Remove class constraint from Interlocked.{Compare}Exchange (#104558)

* 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.

---------

Co-authored-by: Michal Strehovský <[email protected]>

Author: stephentoub

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

@@ -102,21 +102,6 @@ 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 +168,6 @@ 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/jit/importercalls.cpp

@@ -4130,11 +4130,7 @@ GenTree* Compiler::impIntrinsic(CORINFO_CLASS_HANDLE    clsHnd,
             case NI_System_Threading_Interlocked_Exchange:
             case NI_System_Threading_Interlocked_ExchangeAdd:
             {
-                assert(callType != TYP_STRUCT);
-                assert(sig->numArgs == 2);
-
                 var_types retType = JITtype2varType(sig->retType);
-                assert((genTypeSize(retType) >= 4) || (ni == NI_System_Threading_Interlocked_Exchange));
 
                 if (genTypeSize(retType) > TARGET_POINTER_SIZE)
                 {
@@ -4159,6 +4155,10 @@ GenTree* Compiler::impIntrinsic(CORINFO_CLASS_HANDLE    clsHnd,
                     break;
                 }
 
+                assert(callType != TYP_STRUCT);
+                assert(sig->numArgs == 2);
+                assert((genTypeSize(retType) >= 4) || (ni == NI_System_Threading_Interlocked_Exchange));
+
                 GenTree* op2 = impPopStack().val;
                 GenTree* op1 = impPopStack().val;
 

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/NativeAotILProvider.cs

@@ -46,12 +46,6 @@ private static MethodIL TryGetIntrinsicMethodIL(MethodDesc method)
 
             switch (owningType.Name)
             {
-                case "Interlocked":
-                    {
-                        if (owningType.Namespace == "System.Threading")
-                            return InterlockedIntrinsics.EmitIL(method);
-                    }
-                    break;
                 case "Unsafe":
                     {
                         if (owningType.Namespace == "System.Runtime.CompilerServices")
@@ -108,6 +102,12 @@ private static MethodIL TryGetPerInstantiationIntrinsicMethodIL(MethodDesc metho
 
             switch (owningType.Name)
             {
+                case "Interlocked":
+                    {
+                        if (owningType.Namespace == "System.Threading")
+                            return InterlockedIntrinsics.EmitIL(method);
+                    }
+                    break;
                 case "Activator":
                     {
                         TypeSystemContext context = owningType.Context;

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

@@ -19,6 +19,7 @@ public static MethodIL EmitIL(
             MethodDesc method)
         {
             Debug.Assert(((MetadataType)method.OwningType).Name == "Interlocked");
+            Debug.Assert(!method.IsGenericMethodDefinition);
 
             if (method.HasInstantiation && method.Name == "CompareExchange")
             {
@@ -30,22 +31,45 @@ 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.IsPrimitive || tType.IsEnum) && (tType.UnderlyingType.Category is not (TypeFlags.Single or TypeFlags.Double)))
+                    {
+                        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.Int32),
+                            _ => method.Context.GetWellKnownType(WellKnownType.Int64),
+                        };
+                    }
+
+                    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/tools/aot/ILCompiler.ReadyToRun/IL/ReadyToRunILProvider.cs

@@ -85,11 +85,6 @@ private MethodIL TryGetIntrinsicMethodIL(MethodDesc method)
                 return UnsafeIntrinsics.EmitIL(method);
             }
 
-            if (mdType.Name == "Interlocked" && mdType.Namespace == "System.Threading")
-            {
-                return InterlockedIntrinsics.EmitIL(_compilationModuleGroup, method);
-            }
-
             return null;
         }
 
@@ -114,6 +109,11 @@ private MethodIL TryGetPerInstantiationIntrinsicMethodIL(MethodDesc method)
                 return TryGetIntrinsicMethodILForActivator(method);
             }
 
+            if (mdType.Name == "Interlocked" && mdType.Namespace == "System.Threading")
+            {
+                return InterlockedIntrinsics.EmitIL(_compilationModuleGroup, method);
+            }
+
             return null;
         }
 

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_INT,   CompareExchange, SM_RefInt_Int_Int_RetInt)
+DEFINE_METHOD(INTERLOCKED,          COMPARE_EXCHANGE_LONG,  CompareExchange, SM_RefLong_Long_Long_RetLong)
 
 DEFINE_CLASS(RAW_DATA,              CompilerServices,       RawData)
 DEFINE_FIELD(RAW_DATA,              DATA,                   Data)

src/coreclr/vm/jitinterface.cpp

@@ -7154,28 +7154,79 @@ bool getILIntrinsicImplementationForInterlocked(MethodDesc * ftn,
     if (ftn->GetMemberDef() != CoreLibBinder::GetMethod(METHOD__INTERLOCKED__COMPARE_EXCHANGE_T)->GetMemberDef())
         return false;
 
