[PATCH] D142551: [InstCombine] Teach isDeadPHICycle to look through multiple uses

[Luke Lau via Phabricator via llvm-commits]

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Instead of only detecting trivial cycles which must be a PHI with a single use that's a PHI, which in turn has a single use of the first PHI, generalize it to catch more complex cycles.  This means making it look through all of its uses, as well as looking through arbitrary side-effect free instructions.

The motivation behind this is to enable InstCombiner to remove larger dead PHI cycles in one iteration, to prevent cascading iterations in some pathological cases with lots of dead PHI cycles.

Supersedes D142293 <https://reviews.llvm.org/D142293>
Fixes https://github.com/llvm/llvm-project/issues/50564


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D142551

Files:
  llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp
  llvm/test/Transforms/InstCombine/fmul-inseltpoison.ll
  llvm/test/Transforms/InstCombine/fmul.ll
  llvm/test/Transforms/InstCombine/phi-dead-cycle-iteration.ll
  llvm/test/Transforms/InstCombine/phi-dead-cycle-side-effect.ll
  llvm/test/Transforms/InstCombine/pr27703.ll

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