TailRecursion.java

/*
 * fb-contrib - Auxiliary detectors for Java programs
 * Copyright (C) 2005-2017 Dave Brosius
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library 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
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 */
package com.mebigfatguy.fbcontrib.detect;

import org.apache.bcel.classfile.Code;
import org.apache.bcel.classfile.Method;

import com.mebigfatguy.fbcontrib.utils.BugType;
import com.mebigfatguy.fbcontrib.utils.SignatureUtils;
import com.mebigfatguy.fbcontrib.utils.Values;

import edu.umd.cs.findbugs.BugInstance;
import edu.umd.cs.findbugs.BugReporter;
import edu.umd.cs.findbugs.BytecodeScanningDetector;
import edu.umd.cs.findbugs.OpcodeStack;
import edu.umd.cs.findbugs.ba.ClassContext;

/**
 * looks for methods that make a recursive call to itself as the last statement in the method. This tail recursion could be converted into a simple loop which
 * would improve the performance and stack requirements.
 */
public class TailRecursion extends BytecodeScanningDetector {
    public static final int TAILRECURSIONFUDGE = 6;

    private BugReporter bugReporter;
    private OpcodeStack stack;
    private int trPCPos;
    private boolean possibleTailRecursion;
    private boolean isStatic;

    /**
     * constructs a TR detector given the reporter to report bugs on
     *
     * @param bugReporter
     *            the sync of bug reports
     */
    public TailRecursion(BugReporter bugReporter) {
        this.bugReporter = bugReporter;
    }

    /**
     * implements the visitor to create and clear the stack
     *
     * @param classContext
     *            the context object of the currently parsed class
     */
    @Override
    public void visitClassContext(ClassContext classContext) {
        try {
            stack = new OpcodeStack();
            super.visitClassContext(classContext);
        } finally {
            stack = null;
        }
    }

    /**
     * implements the visitor to figure the pc where the method call must occur depending on whether the method returns a value, or not.
     *
     * @param obj
     *            the context object of the currently parsed method
     */
    @Override
    public void visitMethod(Method obj) {
        Code c = obj.getCode();
        if (c != null) {
            byte[] opcodes = c.getCode();
            if (opcodes != null) {
                trPCPos = opcodes.length - 1;
                if (!obj.getSignature().endsWith(Values.SIG_VOID)) {
                    trPCPos -= 1;
                }
                trPCPos -= TAILRECURSIONFUDGE;
                possibleTailRecursion = true;
                isStatic = obj.isStatic();
                stack.resetForMethodEntry(this);
                super.visitMethod(obj);
            }
        }
    }

    /**
     * implements the visitor to find methods that employ tail recursion
     *
     * @param seen
     *            the opcode of the currently parsed instruction
     */
    @Override
    public void sawOpcode(int seen) {
        try {
            stack.precomputation(this);

            if (seen == INVOKEVIRTUAL) {
                checkForTailRecursion();
            }
        } finally {
            stack.sawOpcode(this, seen);
        }
    }

    private void checkForTailRecursion() {
        boolean isRecursion = getMethodName().equals(getNameConstantOperand()) && getMethodSig().equals(getSigConstantOperand())
                && getClassName().equals(getClassConstantOperand());

        if (isRecursion && !isStatic) {
            int numParms = SignatureUtils.getNumParameters(getMethodSig());
            if (stack.getStackDepth() > numParms) {
                OpcodeStack.Item itm = stack.getStackItem(numParms);
                isRecursion = (itm.getRegisterNumber() == 0);
            }
        }

        if (isRecursion && possibleTailRecursion && (getPC() >= trPCPos)) {
            bugReporter.reportBug(new BugInstance(this, BugType.TR_TAIL_RECURSION.name(), NORMAL_PRIORITY).addClass(this).addMethod(this).addSourceLine(this));
        } else {
            possibleTailRecursion = false;
        }
    }

}