libcruft-util/parallel/queue.hpp

/*
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
 *
 * Copyright 2018 Danny Robson <danny@nerdcruft.net>
 */

#ifndef CRUFT_UTIL_PARALLEL_QUEUE_HPP
#define CRUFT_UTIL_PARALLEL_QUEUE_HPP

#include "../maths.hpp"

#include <atomic>
#include <type_traits>


namespace cruft::parallel {
    /// A statically sized thread-safe, lock-free, multi-producer
    /// multi-consumer queue for trivial types.
    ///
    /// \tparam ValueT the type of values to store
    /// \tparam SizeV the number of elements in the underlying store
    template <typename ValueT, uint32_t SizeV = 32>
    class queue {
    public:
        // We optimise specifically for memcpyable types only
        static_assert (
            std::is_trivially_destructible_v<ValueT> && std::is_trivially_copyable_v<ValueT>
        );

        // We need to wrap indices to the bounds of the data array quite often.
        // If it's a power of two then we can use bitmasks to avoid the
        // modulus.
        static_assert (cruft::is_pow2 (SizeV));

        // We need at least 2 data slots; one for read, and one for write
        // pointers. 4 gives a little safety margin for future optimisations.
        static_assert (SizeV >= 4);

        queue ():
            m_write (0),
            m_read { {0}, {0} }
        { ; }


        static constexpr auto capacity (void) { return SizeV; }


        /// attempts to push a value to the queue
        ///
        /// algorithm proceeds as follows:
        ///     * check if there appears to be space
        ///     * increment the write pointer
        ///     * serialise the data
        ///     * update the high read pointer
        ///
        /// if updating the write pointer fails we bail and return false.
        /// else, we will be able to commit the data and return true.
        bool
        push [[nodiscard]] (const ValueT &val)
        {
            do {
                // find our current and next indices, checking there is
                // actually room available to store. if not, retry.
                uint32_t const curr = m_write;
                uint32_t const next = curr + 1;

                // we _must_ wrap the indices here to account for the
                // following case:
                //
                // |lo--------wr|
                //
                // wr's increment won't be arithmetically equal to lo, but
                // they have equivalent wrapped indices.
                if (wrap (next) == wrap (m_read.lo))
                    return false;

                // try to bump the write cursor to claim that slot
                if (uint32_t orig = curr; !m_write.compare_exchange_weak (orig, next)) {
                    asm volatile ("pause;");
                    continue;
                }

                // write the data to the buffer
                m_data[wrap (curr)] = val;

                // keep trying to bump the new high read pointer. if it
                // doesnt succeed then it means that another write is pending
                // and should be realised _very_ soon, so spin on this.
                do {
                    uint32_t orig = curr;
                    if (m_read.hi.compare_exchange_weak (orig, next))
                        break;
                    asm volatile ("pause;");
                } while (1);

                return true;
            } while (1);
        }


        /// attempts to pop a value off the queue into the provided reference
        ///
        /// algorithm proceeds as follows:
        ///     * check if there appears to be space
        ///     * optimistically read the data
        ///     * update the read pointer just past this slot
        ///
        /// if any part of the operation fails then we bail and return false.
        /// importantly: the contents of the provided reference may have been
        /// overwritten, but should not be used unless true was returned.
        bool
        pop [[nodiscard]] (ValueT &out)
        {
            do
            {
                // find the current and next read indices. if there aren't
                // any read slots available then retry.
                uint32_t curr = m_read.lo;
                uint32_t next = curr + 1;

                if (wrap (curr) == wrap (m_read.hi))
                    return false;

                // optimistically read the data into the destination pointer
                out = m_data[wrap (curr)];

                // try to bump the read pointer past the entry we just read.
                // if this succeeds then indicate success, else retry the
                // entire sequence.
                if (uint32_t orig = curr; m_read.lo.compare_exchange_weak (orig, next))
                    return true;

                asm volatile ("pause;");
            } while (1);
        }


    private:
        ValueT m_data[SizeV];

        static constexpr uint32_t
        wrap (uint32_t idx)
        {
            return idx % SizeV;
        }

        /// all indices are not wrapped to the data size. instead they are
        /// allowed to overflow. it all works out the same in the end.
        std::atomic<uint32_t> m_write;

        struct {
            std::atomic<uint32_t> hi;
            std::atomic<uint32_t> lo;
        } m_read;
    };
}

#endif