icsharpcode-ILSpy/ICSharpCode.Decompiler/IL/ControlFlow/AsyncAwaitDecompiler.cs

// Copyright (c) 2017 Daniel Grunwald
// 
// Permission is hereby granted, free of charge, to any person obtaining a copy of this
// software and associated documentation files (the "Software"), to deal in the Software
// without restriction, including without limitation the rights to use, copy, modify, merge,
// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
// to whom the Software is furnished to do so, subject to the following conditions:
// 
// The above copyright notice and this permission notice shall be included in all copies or
// substantial portions of the Software.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
// DEALINGS IN THE SOFTWARE.

using ICSharpCode.Decompiler.CSharp;
using ICSharpCode.Decompiler.DebugInfo;
using ICSharpCode.Decompiler.IL.Transforms;
using ICSharpCode.Decompiler.TypeSystem;
using ICSharpCode.Decompiler.Util;
using System;
using System.Collections.Generic;
using System.Collections.Immutable;
using System.Diagnostics;
using System.Linq;
using System.Reflection.Metadata;

namespace ICSharpCode.Decompiler.IL.ControlFlow
{
	/// <summary>
	/// Decompiler step for C# 5 async/await.
	/// </summary>
	class AsyncAwaitDecompiler : IILTransform
	{
		public static bool IsCompilerGeneratedStateMachine(TypeDefinitionHandle type, MetadataReader metadata)
		{
			TypeDefinition td;
			if (type.IsNil || (td = metadata.GetTypeDefinition(type)).GetDeclaringType().IsNil)
				return false;
			if (!(type.IsCompilerGenerated(metadata) || td.GetDeclaringType().IsCompilerGenerated(metadata)))
				return false;
			foreach (var i in td.GetInterfaceImplementations()) {
				var tr = metadata.GetInterfaceImplementation(i).Interface.GetFullTypeName(metadata);
				if (!tr.IsNested && tr.TopLevelTypeName.Namespace == "System.Runtime.CompilerServices" && tr.TopLevelTypeName.Name == "IAsyncStateMachine")
					return true;
			}
			return false;
		}

		public static bool IsCompilerGeneratedMainMethod(Metadata.PEFile module, MethodDefinitionHandle method)
		{
			var metadata = module.Metadata;
			var definition = metadata.GetMethodDefinition(method);
			var entrypoint = System.Reflection.Metadata.Ecma335.MetadataTokens.MethodDefinitionHandle(module.Reader.PEHeaders.CorHeader.EntryPointTokenOrRelativeVirtualAddress);
			return method == entrypoint && metadata.GetString(definition.Name).Equals("<Main>", StringComparison.Ordinal);
		}

		enum AsyncMethodType
		{
			Void,
			Task,
			TaskOfT,
			AsyncEnumerator,
			AsyncEnumerable
		}

		ILTransformContext context;

		// These fields are set by MatchTaskCreationPattern() or MatchEnumeratorCreationNewObj()
		IType taskType; // return type of the async method; or IAsyncEnumerable{T}/IAsyncEnumerator{T}
		IType underlyingReturnType; // return type of the method (only the "T" for Task{T}), for async enumerators this is the type being yielded
		AsyncMethodType methodType;
		ITypeDefinition stateMachineType;
		ITypeDefinition builderType;
		IField builderField;
		IField stateField;
		int initialState;
		Dictionary<IField, ILVariable> fieldToParameterMap = new Dictionary<IField, ILVariable>();
		Dictionary<ILVariable, ILVariable> cachedFieldToParameterMap = new Dictionary<ILVariable, ILVariable>();
		IField disposeModeField; // 'disposeMode' field (IAsyncEnumerable/IAsyncEnumerator only)

		// These fields are set by AnalyzeMoveNext():
		ILFunction moveNextFunction;
		ILVariable cachedStateVar; // variable in MoveNext that caches the stateField.
		TryCatch mainTryCatch;
		Block setResultAndExitBlock; // block that is jumped to for return statements
									 // Note: for async enumerators, a jump to setResultAndExitBlock is a 'yield break;'
		int finalState;       // final state after the setResultAndExitBlock
		bool finalStateKnown;
		ILVariable resultVar; // the variable that gets returned by the setResultAndExitBlock
		Block setResultYieldBlock; // block that is jumped to for 'yield return' statements
		ILVariable doFinallyBodies;

		// These fields are set by AnalyzeStateMachine():
		int smallestAwaiterVarIndex;
		HashSet<Leave> moveNextLeaves = new HashSet<Leave>();

		// For each block containing an 'await', stores the awaiter variable, and the field storing the awaiter
		// across the yield point.
		Dictionary<Block, (ILVariable awaiterVar, IField awaiterField)> awaitBlocks = new Dictionary<Block, (ILVariable awaiterVar, IField awaiterField)>();

		int catchHandlerOffset;
		List<AsyncDebugInfo.Await> awaitDebugInfos = new List<AsyncDebugInfo.Await>();

		public void Run(ILFunction function, ILTransformContext context)
		{
			if (!context.Settings.AsyncAwait)
				return; // abort if async/await decompilation is disabled
			this.context = context;
			fieldToParameterMap.Clear();
			cachedFieldToParameterMap.Clear();
			awaitBlocks.Clear();
			awaitDebugInfos.Clear();
			moveNextLeaves.Clear();
			if (!MatchTaskCreationPattern(function) && !MatchAsyncEnumeratorCreationPattern(function))
				return;
			try {
				AnalyzeMoveNext();
				ValidateCatchBlock();
				AnalyzeDisposeAsync();
			} catch (SymbolicAnalysisFailedException) {
				return;
			}

			InlineBodyOfMoveNext(function);
			function.CheckInvariant(ILPhase.InAsyncAwait);
			CleanUpBodyOfMoveNext(function);
			function.CheckInvariant(ILPhase.InAsyncAwait);

			AnalyzeStateMachine(function);
			DetectAwaitPattern(function);
			CleanDoFinallyBodies(function);

			context.Step("Translate fields to local accesses", function);
			YieldReturnDecompiler.TranslateFieldsToLocalAccess(function, function, fieldToParameterMap);
			TranslateCachedFieldsToLocals();

			FinalizeInlineMoveNext(function);
			if (methodType == AsyncMethodType.AsyncEnumerable || methodType == AsyncMethodType.AsyncEnumerator) {
				((BlockContainer)function.Body).ExpectedResultType = StackType.Void;
			} else {
				((BlockContainer)function.Body).ExpectedResultType = underlyingReturnType.GetStackType();
			}

			// Re-run control flow simplification over the newly constructed set of gotos,
			// and inlining because TranslateFieldsToLocalAccess() might have opened up new inlining opportunities.
			function.RunTransforms(CSharpDecompiler.EarlyILTransforms(), context);

			AwaitInCatchTransform.Run(function, context);
			AwaitInFinallyTransform.Run(function, context);

			awaitDebugInfos.SortBy(row => row.YieldOffset);
			function.AsyncDebugInfo = new AsyncDebugInfo(catchHandlerOffset, awaitDebugInfos.ToImmutableArray());
		}

		private void CleanUpBodyOfMoveNext(ILFunction function)
		{
			context.StepStartGroup("CleanUpBodyOfMoveNext", function);
			// Copy-propagate stack slots holding an 'ldloca':
			foreach (var stloc in function.Descendants.OfType<StLoc>().Where(s => s.Variable.Kind == VariableKind.StackSlot && s.Variable.IsSingleDefinition && s.Value is LdLoca).ToList()) {
				CopyPropagation.Propagate(stloc, context);
			}

			// Simplify stobj(ldloca) -> stloc
			foreach (var stobj in function.Descendants.OfType<StObj>()) {
				EarlyExpressionTransforms.StObjToStLoc(stobj, context);
			}

