/*
 * 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
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 */
package com.oracle.graal.lir;

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

import java.util.*;

import com.oracle.graal.debug.*;
import com.oracle.graal.nodes.cfg.*;

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

    /**
     * Performs control flow optimizations on the given LIR graph.
     */
    public static <T extends AbstractBlock<T>> void optimize(LIR lir, List<T> codeEmittingOrder) {
        ControlFlowOptimizer.deleteEmptyBlocks(lir, codeEmittingOrder);
    }

    private ControlFlowOptimizer() {
    }

    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 static boolean canDeleteBlock(LIR lir, AbstractBlock<?> 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 static void alignBlock(LIR lir, AbstractBlock<?> 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 static <T extends AbstractBlock<T>> void deleteEmptyBlocks(LIR lir, List<T> blocks) {
        assert verifyBlocks(lir, blocks);
        Iterator<T> iterator = blocks.iterator();
        while (iterator.hasNext()) {
            T block = iterator.next();
            if (canDeleteBlock(lir, block)) {
                // adjust successor and predecessor lists
                T other = block.getSuccessors().iterator().next();
                for (AbstractBlock<T> 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(lir, other);
                }

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