[llvm-bugs] [Bug 40800] New: [SelectionDAG] Add UNDEF handling to SelectionDAG::FoldSetCC

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=40800

            Bug ID: 40800
           Summary: [SelectionDAG] Add UNDEF handling to
                    SelectionDAG::FoldSetCC
           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: llvm-dev at redking.me.uk
                CC: llvm-bugs at lists.llvm.org, spatel+llvm at rotateright.com

SelectionDAG::FoldSetCC currently only folds if both sides are known constants,
but we should be able to handle some cases where one/both sides are UNDEF.

This was noticed while extending SimplifyDemandedVectorElts to support shift
ops - creating UNDEF shift amounts prevented shift by out-of-range/zero/one
tests from being constant folded in X86ISelLowering's LowerShift.

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