			// Copy-propagate temporaries holding a copy of 'this'.
			foreach (var stloc in function.Descendants.OfType<StLoc>().Where(s => s.Variable.IsSingleDefinition && s.Value.MatchLdThis()).ToList()) {
				CopyPropagation.Propagate(stloc, context);
			}
			new RemoveDeadVariableInit().Run(function, context);
			foreach (var block in function.Descendants.OfType<Block>()) {
				// Run inlining, but don't remove dead variables (they might get revived by TranslateFieldsToLocalAccess)
				ILInlining.InlineAllInBlock(function, block, context);
				if (IsAsyncEnumerator) {
					// Remove lone 'ldc.i4', those are sometimes left over after C# compiler
					// optimizes out stores to the state variable.
					block.Instructions.RemoveAll(inst => inst.OpCode == OpCode.LdcI4);
				}
			}
			context.StepEndGroup();
		}

		#region MatchTaskCreationPattern
		bool MatchTaskCreationPattern(ILFunction function)
		{
			if (!(function.Body is BlockContainer blockContainer))
				return false;
			if (blockContainer.Blocks.Count != 1)
				return false;
			var body = blockContainer.EntryPoint.Instructions;
			if (body.Count < 4)
				return false;
			/* Example:
				V_0 is an instance of the compiler-generated struct/class,
				V_1 is an instance of the builder struct/class
				Block IL_0000 (incoming: 1)  {
					...
					stobj AsyncVoidMethodBuilder(ldflda [Field Async+<AwaitYield>d__3.<>t__builder](ldloca V_0), call Create())
					stobj System.Int32(ldflda [Field Async+<AwaitYield>d__3.<>1__state](ldloca V_0), ldc.i4 -1)
					stloc V_1(ldobj System.Runtime.CompilerServices.AsyncVoidMethodBuilder(ldflda [Field Async+<AwaitYield>d__3.<>t__builder](ldloc V_0)))
					call Start(ldloca V_1, ldloca V_0)
					leave IL_0000 (or ret for non-void async methods)
				}
				With custom task types, it's possible that the builder is a reference type.
				In that case, the variable V_1 may be inlined.
			*/

			// Check the second-to-last instruction (the start call) first, as we can get the most information from that
			int pos = body.Count - 2;
			if (!(body[pos] is CallInstruction startCall))
				return false;
			if (startCall.Method.Name != "Start")
				return false;
			taskType = function.Method.ReturnType;
			builderType = startCall.Method.DeclaringTypeDefinition;
			if (taskType.IsKnownType(KnownTypeCode.Void)) {
				methodType = AsyncMethodType.Void;
				underlyingReturnType = taskType;
				if (builderType?.FullTypeName != new TopLevelTypeName("System.Runtime.CompilerServices", "AsyncVoidMethodBuilder"))
					return false;
			} else if (TaskType.IsNonGenericTaskType(taskType, out var builderTypeName)) {
				methodType = AsyncMethodType.Task;
				underlyingReturnType = context.TypeSystem.FindType(KnownTypeCode.Void);
				if (builderType?.FullTypeName != builderTypeName)
					return false;
			} else if (TaskType.IsGenericTaskType(taskType, out builderTypeName)) {
				methodType = AsyncMethodType.TaskOfT;
				if (taskType.IsKnownType(KnownTypeCode.TaskOfT))
					underlyingReturnType = TaskType.UnpackTask(context.TypeSystem, taskType);
				else
					underlyingReturnType = startCall.Method.DeclaringType.TypeArguments[0];
				if (builderType?.FullTypeName != builderTypeName)
					return false;
			} else {
				return false;
			}
			if (startCall.Arguments.Count != 2)
				return false;
			ILInstruction loadBuilderExpr = startCall.Arguments[0];
			if (!startCall.Arguments[1].MatchLdLoca(out ILVariable stateMachineVar))
				return false;
			stateMachineType = stateMachineVar.Type.GetDefinition();
			if (stateMachineType == null)
				return false;
			pos--;

			if (loadBuilderExpr.MatchLdLocRef(out ILVariable builderVar)) {
				// Check third-to-last instruction (copy of builder)
				// stloc builder(ldfld StateMachine::<>t__builder(ldloc stateMachine))
				if (!body[pos].MatchStLoc(builderVar, out loadBuilderExpr))
					return false;
				pos--;
			}
			if (!loadBuilderExpr.MatchLdFld(out var loadStateMachineForBuilderExpr, out builderField))
				return false;
			builderField = (IField)builderField.MemberDefinition;
			if (!(loadStateMachineForBuilderExpr.MatchLdLocRef(stateMachineVar) || loadStateMachineForBuilderExpr.MatchLdLoc(stateMachineVar)))
				return false;

			// Check the last instruction (ret)
			if (methodType == AsyncMethodType.Void) {
				if (!body.Last().MatchLeave(blockContainer))
					return false;
			} else {
				// ret(call(AsyncTaskMethodBuilder::get_Task, ldflda(StateMachine::<>t__builder, ldloca(stateMachine))))
				if (!body.Last().MatchReturn(out var returnValue))
					return false;
				if (!MatchCall(returnValue, "get_Task", out var getTaskArgs) || getTaskArgs.Count != 1)
					return false;
				ILInstruction target;
				IField builderField2;
				if (builderType.IsReferenceType == true) {
					if (!getTaskArgs[0].MatchLdFld(out target, out builderField2))
						return false;
				} else {
					if (!getTaskArgs[0].MatchLdFlda(out target, out builderField2))
						return false;
				}
				if (builderField2.MemberDefinition != builderField)
					return false;
				if (!(target.MatchLdLoc(stateMachineVar) || target.MatchLdLoca(stateMachineVar)))
					return false;
			}

			// Check the last field assignment - this should be the state field
			// stfld <>1__state(ldloca stateField, ldc.i4 -1)
			if (!MatchStFld(body[pos], stateMachineVar, out stateField, out var initialStateExpr))
				return false;
			if (!initialStateExpr.MatchLdcI4(out initialState))
				return false;
			if (initialState != -1)
				return false;
			pos--;

			// Check the second-to-last field assignment - this should be the builder field
			// stfld StateMachine.builder(ldloca stateMachine, call Create())
			if (pos < 0)
				return false;
			if (!MatchStFld(body[pos], stateMachineVar, out var builderField3, out var builderInitialization))
				return false;
			if (builderField3 != builderField)
				return false;
			if (!(builderInitialization is Call createCall))
				return false;
			if (createCall.Method.Name != "Create" || createCall.Arguments.Count != 0)
				return false;

			int stopPos = pos;
			pos = 0;
			if (stateMachineType.Kind == TypeKind.Class) {
				// If state machine is a class, the first instruction creates an instance:
				// stloc stateMachine(newobj StateMachine.ctor())
				if (!body[pos].MatchStLoc(stateMachineVar, out var init))
					return false;
				if (!(init is NewObj newobj && newobj.Arguments.Count == 0 && newobj.Method.DeclaringTypeDefinition == stateMachineType))
					return false;
				pos++;
			}
			for (; pos < stopPos; pos++) {
				// stfld StateMachine.field(ldloca stateMachine, ldvar(param))
				if (!MatchStFld(body[pos], stateMachineVar, out var field, out var fieldInit))
					return false;
				if (fieldInit.MatchLdLoc(out var v) && v.Kind == VariableKind.Parameter) {
					// OK, copies parameter into state machine
					fieldToParameterMap[field] = v;
				} else if (fieldInit is LdObj ldobj && ldobj.Target.MatchLdThis()) {
					// stfld <>4__this(ldloc stateMachine, ldobj AsyncInStruct(ldloc this))
					fieldToParameterMap[field] = ((LdLoc)ldobj.Target).Variable;
				} else {
					return false;
				}
			}

			return true;
		}

		/// <summary>
		/// Matches a (potentially virtual) instance method call.
		/// </summary>
		static bool MatchCall(ILInstruction inst, string name, out InstructionCollection<ILInstruction> args)
		{
			if (inst is CallInstruction call && (call.OpCode == OpCode.Call || call.OpCode == OpCode.CallVirt)
				&& call.Method.Name == name && !call.Method.IsStatic)
			{
				args = call.Arguments;
				return args.Count > 0;
			}
			args = null;
			return false;
		}

