[clang] [llvm] [PGO] Add ability to mark cold functions as optsize/minsize/optnone (PR #69030)

[Arthur Eubanks via cfe-commits]

aeubanks wrote:

> > > Good example. This pass should be run post-inline. @aeubanks, any reason we want to run it early in the pipeline?
> > 
> > 
> > We want the main function simplification pipeline to see these function attributes because some optimizations trigger or don't trigger depending on the presence of the attributes. Modifying function attributes is typically done in CGSCC/module passes since doing so can affect what callers of those functions see (in effect changing other functions), which shouldn't happen in function passes. I suppose it's possible to add this as a CGSCC pass that runs after inlining and before the function simplification pipeline, but this is more of a one time thing and CGSCC passes can revisit functions. So this pass makes the most sense as a module pass, but we can't insert a module pass between inlining and the function simplification pipeline.
> > Can/does the inliner ignore these size attributes when it has call-site profile information?
> 
> Looking at the current change, this new pass is actually after the sample loader (including sample loader inlining) pass, so wenlei@'s concern should be addressed.

Oh I forgot that the sample profile has its own inliner. Yes this pass runs after we load profiling information since it uses profiling information, whether it's sample or instrumented.

Is this patch good to go?

https://github.com/llvm/llvm-project/pull/69030