[PATCH] D72700: [DSE] Add first version of MemorySSA-backed DSE (Bottom up walk).

[Alina Sbirlea via Phabricator via llvm-commits]

asbirlea accepted this revision.
asbirlea added a comment.
This revision is now accepted and ready to land.

I do think there are outstanding issues that need answers, but I believe the way to make progress is to have an initial good version and iterate on it.

1. The major issue is performance, and to start testing this out we need a working version in tree.

There are many MemorySSA and AA calls in this variant, that we may be able to do better. For example: build a walker to do a single getClobbering with all the preconditions on what instructions are safe to skip, instead of doing this in a loop here.

2. Add all missing cases from add-on patches and get parity or better stores eliminated than current DSE. Merge tests when this happens to make this clear.
3. The performance problem's scope goes beyond DSE. The current pass pipeline for both pass managers has a sequence of (gvn, memcpyopt, dse), where all of these use MemDepAnalysis.  Switching DSE to MemorySSA may initially get worse compile times, as we need to build both MemDepAnalysis and MemorySSA, but switching all three (use newgvn and port memcpyopt and dse), may be worthwhile. This is the long term goal I have in mind.

Thanks for all the work on this!


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D72700/new/

https://reviews.llvm.org/D72700