/*
 * Copyright (c) 2009, 2011, 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
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */
package com.oracle.graal.lir;

import static com.oracle.graal.lir.LIR.*;

import java.util.*;

import com.oracle.graal.api.code.*;
import com.oracle.graal.compiler.common.cfg.*;
import com.oracle.graal.lir.gen.*;
import com.oracle.graal.lir.phases.*;
import com.oracle.jvmci.debug.*;

/**
 * This class performs basic optimizations on the control flow graph after LIR generation.
 */
public final class ControlFlowOptimizer extends PostAllocationOptimizationPhase {

    /**
     * Performs control flow optimizations on the given LIR graph.
     */
    @Override
    protected <B extends AbstractBlockBase<B>> void run(TargetDescription target, LIRGenerationResult lirGenRes, List<B> codeEmittingOrder, List<B> linearScanOrder,
                    BenchmarkCounterFactory counterFactory) {
        LIR lir = lirGenRes.getLIR();
        new Optimizer<B>(lir).deleteEmptyBlocks(codeEmittingOrder);
    }

    private static final class Optimizer<B extends AbstractBlockBase<B>> {

        private final LIR lir;

        private Optimizer(LIR lir) {
            this.lir = lir;
        }

        private static final DebugMetric BLOCKS_DELETED = Debug.metric("BlocksDeleted");

        /**
         * Checks whether a block can be deleted. Only blocks with exactly one successor and an
         * unconditional branch to this successor are eligable.
         *
         * @param block the block checked for deletion
         * @return whether the block can be deleted
         */
        private boolean canDeleteBlock(B block) {
            if (block.getSuccessorCount() != 1 || block.getPredecessorCount() == 0 || block.getSuccessors().iterator().next() == block) {
                return false;
            }

            List<LIRInstruction> instructions = lir.getLIRforBlock(block);

            assert instructions.size() >= 2 : "block must have label and branch";
            assert instructions.get(0) instanceof StandardOp.LabelOp : "first instruction must always be a label";
            assert instructions.get(instructions.size() - 1) instanceof StandardOp.JumpOp : "last instruction must always be a branch";
            assert ((StandardOp.JumpOp) instructions.get(instructions.size() - 1)).destination().label() == ((StandardOp.LabelOp) lir.getLIRforBlock(block.getSuccessors().iterator().next()).get(0)).getLabel() : "branch target must be the successor";

            // Block must have exactly one successor.
            return instructions.size() == 2 && !instructions.get(instructions.size() - 1).hasState() && !block.isExceptionEntry();
        }

        private void alignBlock(B block) {
            if (!block.isAligned()) {
                block.setAlign(true);
                List<LIRInstruction> instructions = lir.getLIRforBlock(block);
                assert instructions.get(0) instanceof StandardOp.LabelOp : "first instruction must always be a label";
                StandardOp.LabelOp label = (StandardOp.LabelOp) instructions.get(0);
                instructions.set(0, new StandardOp.LabelOp(label.getLabel(), true));
            }
        }

        private void deleteEmptyBlocks(List<B> blocks) {
            assert verifyBlocks(lir, blocks);
            Iterator<B> iterator = blocks.iterator();
            while (iterator.hasNext()) {
                B block = iterator.next();
                if (canDeleteBlock(block)) {
                    // adjust successor and predecessor lists
                    B other = block.getSuccessors().iterator().next();
                    for (AbstractBlockBase<B> pred : block.getPredecessors()) {
                        Collections.replaceAll(pred.getSuccessors(), block, other);
                    }
                    for (int i = 0; i < other.getPredecessorCount(); i++) {
                        if (other.getPredecessors().get(i) == block) {
                            other.getPredecessors().remove(i);
                            other.getPredecessors().addAll(i, block.getPredecessors());
                        }
                    }
                    block.getSuccessors().clear();
                    block.getPredecessors().clear();

                    if (block.isAligned()) {
                        alignBlock(other);
                    }

                    BLOCKS_DELETED.increment();
                    iterator.remove();
                }
            }
            assert verifyBlocks(lir, blocks);
        }
    }
}