1 /* 2 * Copyright (c) 2024, Oracle and/or its affiliates. All rights reserved. 3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 4 * 5 * This code is free software; you can redistribute it and/or modify it 6 * under the terms of the GNU General Public License version 2 only, as 7 * published by the Free Software Foundation. Oracle designates this 8 * particular file as subject to the "Classpath" exception as provided 9 * by Oracle in the LICENSE file that accompanied this code. 10 * 11 * This code is distributed in the hope that it will be useful, but WITHOUT 12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 14 * version 2 for more details (a copy is included in the LICENSE file that 15 * accompanied this code). 16 * 17 * You should have received a copy of the GNU General Public License version 18 * 2 along with this work; if not, write to the Free Software Foundation, 19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 20 * 21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 22 * or visit www.oracle.com if you need additional information or have any 23 * questions. 24 */ 25 package java.lang.reflect.code.bytecode; 26 27 import java.lang.classfile.CodeBuilder; 28 import java.lang.classfile.CodeElement; 29 import java.lang.classfile.CodeTransform; 30 import java.lang.classfile.PseudoInstruction; 31 import java.lang.classfile.instruction.BranchInstruction; 32 import java.lang.classfile.instruction.LabelTarget; 33 import java.util.ArrayList; 34 import java.util.List; 35 36 /** 37 * BranchCompactor is a CodeTransform skipping redundant branches to immediate targets. 38 */ 39 public final class BranchCompactor implements CodeTransform { 40 41 private BranchInstruction branch; 42 private final List<PseudoInstruction> buffer = new ArrayList<>(); 43 44 public BranchCompactor() { 45 } 46 47 @Override 48 public void accept(CodeBuilder cob, CodeElement coe) { 49 if (branch == null) { 50 if (coe instanceof BranchInstruction bi && bi.opcode().isUnconditionalBranch()) { 51 //unconditional branch is stored 52 branch = bi; 53 } else { 54 //all other elements are passed 55 cob.with(coe); 56 } 57 } else { 58 switch (coe) { 59 case LabelTarget lt when branch.target() == lt.label() -> { 60 //skip branch to immediate target 61 branch = null; 62 //flush the buffer 63 atEnd(cob); 64 //pass the target 65 cob.with(lt); 66 } 67 case PseudoInstruction pi -> { 68 //buffer pseudo instructions 69 buffer.add(pi); 70 } 71 default -> { 72 //any other instruction flushes the branch and buffer 73 atEnd(cob); 74 //replay the code element 75 accept(cob, coe); 76 } 77 } 78 } 79 } 80 81 @Override 82 public void atEnd(CodeBuilder cob) { 83 if (branch != null) { 84 //flush the branch 85 cob.with(branch); 86 branch = null; 87 } 88 //flush the buffer 89 buffer.forEach(cob::with); 90 buffer.clear(); 91 } 92 }