ILSpy/ICSharpCode.Decompiler/IL/Transforms/TransformDisplayClassUsage.cs

// Copyright (c) 2019 Siegfried Pammer
//
// 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 System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using ICSharpCode.Decompiler.TypeSystem;
using ICSharpCode.Decompiler.Util;

namespace ICSharpCode.Decompiler.IL.Transforms
{
	/// <summary>
	/// Transforms closure fields to local variables.
	/// 
	/// This is a post-processing step of <see cref="DelegateConstruction"/>, <see cref="LocalFunctionDecompiler"/>
	/// and <see cref="TransformExpressionTrees"/>.
	/// 
	/// In general we can perform SROA (scalar replacement of aggregates) on any variable that
	/// satisfies the following conditions:
	/// 1) It is initialized by an empty/default constructor call.
	/// 2) The variable is never passed to another method.
	/// 3) The variable is never the target of an invocation.
	/// 
	/// Note that 2) and 3) apply because declarations and uses of lambdas and local functions
	/// are already transformed by the time this transform is applied.
	/// </summary>
	class TransformDisplayClassUsage : ILVisitor, IILTransform
	{
		class VariableToDeclare
		{
			private readonly DisplayClass container;
			private readonly IField field;
			private ILVariable declaredVariable;

			public string Name => field.Name;

			public bool CanPropagate { get; private set; }

			public ILInstruction Initializer { get; set; }

			public VariableToDeclare(DisplayClass container, IField field, ILVariable declaredVariable = null)
			{
				this.container = container;
				this.field = field;
				this.declaredVariable = declaredVariable;

				Debug.Assert(declaredVariable == null || declaredVariable.StateMachineField == field);
			}

			public void Propagate(ILVariable variable)
			{
				Debug.Assert(declaredVariable == null || (variable == null && declaredVariable.StateMachineField == null));
				this.declaredVariable = variable;
				this.CanPropagate = variable != null;
			}

			public ILVariable GetOrDeclare()
			{
				if (declaredVariable != null)
					return declaredVariable;
				declaredVariable = container.Variable.Function.RegisterVariable(VariableKind.Local, field.Type, field.Name);
				declaredVariable.HasInitialValue = true;
				declaredVariable.CaptureScope = container.CaptureScope;
				return declaredVariable;
			}
		}

		[DebuggerDisplay("[DisplayClass {Variable} : {Type}]")]
		class DisplayClass
		{
			public readonly ILVariable Variable;
			public readonly ITypeDefinition Type;
			public readonly Dictionary<IField, VariableToDeclare> VariablesToDeclare;
			public BlockContainer CaptureScope;
			public ILInstruction Initializer;

			public DisplayClass(ILVariable variable, ITypeDefinition type)
			{
				Variable = variable;
				Type = type;
				VariablesToDeclare = new Dictionary<IField, VariableToDeclare>();
			}
		}

		ILTransformContext context;
		ITypeResolveContext decompilationContext;
		readonly Dictionary<ILVariable, DisplayClass> displayClasses = new Dictionary<ILVariable, DisplayClass>();
		readonly List<ILInstruction> instructionsToRemove = new List<ILInstruction>();

		void IILTransform.Run(ILFunction function, ILTransformContext context)
		{
			if (this.context != null)
				throw new InvalidOperationException("Reentrancy in " + nameof(TransformDisplayClassUsage));
			try {
				this.context = context;
				this.decompilationContext = new SimpleTypeResolveContext(context.Function.Method);
				AnalyzeFunction(function);
				Transform(function);
			} finally {
				ClearState();
			}
		}

		void ClearState()
		{
			instructionsToRemove.Clear();
			displayClasses.Clear();
			this.decompilationContext = null;
			this.context = null;
		}

		public void Analyze(ILFunction function, ILTransformContext context)
		{
			ClearState();
			this.context = context;
			this.decompilationContext = new SimpleTypeResolveContext(context.Function.Method);
			AnalyzeFunction(function);
		}

		public static bool AnalyzeVariable(ILVariable v, ILTransformContext context)
		{
			var tdcu = new TransformDisplayClassUsage();
			tdcu.context = context;
			tdcu.decompilationContext = new SimpleTypeResolveContext(context.Function.Method);
			return tdcu.AnalyzeVariable(v) != null;
		}

