atomic/detail/gcc_atomic_memory_order_utils.hpp

0001 /*
0002  * Distributed under the Boost Software License, Version 1.0.
0003  * (See accompanying file LICENSE_1_0.txt or copy at
0004  * http://www.boost.org/LICENSE_1_0.txt)
0005  *
0006  * Copyright (c) 2020 Andrey Semashev
0007  */
0008 /*!
0009  * \file   atomic/detail/gcc_atomic_memory_order_utils.hpp
0010  *
0011  * This header contains utilities for working with gcc atomic memory order constants.
0012  */
0013
0014 #ifndef BOOST_ATOMIC_DETAIL_GCC_ATOMIC_MEMORY_ORDER_UTILS_HPP_INCLUDED_
0015 #define BOOST_ATOMIC_DETAIL_GCC_ATOMIC_MEMORY_ORDER_UTILS_HPP_INCLUDED_
0016
0017 #include <boost/memory_order.hpp>
0018 #include <boost/atomic/detail/config.hpp>
0019 #include <boost/atomic/detail/header.hpp>
0020
0021 #ifdef BOOST_HAS_PRAGMA_ONCE
0022 #pragma once
0023 #endif
0024
0025 namespace boost {
0026 namespace atomics {
0027 namespace detail {
0028
0029 /*!
0030  * The function converts \c boost::memory_order values to the compiler-specific constants.
0031  *
0032  * NOTE: The intention is that the function is optimized away by the compiler, and the
0033  *       compiler-specific constants are passed to the intrinsics. Unfortunately, constexpr doesn't
0034  *       work in this case because the standard atomics interface require memory ordering
0035  *       constants to be passed as function arguments, at which point they stop being constexpr.
0036  *       However, it is crucial that the compiler sees constants and not runtime values,
0037  *       because otherwise it just ignores the ordering value and always uses seq_cst.
0038  *       This is the case with Intel C++ Compiler 14.0.3 (Composer XE 2013 SP1, update 3) and
0039  *       gcc 4.8.2. Intel Compiler issues a warning in this case:
0040  *
0041  *       warning #32013: Invalid memory order specified. Defaulting to seq_cst memory order.
0042  *
0043  *       while gcc acts silently.
0044  *
0045  *       To mitigate the problem ALL functions, including the atomic<> members must be
0046  *       declared with BOOST_FORCEINLINE. In this case the compilers are able to see that
0047  *       all functions are called with constant orderings and call intrinstcts properly.
0048  *
0049  *       Unfortunately, this still doesn't work in debug mode as the compiler doesn't
0050  *       propagate constants even when functions are marked with BOOST_FORCEINLINE. In this case
0051  *       all atomic operaions will be executed with seq_cst semantics.
0052  */
0053 BOOST_FORCEINLINE BOOST_CONSTEXPR int convert_memory_order_to_gcc(memory_order order) BOOST_NOEXCEPT
0054 {
0055     return (order == memory_order_relaxed ? __ATOMIC_RELAXED : (order == memory_order_consume ? __ATOMIC_CONSUME :
0056         (order == memory_order_acquire ? __ATOMIC_ACQUIRE : (order == memory_order_release ? __ATOMIC_RELEASE :
0057         (order == memory_order_acq_rel ? __ATOMIC_ACQ_REL : __ATOMIC_SEQ_CST)))));
0058 }
0059
0060 } // namespace detail
0061 } // namespace atomics
0062 } // namespace boost
0063
0064 #include <boost/atomic/detail/footer.hpp>
0065
0066 #endif // BOOST_ATOMIC_DETAIL_GCC_ATOMIC_MEMORY_ORDER_UTILS_HPP_INCLUDED_