mirror of
https://github.com/icsharpcode/ILSpy.git
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209 lines
8.8 KiB
C#
209 lines
8.8 KiB
C#
// Copyright (c) 2011-2015 Daniel Grunwald
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//
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// Permission is hereby granted, free of charge, to any person obtaining a copy of this
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// software and associated documentation files (the "Software"), to deal in the Software
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// without restriction, including without limitation the rights to use, copy, modify, merge,
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// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
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// to whom the Software is furnished to do so, subject to the following conditions:
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//
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// The above copyright notice and this permission notice shall be included in all copies or
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// substantial portions of the Software.
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//
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// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
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// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
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// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
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// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
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// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
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// DEALINGS IN THE SOFTWARE.
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using System;
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using System.Collections.Generic;
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using System.Diagnostics;
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using System.Linq;
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using ICSharpCode.Decompiler.TypeSystem;
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using ICSharpCode.Decompiler.Util;
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namespace ICSharpCode.Decompiler.IL.Transforms
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{
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/// <summary>
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/// Runs a very simple form of copy propagation.
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/// Copy propagation is used in two cases:
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/// 1) assignments from arguments to local variables
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/// If the target variable is assigned to only once (so always is that argument) and the argument is never changed (no ldarga/starg),
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/// then we can replace the variable with the argument.
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/// 2) assignments of address-loading instructions to local variables
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/// </summary>
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public class CopyPropagation : IILTransform
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{
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public static void Propagate(StLoc store, ILTransformContext context)
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{
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Debug.Assert(store.Variable.IsSingleDefinition);
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Block block = (Block)store.Parent;
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int i = store.ChildIndex;
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DoPropagate(store.Variable, store.Value, block, ref i, context);
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}
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public void Run(ILFunction function, ILTransformContext context)
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{
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var splitVariables = new HashSet<ILVariable>(ILVariableEqualityComparer.Instance);
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foreach (var g in function.Variables.GroupBy(v => v, ILVariableEqualityComparer.Instance))
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{
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if (g.Count() > 1)
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{
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splitVariables.Add(g.Key);
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}
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}
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foreach (var block in function.Descendants.OfType<Block>())
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{
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if (block.Kind != BlockKind.ControlFlow)
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continue;
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RunOnBlock(block, context, splitVariables);
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}
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}
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static void RunOnBlock(Block block, ILTransformContext context, HashSet<ILVariable> splitVariables = null)
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{
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for (int i = 0; i < block.Instructions.Count; i++)
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{
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if (block.Instructions[i].MatchStLoc(out ILVariable v, out ILInstruction copiedExpr))
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{
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if (v.IsSingleDefinition && v.LoadCount == 0 && v.Kind == VariableKind.StackSlot)
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{
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// dead store to stack
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if (SemanticHelper.IsPure(copiedExpr.Flags))
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{
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// no-op -> delete
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context.Step("remove dead store to stack: no-op -> delete", block.Instructions[i]);
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block.Instructions.RemoveAt(i);
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// This can open up new inlining opportunities:
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int c = ILInlining.InlineInto(block, i, InliningOptions.None, context: context);
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i -= c + 1;
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}
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else
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{
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// evaluate the value for its side-effects
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context.Step("remove dead store to stack: evaluate the value for its side-effects", block.Instructions[i]);
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copiedExpr.AddILRange(block.Instructions[i]);
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block.Instructions[i] = copiedExpr;
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context.EndStep(copiedExpr);
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}
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}
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else if (v.IsSingleDefinition && CanPerformCopyPropagation(v, copiedExpr, splitVariables, context.Settings))
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{
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DoPropagate(v, copiedExpr, block, ref i, context);
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}
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}
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}
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}
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static bool CanPerformCopyPropagation(ILVariable target, ILInstruction value, HashSet<ILVariable> splitVariables, DecompilerSettings settings)
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{
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Debug.Assert(target.StackType == value.ResultType);
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if (target.Type.IsSmallIntegerType())
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return false;
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if (splitVariables != null && splitVariables.Contains(target))
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{
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return false; // non-local code move might change semantics when there's split variables
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}
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switch (value.OpCode)
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{
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case OpCode.LdLoca:
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case OpCode.LdsFlda:
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// All address-loading instructions always return the same value for a given operand/argument combination,
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// so they can be safely copied.
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// ... except for LdElema and LdFlda, because those might throw an exception, and we don't want to
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// change the place where the exception is thrown.
