string_concat(register PyStringObject *a, register PyObject *bb)
{
    register Py_ssize_t size;
    register PyStringObject *op;
    if (!PyString_Check(bb)) {
#ifdef Py_USING_UNICODE
        if (PyUnicode_Check(bb))
            return PyUnicode_Concat((PyObject *)a, bb);
#endif
        if (PyByteArray_Check(bb))
            return PyByteArray_Concat((PyObject *)a, bb);
        PyErr_Format(PyExc_TypeError,
                     "cannot concatenate 'str' and '%.200s' objects",
                     Py_TYPE(bb)->tp_name);
        return NULL;
    }
#define b ((PyStringObject *)bb)
    /* Optimize cases with empty left or right operand */
    if ((Py_SIZE(a) == 0 || Py_SIZE(b) == 0) &&
        PyString_CheckExact(a) && PyString_CheckExact(b)) {
        if (Py_SIZE(a) == 0) {
            Py_INCREF(bb);
            return bb;
        }
        Py_INCREF(a);
        return (PyObject *)a;
    }
    /* Check that string sizes are not negative, to prevent an
       overflow in cases where we are passed incorrectly-created
       strings with negative lengths (due to a bug in other code).
    */
    if (Py_SIZE(a) < 0 || Py_SIZE(b) < 0 ||
        Py_SIZE(a) > PY_SSIZE_T_MAX - Py_SIZE(b)) {
        PyErr_SetString(PyExc_OverflowError,
                        "strings are too large to concat");
        return NULL;
    }
    size = Py_SIZE(a) + Py_SIZE(b);

    /* Inline PyObject_NewVar */
    if (size > PY_SSIZE_T_MAX - PyStringObject_SIZE) {
        PyErr_SetString(PyExc_OverflowError,
                        "strings are too large to concat");
        return NULL;
    }
    op = (PyStringObject *)PyObject_MALLOC(PyStringObject_SIZE + size);
    if (op == NULL)
        return PyErr_NoMemory();
    (void)PyObject_INIT_VAR(op, &PyString_Type, size);
    op->ob_shash = -1;
    op->ob_sstate = SSTATE_NOT_INTERNED;
    Py_MEMCPY(op->ob_sval, a->ob_sval, Py_SIZE(a));
    Py_MEMCPY(op->ob_sval + Py_SIZE(a), b->ob_sval, Py_SIZE(b));
    op->ob_sval[size] = '\0';
    return (PyObject *) op;
#undef b
}
Unknown