ILInlining.cs

// Copyright (c) 2011-2017 Daniel Grunwald
// 
// 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.Diagnostics;
using System.Linq;
using System.Reflection;

using ICSharpCode.Decompiler.TypeSystem;
using ICSharpCode.Decompiler.TypeSystem.Implementation;

using NetEscapades.EnumGenerators;

namespace ICSharpCode.Decompiler.IL.Transforms
{
	[Flags]
	[EnumExtensions]
	public enum InliningOptions
	{
		None = 0,
		Aggressive = 1,
		IntroduceNamedArguments = 2,
		FindDeconstruction = 4,
		AllowChangingOrderOfEvaluationForExceptions = 8,
		AllowInliningOfLdloca = 0x10
	}

	/// <summary>
	/// Performs inlining transformations.
	/// </summary>
	public class ILInlining : IILTransform, IBlockTransform, IStatementTransform
	{
		internal InliningOptions options;

		public void Run(ILFunction function, ILTransformContext context)
		{
			foreach (var block in function.Descendants.OfType<Block>())
			{
				InlineAllInBlock(function, block, this.options, context);
			}
			function.Variables.RemoveDead();
		}

		public void Run(Block block, BlockTransformContext context)
		{
			InlineAllInBlock(context.Function, block, this.options, context);
		}

		public void Run(Block block, int pos, StatementTransformContext context)
		{
			var options = this.options | OptionsForBlock(block, pos, context);
			while (InlineOneIfPossible(block, pos, options, context: context))
			{
				// repeat inlining until nothing changes.
			}
		}

		internal static InliningOptions OptionsForBlock(Block block, int pos, ILTransformContext context)
		{
			InliningOptions options = InliningOptions.None;
			if (context.Settings.AggressiveInlining || IsCatchWhenBlock(block))
			{
				options |= InliningOptions.Aggressive;
			}
			else
			{
				var function = block.Ancestors.OfType<ILFunction>().FirstOrDefault();
				var inst = block.Instructions[pos];
				if (IsInConstructorInitializer(function, inst) || PreferExpressionsOverStatements(function))
					options |= InliningOptions.Aggressive;
			}
			if (!context.Settings.UseRefLocalsForAccurateOrderOfEvaluation)
			{
				options |= InliningOptions.AllowChangingOrderOfEvaluationForExceptions;
			}
			return options;
		}

		static bool PreferExpressionsOverStatements(ILFunction function)
		{
			switch (function.Kind)
			{
				case ILFunctionKind.Delegate:
					return function.Parameters.Any(p => CSharp.CSharpDecompiler.IsTransparentIdentifier(p.Name));
				case ILFunctionKind.ExpressionTree:
					return true;
				default:
					return false;
			}
		}

		public static bool InlineAllInBlock(ILFunction function, Block block, InliningOptions options, ILTransformContext context)
		{
			bool modified = false;
			var instructions = block.Instructions;
			for (int i = instructions.Count - 1; i >= 0; i--)
			{
				if (instructions[i] is StLoc inst)
				{
					if (InlineOneIfPossible(block, i, options, context))
					{
						modified = true;
						continue;
					}
				}
			}
			return modified;
		}

		internal static bool IsInConstructorInitializer(ILFunction function, ILInstruction inst)
		{
			int ctorCallStart = function.ChainedConstructorCallILOffset;
			if (inst.EndILOffset > ctorCallStart)
				return false;
			var topLevelInst = inst.Ancestors.LastOrDefault(instr => instr.Parent is Block);
			if (topLevelInst == null)
				return false;
			return topLevelInst.EndILOffset <= ctorCallStart;
		}

		internal static bool IsCatchWhenBlock(Block block)
		{
			var container = BlockContainer.FindClosestContainer(block);
			return container?.Parent is TryCatchHandler handler
				&& handler.Filter == container;
		}

		/// <summary>
		/// Inlines instructions before pos into block.Instructions[pos].
		/// </summary>
		/// <returns>The number of instructions that were inlined.</returns>
		public static int InlineInto(Block block, int pos, InliningOptions options, ILTransformContext context)
		{
			if (pos >= block.Instructions.Count)
				return 0;
			int count = 0;
			while (--pos >= 0)
			{
				if (InlineOneIfPossible(block, pos, options, context))
					count++;
				else
					break;
			}
			return count;
		}

