private string TryReduceJump(BasicBlock root, JumpBasicBlock jumpbb)
{
// Try to find a linear chain of branches of length > 1
var group = new Set<BasicBlock> { jumpbb };
var chain = new Seq<BasicBlock> { jumpbb };
var start = jumpbb;
while (start.Sources.Count == 1 && start.Sources[0] is JumpBasicBlock &&
!group.Contains(start.Sources[0]))
{
// Extend chain upwards
start = (JumpBasicBlock)start.Sources[0];
group.Add(start);
chain.Insert(0, start);
}
var endjump = jumpbb;
while (endjump.Target.Sources.Count == 1 && endjump.Target is JumpBasicBlock && !group.Contains(endjump.Target))
{
// Extend chain downwards
endjump = (JumpBasicBlock)endjump.Target;
group.Add(endjump);
chain.Add(endjump);
}
var end = (BasicBlock)endjump;
if (endjump.Target.Sources.Count == 1 && endjump.Target.AcceptsInstructions && !group.Contains(endjump.Target))
{
// Extend chain downwards to include a final non jump basic block, provided we can put all
// the accumulated instructions there
end = endjump.Target;
group.Add(end);
chain.Add(end);
}
if (chain.Count > 1)
{
// Coalesce and peephole
var instructions = new Seq<Instruction>();
foreach (var bb in chain)
bb.Block.AccumInstructions(instructions);
var newbb = end.CloneWithInstructions(nextBlockId++, Peephole(chain[0].Block.BeforeState, instructions));
root.Coalesce(start, group, newbb);
return String.Format("collapsed chain of {0} jump blocks from B{1}", chain.Count, start.Id);
}
else if (start.Block.Count == 0 && !start.Target.Equals(start))
{
// Short-circuit a trivial jump
root.Delete(start);
return String.Format("short-circuited trivial jump at B{0}", start.Id);
}
else if (start.Target.Equals(start))
{
// loop
var li = new LoopInstruction(NextInstructionId--, start.Block)
{ BeforeState = jumpbb.Block.BeforeState, AfterState = jumpbb.Block.AfterState };
var newbb = new NonReturningBasicBlock(nextBlockId++, new Instructions(li));
root.Coalesce(start, new Set<BasicBlock> { start }, newbb);
return String.Format("trival loop at B{0}", start.Id);
}
else
return null;
}
