[PATCH] D94996: [GVN] do not repeat PRE on failure to split critical edge

[Nick Desaulniers via Phabricator via llvm-commits]

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Fixes an infinite loop encountered in GVN.

GVN will delay PRE if it encounters critical edges, and attempt to split
them later via calls to SplitCriticalEdge(), then restarting.

The caller of GVN::splitCriticalEdges() assumed a return value of true
meant that critical edges were split, that the IR had changed, and that
PRE should be re-attempted, upon which we loop infinitely.

This was exposed after D88438 <https://reviews.llvm.org/D88438>, by compiling the Linux kernel for s390,
but the test case is reproducible on x86.

Fixes: https://github.com/ClangBuiltLinux/linux/issues/1261


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D94996

Files:
  llvm/lib/Transforms/Scalar/GVN.cpp
  llvm/test/Transforms/GVN/critical-edge-split-failure.ll

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