GeezerVison

PyObject*
_Py_bytes_isascii(const char *cptr, Py_ssize_t len)
{
    const char *p = cptr;
    const char *end = p + len;

    while (p < end) {
        /* Fast path, see in STRINGLIB(utf8_decode) in stringlib/codecs.h
           for an explanation. */
        if (_Py_IS_ALIGNED(p, ALIGNOF_SIZE_T)) {
            /* Help allocation */
            const char *_p = p;
            while (_p + SIZEOF_SIZE_T <= end) {
                size_t value = *(const size_t *) _p;
                if (value & ASCII_CHAR_MASK) {
                    Py_RETURN_FALSE;
                }
                _p += SIZEOF_SIZE_T;
            }
            p = _p;
            if (_p == end)
                break;
        }
        if ((unsigned char)*p & 0x80) {
            Py_RETURN_FALSE;
        }
        p++;
    }
    Py_RETURN_TRUE;
}

Py_LOCAL_INLINE(Py_UCS4)
STRINGLIB(find_max_char)(const STRINGLIB_CHAR *begin, const STRINGLIB_CHAR *end)
{
    const unsigned char *p = (const unsigned char *) begin;
    const unsigned char *_end = (const unsigned char *)end;

    while (p < _end) {
        if (_Py_IS_ALIGNED(p, ALIGNOF_SIZE_T)) {
            /* Help register allocation */
            const unsigned char *_p = p;
            while (_p + SIZEOF_SIZE_T <= _end) {
                size_t value = *(const size_t *) _p;
                if (value & UCS1_ASCII_CHAR_MASK)
                    return 255;
                _p += SIZEOF_SIZE_T;
            }
            p = _p;
            if (p == _end)
                break;
        }
        if (*p++ & 0x80)
            return 255;
    }
    return 127;
}

$ python -m timeit -s 'data=b"longstringallascii" *2000' 'data.isascii()'
100000 loops, best of 5: 2.4 usec per loop
$ python -m timeit -s 'data=b"\xff" + b"longstringnotascii" *2000 ' 'data.isascii()'
20000000 loops, best of 5: 17.2 nsec per loop
$ python -m timeit -s 'data=b"longstringnotascii" *2000 + b"\xff"' 'data.isascii()'
100000 loops, best of 5: 2.41 usec per loop
$ python -m timeit -s 'data=b"shortstringallascii"' 'data.isascii()'
20000000 loops, best of 5: 20.5 nsec per loop
$ python -m timeit -s 'data=b"\xffhortstringnotascii"' 'data.isascii()'
20000000 loops, best of 5: 17.3 nsec per loop
$ python -m timeit -s 'data=b"shortstringnotascii/xff"' 'data.isascii()'
10000000 loops, best of 5: 22.4 nsec per loop

$ python -m timeit -s 'data=b"longstringallascii" *2000' 'data.isascii()'
100000 loops, best of 5: 2.4 usec per loop
$ python -m timeit -s 'data=b"\xff" + b"longstringnotascii" *2000 ' 'data.isascii()'
20000000 loops, best of 5: 17.1 nsec per loop
$ python -m timeit -s 'data=b"longstringnotascii" *2000 + b"\xff"' 'data.isascii()'
100000 loops, best of 5: 2.4 usec per loop
$ python -m timeit -s 'data=b"shortstringallascii"' 'data.isascii()'
20000000 loops, best of 5: 20 nsec per loop
$ python -m timeit -s 'data=b"\xffhortstringnotascii"' 'data.isascii()'
20000000 loops, best of 5: 17 nsec per loop
$ python -m timeit -s 'data=b"shortstringnotascii/xff"' 'data.isascii()'
10000000 loops, best of 5: 22.4 nsec per loop