Try to improve List enumerator perf

[stephentoub]

No description provided.

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[stephentoub]

@EgorBot -arm -amd -intel

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

BenchmarkSwitcher.FromAssembly(typeof(Bench).Assembly).Run(args);

[MemoryDiagnoser(false)]
public class Bench
{
    [Benchmark]
    [ArgumentsSource(nameof(GetLists))]
    public int SumList(List<int> list)
    {
        int sum = 0;
        foreach (int item in list)
        {
            sum += item;
        }
        return sum;
    }

    [Benchmark]
    [ArgumentsSource(nameof(GetLists))]
    public int SumEnumerable(IEnumerable<int> list)
    {
        int sum = 0;
        foreach (int item in list)
        {
            sum += item;
        }
        return sum;
    }

    public static IEnumerable<List<int>> GetLists() =>
        from count in new int[] { 1, 10, 1_000 }
        select Enumerable.Range(0, count).ToList();
}

[stephentoub]

@EgorBot -arm -amd -intel

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using System.Runtime.CompilerServices;

BenchmarkSwitcher.FromAssembly(typeof(Bench).Assembly).Run(args);

[DisassemblyDiagnoser]
[MemoryDiagnoser(false)]
public class Bench
{
    [Benchmark]
    [ArgumentsSource(nameof(GetLists))]
    public int SumList(List<string> list)
    {
        int sum = 0;
        foreach (var item in list)
        {
            sum += Process(item);
        }
        return sum;
    }

    [Benchmark]
    [ArgumentsSource(nameof(GetLists))]
    public int SumEnumerable(IEnumerable<string> list)
    {
        int sum = 0;
        foreach (var item in list)
        {
            sum += Process(item);
        }
        return sum;
    }

    [MethodImpl(MethodImplOptions.NoInlining)]
    private static int Process(string item) => item.Length;

    public static IEnumerable<List<string>> GetLists() =>
        from count in new int[] { 1, 10, 1_000 }
        select Enumerable.Range(0, count).Select(i => i.ToString()).ToList();
}

[stephentoub]

@EgorBot -arm -amd -intel

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

BenchmarkSwitcher.FromAssembly(typeof(Bench).Assembly).Run(args);

[MemoryDiagnoser(false)]
public class Bench
{
    [Benchmark]
    [ArgumentsSource(nameof(GetLists))]
    public int SumList(List<int> list)
    {
        int sum = 0;
        foreach (int item in list)
        {
            sum += item;
        }
        return sum;
    }

    [Benchmark]
    [ArgumentsSource(nameof(GetLists))]
    public int SumEnumerable(IEnumerable<int> list)
    {
        int sum = 0;
        foreach (int item in list)
        {
            sum += item;
        }
        return sum;
    }

    public static IEnumerable<List<int>> GetLists() =>
        from count in new int[] { 1, 10, 1_000 }
        select Enumerable.Range(0, count).ToList();
}