ILSpy/ICSharpCode.Decompiler/IL/Transforms/NullCoalescingTransform.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
// 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 System.Text;
using System.Threading.Tasks;

using ICSharpCode.Decompiler.TypeSystem;

namespace ICSharpCode.Decompiler.IL.Transforms
{
	/// <summary>
	/// Transform for constructing the NullCoalescingInstruction (if.notnull(a,b), or in C#: ??)
	/// Note that this transform only handles the case where a,b are reference types.
	/// 
	/// The ?? operator for nullable value types is handled by NullableLiftingTransform.
	/// </summary>
	public class NullCoalescingTransform : IStatementTransform
	{
		public void Run(Block block, int pos, StatementTransformContext context)
		{
			if (TransformRefTypes(block, pos, context))
				return;
			if (TransformHoistedConstructorArgumentNullGuard(block, pos, context))
				return;
			TransformThrowExpressionValueTypes(block, pos, context);
		}

		/// <summary>
		/// Handles NullCoalescingInstruction case 1: reference types.
		/// </summary>
		bool TransformRefTypes(Block block, int pos, StatementTransformContext context)
		{
			if (!(block.Instructions[pos] is StLoc stloc))
				return false;
			if (stloc.Variable.Kind != VariableKind.StackSlot)
				return false;
			if (!block.Instructions[pos + 1].MatchIfInstruction(out var condition, out var trueInst))
				return false;
			if (!(condition.MatchCompEquals(out var left, out var right) && left.MatchLdLoc(stloc.Variable) && right.MatchLdNull()))
				return false;
			trueInst = Block.Unwrap(trueInst);
			// stloc s(valueInst)
			// if (comp(ldloc s == ldnull)) {
			//		stloc s(fallbackInst)
			// }
			// =>
			// stloc s(if.notnull(valueInst, fallbackInst))
			if (trueInst.MatchStLoc(stloc.Variable, out var fallbackValue))
			{
				context.Step("NullCoalescingTransform: simple (reference types)", stloc);
				stloc.Value = new NullCoalescingInstruction(NullCoalescingKind.Ref, stloc.Value, fallbackValue);
				block.Instructions.RemoveAt(pos + 1); // remove if instruction
				ILInlining.InlineOneIfPossible(block, pos, InliningOptions.None, context);
				return true;
			}
			// sometimes the compiler generates:
			// stloc s(valueInst)
			// if (comp(ldloc s == ldnull)) {
			//		stloc v(fallbackInst)
			//      stloc s(ldloc v)
			// }
			// v must be single-assign and single-use.
			if (trueInst is Block trueBlock && trueBlock.Instructions.Count == 2
				&& trueBlock.Instructions[0].MatchStLoc(out var temporary, out fallbackValue)
				&& temporary.IsSingleDefinition && temporary.LoadCount == 1
				&& trueBlock.Instructions[1].MatchStLoc(stloc.Variable, out var useOfTemporary)
				&& useOfTemporary.MatchLdLoc(temporary))
			{
				context.Step("NullCoalescingTransform: with temporary variable (reference types)", stloc);
				stloc.Value = new NullCoalescingInstruction(NullCoalescingKind.Ref, stloc.Value, fallbackValue);
				block.Instructions.RemoveAt(pos + 1); // remove if instruction
				ILInlining.InlineOneIfPossible(block, pos, InliningOptions.None, context);
				return true;
			}
			// stloc obj(valueInst)
			// if (comp(ldloc obj == ldnull)) {
			//		throw(...)
			// }
			// =>
			// stloc obj(if.notnull(valueInst, throw(...)))
			if (context.Settings.ThrowExpressions && trueInst is Throw throwInst)
			{
				context.Step("NullCoalescingTransform (reference types + throw expression)", stloc);
				throwInst.resultType = StackType.O;
				stloc.Value = new NullCoalescingInstruction(NullCoalescingKind.Ref, stloc.Value, throwInst);
				block.Instructions.RemoveAt(pos + 1); // remove if instruction
				ILInlining.InlineOneIfPossible(block, pos, InliningOptions.None, context);
				return true;
			}
			return false;
		}