		/// <summary>
		/// Aggressively inlines the stloc instruction at block.Body[pos] into the next instruction, if possible.
		/// </summary>
		public static bool InlineIfPossible(Block block, int pos, ILTransformContext context)
		{
			return InlineOneIfPossible(block, pos, InliningOptions.Aggressive, context);
		}

		/// <summary>
		/// Inlines the stloc instruction at block.Instructions[pos] into the next instruction, if possible.
		/// </summary>
		public static bool InlineOneIfPossible(Block block, int pos, InliningOptions options, ILTransformContext context)
		{
			context.CancellationToken.ThrowIfCancellationRequested();
			if (block.Instructions[pos] is not StLoc stloc)
				return false;
			if (!VariableCanBeUsedForInlining(stloc.Variable))
				return false;
			// TODO: inlining of small integer types might be semantically incorrect,
			// but we can't avoid it this easily without breaking lots of tests.
			//if (v.Type.IsSmallIntegerType())
			//	return false; // stloc might perform implicit truncation
			return InlineOne(stloc, options, context);
		}

		public static bool VariableCanBeUsedForInlining(ILVariable v)
		{
			if (v.Kind == VariableKind.PinnedLocal)
				return false;
			// ensure the variable is accessed only a single time
			if (v.StoreCount != 1)
				return false;
			if (v.LoadCount + v.AddressCount != 1)
				return false;
			return true;
		}

		/// <summary>
		/// Inlines the stloc instruction at block.Instructions[pos] into the next instruction.
		/// 
		/// Note that this method does not check whether 'v' has only one use;
		/// the caller is expected to validate whether inlining 'v' has any effects on other uses of 'v'.
		/// </summary>
		public static bool InlineOne(StLoc stloc, InliningOptions options, ILTransformContext context)
		{
			ILVariable v = stloc.Variable;
			Block block = (Block)stloc.Parent;
			int pos = stloc.ChildIndex;
			if (DoInline(v, stloc.Value, block.Instructions.ElementAtOrDefault(pos + 1), options, context))
			{
				// Assign the ranges of the stloc instruction:
				stloc.Value.AddILRange(stloc);
				// Remove the stloc instruction:
				Debug.Assert(block.Instructions[pos] == stloc);
				block.Instructions.RemoveAt(pos);
				return true;
			}
			else if (v.LoadCount == 0 && v.AddressCount == 0)
			{
				// The variable is never loaded
				if (SemanticHelper.IsPure(stloc.Value.Flags))
				{
					// Remove completely if the instruction has no effects
					// (except for reading locals)
					context.Step("Remove dead store without side effects", stloc);
					block.Instructions.RemoveAt(pos);
					return true;
				}
				else if (v.Kind == VariableKind.StackSlot)
				{
					context.Step("Remove dead store, but keep expression", stloc);
					// Assign the ranges of the stloc instruction:
					stloc.Value.AddILRange(stloc);
					// Remove the stloc, but keep the inner expression
					stloc.ReplaceWith(stloc.Value);
					return true;
				}
			}
			return false;
		}

		/// <summary>
		/// Inlines 'expr' into 'next', if possible.
		/// 
		/// Note that this method does not check whether 'v' has only one use;
		/// the caller is expected to validate whether inlining 'v' has any effects on other uses of 'v'.
		/// </summary>
		static bool DoInline(ILVariable v, ILInstruction inlinedExpression, ILInstruction next, InliningOptions options, ILTransformContext context)
		{
			var r = FindLoadInNext(next, v, inlinedExpression, options);
			if (r.Type == FindResultType.Found || r.Type == FindResultType.NamedArgument)
			{
				var loadInst = r.LoadInst;
				if (loadInst.OpCode == OpCode.LdLoca)
				{
					if (!IsGeneratedTemporaryForAddressOf((LdLoca)loadInst, v, inlinedExpression, options))
						return false;
				}
				else
				{
					Debug.Assert(loadInst.OpCode == OpCode.LdLoc);
					bool aggressive = (options & InliningOptions.Aggressive) != 0;
					if (!aggressive && v.Kind != VariableKind.StackSlot
						&& !NonAggressiveInlineInto(next, r, inlinedExpression, v))
					{
						return false;
					}
				}

