benchmark.cs

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using System;
using System.Runtime.CompilerServices;

BenchmarkRunner.Run<SkipLocalsInitBenchmark>();

[SimpleJob]
public unsafe class SkipLocalsInitBenchmark
{
    // example numbers taken from BCryptAlgorithmHandle.GetAlgorithmName
    const int StackAllocCharSize = 128;

    private static ReadOnlySpan<char> Payload => "hello, world";

    [Benchmark(Baseline = true)]
    public void WithInit()
    {
        Span<char> buffer = stackalloc char[StackAllocCharSize];
        // just do *something* to justify the local
        Payload.CopyTo(buffer);
    }

    [Benchmark, SkipLocalsInit]
    public void SkipInit()
    {
        Span<char> buffer = stackalloc char[StackAllocCharSize];
        // just do *something* to justify the local
        Payload.CopyTo(buffer);
    }
}

results.txt

BenchmarkDotNet=v0.13.5, OS=Windows 11 (10.0.22621.1555/22H2/2022Update/SunValley2)
Intel Core i9-9900K CPU 3.60GHz (Coffee Lake), 1 CPU, 16 logical and 8 physical cores
.NET SDK=8.0.100-preview.1.23115.2
  [Host]     : .NET 8.0.0 (8.0.23.11008), X64 RyuJIT AVX2
  DefaultJob : .NET 8.0.0 (8.0.23.11008), X64 RyuJIT AVX2


|   Method |     Mean |     Error |    StdDev | Ratio |
|--------- |---------:|----------:|----------:|------:|
| WithInit | 7.396 ns | 0.1415 ns | 0.1324 ns |  1.00 |
| SkipInit | 2.804 ns | 0.0121 ns | 0.0101 ns |  0.38 |