private bool OptimizeBranches(ControlFlowGraph2 cfg)
{
bool hasChanges = false;
foreach (var bb in cfg.BasicBlocks)
{
var ins = bb.Exit;
if (ins.Registers.Count > 0 && ins.Registers[0].PreventOptimization)
{
continue;
}
if (IsComparisonToRegister(ins.Code))
{
hasChanges = OptimizeComparisonToConstZero(ins, bb) || hasChanges;
}
if (ins.Code.IsComparisonBranch() && ins.Next.Code == RCode.Goto)
{
hasChanges = OptimizeBranchFollowedByGoto(ins, cfg) || hasChanges;
}
// Eliminate Predictable branches
if (ins.Code.IsComparisonBranch() || ins.Code == RCode.Goto)
{
hasChanges = OptimizePredictableBranch(ins, bb, cfg) || hasChanges;
}
}
return(hasChanges);
}
private static bool OptimizeBranchFollowedByGoto(Instruction ins, ControlFlowGraph2 cfg)
{
// Redirect branches immediately followed by a goto
// If they jump just after the goto, and we are the only
// instruction reaching the goto.
var branchTarget = (Instruction)ins.Operand;
if (branchTarget != ins.Next.NextOrDefault)
{
return(false);
}
var gotoBlock = cfg.GetBlockFromEntry(ins.Next);
// if there are other instructions reaching the goto, don't do anything.
if (gotoBlock.EntryBlocks.Count() != 1)
{
return(false);
}
// invert the comparison and eliminate the goto
ins.Code = ReverseComparison(ins.Code);
cfg.RerouteBranch(ins, (Instruction)ins.Next.Operand);
ins.Next.ConvertToNop();
return(true);
}
private bool OptimizeBranches(ControlFlowGraph2 cfg)
{
bool hasChanges = false;
foreach (var bb in cfg.BasicBlocks)
{
var ins = bb.Exit;
if (ins.Registers.Count > 0 && ins.Registers[0].PreventOptimization)
continue;
if (IsComparisonToRegister(ins.Code))
hasChanges = OptimizeComparisonToConstZero(ins, bb) || hasChanges;
if (ins.Code.IsComparisonBranch() && ins.Next.Code == RCode.Goto)
hasChanges = OptimizeBranchFollowedByGoto(ins, cfg) || hasChanges;
// Eliminate Predictable branches
if (ins.Code.IsComparisonBranch() || ins.Code == RCode.Goto)
{
hasChanges = OptimizePredictableBranch(ins, bb, cfg) || hasChanges;
}
}
return hasChanges;
}
public bool Transform(Dex target, MethodBody body)
{
bool hasChanges = false;
var graph = new ControlFlowGraph2(body);
var registerUsage = new RegisterUsageMap2(graph);
hasChanges = EliminateRegisterAssigments(registerUsage);
return hasChanges;
}
public bool Transform(Dex target, MethodBody body)
{
bool hasChanges = false;
var graph = new ControlFlowGraph2(body);
var registerUsage = new RegisterUsageMap2(graph);
hasChanges = EliminateRegisterAssigments(registerUsage);
return(hasChanges);
}
/// <summary>
/// This will eliminate or shortcut:
/// (1) zero-comparisons to a just set const
/// (2) branch to a branch when the second branch zero-compares to a
/// just set const before the first branch.
// (3) branch to a const immediatly followed by a zero-comparison
// to that same const.
/// </summary>
private bool OptimizePredictableBranch(Instruction ins, BasicBlock block, ControlFlowGraph2 cfg)
{
// (1) zero-comparisons to a just set const
if (ins != block.Entry &&
IsComparisonWithZero(ins.Code) && ins.Previous.Code == RCode.Const &&
IsSameRegister(ins.Registers, ins.Previous.Registers))
{
if (WillTakeBranch(ins.Code, Convert.ToInt32(ins.Previous.Operand)))
{
ins.Registers.Clear();
ins.Code = RCode.Goto;
return(true);
}
else
{
ins.ConvertToNop();
return(true);
}
}
// (2) branch to a branch when the second branch zero-compares to a
// just set const before the first branch.
var target = (Instruction)ins.Operand;
if (ins != block.Entry &&
IsComparisonWithZero(target.Code) && ins.Previous.Code == RCode.Const &&
IsSameRegister(target.Registers, ins.Previous.Registers))
{
if (WillTakeBranch(target.Code, Convert.ToInt32(ins.Previous.Operand)))
{
cfg.RerouteBranch(ins, (Instruction)target.Operand);
}
else
{
cfg.RerouteBranch(ins, target.Next);
}
return(true);
}
// (3) branch to a const immediatly followed by a zero-comparison to that same const.