				if (r.Type == FindResultType.NamedArgument)
				{
					NamedArgumentTransform.IntroduceNamedArgument(r.CallArgument, context);
					// Now that the argument is evaluated early, we can inline as usual
				}

				context.Step($"Inline variable '{v.Name}'", inlinedExpression);
				// Assign the ranges of the ldloc instruction:
				inlinedExpression.AddILRange(loadInst);

				if (loadInst.OpCode == OpCode.LdLoca)
				{
					// it was an ldloca instruction, so we need to use the pseudo-opcode 'addressof'
					// to preserve the semantics of the compiler-generated temporary
					Debug.Assert(((LdLoca)loadInst).Variable == v);
					loadInst.ReplaceWith(new AddressOf(inlinedExpression, v.Type));
				}
				else
				{
					loadInst.ReplaceWith(inlinedExpression);
				}
				return true;
			}
			return false;
		}

		/// <summary>
		/// Is this a temporary variable generated by the C# compiler for instance method calls on value type values
		/// </summary>
		/// <param name="loadInst">The load instruction (a descendant within 'next')</param>
		/// <param name="v">The variable being inlined.</param>
		static bool IsGeneratedTemporaryForAddressOf(LdLoca loadInst, ILVariable v, ILInstruction inlinedExpression, InliningOptions options)
		{
			Debug.Assert(loadInst.Variable == v);
			if (!options.HasFlag(InliningOptions.AllowInliningOfLdloca))
			{
				return false; // inlining of ldloca is not allowed in the early inlining stage
			}
			// Inlining a value type variable is allowed only if the resulting code will maintain the semantics
			// that the method is operating on a copy.
			// Thus, we have to ensure we're operating on an r-value.
			// Additionally, we cannot inline in cases where the C# compiler prohibits the direct use
			// of the rvalue (e.g. M(ref (MyStruct)obj); is invalid).
			if (IsUsedAsThisPointerInCall(loadInst, out var method, out var constrainedTo))
			{
				if (options.HasFlag(InliningOptions.Aggressive))
				{
					// Inlining might be required in ctor initializers (see #2714).
					// expressionBuilder.VisitAddressOf will handle creating the copy for us.
					return true;
				}

				switch (ClassifyExpression(inlinedExpression))
				{
					case ExpressionClassification.RValue:
						// For struct method calls on rvalues, the C# compiler always generates temporaries.
						return true;
					case ExpressionClassification.MutableLValue:
						// For struct method calls on mutable lvalues, the C# compiler never generates temporaries.
						return false;
					case ExpressionClassification.ReadonlyLValue:
						// For struct method calls on readonly lvalues, the C# compiler
						// only generates a temporary if it isn't a "readonly struct"
						return MethodRequiresCopyForReadonlyLValue(method, constrainedTo);
					default:
						throw new InvalidOperationException("invalid expression classification");
				}
			}
			else if (loadInst.Parent is LdElemaInlineArray)
			{
				return true;
			}
			else if (IsPassedToReadOnlySpanCtor(loadInst))
			{
				// Always inlining is possible here, because it's an 'in' or 'ref readonly' parameter
				// and the C# compiler allows calling it with an rvalue, even though that might produce
				// a warning. Note that we don't need to check the expression classification, because
				// expressionBuilder.VisitAddressOf will handle creating the copy for us.
				// This is necessary, because there are compiler-generated uses of this ctor when
				// passing a single-element array to a params ROS<T> parameter and our following transforms
				// assume the value is already inlined.
				return true;
			}
			else if (IsPassedToInlineArrayAsSpan(loadInst))
			{
				// Inlining is not allowed:
				// <PrivateImplementationDetails>.InlineArrayAsReadOnlySpan(GetInlineArray()) is invalid C# code
				return false;
			}
			else if (IsPassedToInParameter(loadInst))
			{
				if (options.HasFlag(InliningOptions.Aggressive))
				{
					// Inlining might be required in ctor initializers (see #2714).
					// expressionBuilder.VisitAddressOf will handle creating the copy for us.
					return true;
				}

