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Javassist (JAVA programming ASSISTant) makes Java bytecode manipulation
simple. It is a class library for editing bytecodes in Java.
/*
* Javassist, a Java-bytecode translator toolkit.
* Copyright (C) 1999- Shigeru Chiba. All Rights Reserved.
*
* The contents of this file are subject to the Mozilla Public License Version
* 1.1 (the "License"); you may not use this file except in compliance with
* the License. Alternatively, the contents of this file may be used under
* the terms of the GNU Lesser General Public License Version 2.1 or later,
* or the Apache License Version 2.0.
*
* Software distributed under the License is distributed on an "AS IS" basis,
* WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
* for the specific language governing rights and limitations under the
* License.
*/
package javassist.bytecode;
/**
* Utility for computing max_stack.
*/
class CodeAnalyzer implements Opcode {
private ConstPool constPool;
private CodeAttribute codeAttr;
public CodeAnalyzer(CodeAttribute ca) {
codeAttr = ca;
constPool = ca.getConstPool();
}
public int computeMaxStack()
throws BadBytecode
{
/* d = stack[i]
* d == 0: not visited
* d > 0: the depth is d - 1 after executing the bytecode at i.
* d < 0: not visited. the initial depth (before execution) is 1 - d.
*/
CodeIterator ci = codeAttr.iterator();
int length = ci.getCodeLength();
int[] stack = new int[length];
constPool = codeAttr.getConstPool();
initStack(stack, codeAttr);
boolean repeat;
do {
repeat = false;
for (int i = 0; i < length; ++i)
if (stack[i] < 0) {
repeat = true;
visitBytecode(ci, stack, i);
}
} while (repeat);
int maxStack = 1;
for (int i = 0; i < length; ++i)
if (stack[i] > maxStack)
maxStack = stack[i];
return maxStack - 1; // the base is 1.
}
private void initStack(int[] stack, CodeAttribute ca) {
stack[0] = -1;
ExceptionTable et = ca.getExceptionTable();
if (et != null) {
int size = et.size();
for (int i = 0; i < size; ++i)
stack[et.handlerPc(i)] = -2; // an exception is on stack
}
}
private void visitBytecode(CodeIterator ci, int[] stack, int index)
throws BadBytecode
{
int codeLength = stack.length;
ci.move(index);
int stackDepth = -stack[index];
int[] jsrDepth = new int[1];
jsrDepth[0] = -1;
while (ci.hasNext()) {
index = ci.next();
stack[index] = stackDepth;
int op = ci.byteAt(index);
stackDepth = visitInst(op, ci, index, stackDepth);
if (stackDepth < 1)
throw new BadBytecode("stack underflow at " + index);
if (processBranch(op, ci, index, codeLength, stack, stackDepth, jsrDepth))
break;
if (isEnd(op)) // return, ireturn, athrow, ...
break;
if (op == JSR || op == JSR_W)
--stackDepth;
}
}
private boolean processBranch(int opcode, CodeIterator ci, int index,
int codeLength, int[] stack, int stackDepth, int[] jsrDepth)
throws BadBytecode
{
if ((IFEQ <= opcode && opcode <= IF_ACMPNE)
|| opcode == IFNULL || opcode == IFNONNULL) {
int target = index + ci.s16bitAt(index + 1);
checkTarget(index, target, codeLength, stack, stackDepth);
}
else {
int target, index2;
switch (opcode) {
case GOTO :
target = index + ci.s16bitAt(index + 1);
checkTarget(index, target, codeLength, stack, stackDepth);
return true;
case GOTO_W :
target = index + ci.s32bitAt(index + 1);
checkTarget(index, target, codeLength, stack, stackDepth);
return true;
case JSR :
case JSR_W :
if (opcode == JSR)
target = index + ci.s16bitAt(index + 1);
else
target = index + ci.s32bitAt(index + 1);
checkTarget(index, target, codeLength, stack, stackDepth);
/*
* It is unknown which RET comes back to this JSR.
