From 88342e51daeb2b284964ec9e2e2a8a3910fa5c66 Mon Sep 17 00:00:00 2001
From: Ben Adams <thundercat@illyriad.co.uk>
Date: Tue, 8 Jan 2019 11:10:09 +0000
Subject: [PATCH 1/2] Fast-path GetStateMachineBox

---
 .../Runtime/CompilerServices/AsyncMethodBuilder.cs | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/src/System.Private.CoreLib/src/System/Runtime/CompilerServices/AsyncMethodBuilder.cs b/src/System.Private.CoreLib/src/System/Runtime/CompilerServices/AsyncMethodBuilder.cs
index a4c0f7907449..ce5b20b5c391 100644
--- a/src/System.Private.CoreLib/src/System/Runtime/CompilerServices/AsyncMethodBuilder.cs
+++ b/src/System.Private.CoreLib/src/System/Runtime/CompilerServices/AsyncMethodBuilder.cs
@@ -445,6 +445,18 @@ private IAsyncStateMachineBox GetStateMachineBox<TStateMachine>(
                 return stronglyTypedBox;
             }
 
+            // We haven't created the IAsyncStateMachineBox yet, create it
+            return CreateStateMachineBox(ref stateMachine, currentContext);
+        }
+
+        /// <summary>Creates the "boxed" state machine object.</summary>
+        /// <typeparam name="TStateMachine">Specifies the type of the async state machine.</typeparam>
+        /// <param name="stateMachine">The state machine.</param>
+        /// <param name="currentContext">The current <see cref="ExecutionContext"/> to initialize the state machine with.</param>
+        /// <returns>The "boxed" state machine.</returns>
+        private IAsyncStateMachineBox CreateStateMachineBox<TStateMachine>(ref TStateMachine stateMachine,
+            ExecutionContext currentContext) where TStateMachine : IAsyncStateMachine
+        {
             // 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
@@ -479,7 +491,7 @@ private IAsyncStateMachineBox GetStateMachineBox<TStateMachine>(
 
             // At this point, m_task 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-intialized
+            // 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

From 4d6677120a4edc3e7e42af94a3d077bb1de228c2 Mon Sep 17 00:00:00 2001
From: Ben Adams <thundercat@illyriad.co.uk>
Date: Tue, 8 Jan 2019 12:14:42 +0000
Subject: [PATCH 2/2] Be explicit about what is happening

---
 .../src/System/Runtime/CompilerServices/AsyncMethodBuilder.cs   | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/System.Private.CoreLib/src/System/Runtime/CompilerServices/AsyncMethodBuilder.cs b/src/System.Private.CoreLib/src/System/Runtime/CompilerServices/AsyncMethodBuilder.cs
index ce5b20b5c391..c7f45798999c 100644
--- a/src/System.Private.CoreLib/src/System/Runtime/CompilerServices/AsyncMethodBuilder.cs
+++ b/src/System.Private.CoreLib/src/System/Runtime/CompilerServices/AsyncMethodBuilder.cs
@@ -425,6 +425,7 @@ public void AwaitUnsafeOnCompleted<TAwaiter, TStateMachine>(
         /// <typeparam name="TStateMachine">Specifies the type of the async state machine.</typeparam>
         /// <param name="stateMachine">The state machine.</param>
         /// <returns>The "boxed" state machine.</returns>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         private IAsyncStateMachineBox GetStateMachineBox<TStateMachine>(
             ref TStateMachine stateMachine)
             where TStateMachine : IAsyncStateMachine
@@ -454,6 +455,7 @@ private IAsyncStateMachineBox GetStateMachineBox<TStateMachine>(
         /// <param name="stateMachine">The state machine.</param>
         /// <param name="currentContext">The current <see cref="ExecutionContext"/> to initialize the state machine with.</param>
         /// <returns>The "boxed" state machine.</returns>
+        [MethodImpl(MethodImplOptions.NoInlining)]
         private IAsyncStateMachineBox CreateStateMachineBox<TStateMachine>(ref TStateMachine stateMachine,
             ExecutionContext currentContext) where TStateMachine : IAsyncStateMachine
         {