[llvm-bugs] [Bug 30958] New: LoopDistribute domtree handling gives eternal loop

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https://llvm.org/bugs/show_bug.cgi?id=30958

            Bug ID: 30958
           Summary: LoopDistribute domtree handling gives eternal loop
           Product: tools
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: opt
          Assignee: unassignedbugs at nondot.org
          Reporter: jesper.antonsson at ericsson.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Created attachment 17572
  --> https://llvm.org/bugs/attachment.cgi?id=17572&action=edit
Small program provoking the bug

Opt gets stuck in an eternal loop with the attached small program, using these
flags:

opt -simplifycfg -loop-distribute -instcombine loopdist_simplified.ll

The reason is that the LoopDistribute pass mess up the dominator tree and
introduce a cycle, so when the instcombiner needs to walk the dominator tree it
gets stuck in a tight eternal loop:

Program received signal SIGINT, Interrupt.
llvm::DominatorTreeBase<llvm::BasicBlock>::dominatedBySlowTreeWalk
(this=0x343d570, A=0x345e5e0, B=0x3455b70)
    at ../include/llvm/Support/GenericDomTree.h:220
220          B = IDom; // Walk up the tree

If we use this command line:
opt -loop-distribute -verify-dom-info loopdist_simplified.ll

... the verifier complains:

DominatorTree for function: f is not up to date!
Computed:
=============================--------------------------------
Inorder Dominator Tree: DFSNumbers invalid: 0 slow queries.
  [1] %0 {0,5}

Actual:
=============================--------------------------------
Inorder Dominator Tree: 
  [1] %0 {0,17}
    [2] %bb1 {1,16}
      [3] %bb2.lver.check {2,15}
        [4] %bb2.ph.lver.orig {3,6}
          [5] %bb2.lver.orig {4,5}
        [4] %bb2.ph.ldist1 {7,14}
          [5] %bb2.ldist1 {8,13}
            [6] %bb2.ph {9,12}
              [7] %bb2 {10,11}

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