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.algorithmic.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     */
024    public abstract class BytecodeTrimmer {
025    
026        /** Current instructions window. */
027        private AbstractInsnNode[] currentWindow;
028    
029        /** Simple constructor.
030         * @param lookahead number of lookahead instructions
031         */
032        protected BytecodeTrimmer(final int lookahead) {
033            currentWindow = new AbstractInsnNode[lookahead];
034        }
035    
036        /** Trim a list of instructions.
037         * @param instructions list of instructions to optimize
038         */
039        public void trim(final InsnList instructions) {
040    
041            // set up lookahead instructions
042            currentWindow[0] = instructions.get(0);
043            for (int i = 1; (i < currentWindow.length) && (currentWindow[i - 1] != null); ++i) {
044                currentWindow[i] = currentWindow[i - 1].getNext();
045            }
046    
047            // trim the whole instructions set using a sliding window
048            while (currentWindow[currentWindow.length - 1] != null) {
049    
050                // trim current window
051                final boolean updated = trimWindow(instructions, currentWindow);
052    
053                if (!updated) {
054                    // slide the window ourselves one instruction forward
055                    for (int i = 1; i < currentWindow.length; ++i) {
056                        currentWindow[i - 1] = currentWindow[i];
057                    }
058                    if (currentWindow[currentWindow.length - 1] != null) {
059                        currentWindow[currentWindow.length - 1] = currentWindow[currentWindow.length - 1].getNext();
060                    }
061                }
062    
063            }
064    
065        }
066    
067        /** Trim the current window of lookahead instructions.
068         * @param instructions complete instructions list of instructions to trim
069         * @param window current instructions window (belongs to the list)
070         * @return true if instructions and window have been updated and are ready
071         * for next iteration
072         */
073        protected abstract boolean trimWindow(InsnList instructions,
074                                              AbstractInsnNode[] window);
075    
076    }