[perf] Optimise {ActivityTraceId|ActivitySpanId}.CopyTo()

[martincostello]

Optimise ActivityTraceId.CopyTo() and ActivitySpanId.CopyTo() by using Convert.FromHexString() in ActivityTraceId.SetSpanFromHexChars() to benefit from SIMD acceleration, where available, for a ~9x throughput improvement when the IDs realistically vary per invocation.

Adapted from open-telemetry/opentelemetry-dotnet#7775.

Benchmarks

Method	Shared ID?	Before	After	Speedup
ActivityTraceId.CopyTo	No	117.24 ns	12.31 ns	9.5x
ActivitySpanId.CopyTo	No	58.37 ns	6.73 ns	8.7x
ActivityTraceId.CopyTo	Yes	21.80 ns	11.82 ns	1.8x
ActivitySpanId.CopyTo	Yes	11.80 ns	6.53 ns	1.8x

BenchmarkDotNet v0.15.4, Windows 11
13th Gen Intel Core i7-13700H, 1 CPU, 20 logical and 14 physical cores
.NET SDK 11.0.100-rc.1.26425.128
  [Host] : .NET 10.0.12, X64 RyuJIT x86-64-v3

Code

using System.Diagnostics;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

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

[MemoryDiagnoser]
public class ActivityIdCopyToBenchmarks
{
    // Large enough that the branch predictor cannot memorize the full sequence of ids when Shared = false.
    private const int PoolSize = 4096;

    private ActivityTraceId[] _traceIdPool = null!;
    private ActivitySpanId[] _spanIdPool = null!;
    private readonly byte[] _traceIdDestination = new byte[16];
    private readonly byte[] _spanIdDestination = new byte[8];
    private int _index;

    [Params(false, true)]
    public bool Shared { get; set; }

    [GlobalSetup]
    public void Setup()
    {
        _traceIdPool = new ActivityTraceId[PoolSize];
        _spanIdPool = new ActivitySpanId[PoolSize];

        if (Shared)
        {
            ActivityTraceId sharedTraceId = ActivityTraceId.CreateRandom();
            ActivitySpanId sharedSpanId = ActivitySpanId.CreateRandom();

            Array.Fill(_traceIdPool, sharedTraceId);
            Array.Fill(_spanIdPool, sharedSpanId);
        }
        else
        {
            for (int i = 0; i < PoolSize; i++)
            {
                _traceIdPool[i] = ActivityTraceId.CreateRandom();
                _spanIdPool[i] = ActivitySpanId.CreateRandom();
            }
        }

        _index = 0;
    }

    [Benchmark]
    public void TraceIdCopyTo()
    {
        _traceIdPool[_index++ & (PoolSize - 1)].CopyTo(_traceIdDestination);
    }

    [Benchmark]
    public void SpanIdCopyTo()
    {
        _spanIdPool[_index++ & (PoolSize - 1)].CopyTo(_spanIdDestination);
    }
}

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[dotnet-policy-service]

Tagging subscribers to this area: @steveisok, @dotnet/area-system-diagnostics-tracing
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[Copilot]

🟡 Changes recommended

The new exception path can surface an argument name (“outBytes”) that doesn’t match the public CopyTo(... destination ...) parameter, making the resulting error message confusing.

Get a fresh assessment by requesting another Copilot review.

Pull request overview

Optimizes ActivityTraceId.CopyTo() / ActivitySpanId.CopyTo() by switching the internal hex-to-bytes conversion helper to use Convert.FromHexString(...) (on #if NET builds), aiming to leverage SIMD-accelerated hex decoding where available.

Changes:

• Updated ActivityTraceId.SetSpanFromHexChars(...) to decode hex via Convert.FromHexString on NET builds.
• Added a stricter success check (OperationStatus.Done and expected bytesWritten) with an exception on mismatch.
• Clarified the helper’s length precondition in the XML comment.

File summaries

File	Description
src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Activity.cs	Uses span-based Convert.FromHexString in SetSpanFromHexChars to speed up CopyTo() decoding on NET builds.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 1
• Review effort level: Lite

[Copilot]

🟡 Changes recommended

The #else path still relies on Debug.Assert for a public-facing invariant, which can result in partial/truncated writes in Release builds and inconsistent behavior vs the new #if NET throwing behavior.

Get a fresh assessment by requesting another Copilot review.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 1
• Review effort level: Lite

[Copilot]

🟡 Changes recommended

The #if NET path can partially write to destination before throwing on length mismatch, which is inconsistent with the non-NET path and should be guarded up-front (and ideally covered by tests).

Get a fresh assessment by requesting another Copilot review.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 2
• Review effort level: Lite

[Copilot]

🟢 Approval recommended

The change is localized, preserves correct behavior for valid inputs, improves performance via the NET fast path, and adds targeted regression tests for the updated error handling.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0 new
• Review effort level: Lite