Hash#compare_by_identity

VALUE
rb_hash_compare_by_id(VALUE hash)
{
    VALUE tmp;
    st_table *identtable;

    if (rb_hash_compare_by_id_p(hash)) return hash;

    rb_hash_modify_check(hash);
    if (hash_iterating_p(hash)) {
        rb_raise(rb_eRuntimeError, "compare_by_identity during iteration");
    }

    if (RHASH_AR_TABLE_P(hash)) {
        unsigned int bound = RHASH_AR_TABLE_BOUND(hash);
        for (unsigned int i = 0; i < bound; i++) {
            if (ar_cleared_entry(hash, i)) continue;

            ar_table_pair *pair = RHASH_AR_TABLE_REF(hash, i);
            ar_hint_set(hash, i, (st_hash_t)rb_ident_hash(pair->key));
        }
    }
    else if (RHASH_TABLE_EMPTY_P(hash)) {
        // Fast path: There's nothing to rehash, so we don't need a `tmp` table.
        HASH_ASSERT(RHASH_ST_TABLE_P(hash));

        RHASH_ST_TABLE(hash)->type = &identhash;
    }
    else {
        // Slow path: Need to rehash the members of `self` into a new
        // `tmp` table using the new `identhash` compare/hash functions.
        tmp = hash_alloc_capa(0, 0);
        FL_SET_RAW(tmp, RHASH_COMPARE_BY_IDENTITY);
        hash_st_table_init(tmp, RHASH_SIZE(hash));
        identtable = RHASH_ST_TABLE(tmp);

        rb_hash_foreach(hash, rb_hash_rehash_i, (VALUE)tmp);
        rb_hash_free(hash);

        // We know for sure `identtable` is an st table,
        // so we can skip `ar_force_convert_table` here.
        rb_hash_st_table_set(hash, identtable);
        RHASH_ST_CLEAR(tmp);
    }

    FL_SET_RAW(hash, RHASH_COMPARE_BY_IDENTITY);

    rb_gc_register_pinning_obj(hash);

    return hash;
}