		/// <summary>
		/// Matches a store to the state machine.
		/// </summary>
		static bool MatchStFld(ILInstruction stfld, ILVariable stateMachineVar, out IField field, out ILInstruction value)
		{
			if (!stfld.MatchStFld(out var target, out field, out value))
				return false;
			field = field.MemberDefinition as IField;
			return field != null && target.MatchLdLocRef(stateMachineVar);
		}
		#endregion

		#region MatchAsyncEnumeratorCreationPattern
		private bool MatchAsyncEnumeratorCreationPattern(ILFunction function)
		{
			if (!context.Settings.AsyncEnumerator)
				return false;
			taskType = function.ReturnType;
			if (taskType.IsKnownType(KnownTypeCode.IAsyncEnumeratorOfT)) {
				methodType = AsyncMethodType.AsyncEnumerator;
			} else if (taskType.IsKnownType(KnownTypeCode.IAsyncEnumerableOfT)) {
				methodType = AsyncMethodType.AsyncEnumerable;
			} else {
				return false;
			}
			underlyingReturnType = taskType.TypeArguments.Single();
			if (!(function.Body is BlockContainer blockContainer))
				return false;
			if (blockContainer.Blocks.Count != 1)
				return false;
			var body = blockContainer.EntryPoint;
			if (body.Instructions.Count == 1) {
				// No parameters passed to enumerator (not even 'this'):
				// ret(newobj(...))
				if (!body.Instructions[0].MatchReturn(out var newObj))
					return false;
				if (MatchEnumeratorCreationNewObj(newObj, context, out initialState, out stateMachineType)) {
					// HACK: the normal async/await logic expects 'initialState' to be the 'in progress' state
					initialState = -1;
					try {
						AnalyzeEnumeratorCtor(((NewObj)newObj).Method, context, out builderField, out builderType, out stateField);
					} catch (SymbolicAnalysisFailedException) {
						return false;
					}
					return true;
				} else {
					return false;
				}
			} else {

				// stloc v(newobj<CountUpSlowly> d__0..ctor(ldc.i4 - 2))
				// stfld <>4__this(ldloc v, ldloc this)
				// stfld <>3__otherParam(ldloc v, ldloc otherParam)
				// leave IL_0000(ldloc v)
				int pos = 0;
				if (!body.Instructions[pos].MatchStLoc(out var v, out var newObj))
					return false;
				if (!MatchEnumeratorCreationNewObj(newObj, context, out initialState, out stateMachineType))
					return false;
				pos++;

				while (MatchStFld(body.Instructions[pos], v, out var field, out var value)) {
					if (value.MatchLdLoc(out var p) && p.Kind == VariableKind.Parameter) {
						fieldToParameterMap[field] = p;
					} else if (value is LdObj ldobj && ldobj.Target.MatchLdThis()) {
						fieldToParameterMap[field] = ((LdLoc)ldobj.Target).Variable;
					} else {
						return false;
					}
					pos++;
				}
				if (!body.Instructions[pos].MatchReturn(out var returnValue))
					return false;
				if (!returnValue.MatchLdLoc(v))
					return false;
				
				// HACK: the normal async/await logic expects 'initialState' to be the 'in progress' state
				initialState = -1;
				try {
					AnalyzeEnumeratorCtor(((NewObj)newObj).Method, context, out builderField, out builderType, out stateField);
					if (methodType == AsyncMethodType.AsyncEnumerable) {
						ResolveIEnumerableIEnumeratorFieldMapping();
					}
				} catch (SymbolicAnalysisFailedException) {
					return false;
				}
				return true;
			}
		}

		static bool MatchEnumeratorCreationNewObj(ILInstruction inst, ILTransformContext context,
			out int initialState, out ITypeDefinition stateMachineType)
		{
			initialState = default;
			stateMachineType = default;
			// newobj(CurrentType/...::.ctor, ldc.i4(-2))
			if (!(inst is NewObj newObj))
				return false;
			if (newObj.Arguments.Count != 1)
				return false;
			if (!newObj.Arguments[0].MatchLdcI4(out initialState))
				return false;
			stateMachineType = newObj.Method.DeclaringTypeDefinition;
			if (stateMachineType == null)
				return false;
			if (stateMachineType.DeclaringTypeDefinition != context.Function.Method.DeclaringTypeDefinition)
				return false;
			return IsCompilerGeneratorAsyncEnumerator(
				(TypeDefinitionHandle)stateMachineType.MetadataToken,
				context.TypeSystem.MainModule.metadata);
		}

		public static bool IsCompilerGeneratorAsyncEnumerator(TypeDefinitionHandle type, MetadataReader metadata)
		{
			TypeDefinition td;
			if (type.IsNil || !type.IsCompilerGeneratedOrIsInCompilerGeneratedClass(metadata) || (td = metadata.GetTypeDefinition(type)).GetDeclaringType().IsNil)
				return false;
			foreach (var i in td.GetInterfaceImplementations()) {
				var tr = metadata.GetInterfaceImplementation(i).Interface.GetFullTypeName(metadata);
				if (!tr.IsNested && tr.TopLevelTypeName.Namespace == "System.Collections.Generic" && tr.TopLevelTypeName.Name == "IAsyncEnumerator" && tr.TopLevelTypeName.TypeParameterCount == 1)
					return true;
			}
			return false;
		}

		static void AnalyzeEnumeratorCtor(IMethod ctor, ILTransformContext context, out IField builderField, out ITypeDefinition builderType, out IField stateField)
		{
			builderField = null;
			stateField = null;

			var ctorHandle = (MethodDefinitionHandle)ctor.MetadataToken;
			Block body = YieldReturnDecompiler.SingleBlock(YieldReturnDecompiler.CreateILAst(ctorHandle, context).Body);
			if (body == null)
				throw new SymbolicAnalysisFailedException("Missing enumeratorCtor.Body");
			// Block IL_0000 (incoming: 1) {
			//   call Object..ctor(ldloc this)
			// 	 stfld <>1__state(ldloc this, ldloc <>1__state)
			// 	 stfld <>t__builder(ldloc this, call Create())
			// 	 leave IL_0000 (nop)
			// }
			foreach (var inst in body.Instructions) {
				if (inst.MatchStFld(out var target, out var field, out var value)
				&& target.MatchLdThis()
				&& value.MatchLdLoc(out var arg)
				&& arg.Kind == VariableKind.Parameter && arg.Index == 0) {
					stateField = (IField)field.MemberDefinition;
				}
				if (inst.MatchStFld(out target, out field, out value)
					&& target.MatchLdThis()
					&& value is Call call && call.Method.Name == "Create") {
					builderField = (IField)field.MemberDefinition;
				}
			}
			if (stateField == null || builderField == null)
				throw new SymbolicAnalysisFailedException();

			builderType = builderField.Type.GetDefinition();
			if (builderType == null)
				throw new SymbolicAnalysisFailedException();
		}

		private void ResolveIEnumerableIEnumeratorFieldMapping()
		{
			var getAsyncEnumerator = stateMachineType.Methods.FirstOrDefault(m => m.Name.EndsWith(".GetAsyncEnumerator", StringComparison.Ordinal));
			if (getAsyncEnumerator == null)
				throw new SymbolicAnalysisFailedException();
			YieldReturnDecompiler.ResolveIEnumerableIEnumeratorFieldMapping((MethodDefinitionHandle)getAsyncEnumerator.MetadataToken, context, fieldToParameterMap);
		}
		#endregion

