[PATCH] D60479: [MemorySSA] Small fix for the clobber limit.

[Alina Sbirlea via Phabricator via llvm-commits]

asbirlea created this revision.
asbirlea added a reviewer: george.burgess.iv.
Herald added subscribers: Prazek, jlebar.
Herald added a project: LLVM.

After introducing the limit for clobber walking, `walkToPhiOrClobber` would assert that the limit is at least 1 on entry.
The test included triggered that assert.

The callsite in `tryOptimizePhi` making the calls to `walkToPhiOrClobber` is structured like this:

  while (true) {
     if (getBlockingAccess()) { // calls walkToPhiOrClobber
     }
     for (...) {
       walkToPhiOrClobber();
     }
  }

The cleanest fix is to check if the limit was reached inside `walkToPhiOrClobber`, and give an allowence of 1.
This approach not make any alias() calls (no calls to instructionClobbersQuery), so the performance condition is enforced.
The limit is set back to 0 if not used, as this provides info on the fact that we stopped before reaching a true clobber.


Repository:
  rL LLVM

https://reviews.llvm.org/D60479

Files:
  lib/Analysis/MemorySSA.cpp
  test/Analysis/MemorySSA/reduce_clobber_limit.ll

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