		/// <summary>
		/// When an argument of a chained constructor call contains a throwing null-check
		/// (e.g. <c>arg ?? throw ...</c>) and <c>arg</c> is evaluated more than once, the C#
		/// compiler hoists the null-check in front of the chained call:
		/// <code>
		///   if (comp.o(ldloc param == ldnull)) throw(...)
		///   call Base..ctor(..., ldlen(ldloc param), ..., ldloc param, ...)
		/// </code>
		/// The guard then blocks TransformFieldAndConstructorInitializers from lifting the chained
		/// call into a <c>: this(...)</c> / <c>: base(...)</c> clause. Replace the guard with
		/// <code>
		///   stloc temp(if.notnull(ldloc param, throw(...)))
		/// </code>
		/// redirecting the first following use of the parameter (the position the check was hoisted
		/// from) to <c>temp</c>, and leave moving the coalescing expression into the chained call to
		/// ILInlining, which does so only when that preserves the order of evaluation.
		/// Reference-type chains (<c>base/this..ctor</c> calls) are identified by IL offset via
		/// <see cref="ILInlining.IsInConstructorInitializer"/>; value types chain via
		/// <c>this = new TSelf(...)</c>, i.e. <c>stobj(ldthis, newobj TSelf(...))</c>, which
		/// <see cref="ILFunction.ChainedConstructorCallILOffset"/> does not report, so the stobj
		/// shape is matched directly.
		/// </summary>
		bool TransformHoistedConstructorArgumentNullGuard(Block block, int pos, StatementTransformContext context)
		{
			// A throw-expression is the only way to express the folded form.
			if (!context.Settings.ThrowExpressions)
				return false;

			var function = block.Ancestors.OfType<ILFunction>().FirstOrDefault();
			if (function?.Method is not { IsConstructor: true, IsStatic: false })
				return false;

			// Match `if (comp(ldloc param == ldnull)) throw(...)` with no else branch.
			var guard = block.Instructions[pos];
			if (!IsArgumentNullGuard(guard, out var paramLoad, out var throwInst))
				return false;

			if (!GuardPrecedesChainedConstructorCall(block, pos, function, out int searchEndPos))
				return false;

			// Redirect the first following use of the parameter, i.e. the position where the
			// null-check sat before the compiler hoisted it.
			LdLoc firstUse = null;
			for (int i = pos + 1; i <= searchEndPos && firstUse == null; i++)
			{
				firstUse = block.Instructions[i].Descendants.OfType<LdLoc>()
					.FirstOrDefault(ld => ld.Variable == paramLoad.Variable);
			}
			if (firstUse == null)
				return false; // parameter not used up to the chained call -> cannot fold; leave guard in place

			context.Step($"NullCoalescingTransform: fold hoisted null-guard of '{paramLoad.Variable.Name}' into argument of chained constructor call", guard);

			var temp = function.RegisterVariable(VariableKind.StackSlot, paramLoad.Variable.Type);
			firstUse.Variable = temp;
			throwInst.resultType = StackType.O;
			var stloc = new StLoc(temp, new NullCoalescingInstruction(NullCoalescingKind.Ref, paramLoad, throwInst));
			stloc.AddILRange(guard);
			block.Instructions[pos] = stloc;
			context.EndStep(stloc);
			ILInlining.InlineOneIfPossible(block, pos, InliningOptions.None, context);
			return true;
		}

		/// <summary>
		/// Matches a hoisted argument null-guard `if (comp(ldloc param == ldnull)) throw(...)`
		/// (no else branch). Only parameters qualify: nothing else is in scope before the
		/// constructor initializer.
		/// </summary>
		static bool IsArgumentNullGuard(ILInstruction inst, out LdLoc paramLoad, out Throw throwInst)
		{
			paramLoad = null;
			throwInst = null;
			if (!inst.MatchIfInstruction(out var condition, out var trueInst))
				return false;
			if (!(Block.Unwrap(trueInst) is Throw t))
				return false;
			if (!(condition.MatchCompEquals(out var left, out var right) && right.MatchLdNull() && left is LdLoc load))
				return false;
			if (load.Variable.Kind != VariableKind.Parameter)
				return false;
			paramLoad = load;
			throwInst = t;
			return true;
		}

		/// <summary>
		/// Determines whether the guard at <paramref name="pos"/> precedes the constructor's chained
		/// this/base call, i.e. belongs to the hoisted argument evaluation of the constructor
		/// initializer. <paramref name="searchEndPos"/> is the last statement index that may contain
		/// the parameter use to redirect (the statement containing the chained call, if it is in
		/// this block).
		/// </summary>
		static bool GuardPrecedesChainedConstructorCall(Block block, int pos, ILFunction function, out int searchEndPos)
		{
			searchEndPos = -1;
			if (ILInlining.IsInConstructorInitializer(function, block.Instructions[pos]))
			{
				// Reference-type chain: everything before ChainedConstructorCallILOffset is the
				// initializer's argument evaluation. Search up to and including the first statement
				// that reaches past that offset (the statement containing the chained call).
				int ctorCallStart = function.ChainedConstructorCallILOffset;
				for (int i = pos + 1; i < block.Instructions.Count; i++)
				{
					searchEndPos = i;
					if (block.Instructions[i].EndILOffset > ctorCallStart)
						break;
				}
				return searchEndPos > pos;
			}
			// Value-type chain: `this = new TSelf(...)` is not reported by
			// ChainedConstructorCallILOffset, so match the stobj shape directly. Only further
			// hoisted guards may sit between this guard and the chained call; anything else means
			// the stobj is a plain body statement rather than a chain.
			for (int i = pos + 1; i < block.Instructions.Count; i++)
			{
				var inst = block.Instructions[i];
				if (IsValueTypeChainedConstructorCall(inst, function))
				{
					searchEndPos = i;
					return true;
				}
				if (!IsArgumentNullGuard(inst, out _, out _))
					return false;
			}
			return false;
		}

