Source/WTF/wtf/RefCountedWithInlineWeakPtr.h

/*
 * Copyright (C) 2025 Apple Inc. All rights reserved.
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Library General Public
 * License as published by the Free Software Foundation; either
 * version 2 of the License, or (at your option) any later version.
 *
 * This library 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
 * Library General Public License for more details.
 *
 * You should have received a copy of the GNU Library General Public License
 * along with this library; see the file COPYING.LIB.  If not, write to
 * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
 * Boston, MA 02110-1301, USA.
 *
 */

#pragma once

#include <wtf/RefCountDebugger.h>

namespace WTF {

// This base class holds the non-template methods and attributes.
// The RefCountedWithInlineWeakPtr class inherits from it reducing the template bloat
// generated by the compiler (technique called template hoisting).
class WTF_EMPTY_BASE_CLASS RefCountedWithInlineWeakPtrBase {
public:
    void ref() const
    {
        m_refCountDebugger.willRef(m_strongCount);
        ++m_strongCount;
    }

    bool hasOneRef() const { return m_strongCount == 1; }
    unsigned refCount() const { return m_strongCount; }

    void weakRef() const { ++m_weakCount; }

    RefCountDebugger& refCountDebugger() const { return const_cast<RefCountDebugger&>(m_refCountDebugger); }

protected:
    RefCountedWithInlineWeakPtrBase() = default;

    ~RefCountedWithInlineWeakPtrBase()
    {
        m_refCountDebugger.willDestroy(m_strongCount);
        RELEASE_ASSERT(m_strongCount == 1);
        // Use volatile to prevent compilers from eliminating this code. <https://webkit.org/b/304549>
        *static_cast<volatile unsigned*>(&m_strongCount) = 0;
    }

    // Returns true if the pointer should be destroyed.
    bool derefBase() const
    {
        m_refCountDebugger.willDeref(m_strongCount);

        if (m_strongCount != 1) {
            --m_strongCount;
            return false;
        }

        m_refCountDebugger.willDelete();
        return true;
    }

    // Returns true if the pointer should be freed.
    bool weakDerefBase() const
    {
        // FIXME: The macros allow builds to succeed, but a proper fix would better; see https://bugs.webkit.org/show_bug.cgi?id=318247
        IGNORE_GCC_WARNINGS_BEGIN("uninitialized")
        if (m_weakCount != 1) {
            --m_weakCount;
            return false;
        }
        IGNORE_GCC_WARNINGS_END

        return true;
    }

private:
    mutable unsigned m_strongCount { 1 };
    mutable unsigned m_weakCount { 1 }; // The strong counts collectively share one weak count.
    NO_UNIQUE_ADDRESS RefCountDebugger m_refCountDebugger;
};

template<typename T> class RefCountedWithInlineWeakPtr : public RefCountedWithInlineWeakPtrBase {
    WTF_MAKE_NONCOPYABLE(RefCountedWithInlineWeakPtr);
    WTF_DEPRECATED_MAKE_FAST_ALLOCATED(RefCountedWithInlineWeakPtr);
public:
    RefCountedWithInlineWeakPtr() = default;

    void deref() const
    {
        if (!derefBase()) [[likely]]
            return;
        derefSlowCase();
    }

    void weakDeref() const
    {
        if (!weakDerefBase()) [[likely]]
            return;
        weakDerefSlowCase();
    }

private:
    void derefSlowCase() const;
    void weakDerefSlowCase() const;
} SWIFT_RETURNED_AS_UNRETAINED_BY_DEFAULT;

template<typename T> void RefCountedWithInlineWeakPtr<T>::derefSlowCase() const
{
    const_cast<T*>(static_cast<const T*>(this))->~T();
    weakDeref();
}

template<typename T> void RefCountedWithInlineWeakPtr<T>::weakDerefSlowCase() const
{
    T::operator delete(const_cast<T*>(static_cast<const T*>(this)));
}

inline void adopted(RefCountedWithInlineWeakPtrBase* object)
{
    if (!object)
        return;
    object->refCountDebugger().adopted();
}

} // namespace WTF

using WTF::RefCountedWithInlineWeakPtr;