ILSpy/ICSharpCode.Decompiler/IL/ControlFlow/ExitPoints.cs

// Copyright (c) 2014 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;
using System.Diagnostics;
using ICSharpCode.Decompiler.IL.Transforms;
using System.Threading;
using System.Collections.Generic;

namespace ICSharpCode.Decompiler.IL.ControlFlow
{
	/// <summary>
	/// Detect suitable exit points for BlockContainers.
	/// 
	/// An "exit point" is an instruction that causes control flow
	/// to leave the container (a branch or leave instruction).
	/// 
	/// If an "exit point" instruction is placed immediately following a
	/// block container, each equivalent exit point within the container
	/// can be replaced with a "leave container" instruction.
	/// 
	/// This transform performs this replacement: any exit points
	/// equivalent to the exit point following the container are
	/// replaced with a leave instruction.
	/// Additionally, if the container is not yet followed by an exit point,
	/// but has room to introduce such an exit point (i.e. iff the container's 
	/// end point is currently unreachable), we pick one of the non-return
	/// exit points within the container, move it to the position following the
	/// container, and replace all instances within the container with a leave
	/// instruction.
	/// 
	/// This makes it easier for the following transforms to construct
	/// control flow that falls out of blocks instead of using goto/break statements.
	/// </summary>
	public class DetectExitPoints : ILVisitor, IILTransform
	{
		static readonly Nop ExitNotYetDetermined = new Nop { Comment = "ExitNotYetDetermined" };
		static readonly Nop NoExit = new Nop { Comment = "NoExit" };

		bool canIntroduceExitForReturn;

		public DetectExitPoints(bool canIntroduceExitForReturn)
		{
			this.canIntroduceExitForReturn = canIntroduceExitForReturn;
		}

		/// <summary>
		/// Gets the next instruction after <paramref name="inst"/> is executed.
		/// Returns NoExit when the next instruction cannot be identified;
		/// returns <c>null</c> when the end of a Block is reached (so that we could insert an arbitrary instruction)
		/// </summary>
		internal static ILInstruction GetExit(ILInstruction inst)
		{
			SlotInfo slot = inst.SlotInfo;
			if (slot == Block.InstructionSlot) {
				Block block = (Block)inst.Parent;
				return block.Instructions.ElementAtOrDefault(inst.ChildIndex + 1) ?? ExitNotYetDetermined;
			} else if (slot == TryInstruction.TryBlockSlot
				|| slot == TryCatchHandler.BodySlot
				|| slot == TryCatch.HandlerSlot
				|| slot == PinnedRegion.BodySlot)
			{
				return GetExit(inst.Parent);
			}
			return NoExit;
		}

		/// <summary>
		/// Returns true iff exit1 and exit2 are both exit instructions
		/// (branch or leave) and both represent the same exit.
		/// </summary>
		internal static bool CompatibleExitInstruction(ILInstruction exit1, ILInstruction exit2)
		{
			if (exit1 == null || exit2 == null || exit1.OpCode != exit2.OpCode)
				return false;
			switch (exit1.OpCode) {
				case OpCode.Branch:
					Branch br1 = (Branch)exit1;
					Branch br2 = (Branch)exit2;
					return br1.TargetBlock == br2.TargetBlock;
				case OpCode.Leave:
					Leave leave1 = (Leave)exit1;
					Leave leave2 = (Leave)exit2;
					return leave1.TargetContainer == leave2.TargetContainer && leave1.Value.MatchNop() && leave2.Value.MatchNop();
				default:
					return false;
			}
		}

		CancellationToken cancellationToken;
		BlockContainer currentContainer;

		/// <summary>
		/// The instruction that will be executed next after leaving the currentContainer.
		/// <c>ExitNotYetDetermined</c> means the container is last in its parent block, and thus does not
		/// yet have any leave instructions. This means we can move any exit instruction of
		/// our choice our of the container and replace it with a leave instruction.
		/// </summary>
		ILInstruction currentExit;

