Stuffing the return stack buffer

Stuffing the return stack buffer

Actually it is somewhat surprising. CPU designer often put in "chicken bits" for disabling new microarchitectural features, in case they turn out to be buggy. You can then still sell the CPU instead of having to scrap it. And some of these chicken bits have been known to be used over the years (and probably many more were used before the CPUs were released, and the public never heard of them).

But when they designed this fallback from the return stack buffer to the history-based indirect branch predictor into Skylake, they apparently did not put a chicken bit for that in, probably because history-based indirect branch prediction had been present in Intel CPUs for many generations.