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gh-117376: Partial implementation of deferred reference counting (#117696)

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This marks objects as using deferred refrence counting using the
`ob_gc_bits` field in the free-threaded build and collects those objects
during GC.
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Sam Gross 2024-04-12 13:36:20 -04:00 • committed by GitHub
parent c50cb6dd09
commit 4ad8f090cc
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GPG key ID: B5690EEEBB952194
9 changed files with 82 additions and 21 deletions
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19
Python/gc_free_threading.c
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@ -159,6 +159,15 @@ gc_decref(PyObject *op)
op->ob_tid -= 1;
}
static void
disable_deferred_refcounting(PyObject *op)
{
if (_PyObject_HasDeferredRefcount(op)) {
op->ob_gc_bits &= ~_PyGC_BITS_DEFERRED;
op->ob_ref_shared -= (1 << _Py_REF_SHARED_SHIFT);
}
}
static Py_ssize_t
merge_refcount(PyObject *op, Py_ssize_t extra)
{
@ -375,9 +384,10 @@ update_refs(const mi_heap_t *heap, const mi_heap_area_t *area,
}
Py_ssize_t refcount = Py_REFCNT(op);
refcount -= _PyObject_HasDeferredRefcount(op);
_PyObject_ASSERT(op, refcount >= 0);
if (refcount > 0) {
if (refcount > 0 && !_PyObject_HasDeferredRefcount(op)) {
// Untrack tuples and dicts as necessary in this pass, but not objects
// with zero refcount, which we will want to collect.
if (PyTuple_CheckExact(op)) {
@ -466,6 +476,9 @@ mark_heap_visitor(const mi_heap_t *heap, const mi_heap_area_t *area,
return true;
}
_PyObject_ASSERT_WITH_MSG(op, gc_get_refs(op) >= 0,
"refcount is too small");
if (gc_is_unreachable(op) && gc_get_refs(op) != 0) {
// Object is reachable but currently marked as unreachable.
// Mark it as reachable and traverse its pointers to find
@ -499,6 +512,10 @@ scan_heap_visitor(const mi_heap_t *heap, const mi_heap_area_t *area,
struct collection_state *state = (struct collection_state *)args;
if (gc_is_unreachable(op)) {
// Disable deferred refcounting for unreachable objects so that they
// are collected immediately after finalization.
disable_deferred_refcounting(op);
// Merge and add one to the refcount to prevent deallocation while we
// are holding on to it in a worklist.
merge_refcount(op, 1);