		/// <summary>
		/// If <c>currentExit==ExitNotYetDetermined</c>, holds the list of potential exit instructions.
		/// After the currentContainer was visited completely, one of these will be selected as exit instruction.
		/// </summary>
		List<ILInstruction> potentialExits;

		readonly List<Block> blocksPotentiallyMadeUnreachable = new List<Block>();

		public void Run(ILFunction function, ILTransformContext context)
		{
			cancellationToken = context.CancellationToken;
			currentExit = NoExit;
			blocksPotentiallyMadeUnreachable.Clear();
			function.AcceptVisitor(this);
			// It's possible that there are unreachable code blocks which we only
			// detect as such during exit point detection.
			// Clean them up.
			foreach (var block in blocksPotentiallyMadeUnreachable) {
				if (block.IncomingEdgeCount == 0 || block.IncomingEdgeCount == 1 && IsInfiniteLoop(block)) {
					block.Remove();
				}
			}
			blocksPotentiallyMadeUnreachable.Clear();
		}

		static bool IsInfiniteLoop(Block block)
		{
			return block.Instructions.Count == 1
				&& block.Instructions[0] is Branch b
				&& b.TargetBlock == block;
		}

		protected override void Default(ILInstruction inst)
		{
			foreach (var child in inst.Children)
				child.AcceptVisitor(this);
		}
		
		protected internal override void VisitBlockContainer(BlockContainer container)
		{
			var oldExit = currentExit;
			var oldContainer = currentContainer;
			var oldPotentialExits = potentialExits;
			var thisExit = GetExit(container);
			currentExit = thisExit;
			currentContainer = container;
			potentialExits = (thisExit == ExitNotYetDetermined ? new List<ILInstruction>() : null);
			base.VisitBlockContainer(container);
			if (thisExit == ExitNotYetDetermined && potentialExits.Count > 0) {
				// This transform determined an exit point.
				currentExit = ChooseExit(potentialExits);
				foreach (var exit in potentialExits) {
					if (CompatibleExitInstruction(currentExit, exit)) {
						exit.ReplaceWith(new Leave(currentContainer) { ILRange = exit.ILRange });
					}
				}
				Debug.Assert(!currentExit.MatchLeave(currentContainer));
				ILInstruction inst = container;
				// traverse up to the block (we'll always find one because GetExit
				// only returns ExitNotYetDetermined if there's a block)
				while (inst.Parent.OpCode != OpCode.Block)
					inst = inst.Parent;
				Block block = (Block)inst.Parent;
				if (block.HasFlag(InstructionFlags.EndPointUnreachable)) {
					// Special case: despite replacing the exits with leave(currentContainer),
					// we still have an unreachable endpoint.
					// The appended currentExit instruction would not be reachable!
					// This happens in test case ExceptionHandling.ThrowInFinally()
					if (currentExit is Branch b) {
						blocksPotentiallyMadeUnreachable.Add(b.TargetBlock);
					}
				} else {
					block.Instructions.Add(currentExit);
				}
			} else {
				Debug.Assert(thisExit == currentExit);
			}
			currentExit = oldExit;
			currentContainer = oldContainer;
			potentialExits = oldPotentialExits;
		}

		static ILInstruction ChooseExit(List<ILInstruction> potentialExits)
		{
			ILInstruction first = potentialExits[0];
			if (first is Leave l && l.IsLeavingFunction) {
				for (int i = 1; i < potentialExits.Count; i++) {
					var exit = potentialExits[i];
					if (!(exit is Leave l2 && l2.IsLeavingFunction))
						return exit;
				}
			}
			return first;
		}

		protected internal override void VisitBlock(Block block)
		{
			cancellationToken.ThrowIfCancellationRequested();
			// Don't use foreach loop, because the children might add to the block
			for (int i = 0; i < block.Instructions.Count; i++) {
				block.Instructions[i].AcceptVisitor(this);
			}
		}

		void HandleExit(ILInstruction inst)
		{
			if (currentExit == ExitNotYetDetermined && CanIntroduceAsExit(inst)) {
				potentialExits.Add(inst);
			} else if (CompatibleExitInstruction(inst, currentExit)) {
				inst.ReplaceWith(new Leave(currentContainer) { ILRange = inst.ILRange });
			}
		}

		private bool CanIntroduceAsExit(ILInstruction inst)
		{
			if (currentContainer.LeaveCount > 0) {
				// if we're re-running on a block container that already has an exit,
				// we can't introduce any additional exits
				return false;
			}
			if (inst is Leave l && l.IsLeavingFunction) {
				return canIntroduceExitForReturn;
			} else {
				return true;
			}
		}

		protected internal override void VisitBranch(Branch inst)
		{
			if (!inst.TargetBlock.IsDescendantOf(currentContainer)) {
				HandleExit(inst);
			}
		}

		protected internal override void VisitLeave(Leave inst)
		{
			base.VisitLeave(inst);
			if (!inst.Value.MatchNop())
				return;
			HandleExit(inst);
		}
	}
}