Remove unsafe from EqualStartingCharacterCount

[EgorBo]

I wasn't able to detect any difference for Path.GetRelativePath and the codegen is fairly close.
if we want extra perf, we can vectorize this routine trivially for Linux/macOS (where paths are case-sensitives).

Given this API allocates a new string, it probably isn't supposed to be used on a hot path.

[EgorBo]

@MihuBot

[EgorBo]

@EgorBot -windows_intel -intel -arm

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

public class Benchmarks
{
    static void Main(string[] args)
    {
        BenchmarkSwitcher.FromAssembly(typeof(Benchmarks).Assembly).Run(args);
    }

    public static IEnumerable<object> TestData()
    {
        yield return new object[] { @"C:\a", @"C:\a\b" };
        yield return new object[] { @"C:\a\", @"C:\a\b\c" };
        yield return new object[] {
            @"C:\projects\runtime\src\libraries",
            @"C:\projects\runtime\src\libraries\System.Text.Json\tests"
        };
        yield return new object[] {
            @"C:\a\b\c\d\e\f\g\h\i\j\k\l\m\n\o\p",
            @"C:\a\b\c\d\e\f\g\h\i\j\k\l\m\n\o\p\q\r\s\t\u\v\w\x\y\z"
        };
        yield return new object[] {
            @"C:\projects\runtime-main2\src\libraries\Microsoft.Extensions.DependencyInjection.Specification.Tests\src\Fakes",
            @"C:\projects\runtime-main2\src\libraries\Microsoft.Extensions.DependencyInjection.Specification.Tests\src\"
        };
        yield return new object[] { @"C:\foo\bar", @"D:\foo\bar" };
        yield return new object[] { @"C:\foo\bar\baz", @"C:\foo\bar\..\qux\." };
        yield return new object[] { @"\\server\share\folder1", @"\\server\share\folder1\folder2\folder3" };
        yield return new object[] {
            @"C:\Program Files\MyApp",
            @"C:\Program Files\MyApp\bin\Debug\net8.0"
        };
    }

    [Benchmark]
    [ArgumentsSource(nameof(TestData))]
    public string GetFoo(string relativeTo, string path) => 
        Path.GetRelativePath(relativeTo, path);
}

[EgorBo]

@EgorBot -intel -arm

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

public class Benchmarks
{
    static void Main(string[] args)
    {
        BenchmarkSwitcher.FromAssembly(typeof(Benchmarks).Assembly).Run(args);
    }

    public static IEnumerable<object> TestData()
    {
        yield return new object[] { @"C:\a", @"C:\a\b" };
        yield return new object[] { @"C:\a\", @"C:\a\b\c" };
        yield return new object[] {
            @"C:\projects\runtime\src\libraries",
            @"C:\projects\runtime\src\libraries\System.Text.Json\tests"
        };
        yield return new object[] {
            @"C:\a\b\c\d\e\f\g\h\i\j\k\l\m\n\o\p",
            @"C:\a\b\c\d\e\f\g\h\i\j\k\l\m\n\o\p\q\r\s\t\u\v\w\x\y\z"
        };
        yield return new object[] {
            @"C:\projects\runtime-main2\src\libraries\Microsoft.Extensions.DependencyInjection.Specification.Tests\src\Fakes",
            @"C:\projects\runtime-main2\src\libraries\Microsoft.Extensions.DependencyInjection.Specification.Tests\src\"
        };
        yield return new object[] { @"C:\foo\bar", @"D:\foo\bar" };
        yield return new object[] { @"C:\foo\bar\baz", @"C:\foo\bar\..\qux\." };
        yield return new object[] { @"\\server\share\folder1", @"\\server\share\folder1\folder2\folder3" };
        yield return new object[] {
            @"C:\Program Files\MyApp",
            @"C:\Program Files\MyApp\bin\Debug\net8.0"
        };
    }

    [Benchmark]
    [ArgumentsSource(nameof(TestData))]
    public string GetFoo(string relativeTo, string path) => 
        Path.GetRelativePath(relativeTo, path);
}

[Copilot]

Pull Request Overview

This PR modernizes the EqualStartingCharacterCount method by removing unsafe pointer arithmetic and replacing it with safe span-based operations. The implementation now leverages CommonPrefixLength for case-sensitive comparisons and falls back to a simple loop with char.ToUpperInvariant for case-insensitive scenarios.

Key changes:

• Replaced unsafe pointer-based character comparison with safe span operations
• Utilized AsSpan().CommonPrefixLength() for case-sensitive prefix matching
• Simplified case-insensitive comparison logic using indexed access instead of pointer arithmetic

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[EgorBo]

/ba-g deadletter