Another round of speculative-execution vulnerabilities

Another round of speculative-execution vulnerabilities

Even with PGO, control flow still has largely unpredictable regions, which depend upon the details of user input, and can only be predicted at compile time if the exact input the user will use is provided at compile time. This was one component of why Itanium's EPIC never lived up to its performance predictions; as compilers got better at exploiting compile-time known predictability, they also benefited OoO and speculative execution machines, which could exploit predictability that only appears at runtime.

For example, in a H.264 encoder or decoder, black bars are going to send your binary down a highly predictable codepath doing the same thing time and time again; your PGO compiled binary is not going to be set up on the assumption of black bars, because that's just one part of the sorts of input you might get. However, at runtime, the CPU will notice that you're going down the same codepath over and over again as you handle the black bars, and will effectively optimize further based on the behaviour right now. Once you get back to the main picture, it'll change the optimizations it's applying dynamically, because you're no longer going down that specific route through the code.