001    /*
002     * Licensed to the Apache Software Foundation (ASF) under one or more
003     * contributor license agreements.  See the NOTICE file distributed with
004     * this work for additional information regarding copyright ownership.
005     * The ASF licenses this file to You under the Apache License, Version 2.0
006     * (the "License"); you may not use this file except in compliance with
007     * the License.  You may obtain a copy of the License at
008     *
009     *      http://www.apache.org/licenses/LICENSE-2.0
010     *
011     * Unless required by applicable law or agreed to in writing, software
012     * distributed under the License is distributed on an "AS IS" BASIS,
013     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014     * See the License for the specific language governing permissions and
015     * limitations under the License.
016     */
017    package org.apache.commons.nabla.forward.trimming;
018    
019    import org.objectweb.asm.tree.AbstractInsnNode;
020    import org.objectweb.asm.tree.InsnList;
021    
022    /** Base class for code trimmers.
023     * @version $Id$
024     */
025    public abstract class BytecodeTrimmer {
026    
027        /** Current instructions window. */
028        private AbstractInsnNode[] currentWindow;
029    
030        /** Simple constructor.
031         * @param lookahead number of lookahead instructions
032         */
033        protected BytecodeTrimmer(final int lookahead) {
034            currentWindow = new AbstractInsnNode[lookahead];
035        }
036    
037        /** Trim a list of instructions.
038         * @param instructions list of instructions to optimize
039         */
040        public void trim(final InsnList instructions) {
041    
042            // set up lookahead instructions
043            currentWindow[0] = instructions.get(0);
044            for (int i = 1; (i < currentWindow.length) && (currentWindow[i - 1] != null); ++i) {
045                currentWindow[i] = currentWindow[i - 1].getNext();
046            }
047    
048            // trim the whole instructions set using a sliding window
049            while (currentWindow[currentWindow.length - 1] != null) {
050    
051                // trim current window
052                final boolean updated = trimWindow(instructions, currentWindow);
053    
054                if (!updated) {
055                    // slide the window ourselves one instruction forward
056                    for (int i = 1; i < currentWindow.length; ++i) {
057                        currentWindow[i - 1] = currentWindow[i];
058                    }
059                    if (currentWindow[currentWindow.length - 1] != null) {
060                        currentWindow[currentWindow.length - 1] = currentWindow[currentWindow.length - 1].getNext();
061                    }
062                }
063    
064            }
065    
066        }
067    
068        /** Trim the current window of lookahead instructions.
069         * @param instructions complete instructions list of instructions to trim
070         * @param window current instructions window (belongs to the list)
071         * @return true if instructions and window have been updated and are ready
072         * for next iteration
073         */
074        protected abstract boolean trimWindow(InsnList instructions,
075                                              AbstractInsnNode[] window);
076    
077    }