src/java.base/share/classes/jdk/internal/classfile/impl/BlockCodeBuilderImpl.java

/*
 * Copyright (c) 2022, 2025, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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package jdk.internal.classfile.impl;

import java.lang.classfile.CodeBuilder;
import java.lang.classfile.CodeElement;
import java.lang.classfile.Instruction;
import java.lang.classfile.Label;
import java.lang.classfile.TypeKind;
import java.lang.classfile.instruction.LabelTarget;
import java.util.Objects;

import static java.util.Objects.requireNonNull;

public final class BlockCodeBuilderImpl
        extends NonterminalCodeBuilder
        implements CodeBuilder.BlockCodeBuilder {
    private final Label startLabel, endLabel, breakLabel;
    private boolean reachable = true;
    private boolean hasInstructions = false;
    private int topLocal;
    private int terminalMaxLocals;

    public BlockCodeBuilderImpl(CodeBuilder parent, Label breakLabel) {
        super(parent);
        this.startLabel = parent.newLabel();
        this.endLabel = parent.newLabel();
        this.breakLabel = Objects.requireNonNull(breakLabel);
    }

    public void start() {
        topLocal = topLocal(parent);
        terminalMaxLocals = terminal.curTopLocal();
        parent.with((LabelTarget) startLabel);
    }

    public void end() {
        parent.with((LabelTarget) endLabel);
        if (terminalMaxLocals != terminal.curTopLocal()) {
            throw new IllegalStateException("Interference in local variable slot management");
        }
    }

    public boolean reachable() {
        return reachable;
    }

    public boolean isEmpty() {
        return !hasInstructions;
    }

    private int topLocal(CodeBuilder parent) {
        if (parent instanceof BlockCodeBuilderImpl bcb) {
            return bcb.topLocal;
        }
        return findTerminal(parent).curTopLocal();
    }

    @Override
    public CodeBuilder with(CodeElement element) {
        parent.with(requireNonNull(element));

        hasInstructions |= element instanceof Instruction;

        if (reachable) {
            if (element instanceof Instruction i && BytecodeHelpers.isUnconditionalBranch(i.opcode()))
                reachable = false;
        }
        else if (element instanceof LabelTarget) {
            reachable = true;
        }
        return this;
    }

    @Override
    public Label startLabel() {
        return startLabel;
    }

    @Override
    public Label endLabel() {
        return endLabel;
    }

    @Override
    public int allocateLocal(TypeKind typeKind) {
        int retVal = topLocal;
        topLocal += typeKind.slotSize();
        return retVal;
    }

    @Override
    public Label breakLabel() {
        return breakLabel;
    }
}