[PATCH] D15857: [InstCombine] Defend against worst-case exponential execution time

[James Molloy via llvm-commits]

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It is possible to create an IR pattern that results in exponential execution
time in CollectBitParts, by making a tree where every OR refers to one other OR via *both*
operands:

  for (i = 0; i < N; ++i)
      b |= b >> 1;  // Causes 2^N executions of CollectBitParts!

To defend against this (and because ORs are the only node that cause us to
fork control flow) we keep a counter alive between all invocations of
CollectBitParts, expected to be initialized to bitwidth(b).

Repository:
  rL LLVM

http://reviews.llvm.org/D15857

Files:
  lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
  test/Transforms/InstCombine/bitreverse-hang.ll

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