[ack_complete]

PGO also doesn't always optimize in the correct direction. If I have an error handling path in a hot loop, PGO can only optimize around its branch if it actually sees that error occur, and then it will draw wrong conclusions about the branch into the error handler because, absent fudging the tests, it will think the error path has higher importance than it does. I don't want the compiler to optimize for the error path at all, I want it to pessimize it to prioritize the non-error path. But the PGO analysis doesn't know that, it only sees branch patterns and probabilities, and not all error handling paths use exceptions.

PGO is also a pain to use in some situations. You need to be able to regularly exercise all of the main paths in the program under instrumentation, preferably automated, using a configuration as close to release build as possible. That's hard to do when your release build lacks automation support, has nondeterministic behavior by design, cross-compiles to another platform, or requires networked services to exercise main paths. I don't even know how people deal with PGO when there is a requirement for deterministic builds.