[jepler]

See my little test program at https://godbolt.org/g/53SAMq

I believe this program properly handles carry from the low to high part.

The 96- and 128-bit code have the same number of instructions, but the 128-bit code has more instruction bytes due to "REX prefixes" (i.e., 32-bit register add is 3 bytes of opcode, 64-bit register add is 4)

[mmozeiko]

Here's a better variant: https://godbolt.org/g/xRtr4i

[jepler]

nice