template <endianness big_endian>
std::pair<const char *, char16_t *>
sse_convert_latin1_to_utf16(const char *latin1_input, size_t len,
                            char16_t *utf16_output) {
  size_t rounded_len = len & ~0xF; // Round down to nearest multiple of 16
  for (size_t i = 0; i < rounded_len; i += 16) {
    // Load 16 Latin1 characters into a 128-bit register
    __m128i in =
        _mm_loadu_si128(reinterpret_cast<const __m128i *>(&latin1_input[i]));
    __m128i out1 = big_endian ? _mm_unpacklo_epi8(_mm_setzero_si128(), in)
                              : _mm_unpacklo_epi8(in, _mm_setzero_si128());
    __m128i out2 = big_endian ? _mm_unpackhi_epi8(_mm_setzero_si128(), in)
                              : _mm_unpackhi_epi8(in, _mm_setzero_si128());
    // Zero extend each Latin1 character to 16-bit integers and store the
    // results back to memory
    _mm_storeu_si128(reinterpret_cast<__m128i *>(&utf16_output[i]), out1);
    _mm_storeu_si128(reinterpret_cast<__m128i *>(&utf16_output[i + 8]), out2);
  }
  // return pointers pointing to where we left off
  return std::make_pair(latin1_input + rounded_len, utf16_output + rounded_len);
}