Comment by duped

In practice, this code will use a variety of target-specific optimizations or compiler intrinsics blocked behind #ifdefs that need to be periodically updated or added for new targets and toolchains. If it refers to any kind of OS-specific APIs (like RNG) then it will also need to be updated from time to time as those APIs change.

That's not to say that code can't change slowly, just the idea that it never changes is extremely rare in practice.