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/**
* Copyright (C) 2021-present MongoDB, Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the Server Side Public License, version 1,
* as published by MongoDB, Inc.
*
* This program 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
* Server Side Public License for more details.
*
* You should have received a copy of the Server Side Public License
* along with this program. If not, see
* <http://www.mongodb.com/licensing/server-side-public-license>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the Server Side Public License in all respects for
* all of the code used other than as permitted herein. If you modify file(s)
* with this exception, you may extend this exception to your version of the
* file(s), but you are not obligated to do so. If you do not wish to do so,
* delete this exception statement from your version. If you delete this
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
#pragma once
#include <algorithm>
#include <new>
#include <type_traits>
#include "mongo/stdx/new.h"
namespace mongo {
namespace aligned_detail {
template <size_t step>
constexpr size_t roundUpByStep(size_t value) {
return (((value + (step - 1)) / step) * step);
}
} // namespace aligned_detail
/**
* A wrapper holding a `T` value aligned to `alignof(T)` or
* `minAlign`, whichever is greater (i.e. more strict). The value is
* accessed with a pointer-like syntax. Additionally the object will
* be placed in a buffer no smaller than minStorageSize, of which the
* contained T object may use no more than maxObjectSize bytes.
*/
template <typename T,
size_t minAlign = alignof(T),
size_t minStorageSize = sizeof(T),
size_t maxObjectSize = minStorageSize>
class Aligned {
public:
using value_type = T;
static_assert(sizeof(value_type) <= maxObjectSize);
static constexpr size_t kAlignment = std::max(alignof(value_type), minAlign);
static constexpr size_t kStorageSize = std::max(sizeof(value_type), minStorageSize);
template <typename... As>
explicit Aligned(As&&... args) noexcept(std::is_nothrow_constructible_v<value_type, As&&...>) {
new (_raw()) value_type(std::forward<As>(args)...);
}
Aligned(const Aligned& that) noexcept(std::is_nothrow_copy_constructible_v<value_type>) {
new (_raw()) value_type(*that);
}
Aligned& operator=(const Aligned& that) noexcept(
std::is_nothrow_copy_assignable_v<value_type>) {
if (this != &that)
**this = *that;
return *this;
}
Aligned(Aligned&& that) noexcept(std::is_nothrow_move_constructible_v<value_type>) {
new (_raw()) value_type(std::move(*that));
}
Aligned& operator=(Aligned&& that) noexcept(std::is_nothrow_move_assignable_v<value_type>) {
if (this != &that)
**this = std::move(*that);
return *this;
}
~Aligned() {
_value().~value_type();
}
const value_type& operator*() const& noexcept {
return _value();
}
value_type& operator*() & noexcept {
return _value();
}
value_type&& operator*() && noexcept {
return std::move(_value());
}
const value_type* operator->() const noexcept {
return &_value();
}
value_type* operator->() noexcept {
return &_value();
}
private:
static_assert((minAlign & (minAlign - 1)) == 0, "alignments must be a power of two");
const value_type* _raw() const noexcept {
return reinterpret_cast<const value_type*>(&_storage);
}
value_type* _raw() noexcept {
return reinterpret_cast<value_type*>(&_storage);
}
const value_type& _value() const noexcept {
return *stdx::launder(_raw());
}
value_type& _value() noexcept {
return *stdx::launder(_raw());
}
std::aligned_storage_t<kStorageSize, kAlignment> _storage;
};
/**
* Swap the values. Defined out-of-class to work around
* https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89612
*
* TODO: It should be possible to swap Aligned<T> vs Aligned<U> if T
* and U can be swapped, as well to swap instances with varying
* alignment, padding, etc. However, defining that generic swap
* results in ambiguities.
*/
template <typename T, auto... Pack>
void swap(Aligned<T, Pack...>& a, Aligned<T, Pack...>& b) noexcept(std::is_nothrow_swappable_v<T>) {
using std::swap;
swap(*a, *b);
}
/**
* A `CacheExclusive` object is Aligned to the destructive
* interference size, ensuring that it will start at an address that
* will not exhibit false sharing with any objects that precede it in
* memory. Additionally, the storage for the object is padded to a
* sufficient multiple of the destructive interference size to ensure
* that it will not exhibit false sharing with any other objects that
* follow it in memory. However, it is not assured that the embedded T
* object will internally exhibit true sharing with all of itself, as
* the contained object is permitted to be larger than the
* constructive interference size.
*/
template <typename T>
using CacheExclusive =
Aligned<T,
stdx::hardware_destructive_interference_size,
aligned_detail::roundUpByStep<stdx::hardware_destructive_interference_size>(sizeof(T))>;
/**
* A `CacheCombined` object is Aligned to the constructive
* interference size, ensuring that it will start at an address that
* can exhibit true sharing for some forward extent. Additionally, the
* size of the object is constrained to ensure that all of its state
* is eligible for true sharing within the same region of constructive
* interference. However, there is no guarantee that the object will
* not exhibit false sharing with objects that either precede or
* follow it in memory, unless those objects are themselves protected
* from false sharing.
*/
template <typename T>
using CacheCombined = Aligned<T,
stdx::hardware_constructive_interference_size,
sizeof(T),
stdx::hardware_constructive_interference_size>;
/**
* A `CacheCombinedExclusive` object is Aligned to the destructive
* interference size, ensuring that it will start at an address that
* will not exhibit false sharing with objects that precede it in
* memory. Additionally, the storage for the object is padded to the
* destructive interference size, ensuring that it will not exhibit
* false sharing with objects that follow it. Finally, the size of the
* object is constrained to the constructive interference size,
* ensuring that the object will internally exhibit true sharing.
*/
template <typename T>
using CacheCombinedExclusive = Aligned<T,
stdx::hardware_destructive_interference_size,
stdx::hardware_destructive_interference_size,
stdx::hardware_constructive_interference_size>;
} // namespace mongo
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