		#region AnalyzeMoveNext
		/// <summary>
		/// First peek into MoveNext(); analyzes everything outside the big try-catch.
		/// </summary>
		void AnalyzeMoveNext()
		{
			if (stateMachineType.MetadataToken.IsNil)
				throw new SymbolicAnalysisFailedException();
			var metadata = context.PEFile.Metadata;
			var moveNextMethod = metadata.GetTypeDefinition((TypeDefinitionHandle)stateMachineType.MetadataToken)
				.GetMethods().FirstOrDefault(f => metadata.GetString(metadata.GetMethodDefinition(f).Name) == "MoveNext");
			if (moveNextMethod == null)
				throw new SymbolicAnalysisFailedException();
			moveNextFunction = YieldReturnDecompiler.CreateILAst(moveNextMethod, context);
			if (!(moveNextFunction.Body is BlockContainer blockContainer))
				throw new SymbolicAnalysisFailedException();
			if (blockContainer.EntryPoint.IncomingEdgeCount != 1)
				throw new SymbolicAnalysisFailedException();
			cachedStateVar = null;
			int pos = 0;
			while (blockContainer.EntryPoint.Instructions[pos] is StLoc stloc) {
				// stloc V_1(ldfld <>4__this(ldloc this))
				if (!stloc.Value.MatchLdFld(out var target, out var field))
					throw new SymbolicAnalysisFailedException();
				if (!target.MatchLdThis())
					throw new SymbolicAnalysisFailedException();
				if (field.MemberDefinition == stateField && cachedStateVar == null) {
					// stloc(cachedState, ldfld(valuetype StateMachineStruct::<>1__state, ldloc(this)))
					cachedStateVar = stloc.Variable;
				} else if (fieldToParameterMap.TryGetValue((IField)field.MemberDefinition, out var param)) {
					if (!stloc.Variable.IsSingleDefinition)
						throw new SymbolicAnalysisFailedException();
					cachedFieldToParameterMap[stloc.Variable] = param;
				} else {
					throw new SymbolicAnalysisFailedException();
				}
				pos++;
			}
			mainTryCatch = blockContainer.EntryPoint.Instructions[pos] as TryCatch;
			if (mainTryCatch == null)
				throw new SymbolicAnalysisFailedException();
			// CatchHandler will be validated in ValidateCatchBlock()

			if (((BlockContainer)mainTryCatch.TryBlock).EntryPoint.Instructions[0] is StLoc initDoFinallyBodies
				&& initDoFinallyBodies.Variable.Kind == VariableKind.Local
				&& initDoFinallyBodies.Variable.Type.IsKnownType(KnownTypeCode.Boolean)
				&& initDoFinallyBodies.Value.MatchLdcI4(1)) {
				doFinallyBodies = initDoFinallyBodies.Variable;
			}

			Debug.Assert(blockContainer.Blocks[0] == blockContainer.EntryPoint); // already checked this block
			pos = 1;
			if (MatchYieldBlock(blockContainer, pos)) {
				setResultYieldBlock = blockContainer.Blocks[pos];
				pos++;
			} else {
				setResultYieldBlock = null;
			}

			setResultAndExitBlock = blockContainer.Blocks.ElementAtOrDefault(pos);
			CheckSetResultAndExitBlock(blockContainer);

			if (pos + 1 < blockContainer.Blocks.Count)
				throw new SymbolicAnalysisFailedException("too many blocks");
		}

		private bool IsAsyncEnumerator => methodType == AsyncMethodType.AsyncEnumerable || methodType == AsyncMethodType.AsyncEnumerator;

		bool MatchYieldBlock(BlockContainer blockContainer, int pos)
		{
			if (!IsAsyncEnumerator)
				return false;
			var block = blockContainer.Blocks.ElementAtOrDefault(pos);
			if (block == null)
				return false;
			// call SetResult(ldflda <>v__promiseOfValueOrEnd(ldloc this), ldc.i4 1)
			// leave IL_0000(nop)
			if (block.Instructions.Count != 2)
				return false;
			if (!MatchCall(block.Instructions[0], "SetResult", out var args))
				return false;
			if (args.Count != 2)
				return false;
			if (!IsBuilderOrPromiseFieldOnThis(args[0]))
				return false;
			if (!args[1].MatchLdcI4(1))
				return false;
			return block.Instructions[1].MatchLeave(blockContainer);
		}
		void CheckSetResultAndExitBlock(BlockContainer blockContainer)
		{
			if (setResultAndExitBlock == null) {
				// This block can be absent if the function never exits normally,
				// but always throws an exception/loops infinitely.
				resultVar = null;
				finalStateKnown = false; // final state will be detected in ValidateCatchBlock() instead
				return;
			}
			// stfld <>1__state(ldloc this, ldc.i4 -2)
			// [optional] stfld <>u__N(ldloc this, ldnull)
			// call SetResult(ldflda <>t__builder(ldloc this), ldloc result)
			// [optional] call Complete(ldflda <>t__builder(ldloc this))
			// leave IL_0000
			int pos = 0;
			if (!MatchStateAssignment(setResultAndExitBlock.Instructions[pos], out finalState))
				throw new SymbolicAnalysisFailedException();
			finalStateKnown = true;
			pos++;
			MatchHoistedLocalCleanup(setResultAndExitBlock, ref pos);
			if (!MatchCall(setResultAndExitBlock.Instructions[pos], "SetResult", out var args))
				throw new SymbolicAnalysisFailedException();
			if (!IsBuilderOrPromiseFieldOnThis(args[0]))
				throw new SymbolicAnalysisFailedException();
			switch (methodType) {
				case AsyncMethodType.TaskOfT:
					if (args.Count != 2)
						throw new SymbolicAnalysisFailedException();
					if (!args[1].MatchLdLoc(out resultVar))
						throw new SymbolicAnalysisFailedException();
					break;
				case AsyncMethodType.Task:
				case AsyncMethodType.Void:
					resultVar = null;
					if (args.Count != 1)
						throw new SymbolicAnalysisFailedException();
					break;
				case AsyncMethodType.AsyncEnumerable:
				case AsyncMethodType.AsyncEnumerator:
					resultVar = null;
					if (args.Count != 2)
						throw new SymbolicAnalysisFailedException();
					if (!args[1].MatchLdcI4(0))
						throw new SymbolicAnalysisFailedException();
					break;
			}
			pos++;
			if (MatchCall(setResultAndExitBlock.Instructions[pos], "Complete", out args)) {
				if (!(args.Count == 1 && IsBuilderFieldOnThis(args[0])))
					throw new SymbolicAnalysisFailedException();
				pos++;
			}
			if (!setResultAndExitBlock.Instructions[pos].MatchLeave(blockContainer))
				throw new SymbolicAnalysisFailedException();
		}

		private void MatchHoistedLocalCleanup(Block block, ref int pos)
		{
			while (block.Instructions[pos].MatchStFld(out var target, out _, out var value)) {
				// https://github.com/dotnet/roslyn/pull/39735 hoisted local cleanup
				if (!target.MatchLdThis())
					throw new SymbolicAnalysisFailedException();
				if (!(value.MatchLdNull() || value is DefaultValue))
					throw new SymbolicAnalysisFailedException();
				pos++;
			}
		}

		void ValidateCatchBlock()
		{
			// catch E_143 : System.Exception if (ldc.i4 1) BlockContainer {
			// 	Block IL_008f (incoming: 1)  {
			// 		stloc exception(ldloc E_143)
			// 		stfld <>1__state(ldloc this, ldc.i4 -2)
			//      [optional] stfld <>u__N(ldloc this, ldnull)
			// 		call SetException(ldfld <>t__builder(ldloc this), ldloc exception)
			//      [optional] call Complete(ldfld <>t__builder(ldloc this))
			// 		leave IL_0000
			// 	}
			// }
			if (mainTryCatch?.Handlers.Count != 1)
				throw new SymbolicAnalysisFailedException();
			var handler = mainTryCatch.Handlers[0];
			if (!handler.Variable.Type.IsKnownType(KnownTypeCode.Exception))
				throw new SymbolicAnalysisFailedException();
			if (!handler.Filter.MatchLdcI4(1))
				throw new SymbolicAnalysisFailedException();
			var catchBlock = YieldReturnDecompiler.SingleBlock(handler.Body);
			if (catchBlock == null)
				throw new SymbolicAnalysisFailedException();
			catchHandlerOffset = catchBlock.StartILOffset;
			// stloc exception(ldloc E_143)
			if (!(catchBlock.Instructions[0] is StLoc stloc))
				throw new SymbolicAnalysisFailedException();
			if (!stloc.Value.MatchLdLoc(handler.Variable))
				throw new SymbolicAnalysisFailedException();
			// stfld <>1__state(ldloc this, ldc.i4 -2)
			if (!MatchStateAssignment(catchBlock.Instructions[1], out int newState))
				throw new SymbolicAnalysisFailedException();
			if (finalStateKnown) {
				if (newState != finalState)
					throw new SymbolicAnalysisFailedException();
			} else {
				finalState = newState;
				finalStateKnown = true;
			}
			int pos = 2;
			MatchHoistedLocalCleanup(catchBlock, ref pos);
			// call SetException(ldfld <>t__builder(ldloc this), ldloc exception)
			if (!MatchCall(catchBlock.Instructions[pos], "SetException", out var args))
				throw new SymbolicAnalysisFailedException();
			if (args.Count != 2)
				throw new SymbolicAnalysisFailedException();
			if (!IsBuilderOrPromiseFieldOnThis(args[0]))
				throw new SymbolicAnalysisFailedException();
			if (!args[1].MatchLdLoc(stloc.Variable))
				throw new SymbolicAnalysisFailedException();

