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gh-142659: Optimize set_swap_bodies for intersection_update #148155

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gh-142659: Optimize set_swap_bodies for intersection_update
Apr 6, 2026
37a95e6
Add news entry for gh-142659
Apr 6, 2026
1be8796
Use _PyObject_IsUniquelyReferenced instead of Py_REFCNT check
Apr 7, 2026
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3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2026-04-06-11-32-52.gh-issue-142659.44riUp.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
Optimize :meth:`set.intersection_update` by replacing the general-purpose
``set_swap_bodies()`` with a specialized ``set_replace_body()`` that skips
unnecessary atomic operations and checks for the temporary set argument.
102 changes: 48 additions & 54 deletions Objects/setobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1475,74 +1475,68 @@ copy_small_table(setentry *dest, setentry *src)
}
#endif

/* set_swap_bodies() switches the contents of any two sets by moving their
internal data pointers and, if needed, copying the internal smalltables.
Semantically equivalent to:
/* set_replace_body() replaces the contents of dst with those of src,
moving dst's old contents into src for proper cleanup on Py_DECREF.

t=set(a); a.clear(); a.update(b); b.clear(); b.update(t); del t
The caller guarantees that src is a uniquely-referenced temporary set
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@eendebakpt eendebakpt Apr 6, 2026

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If the called is required to cleanup src immediately afterwards, maybe it is cleaner to let the set_replace_body steal the reference and do the decref.

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@Siyet Siyet Apr 7, 2026

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In set_intersection_update_multi_impl, I placed Py_DECREF(tmp) after Py_END_CRITICAL_SECTION().

My reasoning was that moving the decref inside set_replace_body would run the deallocation (and decref of all keys) while still holding the critical section lock, which I thought might be undesirable.

By keeping it outside, the caller has control over when deallocation happens.

This is my first PR though and I'm still getting familiar with the project, so if I'm wrong here - happy to rework it.

that will be discarded immediately afterward. This allows us to skip
atomic operations and shared-marking on src's fields, and to skip the
frozenset hash swap (neither argument is ever a frozenset here).

The function always succeeds and it leaves both objects in a stable state.
Useful for operations that update in-place (by allowing an intermediate
result to be swapped into one of the original inputs).
*/

static void
set_swap_bodies(PySetObject *a, PySetObject *b)
set_replace_body(PySetObject *dst, PySetObject *src)
{
Py_ssize_t t;
setentry *u;
setentry tab[PySet_MINSIZE];
Py_hash_t h;

setentry *a_table = a->table;
setentry *b_table = b->table;
FT_ATOMIC_STORE_PTR_RELEASE(a->table, NULL);
FT_ATOMIC_STORE_PTR_RELEASE(b->table, NULL);

t = a->fill; a->fill = b->fill; b->fill = t;
t = a->used;
FT_ATOMIC_STORE_SSIZE_RELAXED(a->used, b->used);
FT_ATOMIC_STORE_SSIZE_RELAXED(b->used, t);
t = a->mask;
FT_ATOMIC_STORE_SSIZE_RELEASE(a->mask, b->mask);
FT_ATOMIC_STORE_SSIZE_RELEASE(b->mask, t);

u = a_table;
if (a_table == a->smalltable)
u = b->smalltable;
a_table = b_table;
if (b_table == b->smalltable)
a_table = a->smalltable;
b_table = u;

if (a_table == a->smalltable || b_table == b->smalltable) {
memcpy(tab, a->smalltable, sizeof(tab));

assert(!PyType_IsSubtype(Py_TYPE(dst), &PyFrozenSet_Type));
assert(!PyType_IsSubtype(Py_TYPE(src), &PyFrozenSet_Type));
assert(Py_REFCNT(src) == 1);
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@eendebakpt eendebakpt Apr 6, 2026

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Suggested change
assert(Py_REFCNT(src) == 1);
assert(_PyObject_IsUniquelyReferenced(src));

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@colesbury colesbury Apr 6, 2026

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This assertion is going to be prone to gc.get_objects()/gc.get_referrers() issues like in #148180.

I'm not sure if we should do anything about that.

