I would think it's fairly easy to keep all calculations in C in 128-bit and just mask out the top few bytes when retrieving and storing. You could also argue th

I would think it's fairly easy to keep all calculations in C in 128-bit and just mask out the top few bytes when retrieving and storing. You could also argue that >64 bit values will be rare enough that they warrant their own code path if they are encountered as an optimization (perhaps they already do this?).