Remove compiler-generated variable in while (true) loops.

ICSharpCode.Decompiler/IL/ControlFlow/ControlFlowSimplification.cs

@@ -19,6 +19,7 @@ using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
 using ICSharpCode.Decompiler.IL.Transforms;
+using ICSharpCode.Decompiler.TypeSystem;
 
 namespace ICSharpCode.Decompiler.IL.ControlFlow
 {
@@ -52,7 +53,7 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 				SwitchDetection.SimplifySwitchInstruction(block);
 			}
 			SimplifyBranchChains(function, context);
-			CleanUpEmptyBlocks(function);
+			CleanUpEmptyBlocks(function, context);
 		}
 		
 		void InlineVariableInReturnBlock(Block block, ILTransformContext context)
@@ -129,13 +130,13 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 			}
 		}
 		
-		void CleanUpEmptyBlocks(ILFunction function)
+		void CleanUpEmptyBlocks(ILFunction function, ILTransformContext context)
 		{
 			foreach (var container in function.Descendants.OfType<BlockContainer>()) {
 				foreach (var block in container.Blocks) {
 					if (block.Instructions.Count == 0)
 						continue; // block is already marked for deletion
-					while (CombineBlockWithNextBlock(container, block)) {
+					while (CombineBlockWithNextBlock(container, block, context)) {
 						// repeat combining blocks until it is no longer possible
 						// (this loop terminates because a block is deleted in every iteration)
 					}
@@ -153,7 +154,7 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 			return targetBlock.Instructions[0].MatchReturn(out var value) && value is LdLoc;
 		}
 		
-		static bool CombineBlockWithNextBlock(BlockContainer container, Block block)
+		static bool CombineBlockWithNextBlock(BlockContainer container, Block block, ILTransformContext context)
 		{
 			Debug.Assert(container == block.Parent);
 			// Ensure the block will stay a basic block -- we don't want extended basic blocks prior to LoopDetection.
@@ -165,12 +166,31 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 				return false;
 			if (br.TargetBlock == block)
 				return false; // don't inline block into itself
+			context.Step("CombineBlockWithNextBlock", br);
 			var targetBlock = br.TargetBlock;
+			if (targetBlock.ILRange.Start < block.ILRange.Start && IsDeadTrueStore(block)) {
+				// The C# compiler generates a dead store for the condition of while (true) loops.
+				block.Instructions.RemoveRange(block.Instructions.Count - 3, 2);
+			}
 			block.Instructions.Remove(br);
 			block.Instructions.AddRange(targetBlock.Instructions);
 			targetBlock.Instructions.Clear(); // mark targetBlock for deletion
 			return true;
 		}
-		
+
+		/// <summary>
+		/// Returns true if the last two instructions before the branch are storing the value 'true' into an unused variable.
+		/// </summary>
+		private static bool IsDeadTrueStore(Block block)
+		{
+			if (block.Instructions.Count < 3) return false;
+			if (!(block.Instructions.SecondToLastOrDefault() is StLoc deadStore && block.Instructions[block.Instructions.Count - 3] is StLoc tempStore))
+				return false;
+			if (!(deadStore.Variable.LoadCount == 0 && deadStore.Variable.AddressCount == 0))
+				return false;
+			if (!(deadStore.Value.MatchLdLoc(tempStore.Variable) && tempStore.Variable.IsSingleDefinition && tempStore.Variable.LoadCount == 1))
+				return false;
+			return tempStore.Value.MatchLdcI4(1) && deadStore.Variable.Type.IsKnownType(KnownTypeCode.Boolean);
+		}
 	}
 }