM_GENERATIONS - 1, _Py_GC_REASON_MANUAL);
-    _PyErr_SetRaisedException(tstate, exc);
-
-    return n;
-}
-
-Py_ssize_t
-_PyGC_CollectNoFail(PyThreadState *tstate)
-{
-    /* Ideally, this function is only called on interpreter shutdown,
-       and therefore not recursively.  Unfortunately, when there are daemon
-       threads, a daemon thread can start a cyclic garbage collection
-       during interpreter shutdown (and then never finish it).
-       See http://bugs.python.org/issue8713#msg195178 for an example.
-       */
-    return gc_collect_main(tstate, NUM_GENERATIONS - 1, _Py_GC_REASON_SHUTDOWN);
-}
-
-void
-_PyGC_DumpShutdownStats(PyInterpreterState *interp)
-{
-    GCState *gcstate = &interp->gc;
-    if (!(gcstate->debug & DEBUG_SAVEALL)
-        && gcstate->garbage != NULL && PyList_GET_SIZE(gcstate->garbage) > 0) {
-        const char *message;
-        if (gcstate->debug & DEBUG_UNCOLLECTABLE)
-            message = "gc: %zd uncollectable objects at " \
-                "shutdown";
-        else
-            message = "gc: %zd uncollectable objects at " \
-                "shutdown; use gc.set_debug(gc.DEBUG_UNCOLLECTABLE) to list them";
-        /* PyErr_WarnFormat does too many things and we are at shutdown,
-           the warnings module's dependencies (e.g. linecache) may be gone
-           already. */
-        if (PyErr_WarnExplicitFormat(PyExc_ResourceWarning, "gc", 0,
-                                     "gc", NULL, message,
-                                     PyList_GET_SIZE(gcstate->garbage)))
-            PyErr_WriteUnraisable(NULL);
-        if (gcstate->debug & DEBUG_UNCOLLECTABLE) {
-            PyObject *repr = NULL, *bytes = NULL;
-            repr = PyObject_Repr(gcstate->garbage);
-            if (!repr || !(bytes = PyUnicode_EncodeFSDefault(repr)))
-                PyErr_WriteUnraisable(gcstate->garbage);
-            else {
-                PySys_WriteStderr(
-                    "      %s\n",
-                    PyBytes_AS_STRING(bytes)
-                    );
-            }
-            Py_XDECREF(repr);
-            Py_XDECREF(bytes);
-        }
-    }
-}
-
-
-void
-_PyGC_Fini(PyInterpreterState *interp)
-{
-    GCState *gcstate = &interp->gc;
-    Py_CLEAR(gcstate->garbage);
-    Py_CLEAR(gcstate->callbacks);
-
-    /* We expect that none of this interpreters objects are shared
-       with other interpreters.
-       See https://github.com/python/cpython/issues/90228. */
-}
-
-/* for debugging */
-void
-_PyGC_Dump(PyGC_Head *g)
-{
-    _PyObject_Dump(FROM_GC(g));
-}
-
-
-#ifdef Py_DEBUG
-static int
-visit_validate(PyObject *op, void *parent_raw)
-{
-    PyObject *parent = _PyObject_CAST(parent_raw);
-    if (_PyObject_IsFreed(op)) {
-        _PyObject_ASSERT_FAILED_MSG(parent,
-                                    "PyObject_GC_Track() object is not valid");
-    }
-    return 0;
-}
-#endif
-
-
-/* extension modules might be compiled with GC support so these
-   functions must always be available */
-
-void
-PyObject_GC_Track(void *op_raw)
-{
-    PyObject *op = _PyObject_CAST(op_raw);
-    if (_PyObject_GC_IS_TRACKED(op)) {
-        _PyObject_ASSERT_FAILED_MSG(op,
-                                    "object already tracked "
-                                    "by the garbage collector");
-    }
-    _PyObject_GC_TRACK(op);
-
-#ifdef Py_DEBUG
-    /* Check that the object is valid: validate objects traversed
-       by tp_traverse() */
-    traverseproc traverse = Py_TYPE(op)->tp_traverse;
-    (void)traverse(op, visit_validate, op);
-#endif
-}
-
-void
-PyObject_GC_UnTrack(void *op_raw)
-{
-    PyObject *op = _PyObject_CAST(op_raw);
-    /* Obscure:  the Py_TRASHCAN mechanism requires that we be able to
-     * call PyObject_GC_UnTrack twice on an object.
-     */
-    if (_PyObject_GC_IS_TRACKED(op)) {
-        _PyObject_GC_UNTRACK(op);
-    }
-}
-
-int
-PyObject_IS_GC(PyObject *obj)
-{
-    return _PyObject_IS_GC(obj);
-}
-
-void
-_Py_ScheduleGC(PyInterpreterState *interp)
-{
-    _Py_set_eval_breaker_bit(interp, _PY_GC_SCHEDULED_BIT, 1);
-}
-
-void
-_PyObject_GC_Link(PyObject *op)
-{
-    PyGC_Head *g = AS_GC(op);
-    assert(((uintptr_t)g & (sizeof(uintptr_t)-1)) == 0);  // g must be correctly aligned
-
-    PyThreadState *tstate = _PyThreadState_GET();
-    GCState *gcstate = &tstate->interp->gc;
-    g->_gc_next = 0;
-    g->_gc_prev = 0;
-    gcstate->generations[0].count++; /* number of allocated GC objects */
-    if (gcstate->generations[0].count > gcstate->generations[0].threshold &&
-        gcstate->enabled &&
-        gcstate->generations[0].threshold &&
-        !_Py_atomic_load_int_relaxed(&gcstate->collecting) &&
-        !_PyErr_Occurred(tstate))
-    {
-        _Py_ScheduleGC(tstate->interp);
-    }
-}
-
-void
-_Py_RunGC(PyThreadState *tstate)
-{
-    gc_collect_main(tstate, GENERATION_AUTO, _Py_GC_REASON_HEAP);
-}
-
-static PyObject *
-gc_alloc(size_t basicsize, size_t presize)
-{
-    PyThreadState *tstate = _PyThreadState_GET();
-    if (basicsize > PY_SSIZE_T_MAX - presize) {
-        return _PyErr_NoMemory(tstate);
-    }
-    size_t size = presize + basicsize;
-    char *mem = PyObject_Malloc(size);
-    if (mem == NULL) {
-        return _PyErr_NoMemory(tstate);
-    }
-    ((PyObject **)mem)[0] = NULL;
-    ((PyObject **)mem)[1] = NULL;
-    PyObject *op = (PyObject *)(mem + presize);
-    _PyObject_GC_Link(op);
-    return op;
-}
-
-PyObject *
-_PyObject_GC_New(PyTypeObject *tp)
-{
-    size_t presize = _PyType_PreHeaderSize(tp);
-    PyObject *op = gc_alloc(_PyObject_SIZE(tp), presize);
-    if (op == NULL) {
-        return NULL;
-    }
-    _PyObject_Init(op, tp);
-    return op;
-}
-
-PyVarObject *
-_PyObject_GC_NewVar(PyTypeObject *tp, Py_ssize_t nitems)
-{
-    PyVarObject *op;
-
-    if (nitems < 0) {
-        PyErr_BadInternalCall();
-        return NULL;
-    }
-    size_t presize = _PyType_PreHeaderSize(tp);
-    size_t size = _PyObject_VAR_SIZE(tp, nitems);
-    op = (PyVarObject *)gc_alloc(size, presize);
-    if (op == NULL) {
-        return NULL;
-    }
-    _PyObject_InitVar(op, tp, nitems);
-    return op;
-}
-
-PyObject *
-PyUnstable_Object_GC_NewWithExtraData(PyTypeObject *tp, size_t extra_size)
-{
-    size_t presize = _PyType_PreHeaderSize(tp);
-    PyObject *op = gc_alloc(_PyObject_SIZE(tp) + extra_size, presize);
-    if (op == NULL) {
-        return NULL;
-    }
-    memset(op, 0, _PyObject_SIZE(tp) + extra_size);
-    _PyObject_Init(op, tp);
-    return op;
-}
-
-PyVarObject *
-_PyObject_GC_Resize(PyVarObject *op, Py_ssize_t nitems)
-{
-    const size_t basicsize = _PyObject_VAR_SIZE(Py_TYPE(op), nitems);
-    const size_t presize = _PyType_PreHeaderSize(((PyObject *)op)->ob_type);
-    _PyObject_ASSERT((PyObject *)op, !_PyObject_GC_IS_TRACKED(op));
-    if (basicsize > (size_t)PY_SSIZE_T_MAX - presize) {
-        return (PyVarObject *)PyErr_NoMemory();
-    }
-    char *mem = (char *)op - presize;
-    mem = (char *)PyObject_Realloc(mem,  presize + basicsize);
-    if (mem == NULL) {
-        return (PyVarObject *)PyErr_NoMemory();
-    }
-    op = (PyVarObject *) (mem + presize);
-    Py_SET_SIZE(op, nitems);
-    return op;
-}
-
-void
-PyObject_GC_Del(void *op)
-{
-    size_t presize = _PyType_PreHeaderSize(((PyObject *)op)->ob_type);
-    PyGC_Head *g = AS_GC(op);
-    if (_PyObject_GC_IS_TRACKED(op)) {
-        gc_list_remove(g);
-#ifdef Py_DEBUG
-        PyObject *exc = PyErr_GetRaisedException();
-        if (PyErr_WarnExplicitFormat(PyExc_ResourceWarning, "gc", 0,
-                                     "gc", NULL, "Object of type %s is not untracked before destruction",
-                                     ((PyObject*)op)->ob_type->tp_name)) {
-            PyErr_WriteUnraisable(NULL);
-        }
-        PyErr_SetRaisedException(exc);
-#endif
-    }
-    GCState *gcstate = get_gc_state();
-    if (gcstate->generations[0].count > 0) {
-        gcstate->generations[0].count--;
-    }
-    PyObject_Free(((char *)op)-presize);
-}
-
-int
-PyObject_GC_IsTracked(PyObject* obj)
-{
-    if (_PyObject_IS_GC(obj) && _PyObject_GC_IS_TRACKED(obj)) {
-        return 1;
-    }
-    return 0;
-}
-
-int
-PyObject_GC_IsFinalized(PyObject *obj)
-{
-    if (_PyObject_IS_GC(obj) && _PyGC_FINALIZED(obj)) {
-         return 1;
-    }
-    return 0;
-}
-
-void
-PyUnstable_GC_VisitObjects(gcvisitobjects_t callback, void *arg)
-{
-    size_t i;
-    GCState *gcstate = get_gc_state();
-    int origenstate = gcstate->enabled;
-    gcstate->enabled = 0;
-    for (i = 0; i < NUM_GENERATIONS; i++) {
-        PyGC_Head *gc_list, *gc;
-        gc_list = GEN_HEAD(gcstate, i);
-        for (gc = GC_NEXT(gc_list); gc != gc_list; gc = GC_NEXT(gc)) {
-            PyObject *op = FROM_GC(gc);
-            Py_INCREF(op);
-            int res = callback(op, arg);
-            Py_DECREF(op);
-            if (!res) {
-                goto done;
-            }
-        }
-    }
-done:
-    gcstate->enabled = origenstate;
-}
diff --git a/PCbuild/_freeze_module.vcxproj b/PCbuild/_freeze_module.vcxproj
index 292bfa7..f16a763 100644
--- a/PCbuild/_freeze_module.vcxproj
+++ b/PCbuild/_freeze_module.vcxproj
@@ -207,6 +207,7 @@
     <ClCompile Include="..\Python\formatter_unicode.c" />
     <ClCompile Include="..\Python\frame.c" />
     <ClCompile Include="..\Python\future.c" />
+    <ClCompile Include="..\Python\gc.c" />
     <ClCompile Include="..\Python\getargs.c" />
     <ClCompile Include="..\Python\getcompiler.c" />
     <ClCompile Include="..\Python\getcopyright.c" />
diff --git a/PCbuild/_freeze_module.vcxproj.filters b/PCbuild/_freeze_module.vcxproj.filters
index 1c5a6d6..7f03cfe 100644
--- a/PCbuild/_freeze_module.vcxproj.filters
+++ b/PCbuild/_freeze_module.vcxproj.filters
@@ -166,6 +166,9 @@
     <ClCompile Include="..\Python\future.c">
       <Filter>Source Files</Filter>
     </ClCompile>
+    <ClCompile Include="..\Python\gc.c">
+      <Filter>Source Files</Filter>
+    </ClCompile>
     <ClCompile Include="..\Modules\gcmodule.c">
       <Filter>Source Files</Filter>
     </ClCompile>
diff --git a/PCbuild/pythoncore.vcxproj b/PCbuild/pythoncore.vcxproj
index be5b342..163adfd 100644
--- a/PCbuild/pythoncore.vcxproj
+++ b/PCbuild/pythoncore.vcxproj
@@ -566,6 +566,7 @@
       <AdditionalIncludeDirectories>$(GeneratedFrozenModulesDir)Python;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
     </ClCompile>
     <ClCompile Include="..\Python\future.c" />
+    <ClCompile Include="..\Python\gc.c" />
     <ClCompile Include="..\Python\getargs.c" />
     <ClCompile Include="..\Python\getcompiler.c" />
     <ClCompile Include="..\Python\getcopyright.c" />
diff --git a/PCbuild/pythoncore.vcxproj.filters b/PCbuild/pythoncore.vcxproj.filters
index a96ca24..a45a088 100644
--- a/PCbuild/pythoncore.vcxproj.filters
+++ b/PCbuild/pythoncore.vcxproj.filters
@@ -1280,6 +1280,9 @@
     <ClCompile Include="..\Python\future.c">
       <Filter>Python</Filter>
     </ClCompile>
+    <ClCompile Include="..\Python\gc.c">
+      <Filter>Python</Filter>
+    </ClCompile>
     <ClCompile Include="..\Python\getargs.c">
       <Filter>Python</Filter>
     </ClCompile>
diff --git a/Python/gc.c b/Python/gc.c
new file mode 100644
index 0000000..f47c74f
--- /dev/null
+++ b/Python/gc.c
@@ -0,0 +1,1958 @@
+//  This implements the reference cycle garbage collector.
