Add legacy switch-on-nullable transform

ICSharpCode.Decompiler/CSharp/CSharpDecompiler.cs

@@ -83,6 +83,7 @@ namespace ICSharpCode.Decompiler.CSharp
 				new SplitVariables(), // split variables once again, because the stobj(ldloca V, ...) may open up new replacements
 				new SwitchDetection(),
 				new SwitchOnStringTransform(),
+				new SwitchOnNullableTransform(),
 				new BlockILTransform { // per-block transforms
 					PostOrderTransforms = {
 						// Even though it's a post-order block-transform as most other transforms,

ICSharpCode.Decompiler/ICSharpCode.Decompiler.csproj

@@ -277,6 +277,7 @@
     <Compile Include="IL\Instructions\StringToInt.cs" />
     <Compile Include="IL\Instructions\UsingInstruction.cs" />
     <Compile Include="IL\Transforms\InlineReturnTransform.cs" />
+    <Compile Include="IL\Transforms\SwitchOnNullableTransform.cs" />
     <Compile Include="IL\Transforms\SwitchOnStringTransform.cs" />
     <Compile Include="IL\Transforms\UsingTransform.cs" />
     <Compile Include="IL\ControlFlow\AsyncAwaitDecompiler.cs" />

ICSharpCode.Decompiler/IL/Transforms/SwitchOnNullableTransform.cs

@@ -0,0 +1,117 @@
+// Copyright (c) 2017 Siegfried Pammer
+// 
+// Permission is hereby granted, free of charge, to any person obtaining a copy of this
+// software and associated documentation files (the "Software"), to deal in the Software
+// without restriction, including without limitation the rights to use, copy, modify, merge,
+// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
+// to whom the Software is furnished to do so, subject to the following conditions:
+// 
+// The above copyright notice and this permission notice shall be included in all copies or
+// substantial portions of the Software.
+// 
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
+// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
+// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using ICSharpCode.Decompiler.IL.ControlFlow;
+using ICSharpCode.Decompiler.TypeSystem;
+using ICSharpCode.Decompiler.Util;
+
+namespace ICSharpCode.Decompiler.IL.Transforms
+{
+	/// <summary>
+	/// Detects switch-on-nullable patterns employed by the C# compiler and transforms them to an ILAst-switch-instruction.
+	/// </summary>
+	class SwitchOnNullableTransform : IILTransform
+	{
+		public void Run(ILFunction function, ILTransformContext context)
+		{
+			if (!context.Settings.LiftNullables)
+				return;
+
+			HashSet<BlockContainer> changedContainers = new HashSet<BlockContainer>();
+
+			foreach (var block in function.Descendants.OfType<Block>()) {
+				bool changed = false;
+				for (int i = block.Instructions.Count - 1; i >= 0; i--) {
+					SwitchInstruction newSwitch;
+					if (MatchSwitchOnNullable(block.Instructions, i, out newSwitch)) {
+						block.Instructions[i + 1].ReplaceWith(newSwitch);
+						block.Instructions.RemoveRange(i - 2, 3);
+						i -= 2;
+						changed = true;
+						continue;
+					}
+				}
+				if (!changed) continue;
+				SwitchDetection.SimplifySwitchInstruction(block);
+				if (block.Parent is BlockContainer container)
+					changedContainers.Add(container);
+			}
+
+			foreach (var container in changedContainers)
+				container.SortBlocks(deleteUnreachableBlocks: true);
+		}
+
+		bool MatchSwitchOnNullable(InstructionCollection<ILInstruction> instructions, int i, out SwitchInstruction newSwitch)
+		{
+			newSwitch = null;
+			// match first block:
+			// stloc tmp(ldloca switchValueVar)
+			// stloc switchVariable(call GetValueOrDefault(ldloc tmp))
+			// if (logic.not(call get_HasValue(ldloc tmp))) br nullCaseBlock
+			// br switchBlock
+			if (i < 2) return false;
+			if (!instructions[i - 2].MatchStLoc(out var tmp, out var ldloca) ||
+				!instructions[i - 1].MatchStLoc(out var switchVariable, out var getValueOrDefault) ||
+				!instructions[i].MatchIfInstruction(out var condition, out var trueInst))
+				return false;
+			if (!instructions[i + 1].MatchBranch(out var switchBlock) || !trueInst.MatchBranch(out var nullCaseBlock))
+				return false;
+			if (!ldloca.MatchLdLoca(out var switchValueVar))
+				return false;
+			if (!condition.MatchLogicNot(out var getHasValue))
+				return false;
+			if (!(getValueOrDefault is Call getValueOrDefaultCall) || getValueOrDefaultCall.Method.FullName != "System.Nullable.GetValueOrDefault" ||
+				getValueOrDefaultCall.Method.DeclaringType.TypeParameterCount != 1)
+				return false;
+			if (!(getHasValue is Call getHasValueCall) || !getHasValueCall.Method.IsAccessor || getHasValueCall.Method.FullName != "System.Nullable.get_HasValue" ||
+				getHasValueCall.Method.DeclaringType.TypeParameterCount != 1)
+				return false;
+			if (getHasValueCall.Arguments.Count != 1 || getValueOrDefaultCall.Arguments.Count != 1)
+				return false;
+			if (!getHasValueCall.Arguments[0].MatchLdLoc(tmp) || !getValueOrDefaultCall.Arguments[0].MatchLdLoc(tmp))
+				return false;
+			// match second block: switchBlock
+			// switch (ldloc swtichVariable) {
+			// 	case [0..1): br caseBlock1
+			//  ... more cases ...
+			// 	case [long.MinValue..0),[1..5),[6..10),[11..long.MaxValue]: br defaultBlock
+			// }
+			if (switchBlock.Instructions.Count != 1 || switchBlock.IncomingEdgeCount != 1)
+				return false;
+			if (!(switchBlock.Instructions[0] is SwitchInstruction switchInst))
+				return false;
+			newSwitch = new SwitchInstruction(new LdLoc(switchValueVar));
+			newSwitch.IsLifted = true;
+			SwitchSection defaultSection = null;
+			foreach (var section in switchInst.Sections) {
+				if (defaultSection == null || section.Labels.Count() >= defaultSection.Labels.Count())
+					defaultSection = section;
+				newSwitch.Sections.Add(section);
+			}
+			if (defaultSection.Body.MatchBranch(out var defaultBlock) && defaultBlock == nullCaseBlock)
+				defaultSection.HasNullLabel = true;
+			else {
+				newSwitch.Sections.Add(new SwitchSection { Body = new Branch(nullCaseBlock), HasNullLabel = true });
+			}
+			return true;
+		}
+	}
+}