Writing constant time code is hard. We all know that. But I’m always amazed again on how difficult it is. In preparation for making NSS more constant time I looked into certain CPU instructions that are known to be not constant time. So I wrote a little thing to measure the time (CPU cycles) needed for division. div rcx ; eax is now a/b The CPU I’m using in this post is an Intel i7-4790 (haswell).