+//  The Python module inteface to the collector is in gcmodule.c.
+//  See https://devguide.python.org/internals/garbage-collector/
+
+#include "Python.h"
+#include "pycore_ceval.h"         // _Py_set_eval_breaker_bit()
+#include "pycore_context.h"
+#include "pycore_dict.h"          // _PyDict_MaybeUntrack()
+#include "pycore_initconfig.h"
+#include "pycore_interp.h"        // PyInterpreterState.gc
+#include "pycore_object.h"
+#include "pycore_pyerrors.h"
+#include "pycore_pystate.h"       // _PyThreadState_GET()
+#include "pycore_weakref.h"       // _PyWeakref_ClearRef()
+#include "pydtrace.h"
+
+typedef struct _gc_runtime_state GCState;
+
+#ifdef Py_DEBUG
+#  define GC_DEBUG
+#endif
+
+#define GC_NEXT _PyGCHead_NEXT
+#define GC_PREV _PyGCHead_PREV
+
+// update_refs() set this bit for all objects in current generation.
+// subtract_refs() and move_unreachable() uses this to distinguish
+// visited object is in GCing or not.
+//
+// move_unreachable() removes this flag from reachable objects.
+// Only unreachable objects have this flag.
+//
+// No objects in interpreter have this flag after GC ends.
+#define PREV_MASK_COLLECTING   _PyGC_PREV_MASK_COLLECTING
+
+// Lowest bit of _gc_next is used for UNREACHABLE flag.
+//
+// This flag represents the object is in unreachable list in move_unreachable()
+//
+// Although this flag is used only in move_unreachable(), move_unreachable()
+// doesn't clear this flag to skip unnecessary iteration.
+// move_legacy_finalizers() removes this flag instead.
+// Between them, unreachable list is not normal list and we can not use
+// most gc_list_* functions for it.
+#define NEXT_MASK_UNREACHABLE  (1)
+
+#define AS_GC(op) _Py_AS_GC(op)
+#define FROM_GC(gc) _Py_FROM_GC(gc)
+
+// Automatically choose the generation that needs collecting.
+#define GENERATION_AUTO (-1)
+
+static inline int
+gc_is_collecting(PyGC_Head *g)
+{
+    return (g->_gc_prev & PREV_MASK_COLLECTING) != 0;
+}
+
+static inline void
+gc_clear_collecting(PyGC_Head *g)
+{
+    g->_gc_prev &= ~PREV_MASK_COLLECTING;
+}
+
+static inline Py_ssize_t
+gc_get_refs(PyGC_Head *g)
+{
+    return (Py_ssize_t)(g->_gc_prev >> _PyGC_PREV_SHIFT);
+}
+
+static inline void
+gc_set_refs(PyGC_Head *g, Py_ssize_t refs)
+{
+    g->_gc_prev = (g->_gc_prev & ~_PyGC_PREV_MASK)
+        | ((uintptr_t)(refs) << _PyGC_PREV_SHIFT);
+}
+
+static inline void
+gc_reset_refs(PyGC_Head *g, Py_ssize_t refs)
+{
+    g->_gc_prev = (g->_gc_prev & _PyGC_PREV_MASK_FINALIZED)
+        | PREV_MASK_COLLECTING
+        | ((uintptr_t)(refs) << _PyGC_PREV_SHIFT);
+}
+
+static inline void
+gc_decref(PyGC_Head *g)
+{
+    _PyObject_ASSERT_WITH_MSG(FROM_GC(g),
+                              gc_get_refs(g) > 0,
+                              "refcount is too small");
+    g->_gc_prev -= 1 << _PyGC_PREV_SHIFT;
+}
+
+
+#define GEN_HEAD(gcstate, n) (&(gcstate)->generations[n].head)
+
+
+static GCState *
+get_gc_state(void)
+{
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    return &interp->gc;
+}
+
+
+void
+_PyGC_InitState(GCState *gcstate)
+{
+#define INIT_HEAD(GEN) \
+    do { \
+        GEN.head._gc_next = (uintptr_t)&GEN.head; \
+        GEN.head._gc_prev = (uintptr_t)&GEN.head; \
+    } while (0)
+
+    for (int i = 0; i < NUM_GENERATIONS; i++) {
+        assert(gcstate->generations[i].count == 0);
+        INIT_HEAD(gcstate->generations[i]);
+    };
+    gcstate->generation0 = GEN_HEAD(gcstate, 0);
+    INIT_HEAD(gcstate->permanent_generation);
+
+#undef INIT_HEAD
+}
+
+
+PyStatus
+_PyGC_Init(PyInterpreterState *interp)
+{
+    GCState *gcstate = &interp->gc;
+
+    gcstate->garbage = PyList_New(0);
+    if (gcstate->garbage == NULL) {
+        return _PyStatus_NO_MEMORY();
+    }
+
+    gcstate->callbacks = PyList_New(0);
+    if (gcstate->callbacks == NULL) {
+        return _PyStatus_NO_MEMORY();
+    }
+
+    return _PyStatus_OK();
+}
+
+
+/*
+_gc_prev values
+---------------
+
+Between collections, _gc_prev is used for doubly linked list.
+
+Lowest two bits of _gc_prev are used for flags.
+PREV_MASK_COLLECTING is used only while collecting and cleared before GC ends
+or _PyObject_GC_UNTRACK() is called.
+
+During a collection, _gc_prev is temporary used for gc_refs, and the gc list
+is singly linked until _gc_prev is restored.
+
+gc_refs
+    At the start of a collection, update_refs() copies the true refcount
+    to gc_refs, for each object in the generation being collected.
+    subtract_refs() then adjusts gc_refs so that it equals the number of
+    times an object is referenced directly from outside the generation
+    being collected.
+
+PREV_MASK_COLLECTING
+    Objects in generation being collected are marked PREV_MASK_COLLECTING in
+    update_refs().
+
+
+_gc_next values
+---------------
+
+_gc_next takes these values:
+
+0
+    The object is not tracked
+
+!= 0
+    Pointer to the next object in the GC list.
+    Additionally, lowest bit is used temporary for
+    NEXT_MASK_UNREACHABLE flag described below.
+
+NEXT_MASK_UNREACHABLE
+    move_unreachable() then moves objects not reachable (whether directly or
+    indirectly) from outside the generation into an "unreachable" set and
+    set this flag.
+
+    Objects that are found to be reachable have gc_refs set to 1.
+    When this flag is set for the reachable object, the object must be in
+    "unreachable" set.
+    The flag is unset and the object is moved back to "reachable" set.
+
+    move_legacy_finalizers() will remove this flag from "unreachable" set.
+*/
+
+/*** list functions ***/
+
+static inline void
+gc_list_init(PyGC_Head *list)
+{
+    // List header must not have flags.
+    // We can assign pointer by simple cast.
+    list->_gc_prev = (uintptr_t)list;
+    list->_gc_next = (uintptr_t)list;
+}
+
+static inline int
+gc_list_is_empty(PyGC_Head *list)
+{
+    return (list->_gc_next == (uintptr_t)list);
+}
+
+/* Append `node` to `list`. */
+static inline void
+gc_list_append(PyGC_Head *node, PyGC_Head *list)
+{
+    PyGC_Head *last = (PyGC_Head *)list->_gc_prev;
+
+    // last <-> node
+    _PyGCHead_SET_PREV(node, last);
+    _PyGCHead_SET_NEXT(last, node);
+
+    // node <-> list
+    _PyGCHead_SET_NEXT(node, list);
+    list->_gc_prev = (uintptr_t)node;
+}
+
+/* Remove `node` from the gc list it's currently in. */
+static inline void
+gc_list_remove(PyGC_Head *node)
+{
+    PyGC_Head *prev = GC_PREV(node);
+    PyGC_Head *next = GC_NEXT(node);
+
+    _PyGCHead_SET_NEXT(prev, next);
+    _PyGCHead_SET_PREV(next, prev);
+
+    node->_gc_next = 0; /* object is not currently tracked */
+}
+
+/* Move `node` from the gc list it's currently in (which is not explicitly
+ * named here) to the end of `list`.  This is semantically the same as
+ * gc_list_remove(node) followed by gc_list_append(node, list).
