[Suncho]

> each register's carry bits are independent from the ones that get carries added from the last register

No they're not. Let's say B's non-carry bits are all 1. If you carry anything from C, that will affect B's carry bits.

[londons_explore]

You could check if the middle 64-12-12 bits are all 1's, and if they are branch to a slow codepath.

It would be an incredibly rare case, so branch prediction will always get it right, and the cost of a branch nearly nill.

[a1369209993]

If you're trying to add two 256-bit numbers, there's probably cryptography involved, and data-dependant branches are poison to crypto code.