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Java > Open Source Codes > proguard > optimize > peephole > ReachableCodeMarker


1 /*
2  * ProGuard -- shrinking, optimization, obfuscation, and preverification
3  * of Java bytecode.
4  *
5  * Copyright (c) 2002-2007 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  */
21 package proguard.optimize.peephole;
22
23 import proguard.classfile.*;
24 import proguard.classfile.attribute.*;
25 import proguard.classfile.attribute.visitor.*;
26 import proguard.classfile.instruction.*;
27 import proguard.classfile.instruction.visitor.InstructionVisitor;
28 import proguard.classfile.util.SimplifiedVisitor;
29
30 /**
31  * This AttributeVisitor finds all instruction offsets, branch targets, and
32  * exception targets in the CodeAttribute objects that it visits.
33  *
34  * @author Eric Lafortune
35  */
36 public class ReachableCodeMarker
37 extends SimplifiedVisitor
38 implements AttributeVisitor,
39              InstructionVisitor,
40              ExceptionInfoVisitor
41 {
42     private boolean[] isReachable = new boolean[ClassConstants.TYPICAL_CODE_LENGTH];
43
44     private boolean next;
45     private boolean evaluateExceptions;
46
47
48     /**
49      * Returns whether the instruction at the given offset is reachable in
50      * the CodeAttribute that was visited most recently.
51      */
52     public boolean isReachable(int offset)
53     {
54         return isReachable[offset];
55     }
56
57
58     /**
59      * Returns whether any of the instructions at the given offsets are
60      * reachable in the CodeAttribute that was visited most recently.
61      */
62     public boolean isReachable(int startOffset, int endOffset)
63     {
64         // Check if any of the instructions is reachable.
65 for (int offset = startOffset; offset < endOffset; offset++)
66         {
67             if (isReachable[offset])
68             {
69                 return true;
70             }
71         }
72
73         return false;
74     }
75
76
77     // Implementations for AttributeVisitor.
78
79     public void visitAnyAttribute(Clazz clazz, Attribute attribute) {}
80
81
82     public void visitCodeAttribute(Clazz clazz, Method method, CodeAttribute codeAttribute)
83     {
84         // Make sure there is a sufficiently large array.
85 int codeLength = codeAttribute.u4codeLength;
86         if (isReachable.length < codeLength)
87         {
88             // Create a new array.
89 isReachable = new boolean[codeLength];
90         }
91         else
92         {
93             // Reset the array.
94 for (int index = 0; index < codeLength; index++)
95             {
96                 isReachable[index] = false;
97             }
98         }
99
100         // Mark the code, starting at the entry point.
101 markCode(clazz, method, codeAttribute, 0);
102
103         // Mark the exception handlers, iterating as long as necessary.
104 do
105         {
106             evaluateExceptions = false;
107
108             codeAttribute.exceptionsAccept(clazz, method, this);
109         }
110         while (evaluateExceptions);
111     }
112
113
114     // Implementations for InstructionVisitor.
115
116     public void visitSimpleInstruction(Clazz clazz, Method method, CodeAttribute codeAttribute, int offset, SimpleInstruction simpleInstruction)
117     {
118         byte opcode = simpleInstruction.opcode;
119         if (opcode == InstructionConstants.OP_IRETURN ||
120             opcode == InstructionConstants.OP_LRETURN ||
121             opcode == InstructionConstants.OP_FRETURN ||
122             opcode == InstructionConstants.OP_DRETURN ||
123             opcode == InstructionConstants.OP_ARETURN ||
124             opcode == InstructionConstants.OP_RETURN ||
125             opcode == InstructionConstants.OP_ATHROW)
126         {
127             next = false;
128         }
129     }
130
131
132     public void visitConstantInstruction(Clazz clazz, Method method, CodeAttribute codeAttribute, int offset, ConstantInstruction constantInstruction)
133     {
134     }
135
136
137     public void visitVariableInstruction(Clazz clazz, Method method, CodeAttribute codeAttribute, int offset, VariableInstruction variableInstruction)
138     {
139         if (variableInstruction.opcode == InstructionConstants.OP_RET)
140         {
141             next = false;
142         }
143     }
144
145
146     public void visitBranchInstruction(Clazz clazz, Method method, CodeAttribute codeAttribute, int offset, BranchInstruction branchInstruction)
147     {
148         // Mark the branch target.
