Optimized token attachment

Author: sakno

src/DotNext.Threading/Threading/CancellationTokenMultiplexer.CTS.cs

@@ -1,55 +1,59 @@
+using System.Diagnostics;
+using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 
 namespace DotNext.Threading;
 
+using InlinedTokenList = ValueTuple<CancellationTokenRegistration, CancellationTokenRegistration, CancellationTokenRegistration>;
+
 partial class CancellationTokenMultiplexer
 {
     private sealed class PooledCancellationTokenSource : LinkedCancellationTokenSource, IResettable
     {
-        private const int Capacity = 3;
-        private (CancellationTokenRegistration, CancellationTokenRegistration, CancellationTokenRegistration) inlinedList;
-        private byte inlinedTokenCount;
+        private static readonly int InlinedListCapacity = GetCapacity<InlinedTokenList>();
+        
+        private InlinedTokenList inlinedList;
+        private int inlinedTokenCount;
         private List<CancellationTokenRegistration>? extraTokens;
         internal PooledCancellationTokenSource? Next;
 
         public void Add(CancellationToken token)
-            => Add() = Attach(token);
+            => Add(Attach(token));
 
-        private ref CancellationTokenRegistration Add()
+        private void Add(CancellationTokenRegistration registration)
         {
-            Span<CancellationTokenRegistration> registrations;
-            int index;
-            if (inlinedTokenCount < Capacity)
+            if (inlinedTokenCount < InlinedListCapacity)
             {
-                index = inlinedTokenCount++;
-                registrations = inlinedList.AsSpan();
+                Unsafe.Add(ref FirstInlinedRegistration, inlinedTokenCount++) = registration;
             }
             else
             {
                 extraTokens ??= new();
-                index = extraTokens.Count;
-                extraTokens.Add(default);
-                registrations = CollectionsMarshal.AsSpan(extraTokens);
+                extraTokens.Add(registration);
             }
-
-            return ref registrations[index];
         }
 
+        [DebuggerBrowsable(DebuggerBrowsableState.Never)]
+        private ref CancellationTokenRegistration FirstInlinedRegistration
+            => ref Unsafe.As<InlinedTokenList, CancellationTokenRegistration>(ref inlinedList);
+
         public int Count => inlinedTokenCount + extraTokens?.Count ?? 0;
 
         public ref CancellationTokenRegistration this[int index]
         {
             get
             {
+                Debug.Assert((uint)index < (uint)Count);
+                
                 Span<CancellationTokenRegistration> registrations;
-                if (index < Capacity)
+                if (index < InlinedListCapacity)
                 {
                     registrations = inlinedList.AsSpan();
                 }
                 else
                 {
                     registrations = CollectionsMarshal.AsSpan(extraTokens);
-                    index -= Capacity;
+                    index -= InlinedListCapacity;
                 }
 
                 return ref registrations[index];
@@ -63,6 +67,10 @@ public void Reset()
             extraTokens?.Clear();
         }
 
+        private static int GetCapacity<T>()
+            where T : struct, ITuple
+            => default(T).Length;
+
         protected override void Dispose(bool disposing)
         {
             if (disposing)