		/// <summary>
		/// True if <paramref name="inst"/> is a value-type chained constructor call
		/// <c>this = new TSelf(...)</c>, i.e. <c>stobj(ldthis, newobj TSelf(...))</c> where TSelf
		/// is the constructor's declaring type.
		/// </summary>
		static bool IsValueTypeChainedConstructorCall(ILInstruction inst, ILFunction function)
		{
			return inst is StObj { Value: NewObj { Method.IsConstructor: true } newObj } stobj
				&& newObj.Method.DeclaringType.IsReferenceType == false
				&& newObj.Method.DeclaringTypeDefinition == function.Method.DeclaringTypeDefinition
				&& MatchLdThisOrStackSlotCopy(stobj.Target);
		}

		/// <summary>
		/// Matches a load of the this-pointer, either directly or via a single-definition stack slot
		/// that copies it. The compiler spills this to such a slot when a hoisted guard sits between
		/// the this-load and the chained <c>this = new TSelf(...)</c> call.
		/// </summary>
		static bool MatchLdThisOrStackSlotCopy(ILInstruction inst)
		{
			if (inst.MatchLdThis())
				return true;
			return inst.MatchLdLoc(out var v)
				&& v.Kind == VariableKind.StackSlot
				&& v.IsSingleDefinition
				&& v.StoreInstructions.Count == 1
				&& v.StoreInstructions[0] is StLoc { Value: { } storeValue }
				&& storeValue.MatchLdThis();
		}

		/// <summary>
		/// stloc v(value)
		/// if (logic.not(call get_HasValue(ldloca v))) throw(...)
		/// ... Call(arg1, arg2, call GetValueOrDefault(ldloca v), arg4) ...
		/// =>
		/// ... Call(arg1, arg2, if.notnull(value, throw(...)), arg4) ...
		/// </summary>
		bool TransformThrowExpressionValueTypes(Block block, int pos, StatementTransformContext context)
		{
			if (pos + 2 >= block.Instructions.Count)
				return false;
			if (!(block.Instructions[pos] is StLoc stloc))
				return false;
			ILVariable v = stloc.Variable;
			if (!(v.StoreCount == 1 && v.LoadCount == 0 && v.AddressCount == 2))
				return false;
			if (!block.Instructions[pos + 1].MatchIfInstruction(out var condition, out var trueInst))
				return false;
			if (!(Block.Unwrap(trueInst) is Throw throwInst))
				return false;
			if (!condition.MatchLogicNot(out var arg))
				return false;
			if (!(arg is CallInstruction call && NullableLiftingTransform.MatchHasValueCall(call, v)))
				return false;
			var throwInstParent = throwInst.Parent;
			var throwInstChildIndex = throwInst.ChildIndex;
			var nullCoalescingWithThrow = new NullCoalescingInstruction(
				NullCoalescingKind.NullableWithValueFallback,
				stloc.Value,
				throwInst);
			var resultType = NullableType.GetUnderlyingType(call.Method.DeclaringType).GetStackType();
			nullCoalescingWithThrow.UnderlyingResultType = resultType;
			var result = ILInlining.FindLoadInNext(block.Instructions[pos + 2], v, nullCoalescingWithThrow, InliningOptions.None);
			if (result.Type == ILInlining.FindResultType.Found
				&& NullableLiftingTransform.MatchGetValueOrDefault(result.LoadInst.Parent, v))
			{
				context.Step("NullCoalescingTransform (value types + throw expression)", stloc);
				throwInst.resultType = resultType;
				result.LoadInst.Parent.ReplaceWith(nullCoalescingWithThrow);
				block.Instructions.RemoveRange(pos, 2); // remove store(s) and if instruction
				return true;
			}
			else
			{
				// reset the primary position (see remarks on ILInstruction.Parent)
				stloc.Value = stloc.Value;
				var children = throwInstParent.Children;
				children[throwInstChildIndex] = throwInst;
				return false;
			}
		}
	}
}