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// Copyright (c) 2011 AlphaSierraPapa for the SharpDevelop Team
//
// 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.Diagnostics;
using Mono.Cecil.Cil;
namespace ICSharpCode.Decompiler.FlowAnalysis
{
/// <summary>
/// Contains some very simple optimizations that work on the SSA form.
/// </summary>
internal static class SsaOptimization
{
public static void Optimize(SsaForm ssaForm)
{
DirectlyStoreToVariables(ssaForm);
SimpleCopyPropagation(ssaForm);
RemoveDeadAssignments(ssaForm);
}
/// <summary>
/// When any instructions stores its result in a stack location that's used only once in a 'stloc' or 'starg' instruction,
/// we optimize this to directly store in the target location.
/// As optimization this is redundant (does the same as copy propagation), but it'll make us keep the variables named
/// after locals instead of keeping the temps as using only the simple copy propagation would do.
/// </summary>
public static void DirectlyStoreToVariables(SsaForm ssaForm)
{
foreach (SsaBlock block in ssaForm.Blocks) {
block.Instructions.RemoveAll(
inst => {
if (inst.Instruction != null && (inst.Instruction.OpCode == OpCodes.Stloc || inst.Instruction.OpCode == OpCodes.Starg)) {
SsaVariable target = inst.Target;
SsaVariable temp = inst.Operands[0];
if (target.IsSingleAssignment && temp.IsSingleAssignment && temp.Usage.Count == 1 && temp.IsStackLocation) {
temp.Definition.Target = target;
return true;
}
}
return false;
});
}
ssaForm.ComputeVariableUsage(); // update usage after we modified stuff
}
public static void SimpleCopyPropagation(SsaForm ssaForm, bool onlyForStackLocations = true)
{
foreach (SsaBlock block in ssaForm.Blocks) {
foreach (SsaInstruction inst in block.Instructions) {
if (inst.IsMoveInstruction && inst.Target.IsSingleAssignment && inst.Operands[0].IsSingleAssignment) {
if (inst.Target.IsStackLocation || !onlyForStackLocations) {
// replace all uses of 'target' with 'operands[0]'.
foreach (SsaInstruction useInstruction in inst.Target.Usage) {
useInstruction.ReplaceVariableInOperands(inst.Target, inst.Operands[0]);
}
}
}
}
}
ssaForm.ComputeVariableUsage(); // update usage after we modified stuff
}
public static void RemoveDeadAssignments(SsaForm ssaForm)
{
HashSet<SsaVariable> liveVariables = new HashSet<SsaVariable>();
// find variables that are used directly
foreach (SsaBlock block in ssaForm.Blocks) {
foreach (SsaInstruction inst in block.Instructions) {
if (!CanRemoveAsDeadCode(inst)) {
if (inst.Target != null)
liveVariables.Add(inst.Target);
foreach (SsaVariable op in inst.Operands) {
liveVariables.Add(op);
}
}
}
}
Queue<SsaVariable> queue = new Queue<SsaVariable>(liveVariables);
// find variables that are used indirectly
while (queue.Count > 0) {
SsaVariable v = queue.Dequeue();
if (v.IsSingleAssignment) {
foreach (SsaVariable op in v.Definition.Operands) {
if (liveVariables.Add(op))
queue.Enqueue(op);
}
}
}
// remove assignments to all unused variables
foreach (SsaBlock block in ssaForm.Blocks) {
block.Instructions.RemoveAll(
inst => {
if (inst.Target != null && !liveVariables.Contains(inst.Target)) {
Debug.Assert(inst.Target.IsSingleAssignment);
return true;
}
return false;
});
}
ssaForm.ComputeVariableUsage(); // update usage after we modified stuff
}
static bool CanRemoveAsDeadCode(SsaInstruction inst)
{
if (inst.Target != null && !inst.Target.IsSingleAssignment)
return false;
switch (inst.SpecialOpCode) {
case SpecialOpCode.Phi:
case SpecialOpCode.Exception:
case SpecialOpCode.Parameter:
case SpecialOpCode.Uninitialized:
return true;
case SpecialOpCode.None:
return inst.IsMoveInstruction;
default:
return false;
}
}
}
}
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