de.tuberlin.uebb.jbop.optimizer.var.RemoveUnusedLocalVars.java Source code

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Here is the source code for de.tuberlin.uebb.jbop.optimizer.var.RemoveUnusedLocalVars.java

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/*
 * Copyright (C) 2013 uebb.tu-berlin.de.
 * 
 * This file is part of JBOP (Java Bytecode OPtimizer).
 * 
 * JBOP is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 * 
 * JBOP 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 Lesser General Public License for more details.
 * 
 * You should have received a copy of the GNU Lesser General Public License
 * along with JBOP. If not, see <http://www.gnu.org/licenses/>.
 */
package de.tuberlin.uebb.jbop.optimizer.var;

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

import org.objectweb.asm.Opcodes;
import org.objectweb.asm.tree.AbstractInsnNode;
import org.objectweb.asm.tree.IincInsnNode;
import org.objectweb.asm.tree.InsnList;
import org.objectweb.asm.tree.MethodNode;
import org.objectweb.asm.tree.VarInsnNode;

import de.tuberlin.uebb.jbop.optimizer.IOptimizer;
import de.tuberlin.uebb.jbop.optimizer.utils.NodeHelper;

/**
 * Removes unused local variables. <br>
 * Variables are unused if they are stored but never read via load.<br>
 * If a variable is stored and only used by iinc operations,<br>
 * these iinc operations are removed too (not, if this belongs to a loop). <br>
 * eg:
 * 
 * <pre>
 * int i=1;
 * int j=2
 * if(condition){
 *  i=2;
 * }
 * j++;
 * System.out.println(i+2);
 * </pre>
 * 
 * becomes
 * 
 * <pre>
 * int i = 1;
 * if (condition) {
 *   i = 2;
 * }
 * System.out.println(i + 2);
 * </pre>
 * 
 * 
 * @author Christopher Ewest
 */
public class RemoveUnusedLocalVars implements IOptimizer {

    private boolean optimized;

    @Override
    public boolean isOptimized() {
        return optimized;
    }

    @Override
    public InsnList optimize(final InsnList original, final MethodNode methodNode) {
        optimized = false;
        final List<VarInsnNode> stores = new ArrayList<>();
        final List<VarInsnNode> users = new ArrayList<>();
        final List<IincInsnNode> iincs = new ArrayList<>();
        findNodes(original, stores, users, iincs);
        final List<AbstractInsnNode> toBeRemoved = new ArrayList<>();
        for (final VarInsnNode node : stores) {
            if (!usersContains(users, node)) {
                toBeRemoved.add(node);
                toBeRemoved.addAll(iincsContains(iincs, node));
            }
        }
        // final InsnList newList = new InsnList();
        final Iterator<AbstractInsnNode> iterator = original.iterator();
        while (iterator.hasNext()) {
            final AbstractInsnNode currentNode = iterator.next();
            if (toBeRemoved.contains(currentNode)) {
                if (currentNode.getOpcode() == Opcodes.IINC) {
                    original.remove(currentNode);
                    optimized = true;
                    continue;
                }
                final AbstractInsnNode firstOfStack = NodeHelper.getFirstOfStack(currentNode);
                if (firstOfStack != null) {
                    optimized = true;
                    AbstractInsnNode remove = firstOfStack;
                    while (remove != currentNode) {
                        final AbstractInsnNode toRemove = remove;
                        remove = remove.getNext();
                        original.remove(toRemove);
                    }
                    original.remove(currentNode);
                }
            }
        }
        return original;
    }

    private List<AbstractInsnNode> iincsContains(final List<IincInsnNode> iincs, final VarInsnNode node) {
        final List<AbstractInsnNode> nodes = new ArrayList<>();
        for (final IincInsnNode iinc : iincs) {
            if (iinc.var == node.var) {
                nodes.add(iinc);
            }
        }
        return nodes;
    }

    private boolean usersContains(final List<VarInsnNode> users, final VarInsnNode node) {
        for (final VarInsnNode user : users) {
            if (user.var == node.var) {
                return true;
            }
        }
        return false;
    }

    private void findNodes(final InsnList original, final List<VarInsnNode> stores, final List<VarInsnNode> users, //
            final List<IincInsnNode> iincs) {
        final Iterator<AbstractInsnNode> iterator = original.iterator();
        while (iterator.hasNext()) {
            final AbstractInsnNode node = iterator.next();
            if ((node.getOpcode() >= Opcodes.ISTORE) && (node.getOpcode() <= Opcodes.ASTORE)) {
                stores.add((VarInsnNode) node);
            } else if ((node.getOpcode() >= Opcodes.ILOAD) && (node.getOpcode() <= Opcodes.ALOAD)) {
                users.add((VarInsnNode) node);
            } else if (node.getOpcode() == Opcodes.IINC) {
                iincs.add((IincInsnNode) node);
            }
        }
    }

}