		private void AnalyzeFunction(ILFunction function)
		{
			foreach (var f in function.Descendants.OfType<ILFunction>()) {
				foreach (var v in f.Variables.ToArray()) {
					var result = AnalyzeVariable(v);
					if (result == null)
						continue;
					displayClasses.Add(v, result);
				}
			}

			foreach (var displayClass in displayClasses.Values) {
				foreach (var v in displayClass.VariablesToDeclare.Values) {
					if (v.CanPropagate) {
						var variableToPropagate = v.GetOrDeclare();
						if (variableToPropagate.Kind != VariableKind.Parameter && !displayClasses.ContainsKey(variableToPropagate))
							v.Propagate(null);
					}
				}
			}
		}

		private DisplayClass AnalyzeVariable(ILVariable v)
		{
			switch (v.Kind) {
				case VariableKind.Parameter:
					if (context.Settings.YieldReturn && v.Function.StateMachineCompiledWithMono && v.IsThis())
						return HandleMonoStateMachine(v.Function, v);
					return null;
				case VariableKind.StackSlot:
				case VariableKind.Local:
				case VariableKind.DisplayClassLocal:
					return HandleDisplayClass(v);
				case VariableKind.PinnedLocal:
				case VariableKind.UsingLocal:
				case VariableKind.ForeachLocal:
				case VariableKind.InitializerTarget:
				case VariableKind.NamedArgument:
				case VariableKind.ExceptionStackSlot:
				case VariableKind.ExceptionLocal:
					return null;
				default:
					throw new ArgumentOutOfRangeException();
			}
		}

		DisplayClass HandleDisplayClass(ILVariable v)
		{
			ITypeDefinition definition;
			if (v.Kind != VariableKind.StackSlot) {
				definition = v.Type.GetDefinition();
			} else if (v.StoreInstructions.Count > 0 && v.StoreInstructions[0] is StLoc stloc && stloc.Value is NewObj newObj) {
				definition = newObj.Method.DeclaringTypeDefinition;
			} else {
				definition = null;
			}
			if (definition?.DeclaringTypeDefinition == null || definition.ParentModule.PEFile != context.PEFile)
				return null;
			DisplayClass result;
			switch (definition.Kind) {
				case TypeKind.Class:
					if (!v.IsSingleDefinition)
						return null;
					if (!(v.StoreInstructions[0] is StLoc stloc))
						return null;
					// TODO thoroughly validate ctor
					if (!(stloc.Value is NewObj newObj && newObj.Method.Parameters.Count == 0))
						return null;
					result = new DisplayClass(v, definition) {
						CaptureScope = v.CaptureScope,
						Initializer = stloc
					};
					if (stloc.Parent is Block initBlock) {
						for (int i = stloc.ChildIndex + 1; i < initBlock.Instructions.Count; i++) {
							var variable = AddVariable(result, initBlock.Instructions[i], out var field);
							if (variable == null)
								break;
							result.VariablesToDeclare[(IField)field.MemberDefinition] = variable;
						}
					}
					break;
				case TypeKind.Struct:
					if (!v.HasInitialValue)
						return null;
					if (v.StoreCount != 1)
						return null;
					result = new DisplayClass(v, definition) { CaptureScope = v.CaptureScope };
					if (v.Function.Body is BlockContainer b) {
						var block = b.EntryPoint;
						for (int i = 0; i < block.Instructions.Count; i++) {
							var variable = AddVariable(result, block.Instructions[i], out var field);
							if (variable == null)
								break;
							result.VariablesToDeclare[(IField)field.MemberDefinition] = variable;
						}
					}
					break;
				default:
					return null;
			}

			foreach (var ldloc in v.LoadInstructions) {
				if (!ValidateUse(result, ldloc))
					return null;
			}
			foreach (var ldloca in v.AddressInstructions) {
				if (!ValidateUse(result, ldloca))
					return null;
			}
			if (IsMonoNestedCaptureScope(definition)) {
				result.CaptureScope = null;
			}
			return result;
		}

