[llvm-bugs] [Bug 24581] New: optnone function results in MachineBasicBlocks with duplicated predecessors

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=24581

            Bug ID: 24581
           Summary: optnone function results in MachineBasicBlocks with
                    duplicated predecessors
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: matze at braunis.de
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

The machine verifier enforces the property that no block is listed twice in the
successor or predecessor list. For a function with the optnone function this is
not always true after instruction selection. Example:

> cat test.ll
define void @bar() #0 {
entry:
  br i1 undef, label %exit, label %exit

exit:
  ret void
}

attributes #0 = { noinline optnone }
> llc -verify-machineinstrs test.ll
...
*** Bad machine code: MBB has duplicate entries in its successor list. ***
...

I'm not sure why the machine verifier enforces this property, without the
optnone attribute the codegenprepare appears to remove those pointless jumps
though I am not sure yet whether this is on purpose to avoid the verification
problems.

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