[PATCH] D86486: [DSE,MemorySSA] Limit walker steps.

[Florian Hahn via Phabricator via llvm-commits]

fhahn created this revision.
fhahn added reviewers: dmgreen, efriedma, bryant, asbirlea, george.burgess.iv.
Herald added subscribers: dexonsmith, hiraditya, Prazek.
Herald added a project: LLVM.
fhahn requested review of this revision.

For DSE with MemorySSA it is beneficial to only use a single step with
the MemorySSA walker, so we can better control the number of steps
together with other limits and also weed out invalid/unprofitable paths
early on.

This patch requires a follow-up patch to be most effective, which I will
share soon after putting this patch up.

This temporarily XFAIL's the limit tests, because we now explore more
MemoryDefs that may not alias/clobber the killing def. This will be
improved/fixed by the follow-up patch.

This patch allows us to aggressively cut down compile time, geomean
-O3 -0.64%, ReleaseThinLTO -1.65%, at the expense of fewer stores
removed. Subsequent patches will increase the number of removed stores
again, while keeping compile-time in check.

http://llvm-compile-time-tracker.com/compare.php?from=d8e3294118a8c5f3f97688a704d5a05b67646012&to=0a929b6978a068af8ddb02d0d4714a2843dd8ba9&stat=instructions

The new WalkerStepLimit might warrant an additional option.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D86486

Files:
  llvm/include/llvm/Analysis/MemorySSA.h
  llvm/lib/Analysis/MemorySSA.cpp
  llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
  llvm/test/Transforms/DeadStoreElimination/MSSA/debug-counter.ll
  llvm/test/Transforms/DeadStoreElimination/MSSA/memoryssa-scan-limit.ll
  llvm/test/Transforms/DeadStoreElimination/MSSA/multiblock-multipath.ll

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