+ */
+static void
+gc_list_move(PyGC_Head *node, PyGC_Head *list)
+{
+    /* Unlink from current list. */
+    PyGC_Head *from_prev = GC_PREV(node);
+    PyGC_Head *from_next = GC_NEXT(node);
+    _PyGCHead_SET_NEXT(from_prev, from_next);
+    _PyGCHead_SET_PREV(from_next, from_prev);
+
+    /* Relink at end of new list. */
+    // list must not have flags.  So we can skip macros.
+    PyGC_Head *to_prev = (PyGC_Head*)list->_gc_prev;
+    _PyGCHead_SET_PREV(node, to_prev);
+    _PyGCHead_SET_NEXT(to_prev, node);
+    list->_gc_prev = (uintptr_t)node;
+    _PyGCHead_SET_NEXT(node, list);
+}
+
+/* append list `from` onto list `to`; `from` becomes an empty list */
+static void
+gc_list_merge(PyGC_Head *from, PyGC_Head *to)
+{
+    assert(from != to);
+    if (!gc_list_is_empty(from)) {
+        PyGC_Head *to_tail = GC_PREV(to);
+        PyGC_Head *from_head = GC_NEXT(from);
+        PyGC_Head *from_tail = GC_PREV(from);
+        assert(from_head != from);
+        assert(from_tail != from);
+
+        _PyGCHead_SET_NEXT(to_tail, from_head);
+        _PyGCHead_SET_PREV(from_head, to_tail);
+
+        _PyGCHead_SET_NEXT(from_tail, to);
+        _PyGCHead_SET_PREV(to, from_tail);
+    }
+    gc_list_init(from);
+}
+
+static Py_ssize_t
+gc_list_size(PyGC_Head *list)
+{
+    PyGC_Head *gc;
+    Py_ssize_t n = 0;
+    for (gc = GC_NEXT(list); gc != list; gc = GC_NEXT(gc)) {
+        n++;
+    }
+    return n;
+}
+
+/* Walk the list and mark all objects as non-collecting */
+static inline void
+gc_list_clear_collecting(PyGC_Head *collectable)
+{
+    PyGC_Head *gc;
+    for (gc = GC_NEXT(collectable); gc != collectable; gc = GC_NEXT(gc)) {
+        gc_clear_collecting(gc);
+    }
+}
+
+/* Append objects in a GC list to a Python list.
+ * Return 0 if all OK, < 0 if error (out of memory for list)
+ */
+static int
+append_objects(PyObject *py_list, PyGC_Head *gc_list)
+{
+    PyGC_Head *gc;
+    for (gc = GC_NEXT(gc_list); gc != gc_list; gc = GC_NEXT(gc)) {
+        PyObject *op = FROM_GC(gc);
+        if (op != py_list) {
+            if (PyList_Append(py_list, op)) {
+                return -1; /* exception */
+            }
+        }
+    }
+    return 0;
+}
+
+// Constants for validate_list's flags argument.
+enum flagstates {collecting_clear_unreachable_clear,
+                 collecting_clear_unreachable_set,
+                 collecting_set_unreachable_clear,
+                 collecting_set_unreachable_set};
+
+#ifdef GC_DEBUG
+// validate_list checks list consistency.  And it works as document
+// describing when flags are expected to be set / unset.
+// `head` must be a doubly-linked gc list, although it's fine (expected!) if
+// the prev and next pointers are "polluted" with flags.
+// What's checked:
+// - The `head` pointers are not polluted.
+// - The objects' PREV_MASK_COLLECTING and NEXT_MASK_UNREACHABLE flags are all
+//   `set or clear, as specified by the 'flags' argument.
+// - The prev and next pointers are mutually consistent.
+static void
+validate_list(PyGC_Head *head, enum flagstates flags)
+{
+    assert((head->_gc_prev & PREV_MASK_COLLECTING) == 0);
+    assert((head->_gc_next & NEXT_MASK_UNREACHABLE) == 0);
+    uintptr_t prev_value = 0, next_value = 0;
+    switch (flags) {
+        case collecting_clear_unreachable_clear:
+            break;
+        case collecting_set_unreachable_clear:
+            prev_value = PREV_MASK_COLLECTING;
+            break;
+        case collecting_clear_unreachable_set:
+            next_value = NEXT_MASK_UNREACHABLE;
+            break;
+        case collecting_set_unreachable_set:
+            prev_value = PREV_MASK_COLLECTING;
+            next_value = NEXT_MASK_UNREACHABLE;
+            break;
+        default:
+            assert(! "bad internal flags argument");
+    }
+    PyGC_Head *prev = head;
+    PyGC_Head *gc = GC_NEXT(head);
+    while (gc != head) {
+        PyGC_Head *trueprev = GC_PREV(gc);
+        PyGC_Head *truenext = (PyGC_Head *)(gc->_gc_next  & ~NEXT_MASK_UNREACHABLE);
+        assert(truenext != NULL);
+        assert(trueprev == prev);
+        assert((gc->_gc_prev & PREV_MASK_COLLECTING) == prev_value);
+        assert((gc->_gc_next & NEXT_MASK_UNREACHABLE) == next_value);
+        prev = gc;
+        gc = truenext;
+    }
+    assert(prev == GC_PREV(head));
+}
+#else
+#define validate_list(x, y) do{}while(0)
+#endif
+
+/*** end of list stuff ***/
+
+
+/* Set all gc_refs = ob_refcnt.  After this, gc_refs is > 0 and
+ * PREV_MASK_COLLECTING bit is set for all objects in containers.
+ */
+static void
+update_refs(PyGC_Head *containers)
+{
+    PyGC_Head *next;
+    PyGC_Head *gc = GC_NEXT(containers);
+
+    while (gc != containers) {
+        next = GC_NEXT(gc);
+        /* Move any object that might have become immortal to the
+         * permanent generation as the reference count is not accurately
+         * reflecting the actual number of live references to this object
+         */
+        if (_Py_IsImmortal(FROM_GC(gc))) {
+           gc_list_move(gc, &get_gc_state()->permanent_generation.head);
+           gc = next;
+           continue;
+        }
+        gc_reset_refs(gc, Py_REFCNT(FROM_GC(gc)));
+        /* Python's cyclic gc should never see an incoming refcount
+         * of 0:  if something decref'ed to 0, it should have been
+         * deallocated immediately at that time.
+         * Possible cause (if the assert triggers):  a tp_dealloc
+         * routine left a gc-aware object tracked during its teardown
+         * phase, and did something-- or allowed something to happen --
+         * that called back into Python.  gc can trigger then, and may
+         * see the still-tracked dying object.  Before this assert
+         * was added, such mistakes went on to allow gc to try to
+         * delete the object again.  In a debug build, that caused
+         * a mysterious segfault, when _Py_ForgetReference tried
+         * to remove the object from the doubly-linked list of all
+         * objects a second time.  In a release build, an actual
+         * double deallocation occurred, which leads to corruption
+         * of the allocator's internal bookkeeping pointers.  That's
+         * so serious that maybe this should be a release-build
+         * check instead of an assert?
+         */
+        _PyObject_ASSERT(FROM_GC(gc), gc_get_refs(gc) != 0);
+        gc = next;
+    }
+}
+
+/* A traversal callback for subtract_refs. */
+static int
+visit_decref(PyObject *op, void *parent)
+{
+    OBJECT_STAT_INC(object_visits);
+    _PyObject_ASSERT(_PyObject_CAST(parent), !_PyObject_IsFreed(op));
+
+    if (_PyObject_IS_GC(op)) {
+        PyGC_Head *gc = AS_GC(op);
+        /* We're only interested in gc_refs for objects in the
+         * generation being collected, which can be recognized
+         * because only they have positive gc_refs.
+         */
+        if (gc_is_collecting(gc)) {
+            gc_decref(gc);
+        }
+    }
+    return 0;
+}
+
+/* Subtract internal references from gc_refs.  After this, gc_refs is >= 0
+ * for all objects in containers, and is GC_REACHABLE for all tracked gc
+ * objects not in containers.  The ones with gc_refs > 0 are directly
+ * reachable from outside containers, and so can't be collected.
+ */
+static void
+subtract_refs(PyGC_Head *containers)
+{
+    traverseproc traverse;
+    PyGC_Head *gc = GC_NEXT(containers);
+    for (; gc != containers; gc = GC_NEXT(gc)) {
+        PyObject *op = FROM_GC(gc);
+        traverse = Py_TYPE(op)->tp_traverse;
+        (void) traverse(op,
+                        visit_decref,
+                        op);
+    }
+}
+
+/* A traversal callback for move_unreachable. */
+static int
+visit_reachable(PyObject *op, void *arg)
+{
+    PyGC_Head *reachable = arg;
+    OBJECT_STAT_INC(object_visits);
+    if (!_PyObject_IS_GC(op)) {
+        return 0;
+    }
+
+    PyGC_Head *gc = AS_GC(op);
+    const Py_ssize_t gc_refs = gc_get_refs(gc);
+
+    // Ignore objects in other generation.
+    // This also skips objects "to the left" of the current position in
+    // move_unreachable's scan of the 'young' list - they've already been
+    // traversed, and no longer have the PREV_MASK_COLLECTING flag.
+    if (! gc_is_collecting(gc)) {
+        return 0;
+    }
+    // It would be a logic error elsewhere if the collecting flag were set on
+    // an untracked object.
+    assert(gc->_gc_next != 0);
+
+    if (gc->_gc_next & NEXT_MASK_UNREACHABLE) {
+        /* This had gc_refs = 0 when move_unreachable got
+         * to it, but turns out it's reachable after all.
+         * Move it back to move_unreachable's 'young' list,
+         * and move_unreachable will eventually get to it
+         * again.
+         */
+        // Manually unlink gc from unreachable list because the list functions
+        // don't work right in the presence of NEXT_MASK_UNREACHABLE flags.
+        PyGC_Head *prev = GC_PREV(gc);
+        PyGC_Head *next = (PyGC_Head*)(gc->_gc_next & ~NEXT_MASK_UNREACHABLE);
+        _PyObject_ASSERT(FROM_GC(prev),
+                         prev->_gc_next & NEXT_MASK_UNREACHABLE);
+        _PyObject_ASSERT(FROM_GC(next),
+                         next->_gc_next & NEXT_MASK_UNREACHABLE);
+        prev->_gc_next = gc->_gc_next;  // copy NEXT_MASK_UNREACHABLE
+        _PyGCHead_SET_PREV(next, prev);
+
+        gc_list_append(gc, reachable);
+        gc_set_refs(gc, 1);
+    }
+    else if (gc_refs == 0) {
+        /* This is in move_unreachable's 'young' list, but
+         * the traversal hasn't yet gotten to it.  All
+         * we need to do is tell move_unreachable that it's
+         * reachable.
