HighLevelLoopTransform: implement detection of for and do-while

ICSharpCode.Decompiler/IL/Transforms/HighLevelLoopTransform.cs

@@ -20,6 +20,7 @@ using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Text;
+using ICSharpCode.Decompiler.Util;
 
 namespace ICSharpCode.Decompiler.IL.Transforms
 {
@@ -33,8 +34,8 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			foreach (var loop in function.Descendants.OfType<BlockContainer>()) {
 				if (loop.Kind != ContainerKind.Loop)
 					continue;
-				if (MatchWhileLoop(loop, out var loopBody)) {
+				if (MatchWhileLoop(loop, out var condition, out var loopBody)) {
-					MatchForLoop(loop, loopBody);
+					MatchForLoop(loop, condition, loopBody);
 					continue;
 				}
 				if (MatchDoWhileLoop(loop))
@@ -42,7 +43,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			}
 		}
 
-		bool MatchWhileLoop(BlockContainer loop, out Block loopBody)
+		bool MatchWhileLoop(BlockContainer loop, out ILInstruction condition, out Block loopBody)
 		{
 			// while-loop:
 			// if (loop-condition) br loop-content-block
@@ -50,6 +51,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			// -or-
 			// if (loop-condition) block content-block
 			// leave loop-container
+			condition = null;
 			loopBody = null;
 			if (loop.EntryPoint.Instructions.Count != 2)
 				return false;
@@ -57,6 +59,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				return false;
 			if (!ifInstruction.FalseInst.MatchNop())
 				return false;
+			condition = ifInstruction.Condition;
 			var trueInst = ifInstruction.TrueInst;
 			if (!loop.EntryPoint.Instructions[1].MatchLeave(loop))
 				return false;
@@ -79,10 +82,83 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			return true;
 		}
 
+		bool MatchDoWhileLoop(BlockContainer loop)
+		{
+			if (loop.EntryPoint.Instructions.Count < 2)
+				return false;
+			var last = loop.EntryPoint.Instructions.Last();
+			var ifInstruction = loop.EntryPoint.Instructions.SecondToLastOrDefault() as IfInstruction;
+			if (!ifInstruction.FalseInst.MatchNop())
+				return false;
+			bool swapBranches = false;
+			ILInstruction condition = ifInstruction.Condition;
+			while (condition.MatchLogicNot(out var arg)) {
+				swapBranches = !swapBranches;
+				condition = arg;
+			}
+			if (swapBranches) {
+				if (!ifInstruction.TrueInst.MatchLeave(loop))
+					return false;
+				if (!last.MatchBranch(loop.EntryPoint))
+					return false;
+				ifInstruction.FalseInst = ifInstruction.TrueInst;
+				ifInstruction.TrueInst = last;
+				ifInstruction.Condition = condition;
+			} else {
+				if (!ifInstruction.TrueInst.MatchBranch(loop.EntryPoint))
+					return false;
+				if (!last.MatchLeave(loop))
+					return false;
+				ifInstruction.Condition = condition;
+				ifInstruction.FalseInst = last;
+			}
+			Block conditionBlock = new Block();
+			loop.Blocks.Add(conditionBlock);
+			loop.EntryPoint.Instructions.RemoveRange(ifInstruction.ChildIndex, 2);
+			conditionBlock.Instructions.Add(ifInstruction);
+			conditionBlock.AddILRange(ifInstruction.ILRange);
+			conditionBlock.AddILRange(last.ILRange);
+			loop.EntryPoint.Instructions.Add(new Branch(conditionBlock));
+			loop.Kind = ContainerKind.DoWhile;
+			return false;
+		}
+
+		bool MatchForLoop(BlockContainer loop, ILInstruction condition, Block whileLoopBody)
+		{
+			if (loop.EntryPoint.IncomingEdgeCount != 2)
+				return false;
+			var incrementBlock = loop.Blocks.SingleOrDefault(
+				b => b.Instructions.Last().MatchBranch(loop.EntryPoint)
+					 && b.Instructions.SkipLast(1).All(IsSimpleStatement));
+			if (incrementBlock == null) {
+				incrementBlock = whileLoopBody;
+			}
+			var last = incrementBlock.Instructions.LastOrDefault();
+			var secondToLast = incrementBlock.Instructions.SecondToLastOrDefault();
+			if (last == null || secondToLast == null)
+				return false;
+			if (!last.MatchBranch(loop.EntryPoint))
+				return false;
+			if (!MatchIncrement(secondToLast, out var variable))
+				return false;
+			if (!condition.Children.Any(c => c.MatchLdLoc(variable)))
+				return false;
+			int secondToLastIndex = secondToLast.ChildIndex;
+			var newIncremenBlock = new Block();
+			loop.Blocks.Add(newIncremenBlock);
+			newIncremenBlock.Instructions.Add(secondToLast);
+			newIncremenBlock.Instructions.Add(last);
+			newIncremenBlock.AddILRange(secondToLast.ILRange);
+			incrementBlock.Instructions.RemoveRange(secondToLastIndex, 2);
+			incrementBlock.Instructions.Add(new Branch(newIncremenBlock));
+			loop.Kind = ContainerKind.For;
+			return true;
+		}
+
 		/// <summary>
 		/// The last instruction of the while body must be a branch to the loop head.
 		/// </summary>
-		bool MatchWhileLoopBody(Block loopBody, BlockContainer loop)
+		static bool MatchWhileLoopBody(Block loopBody, BlockContainer loop)
 		{
 			if (loopBody.Instructions.Count == 0)
 				return false;
@@ -91,14 +167,35 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			return true;
 		}
 
-		bool MatchDoWhileLoop(BlockContainer loop)
+		static bool MatchIncrement(ILInstruction inst, out ILVariable variable)
 		{
+			if (!inst.MatchStLoc(out variable, out var value))
 				return false;
+			if (!value.MatchBinaryNumericInstruction(BinaryNumericOperator.Add, out var left, out var right)) {
+				if (value is CompoundAssignmentInstruction cai) {
+					left = cai.Target;
+				} else return false;
+			}
+			return left.MatchLdLoc(variable);
 		}
 
-		bool MatchForLoop(BlockContainer loop, Block whileLoopBody)
+		/// <summary>
+		/// Gets whether the statement is 'simple' (usable as for loop iterator):
+		/// Currently we only accept calls and assignments.
+		/// </summary>
+		static bool IsSimpleStatement(ILInstruction inst)
 		{
+			switch (inst.OpCode) {
+				case OpCode.Call:
+				case OpCode.CallVirt:
+				case OpCode.NewObj:
+				case OpCode.StLoc:
+				case OpCode.StObj:
+				case OpCode.CompoundAssignmentInstruction:
+					return true;
+				default:
 					return false;
 			}
 		}
+	}
 }