Remove class constraint from Interlocked.{Compare}Exchange

Today `Interlocked.CompareExchange<T>` and `Interlocked.Exchange<T>` support only reference type `T`s. Now that we have corresponding {Compare}Exchange methods that support types of size 1, 2, 4, and 8, we can remove the constraint and support any `T` that's either a reference type, a primitive type, or an enum type, making the generic overload more useful and avoiding consumers needing to choose less-than-ideal types just because of the need for atomicity with Interlocked.{Compare}Exchange.

src/coreclr/System.Private.CoreLib/src/System/Threading/Interlocked.CoreCLR.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
@@ -102,21 +103,6 @@ public static long Exchange(ref long location1, long value)
         [return: NotNullIfNotNull(nameof(location1))]
         [MethodImpl(MethodImplOptions.InternalCall)]
         private static extern object? ExchangeObject([NotNullIfNotNull(nameof(value))] ref object? location1, object? value);
-
-        // The below whole method reduces to a single call to Exchange(ref object, object) but
-        // the JIT thinks that it will generate more native code than it actually does.
-
-        /// <summary>Sets a variable of the specified type <typeparamref name="T"/> to a specified value and returns the original value, as an atomic operation.</summary>
-        /// <param name="location1">The variable to set to the specified value.</param>
-        /// <param name="value">The value to which the <paramref name="location1"/> parameter is set.</param>
-        /// <returns>The original value of <paramref name="location1"/>.</returns>
-        /// <exception cref="NullReferenceException">The address of location1 is a null pointer.</exception>
-        /// <typeparam name="T">The type to be used for <paramref name="location1"/> and <paramref name="value"/>. This type must be a reference type.</typeparam>
-        [Intrinsic]
-        [return: NotNullIfNotNull(nameof(location1))]
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static T Exchange<T>([NotNullIfNotNull(nameof(value))] ref T location1, T value) where T : class? =>
-            Unsafe.As<T>(Exchange(ref Unsafe.As<T, object?>(ref location1), value));
 #endregion
 
         #region CompareExchange
@@ -183,29 +169,6 @@ public static long CompareExchange(ref long location1, long value, long comparan
         [MethodImpl(MethodImplOptions.InternalCall)]
         [return: NotNullIfNotNull(nameof(location1))]
         private static extern object? CompareExchangeObject(ref object? location1, object? value, object? comparand);
-
-        // Note that getILIntrinsicImplementationForInterlocked() in vm\jitinterface.cpp replaces
-        // the body of the following method with the following IL:
-        //     ldarg.0
-        //     ldarg.1
-        //     ldarg.2
-        //     call System.Threading.Interlocked::CompareExchange(ref Object, Object, Object)
-        //     ret
-        // The workaround is no longer strictly necessary now that we have Unsafe.As but it does
-        // have the advantage of being less sensitive to JIT's inliner decisions.
-
-        /// <summary>Compares two instances of the specified reference type <typeparamref name="T"/> for reference equality and, if they are equal, replaces the first one.</summary>
-        /// <param name="location1">The destination, whose value is compared by reference with <paramref name="comparand"/> and possibly replaced.</param>
-        /// <param name="value">The value that replaces the destination value if the comparison by reference results in equality.</param>
-        /// <param name="comparand">The object that is compared by reference to the value at <paramref name="location1"/>.</param>
-        /// <returns>The original value in <paramref name="location1"/>.</returns>
-        /// <exception cref="NullReferenceException">The address of <paramref name="location1"/> is a null pointer.</exception>
-        /// <typeparam name="T">The type to be used for <paramref name="location1"/>, <paramref name="value"/>, and <paramref name="comparand"/>. This type must be a reference type.</typeparam>
-        [Intrinsic]
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        [return: NotNullIfNotNull(nameof(location1))]
-        public static T CompareExchange<T>(ref T location1, T value, T comparand) where T : class? =>
-            Unsafe.As<T>(CompareExchange(ref Unsafe.As<T, object?>(ref location1), value, comparand));
         #endregion
 
         #region Add

src/coreclr/nativeaot/System.Private.CoreLib/src/System/EventReporter.cs

@@ -171,7 +171,7 @@ private static unsafe void ClrReportEvent(string eventSource, short type, ushort
             Interop.Advapi32.DeregisterEventSource(handle);
         }
 
-        private static byte s_once;
+        private static bool s_once;
 
         public static bool ShouldLogInEventLog
         {
@@ -180,7 +180,7 @@ public static bool ShouldLogInEventLog
                 if (Interop.Kernel32.IsDebuggerPresent())
                     return false;
 
-                if (s_once == 1 || Interlocked.Exchange(ref s_once, 1) == 1)
+                if (s_once || Interlocked.Exchange(ref s_once, true))
                     return false;
 
                 return true;

src/coreclr/nativeaot/System.Private.CoreLib/src/System/Threading/Interlocked.cs

@@ -36,14 +36,6 @@ public static long CompareExchange(ref long location1, long value, long comparan
 #endif
         }
 