		VariableToDeclare AddVariable(DisplayClass result, ILInstruction init, out IField field)
		{
			if (!init.MatchStFld(out var target, out field, out var value))
				return null;
			if (!target.MatchLdLocRef(result.Variable))
				return null;
			if (result.VariablesToDeclare.ContainsKey((IField)field.MemberDefinition))
				return null;
			VariableToDeclare variable = new VariableToDeclare(result, field);
			if (value.MatchLdLoc(out var variableToPropagate) && variableToPropagate.StateMachineField == null && (variableToPropagate.Kind == VariableKind.StackSlot || variableToPropagate.Type.Equals(field.Type))) {
				variable.Propagate(variableToPropagate);
				variable.Initializer = init;
			}
			return variable;
		}

		bool ValidateUse(DisplayClass container, ILInstruction use)
		{
			IField field;
			switch (use.Parent) {
				case LdFlda ldflda when ldflda.MatchLdFlda(out _, out field):
					var keyField = (IField)field.MemberDefinition;
					if (!container.VariablesToDeclare.TryGetValue(keyField, out VariableToDeclare variable) || variable == null) {
						variable = new VariableToDeclare(container, field);
					}
					container.VariablesToDeclare[keyField] = variable;
					return variable != null;
				case StObj stobj when stobj.MatchStObj(out var target, out ILInstruction value, out _) && value == use:
					if (target.MatchLdFlda(out var load, out field) && load.MatchLdLocRef(out var otherVariable) && displayClasses.TryGetValue(otherVariable, out var displayClass)) {
						if (displayClass.VariablesToDeclare.TryGetValue((IField)field.MemberDefinition, out var declaredVar))
							return declaredVar.CanPropagate;
					}
					return true;
				default:
					return false;
			}
		}

		private void Transform(ILFunction function)
		{
			VisitILFunction(function);
			foreach (var displayClass in displayClasses.Values) {
				foreach (var v in displayClass.VariablesToDeclare.Values) {
					if (v.CanPropagate && v.Initializer != null) {
						instructionsToRemove.Add(v.Initializer);
					}
				}
			}
			if (instructionsToRemove.Count > 0) {
				context.Step($"Remove instructions", function);
				foreach (var store in instructionsToRemove) {
					if (store.Parent is Block containingBlock)
						containingBlock.Instructions.Remove(store);
				}
			}
			context.Step($"ResetHasInitialValueFlag", function);
			foreach (var f in TreeTraversal.PostOrder(function, f => f.LocalFunctions)) {
				RemoveDeadVariableInit.ResetHasInitialValueFlag(f, context);
				f.CapturedVariables.RemoveWhere(v => v.IsDead);
			}
		}

		internal static bool IsClosure(ILTransformContext context, ILVariable variable, out ITypeDefinition closureType, out ILInstruction initializer)
		{
			closureType = null;
			initializer = null;
			if (variable.IsSingleDefinition && variable.StoreInstructions.SingleOrDefault() is StLoc inst) {
				initializer = inst;
				if (IsClosureInit(context, inst, out closureType)) {
					return true;
				}
			}
			closureType = variable.Type.GetDefinition();
			if (context.Settings.LocalFunctions && closureType?.Kind == TypeKind.Struct && variable.HasInitialValue && IsPotentialClosure(context, closureType)) {
				initializer = LocalFunctionDecompiler.GetStatement(variable.AddressInstructions.OrderBy(i => i.StartILOffset).First());
				return true;
			}
			return false;
		}

		static bool IsClosureInit(ILTransformContext context, StLoc inst, out ITypeDefinition closureType)
		{
			if (inst.Value is NewObj newObj) {
				closureType = newObj.Method.DeclaringTypeDefinition;
				return closureType != null && IsPotentialClosure(context, newObj);
			}
			closureType = null;
			return false;
		}

		bool IsMonoNestedCaptureScope(ITypeDefinition closureType)
		{
			if (!closureType.Name.Contains("AnonStorey"))
				return false;
			var decompilationContext = new SimpleTypeResolveContext(context.Function.Method);
			return closureType.Fields.Any(f => IsPotentialClosure(decompilationContext.CurrentTypeDefinition, f.ReturnType.GetDefinition()));
		}

