c# - What is it that makes Enum.HasFlag so slow?

Question

I was doing some speed tests and I noticed that Enum.HasFlag is about 16 times slower than using the bitwise operation.

Does anyone know the internals of Enum.HasFlag and why it is so slow? I mean twice as slow wouldn't be too bad but it makes the function unusable when its 16 times slower.

In case anyone is wondering, here is the code I am using to test its speed.

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;

namespace app
{
    public class Program
    {
        [Flags]
        public enum Test
        {
            Flag1 = 1,
            Flag2 = 2,
            Flag3 = 4,
            Flag4 = 8
        }
        static int num = 0;
        static Random rand;
        static void Main(string[] args)
        {
            int seed = (int)DateTime.UtcNow.Ticks;

            var st1 = new SpeedTest(delegate
            {
                Test t = Test.Flag1;
                t |= (Test)rand.Next(1, 9);
                if (t.HasFlag(Test.Flag4))
                    num++;
            });

            var st2 = new SpeedTest(delegate
            {
                Test t = Test.Flag1;
                t |= (Test)rand.Next(1, 9);
                if (HasFlag(t , Test.Flag4))
                    num++;
            });

            rand = new Random(seed);
            st1.Test();
            rand = new Random(seed);
            st2.Test();

            Console.WriteLine("Random to prevent optimizing out things {0}", num);
            Console.WriteLine("HasFlag: {0}ms {1}ms {2}ms", st1.Min, st1.Average, st1.Max);
            Console.WriteLine("Bitwise: {0}ms {1}ms {2}ms", st2.Min, st2.Average, st2.Max);
            Console.ReadLine();
        }
        static bool HasFlag(Test flags, Test flag)
        {
            return (flags & flag) != 0;
        }
    }
    [DebuggerDisplay("Average = {Average}")]
    class SpeedTest
    {
        public int Iterations { get; set; }

        public int Times { get; set; }

        public List<Stopwatch> Watches { get; set; }

        public Action Function { get; set; }

        public long Min { get { return Watches.Min(s => s.ElapsedMilliseconds); } }

        public long Max { get { return Watches.Max(s => s.ElapsedMilliseconds); } }

        public double Average { get { return Watches.Average(s => s.ElapsedMilliseconds); } }

        public SpeedTest(Action func)
        {
            Times = 10;
            Iterations = 100000;
            Function = func;
            Watches = new List<Stopwatch>();
        }

        public void Test()
        {
            Watches.Clear();
            for (int i = 0; i < Times; i++)
            {
                var sw = Stopwatch.StartNew();
                for (int o = 0; o < Iterations; o++)
                {
                    Function();
                }
                sw.Stop();
                Watches.Add(sw);
            }
        }
    }
}

Results:

HasFlag: 52ms 53.6ms 55ms
Bitwise: 3ms 3ms 3ms

Answer 1

Does anyone know the internals of Enum.HasFlag and why it is so slow?

The actual check is just a simple bit check in Enum.HasFlag - it's not the problem here. That being said, it is slower than your own bit check...

There are a couple of reasons for this slowdown:

First, Enum.HasFlag does an explicit check to make sure that the type of the enum and the type of the flag are both the same type, and from the same Enum. There is some cost in this check.

Secondly, there is an unfortunate box and unbox of the value during a conversion to UInt64 that occurs inside of HasFlag. This is, I believe, due to the requirement that Enum.HasFlag work with all enums, regardless of the underlying storage type.

That being said, there is a huge advantage to Enum.HasFlag - it's reliable, clean, and makes the code very obvious and expressive. For the most part, I feel that this makes it worth the cost - but if you're using this in a very performance critical loop, it may be worth doing your own check.