-        [Intrinsic]
-        [return: NotNullIfNotNull(nameof(location1))]
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static T CompareExchange<T>(ref T location1, T value, T comparand) where T : class?
-        {
-            return Unsafe.As<T>(CompareExchange(ref Unsafe.As<T, object?>(ref location1), value, comparand));
-        }
-
         [Intrinsic]
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         [return: NotNullIfNotNull(nameof(location1))]
@@ -92,16 +84,6 @@ public static long Exchange(ref long location1, long value)
 #endif
         }
 
-        [Intrinsic]
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        [return: NotNullIfNotNull(nameof(location1))]
-        public static T Exchange<T>([NotNullIfNotNull(nameof(value))] ref T location1, T value) where T : class?
-        {
-            if (Unsafe.IsNullRef(ref location1))
-                ThrowHelper.ThrowNullReferenceException();
-            return Unsafe.As<T>(RuntimeImports.InterlockedExchange(ref Unsafe.As<T, object?>(ref location1), value));
-        }
-
         [Intrinsic]
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         [return: NotNullIfNotNull(nameof(location1))]

src/coreclr/tools/Common/TypeSystem/IL/Stubs/InterlockedIntrinsics.cs

@@ -30,22 +30,45 @@ public static MethodIL EmitIL(
                 if (compilationModuleGroup.ContainsType(method.OwningType))
 #endif // READYTORUN
                 {
-                    TypeDesc objectType = method.Context.GetWellKnownType(WellKnownType.Object);
-                    MethodDesc compareExchangeObject = method.OwningType.GetKnownMethod("CompareExchange",
-                        new MethodSignature(
-                            MethodSignatureFlags.Static,
-                            genericParameterCount: 0,
-                            returnType: objectType,
-                            parameters: new TypeDesc[] { objectType.MakeByRefType(), objectType, objectType }));
-
-                    ILEmitter emit = new ILEmitter();
-                    ILCodeStream codeStream = emit.NewCodeStream();
-                    codeStream.EmitLdArg(0);
-                    codeStream.EmitLdArg(1);
-                    codeStream.EmitLdArg(2);
-                    codeStream.Emit(ILOpcode.call, emit.NewToken(compareExchangeObject));
-                    codeStream.Emit(ILOpcode.ret);
-                    return emit.Link(method);
+                    // Rewrite the generic Interlocked.CompareExchange<T> to be a call to one of the non-generic overloads.
+                    TypeDesc ceArgType = null;
+
+                    TypeDesc tType = method.Instantiation[0];
+                    if (!tType.IsValueType)
+                    {
+                        ceArgType = method.Context.GetWellKnownType(WellKnownType.Object);
+                    }
+                    else if (tType.IsEnum || tType.IsPrimitive)
+                    {
+                        int size = tType.GetElementSize().AsInt;
+                        Debug.Assert(size is 1 or 2 or 4 or 8);
+                        ceArgType = size switch
+                        {
+                            1 => method.Context.GetWellKnownType(WellKnownType.Byte),
+                            2 => method.Context.GetWellKnownType(WellKnownType.UInt16),
+                            4 => method.Context.GetWellKnownType(WellKnownType.UInt32),
+                            _ => method.Context.GetWellKnownType(WellKnownType.UInt64),
+                        };
+                    }
+
+                    if (ceArgType is not null)
+                    {
+                        MethodDesc compareExchangeNonGeneric = method.OwningType.GetKnownMethod("CompareExchange",
+                            new MethodSignature(
+                                MethodSignatureFlags.Static,
+                                genericParameterCount: 0,
+                                returnType: ceArgType,
+                                parameters: [ceArgType.MakeByRefType(), ceArgType, ceArgType]));
+
+                        ILEmitter emit = new ILEmitter();
+                        ILCodeStream codeStream = emit.NewCodeStream();
+                        codeStream.EmitLdArg(0);
+                        codeStream.EmitLdArg(1);
+                        codeStream.EmitLdArg(2);
+                        codeStream.Emit(ILOpcode.call, emit.NewToken(compareExchangeNonGeneric));
+                        codeStream.Emit(ILOpcode.ret);
+                        return emit.Link(method);
+                    }
                 }
             }
 

