[PATCH] D19743: [SimplifyCFG] Prefer a simplification based on dominating conditions over merging a common destination.

[Chad Rosier via llvm-commits]

mcrosier created this revision.
mcrosier added reviewers: reames, sanjoy, chandlerc, echristo, gberry.
mcrosier added a subscriber: llvm-commits.
Herald added a subscriber: joker.eph.

Often we can simplify the CFG based on dominating conditions.  However, we miss opportunities when we merge blocks with a common destination.

For example, before this change we would need to run -simplifycfg -instcombine -simplifycfg on test4_fold to arrive at the code in test4_no_fold after only a single run of -simplifycfg.

I found a case in Spec2006/soplex where this further removed unreachable code.  As expected, I found no cases where code size increased across Spec2000/Spec2006.  As isImpliedCondition continues to expand in capabilities I expect more opportunities to be exposed.  This may also improve compile-time, but I didn't measure the impact directly.

 Chad

http://reviews.llvm.org/D19743

Files:
  lib/Transforms/Utils/SimplifyCFG.cpp
  test/Transforms/SimplifyCFG/basictest.ll

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