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using ARMeilleure.IntermediateRepresentation;
using ARMeilleure.Translation;
using static ARMeilleure.IntermediateRepresentation.Operation.Factory;
namespace ARMeilleure.CodeGen.Optimizations
{
static class TailMerge
{
public static void RunPass(in CompilerContext cctx)
{
ControlFlowGraph cfg = cctx.Cfg;
BasicBlock mergedReturn = new(cfg.Blocks.Count);
Operand returnValue;
Operation returnOp;
if (cctx.FuncReturnType == OperandType.None)
{
returnValue = default;
returnOp = Operation(Instruction.Return, default);
}
else
{
returnValue = cfg.AllocateLocal(cctx.FuncReturnType);
returnOp = Operation(Instruction.Return, default, returnValue);
}
mergedReturn.Frequency = BasicBlockFrequency.Cold;
mergedReturn.Operations.AddLast(returnOp);
for (BasicBlock block = cfg.Blocks.First; block != null; block = block.ListNext)
{
Operation op = block.Operations.Last;
if (op != default && op.Instruction == Instruction.Return)
{
block.Operations.Remove(op);
if (cctx.FuncReturnType == OperandType.None)
{
PrepareMerge(block, mergedReturn);
}
else
{
Operation copyOp = Operation(Instruction.Copy, returnValue, op.GetSource(0));
PrepareMerge(block, mergedReturn).Append(copyOp);
}
}
}
cfg.Blocks.AddLast(mergedReturn);
cfg.Update();
}
private static BasicBlock PrepareMerge(BasicBlock from, BasicBlock to)
{
BasicBlock fromPred = from.Predecessors.Count == 1 ? from.Predecessors[0] : null;
// If the block is empty, we can try to append to the predecessor and avoid unnecessary jumps.
if (from.Operations.Count == 0 && fromPred != null && fromPred.SuccessorsCount == 1)
{
for (int i = 0; i < fromPred.SuccessorsCount; i++)
{
if (fromPred.GetSuccessor(i) == from)
{
fromPred.SetSuccessor(i, to);
}
}
// NOTE: `from` becomes unreachable and the call to `cfg.Update()` will remove it.
return fromPred;
}
else
{
from.AddSuccessor(to);
return from;
}
}
}
}
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