Benchmark of CollectionsMarshal.AsSpan with PGO

Program.cs

using System.Runtime.InteropServices;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Configs;
using BenchmarkDotNet.Jobs;
using BenchmarkDotNet.Reports;
using BenchmarkDotNet.Running;

Summary? summary = BenchmarkRunner.Run<AsSpanBenchmark>();

[MemoryDiagnoser]
[Config(typeof(MyEnvVars))]
public class AsSpanBenchmark
{
    // Custom config to define "Default vs PGO"
    class MyEnvVars : ManualConfig
    {
        public MyEnvVars()
        {
            // Use .NET 6.0 default mode:
            AddJob(Job.Default.WithId("Default mode"));

            // Use Dynamic PGO mode:
            AddJob(Job.Default.WithId("Dynamic PGO")
                .WithEnvironmentVariables(
                    new EnvironmentVariable("DOTNET_TieredPGO", "1"),
                    new EnvironmentVariable("DOTNET_TC_QuickJitForLoops", "1"),
                    new EnvironmentVariable("DOTNET_ReadyToRun", "0")));
        }
    }
    private const int NumberToWorkOnSoItIsNotOptimizedAwaySeed = 1337;
    private static readonly Random Random = new(60);

    [Params(0, 1, 2, 3, 4)]
    public ushort ListSize { get; set; }

    private List<int> _list = new();
    [GlobalSetup]
    public void GlobalSetup()
    {
        _list = new List<int>(ListSize);
        for (var i = 0; i < ListSize; i++)
            _list.Add(Random.Next(int.MinValue, int.MaxValue));
    }
    [Benchmark]
    public int ForEach()
    {
        int numberToWorkOnSoItIsNotOptimizedAway = NumberToWorkOnSoItIsNotOptimizedAwaySeed;
        foreach (int number in _list)
            numberToWorkOnSoItIsNotOptimizedAway ^= number;
        return numberToWorkOnSoItIsNotOptimizedAway;
    }

    [Benchmark]
    public int For()
    {
        int numberToWorkOnSoItIsNotOptimizedAway = NumberToWorkOnSoItIsNotOptimizedAwaySeed;
        for (var index = 0; index < _list.Count; index++)
            numberToWorkOnSoItIsNotOptimizedAway ^= _list[index];
        return numberToWorkOnSoItIsNotOptimizedAway;
    }
    [Benchmark]
    public int AsSpanForeach()
    {
        int numberToWorkOnSoItIsNotOptimizedAway = NumberToWorkOnSoItIsNotOptimizedAwaySeed;
        foreach (int number in CollectionsMarshal.AsSpan(_list))
            numberToWorkOnSoItIsNotOptimizedAway ^= number;
        return numberToWorkOnSoItIsNotOptimizedAway;
    }
    [Benchmark]
    public int AsSpanFor()
    {
        int numberToWorkOnSoItIsNotOptimizedAway = NumberToWorkOnSoItIsNotOptimizedAwaySeed;
        Span<int> span = CollectionsMarshal.AsSpan(_list);
        for (var index = 0; index < span.Length; index++)
            numberToWorkOnSoItIsNotOptimizedAway ^= span[index];
        return numberToWorkOnSoItIsNotOptimizedAway;
    }

    [Benchmark]
    public int RiderSuggestedLinq()
    {
        return _list.Aggregate(NumberToWorkOnSoItIsNotOptimizedAwaySeed, (current, t) => current ^ t);
    }
}

Results and conclusions can be found here https://www.linkedin.com/pulse/collectionsmarshalasspan-overhead-pgo-henrik-gedionsen