Source/WTF/wtf/CagedPtr.h

/*
 * Copyright (C) 2017 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. AND ITS CONTRIBUTORS ``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 ITS 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 PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
 * THE POSSIBILITY OF SUCH DAMAGE.
 */

#pragma once

#include <wtf/DumbPtrTraits.h>
#include <wtf/Gigacage.h>
#include <wtf/PtrTag.h>

#include <climits>

namespace WTF {

constexpr bool tagCagedPtr = true;

template<Gigacage::Kind passedKind, typename T, bool shouldTag = false, typename PtrTraits = DumbPtrTraits<T>>
class CagedPtr {
public:
    static constexpr Gigacage::Kind kind = passedKind;
    static constexpr unsigned numberOfPACBits = 25;
    static constexpr uintptr_t nonPACBitsMask = (1ull << ((sizeof(T*) * CHAR_BIT) - numberOfPACBits)) - 1;

    CagedPtr() : CagedPtr(nullptr) { }
    CagedPtr(std::nullptr_t)
        : m_ptr(shouldTag ? tagArrayPtr<T>(nullptr, 0) : nullptr)
    { }

    CagedPtr(T* ptr, unsigned size)
        : m_ptr(shouldTag ? tagArrayPtr(ptr, size) : ptr)
    { }

    T* get(unsigned size) const
    {
        ASSERT(m_ptr);
        T* ptr = PtrTraits::unwrap(m_ptr);
        T* cagedPtr = Gigacage::caged(kind, ptr);
        T* untaggedPtr = shouldTag ? untagArrayPtr(mergePointers(ptr, cagedPtr), size) : cagedPtr;
        return untaggedPtr;
    }

    T* getMayBeNull(unsigned size) const
    {
        T* ptr = PtrTraits::unwrap(m_ptr);
        if (!removeArrayPtrTag(ptr))
            return nullptr;
        T* cagedPtr = Gigacage::caged(kind, ptr);
        T* untaggedPtr = shouldTag ? untagArrayPtr(mergePointers(ptr, cagedPtr), size) : cagedPtr;
        return untaggedPtr;
    }

    T* getUnsafe() const
    {
        T* ptr = PtrTraits::unwrap(m_ptr);
        ptr = shouldTag ? removeArrayPtrTag(ptr) : ptr;
        return Gigacage::cagedMayBeNull(kind, ptr);
    }

    // We need the template here so that the type of U is deduced at usage time rather than class time. U should always be T.
    template<typename U = T>
    typename std::enable_if<!std::is_same<void, U>::value, T>::type&
    /* T& */ at(unsigned index, unsigned size) const { return get(size)[index]; }

    void recage(unsigned oldSize, unsigned newSize)
    {
        auto ptr = get(oldSize);
        ASSERT(ptr == getUnsafe());
        *this = CagedPtr(ptr, newSize);
    }

    CagedPtr(CagedPtr& other)
        : m_ptr(other.m_ptr)
    {
    }

    CagedPtr& operator=(const CagedPtr& ptr)
    {
        m_ptr = ptr.m_ptr;
        return *this;
    }

    CagedPtr(CagedPtr&& other)
        : m_ptr(PtrTraits::exchange(other.m_ptr, nullptr))
    {
    }

    CagedPtr& operator=(CagedPtr&& ptr)
    {
        m_ptr = PtrTraits::exchange(ptr.m_ptr, nullptr);
        return *this;
    }

    bool operator==(const CagedPtr& other) const
    {
        bool result = m_ptr == other.m_ptr;
        ASSERT(result == (getUnsafe() == other.getUnsafe()));
        return result;
    }
    
    bool operator!=(const CagedPtr& other) const
    {
        return !(*this == other);
    }
    
    explicit operator bool() const
    {
        return getUnsafe() != nullptr;
    }
    
protected:
    static inline T* mergePointers(T* sourcePtr, T* cagedPtr)
    {
#if CPU(ARM64E)
        return reinterpret_cast<T*>((reinterpret_cast<uintptr_t>(sourcePtr) & ~nonPACBitsMask) | (reinterpret_cast<uintptr_t>(cagedPtr) & nonPACBitsMask));
#else
        UNUSED_PARAM(sourcePtr);
        return cagedPtr;
#endif
    }

    typename PtrTraits::StorageType m_ptr;
};

} // namespace WTF

using WTF::CagedPtr;
using WTF::tagCagedPtr;