Improve performance of Enum's generic IsDefined / GetName / GetValues / GetNames

[stephentoub]

Eliminates the boxing in IsDefined/GetName/GetValues, in GetValues avoids going through the non-generic Array.SetValue for each value, and in GetNames avoids having to go through RuntimeType's GetEnumNames override.

Method	Toolchain	Mean	Ratio	Allocated
IsDefined	\master\corerun.exe	34.39 ns	1.00	24 B
IsDefined	\pr\corerun.exe	24.37 ns	0.70	-
GetName	\master\corerun.exe	52.31 ns	1.00	24 B
GetName	\pr\corerun.exe	25.16 ns	0.48	-
GetNames	\master\corerun.exe	37.25 ns	1.00	80 B
GetNames	\pr\corerun.exe	22.80 ns	0.61	80 B
GetValues	\master\corerun.exe	573.06 ns	1.00	224 B
GetValues	\pr\corerun.exe	19.05 ns	0.03	56 B

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Diagnosers;
using BenchmarkDotNet.Running;
using System;

[MemoryDiagnoser]
public class Program
{
    static void Main(string[] args) => BenchmarkSwitcher.FromAssemblies(new[] { typeof(Program).Assembly }).Run(args);

    private DayOfWeek _value = DayOfWeek.Friday;

    [Benchmark]
    public bool IsDefined() => Enum.IsDefined(_value);

    [Benchmark]
    public string GetName() => Enum.GetName(_value);

    [Benchmark]
    public string[] GetNames() => Enum.GetNames<DayOfWeek>();

    [Benchmark]
    public DayOfWeek[] GetValues() => Enum.GetValues<DayOfWeek>();
}

[EgorBo]

@jkotas @stephentoub In case if you are interested: I've just intrinsified Type.GetTypeCode for typeof(x) or x.GetType() args master...EgorBo:opt-type-gettypecode but the jit-diff is not really exciting, just two methods so I guess it's not worth the complexity:

(-91.67% of base) : System.Collections.Concurrent.dasm - ConcurrentDictionary`2:IsValueWriteAtomic():bool (7 methods)
(-48.73% of base) : System.Private.CoreLib.dasm - Enum:TryParse(String,bool,bool,byref):bool (6 methods)

[stephentoub]

so I guess it's not worth the complexity

Probably not right now, but it's good to have in our back pocket for the future :)