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ProGuard is a free shrinker, optimizer, obfuscator, and preverifier for Java bytecode
There is a newer version: 7.6.0
/*
 * ProGuard -- shrinking, optimization, obfuscation, and preverification
 *             of Java bytecode.
 *
 * Copyright (c) 2002-2020 Guardsquare NV
 *
 * This program is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License as published by the Free
 * Software Foundation; either version 2 of the License, or (at your option)
 * any later version.
 *
 * This program 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 for
 * more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 */
package proguard.optimize.peephole;

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import proguard.classfile.*;
import proguard.classfile.attribute.*;
import proguard.classfile.attribute.visitor.AttributeVisitor;
import proguard.classfile.editor.VariableEditor;
import proguard.classfile.util.*;
import proguard.classfile.visitor.MemberVisitor;
import proguard.optimize.*;
import proguard.optimize.info.*;

/**
 * This MemberVisitor removes unused local variables from the code of the methods
 * that it visits.
 *
 * @see ParameterUsageMarker
 * @see MethodStaticizer
 * @see MethodDescriptorShrinker
 * @author Eric Lafortune
 */
public class VariableShrinker
implements   AttributeVisitor
{
    private static final Logger logger = LogManager.getLogger(VariableShrinker.class);


    private final MemberVisitor extraVariableMemberVisitor;

    private final VariableUsageMarker variableUsageMarker = new VariableUsageMarker();
    private final VariableEditor      variableEditor      = new VariableEditor();


    /**
     * Creates a new VariableShrinker.
     */
    public VariableShrinker()
    {
        this(null);
    }


    /**
     * Creates a new VariableShrinker with an extra visitor.
     * @param extraVariableMemberVisitor an optional extra visitor for all
     *                                   removed variables.
     */
    public VariableShrinker(MemberVisitor extraVariableMemberVisitor)
    {
        this.extraVariableMemberVisitor = extraVariableMemberVisitor;
    }


    // Implementations for AttributeVisitor.

    public void visitAnyAttribute(Clazz clazz, Attribute attribute) {}


    public void visitCodeAttribute(Clazz clazz, Method method, CodeAttribute codeAttribute)
    {
        if ((method.getAccessFlags() & AccessConstants.ABSTRACT) == 0)
        {
            // Compute the parameter size.
            int parameterSize =
                ClassUtil.internalMethodParameterSize(method.getDescriptor(clazz),
                                                      method.getAccessFlags());

            // Get the total size of the local variable frame.
            int maxLocals = codeAttribute.u2maxLocals;

            logger.debug("VariableShrinker: {}.{}{}", clazz.getName(), method.getName(clazz), method.getDescriptor(clazz));
            logger.debug("  Parameter size = {}", parameterSize);
            logger.debug("  Max locals     = {}", maxLocals);

            // Figure out the local variables that are used by the code.
            variableUsageMarker.visitCodeAttribute(clazz, method, codeAttribute);

            // Delete unused local variables from the local variable frame.
            variableEditor.reset(maxLocals);

            for (int variableIndex = parameterSize; variableIndex < maxLocals; variableIndex++)
            {
                // Is the variable not required?
                if (!variableUsageMarker.isVariableUsed(variableIndex))
                {
                    logger.debug("  Deleting local variable #{}", variableIndex);

                    // Delete the unused variable.
                    variableEditor.deleteVariable(variableIndex);

                    // Visit the method, if required.
                    if (extraVariableMemberVisitor != null)
                    {
                        method.accept(clazz, extraVariableMemberVisitor);
                    }
                }
            }

            // Shift all remaining parameters and variables in the byte code.
            variableEditor.visitCodeAttribute(clazz, method, codeAttribute);
        }
    }
}