/*
 * Copyright (c) 2009, 2012, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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 */
package com.oracle.graal.lir;

import java.util.*;

import com.oracle.graal.compiler.common.cfg.*;
import com.oracle.graal.lir.StandardOp.MoveOp;

/**
 * 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
 * {@linkplain #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.
 *
 * 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-&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 {

    /**
     * Optimizes moves on block edges.
     */
    public static void optimize(LIR ir) {
        EdgeMoveOptimizer optimizer = new EdgeMoveOptimizer(ir);

        List<? extends AbstractBlock<?>> blockList = ir.linearScanOrder();
        // ignore the first block in the list (index 0 is not processed)
        for (int i = blockList.size() - 1; i >= 1; i--) {
            AbstractBlock<?> block = blockList.get(i);

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

    private final List<List<LIRInstruction>> edgeInstructionSeqences;
    private LIR ir;

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

    /**
     * Determines if two operations are both {@linkplain MoveOp moves} that have the same
     * {@linkplain MoveOp#getInput() 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 (op1 instanceof MoveOp && op2 instanceof MoveOp) {
            MoveOp move1 = (MoveOp) op1;
            MoveOp move2 = (MoveOp) op2;
            if (move1.getInput().equals(move2.getInput()) && move1.getResult().equals(move2.getResult())) {
                // these moves are exactly equal and can be optimized
                return true;
            }
        }
        return false;
    }

    /**
     * Moves the longest {@linkplain #same common} subsequence at the end all predecessors of
     * {@code block} to the start of {@code block}.
     */
    private void optimizeMovesAtBlockEnd(AbstractBlock<?> block) {
        for (AbstractBlock<?> 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 (AbstractBlock<?> pred : block.getPredecessors()) {
            assert pred != null;
            assert ir.getLIRforBlock(pred) != null;
            List<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().iterator().next() == 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 {@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(AbstractBlock<?> block) {

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

        List<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 (AbstractBlock<?> sux : block.getSuccessors()) {
            List<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().iterator().next() == 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.
     */
    private static LIRInstruction first(List<LIRInstruction> seq) {
        return seq.get(0);
    }

    /**
     * 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.
     */
    private static void removeFirst(List<LIRInstruction> seq) {
        seq.remove(0);
    }

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