+         */
+        gc_set_refs(gc, 1);
+    }
+    /* Else there's nothing to do.
+     * If gc_refs > 0, it must be in move_unreachable's 'young'
+     * list, and move_unreachable will eventually get to it.
+     */
+    else {
+        _PyObject_ASSERT_WITH_MSG(op, gc_refs > 0, "refcount is too small");
+    }
+    return 0;
+}
+
+/* Move the unreachable objects from young to unreachable.  After this,
+ * all objects in young don't have PREV_MASK_COLLECTING flag and
+ * unreachable have the flag.
+ * All objects in young after this are directly or indirectly reachable
+ * from outside the original young; and all objects in unreachable are
+ * not.
+ *
+ * This function restores _gc_prev pointer.  young and unreachable are
+ * doubly linked list after this function.
+ * But _gc_next in unreachable list has NEXT_MASK_UNREACHABLE flag.
+ * So we can not gc_list_* functions for unreachable until we remove the flag.
+ */
+static void
+move_unreachable(PyGC_Head *young, PyGC_Head *unreachable)
+{
+    // previous elem in the young list, used for restore gc_prev.
+    PyGC_Head *prev = young;
+    PyGC_Head *gc = GC_NEXT(young);
+
+    /* Invariants:  all objects "to the left" of us in young are reachable
+     * (directly or indirectly) from outside the young list as it was at entry.
+     *
+     * All other objects from the original young "to the left" of us are in
+     * unreachable now, and have NEXT_MASK_UNREACHABLE.  All objects to the
+     * left of us in 'young' now have been scanned, and no objects here
+     * or to the right have been scanned yet.
+     */
+
+    while (gc != young) {
+        if (gc_get_refs(gc)) {
+            /* gc is definitely reachable from outside the
+             * original 'young'.  Mark it as such, and traverse
+             * its pointers to find any other objects that may
+             * be directly reachable from it.  Note that the
+             * call to tp_traverse may append objects to young,
+             * so we have to wait until it returns to determine
+             * the next object to visit.
+             */
+            PyObject *op = FROM_GC(gc);
+            traverseproc traverse = Py_TYPE(op)->tp_traverse;
+            _PyObject_ASSERT_WITH_MSG(op, gc_get_refs(gc) > 0,
+                                      "refcount is too small");
+            // NOTE: visit_reachable may change gc->_gc_next when
+            // young->_gc_prev == gc.  Don't do gc = GC_NEXT(gc) before!
+            (void) traverse(op,
+                    visit_reachable,
+                    (void *)young);
+            // relink gc_prev to prev element.
+            _PyGCHead_SET_PREV(gc, prev);
+            // gc is not COLLECTING state after here.
+            gc_clear_collecting(gc);
+            prev = gc;
+        }
+        else {
+            /* This *may* be unreachable.  To make progress,
+             * assume it is.  gc isn't directly reachable from
+             * any object we've already traversed, but may be
+             * reachable from an object we haven't gotten to yet.
+             * visit_reachable will eventually move gc back into
+             * young if that's so, and we'll see it again.
+             */
+            // Move gc to unreachable.
+            // No need to gc->next->prev = prev because it is single linked.
+            prev->_gc_next = gc->_gc_next;
+
+            // We can't use gc_list_append() here because we use
+            // NEXT_MASK_UNREACHABLE here.
+            PyGC_Head *last = GC_PREV(unreachable);
+            // NOTE: Since all objects in unreachable set has
+            // NEXT_MASK_UNREACHABLE flag, we set it unconditionally.
+            // But this may pollute the unreachable list head's 'next' pointer
+            // too. That's semantically senseless but expedient here - the
+            // damage is repaired when this function ends.
+            last->_gc_next = (NEXT_MASK_UNREACHABLE | (uintptr_t)gc);
+            _PyGCHead_SET_PREV(gc, last);
+            gc->_gc_next = (NEXT_MASK_UNREACHABLE | (uintptr_t)unreachable);
+            unreachable->_gc_prev = (uintptr_t)gc;
+        }
+        gc = (PyGC_Head*)prev->_gc_next;
+    }
+    // young->_gc_prev must be last element remained in the list.
+    young->_gc_prev = (uintptr_t)prev;
+    // don't let the pollution of the list head's next pointer leak
+    unreachable->_gc_next &= ~NEXT_MASK_UNREACHABLE;
+}
+
+static void
+untrack_tuples(PyGC_Head *head)
+{
+    PyGC_Head *next, *gc = GC_NEXT(head);
+    while (gc != head) {
+        PyObject *op = FROM_GC(gc);
+        next = GC_NEXT(gc);
+        if (PyTuple_CheckExact(op)) {
+            _PyTuple_MaybeUntrack(op);
+        }
+        gc = next;
+    }
+}
+
+/* Try to untrack all currently tracked dictionaries */
+static void
+untrack_dicts(PyGC_Head *head)
+{
+    PyGC_Head *next, *gc = GC_NEXT(head);
+    while (gc != head) {
+        PyObject *op = FROM_GC(gc);
+        next = GC_NEXT(gc);
+        if (PyDict_CheckExact(op)) {
+            _PyDict_MaybeUntrack(op);
+        }
+        gc = next;
+    }
+}
+
+/* Return true if object has a pre-PEP 442 finalization method. */
+static int
+has_legacy_finalizer(PyObject *op)
+{
+    return Py_TYPE(op)->tp_del != NULL;
+}
+
+/* Move the objects in unreachable with tp_del slots into `finalizers`.
+ *
+ * This function also removes NEXT_MASK_UNREACHABLE flag
+ * from _gc_next in unreachable.
+ */
+static void
+move_legacy_finalizers(PyGC_Head *unreachable, PyGC_Head *finalizers)
+{
+    PyGC_Head *gc, *next;
+    assert((unreachable->_gc_next & NEXT_MASK_UNREACHABLE) == 0);
+
+    /* March over unreachable.  Move objects with finalizers into
+     * `finalizers`.
+     */
+    for (gc = GC_NEXT(unreachable); gc != unreachable; gc = next) {
+        PyObject *op = FROM_GC(gc);
+
+        _PyObject_ASSERT(op, gc->_gc_next & NEXT_MASK_UNREACHABLE);
+        gc->_gc_next &= ~NEXT_MASK_UNREACHABLE;
+        next = (PyGC_Head*)gc->_gc_next;
+
+        if (has_legacy_finalizer(op)) {
+            gc_clear_collecting(gc);
+            gc_list_move(gc, finalizers);
+        }
+    }
+}
+
+static inline void
+clear_unreachable_mask(PyGC_Head *unreachable)
+{
+    /* Check that the list head does not have the unreachable bit set */
+    assert(((uintptr_t)unreachable & NEXT_MASK_UNREACHABLE) == 0);
+
+    PyGC_Head *gc, *next;
+    assert((unreachable->_gc_next & NEXT_MASK_UNREACHABLE) == 0);
+    for (gc = GC_NEXT(unreachable); gc != unreachable; gc = next) {
+        _PyObject_ASSERT((PyObject*)FROM_GC(gc), gc->_gc_next & NEXT_MASK_UNREACHABLE);
+        gc->_gc_next &= ~NEXT_MASK_UNREACHABLE;
+        next = (PyGC_Head*)gc->_gc_next;
+    }
+    validate_list(unreachable, collecting_set_unreachable_clear);
+}
+
+/* A traversal callback for move_legacy_finalizer_reachable. */
+static int
+visit_move(PyObject *op, void *arg)
+{
+    PyGC_Head *tolist = arg;
+    OBJECT_STAT_INC(object_visits);
+    if (_PyObject_IS_GC(op)) {
+        PyGC_Head *gc = AS_GC(op);
+        if (gc_is_collecting(gc)) {
+            gc_list_move(gc, tolist);
+            gc_clear_collecting(gc);
+        }
+    }
+    return 0;
+}
+
+/* Move objects that are reachable from finalizers, from the unreachable set
+ * into finalizers set.
+ */
+static void
+move_legacy_finalizer_reachable(PyGC_Head *finalizers)
+{
+    traverseproc traverse;
+    PyGC_Head *gc = GC_NEXT(finalizers);
+    for (; gc != finalizers; gc = GC_NEXT(gc)) {
+        /* Note that the finalizers list may grow during this. */
+        traverse = Py_TYPE(FROM_GC(gc))->tp_traverse;
+        (void) traverse(FROM_GC(gc),
+                        visit_move,
+                        (void *)finalizers);
+    }
+}
+
+/* Clear all weakrefs to unreachable objects, and if such a weakref has a
+ * callback, invoke it if necessary.  Note that it's possible for such
+ * weakrefs to be outside the unreachable set -- indeed, those are precisely
+ * the weakrefs whose callbacks must be invoked.  See gc_weakref.txt for
+ * overview & some details.  Some weakrefs with callbacks may be reclaimed
+ * directly by this routine; the number reclaimed is the return value.  Other
+ * weakrefs with callbacks may be moved into the `old` generation.  Objects
+ * moved into `old` have gc_refs set to GC_REACHABLE; the objects remaining in
+ * unreachable are left at GC_TENTATIVELY_UNREACHABLE.  When this returns,
+ * no object in `unreachable` is weakly referenced anymore.
+ */
+static int
+handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)
+{
+    PyGC_Head *gc;
+    PyObject *op;               /* generally FROM_GC(gc) */
+    PyWeakReference *wr;        /* generally a cast of op */
+    PyGC_Head wrcb_to_call;     /* weakrefs with callbacks to call */
+    PyGC_Head *next;
+    int num_freed = 0;
+
+    gc_list_init(&wrcb_to_call);
+
+    /* Clear all weakrefs to the objects in unreachable.  If such a weakref
+     * also has a callback, move it into `wrcb_to_call` if the callback
+     * needs to be invoked.  Note that we cannot invoke any callbacks until
+     * all weakrefs to unreachable objects are cleared, lest the callback
+     * resurrect an unreachable object via a still-active weakref.  We
+     * make another pass over wrcb_to_call, invoking callbacks, after this
+     * pass completes.
+     */
+    for (gc = GC_NEXT(unreachable); gc != unreachable; gc = next) {
+        PyWeakReference **wrlist;
+
+        op = FROM_GC(gc);
+        next = GC_NEXT(gc);
+
+        if (PyWeakref_Check(op)) {
+            /* A weakref inside the unreachable set must be cleared.  If we
+             * allow its callback to execute inside delete_garbage(), it
+             * could expose objects that have tp_clear already called on
+             * them.  Or, it could resurrect unreachable objects.  One way
+             * this can happen is if some container objects do not implement
+             * tp_traverse.  Then, wr_object can be outside the unreachable
+             * set but can be deallocated as a result of breaking the
+             * reference cycle.  If we don't clear the weakref, the callback
+             * will run and potentially cause a crash.  See bpo-38006 for
+             * one example.