			pos++;
			// [optional] call Complete(ldfld <>t__builder(ldloc this))
			if (MatchCall(catchBlock.Instructions[pos], "Complete", out args)) {
				if (!(args.Count == 1 && IsBuilderFieldOnThis(args[0])))
					throw new SymbolicAnalysisFailedException();
				pos++;
			}

			// leave IL_0000
			if (!catchBlock.Instructions[pos].MatchLeave((BlockContainer)moveNextFunction.Body))
				throw new SymbolicAnalysisFailedException();
		}

		bool IsBuilderFieldOnThis(ILInstruction inst)
		{
			IField field;
			ILInstruction target;
			if (builderType.IsReferenceType == true) {
				// ldfld(StateMachine::<>t__builder, ldloc(this))
				if (!inst.MatchLdFld(out target, out field))
					return false;
			} else {
				// ldflda(StateMachine::<>t__builder, ldloc(this))
				if (!inst.MatchLdFlda(out target, out field))
					return false;
			}
			return target.MatchLdThis() && field.MemberDefinition == builderField;
		}

		bool IsBuilderOrPromiseFieldOnThis(ILInstruction inst)
		{
			if (methodType == AsyncMethodType.AsyncEnumerable || methodType == AsyncMethodType.AsyncEnumerator) {
				return true; // TODO: check against uses of promise fields in other methods?
			} else {
				return IsBuilderFieldOnThis(inst);
			}
		}

		bool MatchStateAssignment(ILInstruction inst, out int newState)
		{
			// stfld(StateMachine::<>1__state, ldloc(this), ldc.i4(stateId))
			if (inst.MatchStFld(out var target, out var field, out var value)
				&& StackSlotValue(target).MatchLdThis()
				&& field.MemberDefinition == stateField
				&& StackSlotValue(value).MatchLdcI4(out newState))
			{
				return true;
			}
			newState = 0;
			return false;
		}

		/// <summary>
		/// Analyse the DisposeAsync() method in order to find the disposeModeField.
		/// </summary>
		private void AnalyzeDisposeAsync()
		{
			disposeModeField = null;
			if (!IsAsyncEnumerator) {
				return;
			}
			var disposeAsync = stateMachineType.Methods.FirstOrDefault(m => m.Name.EndsWith(".DisposeAsync", StringComparison.Ordinal));
			if (disposeAsync == null)
				throw new SymbolicAnalysisFailedException("Could not find DisposeAsync()");
			var disposeAsyncHandle = (MethodDefinitionHandle)disposeAsync.MetadataToken;
			var function = YieldReturnDecompiler.CreateILAst(disposeAsyncHandle, context);
			foreach (var store in function.Descendants) {
				if (!store.MatchStFld(out var target, out var field, out var value))
					continue;
				if (!target.MatchLdThis())
					continue;
				if (!value.MatchLdcI4(1))
					throw new SymbolicAnalysisFailedException();
				if (disposeModeField != null)
					throw new SymbolicAnalysisFailedException("Multiple stores to disposeMode in DisposeAsync()");
				disposeModeField = (IField)field.MemberDefinition;
			}
		}
		#endregion

		#region InlineBodyOfMoveNext
		void InlineBodyOfMoveNext(ILFunction function)
		{
			context.Step("Inline body of MoveNext()", function);
			function.Body = mainTryCatch.TryBlock;
			function.AsyncReturnType = underlyingReturnType;
			function.MoveNextMethod = moveNextFunction.Method;
			function.SequencePointCandidates = moveNextFunction.SequencePointCandidates;
			function.CodeSize = moveNextFunction.CodeSize;
			function.IsIterator = IsAsyncEnumerator;
			moveNextFunction.Variables.Clear();
			moveNextFunction.ReleaseRef();
			foreach (var branch in function.Descendants.OfType<Branch>()) {
				if (branch.TargetBlock == setResultAndExitBlock) {
					branch.ReplaceWith(new Leave((BlockContainer)function.Body, resultVar == null ? null : new LdLoc(resultVar)).WithILRange(branch));
				}
			}
			if (setResultYieldBlock != null) {
				// We still might have branches to this block; and we can't quite yet get rid of it.
				((BlockContainer)function.Body).Blocks.Add(setResultYieldBlock);
			}
			foreach (var leave in function.Descendants.OfType<Leave>()) {
				if (leave.TargetContainer == moveNextFunction.Body) {
					leave.TargetContainer = (BlockContainer)function.Body;
					moveNextLeaves.Add(leave);
				}
			}
			function.Variables.AddRange(function.Descendants.OfType<IInstructionWithVariableOperand>().Select(inst => inst.Variable).Distinct());
			function.Variables.RemoveDead();
			function.Variables.AddRange(fieldToParameterMap.Values);
		}

		void FinalizeInlineMoveNext(ILFunction function)
		{
			context.Step("FinalizeInlineMoveNext()", function);
			foreach (var leave in function.Descendants.OfType<Leave>()) {
				if (moveNextLeaves.Contains(leave)) {
					leave.ReplaceWith(new InvalidBranch {
						Message = "leave MoveNext - await not detected correctly"
					}.WithILRange(leave));
				}
			}
			// Delete dead loads of the state cache variable:
			foreach (var block in function.Descendants.OfType<Block>()) {
				for (int i = block.Instructions.Count - 1; i >= 0; i--) {
					if (block.Instructions[i].MatchStLoc(out var v, out var value)
						&& v.IsSingleDefinition && v.LoadCount == 0
						&& value.MatchLdLoc(cachedStateVar))
					{
						block.Instructions.RemoveAt(i);
					}
				}
			}
		}
		#endregion

		#region AnalyzeStateMachine

		/// <summary>
		/// Analyze the the state machine; and replace 'leave IL_0000' with await+jump to block that gets
		/// entered on the next MoveNext() call.
		/// </summary>
		void AnalyzeStateMachine(ILFunction function)
		{
			context.StepStartGroup("AnalyzeStateMachine()", function);
			smallestAwaiterVarIndex = int.MaxValue;
			foreach (var container in function.Descendants.OfType<BlockContainer>()) {
				// Use a separate state range analysis per container.
				var sra = new StateRangeAnalysis(StateRangeAnalysisMode.AsyncMoveNext, stateField, cachedStateVar);
				sra.CancellationToken = context.CancellationToken;
				sra.doFinallyBodies = doFinallyBodies;
				sra.AssignStateRanges(container, LongSet.Universe);
				var stateToBlockMap = sra.GetBlockStateSetMapping(container);

