[PATCH] D146928: [NaryReassociate] Transform expression (X << C1) + C2 to (X + C3) << C1,

[Evgeniy via Phabricator via llvm-commits]

ebrevnov added a comment.

As I see it, you are trying to solve pass ordering issue (InstCombine prevents GVN/CSE). But your solution assumes there is GVN/CSE follows NaryReassociation what creates another potential pass ordering dependence.
Another point is the way the transformation is implemented right now doesn't fit intent of NaryReassociation pass. In the heart of NaryReassociation  pass is findClosestMatchingDominator call to find and delete common expressions after reassociation. Since you are not checking for existence of common expressions you can easily do the transformation in some other place (ordinary Reassociate pass, or InstSimplify/InstCombine)
In order to solve both mentioned problems and cover your case you will need to use findClosestMatchingDominator and possibly extend it to look for post dominating expressions as well.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D146928/new/

https://reviews.llvm.org/D146928