Source/JavaScriptCore/runtime/CacheableIdentifier.h

/*
 * Copyright (C) 2020 Apple Inc. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
 * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
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#pragma once

#include "JSCJSValue.h"

namespace WTF {

class PrintStream;
class UniquedStringImpl;

} // namespace WTF

using WTF::PrintStream;
using WTF::UniquedStringImpl;

namespace JSC {

class Identifier;
class JSCell;
class SlotVisitor;

class CacheableIdentifier {
public:
    CacheableIdentifier() = default;
    inline CacheableIdentifier(const Identifier&);
    inline CacheableIdentifier(JSCell* identifier);

    CacheableIdentifier(const CacheableIdentifier&) = default;
    CacheableIdentifier(CacheableIdentifier&&) = default;

    CacheableIdentifier(std::nullptr_t)
        : m_bits(0)
    { }

    bool isUid() const { return m_bits & s_uidTag; }
    bool isCell() const { return !isUid(); }
    inline bool isSymbolCell() const;
    inline bool isStringCell() const;

    bool isSymbol() const { return m_bits && uid()->isSymbol(); }

    inline JSCell* cell() const;
    UniquedStringImpl* uid() const;

    explicit operator bool() const { return m_bits; }

    CacheableIdentifier& operator=(const CacheableIdentifier&) = default;
    CacheableIdentifier& operator=(CacheableIdentifier&&) = default;

    bool operator==(const CacheableIdentifier& other) const;
    bool operator!=(const CacheableIdentifier& other) const;
    bool operator==(const Identifier& other) const;

    static inline bool isCacheableIdentifierCell(JSCell*);
    static inline bool isCacheableIdentifierCell(JSValue);

    inline void visitAggregate(SlotVisitor&) const;

    JS_EXPORT_PRIVATE void dump(PrintStream&) const;

private:
    inline void setCellBits(JSCell*);
    inline void setUidBits(UniquedStringImpl*);

    // CacheableIdentifier can either hold a cell pointer or a uid. To discern which
    // it is holding, we tag the low bit if we have a uid. We choose to tag the uid
    // instead of the cell because this keeps the bits of the cell pointer form
    // unpolluted, and therefore, it can be scanned by our conservative GC to keep the
    // cell alive when the CacheableIdentifier is on the stack.
    static constexpr uintptr_t s_uidTag = 1;
    uintptr_t m_bits;
};

} // namespace JSC