Comment by Tiddles-the2nd
19 hours ago
The check isn't important; what's important is being predictable so the CPU can guess which way the check will go. I don't know exactly how it works, but after the first couple of loops, the predictor will assume it's always going to end up in the loop and make that the fast path. It may guess wrong the first couple of loops, and the last check wrong, but the other 997 will be correct.
There is a static branch predictor that is used if there is no statistic on a branching instruction yet, and it's really simple: Jumps backward are assumed to be taken (they usually are from a loop), jumps forward are assumed to be not taken.
So the jump that forms the loop will be predicted correctly for all executions but the very last (when the loop ends).
That's very cute.
I wonder how much more complicated and effective statistical predictors are.
They get much more complicated, but their effectiveness tops out where certain branches just can’t be predicted in advance.