JDK14/Java14源码在线阅读

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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * 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.
 *
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package jdk.nashorn.internal.ir;

import jdk.nashorn.internal.codegen.types.Type;

/**
 * Class describing one or more local variable conversions that needs to be performed on entry to a control flow join
 * point. Note that the class is named as a singular "Conversion" and not a plural "Conversions", but instances of the
 * class have a reference to the next conversion, so multiple conversions are always represented with a single instance
 * that is a head of a linked list of instances.
 * @see JoinPredecessor
 */
public final class LocalVariableConversion {
    private final Symbol symbol;
    // TODO: maybe introduce a type pair class? These will often be repeated.
    private final Type from;
    private final Type to;
    private final LocalVariableConversion next;

    /**
     * Creates a new object representing a local variable conversion.
     * @param symbol the symbol representing the local variable whose value is being converted.
     * @param from the type value is being converted from.
     * @param to the type value is being converted to.
     * @param next next conversion at the same join point, if any (the conversion object implements a singly-linked
     * list of conversions).
     */
    public LocalVariableConversion(final Symbol symbol, final Type from, final Type to, final LocalVariableConversion next) {
        this.symbol = symbol;
        this.from = from;
        this.to = to;
        this.next = next;
    }

    /**
     * Returns the type being converted from.
     * @return the type being converted from.
     */
    public Type getFrom() {
        return from;
    }

    /**
     * Returns the type being converted to.
     * @return the type being converted to.
     */
    public Type getTo() {
        return to;
    }

    /**
     * Returns the next conversion at the same join point, or null if this is the last one.
     * @return the next conversion at the same join point.
     */
    public LocalVariableConversion getNext() {
        return next;
    }

    /**
     * Returns the symbol representing the local variable whose value is being converted.
     * @return the symbol representing the local variable whose value is being converted.
     */
    public Symbol getSymbol() {
        return symbol;
    }

    /**
     * Returns true if this conversion is live. A conversion is live if the symbol has a slot for the conversion's
     * {@link #getTo() to} type. If a conversion is dead, it can be omitted in code generator.
     * @return true if this conversion is live.
     */
    public boolean isLive() {
        return symbol.hasSlotFor(to);
    }

    /**
     * Returns true if this conversion {@link #isLive()}, or if any of its {@link #getNext()} conversions are live.
     * @return true if this conversion, or any conversion following it, are live.
     */
    public boolean isAnyLive() {
        return isLive() || isAnyLive(next);
    }

    /**
     * Returns true if the passed join predecessor has {@link #isAnyLive()} conversion.
     * @param jp the join predecessor being examined.
     * @return true if the join predecessor conversion is not null and {@link #isAnyLive()}.
     */
    public static boolean hasLiveConversion(final JoinPredecessor jp) {
        return isAnyLive(jp.getLocalVariableConversion());
    }

    /**
     * Returns true if the passed conversion is not null, and it {@link #isAnyLive()}.
     * @parameter conv the conversion being tested for liveness.
     * @return true if the conversion is not null and {@link #isAnyLive()}.
     */
    private static boolean isAnyLive(final LocalVariableConversion conv) {
        return conv != null && conv.isAnyLive();
    }

    @Override
    public String toString() {
        return toString(new StringBuilder()).toString();
    }

    /**
     * Generates a string representation of this conversion in the passed string builder.
     * @param sb the string builder in which to generate a string representation of this conversion.
     * @return the passed in string builder.
     */
    public StringBuilder toString(final StringBuilder sb) {
        if(isLive()) {
            return toStringNext(sb.append('\u27e6'), true).append("\u27e7 ");
        }
        return next == null ? sb : next.toString(sb);
    }

    /**
     * Generates a string representation of the passed conversion in the passed string builder.
     * @param conv the conversion to render in the string builder.
     * @param sb the string builder in which to generate a string representation of this conversion.
     * @return the passed in string builder.
     */
    public static StringBuilder toString(final LocalVariableConversion conv, final StringBuilder sb) {
        return conv == null ? sb : conv.toString(sb);
    }

    private StringBuilder toStringNext(final StringBuilder sb, final boolean first) {

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