Python/ceval.c (part 68)

Source:

cpython 3.14 @ ab2d84fe1023/Python/ceval.c ↗

This annotation covers BINARY_SUBSCR specializations. See python_ceval67_detail for COMPARE_OP specializations.

Map

Lines	Symbol	Role
1-80	BINARY_SUBSCR baseline	Generic obj[key] via mp_subscript
81-160	BINARY_SUBSCR_LIST_INT	Fast path: list[integer_index]
161-240	BINARY_SUBSCR_TUPLE_INT	Fast path: tuple[integer_index]
241-320	BINARY_SUBSCR_DICT	Fast path: dict lookup
321-500	STORE_SUBSCR_LIST_INT	Fast path: list[i] = val

Reading

BINARY_SUBSCR_LIST_INT

// CPython: Python/ceval.c:2640 BINARY_SUBSCR_LIST_INT
inst(BINARY_SUBSCR_LIST_INT, (list, sub -- res)) {
    DEOPT_IF(!PyList_CheckExact(list), BINARY_SUBSCR);
    DEOPT_IF(!_PyLong_IsCompact((PyLongObject *)sub), BINARY_SUBSCR);
    Py_ssize_t index = _PyLong_CompactValue((PyLongObject *)sub);
    if (index < 0) index += PyList_GET_SIZE(list);
    DEOPT_IF((size_t)index >= (size_t)PyList_GET_SIZE(list), BINARY_SUBSCR);
    res = PyList_GET_ITEM(list, index);
    Py_INCREF(res);
    Py_DECREF(list); Py_DECREF(sub);
}

lst[i] for a compact integer index directly accesses ob_item[index] without range-check overhead (range is checked via an unsigned comparison which handles negatives too). PyList_GET_ITEM is a direct array dereference.

BINARY_SUBSCR_DICT

// CPython: Python/ceval.c:2720 BINARY_SUBSCR_DICT
inst(BINARY_SUBSCR_DICT, (dict, sub -- res)) {
    DEOPT_IF(!PyDict_CheckExact(dict), BINARY_SUBSCR);
    res = PyDict_GetItemWithError(dict, sub);
    if (res == NULL) {
        if (!_PyErr_Occurred(tstate)) {
            _PyErr_SetKeyError(sub);
        }
        Py_DECREF(dict); Py_DECREF(sub);
        ERROR_IF(true, error);
    }
    Py_INCREF(res);
    Py_DECREF(dict); Py_DECREF(sub);
}

d[key] for a dict goes through PyDict_GetItemWithError (raises KeyError unlike PyDict_GetItem). The specialization checks PyDict_CheckExact to exclude defaultdict and other subclasses which may override __getitem__.

STORE_SUBSCR_LIST_INT

// CPython: Python/ceval.c:2780 STORE_SUBSCR_LIST_INT
inst(STORE_SUBSCR_LIST_INT, (value, list, sub --)) {
    DEOPT_IF(!PyList_CheckExact(list), STORE_SUBSCR);
    DEOPT_IF(!_PyLong_IsCompact((PyLongObject *)sub), STORE_SUBSCR);
    Py_ssize_t index = _PyLong_CompactValue((PyLongObject *)sub);
    if (index < 0) index += PyList_GET_SIZE(list);
    DEOPT_IF((size_t)index >= (size_t)PyList_GET_SIZE(list), STORE_SUBSCR);
    PyObject *old = PyList_GET_ITEM(list, index);
    PyList_SET_ITEM(list, index, value);
    Py_DECREF(old);
    Py_DECREF(list); Py_DECREF(sub);
}

lst[i] = v directly sets ob_item[index] and decrefs the old value. No bounds-check overhead beyond the deopt condition.

gopy notes

BINARY_SUBSCR_LIST_INT is in vm/eval_simple.go. It reads objects.List.Items[index]. BINARY_SUBSCR_DICT calls objects.DictGetWithError. STORE_SUBSCR_LIST_INT replaces objects.List.Items[index]. All deopts fall through to the generic BINARY_SUBSCR handler.