[ncmncm]

More prediction is not, in fact, always better.

Specifically, a prediction is not better when it makes no difference. Then, it is worse, because it consumes a resource that would better be applied in a different place where it could make a difference.

Exceptions are the perfect example of a case where any prediction expenditure would be wasted; except predicting that no exception will be thrown. It is always better to predict no exception is thrown, because only the non-throwing case can benefit.

Running ahead and pre-computing results that would be thrown away if an exception is thrown is a pure win: With no exception, you are far ahead; with an exception, there is no useful work to be sped up, it is all just overhead anyway.

This is similar to a busy-wait: you are better off to have leaving the wait predicted, because that reduces your response latency, even though history predicts that you will continue waiting. This is why there is now a special busy-wait instruction that does not consume a branch prediction slot. (It also consumes no power, because it just sleeps until the cache line being watched shows an update.)