jode/jode/jode/flow/SpecialBlock.java

/* SpecialBlock Copyright (C) 1998-1999 Jochen Hoenicke.
 *
 * 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, 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; see the file COPYING.  If not, write to
 * the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
 *
 * $Id$
 */

package jode.flow;
import jode.decompiler.TabbedPrintWriter;
import jode.expr.*;

/**
 * This is the structured block for atomic instructions.
 */
public class SpecialBlock extends StructuredBlock {

    public static int DUP  = 0;
    public static int SWAP = 1;
    public static int POP  = 2;
    private static String[] output = { "DUP", "SWAP", "POP" };

    /**
     * The type, one of DUP or SWAP
     */
    int type;
    /**
     * The count of stack entries that are transformed. 
     * This is 1 for swap, and 1 or 2 for dup.
     */
    int count;
    /**
     * The depth that the dupped element should be put to (0,1 or 2).
     * For swap this is zero.
     */
    int depth;

    public SpecialBlock(int type, int count, int depth, Jump jump) {
        this.type = type;
        this.count = count;
        this.depth = depth;
	setJump(jump);
    }

    /**
     * This does take the instr into account and modifies stack
     * accordingly.  It then calls super.mapStackToLocal.
     * @param stack the stack before the instruction is called
     * @return stack the stack afterwards.
     */
    public VariableStack mapStackToLocal(VariableStack stack) {
	/* a SpecialBlock is special :-) */
	VariableStack after = stack.executeSpecial(this);
	return super.mapStackToLocal(after);
    }

    public void removePush() {
	/* XXX */
	removeBlock();
    }


    public void dumpInstruction(TabbedPrintWriter writer) 
	throws java.io.IOException
    {
	writer.println(output[type] 
		       + ((count == 1) ? "" : "2")
		       + ((depth == 0) ? "" : "_X"+depth));
    }

    public boolean doTransformations() {
        return (type == SWAP && removeSwap(flowBlock.lastModified))
	    || (type == POP && removePop(flowBlock.lastModified));
    }

    public boolean removeSwap(StructuredBlock last) {

        /* Remove non needed swaps; convert:
         *
         *   PUSH expr1
         *   PUSH expr2
         *   SWAP
         *
         * to:
         *
         *   PUSH expr2
         *   PUSH expr1
         */
        if (last.outer instanceof SequentialBlock
            && last.outer.outer instanceof SequentialBlock
            && last.outer.getSubBlocks()[0] instanceof InstructionBlock
            && last.outer.outer.getSubBlocks()[0] 
            instanceof InstructionBlock) {

            InstructionBlock block1 
                = (InstructionBlock) last.outer.outer.getSubBlocks()[0];
            InstructionBlock block2
                = (InstructionBlock) last.outer.getSubBlocks()[0];

	    Expression expr1 = block1.getInstruction();
	    Expression expr2 = block2.getInstruction();

            if (expr1.isVoid() || expr2.isVoid()
		|| expr1.getFreeOperandCount() != 0
		|| expr2.getFreeOperandCount() != 0
		|| expr1.hasSideEffects(expr2) 
		|| expr2.hasSideEffects(expr1))
                return false;

            /* PUSH expr1 == block1
             * PUSH expr2
             * SWAP
             * ...
             */
            last.outer.replace(block1.outer);
            /* PUSH expr2
             * SWAP
             * ...
             */
            block1.replace(this);
            block1.moveJump(jump);
            /* PUSH expr2
             * PUSH expr1
             */
            block1.flowBlock.lastModified = block1;
            return true;
        }
        return false;
    }

    public boolean removePop(StructuredBlock last) {

	/* There are three possibilities:
         *
         *   PUSH method_invocation()
         *   POP[sizeof PUSH]
         * to:
         *   method_invocation()
	 *
	 *   PUSH arg1
	 *   PUSH arg2
	 *   POP2
	 * to:
	 *   if (arg1 == arg2)
	 *     empty
	 *
	 *   PUSH arg1
	 *   POP
	 * to:
	 *   if (arg1 != 0)
	 *     empty 
         */

        if (last.outer instanceof SequentialBlock
	    && last.outer.getSubBlocks()[0] instanceof InstructionBlock) {

	    if (jump != null && jump.destination == null)
		return false;

            InstructionBlock prev
                = (InstructionBlock) last.outer.getSubBlocks()[0];
	    Expression instr = prev.getInstruction();

	    if (instr.getType().stackSize() == count) {
		StructuredBlock newBlock;
		if (instr instanceof InvokeOperator) {
		    Expression newExpr
			= new PopOperator(instr.getType()).addOperand(instr);
		    prev.setInstruction(newExpr);
		    newBlock = prev;
		} else {
		    Expression newCond = new CompareUnaryOperator
			(instr.getType(), Operator.NOTEQUALS_OP)
			.addOperand(instr);
		    IfThenElseBlock newIfThen = new IfThenElseBlock(newCond);
		    newIfThen.setThenBlock(new EmptyBlock());
		    newBlock = newIfThen;
		}
		// We don't move the definitions of the special block, but
		// it shouldn't have any.
		newBlock.moveDefinitions(last.outer, last);
		newBlock.moveJump(jump);
		if (this == last) {
		    newBlock.replace(last.outer);
		    flowBlock.lastModified = newBlock;
		} else {
		    newBlock.replace(this);
		    last.replace(last.outer);
		}
		return true;
	    }
        }
        return false;
    }
}