[llvm-bugs] [Bug 41020] New: isBitwiseNot / isConstOrConstSplat etc don't support build vector truncation

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=41020

            Bug ID: 41020
           Summary: isBitwiseNot / isConstOrConstSplat etc don't support
                    build vector truncation
           Product: libraries
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: nikita.ppv at gmail.com
                CC: llvm-bugs at lists.llvm.org

The isBitwiseNot() helper currently misses cases where a build_vector of
implicitly truncated constants is used. E.g. an all-ones vNi16 build vector on
AArch64 will use i32 elements of value 65535.

isBitwiseNot() is based on isConstOrConstSplat(), which is where this
limitation ultimately comes from.

It would be possible to easily fix isBitwiseNot() specifically by using the
ISD::isBuildVectorAllOnes() helper (which does handle truncation correctly),
but it would be nice to support this for the whole isConstOrConstSplat()
machinery.

See https://reviews.llvm.org/D59174 for a case where we run into this issue.

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