PopCntBenchmark.cs

using System;
using System.Collections.Generic;
using System.Runtime.CompilerServices;
using System.Text;
using BenchmarkDotNet.Attributes;

namespace PopCountBenchmark
{
    public class Benchmark
    {
        private static readonly ulong[] _Data = GenerateData();

        private static ulong[] GenerateData()
        {
            var result = new ulong[1024];
            var random = new Random(666);
            for (int i = 0; i < result.Length; i++)
            {
                result[i] = (uint) random.Next() | ((ulong) random.Next() << 32);
            }

            return result;
        }

        /// <summary>
        ///     see https://en.wikipedia.org/wiki/Hamming_weight
        /// </summary>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static long BitCount(ulong x)
        {
            x -= (x >> 1) & 0x5555555555555555UL; //put count of each 2 bits into those 2 bits
            x = (x & 0x3333333333333333UL) +
                ((x >> 2) & 0x3333333333333333UL); //put count of each 4 bits into those 4 bits 
            x = (x + (x >> 4)) & 0x0F0F0F0F0F0F0F0FUL; //put count of each 8 bits into those 8 bits 
            return
                (long) ((x * 0x0101010101010101UL) >>
                        56); //returns left 8 bits of x + (x<<8) + (x<<16) + (x<<24) + ... 
        }

        [Params(0x0F0F0F0F0F0F0F0FUL)] public ulong Input { get; set; }

        //[Benchmark]
        //public long BitCount()
        //{
        //    long counter = 0;
        //    for (int i = 0; i < 10000; i++)
        //    {
        //        counter += BitCount(Input);
        //    }

        //    return counter;
        //}

        //[Benchmark]
        //public long PopCount()
        //{
        //    long counter = 0;
        //    for (int i = 0; i < 10000; i++)
        //    {
        //        counter += System.Runtime.Intrinsics.X86.Popcnt.PopCount(Input);
        //    }

        //    return counter;
        //}

        [Benchmark]
        public long PopCountArray()
        {
            long counter = 0;
            for (int i = 0; i < 1024; i++)
            {
                counter += System.Runtime.Intrinsics.X86.Popcnt.PopCount(_Data[i]);
            }

            return counter;

        }

        [Benchmark]
        public long PopCountUnrolledSimple()
        {
            long longResult = 0;
            for (int i = 0; i + 3 <= 1024; i += 3)
            {
                longResult += System.Runtime.Intrinsics.X86.Popcnt.PopCount(_Data[i]);
                longResult += System.Runtime.Intrinsics.X86.Popcnt.PopCount(_Data[i + 1]);
                longResult += System.Runtime.Intrinsics.X86.Popcnt.PopCount(_Data[i + 2]);
            }

            longResult += System.Runtime.Intrinsics.X86.Popcnt.PopCount(_Data[1024 - 1]);
            return (int) longResult;

        }

        [Benchmark]
        public long PopCountUnrolledOriginal()
        {
            long longResult = 0;
            for (int i = 0; i + 3 <= 1024; i += 3)
            {
                var p1 = System.Runtime.Intrinsics.X86.Popcnt.PopCount(_Data[i]);
                var p2 = System.Runtime.Intrinsics.X86.Popcnt.PopCount(_Data[i + 1]);
                var p3 = System.Runtime.Intrinsics.X86.Popcnt.PopCount(_Data[i + 2]);
                longResult += p1 + p2 + p3;
            }

            longResult += System.Runtime.Intrinsics.X86.Popcnt.PopCount(_Data[1024 - 1]);
            return (int)longResult;
        }

        [Benchmark]
        public long PopCountUnrolledReuseVar()
        {
            long p1 = 0, p2 = 0, p3 = 0;
            for (int i = 0; i + 3 <= 1024; i += 3)
            {
                p1 += System.Runtime.Intrinsics.X86.Popcnt.PopCount(_Data[i]);
                p2 += System.Runtime.Intrinsics.X86.Popcnt.PopCount(_Data[i + 1]);
                p3 += System.Runtime.Intrinsics.X86.Popcnt.PopCount(_Data[i + 2]);
            }

            return p1 + p2 + p3 + System.Runtime.Intrinsics.X86.Popcnt.PopCount(_Data[1024 - 1]);
        }
    }
}

BenchmarkDotNet=v0.10.11, OS=Windows 10 Redstone 3 [1709, Fall Creators Update] (10.0.16299.125)
Processor=Intel Core i7-4790K CPU 4.00GHz (Haswell), ProcessorCount=8
Frequency=3906249 Hz, Resolution=256.0001 ns, Timer=TSC
.NET Core SDK=2.2.0-preview1-007813
  [Host]      : .NET Core 2.1.0-preview1-26013-05 (Framework 4.6.26013.03), 64bit RyuJIT
  Core2.1-x64 : .NET Core 2.1.0-preview1-26013-05 (Framework 4.6.26013.03), 64bit RyuJIT

Job=Core2.1-x64  Runtime=Core  Toolchain=.NET Core 2.1  

Method	Input	Mean	Error	StdDev	Allocated
PopCountArray	1085102592571150095	507.1 ns	0.2760 ns	0.2447 ns	0 B
PopCountUnrolledSimple	1085102592571150095	473.3 ns	0.0240 ns	0.0200 ns	0 B
PopCountUnrolledOriginal	1085102592571150095	516.3 ns	0.0744 ns	0.0659 ns	0 B
PopCountUnrolledReuseVar	1085102592571150095	480.4 ns	0.0787 ns	0.0737 ns	0 B

@Tornhoof also copy the _Data into a local variable, you will see far better assembler.