Avoid reference count temporarily dropping to zero while ConditionDetection moves instructions to other blocks.

This avoids recursively un-registering e.g. all LdLocs from their ILVariable.LoadInstructions, etc. (all the ILInstruction.Disconnected logic). This speeds up the example from #1193 by another factor 2.

ICSharpCode.Decompiler/IL/ControlFlow/ConditionDetection.cs

@@ -230,7 +230,9 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 			{
 				context.Step("Embed else-block for goto removal", ifInst);
 				Debug.Assert(IsEmpty(ifInst.FalseInst));
-				ifInst.FalseInst = ExtractBlock(block, block.Instructions.IndexOf(ifInst) + 1, block.Instructions.Count - 1);
+				Block newBlock = new Block();
+				ifInst.FalseInst = newBlock;
+				ExtractBlock(block, block.Instructions.IndexOf(ifInst) + 1, block.Instructions.Count - 1, newBlock);
 			}
 
 			// if (...) { ...; goto blockExit; } blockExit;
@@ -375,6 +377,7 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 
 			//save a copy
 			var thenInst = ifInst.TrueInst;
+			thenInst.AddRef();
 
 			if (ifInst != block.Instructions.SecondToLastOrDefault())
 			{
@@ -382,12 +385,14 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 				// Note that this will only extract instructions that were previously inlined from another block
 				// (via InlineExitBranch), so the instructions are already fully-transformed.
 				// So it's OK to move them into a nested block again (which hides them from the following block transforms).
-				ifInst.TrueInst = ExtractBlock(block, block.Instructions.IndexOf(ifInst) + 1, block.Instructions.Count);
+				var newBlock = new Block();
+				ifInst.TrueInst = newBlock;
+				ExtractBlock(block, block.Instructions.IndexOf(ifInst) + 1, block.Instructions.Count, newBlock);
 			}
 			else
 			{
-				block.Instructions.RemoveAt(block.Instructions.Count - 1);
 				ifInst.TrueInst = exitInst;
+				block.Instructions.RemoveAt(block.Instructions.Count - 1);
 			}
 
 			if (thenInst is Block thenBlock)
@@ -398,6 +403,7 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 			{
 				block.Instructions.Add(thenInst);
 			}
+			thenInst.ReleaseRef();
 
 			ifInst.Condition = Comp.LogicNot(ifInst.Condition);
 			ExpressionTransforms.RunOnSingleStatement(ifInst, context);
@@ -662,9 +668,8 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 		/// <summary>
 		/// Removes a subrange of instructions from a block and returns them in a new Block
 		/// </summary>
-		internal static Block ExtractBlock(Block block, int startIndex, int endIndex)
+		internal static void ExtractBlock(Block block, int startIndex, int endIndex, Block extractedBlock)
 		{
-			var extractedBlock = new Block();
 			for (int i = startIndex; i < endIndex; i++)
 			{
 				var inst = block.Instructions[i];
@@ -672,8 +677,6 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 				extractedBlock.AddILRange(inst);
 			}
 			block.Instructions.RemoveRange(startIndex, endIndex - startIndex);
-
-			return extractedBlock;
 		}
 	}
 }

ICSharpCode.Decompiler/IL/Transforms/HighLevelLoopTransform.cs

@@ -92,8 +92,11 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			ifInstruction.Condition = Comp.LogicNot(ifInstruction.Condition);
 			ifInstruction.FalseInst = ifInstruction.TrueInst;
 			//move the rest of the body into a new block
-			loopBody = ConditionDetection.ExtractBlock(loop.EntryPoint, 1, loop.EntryPoint.Instructions.Count);
+			loopBody = new Block();
+			loopBody.AddRef();
+			ConditionDetection.ExtractBlock(loop.EntryPoint, 1, loop.EntryPoint.Instructions.Count, loopBody);
 			loop.Blocks.Insert(1, loopBody);
+			loopBody.ReleaseRef();
 			if (!loopBody.HasFlag(InstructionFlags.EndPointUnreachable))
 				loopBody.Instructions.Add(new Leave(loop));