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package org.graalvm.compiler.lir;

import java.util.ArrayList;
import java.util.List;

import org.graalvm.compiler.core.common.cfg.AbstractBlockBase;
import org.graalvm.compiler.lir.StandardOp.LoadConstantOp;
import org.graalvm.compiler.lir.StandardOp.MoveOp;
import org.graalvm.compiler.lir.StandardOp.ValueMoveOp;
import org.graalvm.compiler.lir.gen.LIRGenerationResult;
import org.graalvm.compiler.lir.phases.PostAllocationOptimizationPhase;

import jdk.vm.ci.code.TargetDescription;

This class optimizes moves, particularly those that result from eliminating SSA form.
 When a block has more than one predecessor, and all predecessors end with the same sequence of 
move instructions, then these sequences can be replaced with a single copy of the sequence at the beginning of the block. 

Similarly, when a block has more than one successor, then same sequences of moves at the
beginning of the successors can be placed once at the end of the block. But because the moves
must be inserted before all branch instructions, this works only when there is exactly one
conditional branch at the end of the block (because the moves must be inserted before all
branches, but after all compares).

This optimization affects all kind of moves (reg->reg, reg->stack and stack->reg).
Because this optimization works best when a block contains only a few moves, it has a huge impact
on the number of blocks that are totally empty.
/**
 * This class optimizes moves, particularly those that result from eliminating SSA form.
 * <p>
 * When a block has more than one predecessor, and all predecessors end with the
 * {@linkplain Optimizer#same(LIRInstruction, LIRInstruction) same} sequence of {@linkplain MoveOp
 * move} instructions, then these sequences can be replaced with a single copy of the sequence at
 * the beginning of the block.
 * <p>
 * Similarly, when a block has more than one successor, then same sequences of moves at the
 * beginning of the successors can be placed once at the end of the block. But because the moves
 * must be inserted before all branch instructions, this works only when there is exactly one
 * conditional branch at the end of the block (because the moves must be inserted before all
 * branches, but after all compares).
 * <p>
 * This optimization affects all kind of moves (reg-&gt;reg, reg-&gt;stack and stack-&gt;reg).
 * Because this optimization works best when a block contains only a few moves, it has a huge impact
 * on the number of blocks that are totally empty.
 */
public final class EdgeMoveOptimizer extends PostAllocationOptimizationPhase {

    @Override
    protected void run(TargetDescription target, LIRGenerationResult lirGenRes, PostAllocationOptimizationContext context) {
        LIR ir = lirGenRes.getLIR();
        Optimizer optimizer = new Optimizer(ir);

        AbstractBlockBase<?>[] blockList = ir.linearScanOrder();
        // ignore the first block in the list (index 0 is not processed)
        for (int i = blockList.length - 1; i >= 1; i--) {
            AbstractBlockBase<?> block = blockList[i];

            if (block.getPredecessorCount() > 1) {
                optimizer.optimizeMovesAtBlockEnd(block);
            }
            if (block.getSuccessorCount() == 2) {
                optimizer.optimizeMovesAtBlockBegin(block);
            }
        }
    }

    private static final class Optimizer {
        private final List<List<LIRInstruction>> edgeInstructionSeqences;
        private LIR ir;

        Optimizer(LIR ir) {
            this.ir = ir;
            edgeInstructionSeqences = new ArrayList<>(4);
        }

        
Determines if two operations are both moves that have the same source and destination operands. 
Params:
op1 – the first instruction to compare
op2 – the second instruction to compare
Returns:
true if op1 and op2 are the same by the above algorithm/**
         * Determines if two operations are both {@linkplain MoveOp moves} that have the same source
         * and {@linkplain MoveOp#getResult() destination} operands.
         *
         * @param op1 the first instruction to compare
         * @param op2 the second instruction to compare
         * @return {@code true} if {@code op1} and {@code op2} are the same by the above algorithm
         */
        private static boolean same(LIRInstruction op1, LIRInstruction op2) {
            assert op1 != null;
            assert op2 != null;

            if (ValueMoveOp.isValueMoveOp(op1) && ValueMoveOp.isValueMoveOp(op2)) {
                ValueMoveOp move1 = ValueMoveOp.asValueMoveOp(op1);
                ValueMoveOp move2 = ValueMoveOp.asValueMoveOp(op2);
                if (move1.getInput().equals(move2.getInput()) && move1.getResult().equals(move2.getResult())) {
                    // these moves are exactly equal and can be optimized
                    return true;
                }
            } else if (LoadConstantOp.isLoadConstantOp(op1) && LoadConstantOp.isLoadConstantOp(op2)) {
                LoadConstantOp move1 = LoadConstantOp.asLoadConstantOp(op1);
                LoadConstantOp move2 = LoadConstantOp.asLoadConstantOp(op2);
                if (move1.getConstant().equals(move2.getConstant()) && move1.getResult().equals(move2.getResult())) {
                    // these moves are exactly equal and can be optimized
                    return true;
                }
            }
            return false;
        }

        
Moves the longest common subsequence at the end all predecessors of block to the start of block. /**
         * Moves the longest {@linkplain #same common} subsequence at the end all predecessors of
         * {@code block} to the start of {@code block}.
         */
        private void optimizeMovesAtBlockEnd(AbstractBlockBase<?> block) {
            for (AbstractBlockBase<?> pred : block.getPredecessors()) {
                if (pred == block) {
                    // currently we can't handle this correctly.
                    return;
                }
            }

            // clear all internal data structures
            edgeInstructionSeqences.clear();

            int numPreds = block.getPredecessorCount();
            assert numPreds > 1 : "do not call otherwise";

