gh-158816: Skip useless hash table probe when deleting from split dicts (#158819)
delitem_common() unconditionally called lookdict_index() to locate the index slot in dk_indices that points at the entry being removed.
That slot is only used by the combined branch, to overwrite it with DKIX_DUMMY. In split tables the whole probe sequence was redundant work.
Move the call into the combined branch and drop the now-pointless assert() for split tables.
Microbenchmark (free-threaded build, -Og, no PGO; deleting keys from materialized instance dicts; medians over 12 alternating runs):
del d[k] 71.2 -> 69.2 ns/op (-2.8%)
d.pop(k) 98.9 -> 96.1 ns/op (-2.8%)
colliding hashes 78.0 -> 74.4 ns/op (-4.6%)
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-10-04-20-04-55.gh-issue-158816.9ulhWb.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-10-04-20-04-55.gh-issue-158816.9ulhWb.rst
new file mode 100644
index 0000000..fd9997a
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2026-10-04-20-04-55.gh-issue-158816.9ulhWb.rst
@@ -0,0 +1,3 @@
+Speed up deleting items from split dictionaries (for example the
+materialized ``__dict__`` of an instance with inline values) by skipping a
+redundant hash table probe.
diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index 13c9e2e..168a5a4 100644
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -2938,9 +2938,6 @@ delitem_common(PyDictObject *mp, Py_hash_t hash, Py_ssize_t ix,
PyObject *old_key;
- Py_ssize_t hashpos = lookdict_index(mp->ma_keys, hash, ix);
- assert(hashpos >= 0);
-
STORE_USED(mp, mp->ma_used - 1);
if (_PyDict_HasSplitTable(mp)) {
assert(old_value == mp->ma_values->values[ix]);
@@ -2951,6 +2948,9 @@ delitem_common(PyDictObject *mp, Py_hash_t hash, Py_ssize_t ix,
ASSERT_CONSISTENT(mp);
}
else {
+ Py_ssize_t hashpos = lookdict_index(mp->ma_keys, hash, ix);
+ assert(hashpos >= 0);
+
FT_ATOMIC_STORE_UINT32_RELAXED(mp->ma_keys->dk_version, 0);
dictkeys_set_index(mp->ma_keys, hashpos, DKIX_DUMMY);
if (DK_IS_UNICODE(mp->ma_keys)) {
