Comment by MindSpunk

Absolutely, I agree. I suspect that explicit branch for the hot path is doing a lot too.

Separating the hot path into a prefix before calling into a separate cold function should still generate better code. Your prefix only needs to allocate registers and stack space for just that single path. You would only pay the spilling costs in the old code off the hot path rather than every instruction. And I would expect the branch prediction accuracy of that prefix check to be higher than having the hot and cold paths all dispatching through the same tree of branches.

However it's speculation until you measure so I could be wrong.

Good article in any case, I enjoyed reading along.