[PATCH] D70104: [BPI] Improve unreachable/ColdCall heurstics to handle loops.

[Taewook Oh via Phabricator via llvm-commits]

twoh created this revision.
twoh added reviewers: skatkov, chandlerc.
Herald added subscribers: hiraditya, qcolombet.
Herald added a project: LLVM.
twoh updated this revision to Diff 228783.
twoh added a comment.

Add "CHECK" for the test.


While updatePostDominatedByUnreachable attemps to find basic blocks that are post-domianted by unreachable blocks, it currently cannot handle loops precisely, because it doesn't use the actual post dominator tree analysis but relies on heuristics of visiting basic blocks in post-order. More precisely, when the entire loop is post-dominated by the unreachable block, current algorithm fails to detect the entire loop as post-dominated by the unreachable because when the algorithm reaches to the loop latch it fails to tell all its successors (including the loop header) will "eventually" be post-domianted by the unreachable block, because the algorithm hasn't visited the loop header yet. This makes BPI for the loop latch to assume that loop backedges are taken with 100% of probability. And because of this, block frequency info sometimes marks virtually dead loops (which are post dominated by unreachable blocks) super hot, because 100% backedge-taken probability makes the loop iteration count the max value. updatePostDominatedByColdCall has the exact same problem as well.

To address this problem, this patch makes PostDominatedByUnreachable/PostDominatedByColdCall to be computed with the actual post-dominator tree.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D70104

Files:
  llvm/include/llvm/Analysis/BranchProbabilityInfo.h
  llvm/lib/Analysis/BranchProbabilityInfo.cpp
  llvm/test/Analysis/BranchProbabilityInfo/basic.ll
  llvm/test/Analysis/BranchProbabilityInfo/noreturn.ll
  llvm/test/CodeGen/X86/block-placement.ll
  llvm/test/CodeGen/X86/pr37916.ll
  llvm/test/CodeGen/X86/ragreedy-hoist-spill.ll

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