src/coreclr/vm/corelib.h

@@ -694,6 +694,10 @@ DEFINE_METHOD(MEMORY_MARSHAL,       GET_ARRAY_DATA_REFERENCE_MDARRAY, GetArrayDa
 DEFINE_CLASS(INTERLOCKED,           Threading,              Interlocked)
 DEFINE_METHOD(INTERLOCKED,          COMPARE_EXCHANGE_T,     CompareExchange, GM_RefT_T_T_RetT)
 DEFINE_METHOD(INTERLOCKED,          COMPARE_EXCHANGE_OBJECT,CompareExchange, SM_RefObject_Object_Object_RetObject)
+DEFINE_METHOD(INTERLOCKED,          COMPARE_EXCHANGE_BYTE,  CompareExchange, SM_RefByte_Byte_Byte_RetByte)
+DEFINE_METHOD(INTERLOCKED,          COMPARE_EXCHANGE_USHRT, CompareExchange, SM_RefUShrt_UShrt_UShrt_RetUShrt)
+DEFINE_METHOD(INTERLOCKED,          COMPARE_EXCHANGE_UINT,  CompareExchange, SM_RefUInt_UInt_UInt_RetUInt)
+DEFINE_METHOD(INTERLOCKED,          COMPARE_EXCHANGE_ULONG, CompareExchange, SM_RefULong_ULong_ULong_RetULong)
 
 DEFINE_CLASS(RAW_DATA,              CompilerServices,       RawData)
 DEFINE_FIELD(RAW_DATA,              DATA,                   Data)

src/coreclr/vm/jitinterface.cpp

@@ -7248,10 +7248,38 @@ bool getILIntrinsicImplementationForInterlocked(MethodDesc * ftn,
 
     // We are only interested if ftn's token and CompareExchange<T> token match
     if (ftn->GetMemberDef() != CoreLibBinder::GetMethod(METHOD__INTERLOCKED__COMPARE_EXCHANGE_T)->GetMemberDef())
+    {
         return false;
+    }
+
+    // Determine the type of the generic T method parameter
+    _ASSERTE(ftn->HasMethodInstantiation());
+    _ASSERTE(ftn->GetNumGenericMethodArgs() == 1);
+    TypeHandle typeHandle = ftn->GetMethodInstantiation()[0];
+    MethodTable* methodTable = typeHandle.GetMethodTable();
 
-    // Get MethodDesc for non-generic System.Threading.Interlocked.CompareExchange()
-    MethodDesc* cmpxchgObject = CoreLibBinder::GetMethod(METHOD__INTERLOCKED__COMPARE_EXCHANGE_OBJECT);
+    MethodDesc* cmpxchg;
+
+    // Based on the generic method parameter, determine which overload of CompareExchange
+    // to delegate to, or if we can't handle the type at all.
+    if (!typeHandle.IsValueType())
+    {
+        cmpxchg = CoreLibBinder::GetMethod(METHOD__INTERLOCKED__COMPARE_EXCHANGE_OBJECT);
+    }
+    else if (CorTypeInfo::IsPrimitiveType(typeHandle.GetVerifierCorElementType()))
+    {
+        UINT size = typeHandle.GetSize();
+        _ASSERTE(size == 1 || size == 2 || size == 4 || size == 8);
+        cmpxchg =
+            size == 1 ? CoreLibBinder::GetMethod(METHOD__INTERLOCKED__COMPARE_EXCHANGE_BYTE) :
+            size == 2 ? CoreLibBinder::GetMethod(METHOD__INTERLOCKED__COMPARE_EXCHANGE_USHRT) :
+            size == 4 ? CoreLibBinder::GetMethod(METHOD__INTERLOCKED__COMPARE_EXCHANGE_UINT) :
+            CoreLibBinder::GetMethod(METHOD__INTERLOCKED__COMPARE_EXCHANGE_ULONG);
+    }
+    else
+    {
+        return false;
+    }
 
     // Setup up the body of the method
     static BYTE il[] = {
@@ -7262,12 +7290,12 @@ bool getILIntrinsicImplementationForInterlocked(MethodDesc * ftn,
                           CEE_RET
                         };
 
-    // Get the token for non-generic System.Threading.Interlocked.CompareExchange(), and patch [target]
-    mdMethodDef cmpxchgObjectToken = cmpxchgObject->GetMemberDef();
-    il[4] = (BYTE)((int)cmpxchgObjectToken >> 0);
-    il[5] = (BYTE)((int)cmpxchgObjectToken >> 8);
-    il[6] = (BYTE)((int)cmpxchgObjectToken >> 16);
-    il[7] = (BYTE)((int)cmpxchgObjectToken >> 24);
+    // Get the token for the relevant System.Threading.Interlocked.CompareExchange overload, and patch [target]
+    mdMethodDef cmpxchgToken = cmpxchg->GetMemberDef();
+    il[4] = (BYTE)((int)cmpxchgToken >> 0);
+    il[5] = (BYTE)((int)cmpxchgToken >> 8);
+    il[6] = (BYTE)((int)cmpxchgToken >> 16);
+    il[7] = (BYTE)((int)cmpxchgToken >> 24);
 