				foreach (var block in container.Blocks) {
					context.CancellationToken.ThrowIfCancellationRequested();
					if (block.Instructions.Last() is Leave leave && moveNextLeaves.Contains(leave)) {
						// This is likely an 'await' block
						context.Step($"AnalyzeAwaitBlock({block.StartILOffset:x4})", block);
						if (AnalyzeAwaitBlock(block, out var awaiterVar, out var awaiterField, out int state, out int yieldOffset)) {
							block.Instructions.Add(new Await(new LdLoca(awaiterVar)));
							Block targetBlock = stateToBlockMap.GetOrDefault(state);
							if (targetBlock != null) {
								awaitDebugInfos.Add(new AsyncDebugInfo.Await(yieldOffset, targetBlock.StartILOffset));
								block.Instructions.Add(new Branch(targetBlock));
							} else {
								block.Instructions.Add(new InvalidBranch("Could not find block for state " + state));
							}
							awaitBlocks.Add(block, (awaiterVar, awaiterField));
							if (awaiterVar.Index < smallestAwaiterVarIndex) {
								smallestAwaiterVarIndex = awaiterVar.Index.Value;
							}
						}
					} else if (block.Instructions.Last().MatchBranch(setResultYieldBlock)) {
						// This is a 'yield return' in an async enumerator.
						context.Step($"AnalyzeYieldReturn({block.StartILOffset:x4})", block);
						if (AnalyzeYieldReturn(block, out var yieldValue, out int state)) {
							block.Instructions.Add(new YieldReturn(yieldValue));
							Block targetBlock = stateToBlockMap.GetOrDefault(state);
							if (targetBlock != null) {
								block.Instructions.Add(new Branch(targetBlock));
							} else {
								block.Instructions.Add(new InvalidBranch("Could not find block for state " + state));
							}
						} else {
							block.Instructions.Add(new InvalidBranch("Could not detect 'yield return'"));
						}
					}
					TransformYieldBreak(block);
				}
				foreach (var block in container.Blocks) {
					SimplifyIfDisposeMode(block);
				}
				// Skip the state dispatcher and directly jump to the initial state
				var entryPoint = stateToBlockMap.GetOrDefault(initialState);
				if (entryPoint != null) {
					container.Blocks.Insert(0, new Block {
						Instructions = {
							new Branch(entryPoint)
						}
					});
				}
				container.SortBlocks(deleteUnreachableBlocks: true);
			}
			context.StepEndGroup();
		}

		private bool TransformYieldBreak(Block block)
		{
			// stfld disposeMode(ldloc this, ldc.i4 1)
			// br nextBlock
			if (block.Instructions.Count < 2)
				return false;
			if (!(block.Instructions.Last() is Branch branch))
				return false;
			if (!block.Instructions[block.Instructions.Count - 2].MatchStFld(out var target, out var field, out var value))
				return false;
			if (!target.MatchLdThis())
				return false;
			if (field.MemberDefinition != disposeModeField)
				return false;
			if (!value.MatchLdcI4(1))
				return false;

			// Detected a 'yield break;'
			context.Step($"TransformYieldBreak({block.StartILOffset:x4})", block);
			var breakTarget = FindYieldBreakTarget(branch.TargetBlock);
			if (breakTarget is Block targetBlock) {
				branch.TargetBlock = targetBlock;
			} else {
				Debug.Assert(breakTarget is BlockContainer);
				branch.ReplaceWith(new Leave((BlockContainer)breakTarget).WithILRange(branch));
			}
			return true;
		}

		ILInstruction FindYieldBreakTarget(Block block)
		{
			// We'll follow the branch and evaluate the following instructions
			// under the assumption that disposeModeField==1, which lets us follow a series of jumps
			// to determine the final target.
			var visited = new HashSet<Block>();
			var evalContext = new SymbolicEvaluationContext(disposeModeField);
			while (true) {
				for (int i = 0; i < block.Instructions.Count; i++) {
					ILInstruction inst = block.Instructions[i];
					while (inst.MatchIfInstruction(out var condition, out var trueInst, out var falseInst)) {
						var condVal = evalContext.Eval(condition).AsBool();
						if (condVal.Type == SymbolicValueType.IntegerConstant) {
							inst = condVal.Constant != 0 ? trueInst : falseInst;
						} else if (condVal.Type == SymbolicValueType.StateInSet) {
							inst = condVal.ValueSet.Contains(1) ? trueInst : falseInst;
						} else {
							return block;
						}
					}
					if (inst.MatchBranch(out var targetBlock)) {
						if (visited.Add(block)) {
							block = targetBlock;
							break; // continue with next block
						} else {
							return block; // infinite loop detected
						}
					} else if (inst is Leave leave && leave.Value.OpCode == OpCode.Nop) {
						return leave.TargetContainer;
					} else if (inst.OpCode == OpCode.Nop) {
						continue; // continue with next instruction in this block
					} else {
						return block;
					}
				}
			}
		}

		private bool SimplifyIfDisposeMode(Block block)
		{
			// Occasionally Roslyn optimizes out an "if (disposeMode)", but keeps the
			// disposeMode field access. Get rid of those field accesses:
			block.Instructions.RemoveAll(MatchLdDisposeMode);

			// if (logic.not(ldfld disposeMode(ldloc this))) br falseInst
			// br trueInst
			if (!block.MatchIfAtEndOfBlock(out var condition, out _, out var falseInst))
				return false;
			if (!MatchLdDisposeMode(condition))
				return false;
			context.Step($"SimplifyIfDisposeMode({block.StartILOffset:x4})", block);
			block.Instructions[block.Instructions.Count - 2] = falseInst;
			block.Instructions.RemoveAt(block.Instructions.Count - 1);
			return true;

			bool MatchLdDisposeMode(ILInstruction inst)
			{
				if (!inst.MatchLdFld(out var target, out var field))
					return false;
				return target.MatchLdThis() && field.MemberDefinition == disposeModeField;
			}
		}

		bool AnalyzeAwaitBlock(Block block, out ILVariable awaiter, out IField awaiterField, out int state, out int yieldOffset)
		{
			awaiter = null;
			awaiterField = null;
			state = 0;
			yieldOffset = -1;
			int pos = block.Instructions.Count - 2;
			if (pos >= 0 && doFinallyBodies != null && block.Instructions[pos] is StLoc storeDoFinallyBodies) {
				if (!(storeDoFinallyBodies.Variable.Kind == VariableKind.Local
					  && storeDoFinallyBodies.Variable.Type.IsKnownType(KnownTypeCode.Boolean)
					  && storeDoFinallyBodies.Variable.Index == doFinallyBodies.Index)) {
					return false;
				}
				if (!storeDoFinallyBodies.Value.MatchLdcI4(0))
					return false;
				pos--;
			}

			if (pos >= 0 && MatchCall(block.Instructions[pos], "AwaitUnsafeOnCompleted", out var callArgs)) {
				// call AwaitUnsafeOnCompleted(ldflda <>t__builder(ldloc this), ldloca awaiter, ldloc this)
			} else if (pos >= 0 && MatchCall(block.Instructions[pos], "AwaitOnCompleted", out callArgs)) {
				// call AwaitOnCompleted(ldflda <>t__builder(ldloc this), ldloca awaiter, ldloc this)
				// The C# compiler emits the non-unsafe call when the awaiter does not implement
				// ICriticalNotifyCompletion.
			} else {
				return false;
			}
			if (callArgs.Count != 3)
				return false;
			if (!IsBuilderFieldOnThis(callArgs[0]))
				return false;
			if (!callArgs[1].MatchLdLoca(out awaiter))
				return false;
			if (callArgs[2].MatchLdThis()) {
				// OK (if state machine is a struct)
				pos--;
			} else if (callArgs[2].MatchLdLoca(out var tempVar)) {
				// Roslyn, non-optimized uses a class for the state machine.
				// stloc tempVar(ldloc this)
				// call AwaitUnsafeOnCompleted(ldflda <>t__builder](ldloc this), ldloca awaiter, ldloca tempVar)
				if (!(pos > 0 && block.Instructions[pos - 1].MatchStLoc(tempVar, out var tempVal)))
					return false;
				if (!tempVal.MatchLdThis())
					return false;
				pos -= 2;
			} else {
				return false;
			}
			// stfld StateMachine.<>awaiter(ldloc this, ldloc awaiter)
			if (!block.Instructions[pos].MatchStFld(out var target, out awaiterField, out var value))
				return false;
			if (!target.MatchLdThis())
				return false;
			if (!value.MatchLdLoc(awaiter))
				return false;
			pos--;
			// Store IL offset for debug info:
			yieldOffset = block.Instructions[pos].EndILOffset;

