Try to reduce cost of Async

src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/AsyncMethodBuilderCore.cs

@@ -102,6 +102,12 @@ internal static string GetAsyncStateMachineDescription(IAsyncStateMachine stateM
             return sb.ToString();
         }
 
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        internal static void LogTraceOperationBegin(Task t, Type stateMachineType)
+        {
+            TplEventSource.Log.TraceOperationBegin(t.Id, "Async: " + stateMachineType.Name, 0);
+        }
+
         internal static Action CreateContinuationWrapper(Action continuation, Action<Action, Task> invokeAction, Task innerTask) =>
             new ContinuationWrapper(continuation, invokeAction, innerTask).Invoke;
 

src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/AsyncTaskMethodBuilderT.cs

@@ -157,6 +157,8 @@ public struct AsyncTaskMethodBuilder<TResult>
         {
             ExecutionContext? currentContext = ExecutionContext.Capture();
 
+            IAsyncStateMachineBox result;
+
             // Check first for the most common case: not the first yield in an async method.
             // In this case, the first yield will have already "boxed" the state machine in
             // a strongly-typed manner into an AsyncStateMachineBox.  It will already contain
@@ -168,9 +170,8 @@ public struct AsyncTaskMethodBuilder<TResult>
                 {
                     stronglyTypedBox.Context = currentContext;
                 }
-                return stronglyTypedBox;
+                result = stronglyTypedBox;
             }
-
             // The least common case: we have a weakly-typed boxed.  This results if the debugger
             // or some other use of reflection accesses a property like ObjectIdForDebugger or a
             // method like SetNotificationForWaitCompletion prior to the first await happening.  In
@@ -180,64 +181,67 @@ public struct AsyncTaskMethodBuilder<TResult>
             // result in a boxing allocation when storing the TStateMachine if it's a struct, but
             // this only happens in active debugging scenarios where such performance impact doesn't
             // matter.
-            if (taskField is AsyncStateMachineBox<IAsyncStateMachine> weaklyTypedBox)
+            else if (taskField is AsyncStateMachineBox<IAsyncStateMachine> weaklyTypedBox)
             {
                 // If this is the first await, we won't yet have a state machine, so store it.
                 if (weaklyTypedBox.StateMachine == null)
                 {
                     Debugger.NotifyOfCrossThreadDependency(); // same explanation as with usage below
-                    weaklyTypedBox.StateMachine = stateMachine;
+                    weaklyTypedBox.StateMachine = Unsafe.As<IAsyncStateMachine>(RuntimeHelpers.Box(ref Unsafe.As<TStateMachine, byte>(ref stateMachine), typeof(TStateMachine).TypeHandle));
                 }
 