     // Initialize methInfo
     methInfo->ILCode = const_cast<BYTE*>(il);

src/coreclr/vm/metasig.h

@@ -586,6 +586,10 @@ DEFINE_METASIG_T(SM(RefDec_RetVoid, r(g(DECIMAL)), v))
 
 DEFINE_METASIG(GM(RefT_T_T_RetT, IMAGE_CEE_CS_CALLCONV_DEFAULT, 1, r(M(0)) M(0) M(0), M(0)))
 DEFINE_METASIG(SM(RefObject_Object_Object_RetObject, r(j) j j, j))
+DEFINE_METASIG(SM(RefByte_Byte_Byte_RetByte, r(b) b b, b))
+DEFINE_METASIG(SM(RefUShrt_UShrt_UShrt_RetUShrt, r(H) H H, H))
+DEFINE_METASIG(SM(RefUInt_UInt_UInt_RetUInt, r(K) K K, K))
+DEFINE_METASIG(SM(RefULong_ULong_ULong_RetULong, r(L) L L, L))
 
 DEFINE_METASIG_T(SM(RefCleanupWorkListElement_RetVoid, r(C(CLEANUP_WORK_LIST_ELEMENT)), v))
 DEFINE_METASIG_T(SM(RefCleanupWorkListElement_SafeHandle_RetIntPtr, r(C(CLEANUP_WORK_LIST_ELEMENT)) C(SAFE_HANDLE), I))

src/libraries/Common/src/System/Net/StreamBuffer.cs

@@ -292,15 +292,15 @@ private sealed class ResettableValueTaskSource : IValueTaskSource
 
             private ManualResetValueTaskSourceCore<bool> _waitSource; // mutable struct, do not make this readonly
             private CancellationTokenRegistration _waitSourceCancellation;
-            private int _hasWaiter;
+            private bool _hasWaiter;
 
             ValueTaskSourceStatus IValueTaskSource.GetStatus(short token) => _waitSource.GetStatus(token);
 
             void IValueTaskSource.OnCompleted(Action<object?> continuation, object? state, short token, ValueTaskSourceOnCompletedFlags flags) => _waitSource.OnCompleted(continuation, state, token, flags);
 
             void IValueTaskSource.GetResult(short token)
             {
-                Debug.Assert(_hasWaiter == 0);
+                Debug.Assert(!_hasWaiter);
 
                 // Clean up the registration.  This will wait for any in-flight cancellation to complete.
                 _waitSourceCancellation.Dispose();
@@ -312,7 +312,7 @@ void IValueTaskSource.GetResult(short token)
 
             public void SignalWaiter()
             {
-                if (Interlocked.Exchange(ref _hasWaiter, 0) == 1)
+                if (Interlocked.Exchange(ref _hasWaiter, false))
                 {
                     _waitSource.SetResult(true);
                 }
@@ -322,21 +322,21 @@ private void CancelWaiter(CancellationToken cancellationToken)
             {
                 Debug.Assert(cancellationToken.IsCancellationRequested);
 
-                if (Interlocked.Exchange(ref _hasWaiter, 0) == 1)
+                if (Interlocked.Exchange(ref _hasWaiter, false))
                 {
                     _waitSource.SetException(ExceptionDispatchInfo.SetCurrentStackTrace(new OperationCanceledException(cancellationToken)));
                 }
             }
 
             public void Reset()
             {
-                if (_hasWaiter != 0)
+                if (_hasWaiter)
                 {
                     throw new InvalidOperationException("Concurrent use is not supported");
                 }
 
                 _waitSource.Reset();
-                Volatile.Write(ref _hasWaiter, 1);
+                Volatile.Write(ref _hasWaiter, true);
             }
 
             public void Wait()

src/libraries/Common/tests/System/Net/WebSockets/WebSocketStream.cs

@@ -21,8 +21,8 @@ public class WebSocketStream : Stream
         // Used by the class to indicate that the stream is writable.
         private bool _writeable;
 
-        // Whether Dispose has been called. 0 == false, 1 == true
-        private int _disposed;
+        // Whether Dispose has been called.
+        private bool _disposed;
 
         public WebSocketStream(WebSocket socket)
             : this(socket, FileAccess.ReadWrite, ownsSocket: false)
@@ -140,7 +140,7 @@ public void Close(int timeout)
 
         protected override void Dispose(bool disposing)
         {
-            if (Interlocked.Exchange(ref _disposed, 1) != 0)
+            if (Interlocked.Exchange(ref _disposed, true))
             {
                 return;
             }
@@ -269,7 +269,7 @@ public override void SetLength(long value)
 
         private void ThrowIfDisposed()
         {
-            ObjectDisposedException.ThrowIf(_disposed != 0, this);
+            ObjectDisposedException.ThrowIf(_disposed, this);
         }
 
         private static IOException WrapException(string resourceFormatString, Exception innerException)