#3075: Improve performance of CSharpResolver.LookupSimpleNameOrTypeNa…

…me in cases with a large number of local variables.

ICSharpCode.Decompiler/CSharp/Resolver/CSharpResolver.cs

@@ -67,7 +67,7 @@ public CSharpResolver(CSharpTypeResolveContext context)
 				currentTypeDefinitionCache = new TypeDefinitionCache(context.CurrentTypeDefinition);
 		}
 
-		private CSharpResolver(ICompilation compilation, CSharpConversions conversions, CSharpTypeResolveContext context, bool checkForOverflow, bool isWithinLambdaExpression, TypeDefinitionCache currentTypeDefinitionCache, ImmutableStack<IVariable> localVariableStack, ObjectInitializerContext objectInitializerStack)
+		private CSharpResolver(ICompilation compilation, CSharpConversions conversions, CSharpTypeResolveContext context, bool checkForOverflow, bool isWithinLambdaExpression, TypeDefinitionCache currentTypeDefinitionCache, ImmutableStack<Dictionary<string, IVariable>> localVariableStack, ObjectInitializerContext objectInitializerStack)
 		{
 			this.compilation = compilation;
 			this.conversions = conversions;
@@ -228,21 +228,21 @@ public TypeDefinitionCache(ITypeDefinition typeDefinition)
 		// The beginning of a block is marked by a null entry.
 
 		// This data structure is used to allow efficient cloning of the resolver with its local variable context.
-		readonly ImmutableStack<IVariable> localVariableStack = ImmutableStack<IVariable>.Empty;
+		readonly ImmutableStack<Dictionary<string, IVariable>> localVariableStack = ImmutableStack<Dictionary<string, IVariable>>.Empty;
 
-		CSharpResolver WithLocalVariableStack(ImmutableStack<IVariable> stack)
+		CSharpResolver WithLocalVariableStack(ImmutableStack<Dictionary<string, IVariable>> stack)
 		{
 			return new CSharpResolver(compilation, conversions, context, checkForOverflow, isWithinLambdaExpression, currentTypeDefinitionCache, stack, objectInitializerStack);
 		}
 
 		/// <summary>
-		/// Adds a new variable or lambda parameter to the current block.
+		/// Adds new variableŝ or lambda parameters to the current block.
 		/// </summary>
-		public CSharpResolver AddVariable(IVariable variable)
+		public CSharpResolver AddVariables(Dictionary<string, IVariable> variables)
 		{
-			if (variable == null)
-				throw new ArgumentNullException(nameof(variable));
-			return WithLocalVariableStack(localVariableStack.Push(variable));
+			if (variables == null)
+				throw new ArgumentNullException(nameof(variables));
+			return WithLocalVariableStack(localVariableStack.Push(variables));
 		}
 
 		/// <summary>
@@ -251,7 +251,7 @@ public CSharpResolver AddVariable(IVariable variable)
 		/// </summary>
 		public IEnumerable<IVariable> LocalVariables {
 			get {
-				return localVariableStack.Where(v => v != null);
+				return localVariableStack.SelectMany(s => s.Values);
 			}
 		}
 		#endregion
@@ -1482,9 +1482,9 @@ public ResolveResult LookupSimpleNameOrTypeName(string identifier, IReadOnlyList
 				if (lookupMode == NameLookupMode.Expression || lookupMode == NameLookupMode.InvocationTarget)
 				{
 					// Look in local variables
-					foreach (IVariable v in this.LocalVariables)
+					foreach (Dictionary<string, IVariable> variables in localVariableStack)
 					{
-						if (v.Name == identifier)
+						if (variables.TryGetValue(identifier, out var v))
 						{
 							return new LocalResolveResult(v);
 						}

ICSharpCode.Decompiler/CSharp/Transforms/IntroduceUsingDeclarations.cs

@@ -161,11 +161,13 @@ TypeSystemAstBuilder CreateAstBuilder(CSharpTypeResolveContext context, IL.ILFun
 				CSharpResolver resolver = new CSharpResolver(context);
 				if (function != null)
 				{
+					var variables = new Dictionary<string, IVariable>();
 					foreach (var v in function.Variables)
 					{
-						if (v.Kind != IL.VariableKind.Parameter && v.Name != null)
-							resolver = resolver.AddVariable(new DefaultVariable(v.Type, v.Name));
+						if (v.Kind != IL.VariableKind.Parameter && v.Name != null && !variables.ContainsKey(v.Name))
+							variables.Add(v.Name, new DefaultVariable(v.Type, v.Name));
 					}
+					resolver = resolver.AddVariables(variables);
 				}
 
 				return new TypeSystemAstBuilder(resolver) {