[PATCH] D112313: [DSE] Optimize defining access of defs while walking upwards.

[Nikita Popov via Phabricator via llvm-commits]

nikic requested changes to this revision.
nikic added a comment.
This revision now requires changes to proceed.

Marking as changes requested per above comment.

As a more general comment, I'm somewhat concerned about setting and accessing the optimized access outside the caching MSSA walker, as it may result in unexpected behavior changes. E.g. if you run a pass before DSE that uses the walker and which sets an optimized access, but would not be set by this code (e.g. because it requires looking through a MemoryPhi), then that optimized access would be used by D112315 <https://reviews.llvm.org/D112315>. If you did not run the pass beforehand, it wouldn't be used. Conversely, if another pass runs after DSE and uses the caching walker, it will now return the access optimized by DSE, but will it always be the same as the one produced by the walker? Without looking into it in detail, I'm going to guess "no", they can differ due to e.g. different cutoffs. Of course, behavior can already be influenced by adjacent passes because MSSA updating is imprecise (in the sense that it is not equivalent to rebuilding MSSA from scratch), but I think this may make things worse in that a pass dependency would exist even if IR is not modified -- only caches in MSSA are populated/used in an inconsistent manner.


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CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D112313/new/

https://reviews.llvm.org/D112313