[llvm-bugs] [Bug 40394] New: [DAGCombiner] crash when forming shuffle from build vector

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=40394

            Bug ID: 40394
           Summary: [DAGCombiner] crash when forming shuffle from build
                    vector
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: spatel+llvm at rotateright.com
                CC: llvm-bugs at lists.llvm.org

This will crash using llc built from trunk r351752:

target triple = "aarch64-unknown-linux-gnu"

define <4 x i32> @truncating_build_vector_crash(<4 x i16> %t0) {
  %t1 = extractelement <4 x i16> %t0, i32 2
  %vgetq_lane = zext i16 %t1 to i32
  %t2 = insertelement <4 x i32> undef, i32 %vgetq_lane, i64 0
  ret <4 x i32> %t2
}

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The bug was introduced with:
https://reviews.llvm.org/D56281 / https://reviews.llvm.org/rL351198

I committed a fix that limits the transform to avoid the crash here:
https://reviews.llvm.org/rL351753

I think we should pull that commit into the 8.0 release to avoid the crash.

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