[llvm-bugs] [Bug 43595] New: [InstCombine] Failure to recognize sign bit test - shift amount reassociation

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43595

            Bug ID: 43595
           Summary: [InstCombine] Failure to recognize sign bit test -
                    shift amount reassociation
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: lebedev.ri at gmail.com
                CC: llvm-bugs at lists.llvm.org

This is motivated by https://bugs.llvm.org/show_bug.cgi?id=42389
While we succeed to produce sign bit test if we use unsigned:
  https://godbolt.org/z/iSlCpW
We get stuck if we use signed:
  https://godbolt.org/z/VO1-0F

Boo. https://rise4fun.com/Alive/PRU

I'm not sure how to approach this yet.
We can't treat this just as shift amount reassociation,
maybe icmp can be treated as `trunc to i1`.

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