[llvm] [VPlan] Thread plan to VPBuilder (NFC) (PR #125364)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> We should always have a valid instruction point when we create a recipe I think. Can we get the plan when needed on demand?

Makes sense, will check why my previous attempt was crashing again.

> Also, it might be worth doing the ConstantFolding in `simplifyRecipe`, so we catch opportunities after RAUW & co?

Hm, actually I'm not too happy with `simplifyRecipe`: it's currently an ad-hoc way of eliminating trivial patterns that LV introduces. There is currently no disciplined way to figure out which trivial patterns are actually introduced by LV, and I think we have written `simplifyRecipe` by inspecting various outputs. Besides it has other problems like not recursively simplifying patterns (I tried to fix this earlier, if you recall).

What would it mean to write a constant-folder in `simplifyRecipe`? We'd have to match patterns of constants and RAUW them with a constant value. I think the current approach has the advantage of being quite clean, in comparison.

For these reasons, I'm inclined to not touch `simplifyRecipe`, and proceed with the current approach of intercepting create-calls.

https://github.com/llvm/llvm-project/pull/125364