From 300c9cc9a9177413ff0e596cba851c962087b23f Mon Sep 17 00:00:00 2001
From: Siegfried Pammer <siegfriedpammer@gmail.com>
Date: Sat, 11 Nov 2017 03:44:00 +0100
Subject: [PATCH] Do not convert while-true to while-condition loop or do-while
 loop, if the condition uses a variable that is captured inside the loop.

---
 .../TestCases/Correctness/Capturing.cs        | 39 +++++++++++++++++++
 .../IL/Transforms/HighLevelLoopTransform.cs   | 13 +++++++
 2 files changed, 52 insertions(+)

diff --git a/ICSharpCode.Decompiler.Tests/TestCases/Correctness/Capturing.cs b/ICSharpCode.Decompiler.Tests/TestCases/Correctness/Capturing.cs
index 986a154a9..2d5820e6a 100644
--- a/ICSharpCode.Decompiler.Tests/TestCases/Correctness/Capturing.cs
+++ b/ICSharpCode.Decompiler.Tests/TestCases/Correctness/Capturing.cs
@@ -19,6 +19,8 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Correctness
 			OutsideLoopOverArray();
 			OutsideLoopOverArray2();
 			InsideLoopOverArray2();
+			NotWhileDueToVariableInsideLoop();
+			NotDoWhileDueToVariableInsideLoop();
 		}
 
 		static void TestCase1()
@@ -174,5 +176,42 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Correctness
 				Console.WriteLine(func());
 			}
 		}
+
+		static int nextVal;
+
+		static int GetVal()
+		{
+			return ++nextVal & 7;
+		}
+
+		static void NotWhileDueToVariableInsideLoop()
+		{
+			Console.WriteLine("NotWhileDueToVariableInsideLoop:");
+			var functions = new List<Func<int>>();
+			while (true) {
+				int v;
+				if ((v = GetVal()) == 0)
+					break;
+				functions.Add(() => v);
+			}
+			foreach (var f in functions) {
+				Console.WriteLine(f());
+			}
+		}
+
+		static void NotDoWhileDueToVariableInsideLoop()
+		{
+			Console.WriteLine("NotDoWhileDueToVariableInsideLoop:");
+			var functions = new List<Func<int>>();
+			while (true) {
+				int v = GetVal();
+				functions.Add(() => v);
+				if (v == 0)
+					break;
+			}
+			foreach (var f in functions) {
+				Console.WriteLine(f());
+			}
+		}
 	}
 }
diff --git a/ICSharpCode.Decompiler/IL/Transforms/HighLevelLoopTransform.cs b/ICSharpCode.Decompiler/IL/Transforms/HighLevelLoopTransform.cs
index 77eb629b9..955a8b05e 100644
--- a/ICSharpCode.Decompiler/IL/Transforms/HighLevelLoopTransform.cs
+++ b/ICSharpCode.Decompiler/IL/Transforms/HighLevelLoopTransform.cs
@@ -64,6 +64,8 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				return false;
 			if (!ifInstruction.FalseInst.MatchNop())
 				return false;
+			if (UsesVariableCapturedInLoop(loop, ifInstruction.Condition))
+				return false;
 			condition = ifInstruction;
 			var trueInst = ifInstruction.TrueInst;
 			if (!loop.EntryPoint.Instructions[1].MatchLeave(loop))
@@ -188,6 +190,8 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			while (i >= 0 && block.Instructions[i] is IfInstruction ifInst) {
 				if (!ifInst.FalseInst.MatchNop())
 					break;
+				if (UsesVariableCapturedInLoop(loop, ifInst.Condition))
+					break;
 				if (swap) {
 					if (!ifInst.TrueInst.MatchLeave(loop))
 						break;
@@ -203,6 +207,15 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			return list;
 		}
 
+		static bool UsesVariableCapturedInLoop(BlockContainer loop, ILInstruction condition)
+		{
+			foreach (var inst in condition.Descendants.OfType<IInstructionWithVariableOperand>()) {
+				if (inst.Variable.CaptureScope == loop)
+					return true;
+			}
+			return false;
+		}
+
 		static bool MatchDoWhileConditionBlock(BlockContainer loop, Block block, out bool swapBranches)
 		{
 			// match the end of the block: