Neat, but if you're using this in cryptographic code (one of the main consumers of bignums), keep in mind that secret data reaching branches is usually a side-channel risk. Sure, it's only 1 time in 2^64 on random data, but if you're depending on that, then you have to consider whether an attacker can choose data that will make it happen more often.

If you can substitute a cmov without control flow then it's probably safer, e.g. c1 |= c0 & seq(s1,-1) or so, so long as you can make sure the compiler won't turn it into a branch.

It does add a data dependency though ...