Add InvalidInstruction to represent invalid IL opcodes in the ILAst instead of crashing.

ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs

@@ -953,11 +953,23 @@ namespace ICSharpCode.Decompiler.CSharp
 			return true;
 		}
 		
+		protected internal override TranslatedExpression VisitInvalidInstruction(InvalidInstruction inst)
+		{
+			string message = "Invalid IL";
+			if (inst.ILRange.Start != 0) {
+				message += $" near IL_{inst.ILRange.Start:x4}";
+			}
+			if (!string.IsNullOrEmpty(inst.Message)) {
+				message += ": " + inst.Message;
+			}
+			return ErrorExpression(message);
+		}
+		
 		protected override TranslatedExpression Default(ILInstruction inst)
 		{
 			return ErrorExpression("OpCode not supported: " + inst.OpCode);
 		}
-
+		
 		static TranslatedExpression ErrorExpression(string message)
 		{
 			var e = new ErrorExpression();

ICSharpCode.Decompiler/FlowAnalysis/DataFlowVisitor.cs

@@ -278,7 +278,7 @@ namespace ICSharpCode.Decompiler.FlowAnalysis
 			DebugStartPoint(inst);
 			// This method assumes normal control flow and no branches.
 			if ((inst.DirectFlags & (InstructionFlags.ControlFlow | InstructionFlags.MayBranch | InstructionFlags.EndPointUnreachable)) != 0) {
-				throw new NotImplementedException("RDVisitor is missing implementation for " + inst.GetType().Name);
+				throw new NotImplementedException(GetType().Name + " is missing implementation for " + inst.GetType().Name);
 			}
 			
 			// Since this instruction has normal control flow, we can evaluate our children left-to-right.
@@ -438,7 +438,11 @@ namespace ICSharpCode.Decompiler.FlowAnalysis
 		
 		protected internal override void VisitRethrow(Rethrow inst)
 		{
-			PropagateStateOnException();
+			MarkUnreachable();
+		}
+		
+		protected internal override void VisitInvalidInstruction(InvalidInstruction inst)
+		{
 			MarkUnreachable();
 		}
 		

ICSharpCode.Decompiler/IL/ILReader.cs

@@ -242,7 +242,7 @@ namespace ICSharpCode.Decompiler.IL
 				decodedInstruction.ILRange = new Interval(start, reader.Position);
 				UnpackPush(decodedInstruction).ILRange = decodedInstruction.ILRange;
 				instructionBuilder.Add(decodedInstruction);
-				if (decodedInstruction.HasFlag(InstructionFlags.EndPointUnreachable)) {
+				if ((decodedInstruction.DirectFlags & InstructionFlags.EndPointUnreachable) != 0) {
 					if (!stackByOffset.TryGetValue(reader.Position, out currentStack)) {
 						currentStack = ImmutableStack<ILVariable>.Empty;
 					}
@@ -763,7 +763,7 @@ namespace ICSharpCode.Decompiler.IL
 				case ILOpCode.Unbox_Any:
 					return Push(new UnboxAny(Pop(), ReadAndDecodeTypeReference()));
 				default:
-					throw new NotImplementedException(ilOpCode.ToString());
+					return new InvalidInstruction("Unknown opcode: " + ilOpCode.ToString());
 			}
 		}
 
@@ -832,17 +832,19 @@ namespace ICSharpCode.Decompiler.IL
 			return new StLoc(v, inst);
 		}
 		
-		LdLoc Peek()
+		ILInstruction Peek()
 		{
-			Debug.Assert(!currentStack.IsEmpty);
-			// TODO: handle stack underflow?
+			if (currentStack.IsEmpty) {
+				return new InvalidInstruction("Stack underflow") { ILRange = new Interval(reader.Position, reader.Position) };
+			}
 			return new LdLoc(currentStack.Peek());
 		}
 		
-		LdLoc Pop()
+		ILInstruction Pop()
 		{
-			Debug.Assert(!currentStack.IsEmpty);
-			// TODO: handle stack underflow?
+			if (currentStack.IsEmpty) {
+				return new InvalidInstruction("Stack underflow") { ILRange = new Interval(reader.Position, reader.Position) };
+			}
 			ILVariable v;
 			currentStack = currentStack.Pop(out v);
 			return new LdLoc(v);

ICSharpCode.Decompiler/IL/Instructions.cs

@@ -29,6 +29,8 @@ namespace ICSharpCode.Decompiler.IL
 	/// </summary>
 	public enum OpCode
 	{
+		/// <summary>Represents invalid IL. Semantically, this instruction is considered to throw some kind of exception.</summary>
+		InvalidInstruction,
 		/// <summary>No operation. Takes 0 arguments and returns void.</summary>
 		Nop,
 		/// <summary>A container of IL blocks.</summary>
@@ -440,6 +442,32 @@ namespace ICSharpCode.Decompiler.IL
 		}
 	}
 
