Bug report

Bug description:

Python's ceval.c contains the function

PyObject *
_Py_CallBuiltinClass_StackRefSteal(
    _PyStackRef callable,
    _PyStackRef *arguments,
    int total_args)
{
    PyObject *res;
    STACKREFS_TO_PYOBJECTS(arguments, total_args, args_o);
    if (CONVERSION_FAILED(args_o)) {
        res = NULL;
        goto cleanup;
    }
    PyTypeObject *tp = (PyTypeObject *)PyStackRef_AsPyObjectBorrow(callable);
    res = tp->tp_vectorcall((PyObject *)tp, args_o, total_args, NULL);       /* <----- relevant line */
   ....
}

The call to tp_vectorcall does not declare PY_VECTORCALL_ARGUMENTS_OFFSET despite STACKREFS_TO_PYOBJECTS (via _PyObjectArray_FromStackRefArray) always allocating an args_o that has an extra scratch space at position -1.

Strictly speaking, this isn't incorrect. But it can force the caller to make a redundant copy of the arguments, which is inefficient. Best practice to allocate the extra entry and then communicate its presence to the vector-callee.

This inefficiency is not restricted to Python-internal types. It also affects custom heap types that are themselves vector-callable. In this case, Python's adaptive optimizing interpreter replaces CALL_NON_PY_GENERAL with CALL_BUILTIN_CLASS that then goes through this code path.

cc @markshannon @vstinner

CPython versions tested on:

3.14

Operating systems tested on:

macOS

Linked PRs

• gh-143361: Pass PY_VECTORCALL_ARGUMENTS_OFFSET in _Py_CallBuiltinClass_StackRefSteal #143367