[llvm-bugs] [Bug 43898] New: [PeepholeOptimizer] foldRedundantCopy breaks if Copy is changed by TII.foldImmediate

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43898

            Bug ID: 43898
           Summary: [PeepholeOptimizer] foldRedundantCopy breaks if Copy
                    is changed by TII.foldImmediate
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: alex.davies at iinet.net.au
                CC: llvm-bugs at lists.llvm.org

For all MIs per basic block:

  foldRedundantCopy tracks copy instructions and places them in DenseMap
CopyMIs.

  foldImmediate tracks isMoveImm, and calls foldImmediate on uses of these.
foldImmediate is allowed to change the desc of UseMI - see
ARMBaseInstrInfo.cpp.

On seeing the description for "foldImmediate", it seems logical to optimize
copy instructions - but if you try to do so, it's only a matter of time until
compilation fails, as foldRedundantCopy does not verify that instructions in
CopyMI is still actually a copy instruction.

I figure either (1) do not call foldImmediate on COPYs, if it is recommended
that targets do not perform this optimization here, or (2) add:

  if (!PrevCopy->isCopy())
    return false;

to line 1410 of PeepholeOptimizer.cpp

Cheers :)

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