ILSpy/ICSharpCode.Decompiler/IL/Transforms/SwitchOnNullableTransform.cs

// 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
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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 (MatchRoslynSwitchOnNullable(block.Instructions, i, out newSwitch)) {
						block.Instructions[i - 1].ReplaceWith(newSwitch);
						block.Instructions.RemoveRange(i, 2);
						i--;
						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);
		}

		/// <summary>
		/// Matches legacy C# switch on nullable.
		/// </summary>
		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 (!tmp.IsSingleDefinition || tmp.LoadCount != 2)
				return false;
			if (!switchVariable.IsSingleDefinition || switchVariable.LoadCount != 1)
				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 (!NullableLiftingTransform.MatchGetValueOrDefault(getValueOrDefault, out ILInstruction getValueOrDefaultArg))
				return false;
			if (!NullableLiftingTransform.MatchHasValueCall(getHasValue, out ILInstruction getHasValueArg))
				return false;
			if (!(getHasValueArg.MatchLdLoc(tmp) && getValueOrDefaultArg.MatchLdLoc(tmp)))
				return false;
			// match second block: switchBlock
			// switch (ldloc switchVariable) {
			// 	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 = BuildLiftedSwitch(nullCaseBlock, switchInst, new LdLoc(switchValueVar));
			return true;
		}

		static SwitchInstruction BuildLiftedSwitch(Block nullCaseBlock, SwitchInstruction switchInst, ILInstruction switchValue)
		{
			SwitchInstruction newSwitch = new SwitchInstruction(switchValue);
			newSwitch.IsLifted = true;
			newSwitch.Sections.AddRange(switchInst.Sections);
			newSwitch.Sections.Add(new SwitchSection { Body = new Branch(nullCaseBlock), HasNullLabel = true });
			return newSwitch;
		}

		/// <summary>
		/// Matches Roslyn C# switch on nullable.
		/// </summary>
		bool MatchRoslynSwitchOnNullable(InstructionCollection<ILInstruction> instructions, int i, out SwitchInstruction newSwitch)
		{
			newSwitch = null;
			// match first block:
			// stloc tmp(ldloc switchValueVar)
			// if (logic.not(call get_HasValue(ldloca tmp))) br nullCaseBlock
			// br switchBlock
			if (i < 1) return false;
			if (!instructions[i - 1].MatchStLoc(out var tmp, out var switchValue) ||
				!instructions[i].MatchIfInstruction(out var condition, out var trueInst))
				return false;
			if (tmp.StoreCount != 1 || tmp.AddressCount != 2 || tmp.LoadCount != 0)
				return false;
			if (!instructions[i + 1].MatchBranch(out var switchBlock) || !trueInst.MatchBranch(out var nullCaseBlock))
				return false;
			if (!condition.MatchLogicNot(out var getHasValue) || !NullableLiftingTransform.MatchHasValueCall(getHasValue, out ILVariable target1) || target1 != tmp)
				return false;
			// match second block: switchBlock
			// note: I have seen cases where switchVar is inlined into the switch.
			// stloc switchVar(call GetValueOrDefault(ldloca tmp))
			// switch (ldloc switchVar) {
			// 	case [0..1): br caseBlock1
			// ... more cases ...
			// 	case [long.MinValue..0),[1..5),[6..10),[11..long.MaxValue]: br defaultBlock
			// }
			if (switchBlock.IncomingEdgeCount != 1)
				return false;
			SwitchInstruction switchInst;
			switch (switchBlock.Instructions.Count) {
				case 2: {
					// this is the normal case described by the pattern above
					if (!switchBlock.Instructions[0].MatchStLoc(out var switchVar, out var getValueOrDefault))
						return false;
					if (!switchVar.IsSingleDefinition || switchVar.LoadCount != 1)
						return false;
					if (!NullableLiftingTransform.MatchGetValueOrDefault(getValueOrDefault, tmp))
						return false;
					if (!(switchBlock.Instructions[1] is SwitchInstruction si))
						return false;
					switchInst = si;
					break;
				}
				case 1: {
					// this is the special case where `call GetValueOrDefault(ldloca tmp)` is inlined into the switch.
					if (!(switchBlock.Instructions[0] is SwitchInstruction si))
						return false;
					if (!NullableLiftingTransform.MatchGetValueOrDefault(si.Value, tmp))
						return false;
					switchInst = si;
					break;
				}
				default: {
					return false;
				}
			}
			newSwitch = BuildLiftedSwitch(nullCaseBlock, switchInst, switchValue);
			return true;
		}
	}
}