BranchInstruction.java revision b9cc48a43ed984587c939d02fba5316bf5c0df6e 
1/*
2 * ProGuard -- shrinking, optimization, obfuscation, and preverification
3 *             of Java bytecode.
4 *
5 * Copyright (c) 2002-2013 Eric Lafortune (eric@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 */
21package proguard.classfile.instruction;
22
23import proguard.classfile.*;
24import proguard.classfile.attribute.CodeAttribute;
25import proguard.classfile.instruction.visitor.InstructionVisitor;
26
27/**
28 * This interface describes an instruction that branches to a given offset in
29 * the code.
30 *
31 * @author Eric Lafortune
32 */
33public class BranchInstruction extends Instruction
34{
35    public int branchOffset;
36
37
38    /**
39     * Creates an uninitialized BranchInstruction.
40     */
41    public BranchInstruction() {}
42
43
44    public BranchInstruction(byte opcode, int branchOffset)
45    {
46        this.opcode       = opcode;
47        this.branchOffset = branchOffset;
48    }
49
50
51    /**
52     * Copies the given instruction into this instruction.
53     * @param branchInstruction the instruction to be copied.
54     * @return this instruction.
55     */
56    public BranchInstruction copy(BranchInstruction branchInstruction)
57    {
58        this.opcode       = branchInstruction.opcode;
59        this.branchOffset = branchInstruction.branchOffset;
60
61        return this;
62    }
63
64
65    // Implementations for Instruction.
66
67    public byte canonicalOpcode()
68    {
69        // Remove the _w extension, if any.
70        switch (opcode)
71        {
72            case InstructionConstants.OP_GOTO_W: return InstructionConstants.OP_GOTO;
73
74            case InstructionConstants.OP_JSR_W: return InstructionConstants.OP_JSR;
75
76            default: return opcode;
77        }
78    }
79
80    public Instruction shrink()
81    {
82        // Do we need an ordinary branch or a wide branch?
83        if (requiredBranchOffsetSize() == 2)
84        {
85            // Can we replace the wide branch by an ordinary branch?
86            if      (opcode == InstructionConstants.OP_GOTO_W)
87            {
88                opcode = InstructionConstants.OP_GOTO;
89            }
90            else if (opcode == InstructionConstants.OP_JSR_W)
91            {
92                opcode = InstructionConstants.OP_JSR;
93            }
94        }
95        else
96        {
97            // Should we replace the ordinary branch by a wide branch?
98            if      (opcode == InstructionConstants.OP_GOTO)
99            {
100                opcode = InstructionConstants.OP_GOTO_W;
101            }
102            else if (opcode == InstructionConstants.OP_JSR)
103            {
104                opcode = InstructionConstants.OP_JSR_W;
105            }
106            else
107            {
108                throw new IllegalArgumentException("Branch instruction can't be widened ("+this.toString()+")");
109            }
110        }
111
112        return this;
113    }
114
115    protected void readInfo(byte[] code, int offset)
116    {
117        branchOffset = readSignedValue(code, offset, branchOffsetSize());
118    }
119
120
121    protected void writeInfo(byte[] code, int offset)
122    {
123        if (requiredBranchOffsetSize() > branchOffsetSize())
124        {
125            throw new IllegalArgumentException("Instruction has invalid branch offset size ("+this.toString(offset)+")");
126        }
127
128        writeSignedValue(code, offset, branchOffset, branchOffsetSize());
129    }
130
131
132    public int length(int offset)
133    {
134        return 1 + branchOffsetSize();
135    }
136
137
138    public void accept(Clazz clazz, Method method, CodeAttribute codeAttribute, int offset, InstructionVisitor instructionVisitor)
139    {
140        instructionVisitor.visitBranchInstruction(clazz, method, codeAttribute, offset, this);
141    }
142
143
144    public String toString(int offset)
145    {
146        return "["+offset+"] "+toString()+" (target="+(offset+branchOffset)+")";
147    }
148
149
150    // Implementations for Object.
151
152    public String toString()
153    {
154        return getName()+" "+(branchOffset >= 0 ? "+" : "")+branchOffset;
155    }
156
157
158    // Small utility methods.
159
160    /**
161     * Returns the branch offset size for this instruction.
162     */
163    private int branchOffsetSize()
164    {
165        return opcode == InstructionConstants.OP_GOTO_W ||
166               opcode == InstructionConstants.OP_JSR_W  ? 4 :
167                                                          2;
168    }
169
170
171    /**
172     * Computes the required branch offset size for this instruction's branch
173     * offset.
174     */
175    private int requiredBranchOffsetSize()
176    {
177        return (short)branchOffset == branchOffset ? 2 : 4;
178    }
179}
180