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      1 /*
      2  * ProGuard -- shrinking, optimization, obfuscation, and preverification
      3  *             of Java bytecode.
      4  *
      5  * Copyright (c) 2002-2009 Eric Lafortune (eric (at) graphics.cornell.edu)
      6  *
      7  * This program is free software; you can redistribute it and/or modify it
      8  * under the terms of the GNU General Public License as published by the Free
      9  * Software Foundation; either version 2 of the License, or (at your option)
     10  * any later version.
     11  *
     12  * This program is distributed in the hope that it will be useful, but WITHOUT
     13  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
     14  * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
     15  * more details.
     16  *
     17  * You should have received a copy of the GNU General Public License along
     18  * with this program; if not, write to the Free Software Foundation, Inc.,
     19  * 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
     20  */
     21 package proguard.optimize.peephole;
     22 
     23 import proguard.classfile.*;
     24 import proguard.classfile.attribute.CodeAttribute;
     25 import proguard.classfile.editor.CodeAttributeEditor;
     26 import proguard.classfile.instruction.*;
     27 import proguard.classfile.instruction.visitor.InstructionVisitor;
     28 import proguard.classfile.util.SimplifiedVisitor;
     29 
     30 /**
     31  * This InstructionVisitor simplifies unconditional branches to other
     32  * unconditional branches.
     33  *
     34  * @author Eric Lafortune
     35  */
     36 public class GotoGotoReplacer
     37 extends      SimplifiedVisitor
     38 implements   InstructionVisitor
     39 {
     40     private final CodeAttributeEditor codeAttributeEditor;
     41     private final InstructionVisitor  extraInstructionVisitor;
     42 
     43 
     44     /**
     45      * Creates a new GotoGotoReplacer.
     46      * @param codeAttributeEditor     a code editor that can be used for
     47      *                                accumulating changes to the code.
     48      */
     49     public GotoGotoReplacer(CodeAttributeEditor codeAttributeEditor)
     50     {
     51         this(codeAttributeEditor, null);
     52     }
     53 
     54 
     55     /**
     56      * Creates a new GotoGotoReplacer.
     57      * @param codeAttributeEditor     a code editor that can be used for
     58      *                                accumulating changes to the code.
     59      * @param extraInstructionVisitor an optional extra visitor for all replaced
     60      *                                goto instructions.
     61      */
     62     public GotoGotoReplacer(CodeAttributeEditor codeAttributeEditor,
     63                             InstructionVisitor  extraInstructionVisitor)
     64     {
     65         this.codeAttributeEditor     = codeAttributeEditor;
     66         this.extraInstructionVisitor = extraInstructionVisitor;
     67     }
     68 
     69 
     70     // Implementations for InstructionVisitor.
     71 
     72     public void visitAnyInstruction(Clazz clazz, Method method, CodeAttribute codeAttribute, int offset, Instruction instruction) {}
     73 
     74 
     75     public void visitBranchInstruction(Clazz clazz, Method method, CodeAttribute codeAttribute, int offset, BranchInstruction branchInstruction)
     76     {
     77         // Check if the instruction is an unconditional goto instruction.
     78         byte opcode = branchInstruction.opcode;
     79         if (opcode == InstructionConstants.OP_GOTO ||
     80             opcode == InstructionConstants.OP_GOTO_W)
     81         {
     82             // Check if the goto instruction points to another simple goto
     83             // instruction.
     84             int branchOffset = branchInstruction.branchOffset;
     85             int targetOffset = offset + branchOffset;
     86 
     87             if (branchOffset != branchInstruction.length(offset) &&
     88                 !codeAttributeEditor.isModified(offset) &&
     89                 !codeAttributeEditor.isModified(targetOffset))
     90             {
     91                 Instruction targetInstruction =
     92                     InstructionFactory.create(codeAttribute.code, targetOffset);
     93 
     94                 if (targetInstruction.opcode == InstructionConstants.OP_GOTO)
     95                 {
     96                     // Simplify the goto instruction.
     97                     int targetBranchOffset = ((BranchInstruction)targetInstruction).branchOffset;
     98 
     99                     Instruction newBranchInstruction =
    100                          new BranchInstruction(opcode,
    101                                                (branchOffset + targetBranchOffset));
    102                     codeAttributeEditor.replaceInstruction(offset,
    103                                                            newBranchInstruction);
    104 
    105                     // Visit the instruction, if required.
    106                     if (extraInstructionVisitor != null)
    107                     {
    108                         extraInstructionVisitor.visitBranchInstruction(clazz, method, codeAttribute, offset, branchInstruction);
    109                     }
    110                 }
    111             }
    112         }
    113     }
    114 }
    115