[LLVMbugs] [Bug 18319] New: Function ConstantFoldSelectInstruction doesn't handle vector select constant expression with "undefs"

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=18319

            Bug ID: 18319
           Summary: Function ConstantFoldSelectInstruction doesn't handle
                    vector select constant expression with "undefs"
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: ili.filippov at gmail.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

The problem is reproducible with LLVM trunk.

The reproducer:

; Function Attrs: nounwind
define void @function(<8 x i8>* %RET) #6 {
allocas:
  %m8.i161 = trunc <8 x i16> select (<8 x i1> <i1 true, i1 true, i1 true, i1
true, i1 true, i1 undef, i1 undef, i1 undef>, <8 x i16> <i16 -1, i16 -1, i16
-1, i16 -1, i16 -1, i16 0, i16 0, i16 0>, <8 x i16> zeroinitializer) to <8 x
i8>
  store <8 x i8> %m8.i161, <8 x i8>* %RET
  ret void
}

opt reproducer.ll -instcombine > reproducer.bc

The command fails in assert. This happens because vector select constant
expression containing "undefs" is not transformed to regular vector constant.
And then cast optimization doesn’t know how to work with it.

Particularly, in function ConstantFoldSelectInstruction const vector select is
converted to vector select only if it contains true or false value (which seems
reasonable from the first glance), but it also needs to handle "undef" somehow.

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