bpo-46541: Replace core use of _Py_IDENTIFIER() with statically initialized global objects.

[ericsnowcurrently]

In this PR we're no longer using _Py_IDENTIFIER() (or _Py_static_string()) in any core CPython code. It is still used in a number of non-builtin stdlib modules.

The replacement is: PyUnicodeObject (not pointer) fields under _PyRuntimeState, statically initialized as part of _PyRuntime. A new _Py_GET_GLOBAL_IDENTIFIER() macro facilitates lookup of the fields (along with _Py_GET_GLOBAL_STRING() for non-identifier strings).

https://bugs.python.org/issue46541#msg411799 explains the rationale for this change.

The core of the change is in:

• (new) Include/internal/pycore_global_strings.h - the declarations for the global strings, along with the macros
• Include/internal/pycore_runtime_init.h - added the static initializers for the global strings
• Include/internal/pycore_global_objects.h - where the struct in pycore_global_strings.h is hooked into _PyRuntimeState
• Tools/scripts/generate_global_objects.py - added generation of the global string declarations and static initializers

I've also added a --check flag to generate_global_objects.py (along with make check-global-objects) to check for unused global strings. That check is added to the PR CI config.

The remainder of the PR updates the core code to use _Py_GET_GLOBAL_IDENTIFIER() instead of _Py_IDENTIFIER() and the related _Py*Id functions (likewise for _Py_GET_GLOBAL_STRING() instead of _Py_static_string()). This includes adding a few functions where there wasn't already an alternative to _Py*Id(), replacing the _Py_Identifier * parameter with PyObject *.

I'm planning on addressing the following separately:

• stop using _Py_IDENTIFIER() in the stdlib modules
• (maybe) get rid of _Py_IDENTIFIER(), etc. entirely -- this may not be doable as at least one package on PyPI using this (private) API
• (maybe) intern the strings during runtime init

https://bugs.python.org/issue46541

[sweeneyde]

Just a half-baked idea: would it be possible to initialize all hashes of global strings at interpreter startup, then eliminate some branching with a special

int
_PyDict_GetItemStrWithHashInitialized(PyDictObject *mp, PyUnicodeObject *key)
{
    assert(PyDict_CheckExact(mp));
    assert(PyUnicode_CheckExact(key));
    Py_hash_t hash = ((PyASCIIObject *)key)->hash;
    assert(hash != -1);
    // Inline _PyDict_GetItem_KnownHash --> _Py_dict_lookup
    PyDictKeysObject *dk = mp->ma_keys;
    Py_ssize_t ix = dictkeys_stringlookup(dk, key, hash);
    if (ix == DKIX_EMPTY) {
        return NULL;
    }
    else if (kind == DICT_KEYS_SPLIT) {
        return mp->ma_values->values[ix];
    }
    else {
        return DK_ENTRIES(dk)[ix].me_value;
    }
}

[markshannon]

Maybe add this to https://github.com/faster-cpython/ideas/discussions?

[ericsnowcurrently]

@markshannon approved this offline.