+             */
+            _PyWeakref_ClearRef((PyWeakReference *)op);
+        }
+
+        if (! _PyType_SUPPORTS_WEAKREFS(Py_TYPE(op))) {
+            continue;
+        }
+
+        /* It supports weakrefs.  Does it have any?
+         *
+         * This is never triggered for static types so we can avoid the
+         * (slightly) more costly _PyObject_GET_WEAKREFS_LISTPTR().
+         */
+        wrlist = _PyObject_GET_WEAKREFS_LISTPTR_FROM_OFFSET(op);
+
+        /* `op` may have some weakrefs.  March over the list, clear
+         * all the weakrefs, and move the weakrefs with callbacks
+         * that must be called into wrcb_to_call.
+         */
+        for (wr = *wrlist; wr != NULL; wr = *wrlist) {
+            PyGC_Head *wrasgc;                  /* AS_GC(wr) */
+
+            /* _PyWeakref_ClearRef clears the weakref but leaves
+             * the callback pointer intact.  Obscure:  it also
+             * changes *wrlist.
+             */
+            _PyObject_ASSERT((PyObject *)wr, wr->wr_object == op);
+            _PyWeakref_ClearRef(wr);
+            _PyObject_ASSERT((PyObject *)wr, wr->wr_object == Py_None);
+            if (wr->wr_callback == NULL) {
+                /* no callback */
+                continue;
+            }
+
+            /* Headache time.  `op` is going away, and is weakly referenced by
+             * `wr`, which has a callback.  Should the callback be invoked?  If wr
+             * is also trash, no:
+             *
+             * 1. There's no need to call it.  The object and the weakref are
+             *    both going away, so it's legitimate to pretend the weakref is
+             *    going away first.  The user has to ensure a weakref outlives its
+             *    referent if they want a guarantee that the wr callback will get
+             *    invoked.
+             *
+             * 2. It may be catastrophic to call it.  If the callback is also in
+             *    cyclic trash (CT), then although the CT is unreachable from
+             *    outside the current generation, CT may be reachable from the
+             *    callback.  Then the callback could resurrect insane objects.
+             *
+             * Since the callback is never needed and may be unsafe in this case,
+             * wr is simply left in the unreachable set.  Note that because we
+             * already called _PyWeakref_ClearRef(wr), its callback will never
+             * trigger.
+             *
+             * OTOH, if wr isn't part of CT, we should invoke the callback:  the
+             * weakref outlived the trash.  Note that since wr isn't CT in this
+             * case, its callback can't be CT either -- wr acted as an external
+             * root to this generation, and therefore its callback did too.  So
+             * nothing in CT is reachable from the callback either, so it's hard
+             * to imagine how calling it later could create a problem for us.  wr
+             * is moved to wrcb_to_call in this case.
+             */
+            if (gc_is_collecting(AS_GC((PyObject *)wr))) {
+                /* it should already have been cleared above */
+                assert(wr->wr_object == Py_None);
+                continue;
+            }
+
+            /* Create a new reference so that wr can't go away
+             * before we can process it again.
+             */
+            Py_INCREF(wr);
+
+            /* Move wr to wrcb_to_call, for the next pass. */
+            wrasgc = AS_GC((PyObject *)wr);
+            assert(wrasgc != next); /* wrasgc is reachable, but
+                                       next isn't, so they can't
+                                       be the same */
+            gc_list_move(wrasgc, &wrcb_to_call);
+        }
+    }
+
+    /* Invoke the callbacks we decided to honor.  It's safe to invoke them
+     * because they can't reference unreachable objects.
+     */
+    while (! gc_list_is_empty(&wrcb_to_call)) {
+        PyObject *temp;
+        PyObject *callback;
+
+        gc = (PyGC_Head*)wrcb_to_call._gc_next;
+        op = FROM_GC(gc);
+        _PyObject_ASSERT(op, PyWeakref_Check(op));
+        wr = (PyWeakReference *)op;
+        callback = wr->wr_callback;
+        _PyObject_ASSERT(op, callback != NULL);
+
+        /* copy-paste of weakrefobject.c's handle_callback() */
+        temp = PyObject_CallOneArg(callback, (PyObject *)wr);
+        if (temp == NULL) {
+            PyErr_WriteUnraisable(callback);
+        }
+        else {
+            Py_DECREF(temp);
+        }
+
+        /* Give up the reference we created in the first pass.  When
+         * op's refcount hits 0 (which it may or may not do right now),
+         * op's tp_dealloc will decref op->wr_callback too.  Note
+         * that the refcount probably will hit 0 now, and because this
+         * weakref was reachable to begin with, gc didn't already
+         * add it to its count of freed objects.  Example:  a reachable
+         * weak value dict maps some key to this reachable weakref.
+         * The callback removes this key->weakref mapping from the
+         * dict, leaving no other references to the weakref (excepting
+         * ours).
+         */
+        Py_DECREF(op);
+        if (wrcb_to_call._gc_next == (uintptr_t)gc) {
+            /* object is still alive -- move it */
+            gc_list_move(gc, old);
+        }
+        else {
+            ++num_freed;
+        }
+    }
+
+    return num_freed;
+}
+
+static void
+debug_cycle(const char *msg, PyObject *op)
+{
+    PySys_FormatStderr("gc: %s <%s %p>\n",
+                       msg, Py_TYPE(op)->tp_name, op);
+}
+
+/* Handle uncollectable garbage (cycles with tp_del slots, and stuff reachable
+ * only from such cycles).
+ * If _PyGC_DEBUG_SAVEALL, all objects in finalizers are appended to the module
+ * garbage list (a Python list), else only the objects in finalizers with
+ * __del__ methods are appended to garbage.  All objects in finalizers are
+ * merged into the old list regardless.
+ */
+static void
+handle_legacy_finalizers(PyThreadState *tstate,
+                         GCState *gcstate,
+                         PyGC_Head *finalizers, PyGC_Head *old)
+{
+    assert(!_PyErr_Occurred(tstate));
+    assert(gcstate->garbage != NULL);
+
+    PyGC_Head *gc = GC_NEXT(finalizers);
+    for (; gc != finalizers; gc = GC_NEXT(gc)) {
+        PyObject *op = FROM_GC(gc);
+
+        if ((gcstate->debug & _PyGC_DEBUG_SAVEALL) || has_legacy_finalizer(op)) {
+            if (PyList_Append(gcstate->garbage, op) < 0) {
+                _PyErr_Clear(tstate);
+                break;
+            }
+        }
+    }
+
+    gc_list_merge(finalizers, old);
+}
+
+/* Run first-time finalizers (if any) on all the objects in collectable.
+ * Note that this may remove some (or even all) of the objects from the
+ * list, due to refcounts falling to 0.
+ */
+static void
+finalize_garbage(PyThreadState *tstate, PyGC_Head *collectable)
+{
+    destructor finalize;
+    PyGC_Head seen;
+
+    /* While we're going through the loop, `finalize(op)` may cause op, or
+     * other objects, to be reclaimed via refcounts falling to zero.  So
+     * there's little we can rely on about the structure of the input
+     * `collectable` list across iterations.  For safety, we always take the
+     * first object in that list and move it to a temporary `seen` list.
+     * If objects vanish from the `collectable` and `seen` lists we don't
+     * care.
+     */
+    gc_list_init(&seen);
+
+    while (!gc_list_is_empty(collectable)) {
+        PyGC_Head *gc = GC_NEXT(collectable);
+        PyObject *op = FROM_GC(gc);
+        gc_list_move(gc, &seen);
+        if (!_PyGCHead_FINALIZED(gc) &&
+            (finalize = Py_TYPE(op)->tp_finalize) != NULL)
+        {
+            _PyGCHead_SET_FINALIZED(gc);
+            Py_INCREF(op);
+            finalize(op);
+            assert(!_PyErr_Occurred(tstate));
+            Py_DECREF(op);
+        }
+    }
+    gc_list_merge(&seen, collectable);
+}
+
+/* Break reference cycles by clearing the containers involved.  This is
+ * tricky business as the lists can be changing and we don't know which
+ * objects may be freed.  It is possible I screwed something up here.
+ */
+static void
+delete_garbage(PyThreadState *tstate, GCState *gcstate,
+               PyGC_Head *collectable, PyGC_Head *old)
+{
+    assert(!_PyErr_Occurred(tstate));
+
+    while (!gc_list_is_empty(collectable)) {
+        PyGC_Head *gc = GC_NEXT(collectable);
+        PyObject *op = FROM_GC(gc);
+
+        _PyObject_ASSERT_WITH_MSG(op, Py_REFCNT(op) > 0,
+                                  "refcount is too small");
+
+        if (gcstate->debug & _PyGC_DEBUG_SAVEALL) {
+            assert(gcstate->garbage != NULL);
+            if (PyList_Append(gcstate->garbage, op) < 0) {
+                _PyErr_Clear(tstate);
+            }
+        }
+        else {
+            inquiry clear;
+            if ((clear = Py_TYPE(op)->tp_clear) != NULL) {
+                Py_INCREF(op);
+                (void) clear(op);
+                if (_PyErr_Occurred(tstate)) {
+                    PyErr_FormatUnraisable("Exception ignored in tp_clear of %s",
+                                           Py_TYPE(op)->tp_name);
+                }
+                Py_DECREF(op);
+            }
+        }
+        if (GC_NEXT(collectable) == gc) {
+            /* object is still alive, move it, it may die later */
+            gc_clear_collecting(gc);
+            gc_list_move(gc, old);
+        }
+    }
+}
+
+/* Clear all free lists
+ * All free lists are cleared during the collection of the highest generation.
+ * Allocated items in the free list may keep a pymalloc arena occupied.
+ * Clearing the free lists may give back memory to the OS earlier.
+ */
+static void
+clear_freelists(PyInterpreterState *interp)
+{
+    _PyTuple_ClearFreeList(interp);
+    _PyFloat_ClearFreeList(interp);
+    _PyList_ClearFreeList(interp);
+    _PyDict_ClearFreeList(interp);
+    _PyAsyncGen_ClearFreeLists(interp);
+    _PyContext_ClearFreeList(interp);
+}
+
+// Show stats for objects in each generations
+static void
+show_stats_each_generations(GCState *gcstate)
+{
+    char buf[100];
+    size_t pos = 0;
+
+    for (int i = 0; i < NUM_GENERATIONS && pos < sizeof(buf); i++) {
+        pos += PyOS_snprintf(buf+pos, sizeof(buf)-pos,
+                             " %zd",
+                             gc_list_size(GEN_HEAD(gcstate, i)));
+    }
+
+    PySys_FormatStderr(
+        "gc: objects in each generation:%s\n"
+        "gc: objects in permanent generation: %zd\n",
+        buf, gc_list_size(&gcstate->permanent_generation.head));
+}
+
+/* Deduce which objects among "base" are unreachable from outside the list
+   and move them to 'unreachable'. The process consist in the following steps:
+
+1. Copy all reference counts to a different field (gc_prev is used to hold
+   this copy to save memory).
