Peephole.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License. 
 *
 */
import java.util.Iterator;
import org.apache.bcel.Repository;
import org.apache.bcel.classfile.JavaClass;
import org.apache.bcel.classfile.Method;
import org.apache.bcel.generic.ConstantPoolGen;
import org.apache.bcel.generic.InstructionHandle;
import org.apache.bcel.generic.InstructionList;
import org.apache.bcel.generic.InstructionTargeter;
import org.apache.bcel.generic.MethodGen;
import org.apache.bcel.generic.TargetLostException;
import org.apache.bcel.util.InstructionFinder;

/**
 * Remove NOPs from given class
 *
 * @version $Id: Peephole.java 1589381 2014-04-23 12:22:36Z ebourg $
 * @author <A HREF="mailto:m.dahm@gmx.de">M. Dahm</A>
 */
public class Peephole {
  public static void main(String[] argv) {
    try {
      /* Load the class from CLASSPATH.
       */
      JavaClass       clazz   = Repository.lookupClass(argv[0]);
      Method[]        methods = clazz.getMethods();
      ConstantPoolGen cp      = new ConstantPoolGen(clazz.getConstantPool());

      for(int i=0; i < methods.length; i++) {
    if(!(methods[i].isAbstract() || methods[i].isNative())) {
      MethodGen mg       = new MethodGen(methods[i],
                         clazz.getClassName(), cp);
      Method    stripped = removeNOPs(mg);
      
      if(stripped != null) {
        methods[i] = stripped; // Overwrite with stripped method
    }
    }
      }

      /* Dump the class to <class name>_.class
       */
      clazz.setConstantPool(cp.getFinalConstantPool());
      clazz.dump(clazz.getClassName() + "_.class");
    } catch(Exception e) { e.printStackTrace(); }
  }

  private static Method removeNOPs(MethodGen mg) {
    InstructionList   il    = mg.getInstructionList();
    InstructionFinder f     = new InstructionFinder(il);
    String            pat   = "NOP+"; // Find at least one NOP
    InstructionHandle next  = null;
    int               count = 0;

    for(Iterator<InstructionHandle[]> e = f.search(pat); e.hasNext(); ) {
      InstructionHandle[] match = e.next();
      InstructionHandle   first = match[0];
      InstructionHandle   last  = match[match.length - 1];
      
      /* Some nasty Java compilers may add NOP at end of method.
       */
      if((next = last.getNext()) == null) {
        break;
    }

      count += match.length;

      /* Delete NOPs and redirect any references to them to the following
       * (non-nop) instruction.
       */
      try {
    il.delete(first, last);
      } catch(TargetLostException e2) {
    InstructionHandle[] targets = e2.getTargets();
    for(int i=0; i < targets.length; i++) {
      InstructionTargeter[] targeters = targets[i].getTargeters();
      
      for(int j=0; j < targeters.length; j++) {
        targeters[j].updateTarget(targets[i], next);
    }
    }
      }
    }

    Method m = null;
    
    if(count > 0) {
      System.out.println("Removed " + count + " NOP instructions from method " +
             mg.getName());
      m = mg.getMethod();
    }

    il.dispose(); // Reuse instruction handles
    return m;
  }
}