[llvm] [SCEV] Try to re-use existing LCSSA phis when expanding SCEVAddRecExpr. (PR #147214)

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

I'm trying to understand the implementation approach here. At a high level, I'd expect the reuse to be for the in-loop expansion, and then reuse of an existing LCSSA phi node to be an optimization during LCSSA fixup. Is the idea here that we can't find existing expansions (that are not part of the ExprValue map) in the loop efficiently, so instead we focus on only the out-of-loop case, because that one only has to inspect LCSSA phi nodes? And this is also limited to addrecs only for the same reason?

https://github.com/llvm/llvm-project/pull/147214