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 jdk.incubator.code.bytecode;
26 
27 import jdk.internal.classfile.impl.BytecodeHelpers;
28 
29 import java.lang.classfile.CodeBuilder;
30 import java.lang.classfile.CodeElement;
31 import java.lang.classfile.CodeTransform;
32 import java.lang.classfile.PseudoInstruction;
33 import java.lang.classfile.instruction.BranchInstruction;
34 import java.lang.classfile.instruction.LabelTarget;
35 import java.util.ArrayList;
36 import java.util.List;
37 
38 /**
39  * BranchCompactor is a CodeTransform skipping redundant branches to immediate targets.
40  */
41 public final class BranchCompactor implements CodeTransform {
42 
43     private BranchInstruction branch;
44     private final List<PseudoInstruction> buffer = new ArrayList<>();
45 
46     public BranchCompactor() {
47     }
48 
49     @Override
50     public void accept(CodeBuilder cob, CodeElement coe) {
51         if (branch == null) {
52             if (coe instanceof BranchInstruction bi && BytecodeHelpers.isUnconditionalBranch(bi.opcode())) {
53                 //unconditional branch is stored
54                 branch = bi;
55             } else {
56                 //all other elements are passed
57                 cob.with(coe);
58             }
59         } else {
60             switch (coe) {
61                 case LabelTarget lt when branch.target() == lt.label() -> {
62                     //skip branch to immediate target
63                     branch = null;
64                     //flush the buffer
65                     atEnd(cob);
66                     //pass the target
67                     cob.with(lt);
68                 }
69                 case PseudoInstruction pi -> {
70                     //buffer pseudo instructions
71                     buffer.add(pi);
72                 }
73                 default -> {
74                     //any other instruction flushes the branch and buffer
75                     atEnd(cob);
76                     //replay the code element
77                     accept(cob, coe);
78                 }
79             }
80         }
81     }
82 
83     @Override
84     public void atEnd(CodeBuilder cob) {
85         if (branch != null) {
86             //flush the branch
87             cob.with(branch);
88             branch = null;
89         }
90         //flush the buffer
91         buffer.forEach(cob::with);
92         buffer.clear();
93     }
94 }