				switch (ClassifyExpression(inlinedExpression))
				{
					case ExpressionClassification.RValue:
						// For rvalues passed to in parameters, the C# compiler generates a temporary.
						return true;
					case ExpressionClassification.MutableLValue:
					case ExpressionClassification.ReadonlyLValue:
						// For lvalues passed to in parameters, the C# compiler never generates temporaries.
						return false;
					default:
						throw new InvalidOperationException("invalid expression classification");
				}
			}
			else if (IsUsedAsThisPointerInFieldRead(loadInst))
			{
				// mcs generated temporaries for field reads on rvalues (#1555)
				return ClassifyExpression(inlinedExpression) == ExpressionClassification.RValue;
			}
			else
			{
				return false;
			}
		}

		private static bool IsPassedToInlineArrayAsSpan(LdLoca loadInst)
		{
			if (loadInst.Parent is not Call call)
				return false;
			var method = call.Method;
			var declaringType = method.DeclaringType;
			return declaringType.ReflectionName == "<PrivateImplementationDetails>"
				&& method.Name is "InlineArrayAsReadOnlySpan" or "InlineArrayAsSpan"
				&& method.Parameters is [var arg, var length]
				&& (method.ReturnType.IsKnownType(KnownTypeCode.SpanOfT) || method.ReturnType.IsKnownType(KnownTypeCode.ReadOnlySpanOfT))
				&& arg.Type is ByReferenceType
				&& length.Type.IsKnownType(KnownTypeCode.Int32);
		}

		internal static bool MethodRequiresCopyForReadonlyLValue(IMethod method, IType constrainedTo = null)
		{
			if (method == null)
				return true;
			var type = constrainedTo ?? method.DeclaringType;
			if (type.IsReferenceType == true)
				return false; // reference types are never implicitly copied
			if (method.ThisIsRefReadOnly)
				return false; // no copies for calls on readonly structs
			return true;
		}

		internal static bool IsUsedAsThisPointerInCall(LdLoca ldloca)
		{
			return IsUsedAsThisPointerInCall(ldloca, out _, out _);
		}

		static bool IsUsedAsThisPointerInCall(LdLoca ldloca, out IMethod method, out IType constrainedType)
		{
			method = null;
			constrainedType = null;
			if (ldloca.Variable.Type.IsReferenceType ?? false)
				return false;
			ILInstruction inst = ldloca;
			if (inst.Parent is LdObjIfRef)
			{
				inst = inst.Parent;
			}
			while (inst.Parent is LdFlda ldflda)
			{
				inst = ldflda;
			}
			if (inst.ChildIndex != 0)
				return false;
			switch (inst.Parent.OpCode)
			{
				case OpCode.Call:
				case OpCode.CallVirt:
					var callInst = (CallInstruction)inst.Parent;
					method = callInst.Method;
					constrainedType = callInst.ConstrainedTo;
					if (method.IsAccessor)
					{
						if (method.AccessorKind == MethodSemanticsAttributes.Getter)
						{
							// C# doesn't allow property compound assignments on temporary structs
							return !(inst.Parent.Parent is CompoundAssignmentInstruction cai
								&& cai.TargetKind == CompoundTargetKind.Property
								&& cai.Target == inst.Parent);
						}
						else
						{
							// C# doesn't allow calling setters on temporary structs
							return false;
						}
					}
					return !method.IsStatic;
				case OpCode.Await:
					method = ((Await)inst.Parent).GetAwaiterMethod;
					return true;
				case OpCode.NullableUnwrap:
					return ((NullableUnwrap)inst.Parent).RefInput;
				case OpCode.MatchInstruction:
					method = ((MatchInstruction)inst.Parent).Method;
					return true;
				default:
					return false;
			}
		}

		static bool IsUsedAsThisPointerInFieldRead(LdLoca ldloca)
		{
			if (ldloca.Variable.Type.IsReferenceType ?? false)
				return false;
			ILInstruction inst = ldloca;
			while (inst.Parent is LdFlda ldflda)
			{
				inst = ldflda;
			}
			return inst != ldloca && inst.Parent is LdObj;
		}

		internal static bool IsPassedToInParameter(LdLoca ldloca)
		{
			if (ldloca.Parent is not CallInstruction call)
			{
				return false;
			}
			return call.GetParameter(ldloca.ChildIndex)?.ReferenceKind is ReferenceKind.In;
		}

		static bool IsPassedToReadOnlySpanCtor(LdLoca ldloca)
		{
			if (ldloca.Parent is not NewObj call)
			{
				return false;
			}
			var method = call.Method;
			return method.IsConstructor
				&& method.Parameters.Count == 1
				&& method.DeclaringType.IsKnownType(KnownTypeCode.ReadOnlySpanOfT);
		}

		internal static bool IsReadOnlySpanCharCtor(IMethod method)
		{
			return method.IsConstructor
				&& method.Parameters.Count == 1
				&& method.DeclaringType.IsKnownType(KnownTypeCode.ReadOnlySpanOfT)
				&& method.DeclaringType.TypeArguments[0].IsKnownType(KnownTypeCode.Char)
				&& method.Parameters[0].Type is ByReferenceType brt
				&& brt.ElementType.IsKnownType(KnownTypeCode.Char);
		}

