Streamline String.Substring

[stephentoub]

• Split the one-arg Substring from the two-arg Substring to avoid unnecessary checks in the former
• Employ the same argument validation checks as Span, and then delegate to a helper that does more detailed checking to throw the right exception
• Avoid duplicative checks in the body
• Reorder checks in one-arg overload to do success paths before error paths where possible

Method	Job	Toolchain	StartIndex	Mean	Error	StdDev	Ratio	RatioSD
OneArg	Job-LMVHFB	\main\corerun.exe	0	2.106 ns	0.0840 ns	0.0825 ns	1.00	0.00
OneArg	Job-BIJWCO	\pr\corerun.exe	0	1.418 ns	0.0337 ns	0.0315 ns	0.67	0.02
TwoArgs	Job-LMVHFB	\main\corerun.exe	0	1.927 ns	0.0137 ns	0.0114 ns	1.00	0.00
TwoArgs	Job-BIJWCO	\pr\corerun.exe	0	1.750 ns	0.0300 ns	0.0266 ns	0.91	0.02
OneArg	Job-LMVHFB	\main\corerun.exe	6	10.272 ns	0.0776 ns	0.0688 ns	1.00	0.00
OneArg	Job-BIJWCO	\pr\corerun.exe	6	9.869 ns	0.0226 ns	0.0189 ns	0.96	0.01
TwoArgs	Job-LMVHFB	\main\corerun.exe	6	10.231 ns	0.0793 ns	0.0703 ns	1.00	0.00
TwoArgs	Job-BIJWCO	\pr\corerun.exe	6	10.185 ns	0.2332 ns	0.1947 ns	0.99	0.02
OneArg	Job-LMVHFB	\main\corerun.exe	11	2.040 ns	0.0252 ns	0.0197 ns	1.00	0.00
OneArg	Job-BIJWCO	\pr\corerun.exe	11	1.838 ns	0.0572 ns	0.0507 ns	0.90	0.03
TwoArgs	Job-LMVHFB	\main\corerun.exe	11	2.323 ns	0.0313 ns	0.0293 ns	1.00	0.00
TwoArgs	Job-BIJWCO	\pr\corerun.exe	11	1.640 ns	0.0872 ns	0.1071 ns	0.71	0.05

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

public partial class Program
{
    static void Main(string[] args) => BenchmarkSwitcher.FromAssembly(typeof(Program).Assembly).Run(args);

    private string _str = "hello world";

    [Params(0, 6, 11)]
    public int StartIndex { get; set; }

    [Benchmark]
    public string OneArg() => _str.Substring(StartIndex);

    [Benchmark]
    public string TwoArgs()
    {
        string s = _str;
        int startIndex = StartIndex;
        return s.Substring(startIndex, s.Length - startIndex);
    }
}

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Issue Details

---

• Split the one-arg Substring from the two-arg Substring to avoid unnecessary checks in the former
• Employ the same argument validation checks as Span, and then delegate to a helper that does more detailed checking to throw the right exception
• Avoid duplicative checks in the body
• Reorder checks in one-arg overload to do success paths before error paths where possible

Method	Job	Toolchain	StartIndex	Mean	Error	StdDev	Ratio	RatioSD
OneArg	Job-LMVHFB	\main\corerun.exe	0	2.106 ns	0.0840 ns	0.0825 ns	1.00	0.00
OneArg	Job-BIJWCO	\pr\corerun.exe	0	1.418 ns	0.0337 ns	0.0315 ns	0.67	0.02
TwoArgs	Job-LMVHFB	\main\corerun.exe	0	1.927 ns	0.0137 ns	0.0114 ns	1.00	0.00
TwoArgs	Job-BIJWCO	\pr\corerun.exe	0	1.750 ns	0.0300 ns	0.0266 ns	0.91	0.02
OneArg	Job-LMVHFB	\main\corerun.exe	6	10.272 ns	0.0776 ns	0.0688 ns	1.00	0.00
OneArg	Job-BIJWCO	\pr\corerun.exe	6	9.869 ns	0.0226 ns	0.0189 ns	0.96	0.01
TwoArgs	Job-LMVHFB	\main\corerun.exe	6	10.231 ns	0.0793 ns	0.0703 ns	1.00	0.00
TwoArgs	Job-BIJWCO	\pr\corerun.exe	6	10.185 ns	0.2332 ns	0.1947 ns	0.99	0.02
OneArg	Job-LMVHFB	\main\corerun.exe	11	2.040 ns	0.0252 ns	0.0197 ns	1.00	0.00
OneArg	Job-BIJWCO	\pr\corerun.exe	11	1.838 ns	0.0572 ns	0.0507 ns	0.90	0.03
TwoArgs	Job-LMVHFB	\main\corerun.exe	11	2.323 ns	0.0313 ns	0.0293 ns	1.00	0.00
TwoArgs	Job-BIJWCO	\pr\corerun.exe	11	1.640 ns	0.0872 ns	0.1071 ns	0.71	0.05

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

public partial class Program
{
    static void Main(string[] args) => BenchmarkSwitcher.FromAssembly(typeof(Program).Assembly).Run(args);

    private string _str = "hello world";

    [Params(0, 6, 11)]
    public int StartIndex { get; set; }

    [Benchmark]
    public string OneArg() => _str.Substring(StartIndex);

    [Benchmark]
    public string TwoArgs()
    {
        string s = _str;
        int startIndex = StartIndex;
        return s.Substring(startIndex, s.Length - startIndex);
    }
}

Author:	stephentoub
Assignees:	-
Labels:	area-System.Runtime, tenet-performance
Milestone:	7.0.0

[kunalspathak]

Improvements: dotnet/perf-autofiling-issues#7331

[nietras]

I guess #62577 was relevant after all. 😅

[stephentoub]

Heh, yeah, I don't remember that one, but it looks like we did very similar things.