com.github.javalbert.bytecode.utils.AsmUtils.java Source code

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/*******************************************************************************
 * Copyright 2017 Albert Shun-Dat Chan
 *
 * Licensed 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.
 *******************************************************************************/
package com.github.javalbert.bytecode.utils;

import static org.objectweb.asm.Opcodes.*;

import java.util.ArrayList;
import java.util.List;

import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.Label;
import org.objectweb.asm.MethodVisitor;

public final class AsmUtils {
    /**
     * 
     * @param defaultCaseLabel
     * @param cases should be a sorted int array
     * @return an array of {@link Label}s when calling {@link MethodVisitor#visitTableSwitchInsn(int, int, Label, Label...)}
     */
    public static Label[] getTableSwitchLabels(Label defaultCaseLabel, int[] cases) {
        List<Label> labels = new ArrayList<>();

        int currentCase = 0;
        int caseValue = cases[0];

        int hi = cases[cases.length - 1];
        int lo = cases[0];

        for (int i = lo; i <= hi; i++) {
            int currentCaseValue = cases[currentCase];

            if (caseValue < currentCaseValue) {
                labels.add(defaultCaseLabel);
                caseValue++;
            } else {
                labels.add(new Label());
                caseValue = currentCaseValue + 1;
                currentCase++;
            }
        }
        return labels.stream().toArray(s -> new Label[s]);
    }

    /**
     * Determines what <code>switch</code> Java instruction to use:
     * <br>{@link MethodVisitor#visitTableSwitchInsn(int, int, Label, Label...)}
     * <br>or
     * <br>{@link MethodVisitor#visitLookupSwitchInsn(Label, int[], Label[])}
     * @param cases should be a sorted int array
     * @return
     */
    public static boolean useTableSwitch(int[] cases) {
        int hi = cases[cases.length - 1];
        int lo = cases[0];
        int nlabels = cases.length;

        // CREDIT: http://stackoverflow.com/a/31032054
        // CREDIT: http://hg.openjdk.java.net/jdk8/jdk8/langtools/file/30db5e0aaf83/src/share/classes/com/sun/tools/javac/jvm/Gen.java#l1153
        // Determine whether to issue a tableswitch or a lookupswitch
        // instruction.
        long table_space_cost = 4 + ((long) hi - lo + 1); // words
        long table_time_cost = 3; // comparisons
        long lookup_space_cost = 3 + 2 * (long) nlabels;
        long lookup_time_cost = nlabels;
        return nlabels > 0 && table_space_cost + 3 * table_time_cost <= lookup_space_cost + 3 * lookup_time_cost;
    }

    public static void visitDefaultConstructor(ClassWriter cw, String classTypeDescriptor) {
        MethodVisitor mv = cw.visitMethod(ACC_PUBLIC, "<init>", "()V", null, null);
        mv.visitCode();
        Label l0 = new Label();
        mv.visitLabel(l0);
        mv.visitVarInsn(ALOAD, 0);
        mv.visitMethodInsn(INVOKESPECIAL, "java/lang/Object", "<init>", "()V", false);
        mv.visitInsn(RETURN);
        Label l1 = new Label();
        mv.visitLabel(l1);
        mv.visitLocalVariable("this", classTypeDescriptor, null, l0, l1, 0);
        mv.visitMaxs(1, 1);
        mv.visitEnd();
    }

    /**
     * 
     * @param mv
     * @param i the int value
     */
    public static void visitZeroOperandInt(MethodVisitor mv, int i) {
        if (i > 5) {
            mv.visitIntInsn(BIPUSH, i);
        } else {
            int opcode = NOP;
            switch (i) {
            case 0:
                opcode = ICONST_0;
                break;
            case 1:
                opcode = ICONST_1;
                break;
            case 2:
                opcode = ICONST_2;
                break;
            case 3:
                opcode = ICONST_3;
                break;
            case 4:
                opcode = ICONST_4;
                break;
            case 5:
                opcode = ICONST_5;
                break;
            }
            mv.visitInsn(opcode);
        }
    }

    private AsmUtils() {
    }
}