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/*
* Copyright (C) 2024 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/Noncopyable.h>
#include <wtf/Nonmovable.h>
#include <wtf/StdLibExtras.h>
#include <wtf/UniqueRef.h>
namespace WTF {
template<typename OwnerType, typename T>
class LazyUniqueRef {
WTF_MAKE_NONCOPYABLE(LazyUniqueRef);
WTF_MAKE_NONMOVABLE(LazyUniqueRef);
public:
LazyUniqueRef() = default;
template<typename Func>
LazyUniqueRef(const Func& func)
{
initLater(func);
}
typedef T* (*FuncType)(OwnerType&, LazyUniqueRef&);
~LazyUniqueRef()
{
ASSERT(!(m_pointer & initializingTag));
if (m_pointer & lazyTag)
return;
uintptr_t pointer = std::exchange(m_pointer, 0);
if (pointer)
delete std::bit_cast<T*>(pointer);
}
bool isInitialized() const { return !(m_pointer & lazyTag); }
const T& get(const OwnerType& owner) const
{
return const_cast<LazyUniqueRef&>(*this).get(const_cast<OwnerType&>(owner));
}
T& get(OwnerType& owner)
{
ASSERT(m_pointer);
ASSERT(!(m_pointer & initializingTag));
if (UNLIKELY(m_pointer & lazyTag)) {
FuncType func = *std::bit_cast<FuncType*>(m_pointer & ~(lazyTag | initializingTag));
return *func(owner, *this);
}
return *std::bit_cast<T*>(m_pointer);
}
const T* getIfExists() const
{
return const_cast<LazyUniqueRef&>(*this).getIfExists();
}
T* getIfExists()
{
ASSERT(m_pointer);
if (m_pointer & lazyTag)
return nullptr;
return std::bit_cast<T*>(m_pointer);
}
T* ptr(OwnerType& owner) RETURNS_NONNULL { &get(owner); }
T* ptr(const OwnerType& owner) const RETURNS_NONNULL { return &get(owner); }
template<typename Func>
void initLater(const Func&)
{
static_assert(alignof(T) >= 4);
static_assert(isStatelessLambda<Func>());
// Logically we just want to stuff the function pointer into m_pointer, but then we'd be sad
// because a function pointer is not guaranteed to be a multiple of anything. The tag bits
// may be used for things. We address this problem by indirecting through a global const
// variable. The "theFunc" variable is guaranteed to be native-aligned, i.e. at least a
// multiple of 4.
static constexpr FuncType theFunc = &callFunc<Func>;
m_pointer = lazyTag | std::bit_cast<uintptr_t>(&theFunc);
}
void set(UniqueRef<T>&& ref)
{
UniqueRef<T> local = WTFMove(ref);
m_pointer = std::bit_cast<uintptr_t>(local.moveToUniquePtr().release());
}
private:
static const uintptr_t lazyTag = 1;
static const uintptr_t initializingTag = 2;
template<typename Func>
static T* callFunc(OwnerType& owner, LazyUniqueRef& ref)
{
ref.m_pointer |= initializingTag;
callStatelessLambda<void, Func>(owner, ref);
RELEASE_ASSERT(!(ref.m_pointer & lazyTag));
RELEASE_ASSERT(!(ref.m_pointer & initializingTag));
return std::bit_cast<T*>(ref.m_pointer);
}
uintptr_t m_pointer { 0 };
};
} // namespace WTF
using WTF::LazyUniqueRef;
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