icsharpcode-ILSpy/ICSharpCode.Decompiler/IL/Transforms/RemoveDeadVariableInit.cs

// Copyright (c) 2016 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
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// DEALINGS IN THE SOFTWARE.

using System.Collections.Generic;
using System.Linq;

using ICSharpCode.Decompiler.FlowAnalysis;
using ICSharpCode.Decompiler.TypeSystem;

namespace ICSharpCode.Decompiler.IL.Transforms
{
	/// <summary>
	/// Remove <c>HasInitialValue</c> from locals that are definitely assigned before every use
	/// (=the initial value is a dead store).
	/// 
	/// In yield return generators, additionally removes dead 'V = null;' assignments.
	/// 
	/// Additionally infers IType of stack slots that have StackType.Ref
	/// </summary>
	public class RemoveDeadVariableInit : IILTransform
	{
		public void Run(ILFunction function, ILTransformContext context)
		{
			ResetUsesInitialValueFlag(function, context);
			// Remove dead stores to variables that are never read from.
			// If the stored value has some side-effect, the value is unwrapped.
			// This is necessary to remove useless stores generated by some compilers, e.g., the F# compiler.
			// In yield return + async, the C# compiler tends to store null/default(T) to variables
			// when the variable goes out of scope.
			bool removeDeadStores = function.IsAsync || function.IsIterator || context.Settings.RemoveDeadStores;

			var variableQueue = new Queue<ILVariable>(function.Variables);
			while (variableQueue.Count > 0)
			{
				var v = variableQueue.Dequeue();
				if (v.Kind != VariableKind.Local && v.Kind != VariableKind.StackSlot)
					continue;
				if (!(v.RemoveIfRedundant || removeDeadStores))
					continue;
				// Skip variables that are captured in a mcs yield state-machine
				// loads of these will only be visible after DelegateConstruction step.
				if (function.StateMachineCompiledWithMono && v.StateMachineField != null)
					continue;
				if (v.LoadCount != 0 || v.AddressCount != 0)
					continue;
				foreach (var stloc in v.StoreInstructions.OfType<StLoc>().ToArray())
				{
					if (stloc.Parent is Block block)
					{
						context.Step($"Dead store to {v.Name}", stloc);
						if (SemanticHelper.IsPure(stloc.Value.Flags))
						{
							block.Instructions.Remove(stloc);
						}
						else
						{
							stloc.ReplaceWith(stloc.Value);
						}
						if (stloc.Value is LdLoc ldloc)
						{
							variableQueue.Enqueue(ldloc.Variable);
						}
					}
				}
			}


			// Try to infer IType of stack slots that are of StackType.Ref:
			foreach (var v in function.Variables)
			{
				if (v.Kind == VariableKind.StackSlot && v.StackType == StackType.Ref && v.AddressCount == 0)
				{
					IType newType = null;
					// Multiple store are possible in case of (c ? ref a : ref b) += 1, for example.
					foreach (var stloc in v.StoreInstructions.OfType<StLoc>())
					{
						var inferredType = stloc.Value.InferType(context.TypeSystem);
						// cancel, if types of values do not match exactly
						if (newType != null && !newType.Equals(inferredType))
						{
							newType = SpecialType.UnknownType;
							break;
						}
						newType = inferredType;
					}
					// Only overwrite existing type, if a "better" type was found.
					if (newType != null && newType != SpecialType.UnknownType)
						v.Type = newType;
				}
			}
		}

		internal static void ResetUsesInitialValueFlag(ILFunction function, ILTransformContext context)
		{
			var visitor = new DefiniteAssignmentVisitor(function, context.CancellationToken);
			function.AcceptVisitor(visitor);
			foreach (var v in function.Variables)
			{
				if (v.Kind != VariableKind.Parameter && !visitor.IsPotentiallyUsedUninitialized(v))
				{
					v.UsesInitialValue = false;
				}
			}
		}
	}
}