+2. Traverse all objects in "base" and visit all referred objects using
+   "tp_traverse" and for every visited object, subtract 1 to the reference
+   count (the one that we copied in the previous step). After this step, all
+   objects that can be reached directly from outside must have strictly positive
+   reference count, while all unreachable objects must have a count of exactly 0.
+3. Identify all unreachable objects (the ones with 0 reference count) and move
+   them to the "unreachable" list. This step also needs to move back to "base" all
+   objects that were initially marked as unreachable but are referred transitively
+   by the reachable objects (the ones with strictly positive reference count).
+
+Contracts:
+
+    * The "base" has to be a valid list with no mask set.
+
+    * The "unreachable" list must be uninitialized (this function calls
+      gc_list_init over 'unreachable').
+
+IMPORTANT: This function leaves 'unreachable' with the NEXT_MASK_UNREACHABLE
+flag set but it does not clear it to skip unnecessary iteration. Before the
+flag is cleared (for example, by using 'clear_unreachable_mask' function or
+by a call to 'move_legacy_finalizers'), the 'unreachable' list is not a normal
+list and we can not use most gc_list_* functions for it. */
+static inline void
+deduce_unreachable(PyGC_Head *base, PyGC_Head *unreachable) {
+    validate_list(base, collecting_clear_unreachable_clear);
+    /* Using ob_refcnt and gc_refs, calculate which objects in the
+     * container set are reachable from outside the set (i.e., have a
+     * refcount greater than 0 when all the references within the
+     * set are taken into account).
+     */
+    update_refs(base);  // gc_prev is used for gc_refs
+    subtract_refs(base);
+
+    /* Leave everything reachable from outside base in base, and move
+     * everything else (in base) to unreachable.
+     *
+     * NOTE:  This used to move the reachable objects into a reachable
+     * set instead.  But most things usually turn out to be reachable,
+     * so it's more efficient to move the unreachable things.  It "sounds slick"
+     * to move the unreachable objects, until you think about it - the reason it
+     * pays isn't actually obvious.
+     *
+     * Suppose we create objects A, B, C in that order.  They appear in the young
+     * generation in the same order.  If B points to A, and C to B, and C is
+     * reachable from outside, then the adjusted refcounts will be 0, 0, and 1
+     * respectively.
+     *
+     * When move_unreachable finds A, A is moved to the unreachable list.  The
+     * same for B when it's first encountered.  Then C is traversed, B is moved
+     * _back_ to the reachable list.  B is eventually traversed, and then A is
+     * moved back to the reachable list.
+     *
+     * So instead of not moving at all, the reachable objects B and A are moved
+     * twice each.  Why is this a win?  A straightforward algorithm to move the
+     * reachable objects instead would move A, B, and C once each.
+     *
+     * The key is that this dance leaves the objects in order C, B, A - it's
+     * reversed from the original order.  On all _subsequent_ scans, none of
+     * them will move.  Since most objects aren't in cycles, this can save an
+     * unbounded number of moves across an unbounded number of later collections.
+     * It can cost more only the first time the chain is scanned.
+     *
+     * Drawback:  move_unreachable is also used to find out what's still trash
+     * after finalizers may resurrect objects.  In _that_ case most unreachable
+     * objects will remain unreachable, so it would be more efficient to move
+     * the reachable objects instead.  But this is a one-time cost, probably not
+     * worth complicating the code to speed just a little.
+     */
+    gc_list_init(unreachable);
+    move_unreachable(base, unreachable);  // gc_prev is pointer again
+    validate_list(base, collecting_clear_unreachable_clear);
+    validate_list(unreachable, collecting_set_unreachable_set);
+}
+
+/* Handle objects that may have resurrected after a call to 'finalize_garbage', moving
+   them to 'old_generation' and placing the rest on 'still_unreachable'.
+
+   Contracts:
+       * After this function 'unreachable' must not be used anymore and 'still_unreachable'
+         will contain the objects that did not resurrect.
+
+       * The "still_unreachable" list must be uninitialized (this function calls
+         gc_list_init over 'still_unreachable').
+
+IMPORTANT: After a call to this function, the 'still_unreachable' set will have the
+PREV_MARK_COLLECTING set, but the objects in this set are going to be removed so
+we can skip the expense of clearing the flag to avoid extra iteration. */
+static inline void
+handle_resurrected_objects(PyGC_Head *unreachable, PyGC_Head* still_unreachable,
+                           PyGC_Head *old_generation)
+{
+    // Remove the PREV_MASK_COLLECTING from unreachable
+    // to prepare it for a new call to 'deduce_unreachable'
+    gc_list_clear_collecting(unreachable);
+
+    // After the call to deduce_unreachable, the 'still_unreachable' set will
+    // have the PREV_MARK_COLLECTING set, but the objects are going to be
+    // removed so we can skip the expense of clearing the flag.
+    PyGC_Head* resurrected = unreachable;
+    deduce_unreachable(resurrected, still_unreachable);
+    clear_unreachable_mask(still_unreachable);
+
+    // Move the resurrected objects to the old generation for future collection.
+    gc_list_merge(resurrected, old_generation);
+}
+
+
+/* Invoke progress callbacks to notify clients that garbage collection
+ * is starting or stopping
+ */
+static void
+invoke_gc_callback(PyThreadState *tstate, const char *phase,
+                   int generation, Py_ssize_t collected,
+                   Py_ssize_t uncollectable)
+{
+    assert(!_PyErr_Occurred(tstate));
+
+    /* we may get called very early */
+    GCState *gcstate = &tstate->interp->gc;
+    if (gcstate->callbacks == NULL) {
+        return;
+    }
+
+    /* The local variable cannot be rebound, check it for sanity */
+    assert(PyList_CheckExact(gcstate->callbacks));
+    PyObject *info = NULL;
+    if (PyList_GET_SIZE(gcstate->callbacks) != 0) {
+        info = Py_BuildValue("{sisnsn}",
+            "generation", generation,
+            "collected", collected,
+            "uncollectable", uncollectable);
+        if (info == NULL) {
+            PyErr_FormatUnraisable("Exception ignored on invoking gc callbacks");
+            return;
+        }
+    }
+
+    PyObject *phase_obj = PyUnicode_FromString(phase);
+    if (phase_obj == NULL) {
+        Py_XDECREF(info);
+        PyErr_FormatUnraisable("Exception ignored on invoking gc callbacks");
+        return;
+    }
+
+    PyObject *stack[] = {phase_obj, info};
+    for (Py_ssize_t i=0; i<PyList_GET_SIZE(gcstate->callbacks); i++) {
+        PyObject *r, *cb = PyList_GET_ITEM(gcstate->callbacks, i);
+        Py_INCREF(cb); /* make sure cb doesn't go away */
+        r = PyObject_Vectorcall(cb, stack, 2, NULL);
+        if (r == NULL) {
+            PyErr_WriteUnraisable(cb);
+        }
+        else {
+            Py_DECREF(r);
+        }
+        Py_DECREF(cb);
+    }
+    Py_DECREF(phase_obj);
+    Py_XDECREF(info);
+    assert(!_PyErr_Occurred(tstate));
+}
+
+
+/* Find the oldest generation (highest numbered) where the count
+ * exceeds the threshold.  Objects in the that generation and
+ * generations younger than it will be collected. */
+static int
+gc_select_generation(GCState *gcstate)
+{
+    for (int i = NUM_GENERATIONS-1; i >= 0; i--) {
+        if (gcstate->generations[i].count > gcstate->generations[i].threshold) {
+            /* Avoid quadratic performance degradation in number
+               of tracked objects (see also issue #4074):
+
+               To limit the cost of garbage collection, there are two strategies;
+                 - make each collection faster, e.g. by scanning fewer objects
+                 - do less collections
+               This heuristic is about the latter strategy.
+
+               In addition to the various configurable thresholds, we only trigger a
+               full collection if the ratio
+
+                long_lived_pending / long_lived_total
+
+               is above a given value (hardwired to 25%).
+
+               The reason is that, while "non-full" collections (i.e., collections of
+               the young and middle generations) will always examine roughly the same
+               number of objects -- determined by the aforementioned thresholds --,
+               the cost of a full collection is proportional to the total number of
+               long-lived objects, which is virtually unbounded.
+
+               Indeed, it has been remarked that doing a full collection every
+               <constant number> of object creations entails a dramatic performance
+               degradation in workloads which consist in creating and storing lots of
+               long-lived objects (e.g. building a large list of GC-tracked objects would
+               show quadratic performance, instead of linear as expected: see issue #4074).
+
+               Using the above ratio, instead, yields amortized linear performance in
+               the total number of objects (the effect of which can be summarized
+               thusly: "each full garbage collection is more and more costly as the
+               number of objects grows, but we do fewer and fewer of them").
+
+               This heuristic was suggested by Martin von Löwis on python-dev in
+               June 2008. His original analysis and proposal can be found at:
+               http://mail.python.org/pipermail/python-dev/2008-June/080579.html
+            */
+            if (i == NUM_GENERATIONS - 1
+                && gcstate->long_lived_pending < gcstate->long_lived_total / 4)
+            {
+                continue;
+            }
+            return i;
+        }
+    }
+    return -1;
+}
+
+
+/* This is the main function.  Read this to understand how the
+ * collection process works. */
+static Py_ssize_t
+gc_collect_main(PyThreadState *tstate, int generation, _PyGC_Reason reason)
+{
+    int i;
+    Py_ssize_t m = 0; /* # objects collected */
+    Py_ssize_t n = 0; /* # unreachable objects that couldn't be collected */
+    PyGC_Head *young; /* the generation we are examining */
+    PyGC_Head *old; /* next older generation */
+    PyGC_Head unreachable; /* non-problematic unreachable trash */
+    PyGC_Head finalizers;  /* objects with, & reachable from, __del__ */
+    PyGC_Head *gc;
+    _PyTime_t t1 = 0;   /* initialize to prevent a compiler warning */
+    GCState *gcstate = &tstate->interp->gc;
+
+    // gc_collect_main() must not be called before _PyGC_Init
+    // or after _PyGC_Fini()
+    assert(gcstate->garbage != NULL);
+    assert(!_PyErr_Occurred(tstate));
+
+    int expected = 0;
+    if (!_Py_atomic_compare_exchange_int(&gcstate->collecting, &expected, 1)) {
+        // Don't start a garbage collection if one is already in progress.
+        return 0;
+    }
+
+    if (generation == GENERATION_AUTO) {
+        // Select the oldest generation that needs collecting. We will collect
+        // objects from that generation and all generations younger than it.
+        generation = gc_select_generation(gcstate);
+        if (generation < 0) {
+            // No generation needs to be collected.
