attribute/visitor/StackSizeComputer.java

/*
 * ProGuard -- shrinking, optimization, obfuscation, and preverification
 *             of Java bytecode.
 *
 * Copyright (c) 2002-2009 Eric Lafortune (eric@graphics.cornell.edu)
 *
 * 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.classfile.attribute.visitor;

import proguard.classfile.*;
import proguard.classfile.visitor.ClassPrinter;
import proguard.classfile.attribute.*;
import proguard.classfile.instruction.*;
import proguard.classfile.instruction.visitor.InstructionVisitor;
import proguard.classfile.util.SimplifiedVisitor;

/**
 * This AttributeVisitor computes the stack sizes at all instruction offsets
 * of the code attributes that it visits.
 *
 * @author Eric Lafortune
 */
public class StackSizeComputer
extends      SimplifiedVisitor
implements   AttributeVisitor,
             InstructionVisitor,
             ExceptionInfoVisitor
{
    //*
    private static final boolean DEBUG = false;
    /*/
    private static       boolean DEBUG = true;
    //*/


    private boolean[] evaluated  = new boolean[ClassConstants.TYPICAL_CODE_LENGTH];
    private int[]     stackSizes = new int[ClassConstants.TYPICAL_CODE_LENGTH];

    private boolean exitInstructionBlock;

    private int stackSize;
    private int maxStackSize;


    /**
     * Returns whether the instruction at the given offset is reachable in the
     * most recently visited code attribute.
     */
    public boolean isReachable(int instructionOffset)
    {
        return evaluated[instructionOffset];
    }


    /**
     * Returns the stack size at the given instruction offset of the most
     * recently visited code attribute.
     */
    public int getStackSize(int instructionOffset)
    {
        if (!evaluated[instructionOffset])
        {
            throw new IllegalArgumentException("Unknown stack size at unreachable instruction offset ["+instructionOffset+"]");
        }

        return stackSizes[instructionOffset];
    }


    /**
     * Returns the maximum stack size of the most recently visited code attribute.
     */
    public int getMaxStackSize()
    {
        return maxStackSize;
    }


    // 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)
        {
            System.err.println("Unexpected error while computing stack sizes:");
            System.err.println("  Class       = ["+clazz.getName()+"]");
            System.err.println("  Method      = ["+method.getName(clazz)+method.getDescriptor(clazz)+"]");
            System.err.println("  Exception   = ["+ex.getClass().getName()+"] ("+ex.getMessage()+")");

            if (DEBUG)
            {
                method.accept(clazz, new ClassPrinter());
            }

            throw ex;
        }
    }


    public void visitCodeAttribute0(Clazz clazz, Method method, CodeAttribute codeAttribute)
    {
        if (DEBUG)
        {
            System.out.println("StackSizeComputer: "+clazz.getName()+"."+method.getName(clazz)+method.getDescriptor(clazz));
        }

        // Try to reuse the previous array.
        int codeLength = codeAttribute.u4codeLength;
        if (evaluated.length < codeLength)
        {
            evaluated  = new boolean[codeLength];
            stackSizes = new int[codeLength];
        }
        else
        {
            for (int index = 0; index < codeLength; index++)
            {
                evaluated[index] = false;
            }
        }

        // The initial stack is always empty.
        stackSize    = 0;
        maxStackSize = 0;

        // Evaluate the instruction block starting at the entry point of the method.
        evaluateInstructionBlock(clazz, method, codeAttribute, 0);

        // Evaluate the exception handlers.
        codeAttribute.exceptionsAccept(clazz, method, this);
    }


    // Implementations for InstructionVisitor.

    public void visitSimpleInstruction(Clazz clazz, Method method, CodeAttribute codeAttribute, int offset, SimpleInstruction simpleInstruction)
    {
        byte opcode = simpleInstruction.opcode;

        // Some simple instructions exit from the current instruction block.
        exitInstructionBlock =
            opcode == InstructionConstants.OP_IRETURN ||
            opcode == InstructionConstants.OP_LRETURN ||
            opcode == InstructionConstants.OP_FRETURN ||
            opcode == InstructionConstants.OP_DRETURN ||
            opcode == InstructionConstants.OP_ARETURN ||
            opcode == InstructionConstants.OP_RETURN  ||
            opcode == InstructionConstants.OP_ATHROW;
    }

    public void visitConstantInstruction(Clazz clazz, Method method, CodeAttribute codeAttribute, int offset, ConstantInstruction constantInstruction)
    {
        // Constant pool instructions never end the current instruction block.
        exitInstructionBlock = false;
    }

    public void visitVariableInstruction(Clazz clazz, Method method, CodeAttribute codeAttribute, int offset, VariableInstruction variableInstruction)
    {
        byte opcode = variableInstruction.opcode;

        // The ret instruction end the current instruction block.
        exitInstructionBlock =
            opcode == InstructionConstants.OP_RET;
    }

    public void visitBranchInstruction(Clazz clazz, Method method, CodeAttribute codeAttribute, int offset, BranchInstruction branchInstruction)
    {
        byte opcode = branchInstruction.opcode;

        // Evaluate the target instruction blocks.
        evaluateInstructionBlock(clazz,
                                 method,
                                 codeAttribute,
                                 offset +
                                 branchInstruction.branchOffset);