		/// <summary>
		/// Gets whether the instruction, when converted into C#, turns into an l-value that can
		/// be used to mutate a value-type.
		/// If this function returns false, the C# compiler would introduce a temporary copy
		/// when calling a method on a value-type (and any mutations performed by the method will be lost)
		/// </summary>
		internal static ExpressionClassification ClassifyExpression(ILInstruction inst)
		{
			switch (inst.OpCode)
			{
				case OpCode.LdLoc:
				case OpCode.StLoc:
					ILVariable v = ((IInstructionWithVariableOperand)inst).Variable;
					if (v.IsRefReadOnly
						|| v.Kind == VariableKind.ForeachLocal
						|| v.Kind == VariableKind.UsingLocal)
					{
						return ExpressionClassification.ReadonlyLValue;
					}
					else
					{
						return ExpressionClassification.MutableLValue;
					}
				case OpCode.LdObj:
					// ldobj typically refers to a storage location,
					// but readonly fields are an exception.
					if (IsReadonlyReference(((LdObj)inst).Target))
					{
						return ExpressionClassification.ReadonlyLValue;
					}
					else
					{
						return ExpressionClassification.MutableLValue;
					}
				case OpCode.StObj:
					// stobj is the same as ldobj.
					if (IsReadonlyReference(((StObj)inst).Target))
					{
						return ExpressionClassification.ReadonlyLValue;
					}
					else
					{
						return ExpressionClassification.MutableLValue;
					}
				case OpCode.Call:
					var m = ((CallInstruction)inst).Method;
					// multi-dimensional array getters are lvalues,
					// everything else is an rvalue.
					if (m.DeclaringType.Kind == TypeKind.Array)
					{
						return ExpressionClassification.MutableLValue;
					}
					else
					{
						return ExpressionClassification.RValue;
					}
				default:
					return ExpressionClassification.RValue; // most instructions result in an rvalue
			}
		}

		/// <summary>
		/// Gets whether the ILInstruction will turn into a C# expresion that is considered readonly by the C# compiler.
		/// </summary>
		internal static bool IsReadonlyReference(ILInstruction addr)
		{
			switch (addr)
			{
				case LdFlda ldflda:
					return ldflda.Field.IsReadOnly
						|| (ldflda.Field.DeclaringType.Kind == TypeKind.Struct && IsReadonlyReference(ldflda.Target));
				case LdsFlda ldsflda:
					return ldsflda.Field.IsReadOnly;
				case LdLoc ldloc:
					return ldloc.Variable.IsRefReadOnly;
				case Call call:
					return call.Method.ReturnTypeIsRefReadOnly;
				case CallVirt call:
					return call.Method.ReturnTypeIsRefReadOnly;
				case CallIndirect calli:
					return calli.FunctionPointerType.ReturnIsRefReadOnly;
				case AddressOf _:
					// C# doesn't allow mutation of value-type temporaries
					return true;
				default:
					if (addr.MatchLdFld(out _, out var field))
						return field.ReturnTypeIsRefReadOnly;
					return false;
			}
		}

		/// <summary>
		/// Determines whether a variable should be inlined in non-aggressive mode, even though it is not a generated variable.
		/// </summary>
		/// <param name="next">The next top-level expression</param>
		/// <param name="v">The variable being eliminated by inlining.</param>
		/// <param name="inlinedExpression">The expression being inlined</param>
		static bool NonAggressiveInlineInto(ILInstruction next, FindResult findResult, ILInstruction inlinedExpression, ILVariable v)
		{
			if (findResult.Type == FindResultType.NamedArgument)
			{
				var originalStore = (StLoc)inlinedExpression.Parent;
				return !originalStore.ILStackWasEmpty;
			}
			Debug.Assert(findResult.Type == FindResultType.Found);

			var loadInst = findResult.LoadInst;
			Debug.Assert(loadInst.IsDescendantOf(next));

