[llvm-bugs] [Bug 43523] New: [InstCombine] Extract high bits + sext -> Extract high bits with sext

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43523

            Bug ID: 43523
           Summary: [InstCombine] Extract high bits + sext -> Extract high
                    bits with sext
           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

https://godbolt.org/z/qhpAcX

int extract_high_bits_with_signext_BAD(unsigned long data, int nbits) {
    return (int(data >> (64-nbits)) << (32-nbits)) >> (32-nbits);
}
int extract_high_bits_with_signext_GOOD(unsigned long data, int nbits) {
    return (signed long)(data) >> (64-nbits);
}

I'm not sure yet what generalization of the pattern is there,
maybe here i can get away with just looking for that specific pattern..

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