[llvm-bugs] [Bug 27523] New: loop-unroll and simplifycfg cause oscillation with phi node arguments

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=27523

            Bug ID: 27523
           Summary: loop-unroll and simplifycfg cause oscillation with phi
                    node arguments
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Loop Optimizer
          Assignee: unassignedbugs at nondot.org
          Reporter: nwilson20 at gmail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Created attachment 16269
  --> https://llvm.org/bugs/attachment.cgi?id=16269&action=edit
scfg_loop_unroll.ll

When using the flags -simplifycfg and -loop-unroll on the attached IR
(scfg_loop_unroll.ll) the phi nodes will keep flipping. This was produced by
using the attached IR and using the output as input back to opt.

For example:
$./opt -simplifycfg -loop-unroll -S scfg_loop_unroll.ll > out1.ll
$ ./opt -simplifycfg -loop-unroll -S out1.ll > out2.ll
$./opt -simplifycfg -loop-unroll -S out2.ll > out3.ll


$diff out2.ll out1.ll
1c1
< ; ModuleID = 'out1.ll'
---
> ; ModuleID = 'simplifycfg_loop-unroll_osc.ll'
130c130
<   %.ph = phi i32 [ %3, %if.then20 ], [ undef, %if.end27 ]
---
>   %.ph = phi i32 [ undef, %if.end27 ], [ %3, %if.then20 ]


$diff out3.ll out2.ll
1c1
< ; ModuleID = 'out2.ll'
---
> ; ModuleID = 'out1.ll'
130c130
<   %.ph = phi i32 [ undef, %if.end27 ], [ %3, %if.then20 ]
---
>   %.ph = phi i32 [ %3, %if.then20 ], [ undef, %if.end27 ]

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