149 markBranchTarget(clazz,
150                          method,
151                          codeAttribute,
152                          offset + branchInstruction.branchOffset);
153
154         byte opcode = branchInstruction.opcode;
155         if (opcode == InstructionConstants.OP_GOTO ||
156             opcode == InstructionConstants.OP_GOTO_W)
157         {
158             next = false;
159         }
160     }
161
162
163     public void visitAnySwitchInstruction(Clazz clazz, Method method, CodeAttribute codeAttribute, int offset, SwitchInstruction switchInstruction)
164     {
165         // Mark the branch targets of the default jump offset.
166 markBranchTarget(clazz,
167                          method,
168                          codeAttribute,
169                          offset + switchInstruction.defaultOffset);
170
171         // Mark the branch targets of the jump offsets.
172 markBranchTargets(clazz,
173                           method,
174                           codeAttribute,
175                           offset,
176                           switchInstruction.jumpOffsets);
177
178         next = false;
179     }
180
181
182     // Implementations for ExceptionInfoVisitor.
183
184     public void visitExceptionInfo(Clazz clazz, Method method, CodeAttribute codeAttribute, ExceptionInfo exceptionInfo)
185     {
186         // Mark the exception handler if it's relevant.
187 if (!isReachable(exceptionInfo.u2handlerPC) &&
188             isReachable(exceptionInfo.u2startPC, exceptionInfo.u2endPC))
189         {
190             markCode(clazz, method, codeAttribute, exceptionInfo.u2handlerPC);
191
192             evaluateExceptions = true;
193         }
194     }
195
196
197     // Small utility methods.
198
199     /**
200      * Marks the branch targets of the given jump offsets for the instruction
201      * at the given offset.
202      */
203     private void markBranchTargets(Clazz clazz, Method method, CodeAttribute codeAttribute, int offset, int[] jumpOffsets)
204     {
205         for (int index = 0; index < jumpOffsets.length; index++)
206         {
207             markCode(clazz, method, codeAttribute, offset + jumpOffsets[index]);
208         }
209     }
210
211
212     /**
213      * Marks the branch target at the given offset.
214      */
215     private void markBranchTarget(Clazz clazz, Method method, CodeAttribute codeAttribute, int offset)
216     {
217         boolean oldNext = next;
218
219         markCode(clazz, method, codeAttribute, offset);
220
221         next = oldNext;
222     }
223
224
225     /**
226      * Marks the code starting at the given offset.
227      */
228     private void markCode(Clazz clazz, Method method, CodeAttribute codeAttribute, int offset)
229     {
230         boolean oldNext = next;
231
232         byte[] code = codeAttribute.code;
233
234         // Continue with the current instruction as long as we haven't marked it
235 // yet.
236 while (!isReachable[offset])
237         {
238             // Get the current instruction.
239 Instruction instruction = InstructionFactory.create(code, offset);
240
241             // Mark it as reachable.
242 isReachable[offset] = true;
243
244             // By default, we'll assume we can continue with the next
245 // instruction in a moment.
246 next = true;
247
248             // Mark the branch targets, if any.
249 instruction.accept(clazz, method, codeAttribute, offset, this);
250
251             // Can we really continue with the next instruction?
252 if (!next)
253             {
254                 break;
255             }
256
257             // Go to the next instruction.
258 offset += instruction.length(offset);
259         }
260
261         next = oldNext;
262     }
263 }
264
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