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@zhuyifei1999 zhuyifei1999 Apr 7, 2026

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Will gc.get_objects and similar ever see this set? They use a stop-the-world heap traversal, which can't happen until set_intersection_update / set_intersection_update_multi_impl returns, and by the time that happens, this src set is already gone.

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@Siyet Siyet Apr 7, 2026

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I agree with @zhuyifei1999 - src is a local variable that gets freed before the function returns, so gc can't observe it.

Switched to _PyObject_IsUniquelyReferenced per @eendebakpt's suggestion regardless, since I believe it's the correct API for free-threaded builds.


setentry *dst_table = dst->table;
setentry *src_table = src->table;
FT_ATOMIC_STORE_PTR_RELEASE(dst->table, NULL);
src->table = NULL;

t = dst->fill; dst->fill = src->fill; src->fill = t;
t = dst->used;
FT_ATOMIC_STORE_SSIZE_RELAXED(dst->used, src->used);
src->used = t;
t = dst->mask;
FT_ATOMIC_STORE_SSIZE_RELEASE(dst->mask, src->mask);
src->mask = t;

u = dst_table;
if (dst_table == dst->smalltable)
u = src->smalltable;
dst_table = src_table;
if (src_table == src->smalltable)
dst_table = dst->smalltable;
src_table = u;

if (dst_table == dst->smalltable || src_table == src->smalltable) {
memcpy(tab, dst->smalltable, sizeof(tab));
#ifndef Py_GIL_DISABLED
memcpy(a->smalltable, b->smalltable, sizeof(tab));
memcpy(b->smalltable, tab, sizeof(tab));
memcpy(dst->smalltable, src->smalltable, sizeof(tab));
memcpy(src->smalltable, tab, sizeof(tab));
#else
copy_small_table(a->smalltable, b->smalltable);
copy_small_table(b->smalltable, tab);
copy_small_table(dst->smalltable, src->smalltable);
memcpy(src->smalltable, tab, sizeof(tab));
#endif
}

if (PyType_IsSubtype(Py_TYPE(a), &PyFrozenSet_Type) &&
PyType_IsSubtype(Py_TYPE(b), &PyFrozenSet_Type)) {
h = FT_ATOMIC_LOAD_SSIZE_RELAXED(a->hash);
FT_ATOMIC_STORE_SSIZE_RELAXED(a->hash, FT_ATOMIC_LOAD_SSIZE_RELAXED(b->hash));
FT_ATOMIC_STORE_SSIZE_RELAXED(b->hash, h);
} else {
FT_ATOMIC_STORE_SSIZE_RELAXED(a->hash, -1);
FT_ATOMIC_STORE_SSIZE_RELAXED(b->hash, -1);
}
if (!SET_IS_SHARED(b) && SET_IS_SHARED(a)) {
SET_MARK_SHARED(b);
}
if (!SET_IS_SHARED(a) && SET_IS_SHARED(b)) {
SET_MARK_SHARED(a);
FT_ATOMIC_STORE_SSIZE_RELAXED(dst->hash, -1);

if (SET_IS_SHARED(dst)) {
SET_MARK_SHARED(src);
}
FT_ATOMIC_STORE_PTR_RELEASE(a->table, a_table);
FT_ATOMIC_STORE_PTR_RELEASE(b->table, b_table);

FT_ATOMIC_STORE_PTR_RELEASE(dst->table, dst_table);
src->table = src_table;
}

/*[clinic input]
Expand Down Expand Up @@ -1797,7 +1791,7 @@ set_intersection_update(PySetObject *so, PyObject *other)
tmp = set_intersection(so, other);
if (tmp == NULL)
return NULL;
set_swap_bodies(so, (PySetObject *)tmp);
set_replace_body(so, (PySetObject *)tmp);
Py_DECREF(tmp);
Py_RETURN_NONE;
}
Expand All @@ -1821,7 +1815,7 @@ set_intersection_update_multi_impl(PySetObject *so, PyObject * const *others,
if (tmp == NULL)
return NULL;
Py_BEGIN_CRITICAL_SECTION(so);
set_swap_bodies(so, (PySetObject *)tmp);
set_replace_body(so, (PySetObject *)tmp);
Py_END_CRITICAL_SECTION();
Py_DECREF(tmp);
Py_RETURN_NONE;
Expand Down
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