			// decide based on the source expression being inlined
			switch (inlinedExpression.OpCode)
			{
				case OpCode.DefaultValue:
				case OpCode.StObj:
				case OpCode.NumericCompoundAssign:
				case OpCode.UserDefinedCompoundAssign:
				case OpCode.Await:
				case OpCode.SwitchInstruction:
					return true;
				case OpCode.LdLoc:
					if (v.StateMachineField == null && ((LdLoc)inlinedExpression).Variable.StateMachineField != null)
					{
						// Roslyn likes to put the result of fetching a state machine field into a temporary variable,
						// so inline more aggressively in such cases.
						return true;
					}
					break;
			}
			if (inlinedExpression.ResultType == StackType.Ref)
			{
				// VB likes to use ref locals for compound assignment
				// (the C# compiler uses ref stack slots instead).
				// We want to avoid unnecessary ref locals, so we'll always inline them if possible.
				return true;
			}

			var parent = loadInst.Parent;
			if (NullableLiftingTransform.MatchNullableCtor(parent, out _, out _))
			{
				// inline into nullable ctor call in lifted operator
				parent = parent.Parent;
			}
			if (parent is ILiftableInstruction liftable && liftable.IsLifted)
			{
				return true; // inline into lifted operators
			}
			// decide based on the new parent into which we are inlining:
			switch (parent.OpCode)
			{
				case OpCode.NullCoalescingInstruction:
					if (NullableType.IsNullable(v.Type))
						return true; // inline nullables into ?? operator
					break;
				case OpCode.NullableUnwrap:
					return true; // inline into ?. operator
				case OpCode.UserDefinedLogicOperator:
				case OpCode.DynamicLogicOperatorInstruction:
					return true; // inline into (left slot of) user-defined && or || operator
				case OpCode.DynamicGetMemberInstruction:
				case OpCode.DynamicGetIndexInstruction:
					if (parent.Parent.OpCode == OpCode.DynamicCompoundAssign)
						return true; // inline into dynamic compound assignments
					break;
				case OpCode.DynamicCompoundAssign:
					return true;
				case OpCode.GetPinnableReference:
				case OpCode.LocAllocSpan:
					return true; // inline size-expressions into localloc.span
				case OpCode.Call:
				case OpCode.CallVirt:
					// Aggressive inline into property/indexer getter calls for compound assignment calls
					// (The compiler generates locals for these because it doesn't want to evalute the args twice for getter+setter)
					if (parent.SlotInfo == CompoundAssignmentInstruction.TargetSlot)
					{
						return true;
					}
					if (((CallInstruction)parent).Method is SyntheticRangeIndexAccessor)
					{
						return true;
					}
					break;
				case OpCode.CallIndirect when loadInst.SlotInfo == CallIndirect.FunctionPointerSlot:
					return true;
				case OpCode.LdElema:
					if (((LdElema)parent).WithSystemIndex)
					{
						return true;
					}
					break;
				case OpCode.Leave:
				case OpCode.YieldReturn:
					return true;
				case OpCode.SwitchInstruction:
					// Preserve type info on switch instruction, if we're inlining a local variable into the switch-value slot.
					if (v.Kind != VariableKind.StackSlot && loadInst.SlotInfo == SwitchInstruction.ValueSlot)
					{
						((SwitchInstruction)parent).Type ??= v.Type;
					}
					return true;
				//case OpCode.BinaryNumericInstruction when parent.SlotInfo == SwitchInstruction.ValueSlot:
				case OpCode.StringToInt when parent.SlotInfo == SwitchInstruction.ValueSlot:
					return true;
				case OpCode.MatchInstruction:
					var match = (MatchInstruction)parent;
					if (match.IsDeconstructTuple
						|| (match.CheckType && match.Variable.Type.IsReferenceType != true))
					{
						return true;
					}
					break;
			}
			// decide based on the top-level target instruction into which we are inlining:
			switch (next.OpCode)
			{
				case OpCode.IfInstruction:
					while (parent.MatchLogicNot(out _))
					{
						parent = parent.Parent;
					}
					return parent == next;
				default:
					return false;
			}
		}

