[llvm] [llvm-profgen] Support creating profiles of arbitrary events (PR #99026)

[via llvm-commits]

WenleiHe wrote:

@tcreech-intel @williamweixiao I was there at the EuroLLVM talk. While I appreciate the work you guys are doing, I think what's missing is practical result beyond hand-crafted microbenchmark. It's one thing to explore ideas with prototype leveraging LLVM project, and then a different discussion when it comes to making prototypes into LLVM project itself, which means the entire community will be committed to carrying and maintaining such features/optimizations going forward. 

For that to happen, generally we need 1) practical benefit demonstrated beyond novelty, beyond toy-program showcasing the idea, 2) we need to have a coherent design including its interaction with other existing features. We we have a ton of prototypes that we didn't upstream too, including PEBS value profiling work.

> We plan to prepare an RFC in the near future as requested. In the very short term our intent was to provide the branch mispredict parts of HWPGO with LLVM19 as a follow-up to our EuroLLVM presentation on HWPGO. However, that is not to say that we mean to rush the review process.

It's pointless to send RFC after the fact. The purpose of RFC is to seek feedback, build consensus on worthiness of feature/optimization, and also on design/implementation, all before anything is committed upstream. This is a convention of how everyone works in LLVM community.

I'd like to suggest pausing committing any of these changes related to HWPGO before RFC is out and consensus is built.

https://github.com/llvm/llvm-project/pull/99026