+            _Py_atomic_store_int(&gcstate->collecting, 0);
+            return 0;
+        }
+    }
+
+    assert(generation >= 0 && generation < NUM_GENERATIONS);
+
+#ifdef Py_STATS
+    if (_Py_stats) {
+        _Py_stats->object_stats.object_visits = 0;
+    }
+#endif
+    GC_STAT_ADD(generation, collections, 1);
+
+    if (reason != _Py_GC_REASON_SHUTDOWN) {
+        invoke_gc_callback(tstate, "start", generation, 0, 0);
+    }
+
+    if (gcstate->debug & _PyGC_DEBUG_STATS) {
+        PySys_WriteStderr("gc: collecting generation %d...\n", generation);
+        show_stats_each_generations(gcstate);
+        t1 = _PyTime_GetPerfCounter();
+    }
+
+    if (PyDTrace_GC_START_ENABLED()) {
+        PyDTrace_GC_START(generation);
+    }
+
+    /* update collection and allocation counters */
+    if (generation+1 < NUM_GENERATIONS) {
+        gcstate->generations[generation+1].count += 1;
+    }
+    for (i = 0; i <= generation; i++) {
+        gcstate->generations[i].count = 0;
+    }
+
+    /* merge younger generations with one we are currently collecting */
+    for (i = 0; i < generation; i++) {
+        gc_list_merge(GEN_HEAD(gcstate, i), GEN_HEAD(gcstate, generation));
+    }
+
+    /* handy references */
+    young = GEN_HEAD(gcstate, generation);
+    if (generation < NUM_GENERATIONS-1) {
+        old = GEN_HEAD(gcstate, generation+1);
+    }
+    else {
+        old = young;
+    }
+    validate_list(old, collecting_clear_unreachable_clear);
+
+    deduce_unreachable(young, &unreachable);
+
+    untrack_tuples(young);
+    /* Move reachable objects to next generation. */
+    if (young != old) {
+        if (generation == NUM_GENERATIONS - 2) {
+            gcstate->long_lived_pending += gc_list_size(young);
+        }
+        gc_list_merge(young, old);
+    }
+    else {
+        /* We only un-track dicts in full collections, to avoid quadratic
+           dict build-up. See issue #14775. */
+        untrack_dicts(young);
+        gcstate->long_lived_pending = 0;
+        gcstate->long_lived_total = gc_list_size(young);
+    }
+
+    /* All objects in unreachable are trash, but objects reachable from
+     * legacy finalizers (e.g. tp_del) can't safely be deleted.
+     */
+    gc_list_init(&finalizers);
+    // NEXT_MASK_UNREACHABLE is cleared here.
+    // After move_legacy_finalizers(), unreachable is normal list.
+    move_legacy_finalizers(&unreachable, &finalizers);
+    /* finalizers contains the unreachable objects with a legacy finalizer;
+     * unreachable objects reachable *from* those are also uncollectable,
+     * and we move those into the finalizers list too.
+     */
+    move_legacy_finalizer_reachable(&finalizers);
+
+    validate_list(&finalizers, collecting_clear_unreachable_clear);
+    validate_list(&unreachable, collecting_set_unreachable_clear);
+
+    /* Print debugging information. */
+    if (gcstate->debug & _PyGC_DEBUG_COLLECTABLE) {
+        for (gc = GC_NEXT(&unreachable); gc != &unreachable; gc = GC_NEXT(gc)) {
+            debug_cycle("collectable", FROM_GC(gc));
+        }
+    }
+
+    /* Clear weakrefs and invoke callbacks as necessary. */
+    m += handle_weakrefs(&unreachable, old);
+
+    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);
+
+    /* 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);
+
+    /* Call tp_clear on objects in the final_unreachable set.  This will cause
+    * the reference cycles to be broken.  It may also cause some objects
+    * in finalizers to be freed.
+    */
+    m += gc_list_size(&final_unreachable);
+    delete_garbage(tstate, gcstate, &final_unreachable, old);
+
+    /* Collect statistics on uncollectable objects found and print
+     * debugging information. */
+    for (gc = GC_NEXT(&finalizers); gc != &finalizers; gc = GC_NEXT(gc)) {
+        n++;
+        if (gcstate->debug & _PyGC_DEBUG_UNCOLLECTABLE)
+            debug_cycle("uncollectable", FROM_GC(gc));
+    }
+    if (gcstate->debug & _PyGC_DEBUG_STATS) {
+        double d = _PyTime_AsSecondsDouble(_PyTime_GetPerfCounter() - t1);
+        PySys_WriteStderr(
+            "gc: done, %zd unreachable, %zd uncollectable, %.4fs elapsed\n",
+            n+m, n, d);
+    }
+
+    /* Append instances in the uncollectable set to a Python
+     * reachable list of garbage.  The programmer has to deal with
+     * this if they insist on creating this type of structure.
+     */
+    handle_legacy_finalizers(tstate, gcstate, &finalizers, old);
+    validate_list(old, collecting_clear_unreachable_clear);
+
+    /* Clear free list only during the collection of the highest
+     * generation */
+    if (generation == NUM_GENERATIONS-1) {
+        clear_freelists(tstate->interp);
+    }
+
+    if (_PyErr_Occurred(tstate)) {
+        if (reason == _Py_GC_REASON_SHUTDOWN) {
+            _PyErr_Clear(tstate);
+        }
+        else {
+            PyErr_FormatUnraisable("Exception ignored in garbage collection");
+        }
+    }
+
+    /* Update stats */
+    struct gc_generation_stats *stats = &gcstate->generation_stats[generation];
+    stats->collections++;
+    stats->collected += m;
+    stats->uncollectable += n;
+
+    GC_STAT_ADD(generation, objects_collected, m);
+#ifdef Py_STATS
+    if (_Py_stats) {
+        GC_STAT_ADD(generation, object_visits,
+            _Py_stats->object_stats.object_visits);
+        _Py_stats->object_stats.object_visits = 0;
+    }
+#endif
+
+    if (PyDTrace_GC_DONE_ENABLED()) {
+        PyDTrace_GC_DONE(n + m);
+    }
+
+    if (reason != _Py_GC_REASON_SHUTDOWN) {
+        invoke_gc_callback(tstate, "stop", generation, m, n);
+    }
+
+    assert(!_PyErr_Occurred(tstate));
+    _Py_atomic_store_int(&gcstate->collecting, 0);
+    return n + m;
+}
+
+static int
+referrersvisit(PyObject* obj, void *arg)
+{
+    PyObject *objs = arg;
+    Py_ssize_t i;
+    for (i = 0; i < PyTuple_GET_SIZE(objs); i++) {
+        if (PyTuple_GET_ITEM(objs, i) == obj) {
+            return 1;
+        }
+    }
+    return 0;
+}
+
+static int
+gc_referrers_for(PyObject *objs, PyGC_Head *list, PyObject *resultlist)
+{
+    PyGC_Head *gc;
+    PyObject *obj;
+    traverseproc traverse;
+    for (gc = GC_NEXT(list); gc != list; gc = GC_NEXT(gc)) {
+        obj = FROM_GC(gc);
+        traverse = Py_TYPE(obj)->tp_traverse;
+        if (obj == objs || obj == resultlist) {
+            continue;
+        }
+        if (traverse(obj, referrersvisit, objs)) {
+            if (PyList_Append(resultlist, obj) < 0) {
+                return 0; /* error */
+            }
+        }
+    }
+    return 1; /* no error */
+}
+
+PyObject *
+_PyGC_GetReferrers(PyInterpreterState *interp, PyObject *objs)
+{
+    PyObject *result = PyList_New(0);
+    if (!result) {
+        return NULL;
+    }
+
+    GCState *gcstate = &interp->gc;
+    for (int i = 0; i < NUM_GENERATIONS; i++) {
+        if (!(gc_referrers_for(objs, GEN_HEAD(gcstate, i), result))) {
+            Py_DECREF(result);
+            return NULL;
+        }
+    }
+    return result;
+}
+
+PyObject *
+_PyGC_GetObjects(PyInterpreterState *interp, Py_ssize_t generation)
+{
+    assert(generation >= -1 && generation < NUM_GENERATIONS);
+    GCState *gcstate = &interp->gc;
+
+    PyObject *result = PyList_New(0);
+    if (result == NULL) {
+        return NULL;
+    }
+
+    if (generation == -1) {
+        /* If generation is -1, get all objects from all generations */
+        for (int i = 0; i < NUM_GENERATIONS; i++) {
+            if (append_objects(result, GEN_HEAD(gcstate, i))) {
+                goto error;
+            }
+        }
+    }
+    else {
+        if (append_objects(result, GEN_HEAD(gcstate, generation))) {
+            goto error;
+        }
+    }
+
+    return result;
+error:
+    Py_DECREF(result);
+    return NULL;
+}
+
+void
+_PyGC_Freeze(PyInterpreterState *interp)
+{
+    GCState *gcstate = &interp->gc;
+    for (int i = 0; i < NUM_GENERATIONS; ++i) {
+        gc_list_merge(GEN_HEAD(gcstate, i), &gcstate->permanent_generation.head);
+        gcstate->generations[i].count = 0;
+    }
+}
+
+void
+_PyGC_Unfreeze(PyInterpreterState *interp)
+{
+    GCState *gcstate = &interp->gc;
+    gc_list_merge(&gcstate->permanent_generation.head,
+                  GEN_HEAD(gcstate, NUM_GENERATIONS-1));
+}
+
+Py_ssize_t
+_PyGC_GetFreezeCount(PyInterpreterState *interp)
+{
+    GCState *gcstate = &interp->gc;
+    return gc_list_size(&gcstate->permanent_generation.head);
+}
+
+/* C API for controlling the state of the garbage collector */
+int
+PyGC_Enable(void)
+{
+    GCState *gcstate = get_gc_state();
+    int old_state = gcstate->enabled;
+    gcstate->enabled = 1;
+    return old_state;
+}
+
+int
+PyGC_Disable(void)
+{
+    GCState *gcstate = get_gc_state();
+    int old_state = gcstate->enabled;
+    gcstate->enabled = 0;
+    return old_state;
+}
+
+int
+PyGC_IsEnabled(void)
+{
+    GCState *gcstate = get_gc_state();
+    return gcstate->enabled;
+}
+
+/* Public API to invoke gc.collect() from C */
+Py_ssize_t
+PyGC_Collect(void)
+{
+    PyThreadState *tstate = _PyThreadState_GET();
+    GCState *gcstate = &tstate->interp->gc;
+
+    if (!gcstate->enabled) {
+        return 0;
+    }
+
+    Py_ssize_t n;
+    PyObject *exc = _PyErr_GetRaisedException(tstate);
+    n = gc_collect_main(tstate, NUM_GENERATIONS - 1, _Py_GC_REASON_MANUAL);
+    _PyErr_SetRaisedException(tstate, exc);
+
+    return n;
+}
+
+Py_ssize_t
+_PyGC_Collect(PyThreadState *tstate, int generation, _PyGC_Reason reason)
+{
+    return gc_collect_main(tstate, generation, reason);
+}
+
+Py_ssize_t
+_PyGC_CollectNoFail(PyThreadState *tstate)
+{
+    /* Ideally, this function is only called on interpreter shutdown,
+       and therefore not recursively.  Unfortunately, when there are daemon
+       threads, a daemon thread can start a cyclic garbage collection
+       during interpreter shutdown (and then never finish it).