* So we assume that if the stack depth at one JSR instruction
* is N, then it is also N at other JSRs and N - 1 at all RET
* instructions. Note that STACK_GROW[JSR] is 1 since it pushes
* a return address on the operand stack.
*/
if (jsrDepth[0] < 0) {
jsrDepth[0] = stackDepth;
return false;
}
else if (stackDepth == jsrDepth[0])
return false;
else
throw new BadBytecode(
"sorry, cannot compute this data flow due to JSR: "
+ stackDepth + "," + jsrDepth[0]);
case RET :
if (jsrDepth[0] < 0) {
jsrDepth[0] = stackDepth + 1;
return false;
}
else if (stackDepth + 1 == jsrDepth[0])
return true;
else
throw new BadBytecode(
"sorry, cannot compute this data flow due to RET: "
+ stackDepth + "," + jsrDepth[0]);
case LOOKUPSWITCH :
case TABLESWITCH :
index2 = (index & ~3) + 4;
target = index + ci.s32bitAt(index2);
checkTarget(index, target, codeLength, stack, stackDepth);
if (opcode == LOOKUPSWITCH) {
int npairs = ci.s32bitAt(index2 + 4);
index2 += 12;
for (int i = 0; i < npairs; ++i) {
target = index + ci.s32bitAt(index2);
checkTarget(index, target, codeLength,
stack, stackDepth);
index2 += 8;
}
}
else {
int low = ci.s32bitAt(index2 + 4);
int high = ci.s32bitAt(index2 + 8);
int n = high - low + 1;
index2 += 12;
for (int i = 0; i < n; ++i) {
target = index + ci.s32bitAt(index2);
checkTarget(index, target, codeLength,
stack, stackDepth);
index2 += 4;
}
}
return true; // always branch.
}
}
return false; // may not branch.
}
private void checkTarget(int opIndex, int target, int codeLength,
int[] stack, int stackDepth)
throws BadBytecode
{
if (target < 0 || codeLength <= target)
throw new BadBytecode("bad branch offset at " + opIndex);
int d = stack[target];
if (d == 0)
stack[target] = -stackDepth;
else if (d != stackDepth && d != -stackDepth)
throw new BadBytecode("verification error (" + stackDepth +
"," + d + ") at " + opIndex);
}
private static boolean isEnd(int opcode) {
return (IRETURN <= opcode && opcode <= RETURN) || opcode == ATHROW;
}
/**
* Visits an instruction.
*/
private int visitInst(int op, CodeIterator ci, int index, int stack)
throws BadBytecode
{
String desc;
switch (op) {
case GETFIELD :
stack += getFieldSize(ci, index) - 1;
break;
case PUTFIELD :
stack -= getFieldSize(ci, index) + 1;
break;
case GETSTATIC :
stack += getFieldSize(ci, index);
break;
case PUTSTATIC :
stack -= getFieldSize(ci, index);
break;
case INVOKEVIRTUAL :
case INVOKESPECIAL :
desc = constPool.getMethodrefType(ci.u16bitAt(index + 1));
stack += Descriptor.dataSize(desc) - 1;
break;
case INVOKESTATIC :
desc = constPool.getMethodrefType(ci.u16bitAt(index + 1));
stack += Descriptor.dataSize(desc);
break;
case INVOKEINTERFACE :
desc = constPool.getInterfaceMethodrefType(
ci.u16bitAt(index + 1));
stack += Descriptor.dataSize(desc) - 1;
break;
case INVOKEDYNAMIC :
desc = constPool.getInvokeDynamicType(ci.u16bitAt(index + 1));
stack += Descriptor.dataSize(desc); // assume CosntPool#REF_invokeStatic
break;
case ATHROW :
stack = 1; // the stack becomes empty (1 means no values).
break;
case MULTIANEWARRAY :
stack += 1 - ci.byteAt(index + 3);
break;
case WIDE :
op = ci.byteAt(index + 1);
// don't break here.
default :
stack += STACK_GROW[op];
}
return stack;
}
private int getFieldSize(CodeIterator ci, int index) {
String desc = constPool.getFieldrefType(ci.u16bitAt(index + 1));
return Descriptor.dataSize(desc);
}
}