DetectExitPoints: introduce exit points for loops+switch

This allows reverting the changes to HighLevelLoopTransform+ReduceNestingTransform from the previous commit, which fixes a bug in loop detection (the previous commit did not handle loops where the loop BlockContainer didn't have a Block as parent).

ICSharpCode.Decompiler.Tests/TestCases/Pretty/ReduceNesting.cs

@@ -198,15 +198,16 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 					case 1:
 						Console.WriteLine("case 1");
 						return;
-					default:
-						if (B(1))
-						{
-							Console.WriteLine(1);
-						}
-						return;
+				}
+				if (B(1))
+				{
+					Console.WriteLine(1);
 				}
 			}
-			Console.WriteLine("else");
+			else
+			{
+				Console.WriteLine("else");
+			}
 		}
 
 		// nesting should not be reduced as maximum nesting level is 1
@@ -346,13 +347,12 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 
 				Console.WriteLine();
 
-				if (!B(1))
-				{
-					return;
-				}
-				for (int i = 0; i < 10; i++)
+				if (B(1))
 				{
-					Console.WriteLine(i);
+					for (int i = 0; i < 10; i++)
+					{
+						Console.WriteLine(i);
+					}
 				}
 			}
 			catch

ICSharpCode.Decompiler/CSharp/CSharpDecompiler.cs

@@ -93,7 +93,7 @@ namespace ICSharpCode.Decompiler.CSharp
 				new YieldReturnDecompiler(), // must run after inlining but before loop detection
 				new AsyncAwaitDecompiler(),  // must run after inlining but before loop detection
 				new DetectCatchWhenConditionBlocks(), // must run after inlining but before loop detection
-				new DetectExitPoints(canIntroduceExitForReturn: false),
+				new DetectExitPoints(),
 				new EarlyExpressionTransforms(),
 				// RemoveDeadVariableInit must run after EarlyExpressionTransforms so that stobj(ldloca V, ...)
 				// is already collapsed into stloc(V, ...).
@@ -117,7 +117,7 @@ namespace ICSharpCode.Decompiler.CSharp
 					}
 				},
 				// re-run DetectExitPoints after loop detection
-				new DetectExitPoints(canIntroduceExitForReturn: false),
+				new DetectExitPoints(),
 				new BlockILTransform { // per-block transforms
 					PostOrderTransforms = {
 						new ConditionDetection(),

ICSharpCode.Decompiler/IL/ControlFlow/ExitPoints.cs

@@ -52,13 +52,6 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 		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;
@@ -214,12 +207,12 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 		static ILInstruction ChooseExit(List<ILInstruction> potentialExits)
 		{
 			ILInstruction first = potentialExits[0];
-			if (first is Leave l && l.IsLeavingFunction)
+			if (first is Leave { IsLeavingFunction: true })
 			{
 				for (int i = 1; i < potentialExits.Count; i++)
 				{
 					var exit = potentialExits[i];
-					if (!(exit is Leave l2 && l2.IsLeavingFunction))
+					if (!(exit is Leave { IsLeavingFunction: true }))
 						return exit;
 				}
 			}
@@ -256,9 +249,11 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 				// we can't introduce any additional exits
 				return false;
 			}
-			if (inst is Leave l && l.IsLeavingFunction)
+			if (inst is Leave { IsLeavingFunction: true })
 			{
-				return canIntroduceExitForReturn;
+				// Only convert 'return;' to an exit in a context where we can turn it into 'break;'.
+				// In other contexts we risk turning it into 'goto'.
+				return currentContainer.Kind != ContainerKind.Normal;
 			}
 			else
 			{

ICSharpCode.Decompiler/IL/Transforms/HighLevelLoopTransform.cs

@@ -38,25 +38,18 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		{
 			this.context = context;
 
-			foreach (var block in function.Descendants.OfType<Block>())
+			foreach (BlockContainer loop in function.Descendants.OfType<BlockContainer>())
 			{
-				for (int i = 0; i < block.Instructions.Count; i++)
+				if (loop.Kind != ContainerKind.Loop)
+					continue;
+				if (MatchWhileLoop(loop, out var condition, out var loopBody))
 				{
-					var loop = block.Instructions[i] as BlockContainer;
-					if (loop == null || loop.Kind != ContainerKind.Loop)
-						continue;
-					// convert a "return" to a "break" so that we can match the high-level
-					// loop patterns
-					RemoveRedundantReturn.ReturnToBreak(block, loop, context);
-					if (MatchWhileLoop(loop, out var condition, out var loopBody))
-					{
-						if (context.Settings.ForStatement)
-							MatchForLoop(loop, condition, loopBody);
-						continue;
-					}
-					if (context.Settings.DoWhileStatement && MatchDoWhileLoop(loop))
-						continue;
+					if (context.Settings.ForStatement)
+						MatchForLoop(loop, condition, loopBody);
+					continue;
 				}
+				if (context.Settings.DoWhileStatement && MatchDoWhileLoop(loop))
+					continue;
 			}
 		}
 

ICSharpCode.Decompiler/IL/Transforms/ReduceNestingTransform.cs

@@ -103,10 +103,6 @@ namespace ICSharpCode.Decompiler.IL
 						// visit the contents of the container
 						Visit(container, continueTarget);
 
-						if (container.Kind == ContainerKind.Switch)
-						{
-							RemoveRedundantReturn.ReturnToBreak(block, container, context);
-						}
 						// reduce nesting in switch blocks
 						if (container.Kind == ContainerKind.Switch &&
 							CanDuplicateExit(NextInsn(), continueTarget, out var keywordExit1) &&