[llvm-bugs] [Bug 52094] New: [InstCombine] failed to set/use popcount range metadata

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=52094

            Bug ID: 52094
           Summary: [InstCombine] failed to set/use popcount range
                    metadata
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: spatel+llvm at rotateright.com
                CC: llvm-bugs at lists.llvm.org

https://alive2.llvm.org/ce/z/c2tUxf

define i32 @src(i32 %0) {
  %2 = and i32 %0, 15
  %3 = tail call i32 @llvm.ctpop.i32(i32 %2)
  %4 = and i32 %3, 5
  %5 = icmp eq i32 %4, 1
  %6 = zext i1 %5 to i32
  ret i32 %6
}

define i32 @tgt(i32 %0) {
  %2 = and i32 %0, 15
  %3 = tail call i32 @llvm.ctpop.i32(i32 %2)
  %4 = and i32 %3, 1
  ret i32 %4
}
declare i32 @llvm.ctpop.i32(i32) #2

---------------------------------------------------------------------------

We fail to recognize this as a parity check which could lead to far worse
codegen (especially for a target that does not have a popcount instruction):

IR corresponds to source like this:

int xor4_popcount(int num) {
    int masknum = num & 0xf;
    int pop = __builtin_popcount(masknum);
    return pop == 1 || pop == 3; 
}

This example is derived from the post-commit discussion in:
https://reviews.llvm.org/D110170

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