Insulation from performance concerns also makes your performance model less predictable. Because the ends of optimisations are observable, but their means are not, people resort to “butterfly programming”. As a small example, in ActionScript 3, people used to write their conditionals like this:

    if (a) if (b) {
      ...
    }

Rather than this:

    if (a && b) {
      ...
    }

Because the dumb compiler generated better code for the former.

In the real world, I’ve seen seemingly benign code changes prevent optimisations from firing—optimisations we didn’t know we were relying on. So people start coding to the implementation, rather than the language or the human, without even being able to see what they’re doing. I really don’t like that.

Optimisers are like garbage collectors—they give you better ergonomics than, and equal average performance to the manual alternative, if you’re willing to relinquish predictability and a non-voodoo mental model of your code’s performance.