+       See http://bugs.python.org/issue8713#msg195178 for an example.
+       */
+    return gc_collect_main(tstate, NUM_GENERATIONS - 1, _Py_GC_REASON_SHUTDOWN);
+}
+
+void
+_PyGC_DumpShutdownStats(PyInterpreterState *interp)
+{
+    GCState *gcstate = &interp->gc;
+    if (!(gcstate->debug & _PyGC_DEBUG_SAVEALL)
+        && gcstate->garbage != NULL && PyList_GET_SIZE(gcstate->garbage) > 0) {
+        const char *message;
+        if (gcstate->debug & _PyGC_DEBUG_UNCOLLECTABLE) {
+            message = "gc: %zd uncollectable objects at shutdown";
+        }
+        else {
+            message = "gc: %zd uncollectable objects at shutdown; " \
+                "use gc.set_debug(gc.DEBUG_UNCOLLECTABLE) to list them";
+        }
+        /* PyErr_WarnFormat does too many things and we are at shutdown,
+           the warnings module's dependencies (e.g. linecache) may be gone
+           already. */
+        if (PyErr_WarnExplicitFormat(PyExc_ResourceWarning, "gc", 0,
+                                     "gc", NULL, message,
+                                     PyList_GET_SIZE(gcstate->garbage)))
+        {
+            PyErr_WriteUnraisable(NULL);
+        }
+        if (gcstate->debug & _PyGC_DEBUG_UNCOLLECTABLE) {
+            PyObject *repr = NULL, *bytes = NULL;
+            repr = PyObject_Repr(gcstate->garbage);
+            if (!repr || !(bytes = PyUnicode_EncodeFSDefault(repr))) {
+                PyErr_WriteUnraisable(gcstate->garbage);
+            }
+            else {
+                PySys_WriteStderr(
+                    "      %s\n",
+                    PyBytes_AS_STRING(bytes)
+                    );
+            }
+            Py_XDECREF(repr);
+            Py_XDECREF(bytes);
+        }
+    }
+}
+
+
+void
+_PyGC_Fini(PyInterpreterState *interp)
+{
+    GCState *gcstate = &interp->gc;
+    Py_CLEAR(gcstate->garbage);
+    Py_CLEAR(gcstate->callbacks);
+
+    /* We expect that none of this interpreters objects are shared
+       with other interpreters.
+       See https://github.com/python/cpython/issues/90228. */
+}
+
+/* for debugging */
+void
+_PyGC_Dump(PyGC_Head *g)
+{
+    _PyObject_Dump(FROM_GC(g));
+}
+
+
+#ifdef Py_DEBUG
+static int
+visit_validate(PyObject *op, void *parent_raw)
+{
+    PyObject *parent = _PyObject_CAST(parent_raw);
+    if (_PyObject_IsFreed(op)) {
+        _PyObject_ASSERT_FAILED_MSG(parent,
+                                    "PyObject_GC_Track() object is not valid");
+    }
+    return 0;
+}
+#endif
+
+
+/* extension modules might be compiled with GC support so these
+   functions must always be available */
+
+void
+PyObject_GC_Track(void *op_raw)
+{
+    PyObject *op = _PyObject_CAST(op_raw);
+    if (_PyObject_GC_IS_TRACKED(op)) {
+        _PyObject_ASSERT_FAILED_MSG(op,
+                                    "object already tracked "
+                                    "by the garbage collector");
+    }
+    _PyObject_GC_TRACK(op);
+
+#ifdef Py_DEBUG
+    /* Check that the object is valid: validate objects traversed
+       by tp_traverse() */
+    traverseproc traverse = Py_TYPE(op)->tp_traverse;
+    (void)traverse(op, visit_validate, op);
+#endif
+}
+
+void
+PyObject_GC_UnTrack(void *op_raw)
+{
+    PyObject *op = _PyObject_CAST(op_raw);
+    /* Obscure:  the Py_TRASHCAN mechanism requires that we be able to
+     * call PyObject_GC_UnTrack twice on an object.
+     */
+    if (_PyObject_GC_IS_TRACKED(op)) {
+        _PyObject_GC_UNTRACK(op);
+    }
+}
+
+int
+PyObject_IS_GC(PyObject *obj)
+{
+    return _PyObject_IS_GC(obj);
+}
+
+void
+_Py_ScheduleGC(PyInterpreterState *interp)
+{
+    _Py_set_eval_breaker_bit(interp, _PY_GC_SCHEDULED_BIT, 1);
+}
+
+void
+_PyObject_GC_Link(PyObject *op)
+{
+    PyGC_Head *g = AS_GC(op);
+    assert(((uintptr_t)g & (sizeof(uintptr_t)-1)) == 0);  // g must be correctly aligned
+
+    PyThreadState *tstate = _PyThreadState_GET();
+    GCState *gcstate = &tstate->interp->gc;
+    g->_gc_next = 0;
+    g->_gc_prev = 0;
+    gcstate->generations[0].count++; /* number of allocated GC objects */
+    if (gcstate->generations[0].count > gcstate->generations[0].threshold &&
+        gcstate->enabled &&
+        gcstate->generations[0].threshold &&
+        !_Py_atomic_load_int_relaxed(&gcstate->collecting) &&
+        !_PyErr_Occurred(tstate))
+    {
+        _Py_ScheduleGC(tstate->interp);
+    }
+}
+
+void
+_Py_RunGC(PyThreadState *tstate)
+{
+    gc_collect_main(tstate, GENERATION_AUTO, _Py_GC_REASON_HEAP);
+}
+
+static PyObject *
+gc_alloc(size_t basicsize, size_t presize)
+{
+    PyThreadState *tstate = _PyThreadState_GET();
+    if (basicsize > PY_SSIZE_T_MAX - presize) {
+        return _PyErr_NoMemory(tstate);
+    }
+    size_t size = presize + basicsize;
+    char *mem = PyObject_Malloc(size);
+    if (mem == NULL) {
+        return _PyErr_NoMemory(tstate);
+    }
+    ((PyObject **)mem)[0] = NULL;
+    ((PyObject **)mem)[1] = NULL;
+    PyObject *op = (PyObject *)(mem + presize);
+    _PyObject_GC_Link(op);
+    return op;
+}
+
+PyObject *
+_PyObject_GC_New(PyTypeObject *tp)
+{
+    size_t presize = _PyType_PreHeaderSize(tp);
+    PyObject *op = gc_alloc(_PyObject_SIZE(tp), presize);
+    if (op == NULL) {
+        return NULL;
+    }
+    _PyObject_Init(op, tp);
+    return op;
+}
+
+PyVarObject *
+_PyObject_GC_NewVar(PyTypeObject *tp, Py_ssize_t nitems)
+{
+    PyVarObject *op;
+
+    if (nitems < 0) {
+        PyErr_BadInternalCall();
+        return NULL;
+    }
+    size_t presize = _PyType_PreHeaderSize(tp);
+    size_t size = _PyObject_VAR_SIZE(tp, nitems);
+    op = (PyVarObject *)gc_alloc(size, presize);
+    if (op == NULL) {
+        return NULL;
+    }
+    _PyObject_InitVar(op, tp, nitems);
+    return op;
+}
+
+PyObject *
+PyUnstable_Object_GC_NewWithExtraData(PyTypeObject *tp, size_t extra_size)
+{
+    size_t presize = _PyType_PreHeaderSize(tp);
+    PyObject *op = gc_alloc(_PyObject_SIZE(tp) + extra_size, presize);
+    if (op == NULL) {
+        return NULL;
+    }
+    memset(op, 0, _PyObject_SIZE(tp) + extra_size);
+    _PyObject_Init(op, tp);
+    return op;
+}
+
+PyVarObject *
+_PyObject_GC_Resize(PyVarObject *op, Py_ssize_t nitems)
+{
+    const size_t basicsize = _PyObject_VAR_SIZE(Py_TYPE(op), nitems);
+    const size_t presize = _PyType_PreHeaderSize(((PyObject *)op)->ob_type);
+    _PyObject_ASSERT((PyObject *)op, !_PyObject_GC_IS_TRACKED(op));
+    if (basicsize > (size_t)PY_SSIZE_T_MAX - presize) {
+        return (PyVarObject *)PyErr_NoMemory();
+    }
+    char *mem = (char *)op - presize;
+    mem = (char *)PyObject_Realloc(mem,  presize + basicsize);
+    if (mem == NULL) {
+        return (PyVarObject *)PyErr_NoMemory();
+    }
+    op = (PyVarObject *) (mem + presize);
+    Py_SET_SIZE(op, nitems);
+    return op;
+}
+
+void
+PyObject_GC_Del(void *op)
+{
+    size_t presize = _PyType_PreHeaderSize(((PyObject *)op)->ob_type);
+    PyGC_Head *g = AS_GC(op);
+    if (_PyObject_GC_IS_TRACKED(op)) {
+        gc_list_remove(g);
+#ifdef Py_DEBUG
+        PyObject *exc = PyErr_GetRaisedException();
+        if (PyErr_WarnExplicitFormat(PyExc_ResourceWarning, "gc", 0,
+                                     "gc", NULL, "Object of type %s is not untracked before destruction",
+                                     ((PyObject*)op)->ob_type->tp_name)) {
+            PyErr_WriteUnraisable(NULL);
+        }
+        PyErr_SetRaisedException(exc);
+#endif
+    }
+    GCState *gcstate = get_gc_state();
+    if (gcstate->generations[0].count > 0) {
+        gcstate->generations[0].count--;
+    }
+    PyObject_Free(((char *)op)-presize);
+}
+
+int
+PyObject_GC_IsTracked(PyObject* obj)
+{
+    if (_PyObject_IS_GC(obj) && _PyObject_GC_IS_TRACKED(obj)) {
+        return 1;
+    }
+    return 0;
+}
+
+int
+PyObject_GC_IsFinalized(PyObject *obj)
+{
+    if (_PyObject_IS_GC(obj) && _PyGC_FINALIZED(obj)) {
+         return 1;
+    }
+    return 0;
+}
+
+void
+PyUnstable_GC_VisitObjects(gcvisitobjects_t callback, void *arg)
+{
+    size_t i;
+    GCState *gcstate = get_gc_state();
+    int origenstate = gcstate->enabled;
+    gcstate->enabled = 0;
+    for (i = 0; i < NUM_GENERATIONS; i++) {
+        PyGC_Head *gc_list, *gc;
+        gc_list = GEN_HEAD(gcstate, i);
+        for (gc = GC_NEXT(gc_list); gc != gc_list; gc = GC_NEXT(gc)) {
+            PyObject *op = FROM_GC(gc);
+            Py_INCREF(op);
+            int res = callback(op, arg);
+            Py_DECREF(op);
+            if (!res) {
+                goto done;
+            }
+        }
+    }
+done:
+    gcstate->enabled = origenstate;
+}