Improve throughput of String.Split(char / char[], ...)

[stephentoub]

Also reduces the amount of unsafe code used.

Benchmark Results:

Benchmark	Mean Before	Mean After	Improvement
Split_NoSeps_NoSplits	94.65	69.33	26.75%
Split_1Sep_NoSplits	47.11	34.61	26.53%
Split_2Seps_NoSplits	61.28	35.21	42.54%
Split_3Seps_NoSplits	91.39	43.14	52.80%
Split_ManySeps_NoSplits	118.73	53.75	54.73%
Split_NoSeps_Splits	123.89	118.58	4.29%
Split_1Sep_Splits	92.72	80.92	12.73%
Split_2Seps_Splits	100.64	88.27	12.29%
Split_3Seps_Splits	131.76	104.92	20.37%
Split_ManySeps_Splits	182.5	130.35	28.58%
Split_NoSeps_ManySplits	275.11	281.97	-2.49%
Split_1Sep_ManySplits	251.72	242.38	3.71%
Split_2Seps_ManySplits	255.53	244.98	4.13%
Split_3Seps_ManySplits	271.73	250.99	7.63%
Split_ManySeps_ManySplits	314.8	308.13	2.12%

Benchmark

using System;
using BenchmarkDotNet.Attributes.Jobs;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

[InProcess]
public class Program
{
    public static void Main() => BenchmarkRunner.Run<Program>();
    private const int Iters = 1_000;
    private static readonly char[] s_commaSpace = new char[] { ',', ' ' };
    private static readonly char[] s_commaSpaceColon = new char[] { ',', ' ', ':' };
    private static readonly char[] s_letters = new char[] { 'a','b','c','d','e','f','g' };

    [Benchmark]
    public unsafe void Split_NoSeps_NoSplits()
    {
        for (int i = 0; i < Iters; i++) "123456789123456789123456789".Split();
    }

    [Benchmark]
    public unsafe void Split_1Sep_NoSplits()
    {
        for (int i = 0; i < Iters; i++) "123456789123456789123456789".Split(',');
    }

    [Benchmark]
    public unsafe void Split_2Seps_NoSplits()
    {
        char[] seps = s_commaSpace;
        for (int i = 0; i < Iters; i++) "123456789123456789123456789".Split(seps);
    }

    [Benchmark]
    public unsafe void Split_3Seps_NoSplits()
    {
        char[] seps = s_commaSpaceColon;
        for (int i = 0; i < Iters; i++) "123456789123456789123456789".Split(seps);
    }

    [Benchmark]
    public unsafe void Split_ManySeps_NoSplits()
    {
        char[] seps = s_letters;
        for (int i = 0; i < Iters; i++) "123456789123456789123456789".Split(seps);
    }


    [Benchmark]
    public unsafe void Split_NoSeps_Splits()
    {
        for (int i = 0; i < Iters; i++) "12345678 12345678 123456789".Split();
    }

    [Benchmark]
    public unsafe void Split_1Sep_Splits()
    {
        for (int i = 0; i < Iters; i++) "12345678,12345678,123456789".Split(',');
    }

    [Benchmark]
    public unsafe void Split_2Seps_Splits()
    {
        char[] seps = s_commaSpace;
        for (int i = 0; i < Iters; i++) "12345678,12345678 123456789".Split(seps);
    }

    [Benchmark]
    public unsafe void Split_3Seps_Splits()
    {
        char[] seps = s_commaSpaceColon;
        for (int i = 0; i < Iters; i++) "1234567,91234567 91234567:9".Split(seps);
    }

    [Benchmark]
    public unsafe void Split_ManySeps_Splits()
    {
        char[] seps = s_letters;
        for (int i = 0; i < Iters; i++) "1234567a91234567d91234567g9".Split(seps);
    }


    [Benchmark]
    public unsafe void Split_NoSeps_ManySplits()
    {
        for (int i = 0; i < Iters; i++) "1 3 5 7 9 2 4 6 8 1 3 5 7 9".Split();
    }

    [Benchmark]
    public unsafe void Split_1Sep_ManySplits()
    {
        for (int i = 0; i < Iters; i++) "1,3,5,7,9,2,4,6,8,1,3,5,7,9".Split(',');
    }

    [Benchmark]
    public unsafe void Split_2Seps_ManySplits()
    {
        char[] seps = s_commaSpace;
        for (int i = 0; i < Iters; i++) "1,3 5,7 9,2 4,6 8,1 3,5 7,9".Split(seps);
    }

    [Benchmark]
    public unsafe void Split_3Seps_ManySplits()
    {
        char[] seps = s_commaSpaceColon;
        for (int i = 0; i < Iters; i++) "1,3 5:7,9 2:4,6 8:1,3 5:7,9".Split(seps);
    }

    [Benchmark]
    public unsafe void Split_ManySeps_ManySplits()
    {
        char[] seps = s_letters;
        for (int i = 0; i < Iters; i++) "1a3b5c7d9e2f4g6a8b1c3d5e7f9".Split(seps);
    }
}

cc: @jkotas, @bbowyersmyth, @joperezr, @AlexGhiondea

[justinvp]

unsafe can now be removed here and below, right?

[stephentoub]

Yup

[jkotas]

@dotnet-bot test OSX10.12 x64 Checked Innerloop Build and Test please