There is no special shortcut for str_rprimitive in builtin_len currently.

We may:

• Add a branch for str_rpimitive
• Reduce redundant codes

We may directly get length from PyUnicodeObject using LoadMem method.

#define PyUnicode_GET_LENGTH(op)                \
    (assert(PyUnicode_Check(op)),               \
     assert(PyUnicode_IS_READY(op)),            \
     ((PyASCIIObject *)(op))->length)

#define PyUnicode_IS_READY(op) (((PyASCIIObject*)op)->state.ready)

According to CPython's comments, the length member would be 0 when PyUnicode is not ready. Maybe it's ok to omit the IS_READY check?