			// stloc S_10(ldloc this)
			// stloc S_11(ldc.i4 0)
			// stloc cachedStateVar(ldloc S_11)
			// stfld <>1__state(ldloc S_10, ldloc S_11)
			if (!block.Instructions[pos].MatchStFld(out target, out var field, out value))
				return false;
			if (!StackSlotValue(target).MatchLdThis())
				return false;
			if (field.MemberDefinition != stateField)
				return false;
			if (!StackSlotValue(value).MatchLdcI4(out state))
				return false;
			if (pos > 0 && block.Instructions[pos - 1] is StLoc stloc
				&& stloc.Variable.Kind == VariableKind.Local && stloc.Variable.Index == cachedStateVar.Index
				&& StackSlotValue(stloc.Value).MatchLdcI4(state)) {
				// also delete the assignment to cachedStateVar
				pos--;
			}
			block.Instructions.RemoveRange(pos, block.Instructions.Count - pos);
			// delete preceding dead stores:
			while (pos > 0 && block.Instructions[pos - 1] is StLoc stloc2
				&& stloc2.Variable.IsSingleDefinition && stloc2.Variable.LoadCount == 0
				&& stloc2.Variable.Kind == VariableKind.StackSlot
				&& SemanticHelper.IsPure(stloc2.Value.Flags))
			{
				pos--;
			}
			block.Instructions.RemoveRange(pos, block.Instructions.Count - pos);
			return true;
		}

		static ILInstruction StackSlotValue(ILInstruction inst)
		{
			if (inst.MatchLdLoc(out var v) && v.Kind == VariableKind.StackSlot && v.IsSingleDefinition) {
				if (v.StoreInstructions[0] is StLoc stloc) {
					return stloc.Value;
				}
			}
			return inst;
		}

		private bool AnalyzeYieldReturn(Block block, out ILInstruction yieldValue, out int newState)
		{
			yieldValue = default;
			newState = default;
			Debug.Assert(block.Instructions.Last().MatchBranch(setResultYieldBlock));
			// stfld current(ldloc this, ldstr "yieldValue")
			// stloc S_45(ldloc this)
			// stloc S_46(ldc.i4 -5)
			// stloc V_0(ldloc S_46)
			// stfld stateField(ldloc S_45, ldloc S_46)
			// br setResultYieldBlock

			int pos = block.Instructions.Count - 2;
			// Immediately before the 'yield return', there should be a state assignment:
			if (pos < 0 || !MatchStateAssignment(block.Instructions[pos], out newState))
				return false;
			pos--;

			if (pos >= 0 && block.Instructions[pos].MatchStLoc(cachedStateVar, out var cachedStateNewValue)) {
				if (StackSlotValue(cachedStateNewValue).MatchLdcI4(newState)) {
					pos--; // OK, ignore V_0 store
				} else {
					return false;
				}
			}

			while (pos >= 0 && block.Instructions[pos] is StLoc stloc) {
				if (stloc.Variable.Kind != VariableKind.StackSlot)
					return false;
				if (!SemanticHelper.IsPure(stloc.Value.Flags))
					return false;
				pos--;
			}

			if (pos < 0 || !block.Instructions[pos].MatchStFld(out var target, out var field, out yieldValue))
				return false;
			if (!StackSlotValue(target).MatchLdThis())
				return false;
			// TODO: check that we are accessing the current field (compare with get_Current)

			block.Instructions.RemoveRange(pos, block.Instructions.Count - pos);
			return true;
		}
		#endregion

		#region DetectAwaitPattern
		void DetectAwaitPattern(ILFunction function)
		{
			context.StepStartGroup("DetectAwaitPattern", function);
			foreach (var container in function.Descendants.OfType<BlockContainer>()) {
				foreach (var block in container.Blocks) {
					context.CancellationToken.ThrowIfCancellationRequested();
					DetectAwaitPattern(block);
				}
				container.SortBlocks(deleteUnreachableBlocks: true);
			}
			context.StepEndGroup(keepIfEmpty: true);
		}

		void DetectAwaitPattern(Block block)
		{
			// block:
			//   stloc awaiterVar(callvirt GetAwaiter(...))
			//   if (call get_IsCompleted(ldloca awaiterVar)) br completedBlock
			//   br awaitBlock
			// awaitBlock:
			//   ..
			//   br resumeBlock
			// resumeBlock:
			//   ..
			//   br completedBlock
			if (block.Instructions.Count < 3)
				return;
			// stloc awaiterVar(callvirt GetAwaiter(...))
			if (!(block.Instructions[block.Instructions.Count - 3] is StLoc stLocAwaiter))
				return;
			ILVariable awaiterVar = stLocAwaiter.Variable;
			if (!(stLocAwaiter.Value is CallInstruction getAwaiterCall))
				return;
			if (!(getAwaiterCall.Method.Name == "GetAwaiter" && (!getAwaiterCall.Method.IsStatic || getAwaiterCall.Method.IsExtensionMethod)))
				return;
			if (getAwaiterCall.Arguments.Count != 1)
				return;
			// if (call get_IsCompleted(ldloca awaiterVar)) br completedBlock
			if (!block.Instructions[block.Instructions.Count - 2].MatchIfInstruction(out var condition, out var trueInst))
				return;
			if (!trueInst.MatchBranch(out var completedBlock))
				return;
			// br awaitBlock
			if (!block.Instructions.Last().MatchBranch(out var awaitBlock))
				return;
			// condition might be inverted, swap branches:
			if (condition.MatchLogicNot(out var negatedCondition)) {
				condition = negatedCondition;
				ExtensionMethods.Swap(ref completedBlock, ref awaitBlock);
			}
			// continue matching call get_IsCompleted(ldloca awaiterVar)
			if (!MatchCall(condition, "get_IsCompleted", out var isCompletedArgs) || isCompletedArgs.Count != 1)
				return;
			if (!UnwrapConvUnknown(isCompletedArgs[0]).MatchLdLocRef(awaiterVar))
				return;
			// Check awaitBlock and resumeBlock:
			if (!awaitBlocks.TryGetValue(awaitBlock, out var awaitBlockData))
				return;
			if (awaitBlockData.awaiterVar != awaiterVar)
				return;
			if (!CheckAwaitBlock(awaitBlock, out var resumeBlock, out var stackField))
				return;
			if (!CheckResumeBlock(resumeBlock, awaiterVar, awaitBlockData.awaiterField, completedBlock, stackField))
				return;
			// Check completedBlock. The first instruction involves the GetResult call, but it might have
			// been inlined into another instruction.
			var getResultCall = ILInlining.FindFirstInlinedCall(completedBlock.Instructions[0]);
			if (getResultCall == null)
				return;
			if (!MatchCall(getResultCall, "GetResult", out var getResultArgs) || getResultArgs.Count != 1)
				return;
			if (!UnwrapConvUnknown(getResultArgs[0]).MatchLdLocRef(awaiterVar))
				return;
			// All checks successful, let's transform.
			context.Step("Transform await pattern", block);
			block.Instructions.RemoveAt(block.Instructions.Count - 3); // remove getAwaiter call
			block.Instructions.RemoveAt(block.Instructions.Count - 2); // remove if (isCompleted)
			((Branch)block.Instructions.Last()).TargetBlock = completedBlock; // instead, directly jump to completed block
			Await awaitInst = new Await(UnwrapConvUnknown(getAwaiterCall.Arguments.Single()));
			awaitInst.GetResultMethod = getResultCall.Method;
			awaitInst.GetAwaiterMethod = getAwaiterCall.Method;
			getResultCall.ReplaceWith(awaitInst);

