When Compilers Disagree About UTF‑8

In the summer 2006 I started an open-source C++ library for handling UTF-8 strings. I wanted it to be portable and to work well with STL but did not go crazy with optimizations at the time.

Recently, as I started writing about UTF-8 decoding, I spent more time revisiting the internals of the library. The function I decided to optimize decodes a UTF-8 encoded code point:

    template 
    utf_error validate_next(octet_iterator& it, octet_iterator end, utfchar32_t& code_point)
    {
        if (it == end)
            return NOT_ENOUGH_ROOM;

        // Save the original value of it so we can go back in case of failure
        // Of course, it does not make much sense with i.e. stream iterators
        octet_iterator original_it = it;

        utfchar32_t cp = 0;
        // Determine the sequence length based on the lead octet
        const int length = utf8::internal::sequence_length(it);

        // Get trail octets and calculate the code point
        utf_error err = UTF8_OK;
        switch (length) {
            case 0:
                return INVALID_LEAD;
            case 1:
                err = utf8::internal::get_sequence_1(it, end, cp);
                break;
            case 2:
                err = utf8::internal::get_sequence_2(it, end, cp);
            break;
            case 3:
                err = utf8::internal::get_sequence_3(it, end, cp);
            break;
            case 4:
                err = utf8::internal::get_sequence_4(it, end, cp);
            break;
        }

        if (err == UTF8_OK) {
            // Decoding succeeded. Now, security checks...
            if (utf8::internal::is_code_point_valid(cp)) {
                if (!utf8::internal::is_overlong_sequence(cp, length)){
                    // Passed! Return here.
                    code_point = cp;
                    ++it;
                    return UTF8_OK;
                }
                else
                    err = OVERLONG_SEQUENCE;
            }…