                 // Update the context.  This only happens with a debugger, so no need to spend
                 // extra IL checking for equality before doing the assignment.
                 weaklyTypedBox.Context = currentContext;
-                return weaklyTypedBox;
+                result = weaklyTypedBox;
             }
-
-            // Alert a listening debugger that we can't make forward progress unless it slips threads.
-            // If we don't do this, and a method that uses "await foo;" is invoked through funceval,
-            // we could end up hooking up a callback to push forward the async method's state machine,
-            // the debugger would then abort the funceval after it takes too long, and then continuing
-            // execution could result in another callback being hooked up.  At that point we have
-            // multiple callbacks registered to push the state machine, which could result in bad behavior.
-            Debugger.NotifyOfCrossThreadDependency();
-
-            // At this point, taskField should really be null, in which case we want to create the box.
-            // However, in a variety of debugger-related (erroneous) situations, it might be non-null,
-            // e.g. if the Task property is examined in a Watch window, forcing it to be lazily-initialized
-            // as a Task<TResult> rather than as an AsyncStateMachineBox.  The worst that happens in such
-            // cases is we lose the ability to properly step in the debugger, as the debugger uses that
-            // object's identity to track this specific builder/state machine.  As such, we proceed to
-            // overwrite whatever's there anyway, even if it's non-null.
+            else
+            {
+                // Alert a listening debugger that we can't make forward progress unless it slips threads.
+                // If we don't do this, and a method that uses "await foo;" is invoked through funceval,
+                // we could end up hooking up a callback to push forward the async method's state machine,
+                // the debugger would then abort the funceval after it takes too long, and then continuing
+                // execution could result in another callback being hooked up.  At that point we have
+                // multiple callbacks registered to push the state machine, which could result in bad behavior.
+                Debugger.NotifyOfCrossThreadDependency();
+
+                // At this point, taskField should really be null, in which case we want to create the box.
+                // However, in a variety of debugger-related (erroneous) situations, it might be non-null,
+                // e.g. if the Task property is examined in a Watch window, forcing it to be lazily-initialized
+                // as a Task<TResult> rather than as an AsyncStateMachineBox.  The worst that happens in such
+                // cases is we lose the ability to properly step in the debugger, as the debugger uses that
+                // object's identity to track this specific builder/state machine.  As such, we proceed to
+                // overwrite whatever's there anyway, even if it's non-null.
 #if NATIVEAOT
-            // DebugFinalizableAsyncStateMachineBox looks like a small type, but it actually is not because
-            // it will have a copy of all the slots from its parent. It will add another hundred(s) bytes
-            // per each async method in NativeAOT binaries without adding much value. Avoid
-            // generating this extra code until a better solution is implemented.
-            var box = new AsyncStateMachineBox<TStateMachine>();
+                // DebugFinalizableAsyncStateMachineBox looks like a small type, but it actually is not because
+                // it will have a copy of all the slots from its parent. It will add another hundred(s) bytes
+                // per each async method in NativeAOT binaries without adding much value. Avoid
+                // generating this extra code until a better solution is implemented.
+                var box = new AsyncStateMachineBox<TStateMachine>();
 #else
-            AsyncStateMachineBox<TStateMachine> box = AsyncMethodBuilderCore.TrackAsyncMethodCompletion ?
-                CreateDebugFinalizableAsyncStateMachineBox<TStateMachine>() :
-                new AsyncStateMachineBox<TStateMachine>();
+                AsyncStateMachineBox<TStateMachine> box = AsyncMethodBuilderCore.TrackAsyncMethodCompletion ?
+                    CreateDebugFinalizableAsyncStateMachineBox<TStateMachine>() :
+                    new AsyncStateMachineBox<TStateMachine>();
 #endif
-            taskField = box; // important: this must be done before storing stateMachine into box.StateMachine!
-            box.StateMachine = stateMachine;
-            box.Context = currentContext;
+                taskField = box; // important: this must be done before storing stateMachine into box.StateMachine!
+                box.StateMachine = stateMachine;
+                box.Context = currentContext;
 
-            // Log the creation of the state machine box object / task for this async method.
-            if (TplEventSource.Log.IsEnabled())
-            {
-                TplEventSource.Log.TraceOperationBegin(box.Id, "Async: " + stateMachine.GetType().Name, 0);
-            }
+                // Log the creation of the state machine box object / task for this async method.
+                if (TplEventSource.Log.IsEnabled())
+                {
+                    AsyncMethodBuilderCore.LogTraceOperationBegin(box, stateMachine.GetType());
+                }
 
-            // And if async debugging is enabled, track the task.
-            if (Threading.Tasks.Task.s_asyncDebuggingEnabled)
-            {
-                Threading.Tasks.Task.AddToActiveTasks(box);
+                // And if async debugging is enabled, track the task.
+                if (Threading.Tasks.Task.s_asyncDebuggingEnabled)
+                {
+                    Threading.Tasks.Task.AddToActiveTasks(box);
+                }
+                result = box;
             }
 
-            return box;
+            return result;
         }
 
 #if !NATIVEAOT