Add PyLATIN1Object for single byte latin1 strings

[kumaraditya303]

While working on python/cpython#31616, I noticed that only ascii strings use PyASCIIObject whereas latin1 strings use PyCompactUnicodeObject with utf8_length = 0, utf8 = 0x0, wstr_length = 0 set which wastes space, so instead of PyASCIIObject, we can have PyLATIN1Object since latin1 is in range(255) it can be stored in the uint8_t and not waste space.
This is only for strings of size 1.

Layout:

New layout of PyLATIN1Object:

typedef struct {
    /* There are 4 forms of Unicode strings:

       - compact latin1:

         * structure = PyLATIN1Object
         * test: PyUnicode_IS_COMPACT_LATIN1(op)
         * kind = PyUnicode_1BYTE_KIND
         * compact = 1
         * latin1 = 1
         * ready = 1
         * (length is the length of the utf8 and wstr strings)
         * (data starts just after the structure)
         * (since latin1 is decoded from UTF-8, the utf8 string are the data)

       - compact:

         * structure = PyCompactUnicodeObject
         * test: PyUnicode_IS_COMPACT(op) && !PyUnicode_IS_ASCII(op)
         * kind = PyUnicode_1BYTE_KIND, PyUnicode_2BYTE_KIND or
           PyUnicode_4BYTE_KIND
         * compact = 1
         * ready = 1
         * latin1 = 0
         * utf8 is not shared with data
         * utf8_length = 0 if utf8 is NULL
         * wstr is shared with data and wstr_length=length
           if kind=PyUnicode_2BYTE_KIND and sizeof(wchar_t)=2
           or if kind=PyUnicode_4BYTE_KIND and sizeof(wchar_t)=4
         * wstr_length = 0 if wstr is NULL
         * (data starts just after the structure)

       - legacy string, not ready:

         * structure = PyUnicodeObject
         * test: kind == PyUnicode_WCHAR_KIND
         * length = 0 (use wstr_length)
         * hash = -1
         * kind = PyUnicode_WCHAR_KIND
         * compact = 0
         * latin1 = 0
         * ready = 0
         * interned = SSTATE_NOT_INTERNED
         * wstr is not NULL
         * data.any is NULL
         * utf8 is NULL
         * utf8_length = 0

       - legacy string, ready:

         * structure = PyUnicodeObject structure
         * test: !PyUnicode_IS_COMPACT(op) && kind != PyUnicode_WCHAR_KIND
         * kind = PyUnicode_1BYTE_KIND, PyUnicode_2BYTE_KIND or
           PyUnicode_4BYTE_KIND
         * compact = 0
         * ready = 1
         * data.any is not NULL
         * utf8 is shared and utf8_length = length with data.any if ascii = 1
         * utf8_length = 0 if utf8 is NULL
         * wstr is shared with data.any and wstr_length = length
           if kind=PyUnicode_2BYTE_KIND and sizeof(wchar_t)=2
           or if kind=PyUnicode_4BYTE_KIND and sizeof(wchar_4)=4
         * wstr_length = 0 if wstr is NULL

       Compact strings use only one memory block (structure + characters),
       whereas legacy strings use one block for the structure and one block
       for characters.

       Legacy strings are created by PyUnicode_FromUnicode() and
       PyUnicode_FromStringAndSize(NULL, size) functions. They become ready
       when PyUnicode_READY() is called.

       See also _PyUnicode_CheckConsistency().
    */
    PyObject_HEAD
    Py_ssize_t length;          /* Number of code points in the string */
    Py_hash_t hash;             /* Hash value; -1 if not set */
    struct {
        /*
           SSTATE_NOT_INTERNED (0)
           SSTATE_INTERNED_MORTAL (1)
           SSTATE_INTERNED_IMMORTAL (2)

           If interned != SSTATE_NOT_INTERNED, the two references from the
           dictionary to this object are *not* counted in ob_refcnt.
         */
        unsigned int interned:2;
        /* Character size:

           - PyUnicode_WCHAR_KIND (0):

             * character type = wchar_t (16 or 32 bits, depending on the
               platform)

           - PyUnicode_1BYTE_KIND (1):

             * character type = Py_UCS1 (8 bits, unsigned)
             * all characters are in the range U+0000-U+00FF (latin1)

           - PyUnicode_2BYTE_KIND (2):

             * character type = Py_UCS2 (16 bits, unsigned)
             * all characters are in the range U+0000-U+FFFF (BMP)
             * at least one character is in the range U+0100-U+FFFF

           - PyUnicode_4BYTE_KIND (4):

             * character type = Py_UCS4 (32 bits, unsigned)
             * all characters are in the range U+0000-U+10FFFF
             * at least one character is in the range U+10000-U+10FFFF
         */
        unsigned int kind:3;
        /* Compact is with respect to the allocation scheme. Compact unicode
           objects only require one memory block while non-compact objects use
           one block for the PyUnicodeObject struct and another for its data
           buffer. */
        unsigned int compact:1;
        /* The string only contains characters in the range U+0000-U+00FF (latin1)
           and the kind is PyUnicode_1BYTE_KIND. If latin1 is set and compact is
           set, use the PyLATIN1Object structure. */
        unsigned int latin1:1;
        /* The ready flag indicates whether the object layout is initialized
           completely. This means that this is either a compact object, or
           the data pointer is filled out. The bit is redundant, and helps
           to minimize the test in PyUnicode_IS_READY(). */
        unsigned int ready:1;
        /* Padding to ensure that PyUnicode_DATA() is always aligned to
           4 bytes (see issue #19537 on m68k). */
        unsigned int :24;
    } state;
    wchar_t *wstr;              /* wchar_t representation (null-terminated) */
} 

[methane]

latin1 != utf8. We need utf8 cache for latin1 string.

[kumaraditya303]

Is utf8 cache needed for string of length of 1 too?

[methane]

>>> s="\u0080"
>>> s.encode('utf-8')
b'\xc2\x80'
>>> s.encode('latin1')
b'\x80'

[kumaraditya303]

Thanks for the clarification.

[methane]

Another idea. Limit unicode length to 56bit on 64bit platform.
Put state+length into 8bytes.