[pm215]

Wikipedia reckons 10-20 cycle penalty for a branch mispredict (which sounds plausible given it fills the pipeline with useless junk). Given that branches are quite common, that's painful enough to want to avoid, but definitely not so painful that you'd want to devote as much silicon to solving it as you do to, say, L1 cache.

I do recall a bit of research (published by a Nokia R&D team I think) that reckoned you could get a mostly-ok performance estimate by tracking about half a dozen indicators including instructions executed, cache misses, tlb misses and brancb mispredicts and weighting them appropriately. The trouble there is nobody wants a performance model that's right 90% of the time but significantly wrong 10% of the time with no way to tell if the workload you want to test is in the 10%. But it's an indication of the importance of branch prediction still, I think.