			// Remove useless reset of awaiterVar.
			if (completedBlock.Instructions.ElementAtOrDefault(1) is StObj stobj) {
				if (stobj.Target.MatchLdLoca(awaiterVar) && stobj.Value.OpCode == OpCode.DefaultValue) {
					completedBlock.Instructions.RemoveAt(1);
				}
			}
		}

		static ILInstruction UnwrapConvUnknown(ILInstruction inst)
		{
			if (inst is Conv conv && conv.TargetType == PrimitiveType.Unknown) {
				return conv.Argument;
			}
			return inst;
		}

		bool CheckAwaitBlock(Block block, out Block resumeBlock, out IField stackField)
		{
			// awaitBlock:
			//   (pre-roslyn: save stack)
			//   await(ldloca V_2)
			//   br resumeBlock
			resumeBlock = null;
			stackField = null;
			if (block.Instructions.Count < 2)
				return false;
			int pos = 0;
			if (block.Instructions[pos] is StLoc stloc && stloc.Variable.IsSingleDefinition) {
				if (!block.Instructions[pos + 1].MatchStFld(out var target, out stackField, out var value))
					return false;
				if (!target.MatchLdThis())
					return false;
				pos += 2;
			}
			// await(ldloca awaiterVar)
			if (block.Instructions[pos].OpCode != OpCode.Await)
				return false;
			// br resumeBlock
			return block.Instructions[pos + 1].MatchBranch(out resumeBlock);
		}

		bool CheckResumeBlock(Block block, ILVariable awaiterVar, IField awaiterField, Block completedBlock, IField stackField)
		{
			int pos = 0;
			if (!RestoreStack(block, ref pos, stackField))
				return false;

			// stloc awaiterVar(ldfld awaiterField(ldloc this))
			if (!block.Instructions[pos].MatchStLoc(awaiterVar, out var value))
				return false;
			if (value is CastClass cast && cast.Type.Equals(awaiterVar.Type)) {
				// If the awaiter is a reference type, it might get stored in a field of type `object`
				// and cast back to the awaiter type in the resume block
				value = cast.Argument;
			}
			if (!value.MatchLdFld(out var target, out var field))
				return false;
			if (!target.MatchLdThis())
				return false;
			if (!field.Equals(awaiterField))
				return false;
			pos++;

			// stfld awaiterField(ldloc this, default.value)
			if (block.Instructions[pos].MatchStFld(out target, out field, out value)
				&& target.MatchLdThis()
				&& field.Equals(awaiterField)
				&& (value.OpCode == OpCode.DefaultValue || value.OpCode == OpCode.LdNull))
			{
				pos++;
			} else {
				// {stloc V_6(default.value System.Runtime.CompilerServices.TaskAwaiter)}
				// {stobj System.Runtime.CompilerServices.TaskAwaiter`1[[System.Int32]](ldflda <>u__$awaiter4(ldloc this), ldloc V_6) at IL_0163}
				if (block.Instructions[pos].MatchStLoc(out var variable, out value) && value.OpCode == OpCode.DefaultValue
					&& block.Instructions[pos + 1].MatchStFld(out target, out field, out value)
					&& field.Equals(awaiterField)
					&& value.MatchLdLoc(variable)) {
					pos += 2;
				}
			}

			// stloc S_28(ldc.i4 -1)
			// stloc cachedStateVar(ldloc S_28)
			// stfld <>1__state(ldloc this, ldloc S_28)
			ILVariable m1Var = null;
			if (block.Instructions[pos] is StLoc stlocM1 && stlocM1.Value.MatchLdcI4(initialState) && stlocM1.Variable.Kind == VariableKind.StackSlot) {
				m1Var = stlocM1.Variable;
				pos++;
			}
			if (block.Instructions[pos] is StLoc stlocCachedState) {
				if (stlocCachedState.Variable.Kind == VariableKind.Local && stlocCachedState.Variable.Index == cachedStateVar?.Index) { 
					if (stlocCachedState.Value.MatchLdLoc(m1Var) || stlocCachedState.Value.MatchLdcI4(initialState))
						pos++;
				}
			}
			if (block.Instructions[pos].MatchStFld(out target, out field, out value)) {
				if (!target.MatchLdThis())
					return false;
				if (!field.MemberDefinition.Equals(stateField.MemberDefinition))
					return false;
				if (!(value.MatchLdcI4(initialState) || value.MatchLdLoc(m1Var)))
					return false;
				pos++;
			} else {
				return false;
			}
			
			return block.Instructions[pos].MatchBranch(completedBlock);
		}

		private bool RestoreStack(Block block, ref int pos, IField stackField)
		{
			if (stackField == null) {
				return true; // nothing to restore
			}
			// stloc temp(unbox.any T(ldfld <>t__stack(ldloc this)))
			if (!(block.Instructions[pos] is StLoc stloc))
				return false;
			if (!stloc.Variable.IsSingleDefinition)
				return false;
			if (!(stloc.Value is UnboxAny unbox))
				return false;
			if (!unbox.Argument.MatchLdFld(out var target, out var field))
				return false;
			if (!target.MatchLdThis())
				return false;
			if (!field.Equals(stackField))
				return false;
			pos++;
			// restoring stack slots
			while (block.Instructions[pos].MatchStLoc(out var v) && v.Kind == VariableKind.StackSlot) {
				pos++;
			}
			// stfld <>t__stack(ldloc this, ldnull)
			if (block.Instructions[pos].MatchStFld(out target, out field, out var value)) {
				if (target.MatchLdThis() && field.Equals(stackField) && value.MatchLdNull()) {
					pos++;
				}
			}
			return true;
		}
		#endregion

		/// <summary>
		/// Eliminates usage of doFinallyBodies
		/// </summary>
		private void CleanDoFinallyBodies(ILFunction function)
		{
			if (doFinallyBodies == null) {
				return; // roslyn-compiled code doesn't use doFinallyBodies
			}
			context.StepStartGroup("CleanDoFinallyBodies", function);
			Block entryPoint = GetBodyEntryPoint(function.Body as BlockContainer);
			if (entryPoint != null && entryPoint.Instructions[0].MatchStLoc(doFinallyBodies, out var value) && value.MatchLdcI4(1)) {
				// Remove initial doFinallyBodies assignment, if it wasn't already removed when
				// we rearranged the control flow.
				entryPoint.Instructions.RemoveAt(0);
			}
			if (doFinallyBodies.StoreInstructions.Count != 0 || doFinallyBodies.AddressCount != 0) {
				// misdetected another variable as doFinallyBodies?
				// reintroduce the initial store of ldc.i4(1)
				context.Step("Re-introduce misdetected doFinallyBodies", function);
				((BlockContainer)function.Body).EntryPoint.Instructions.Insert(0,
					new StLoc(doFinallyBodies, new LdcI4(1)));
				return;
			}
			foreach (var tryFinally in function.Descendants.OfType<TryFinally>()) {
				entryPoint = GetBodyEntryPoint(tryFinally.FinallyBlock as BlockContainer);
				if (entryPoint?.Instructions[0] is IfInstruction ifInst) {
					if (ifInst.Condition.MatchLogicNot(out var logicNotArg) && logicNotArg.MatchLdLoc(doFinallyBodies)) {
						context.Step("Remove if(doFinallyBodies) from try-finally", tryFinally);
						// condition will always be false now that we're using 'await' instructions
						entryPoint.Instructions.RemoveAt(0);
					}
				}
			}
			// if there's any remaining loads (there shouldn't be), replace them with the constant 1
			foreach (LdLoc load in doFinallyBodies.LoadInstructions.ToArray()) {
				load.ReplaceWith(new LdcI4(1).WithILRange(load));
			}
			context.StepEndGroup(keepIfEmpty: true);
		}

		internal static Block GetBodyEntryPoint(BlockContainer body)
		{
			if (body == null)
				return null;
			Block entryPoint = body.EntryPoint;
			while (entryPoint.Instructions[0].MatchBranch(out var targetBlock) && targetBlock.IncomingEdgeCount == 1 && targetBlock.Parent == body) {
				entryPoint = targetBlock;
			}
			return entryPoint;
		}

		void TranslateCachedFieldsToLocals()
		{
			foreach (var (cachedVar, param) in cachedFieldToParameterMap) {
				Debug.Assert(cachedVar.StoreCount <= 1);
				foreach (var inst in cachedVar.LoadInstructions.ToArray()) {
					inst.Variable = param;
				}
				foreach (var inst in cachedVar.AddressInstructions.ToArray()) {
					inst.Variable = param;
				}
			}
		}
	}
}