+	/// <summary>Represents invalid IL. Semantically, this instruction is considered to throw some kind of exception.</summary>
+	public sealed partial class InvalidInstruction : SimpleInstruction
+	{
+		public InvalidInstruction() : base(OpCode.InvalidInstruction)
+		{
+		}
+		public override StackType ResultType { get { return StackType.Void; } }
+		protected override InstructionFlags ComputeFlags()
+		{
+			return InstructionFlags.MayThrow | InstructionFlags.EndPointUnreachable;
+		}
+		public override InstructionFlags DirectFlags {
+			get {
+				return InstructionFlags.MayThrow | InstructionFlags.EndPointUnreachable;
+			}
+		}
+		public override void AcceptVisitor(ILVisitor visitor)
+		{
+			visitor.VisitInvalidInstruction(this);
+		}
+		public override T AcceptVisitor<T>(ILVisitor<T> visitor)
+		{
+			return visitor.VisitInvalidInstruction(this);
+		}
+	}
+
 	/// <summary>No operation. Takes 0 arguments and returns void.</summary>
 	public sealed partial class Nop : SimpleInstruction
 	{
@@ -3068,6 +3096,10 @@ namespace ICSharpCode.Decompiler.IL
 		/// <summary>Called by Visit*() methods that were not overridden</summary>
 		protected abstract void Default(ILInstruction inst);
 		
+		protected internal virtual void VisitInvalidInstruction(InvalidInstruction inst)
+		{
+			Default(inst);
+		}
 		protected internal virtual void VisitNop(Nop inst)
 		{
 			Default(inst);
@@ -3366,6 +3398,10 @@ namespace ICSharpCode.Decompiler.IL
 		/// <summary>Called by Visit*() methods that were not overridden</summary>
 		protected abstract T Default(ILInstruction inst);
 		
+		protected internal virtual T VisitInvalidInstruction(InvalidInstruction inst)
+		{
+			return Default(inst);
+		}
 		protected internal virtual T VisitNop(Nop inst)
 		{
 			return Default(inst);
@@ -3701,6 +3737,7 @@ namespace ICSharpCode.Decompiler.IL
 	partial class InstructionOutputExtensions
 	{
 		static readonly string[] originalOpCodeNames = {
+			"invalid",
 			"nop",
 			"ILFunction",
 			"BlockContainer",
@@ -3778,6 +3815,14 @@ namespace ICSharpCode.Decompiler.IL
 	
 	partial class ILInstruction
 	{
+		public bool MatchInvalidInstruction()
+		{
+			var inst = this as InvalidInstruction;
+			if (inst != null) {
+				return true;
+			}
+			return false;
+		}
 		public bool MatchNop()
 		{
 			var inst = this as Nop;

ICSharpCode.Decompiler/IL/Instructions.tt

@@ -36,6 +36,8 @@
 	};
 	
 	OpCode[] opCodes = {
+		new OpCode("invalid", "Represents invalid IL. Semantically, this instruction is considered to throw some kind of exception.",
+			CustomClassName("InvalidInstruction"), NoArguments, MayThrow, UnconditionalBranch),
 		new OpCode("nop", "No operation. Takes 0 arguments and returns void.",
 			VoidResult, NoArguments),
 		new OpCode("ILFunction", "A container of IL blocks.",

ICSharpCode.Decompiler/IL/Instructions/Branch.cs

@@ -55,6 +55,12 @@ namespace ICSharpCode.Decompiler.IL
 			return InstructionFlags.MayBranch | InstructionFlags.EndPointUnreachable;
 		}
 		
+		public override InstructionFlags DirectFlags {
+			get {
+				return InstructionFlags.MayBranch | InstructionFlags.EndPointUnreachable;
+			}
+		}
+		
 		public int TargetILOffset {
 			get { return targetBlock != null ? targetBlock.ILRange.Start : targetILOffset; }
 		}

ICSharpCode.Decompiler/IL/Instructions/Leave.cs

@@ -52,6 +52,12 @@ namespace ICSharpCode.Decompiler.IL
 			return InstructionFlags.MayBranch | InstructionFlags.EndPointUnreachable;
 		}
 		
+		public override InstructionFlags DirectFlags {
+			get {
+				return InstructionFlags.MayBranch | InstructionFlags.EndPointUnreachable;
+			}
+		}
+		
 		public BlockContainer TargetContainer {
 			get { return targetContainer; }
 			set {

ICSharpCode.Decompiler/IL/Instructions/SimpleInstruction.cs

@@ -36,4 +36,24 @@ namespace ICSharpCode.Decompiler.IL
 			// the non-custom WriteTo would add useless parentheses
 		}
 	}
+	
+	partial class InvalidInstruction : SimpleInstruction
+	{
+		public string Message;
+		
+		public InvalidInstruction(string message) : this()
+		{
+			this.Message = message;
+		}
+		
+		public override void WriteTo(ITextOutput output)
+		{
+			output.Write(OpCode);
+			if (!string.IsNullOrEmpty(Message)) {
+				output.Write('(');
+				output.Write(Message);
+				output.Write(')');
+			}
+		}
+	}
 }