[llvm-bugs] [Bug 40766] New: Complete implementation of fewerElementsVector legalization method in GlobalISel

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https://bugs.llvm.org/show_bug.cgi?id=40766

            Bug ID: 40766
           Summary: Complete implementation of fewerElementsVector
                    legalization method in GlobalISel
           Product: libraries
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: GlobalISel
          Assignee: unassignedbugs at nondot.org
          Reporter: rtereshin at apple.com
                CC: llvm-bugs at lists.llvm.org, quentin.colombet at gmail.com

It looks like the current version of the fewerElementsVector legalization
method in GlobalISel can handle its inputs iff originalNumberOfElements %
requestedNumberOfElements is either 0 or 1 (the vector could be split into
equally sized parts or the reminder is exactly a single scalar). It's hard to
call such a behavior orthogonal and well expected, and it looks like it may
cause subtle and hard to notice bugs and / or force people to do non-obvious
work-arounds, maybe we need to improve this. See
https://reviews.llvm.org/D58123#inline-516061 and around for more details.

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