		/// <summary>
		/// mcs likes to optimize closures in yield state machines away by moving the captured variables' fields into the state machine type,
		/// We construct a <see cref="DisplayClass"/> that spans the whole method body.
		/// </summary>
		DisplayClass HandleMonoStateMachine(ILFunction function, ILVariable thisVariable)
		{
			if (!(function.StateMachineCompiledWithMono && thisVariable.IsThis()))
				return null;
			// Special case for Mono-compiled yield state machines
			ITypeDefinition closureType = thisVariable.Type.GetDefinition();
			if (!(closureType != decompilationContext.CurrentTypeDefinition
				&& IsPotentialClosure(decompilationContext.CurrentTypeDefinition, closureType, allowTypeImplementingInterfaces: true)))
				return null;

			var displayClass = new DisplayClass(thisVariable, thisVariable.Type.GetDefinition());
			displayClass.CaptureScope = (BlockContainer)function.Body;
			foreach (var stateMachineVariable in function.Variables) {
				if (stateMachineVariable.StateMachineField == null || displayClass.VariablesToDeclare.ContainsKey(stateMachineVariable.StateMachineField))
					continue;
				VariableToDeclare variableToDeclare = new VariableToDeclare(displayClass, stateMachineVariable.StateMachineField, stateMachineVariable);
				displayClass.VariablesToDeclare.Add(stateMachineVariable.StateMachineField, variableToDeclare);
			}
			if (!function.Method.IsStatic && FindThisField(out var thisField)) {
				var thisVar = function.Variables
					.FirstOrDefault(t => t.IsThis() && t.Type.GetDefinition() == decompilationContext.CurrentTypeDefinition);
				if (thisVar == null) {
					thisVar = new ILVariable(VariableKind.Parameter, decompilationContext.CurrentTypeDefinition, -1) { Name = "this" };
					function.Variables.Add(thisVar);
				}
				VariableToDeclare variableToDeclare = new VariableToDeclare(displayClass, thisField, thisVar);
				displayClass.VariablesToDeclare.Add(thisField, variableToDeclare);
			}
			return displayClass;

			bool FindThisField(out IField foundField)
			{
				foundField = null;
				foreach (var field in closureType.GetFields(f2 => !f2.IsStatic && !displayClass.VariablesToDeclare.ContainsKey(f2) && f2.Type.GetDefinition() == decompilationContext.CurrentTypeDefinition)) {
					thisField = field;
					return true;
				}
				return false;
			}
		}

		internal static bool IsSimpleDisplayClass(IType type)
		{
			if (!type.HasGeneratedName() || (!type.Name.Contains("DisplayClass") && !type.Name.Contains("AnonStorey")))
				return false;
			if (type.DirectBaseTypes.Any(t => !t.IsKnownType(KnownTypeCode.Object)))
				return false;
			return true;
		}

		internal static bool IsPotentialClosure(ILTransformContext context, NewObj inst)
		{
			var decompilationContext = new SimpleTypeResolveContext(context.Function.Ancestors.OfType<ILFunction>().Last().Method);
			return IsPotentialClosure(decompilationContext.CurrentTypeDefinition, inst.Method.DeclaringTypeDefinition);
		}

		internal static bool IsPotentialClosure(ILTransformContext context, ITypeDefinition potentialDisplayClass)
		{
			var decompilationContext = new SimpleTypeResolveContext(context.Function.Ancestors.OfType<ILFunction>().Last().Method);
			return IsPotentialClosure(decompilationContext.CurrentTypeDefinition, potentialDisplayClass);
		}

		internal static bool IsPotentialClosure(ITypeDefinition decompiledTypeDefinition, ITypeDefinition potentialDisplayClass, bool allowTypeImplementingInterfaces = false)
		{
			if (potentialDisplayClass == null || !potentialDisplayClass.IsCompilerGeneratedOrIsInCompilerGeneratedClass())
				return false;
			switch (potentialDisplayClass.Kind) {
				case TypeKind.Struct:
					break;
				case TypeKind.Class:
					if (!potentialDisplayClass.IsSealed)
						return false;
					if (!allowTypeImplementingInterfaces) {
						if (!potentialDisplayClass.DirectBaseTypes.All(t => t.IsKnownType(KnownTypeCode.Object)))
							return false;
					}
					break;
				default:
					return false;
			}
			
			while (potentialDisplayClass != decompiledTypeDefinition) {
				potentialDisplayClass = potentialDisplayClass.DeclaringTypeDefinition;
				if (potentialDisplayClass == null)
					return false;
			}
			return true;
		}

