Enable packed UTF-16 search on ARM64

[EgorBo]

[Experiment for EgorBot]

Enables packed UTF-16 search on ARM64 for long inputs.

Note

This description was generated by GitHub Copilot.

[EgorBo]

@EgorBot -macos_arm -linux_arm -aws_graviton5

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

BenchmarkSwitcher.FromAssembly(typeof(PackedUtf16Search).Assembly).Run(args);

public class PackedUtf16Search
{
    private string _absent = null!;
    private string _late = null!;

    [Params(64, 511, 512, 1024, 4096)]
    public int Length { get; set; }

    [GlobalSetup]
    public void Setup()
    {
        _absent = new string('a', Length);
        _late = string.Concat(new string('a', Length - 1), "z");
    }

    [Benchmark]
    public int IndexOfAbsent() => _absent.AsSpan().IndexOf('z');

    [Benchmark]
    public int IndexOfLate() => _late.AsSpan().IndexOf('z');

    [Benchmark]
    public int IndexOfRangeAbsent() =>
        _absent.AsSpan().IndexOfAnyInRange('x', 'z');
}

Note

This benchmark comment was generated by GitHub Copilot.

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[EgorBo]

@MihaZupan this is mostly AI slop, but it seems to improve performance for real-world uses. Can you take over when you have time? We need this for arm64 customers.

[MihaZupan]

I ripped it out because it regressed early exits (i.e. Regex) benchmarks.
Might be worth looking into splitting by length like you're doing here ...