Use LastIndexOf{Any} in Path.GetFileName

[stephentoub]

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using System;
using System.IO;

[HideColumns("Error", "StdDev", "Median", "RatioSD")]
public partial class Program
{
    static void Main(string[] args) => BenchmarkSwitcher.FromAssembly(typeof(Program).Assembly).Run(args);

    [Benchmark]
    [Arguments(@"C:\test.txt")]
    [Arguments(@"C:\Users\stoub\Desktop\Benchmarks\Program.cs")]
    [Arguments(@"D:\repos\runtime\src\libraries\System.Private.CoreLib\src\System\IO\MemoryStream.cs")]
    public ReadOnlySpan<char> GetFileName(string path) => Path.GetFileName(path.AsSpan());
}

Method	Job	Toolchain	path	Mean	Ratio
GetFileName	Job-ZOQARC	\main\corerun.exe	C:\Us(...)am.cs [44]	16.41 ns	1.00
GetFileName	Job-UTQVEC	\pr\corerun.exe	C:\Us(...)am.cs [44]	12.22 ns	0.74
GetFileName	Job-ZOQARC	\main\corerun.exe	C:\test.txt	14.39 ns	1.00
GetFileName	Job-UTQVEC	\pr\corerun.exe	C:\test.txt	12.00 ns	0.83
GetFileName	Job-ZOQARC	\main\corerun.exe	D:\re(...)am.cs [83]	20.40 ns	1.00
GetFileName	Job-UTQVEC	\pr\corerun.exe	D:\re(...)am.cs [83]	12.27 ns	0.60

[ghost]

Tagging subscribers to this area: @dotnet/area-system-io
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Issue Details

---

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using System;
using System.IO;

[HideColumns("Error", "StdDev", "Median", "RatioSD")]
public partial class Program
{
    static void Main(string[] args) => BenchmarkSwitcher.FromAssembly(typeof(Program).Assembly).Run(args);

    [Benchmark]
    [Arguments(@"C:\test.txt")]
    [Arguments(@"C:\Users\stoub\Desktop\Benchmarks\Program.cs")]
    [Arguments(@"D:\repos\runtime\src\libraries\System.Private.CoreLib\src\System\IO\MemoryStream.cs")]
    public ReadOnlySpan<char> GetFileName(string path) => Path.GetFileName(path.AsSpan());
}

Method	Job	Toolchain	path	Mean	Ratio
GetFileName	Job-ZOQARC	\main\corerun.exe	C:\Us(...)am.cs [44]	16.41 ns	1.00
GetFileName	Job-UTQVEC	\pr\corerun.exe	C:\Us(...)am.cs [44]	12.22 ns	0.74
GetFileName	Job-ZOQARC	\main\corerun.exe	C:\test.txt	14.39 ns	1.00
GetFileName	Job-UTQVEC	\pr\corerun.exe	C:\test.txt	12.00 ns	0.83
GetFileName	Job-ZOQARC	\main\corerun.exe	D:\re(...)am.cs [83]	20.40 ns	1.00
GetFileName	Job-UTQVEC	\pr\corerun.exe	D:\re(...)am.cs [83]	12.27 ns	0.60

Author:	stephentoub
Assignees:	-
Labels:	area-System.IO, tenet-performance
Milestone:	8.0.0

[adamsitnik]

LGTM, thank you @stephentoub !

[stephentoub]

Failure is #75391

[AndyAyersMS]

Improvements:

• [Perf] Windows/x64: 2 Improvements on 9/10/2022 3:01:51 PM perf-autofiling-issues#8540
• [Perf] Windows/arm64: 1 Improvement on 9/10/2022 3:01:51 PM perf-autofiling-issues#8572
• [Perf] Windows/arm64: 1 Improvement on 9/10/2022 3:01:51 PM perf-autofiling-issues#8594