When implementing bignums on x86, obviously the most efficient choice for digit size is 32 bits. However, you need arithmetic up to twice the digit size (i.e. 32+32=33, 32\*32=64, 64/32=32). Fortunately, not only does x86 provide this, but it's also accessible from portable C (`uint64_t`).

Similarly, on x64 it would be desirable to use 64-bit digits. This would require 128 bit arithmetic (i.e. 64+64=65, 64\*64=128, 128/64=64). Fortunately, x64 provides this. Unfortunately, it's not accessible from portable C, though obviously one could dip into assembly.

So my question is whether it's accessible from non-portable C. Do any C compilers on x64 provide access to this, and if so, what's the syntax?

(Note that I'm not talking about 128-bit vectors that are strictly treated as collections of 32 or 64-bit words with no carry propagation between them, but about actual 128-bit integer operations.)