[llvm-bugs] [Bug 42391] New: InstCombine: shift amount reassociation

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=42391

            Bug ID: 42391
           Summary: InstCombine: 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

https://godbolt.org/z/S9Zn-y
https://rise4fun.com/Alive/plLm

int bad2(unsigned diff, unsigned len1, unsigned len2) {
    return ((diff >> (32-len1)) >> (len1 - 1));
}
int good2(unsigned diff, unsigned len1, unsigned len2) {
    return diff >> ((32-len1)+(len1-1));
}

Name: dd
Pre: C > 0 ; signed positive
  %4 = sub i32 32, %1
  %5 = lshr i32 %0, %4
  %6 = sub i32 %1, C
  %r = lshr i32 %5, %6
  =>
  %z4 = sub i32 32, C
  %r = lshr i32 %0, %z4

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