Comment by haberman

> One thing that people advocating FDO often forget: this is statically tuning the code for a specific use case.

Yes, I meant to mention this but forgot. The numbers I posted are a best-case example for FDO, because the FDO is specific to the one benchmark I'm testing.

> Ok, so feed it with a huge mix of benchmarks that simulate typical usage. But then the profile gets flatter and FDO becomes much less effective.

True, though I think one clear win with FDO is helping the compiler tell fast-paths from slow-paths in each opcode. I would expect this distinction to be relatively universal regardless of workload.

The theory of fast-path/slow-path optimization would say that fast-paths are much more common than slow-paths. Otherwise optimizing fast-paths would be useless because slow-paths would dominate.

The fact that a compiler can't statically tell a fast-path from a slow-path is a big weakness. FDO does seem like it should be able to help mitigate that weakness.