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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.CodeAttributeEditor;
import proguard.classfile.instruction.Instruction;
import proguard.classfile.instruction.visitor.InstructionVisitor;

/**
 * This InstructionVisitor deletes blocks of code that can never be reached by
 * regular calls or branches.
 *
 * @author Eric Lafortune
 */
public class UnreachableCodeRemover
implements   AttributeVisitor,
             InstructionVisitor
{
    private static final Logger logger = LogManager.getLogger(UnreachableCodeRemover.class);

    private final InstructionVisitor  extraInstructionVisitor;

    private final ReachableCodeMarker reachableCodeMarker = new ReachableCodeMarker();
    private final CodeAttributeEditor codeAttributeEditor = new CodeAttributeEditor();


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


    /**
     * Creates a new UnreachableCodeRemover.
     * @param extraInstructionVisitor an optional extra visitor for all
     *                                deleted instructions.
     */
    public UnreachableCodeRemover(InstructionVisitor  extraInstructionVisitor)
    {
        this.extraInstructionVisitor = extraInstructionVisitor;
    }


    // Implementations for AttributeVisitor.

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


    public void visitCodeAttribute(Clazz clazz, Method method, CodeAttribute codeAttribute)
    {
//        DEBUG =
//            clazz.getName().equals("abc/Def") &&
//            method.getName(clazz).equals("abc");

        // TODO: Remove this when the code has stabilized.
        // Catch any unexpected exceptions from the actual visiting method.
        try
        {
            // Process the code.
            visitCodeAttribute0(clazz, method, codeAttribute);
        }
        catch (RuntimeException ex)
        {
            logger.error("Unexpected error while removing unreachable code:");
            logger.error("  Class       = [{}]", clazz.getName());
            logger.error("  Method      = [{}{}]", method.getName(clazz), method.getDescriptor(clazz));
            logger.error("  Exception   = [{}] ({})", ex.getClass().getName(), ex.getMessage());

            throw ex;
        }
    }


    public void visitCodeAttribute0(Clazz clazz, Method method, CodeAttribute codeAttribute)
    {
        logger.debug("UnreachableCodeRemover: {}.{}{}",
                     clazz.getName(),
                     method.getName(clazz),
                     method.getDescriptor(clazz)
        );

        reachableCodeMarker.visitCodeAttribute(clazz, method, codeAttribute);

        codeAttributeEditor.reset(codeAttribute.u4codeLength);

        codeAttribute.instructionsAccept(clazz, method, this);

        codeAttributeEditor.visitCodeAttribute(clazz, method, codeAttribute);
    }


    // Implementations for InstructionVisitor.

    public void visitAnyInstruction(Clazz clazz, Method method, CodeAttribute codeAttribute, int offset, Instruction instruction)
    {
        logger.debug("  {} {}",
                     reachableCodeMarker.isReachable(offset) ? "+" : "-",
                     instruction.toString(offset)
        );

        // Is this instruction unreachable?
        if (!reachableCodeMarker.isReachable(offset))
        {
            // Then delete it.
            codeAttributeEditor.deleteInstruction(offset);

            // Visit the instruction, if required.
            if (extraInstructionVisitor != null)
            {
                instruction.accept(clazz, method, codeAttribute, offset, extraInstructionVisitor);
            }
        }
    }
}