		/// <summary>
		/// Gets whether 'expressionBeingMoved' can be inlined into 'expr'.
		/// </summary>
		public static bool CanInlineInto(ILInstruction expr, ILVariable v, ILInstruction expressionBeingMoved)
		{
			return FindLoadInNext(expr, v, expressionBeingMoved, InliningOptions.None).Type == FindResultType.Found;
		}

		internal enum FindResultType
		{
			/// <summary>
			/// Found a load; inlining is possible.
			/// </summary>
			Found,
			/// <summary>
			/// Load not found and re-ordering not possible. Stop the search.
			/// </summary>
			Stop,
			/// <summary>
			/// Load not found, but the expressionBeingMoved can be re-ordered with regards to the
			/// tested expression, so we may continue searching for the matching load.
			/// </summary>
			Continue,
			/// <summary>
			/// Found a load in call, but re-ordering not possible with regards to the
			/// other call arguments.
			/// Inlining is not possible, but we might convert the call to named arguments.
			/// Only used with <see cref="InliningOptions.IntroduceNamedArguments"/>.
			/// </summary>
			NamedArgument,
			/// <summary>
			/// Found a deconstruction.
			/// Only used with <see cref="InliningOptions.FindDeconstruction"/>.
			/// </summary>
			Deconstruction,
		}

		internal readonly struct FindResult
		{
			public readonly FindResultType Type;
			public readonly ILInstruction LoadInst; // ldloc or ldloca instruction that loads the variable to be inlined
			public readonly ILInstruction CallArgument; // argument of call that needs to be promoted to a named argument

			private FindResult(FindResultType type, ILInstruction loadInst, ILInstruction callArg)
			{
				this.Type = type;
				this.LoadInst = loadInst;
				this.CallArgument = callArg;
			}

			public static readonly FindResult Stop = new FindResult(FindResultType.Stop, null, null);
			public static readonly FindResult Continue = new FindResult(FindResultType.Continue, null, null);

			public static FindResult Found(ILInstruction loadInst)
			{
				Debug.Assert(loadInst.OpCode == OpCode.LdLoc || loadInst.OpCode == OpCode.LdLoca);
				return new FindResult(FindResultType.Found, loadInst, null);
			}

			public static FindResult NamedArgument(ILInstruction loadInst, ILInstruction callArg)
			{
				Debug.Assert(loadInst.OpCode == OpCode.LdLoc || loadInst.OpCode == OpCode.LdLoca);
				Debug.Assert(callArg.Parent is CallInstruction);
				return new FindResult(FindResultType.NamedArgument, loadInst, callArg);
			}

			public static FindResult Deconstruction(DeconstructInstruction deconstruction)
			{
				return new FindResult(FindResultType.Deconstruction, deconstruction, null);
			}
		}

		/// <summary>
		/// Finds the position to inline to.
		/// </summary>
		/// <returns>true = found; false = cannot continue search; null = not found</returns>
		internal static FindResult FindLoadInNext(ILInstruction expr, ILVariable v, ILInstruction expressionBeingMoved, InliningOptions options)
		{
			if (expr == null)
				return FindResult.Stop;
			if (expr.MatchLdLoc(v) || expr.MatchLdLoca(v))
			{
				// Match found, we can inline unless there are slot restrictions.
				if (expr.Parent.SatisfiesSlotRestrictionForInlining(expr.ChildIndex, expressionBeingMoved))
				{
					return FindResult.Found(expr);
				}
				// We cannot inline because the targets slot restrictions are not satisfied
				if ((options & InliningOptions.AllowChangingOrderOfEvaluationForExceptions) != 0 && expr.SlotInfo == StObj.TargetSlot)
				{
					// Special case: inlining will change code semantics,
					// but we accept that so that we can avoid a ref local.
					if (expressionBeingMoved is LdFlda ldflda)
						ldflda.DelayExceptions = true;
					else if (expressionBeingMoved is LdElema ldelema)
						ldelema.DelayExceptions = true;
					return FindResult.Found(expr);
				}
				return FindResult.Stop;
			}
			else if (expr is Block { Kind: BlockKind.CallWithNamedArgs } block)
			{
				return NamedArgumentTransform.CanExtendNamedArgument(block, v, expressionBeingMoved);
			}
			else if (options.HasFlag(InliningOptions.FindDeconstruction) && expr is DeconstructInstruction di)
			{
				return FindResult.Deconstruction(di);
			}
			foreach (var child in expr.Children)
			{
				if (!expr.CanInlineIntoSlot(child.ChildIndex, expressionBeingMoved))
					return FindResult.Stop;

