[clang] [llvm] [CGData][ThinLTO] Global Outlining with Two-CodeGen Rounds (PR #90933)

[Teresa Johnson via llvm-commits]

teresajohnson wrote:

> > @teresajohnson Do you have any concern or comment on this direction?
> 
> Just a quick reply to say that I am taking a look today (unfamiliar with this approach so need to read through the RFC etc) and will get back later today

Sorry I was obviously over optimistic on when I could do this! A couple of high level comments / questions:

- Looking at the NFC, this seems like it has very similar issues to Propeller, which wants to redo just the codegen with a new injected profile and BB ordering. It would be good to see if we can converge to similar approaches. I asked @rlavaee to take a look and he is reading through the background on this work. @rlavaee do you think Propeller could use a similar approach to this where it saves the pre-codegen bitcode and re-loads it instead of redoing opt? This isn't necessarily an action item for this PR, but I wanted Rahman to take a look since he is more familiar with codegen.

- I think this should be doable to implement with distributed ThinLTO if we want in the future, from what I can tell. But we'll need a way to save the bitcode before codegen from a non-in-process backend (e.g. the thinBackend invoked from clang). Not asking you to do anything for this PR on that, but just thinking through it. Seems doable...

- My biggest concern about this patch as written is whether it will break down under LTO's caching mechanism - have you tested it with caching? It seems like it might just skip the backend completely since you aren't adding anything to the cache key.



https://github.com/llvm/llvm-project/pull/90933