libcruft-util/pool.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 2011-2019 Danny Robson <danny@nerdcruft.net>
 */

#pragma once

#include "debug/assert.hpp"
#include "cast.hpp"

#include <atomic>
#include <new>

#include <cstddef>
#include <cstdint>

namespace cruft {
    /// a simple pre-allocated pool for storage of PODs.
    ///
    /// non-POD types can be stored, but there are no guarantees for calling
    /// item destructors at pool destruction time.
    template <typename T>
    class pool {
    protected:
        union node;

        union node {
            alignas(node*) std::atomic<node*> next;
            alignas(node*) node* raw;
            alignas(T)     char data[sizeof(T)];
        };

        static_assert (std::atomic<node*>::is_always_lock_free);

        // root address of allocation. used in deletion at destruction time.
        node* m_head;

        // the next available entry in the linked list
        std::atomic<node *> m_next;

        // the total number of items that could be stored
        std::size_t m_capacity;

        // the number of items currently stored.
        std::atomic<size_t> m_size;

    public:
        pool (const pool&) = delete;
        pool& operator= (const pool&) = delete;

        pool (pool &&rhs) noexcept
            : m_head (nullptr)
            , m_next (nullptr)
            , m_capacity (0)
            , m_size (0)
        {
            std::swap (m_head, rhs.m_head);

            m_next = rhs.m_next.load ();
            rhs.m_next = nullptr;

            std::swap (m_capacity, rhs.m_capacity);

            m_size = rhs.m_size.load ();
            rhs.m_size = 0;
        }

        pool& operator= (pool&&);

        explicit
        pool (std::size_t _capacity):
            m_capacity (_capacity),
            m_size (0u)
        {
            // allocate the memory and note the base address for deletion in destructor
            m_next = m_head = new node[m_capacity];

            relink ();
        }

        ~pool ()
        {
            clear ();

            // don't check if everything's been returned as pools are often used
            // for PODs which don't need to be destructed via calling release.
            delete [] m_head;
        }


        // Data management
        [[nodiscard]] T*
        allocate [[gnu::malloc]] [[gnu::returns_nonnull]] (void)
        {
            // double check we have enough capacity left
            if (!m_next)
                throw std::bad_alloc ();
            CHECK_LT (m_size, m_capacity);

            // unlink the current cursor
            do {
                node* curr = m_next;
                node* soon = curr->next;

                if (m_next.compare_exchange_weak (curr, soon)) {
                    ++m_size;
                    return std::launder (cruft::cast::alignment<T*> (curr));
                }
            } while (1);
        }

        void
        deallocate (T *base)
        {
            auto soon = cruft::cast::alignment<node*> (base);

            do {
                node *curr = m_next;
                soon->next = curr;
                if (m_next.compare_exchange_weak (curr, soon)) {
                    --m_size;
                    return;
                }
            } while (1);
        }


        template <typename ...Args>
        T*
        construct (Args &&...args)
        {
            auto ptr = allocate ();
            try {
                return new (ptr) T (std::forward<Args> (args)...);
            } catch (...) {
                deallocate (ptr);
                throw;
            }
        }


        void
        destroy (T *ptr)
        {
            ptr->~T ();
            deallocate (ptr);
        }


        void destroy (size_t idx)
        {
            return destroy (&(*this)[idx]);
        }


        auto capacity (void) const { return m_capacity; }
        auto size     (void) const { return m_size.load (); }
        bool empty    (void) const { return size () == 0; }
        bool full     (void) const { return size () == capacity (); }


        /// Destroys all objects that have been allocated, frees the
        /// associated memory, and then rebuilds the free node list ready for
        /// allocations again.
        ///
        /// NOTE: All bets are off if any object throws an exception out of
        /// their destructor. We provide no exception guarantees.
        ///
        /// This call is NOT thread safe. No users should be accessing this
        /// object for the duration of this call.
        void clear (void)
        {
            // Create a fake root so that we can always point to the parent
            // of every node in the system. Hopefully this isn't too large for
            // the stack.
            node container;
            container.next.store (m_next.load ());

            // Sort the node list. We walk the list, and at each step reparent
            // the child at the lowest memory address to the cursor.
            for (node* start = container.raw; start; start = start->raw) {
                node* parent = start;

                // Find the node whose child is the lowest pointer
                int count = 0;
                for (auto cursor = parent; cursor->raw; cursor = cursor->raw) {
                    ++count;
                    CHECK_NEQ (cursor->raw, start);
                    if (cursor->raw < parent)
                        parent = cursor;
                }

                // Parent the lowest child to the start of the sorted list
                auto tmp = start->raw;
                start->raw = parent->raw;

                // Remove the lowest child from their old parent
                auto parent_next = parent->raw;
                parent->raw = parent_next ? parent_next->raw : nullptr;

                // Parent the old successor of the start to the lowest child
                start->raw = tmp;
            }

            // Now that we've ordered the nodes we can walk the list from
            // start to finish and find nodes that aren't in the free list.
            // Call the destructors on the data contained in these.
            auto node_cursor = m_next.load (std::memory_order_relaxed);
            auto data_cursor = m_head;

            while (node_cursor) {
                while (data_cursor < node_cursor) {
                    cruft::cast::alignment<T*> (data_cursor->data)->~T ();
                    ++data_cursor;
                }

                node_cursor = node_cursor->raw;
                ++data_cursor;
            }

            while (data_cursor < m_head + m_capacity) {
                cruft::cast::alignment<T*> (data_cursor->data)->~T ();
                ++data_cursor;
            }

            relink ();
        }

    private:
        void relink (void)
        {
            // Reset the allocation cursor to point to the start of the region
            m_next = m_head;

            // build out the linked list from all the nodes.
            for (size_t i = 0; i < m_capacity - 1; ++i)
                m_next[i].next = m_next + i + 1;
            m_next[m_capacity - 1].next = nullptr;
        }

    public:
        // Indexing
        size_t index (T const *ptr) const
        {
            CHECK_LIMIT (cruft::cast::alignment<node const*> (ptr), m_head, m_head + m_capacity);
            return cruft::cast::alignment<node const*> (ptr) - m_head;
        }


        /// returns the base address of the allocation.
        ///
        /// guaranteed to point to the first _possible_ allocated value;
        /// however it may not be _live_ at any given moment.
        ///
        /// DO NOT use this pointer for indexing as you will be unable to
        /// account for internal node sizes, alignment, or padding.
        void      * base (void)      & { return m_head; }
        void const* base (void) const& { return m_head; }

        T& operator[] (size_t idx) &
        {
            CHECK_LIMIT (idx, 0u, capacity ());
            return *cruft::cast::alignment<T*> (&m_head[idx].data[0]);
        }


        T const& operator[] (size_t idx) const&
        {
            CHECK_LIMIT (idx, 0u, capacity ());
            return *cruft::cast::alignment<T const*> (&m_head[idx].data[0]);
        }
    };
}