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return true;
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case OpCode.LdElema:
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case OpCode.LdFlda:
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return !settings.UseRefLocalsForAccurateOrderOfEvaluation;
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case OpCode.LdLoc:
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var v = ((LdLoc)value).Variable;
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if (splitVariables != null && splitVariables.Contains(v))
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{
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return false; // non-local code move might change semantics when there's split variables
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}
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switch (v.Kind)
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{
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case VariableKind.Parameter:
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// Parameters can be copied only if they aren't assigned to (directly or indirectly via ldarga)
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// note: the initialization by the caller is the first store -> StoreCount must be 1
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return v.IsSingleDefinition;
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default:
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// Variables can be copied if both are single-definition.
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// To avoid removing too many variables, we do this only if the target
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// is either a stackslot or a ref local.
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Debug.Assert(target.IsSingleDefinition);
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return v.IsSingleDefinition && (target.Kind == VariableKind.StackSlot || target.StackType == StackType.Ref);
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}
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default:
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// All instructions without special behavior that target a stack-variable can be copied.
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return value.Flags == InstructionFlags.None && value.Children.Count == 0 && target.Kind == VariableKind.StackSlot;
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}
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}
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static void DoPropagate(ILVariable v, ILInstruction copiedExpr, Block block, ref int i, ILTransformContext context)
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{
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context.Step($"Copy propagate {v.Name}", copiedExpr);
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int firstUninlinedArg = i;
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// un-inline the arguments of the ldArg instruction
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ILVariable[] uninlinedArgs = new ILVariable[copiedExpr.Children.Count];
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for (int j = 0; j < uninlinedArgs.Length; j++)
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{
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var arg = copiedExpr.Children[j];
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var type = arg.InferType(context.TypeSystem);
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uninlinedArgs[j] = new ILVariable(VariableKind.StackSlot, type, arg.ResultType) {
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Name = "C_" + arg.StartILOffset,
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HasGeneratedName = true,
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};
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block.Instructions.Insert(i++, new StLoc(uninlinedArgs[j], arg));
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}
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v.Function.Variables.AddRange(uninlinedArgs);
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// perform copy propagation:
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foreach (var expr in v.LoadInstructions.ToArray())
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{
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var clone = copiedExpr.Clone();
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for (int j = 0; j < uninlinedArgs.Length; j++)
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{
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clone.Children[j].ReplaceWith(new LdLoc(uninlinedArgs[j]));
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}
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// We are copying an expression from far away, reusing the ILRange would result in incorrect sequence points.
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clone.SetILRange(new Interval());
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if (expr.SlotInfo == StObj.TargetSlot && clone.HasDirectFlag(InstructionFlags.MayThrow)
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&& !expr.Parent.SatisfiesSlotRestrictionForInlining(expr.ChildIndex, clone))
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{
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// A LdFlda/LdElema used as StObj target has to delay its exception, because C#
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// computes the value to be stored before dereferencing the target. Accept the
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// changed point at which the exception is thrown, mirroring
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// InliningOptions.AllowChangingOrderOfEvaluationForExceptions in ILInlining.
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// Unlike inlining, copy propagation has no third option of just giving up: the
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// defining store is gone by the end of this loop, so every load must be replaced.
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// Refusing the copy is only possible before propagation starts, which is what
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// CanPerformCopyPropagation does when ref locals are requested; the assert
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// guards the public Propagate() entry point, which bypasses that check.
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Debug.Assert(!context.Settings.UseRefLocalsForAccurateOrderOfEvaluation);
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if (clone is LdFlda ldflda)
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ldflda.DelayExceptions = true;
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else if (clone is LdElema ldelema)
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ldelema.DelayExceptions = true;
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}
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expr.ReplaceWith(clone);
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}
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block.Instructions.RemoveAt(i);
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int c = ILInlining.InlineInto(block, i, InliningOptions.None, context: context);
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i -= c + 1;
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if (uninlinedArgs.Length > 0)
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{
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// The stores holding the un-inlined arguments are themselves copy-propagation
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// candidates: copying an ldflda leaves its target address behind in a stack slot,
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// and if that address is e.g. an ldloca it can be copied as well. They sit before
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// the current position, so rewind far enough for the caller's loop to visit them;
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// otherwise a multiply-loaded stack slot survives as a spurious ref local.
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i = Math.Min(i, firstUninlinedArg - 1);
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}
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}
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}
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}
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