[llvm-bugs] [Bug 52093] New: [x86] parity op gets transformed to popcount

via llvm-bugs llvm-bugs at lists.llvm.org
Wed Oct 6 10:06:56 PDT 2021


            Bug ID: 52093
           Summary: [x86] parity op gets transformed to popcount
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: spatel+llvm at rotateright.com
                CC: craig.topper at gmail.com, llvm-bugs at lists.llvm.org,
                    llvm-dev at redking.me.uk, pengfei.wang at intel.com,
                    spatel+llvm at rotateright.com

define i32 @xor4_popcount(i32 %x) {
  %m = and i32 %x, 15
  %p = tail call i32 @llvm.ctpop.i32(i32 %m)
  %r = and i32 %p, 1
  ret i32 %r

declare i32 @llvm.ctpop.i32(i32)

% llc -o - pop.ll              
        xorl    %eax, %eax
        testb   $15, %dil
        setnp   %al

% llc -o - pop.ll -mattr=popcnt
        andl    $15, %edi
        popcntl %edi, %eax
        andl    $1, %eax


Debug spew shows that we convert to a parity node either way, but then convert
back to ctpop for a target that supports that instruction.

Test and set likely has better latency/throughput than popcnt + 2 mask
instructions on all recent x86 CPUs.

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