[llvm-bugs] [Bug 34935] New: Cyclic PHI nodes

via llvm-bugs llvm-bugs at lists.llvm.org
Thu Oct 12 16:34:24 PDT 2017


            Bug ID: 34935
           Summary: Cyclic PHI nodes
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: davide at freebsd.org
          Reporter: davide at freebsd.org
                CC: dberlin at dberlin.org, llvm-bugs at lists.llvm.org

define void @patatino() {
  br label %bb1

  %tmp = phi i1 [ undef, %bb ], [ %tmp3, %bb1 ]
  %tmp3 = and i1 %tmp, true
  br label %bb1

This is the bug that we think we defeated, many times, and instead, keeps
coming back.
NewGVN keeps moving instructions between classes and never reaches convergence.
Eventually we hit an assertion because we processed an instruction too many

I'll take a stab at debugging it, but this is a FYI.

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