        // Evaluate the instructions after a subroutine branch.
        if (opcode == InstructionConstants.OP_JSR ||
            opcode == InstructionConstants.OP_JSR_W)
        {
            // We assume subroutine calls (jsr and jsr_w instructions) don't
            // change the stack, other than popping the return value.
            stackSize -= 1;

            evaluateInstructionBlock(clazz,
                                     method,
                                     codeAttribute,
                                     offset + branchInstruction.length(offset));
        }

        // Some branch instructions always end the current instruction block.
        exitInstructionBlock =
            opcode == InstructionConstants.OP_GOTO   ||
            opcode == InstructionConstants.OP_GOTO_W ||
            opcode == InstructionConstants.OP_JSR    ||
            opcode == InstructionConstants.OP_JSR_W;
    }


    public void visitAnySwitchInstruction(Clazz clazz, Method method, CodeAttribute codeAttribute, int offset, SwitchInstruction switchInstruction)
    {
        // Evaluate the target instruction blocks.

        // Loop over all jump offsets.
        int[] jumpOffsets = switchInstruction.jumpOffsets;

        for (int index = 0; index < jumpOffsets.length; index++)
        {
            // Evaluate the jump instruction block.
            evaluateInstructionBlock(clazz,
                                     method,
                                     codeAttribute,
                                     offset + jumpOffsets[index]);
        }

        // Also evaluate the default instruction block.
        evaluateInstructionBlock(clazz,
                                 method,
                                 codeAttribute,
                                 offset + switchInstruction.defaultOffset);

        // The switch instruction always ends the current instruction block.
        exitInstructionBlock = true;
    }


    // Implementations for ExceptionInfoVisitor.

    public void visitExceptionInfo(Clazz clazz, Method method, CodeAttribute codeAttribute, ExceptionInfo exceptionInfo)
    {
        if (DEBUG)
        {
            System.out.println("Exception:");
        }

        // The stack size when entering the exception handler is always 1.
        stackSize = 1;

        // Evaluate the instruction block starting at the entry point of the
        // exception handler.
        evaluateInstructionBlock(clazz,
                                 method,
                                 codeAttribute,
                                 exceptionInfo.u2handlerPC);
    }


    // Small utility methods.

    /**
     * Evaluates a block of instructions that hasn't been handled before,
     * starting at the given offset and ending at a branch instruction, a return
     * instruction, or a throw instruction. Branch instructions are handled
     * recursively.
     */
    private void evaluateInstructionBlock(Clazz         clazz,
                                          Method        method,
                                          CodeAttribute codeAttribute,
                                          int           instructionOffset)
    {
        if (DEBUG)
        {
            if (evaluated[instructionOffset])
            {
                System.out.println("-- (instruction block at "+instructionOffset+" already evaluated)");
            }
            else
            {
                System.out.println("-- instruction block:");
            }
        }

        // Remember the initial stack size.
        int initialStackSize = stackSize;

        // Remember the maximum stack size.
        if (maxStackSize < stackSize)
        {
            maxStackSize = stackSize;
        }

        // Evaluate any instructions that haven't been evaluated before.
        while (!evaluated[instructionOffset])
        {
            // Mark the instruction as evaluated.
            evaluated[instructionOffset] = true;

            Instruction instruction = InstructionFactory.create(codeAttribute.code,
                                                                instructionOffset);

            if (DEBUG)
            {
                int stackPushCount = instruction.stackPushCount(clazz);
                int stackPopCount  = instruction.stackPopCount(clazz);
                System.out.println("["+instructionOffset+"]: "+
                                   stackSize+" - "+
                                   stackPopCount+" + "+
                                   stackPushCount+" = "+
                                   (stackSize+stackPushCount-stackPopCount)+": "+
                                   instruction.toString(instructionOffset));
            }

            // Compute the instruction's effect on the stack size.
            stackSize -= instruction.stackPopCount(clazz);

            if (stackSize < 0)
            {
                throw new IllegalArgumentException("Stack size becomes negative after instruction "+
                                                   instruction.toString(instructionOffset)+" in ["+
                                                   clazz.getName()+"."+
                                                   method.getName(clazz)+
                                                   method.getDescriptor(clazz)+"]");
            }

            stackSizes[instructionOffset] =
            stackSize += instruction.stackPushCount(clazz);

            // Remember the maximum stack size.
            if (maxStackSize < stackSize)
            {
                maxStackSize = stackSize;
            }

            // Remember the next instruction offset.
            int nextInstructionOffset = instructionOffset +
                                        instruction.length(instructionOffset);

            // Visit the instruction, in order to handle branches.
            instruction.accept(clazz, method, codeAttribute, instructionOffset, this);

            // Stop evaluating after a branch.
            if (exitInstructionBlock)
            {
                break;
            }

            // Continue with the next instruction.
            instructionOffset = nextInstructionOffset;

            if (DEBUG)
            {
                if (evaluated[instructionOffset])
                {
                    System.out.println("-- (instruction at "+instructionOffset+" already evaluated)");
                }
            }
        }

        // Restore the stack size for possible subsequent instruction blocks.
        this.stackSize = initialStackSize;
    }
}