[llvm-bugs] [Bug 52092] New: convert shifts and logic to popcount

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=52092

            Bug ID: 52092
           Summary: convert shifts and logic to popcount
           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

define i32 @src(i32 %0) {
  %2 = lshr i32 %0, 1
  %3 = xor i32 %2, %0
  %4 = lshr i32 %0, 2
  %5 = xor i32 %3, %4
  %6 = lshr i32 %0, 3
  %7 = xor i32 %5, %6
  %8 = and i32 %7, 1
  ret i32 %8
}

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)

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

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

The number of masked bits is not limited to 4 specifically, and those bits
don't have to be sequential. There's likely some related transforms for 'and'
or 'or' rather than 'xor'.

This may not be a win in codegen for a target that doesn't support popcount, so
backend work may be needed if this is implemented in IR. 

Since the pattern is not limited to a fixed number of bits/ops, this could be
tried in AggressiveInstCombine to avoid compile-time concerns.

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