> Unfortunately no C compiler can do this optimization automatically:

> The functions recip and recip⁺ and not equivalent.

This is one of those examples of how optimizing code can improve legibility, robustness, or both.

The first implementation allows for side effects to change the outcome of the function. But the problem is that the code is not written expecting someone to modify the values in the middle of the loop. It's incorrect behavior, and you're paying a performance penalty for it to boot.

Functional Core code tends not to have this problem, in that we pass in a snapshot of data and it either gets an answer or an error.

I've seen too much code that checks 3 times if a user is either still logged in or has permission to do a task, and not one of them was set up to deal with one answer for the first call and a different one for any of the subsequent ones. They just go into undefined behavior.