            // setup a list with the LIR instructions of all predecessors
            for (AbstractBlockBase<?> pred : block.getPredecessors()) {
                assert pred != null;
                assert ir.getLIRforBlock(pred) != null;
                ArrayList<LIRInstruction> predInstructions = ir.getLIRforBlock(pred);

                if (pred.getSuccessorCount() != 1) {
                    // this can happen with switch-statements where multiple edges are between
                    // the same blocks.
                    return;
                }

                assert pred.getSuccessors()[0] == block : "invalid control flow";
                assert predInstructions.get(predInstructions.size() - 1) instanceof StandardOp.JumpOp : "block must end with unconditional jump";

                if (predInstructions.get(predInstructions.size() - 1).hasState()) {
                    // can not optimize instructions that have debug info
                    return;
                }

                // ignore the unconditional branch at the end of the block
                List<LIRInstruction> seq = predInstructions.subList(0, predInstructions.size() - 1);
                edgeInstructionSeqences.add(seq);
            }

            // process lir-instructions while all predecessors end with the same instruction
            while (true) {
                List<LIRInstruction> seq = edgeInstructionSeqences.get(0);
                if (seq.isEmpty()) {
                    return;
                }

                LIRInstruction op = last(seq);
                for (int i = 1; i < numPreds; ++i) {
                    List<LIRInstruction> otherSeq = edgeInstructionSeqences.get(i);
                    if (otherSeq.isEmpty() || !same(op, last(otherSeq))) {
                        return;
                    }
                }

                // insert the instruction at the beginning of the current block
                ir.getLIRforBlock(block).add(1, op);

                // delete the instruction at the end of all predecessors
                for (int i = 0; i < numPreds; i++) {
                    seq = edgeInstructionSeqences.get(i);
                    removeLast(seq);
                }
            }
        }

        
Moves the longest common subsequence at the start of all successors of block to the end of block just prior to the branch instruction ending block. /**
         * Moves the longest {@linkplain #same common} subsequence at the start of all successors of
         * {@code block} to the end of {@code block} just prior to the branch instruction ending
         * {@code block}.
         */
        private void optimizeMovesAtBlockBegin(AbstractBlockBase<?> block) {

            edgeInstructionSeqences.clear();
            int numSux = block.getSuccessorCount();

            ArrayList<LIRInstruction> instructions = ir.getLIRforBlock(block);

            assert numSux == 2 : "method should not be called otherwise";

            LIRInstruction lastInstruction = instructions.get(instructions.size() - 1);
            if (lastInstruction.hasState()) {
                // cannot optimize instructions when debug info is needed
                return;
            }

            LIRInstruction branch = lastInstruction;
            if (!(branch instanceof StandardOp.BranchOp) || branch.hasOperands()) {
                // Only blocks that end with a conditional branch are optimized.
                // In addition, a conditional branch with operands (including state) cannot
                // be optimized. Moving a successor instruction before such a branch may
                // interfere with the operands of the branch. For example, a successive move
                // instruction may redefine an input operand of the branch.
                return;
            }

            // Now it is guaranteed that the block ends with a conditional branch.
            // The instructions are inserted at the end of the block before the branch.
            int insertIdx = instructions.size() - 1;

            // setup a list with the lir-instructions of all successors
            for (AbstractBlockBase<?> sux : block.getSuccessors()) {
                ArrayList<LIRInstruction> suxInstructions = ir.getLIRforBlock(sux);

                assert suxInstructions.get(0) instanceof StandardOp.LabelOp : "block must start with label";

                if (sux.getPredecessorCount() != 1) {
                    // this can happen with switch-statements where multiple edges are between
                    // the same blocks.
                    return;
                }
                assert sux.getPredecessors()[0] == block : "invalid control flow";

                // ignore the label at the beginning of the block
                List<LIRInstruction> seq = suxInstructions.subList(1, suxInstructions.size());
                edgeInstructionSeqences.add(seq);
            }

            // process LIR instructions while all successors begin with the same instruction
            while (true) {
                List<LIRInstruction> seq = edgeInstructionSeqences.get(0);
                if (seq.isEmpty()) {
                    return;
                }

                LIRInstruction op = first(seq);
                for (int i = 1; i < numSux; i++) {
                    List<LIRInstruction> otherSeq = edgeInstructionSeqences.get(i);
                    if (otherSeq.isEmpty() || !same(op, first(otherSeq))) {
                        // these instructions are different and cannot be optimized .
                        // no further optimization possible
                        return;
                    }
                }

                // insert instruction at end of current block
                ir.getLIRforBlock(block).add(insertIdx, op);
                insertIdx++;

                // delete the instructions at the beginning of all successors
                for (int i = 0; i < numSux; i++) {
                    seq = edgeInstructionSeqences.get(i);
                    removeFirst(seq);
                }
            }
        }

        
Gets the first element from a LIR instruction sequence.
/**
         * Gets the first element from a LIR instruction sequence.
         */
        private static LIRInstruction first(List<LIRInstruction> seq) {
            return seq.get(0);
        }

        
Gets the last element from a LIR instruction sequence.
/**
         * Gets the last element from a LIR instruction sequence.
         */
        private static LIRInstruction last(List<LIRInstruction> seq) {
            return seq.get(seq.size() - 1);
        }

        
Removes the first element from a LIR instruction sequence.
/**
         * Removes the first element from a LIR instruction sequence.
         */
        private static void removeFirst(List<LIRInstruction> seq) {
            seq.remove(0);
        }

        
Removes the last element from a LIR instruction sequence.
/**
         * Removes the last element from a LIR instruction sequence.
         */
        private static void removeLast(List<LIRInstruction> seq) {
            seq.remove(seq.size() - 1);
        }

    }
}