		ILFunction currentFunction;

		protected internal override void VisitILFunction(ILFunction function)
		{
			var oldFunction = this.currentFunction;
			context.StepStartGroup("Visit " + function.Name);
			try {
				this.currentFunction = function;
				base.VisitILFunction(function);
			} finally {
				this.currentFunction = oldFunction;
				context.StepEndGroup(keepIfEmpty: true);
			}
		}

		protected override void Default(ILInstruction inst)
		{
			foreach (var child in inst.Children) {
				child.AcceptVisitor(this);
			}
		}

		protected internal override void VisitStLoc(StLoc inst)
		{
			base.VisitStLoc(inst);

			if (inst.Parent is Block && inst.Variable.IsSingleDefinition) {
				if ((inst.Variable.Kind == VariableKind.Local || inst.Variable.Kind == VariableKind.StackSlot) && inst.Variable.LoadCount == 0 && (inst.Value is StLoc || inst.Value is CompoundAssignmentInstruction)) {
					context.Step($"Remove unused variable assignment {inst.Variable.Name}", inst);
					inst.ReplaceWith(inst.Value);
					return;
				}
				if (displayClasses.TryGetValue(inst.Variable, out _) && inst.Value is NewObj) {
					instructionsToRemove.Add(inst);
					return;
				}
			}
		}

		protected internal override void VisitStObj(StObj inst)
		{
			base.VisitStObj(inst);
			var instructions = inst.Parent.Children;
			int childIndex = inst.ChildIndex;
			EarlyExpressionTransforms.StObjToStLoc(inst, context);
			if (inst != instructions[childIndex]) {
				foreach (var displayClass in displayClasses.Values) {
					foreach (var v in displayClass.VariablesToDeclare.Values) {
						if (v.CanPropagate && v.Initializer == inst) {
							v.Initializer = instructions[childIndex];
						}
					}
				}
				if (instructions[childIndex].MatchStLoc(out var innerDisplayClassVar, out var value) && value.MatchLdLoc(out var outerDisplayClassVar)) {
					if (!displayClasses.ContainsKey(innerDisplayClassVar) && displayClasses.TryGetValue(outerDisplayClassVar, out var displayClass)) {
						displayClasses.Add(innerDisplayClassVar, displayClass);
						instructionsToRemove.Add(instructions[childIndex]);
					}
				}
			}
		}

		protected internal override void VisitLdObj(LdObj inst)
		{
			base.VisitLdObj(inst);
			EarlyExpressionTransforms.LdObjToLdLoc(inst, context);
		}

		private bool IsDisplayClassLoad(ILInstruction target, out ILVariable variable)
		{
			// We cannot use MatchLdLocRef here because local functions use ref parameters
			if (target.MatchLdLoc(out variable) || target.MatchLdLoca(out variable))
				return true;
			return false;
		}

		private bool IsDisplayClassFieldAccess(ILInstruction inst, out ILVariable displayClassVar, out DisplayClass displayClass, out IField field)
		{
			displayClass = null;
			displayClassVar = null;
			field = null;
			if (!(inst is LdFlda ldflda))
				return false;
			field = ldflda.Field;
			return IsDisplayClassLoad(ldflda.Target, out displayClassVar)
				&& displayClasses.TryGetValue(displayClassVar, out displayClass);
		}

		protected internal override void VisitLdFlda(LdFlda inst)
		{
			base.VisitLdFlda(inst);
			// Get display class info
			if (!IsDisplayClassFieldAccess(inst, out _, out DisplayClass displayClass, out IField field))
				return;
			var keyField = (IField)field.MemberDefinition;
			var v = displayClass.VariablesToDeclare[keyField];
			context.Step($"Replace {field.Name} with captured variable {v.Name}", inst);
			ILVariable variable = v.GetOrDeclare();
			inst.ReplaceWith(new LdLoca(variable).WithILRange(inst));
			if (variable.Function != currentFunction && variable.Kind != VariableKind.DisplayClassLocal) {
				currentFunction.CapturedVariables.Add(variable);
			}
		}
	}
}