// We can only shortcut if the register is not read again after the branch.
if (target.Code == RCode.Const && IsComparisonWithZero(target.Next.Code) &&
IsSameRegister(target.Registers, target.Next.Registers))
{
var secondBranch = target.Next;
var secondBlock = cfg.GetBlockFromExit(secondBranch);
var visited = new HashSet <BasicBlock> {
secondBlock
};
// we know the second block contains only two instructions.
if (!IsRegisterReadAgain(target.Registers[0], secondBlock.ExitBlocks, visited))
{
if (WillTakeBranch(secondBranch.Code, Convert.ToInt32(target.Operand)))
{
cfg.RerouteBranch(ins, (Instruction)target.Next.Operand);
}
else
{
cfg.RerouteBranch(ins, target.Next.Next);
}
return(true);
}
}
return(false);
}
public bool Transform(Dex target, MethodBody body)
{
var cfg = new ControlFlowGraph2(body);
return(OptimizeBranches(cfg));
}
/// <summary>
/// This will eliminate or shortcut:
/// (1) zero-comparisons to a just set const
/// (2) branch to a branch when the second branch zero-compares to a
/// just set const before the first branch.
// (3) branch to a const immediatly followed by a zero-comparison
// to that same const.
/// </summary>
private bool OptimizePredictableBranch(Instruction ins, BasicBlock block, ControlFlowGraph2 cfg)
{
// (1) zero-comparisons to a just set const
if (ins != block.Entry
&& IsComparisonWithZero(ins.Code) && ins.Previous.Code == RCode.Const
&& IsSameRegister(ins.Registers, ins.Previous.Registers))
{
if (WillTakeBranch(ins.Code, Convert.ToInt32(ins.Previous.Operand)))
{
ins.Registers.Clear();
ins.Code = RCode.Goto;
return true;
}
else
{
ins.ConvertToNop();
return true;
}
}
// (2) branch to a branch when the second branch zero-compares to a
// just set const before the first branch.
var target = (Instruction)ins.Operand;
if (ins != block.Entry
&& IsComparisonWithZero(target.Code) && ins.Previous.Code == RCode.Const
&& IsSameRegister(target.Registers, ins.Previous.Registers))
{
if (WillTakeBranch(target.Code, Convert.ToInt32(ins.Previous.Operand)))
cfg.RerouteBranch(ins, (Instruction)target.Operand);
else
cfg.RerouteBranch(ins, target.Next);
return true;
}
// (3) branch to a const immediatly followed by a zero-comparison to that same const.
// We can only shortcut if the register is not read again after the branch.
if (target.Code == RCode.Const && IsComparisonWithZero(target.Next.Code)
&& IsSameRegister(target.Registers, target.Next.Registers))
{
var secondBranch = target.Next;
var secondBlock = cfg.GetBlockFromExit(secondBranch);
var visited = new HashSet<BasicBlock> {secondBlock};
// we know the second block contains only two instructions.
if (!IsRegisterReadAgain(target.Registers[0], secondBlock.ExitBlocks, visited))
{
if (WillTakeBranch(secondBranch.Code, Convert.ToInt32(target.Operand)))
cfg.RerouteBranch(ins, (Instruction)target.Next.Operand);
else
cfg.RerouteBranch(ins, target.Next.Next);
return true;
}
}
return false;
}
private static bool OptimizeBranchFollowedByGoto(Instruction ins, ControlFlowGraph2 cfg)
{
// Redirect branches immediately followed by a goto
// If they jump just after the goto, and we are the only
// instruction reaching the goto.
var branchTarget = (Instruction) ins.Operand;
if (branchTarget != ins.Next.NextOrDefault)
return false;
var gotoBlock = cfg.GetBlockFromEntry(ins.Next);
// if there are other instructions reaching the goto, don't do anything.
if (gotoBlock.EntryBlocks.Count() != 1)
return false;
// invert the comparison and eliminate the goto
ins.Code = ReverseComparison(ins.Code);
cfg.RerouteBranch(ins, (Instruction)ins.Next.Operand);
ins.Next.ConvertToNop();
return true;
}
public bool Transform(Dex target, MethodBody body)
{
var cfg = new ControlFlowGraph2(body);
return OptimizeBranches(cfg);
}