-    // Get MethodDesc for non-generic System.Threading.Interlocked.CompareExchange()
-    MethodDesc* cmpxchgObject = CoreLibBinder::GetMethod(METHOD__INTERLOCKED__COMPARE_EXCHANGE_OBJECT);
-
-    // Setup up the body of the method
-    static BYTE il[] = {
-                          CEE_LDARG_0,
-                          CEE_LDARG_1,
-                          CEE_LDARG_2,
-                          CEE_CALL,0,0,0,0,
-                          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);
+    // Determine the type of the generic T method parameter
+    _ASSERTE(ftn->HasMethodInstantiation());
+    _ASSERTE(ftn->GetNumGenericMethodArgs() == 1);
+    TypeHandle typeHandle = ftn->GetMethodInstantiation()[0];
+
+    // Setup up the body of the CompareExchange methods; the method token will be patched on first use.
+    static BYTE il[5][9] =
+    {
+        { CEE_LDARG_0, CEE_LDARG_1, CEE_LDARG_2, CEE_CALL, 0, 0, 0, 0, CEE_RET }, // object
+        { CEE_LDARG_0, CEE_LDARG_1, CEE_LDARG_2, CEE_CALL, 0, 0, 0, 0, CEE_RET }, // byte
+        { CEE_LDARG_0, CEE_LDARG_1, CEE_LDARG_2, CEE_CALL, 0, 0, 0, 0, CEE_RET }, // ushort
+        { CEE_LDARG_0, CEE_LDARG_1, CEE_LDARG_2, CEE_CALL, 0, 0, 0, 0, CEE_RET }, // int
+        { CEE_LDARG_0, CEE_LDARG_1, CEE_LDARG_2, CEE_CALL, 0, 0, 0, 0, CEE_RET }, // long
+    };
+
+    // Based on the generic method parameter, determine which overload of CompareExchange
+    // to delegate to, or if we can't handle the type at all.
+    int ilIndex;
+    MethodDesc* cmpxchgMethod;
+    if (!typeHandle.IsValueType())
+    {
+        ilIndex = 0;
+        cmpxchgMethod = CoreLibBinder::GetMethod(METHOD__INTERLOCKED__COMPARE_EXCHANGE_OBJECT);
+    }
+    else
+    {
+        CorElementType elementType = typeHandle.GetVerifierCorElementType();
+        if (!CorTypeInfo::IsPrimitiveType(elementType) ||
+            elementType == ELEMENT_TYPE_R4 ||
+            elementType == ELEMENT_TYPE_R8)
+        {
+            return false;
+        }
+        else
+        {
+            switch (typeHandle.GetSize())
+            {
+                case 1:
+                    ilIndex = 1;
+                    cmpxchgMethod = CoreLibBinder::GetMethod(METHOD__INTERLOCKED__COMPARE_EXCHANGE_BYTE);
+                    break;
+
+                case 2:
+                    ilIndex = 2;
+                    cmpxchgMethod = CoreLibBinder::GetMethod(METHOD__INTERLOCKED__COMPARE_EXCHANGE_USHRT);
+                    break;
+
+                case 4:
+                    ilIndex = 3;
+                    cmpxchgMethod = CoreLibBinder::GetMethod(METHOD__INTERLOCKED__COMPARE_EXCHANGE_INT);
+                    break;
+
+                case 8:
+                    ilIndex = 4;
+                    cmpxchgMethod = CoreLibBinder::GetMethod(METHOD__INTERLOCKED__COMPARE_EXCHANGE_LONG);
+                    break;
+
+                default:
+                    _ASSERT(!"Unexpected primitive type size");
+                    return false;
+            }
+        }
+    }
+
+    mdMethodDef cmpxchgToken = cmpxchgMethod->GetMemberDef();
+    il[ilIndex][4] = (BYTE)((int)cmpxchgToken >> 0);
+    il[ilIndex][5] = (BYTE)((int)cmpxchgToken >> 8);
+    il[ilIndex][6] = (BYTE)((int)cmpxchgToken >> 16);
+    il[ilIndex][7] = (BYTE)((int)cmpxchgToken >> 24);
 
     // Initialize methInfo
-    methInfo->ILCode = const_cast<BYTE*>(il);
-    methInfo->ILCodeSize = sizeof(il);
+    methInfo->ILCode = const_cast<BYTE*>(il[ilIndex]);
+    methInfo->ILCodeSize = sizeof(il[ilIndex]);
     methInfo->maxStack = 3;
     methInfo->EHcount = 0;
     methInfo->options = (CorInfoOptions)0;

src/coreclr/vm/metasig.h

@@ -586,6 +586,8 @@ 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_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()