				// Recursively try to find the load instruction
				FindResult r = FindLoadInNext(child, v, expressionBeingMoved, options);
				if (r.Type != FindResultType.Continue)
				{
					if (r.Type == FindResultType.Stop && (options & InliningOptions.IntroduceNamedArguments) != 0 && expr is CallInstruction call)
						return NamedArgumentTransform.CanIntroduceNamedArgument(call, child, v, expressionBeingMoved);
					return r;
				}
			}
			if (IsSafeForInlineOver(expr, expressionBeingMoved))
				return FindResult.Continue; // continue searching
			else
				return FindResult.Stop; // abort, inlining not possible
		}

		/// <summary>
		/// Determines whether it is safe to move 'expressionBeingMoved' past 'expr'
		/// </summary>
		static bool IsSafeForInlineOver(ILInstruction expr, ILInstruction expressionBeingMoved)
		{
			return SemanticHelper.MayReorder(expressionBeingMoved, expr);
		}

		/// <summary>
		/// Finds the first call instruction within the instructions that were inlined into inst.
		/// </summary>
		internal static CallInstruction FindFirstInlinedCall(ILInstruction inst)
		{
			foreach (var child in inst.Children)
			{
				if (!child.SlotInfo.CanInlineInto)
					break;
				var call = FindFirstInlinedCall(child);
				if (call != null)
				{
					return call;
				}
			}
			return inst as CallInstruction;
		}

		/// <summary>
		/// Gets whether 'expressionBeingMoved' can be moved from somewhere before 'stmt' to become the replacement of 'targetLoad'.
		/// </summary>
		public static bool CanMoveInto(ILInstruction expressionBeingMoved, ILInstruction stmt, ILInstruction targetLoad)
		{
			Debug.Assert(targetLoad.IsDescendantOf(stmt));
			for (ILInstruction inst = targetLoad; inst != stmt; inst = inst.Parent)
			{
				if (!inst.Parent.CanInlineIntoSlot(inst.ChildIndex, expressionBeingMoved))
					return false;
				// Check whether re-ordering with predecessors is valid:
				int childIndex = inst.ChildIndex;
				for (int i = 0; i < childIndex; ++i)
				{
					ILInstruction predecessor = inst.Parent.Children[i];
					if (!IsSafeForInlineOver(predecessor, expressionBeingMoved))
						return false;
				}
			}
			return true;
		}

		/// <summary>
		/// Gets whether 'expressionBeingMoved' can be moved from somewhere before 'stmt' to become the replacement of 'targetLoad'.
		/// </summary>
		public static bool CanMoveIntoCallVirt(ILInstruction expressionBeingMoved, ILInstruction stmt, CallVirt nestedCallVirt, ILInstruction targetLoad)
		{
			Debug.Assert(targetLoad.IsDescendantOf(stmt) && nestedCallVirt.IsDescendantOf(stmt));
			ILInstruction thisArg = nestedCallVirt.Arguments[0];
			Debug.Assert(thisArg is LdObjIfRef);
			for (ILInstruction inst = targetLoad; inst != stmt; inst = inst.Parent)
			{
				if (!inst.Parent.CanInlineIntoSlot(inst.ChildIndex, expressionBeingMoved))
					return false;
				// Check whether re-ordering with predecessors is valid:
				int childIndex = inst.ChildIndex;
				for (int i = 0; i < childIndex; ++i)
				{
					ILInstruction predecessor = inst.Parent.Children[i];
					if (predecessor != thisArg && !IsSafeForInlineOver(predecessor, expressionBeingMoved))
						return false;
				}
			}
			return true;
		}

		/// <summary>
		/// Gets whether arg can be un-inlined out of stmt.
		/// </summary>
		/// <seealso cref="ILInstruction.Extract"/>
		internal static bool CanUninline(ILInstruction arg, ILInstruction stmt)
		{
			// moving into and moving out-of are equivalent
			return CanMoveInto(arg, stmt, arg);
		}
	}

	internal enum ExpressionClassification
	{
		RValue,
		MutableLValue,
		ReadonlyLValue,
	}
}