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gh-142659: Optimize set_swap_bodies for intersection_update #148155

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554d5c1
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
100c491
gh-142659: Move GIL/free-threading branch into copy_small_table
Apr 12, 2026
0c0a00a
gh-142659: Use PySet_Check in set_replace_body assertions
Apr 12, 2026
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109 changes: 50 additions & 59 deletions Objects/setobject.c
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Original file line numberDiff line numberDiff line change
Expand Up@@ -1464,85 +1464,76 @@ set_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
return make_new_set(type, NULL);
}

#ifdef Py_GIL_DISABLED
static void
copy_small_table(setentry *dest, setentry *src)
{
#ifdef Py_GIL_DISABLED
for (Py_ssize_t i = 0; i < PySet_MINSIZE; i++) {
_Py_atomic_store_ptr_release(&dest[i].key, src[i].key);
_Py_atomic_store_ssize_relaxed(&dest[i].hash, src[i].hash);
}
}
#else
memcpy(dest, src, PySet_MINSIZE * sizeof(setentry));
#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

@eendebakpteendebakptApr 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.

@SiyetSiyetApr 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)

@eendebakpteendebakptMay 18, 2026

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dst is locked in all usages of set_replace_body afaics. Can we update the name to reflect this? And maybe add an assert on dst.

{
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));
#ifndef Py_GIL_DISABLED
memcpy(a->smalltable, b->smalltable, sizeof(tab));
memcpy(b->smalltable, tab, sizeof(tab));
#else
copy_small_table(a->smalltable, b->smalltable);
copy_small_table(b->smalltable, 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);
assert(PySet_Check((PyObject *)dst));
assert(PySet_Check((PyObject *)src));
assert(_PyObject_IsUniquelyReferenced((PyObject *)src));

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));
copy_small_table(dst->smalltable, src->smalltable);
memcpy(src->smalltable, tab, sizeof(tab));
}
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 DownExpand Up@@ -1797,7 +1788,7 @@ 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 +1812,7 @@ 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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