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// Copyright Yahoo. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
#pragma once
#include <atomic>
#include <type_traits>
#include <version>
/**
* Utility functions for single value atomic memory accesses.
*
* store/load_ref_* functions can be used to provide well-defined atomic
* memory access to memory locations that aren't explicitly wrapped in std::atomic
* objects. In this case, all potentially racing loads/stores _must_ be through
* atomic utility functions (or atomic_ref).
*
* Non-ref store/load_* functions are just syntactic sugar to make code using
* atomics more readable, but additionally adds sanity checks that all atomics
* are always lock-free.
*/
namespace vespalib::atomic {
//
// std::atomic_ref<T> helpers
//
namespace detail {
template <typename T> struct is_std_atomic : std::false_type {};
template <typename T> struct is_std_atomic<std::atomic<T>> : std::true_type {};
template <typename T> inline constexpr bool is_std_atomic_v = is_std_atomic<T>::value;
}
// TODO can generalize atomic_ref code once no special casing is needed
template <typename T1, typename T2>
constexpr void store_ref_relaxed(T1& lhs, T2&& v) noexcept {
static_assert(!detail::is_std_atomic_v<T1>, "atomic ref function invoked with a std::atomic, probably not intended");
#if __cpp_lib_atomic_ref
static_assert(std::atomic_ref<T1>::is_always_lock_free);
std::atomic_ref<T1>(lhs).store(std::forward<T2>(v), std::memory_order_relaxed);
#else
// TODO replace with compiler intrinsic
lhs = std::forward<T2>(v);
#endif
}
template <typename T1, typename T2>
constexpr void store_ref_release(T1& lhs, T2&& v) noexcept {
static_assert(!detail::is_std_atomic_v<T1>, "atomic ref function invoked with a std::atomic, probably not intended");
#if __cpp_lib_atomic_ref
static_assert(std::atomic_ref<T1>::is_always_lock_free);
std::atomic_ref<T1>(lhs).store(std::forward<T2>(v), std::memory_order_release);
#else
// TODO replace with compiler intrinsic
lhs = std::forward<T2>(v);
std::atomic_thread_fence(std::memory_order_release);
#endif
}
template <typename T1, typename T2>
constexpr void store_ref_seq_cst(T1& lhs, T2&& v) noexcept {
static_assert(!detail::is_std_atomic_v<T1>, "atomic ref function invoked with a std::atomic, probably not intended");
#if __cpp_lib_atomic_ref
static_assert(std::atomic_ref<T1>::is_always_lock_free);
std::atomic_ref<T1>(lhs).store(std::forward<T2>(v), std::memory_order_seq_cst);
#else
// TODO replace with compiler intrinsic
lhs = std::forward<T2>(v);
std::atomic_thread_fence(std::memory_order_seq_cst);
#endif
}
template <typename T>
[[nodiscard]] constexpr T load_ref_relaxed(const T& a) noexcept {
static_assert(!detail::is_std_atomic_v<T>, "atomic ref function invoked with a std::atomic, probably not intended");
#if __cpp_lib_atomic_ref
static_assert(std::atomic_ref<const T>::is_always_lock_free);
return std::atomic_ref<const T>(a).load(std::memory_order_relaxed);
#else
// TODO replace with compiler intrinsic
return a;
#endif
}
template <typename T>
[[nodiscard]] constexpr T load_ref_acquire(const T& a) noexcept {
static_assert(!detail::is_std_atomic_v<T>, "atomic ref function invoked with a std::atomic, probably not intended");
#if __cpp_lib_atomic_ref
static_assert(std::atomic_ref<const T>::is_always_lock_free);
return std::atomic_ref<const T>(a).load(std::memory_order_acquire);
#else
// TODO replace with compiler intrinsic
std::atomic_thread_fence(std::memory_order_acquire);
return a;
#endif
}
template <typename T>
[[nodiscard]] constexpr T load_ref_seq_cst(const T& a) noexcept {
static_assert(!detail::is_std_atomic_v<T>, "atomic ref function invoked with a std::atomic, probably not intended");
#if __cpp_lib_atomic_ref
static_assert(std::atomic_ref<const T>::is_always_lock_free);
return std::atomic_ref<const T>(a).load(std::memory_order_seq_cst);
#else
// TODO replace with compiler intrinsic
std::atomic_thread_fence(std::memory_order_seq_cst);
return a;
#endif
}
//
// std::atomic<T> helpers
//
template <typename T1, typename T2>
constexpr void store_relaxed(std::atomic<T1>& lhs, T2&& v) noexcept {
static_assert(std::atomic<T1>::is_always_lock_free);
lhs.store(std::forward<T2>(v), std::memory_order_relaxed);
}
template <typename T1, typename T2>
constexpr void store_release(std::atomic<T1>& lhs, T2&& v) noexcept {
static_assert(std::atomic<T1>::is_always_lock_free);
lhs.store(std::forward<T2>(v), std::memory_order_release);
}
template <typename T1, typename T2>
constexpr void store_seq_cst(std::atomic<T1>& lhs, T2&& v) noexcept {
static_assert(std::atomic<T1>::is_always_lock_free);
lhs.store(std::forward<T2>(v), std::memory_order_seq_cst);
}
template <typename T>
[[nodiscard]] constexpr T load_relaxed(const std::atomic<T>& a) noexcept {
static_assert(std::atomic<T>::is_always_lock_free);
return a.load(std::memory_order_relaxed);
}
template <typename T>
[[nodiscard]] constexpr T load_acquire(const std::atomic<T>& a) noexcept {
static_assert(std::atomic<T>::is_always_lock_free);
return a.load(std::memory_order_acquire);
}
template <typename T>
[[nodiscard]] constexpr T load_seq_cst(const std::atomic<T>& a) noexcept {
static_assert(std::atomic<T>::is_always_lock_free);
return a.load(std::memory_order_seq_cst);
}
} // vespalib::atomic
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