Comment by akoboldfrying

Branch prediction is great for predictable branches, which is often what you have, or a good approximation to it. I forget the exact criteria, but even quite old chips could learn, e.g., all repeating patterns of length up to 4, most repeating patterns of length up to 8 and fixed-length loop patterns (n YESes followed by 1 NO) of any length.

Quite often, though, you don't have predictable branches, and then you'll pay half the misprediction cost each time on average. If you're really unlucky, you could hit inputs where the branch predictor gets it wrong more than 50% of the time.