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//
// immer: immutable data structures for C++
// Copyright (C) 2016, 2017, 2018 Juan Pedro Bolivar Puente
//
// This software is distributed under the Boost Software License, Version 1.0.
// See accompanying file LICENSE or copy at http://boost.org/LICENSE_1_0.txt
//
#pragma once
#include <immer/heap/free_list_node.hpp>
#include <immer/heap/with_data.hpp>
#include <atomic>
#include <cassert>
namespace immer {
/*!
* Adaptor that does not release the memory to the parent heap but
* instead it keeps the memory in a thread-safe global free list. Must
* be preceded by a `with_data<free_list_node, ...>` heap adaptor.
*
* @tparam Size Maximum size of the objects to be allocated.
* @tparam Base Type of the parent heap.
*/
template <std::size_t Size, std::size_t Limit, typename Base>
struct free_list_heap : Base
{
using base_t = Base;
template <typename... Tags>
static void* allocate(std::size_t size, Tags...)
{
assert(size <= sizeof(free_list_node) + Size);
assert(size >= sizeof(free_list_node));
free_list_node* n;
do {
n = head().data;
if (!n) {
auto p = base_t::allocate(Size + sizeof(free_list_node));
return static_cast<free_list_node*>(p);
}
} while (!head().data.compare_exchange_weak(n, n->next));
head().count.fetch_sub(1u, std::memory_order_relaxed);
return n;
}
template <typename... Tags>
static void deallocate(std::size_t size, void* data, Tags...)
{
assert(size <= sizeof(free_list_node) + Size);
assert(size >= sizeof(free_list_node));
// we use relaxed, because we are fine with temporarily having
// a few more/less buffers in free list
if (head().count.load(std::memory_order_relaxed) >= Limit) {
base_t::deallocate(Size + sizeof(free_list_node), data);
} else {
auto n = static_cast<free_list_node*>(data);
do {
n->next = head().data;
} while (!head().data.compare_exchange_weak(n->next, n));
head().count.fetch_add(1u, std::memory_order_relaxed);
}
}
private:
struct head_t
{
std::atomic<free_list_node*> data;
std::atomic<std::size_t> count;
};
static head_t& head()
{
static head_t head_{{nullptr}, {0}};
return head_;
}
};
} // namespace immer
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