277 lines
8.3 KiB
C++
277 lines
8.3 KiB
C++
/*
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* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/.
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*
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* Copyright 2011-2019 Danny Robson <danny@nerdcruft.net>
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*/
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#pragma once
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#include "debug/assert.hpp"
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#include "cast.hpp"
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#include <atomic>
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#include <new>
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#include <cstddef>
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#include <cstdint>
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namespace cruft {
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/// a simple pre-allocated pool for storage of PODs.
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///
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/// non-POD types can be stored, but there are no guarantees for calling
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/// item destructors at pool destruction time.
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template <typename T>
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class pool {
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protected:
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union node;
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union node {
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alignas(node*) std::atomic<node*> next;
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alignas(node*) node* raw;
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alignas(T) char data[sizeof(T)];
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};
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static_assert (std::atomic<node*>::is_always_lock_free);
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// root address of allocation. used in deletion at destruction time.
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node* m_head;
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// the next available entry in the linked list
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std::atomic<node *> m_next;
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// the total number of items that could be stored
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std::size_t m_capacity;
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// the number of items currently stored.
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std::atomic<size_t> m_size;
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public:
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pool (const pool&) = delete;
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pool& operator= (const pool&) = delete;
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pool (pool &&rhs) noexcept
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: m_head (nullptr)
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, m_next (nullptr)
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, m_capacity (0)
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, m_size (0)
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{
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std::swap (m_head, rhs.m_head);
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m_next = rhs.m_next.load ();
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rhs.m_next = nullptr;
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std::swap (m_capacity, rhs.m_capacity);
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m_size = rhs.m_size.load ();
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rhs.m_size = 0;
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}
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pool& operator= (pool&&);
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explicit
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pool (std::size_t _capacity):
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m_capacity (_capacity),
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m_size (0u)
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{
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// allocate the memory and note the base address for deletion in destructor
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m_next = m_head = new node[m_capacity];
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relink ();
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}
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~pool ()
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{
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clear ();
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// don't check if everything's been returned as pools are often used
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// for PODs which don't need to be destructed via calling release.
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delete [] m_head;
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}
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// Data management
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[[nodiscard]] T*
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allocate [[gnu::malloc]] [[gnu::returns_nonnull]] (void)
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{
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// double check we have enough capacity left
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if (!m_next)
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throw std::bad_alloc ();
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CHECK_LT (m_size, m_capacity);
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// unlink the current cursor
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do {
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node* curr = m_next;
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node* soon = curr->next;
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if (m_next.compare_exchange_weak (curr, soon)) {
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++m_size;
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return std::launder (cruft::cast::alignment<T*> (curr));
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}
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} while (1);
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}
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void
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deallocate (T *base)
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{
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auto soon = cruft::cast::alignment<node*> (base);
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do {
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node *curr = m_next;
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soon->next = curr;
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if (m_next.compare_exchange_weak (curr, soon)) {
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--m_size;
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return;
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}
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} while (1);
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}
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template <typename ...Args>
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T*
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construct (Args &&...args)
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{
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auto ptr = allocate ();
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try {
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return new (ptr) T (std::forward<Args> (args)...);
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} catch (...) {
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deallocate (ptr);
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throw;
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}
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}
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void
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destroy (T *ptr)
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{
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ptr->~T ();
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deallocate (ptr);
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}
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void destroy (size_t idx)
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{
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return destroy (&(*this)[idx]);
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}
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auto capacity (void) const { return m_capacity; }
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auto size (void) const { return m_size.load (); }
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bool empty (void) const { return size () == 0; }
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bool full (void) const { return size () == capacity (); }
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/// Destroys all objects that have been allocated, frees the
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/// associated memory, and then rebuilds the free node list ready for
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/// allocations again.
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///
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/// NOTE: All bets are off if any object throws an exception out of
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/// their destructor. We provide no exception guarantees.
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///
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/// This call is NOT thread safe. No users should be accessing this
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/// object for the duration of this call.
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void clear (void)
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{
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// Create a fake root so that we can always point to the parent
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// of every node in the system. Hopefully this isn't too large for
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// the stack.
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node container;
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container.next.store (m_next.load ());
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// Sort the node list. We walk the list, and at each step reparent
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// the child at the lowest memory address to the cursor.
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for (node* start = container.raw; start; start = start->raw) {
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node* parent = start;
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// Find the node whose child is the lowest pointer
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int count = 0;
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for (auto cursor = parent; cursor->raw; cursor = cursor->raw) {
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++count;
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CHECK_NEQ (cursor->raw, start);
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if (cursor->raw < parent)
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parent = cursor;
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}
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// Parent the lowest child to the start of the sorted list
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auto tmp = start->raw;
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start->raw = parent->raw;
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// Remove the lowest child from their old parent
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auto parent_next = parent->raw;
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parent->raw = parent_next ? parent_next->raw : nullptr;
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// Parent the old successor of the start to the lowest child
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start->raw = tmp;
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}
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// Now that we've ordered the nodes we can walk the list from
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// start to finish and find nodes that aren't in the free list.
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// Call the destructors on the data contained in these.
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auto node_cursor = m_next.load (std::memory_order_relaxed);
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auto data_cursor = m_head;
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while (node_cursor) {
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while (data_cursor < node_cursor) {
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cruft::cast::alignment<T*> (data_cursor->data)->~T ();
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++data_cursor;
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}
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node_cursor = node_cursor->raw;
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++data_cursor;
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}
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while (data_cursor < m_head + m_capacity) {
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cruft::cast::alignment<T*> (data_cursor->data)->~T ();
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++data_cursor;
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}
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relink ();
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}
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private:
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void relink (void)
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{
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// Reset the allocation cursor to point to the start of the region
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m_next = m_head;
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// build out the linked list from all the nodes.
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for (size_t i = 0; i < m_capacity - 1; ++i)
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m_next[i].next = m_next + i + 1;
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m_next[m_capacity - 1].next = nullptr;
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}
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public:
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// Indexing
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size_t index (T const *ptr) const
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{
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CHECK_LIMIT (cruft::cast::alignment<node const*> (ptr), m_head, m_head + m_capacity);
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return cruft::cast::alignment<node const*> (ptr) - m_head;
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}
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/// returns the base address of the allocation.
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///
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/// guaranteed to point to the first _possible_ allocated value;
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/// however it may not be _live_ at any given moment.
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///
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/// DO NOT use this pointer for indexing as you will be unable to
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/// account for internal node sizes, alignment, or padding.
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void * base (void) & { return m_head; }
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void const* base (void) const& { return m_head; }
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T& operator[] (size_t idx) &
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{
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CHECK_LIMIT (idx, 0u, capacity ());
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return *cruft::cast::alignment<T*> (&m_head[idx].data[0]);
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}
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T const& operator[] (size_t idx) const&
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{
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CHECK_LIMIT (idx, 0u, capacity ());
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return *cruft::cast::alignment<T const*> (&m_head[idx].data[0]);
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}
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};
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}
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