[llvm] [CodeGenPrepare] Cache known-live PHIs when deleting dead PHI chains (PR #207191)

[via llvm-commits]

Jolyon0202 wrote:

> > CodeGenPrepare enables the cache only for huge functions. Other callers keep
> > the default nullptr behavior.
> 
> Why?

Thanks for review. In the CodeGenPrepare pass, we traverse all basic blocks and invoke DeleteDeadPHIs for each block. This process performs an in-depth scan of cross-BB PHI dependency chains. In this pass, we first determine whether a function is a huge function; if so, we pass a nullptr KnownNonDeadPHIsPtr to enable this optimization. Other Caller don’t seem to carry such high complexity（loop header or bbchanged）, but they also have this risk. For this reason, I added a local localKnownNonDeadPHIs variable inside DeleteDeadPHIs to optimize this workflow as well. What do you think? @nikic 



https://github.com/llvm/llvm-project/pull/207191