// Copyright (c) 2022 The Bitcoin Core developers
// Distributed under the MIT software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.
#ifndef BITCOIN_SUPPORT_ALLOCATORS_POOL_H
#define BITCOIN_SUPPORT_ALLOCATORS_POOL_H
#include <array>
#include <cassert>
#include <cstddef>
#include <list>
#include <memory>
#include <new>
#include <type_traits>
#include <utility>
/**
* A memory resource similar to std::pmr::unsynchronized_pool_resource, but
* optimized for node-based containers. It has the following properties:
*
* * Owns the allocated memory and frees it on destruction, even when deallocate
* has not been called on the allocated blocks.
*
* * Consists of a number of pools, each one for a different block size.
* Each pool holds blocks of uniform size in a freelist.
*
* * Exhausting memory in a freelist causes a new allocation of a fixed size chunk.
* This chunk is used to carve out blocks.
*
* * Block sizes or alignments that can not be served by the pools are allocated
* and deallocated by operator new().
*
* PoolResource is not thread-safe. It is intended to be used by PoolAllocator.
*
* @tparam MAX_BLOCK_SIZE_BYTES Maximum size to allocate with the pool. If larger
* sizes are requested, allocation falls back to new().
*
* @tparam ALIGN_BYTES Required alignment for the allocations.
*
* An example: If you create a PoolResource<128, 8>(262144) and perform a bunch of
* allocations and deallocate 2 blocks with size 8 bytes, and 3 blocks with size 16,
* the members will look like this:
*
* m_free_lists m_allocated_chunks
* ┌───┐ ┌───┐ ┌────────────-------──────┐
* │ │ blocks │ ├─►│ 262144 B │
* │ │ ┌─────┐ ┌─────┐ └─┬─┘ └────────────-------──────┘
* │ 1 ├─►│ 8 B ├─►│ 8 B │ │
* │ │ └─────┘ └─────┘ :
* │ │ │
* │ │ ┌─────┐ ┌─────┐ ┌─────┐ ▼
* │ 2 ├─►│16 B ├─►│16 B ├─►│16 B │ ┌───┐ ┌─────────────────────────┐
* │ │ └─────┘ └─────┘ └─────┘ │ ├─►│ ▲ │ ▲
* │ │ └───┘ └──────────┬──────────────┘ │
* │ . │ │ m_available_memory_end
* │ . │ m_available_memory_it
* │ . │
* │ │
* │ │
* │16 │
* └───┘
*
* Here m_free_lists[1] holds the 2 blocks of size 8 bytes, and m_free_lists[2]
* holds the 3 blocks of size 16. The blocks came from the data stored in the
* m_allocated_chunks list. Each chunk has bytes 262144. The last chunk has still
* some memory available for the blocks, and when m_available_memory_it is at the
* end, a new chunk will be allocated and added to the list.
*/
template <std::size_t MAX_BLOCK_SIZE_BYTES, std::size_t ALIGN_BYTES>
class PoolResource final
{
static_assert(ALIGN_BYTES > 0, "ALIGN_BYTES must be nonzero");
static_assert((ALIGN_BYTES & (ALIGN_BYTES - 1)) == 0, "ALIGN_BYTES must be a power of two");
/**
* In-place linked list of the allocations, used for the freelist.
*/
struct ListNode {
ListNode* m_next;
explicit ListNode(ListNode* next) : m_next(next) {}
};
static_assert(std::is_trivially_destructible_v<ListNode>, "Make sure we don't need to manually call a destructor");
/**
* Internal alignment value. The larger of the requested ALIGN_BYTES and alignof(FreeList).
*/
static constexpr std::size_t ELEM_ALIGN_BYTES = std::max(alignof(ListNode), ALIGN_BYTES);
static_assert((ELEM_ALIGN_BYTES & (ELEM_ALIGN_BYTES - 1)) == 0, "ELEM_ALIGN_BYTES must be a power of two");
static_assert(sizeof(ListNode) <= ELEM_ALIGN_BYTES, "Units of size ELEM_SIZE_ALIGN need to be able to store a ListNode");
static_assert((MAX_BLOCK_SIZE_BYTES & (ELEM_ALIGN_BYTES - 1)) == 0, "MAX_BLOCK_SIZE_BYTES needs to be a multiple of the alignment.");
/**
* Size in bytes to allocate per chunk
*/
const size_t m_chunk_size_bytes;
/**
* Contains all allocated pools of memory, used to free the data in the destructor.
*/
std::list<std::byte*> m_allocated_chunks{};
/**
* Single linked lists of all data that came from deallocating.
* m_free_lists[n] will serve blocks of size n*ELEM_ALIGN_BYTES.
*/
std::array<ListNode*, MAX_BLOCK_SIZE_BYTES / ELEM_ALIGN_BYTES + 1> m_free_lists{};
/**
* Points to the beginning of available memory for carving out allocations.
*/
std::byte* m_available_memory_it = nullptr;
/**
* Points to the end of available memory for carving out allocations.
*
* That member variable is redundant, and is always equal to `m_allocated_chunks.back() + m_chunk_size_bytes`
* whenever it is accessed, but `m_available_memory_end` caches this for clarity and efficiency.
*/
std::byte* m_available_memory_end = nullptr;
/**
* How many multiple of ELEM_ALIGN_BYTES are necessary to fit bytes. We use that result directly as an index
* into m_free_lists. Round up for the special case when bytes==0.
*/
[[nodiscard]] static constexpr std::size_t NumElemAlignBytes(std::size_t bytes)
{
return (bytes + ELEM_ALIGN_BYTES - 1) / ELEM_ALIGN_BYTES + (bytes == 0);
}
/**
* True when it is possible to make use of the freelist
*/
[[nodiscard]] static constexpr bool IsFreeListUsable(std::size_t bytes, std::size_t alignment)
{
return alignment <= ELEM_ALIGN_BYTES && bytes <= MAX_BLOCK_SIZE_BYTES;
}
/**
* Replaces node with placement constructed ListNode that points to the previous node
*/
void PlacementAddToList(void* p, ListNode*& node)
{
node = new (p) ListNode{node};
}
/**
* Allocate one full memory chunk which will be used to carve out allocations.
* Also puts any leftover bytes into the freelist.
*
* Precondition: leftover bytes are either 0 or few enough to fit into a place in the freelist
*/
void AllocateChunk()
{
// if there is still any available memory left, put it into the freelist.
size_t remaining_available_bytes = std::distance(m_available_memory_it, m_available_memory_end);
if (0 != remaining_available_bytes) {
PlacementAddToList(m_available_memory_it, m_free_lists[remaining_available_bytes / ELEM_ALIGN_BYTES]);
}
void* storage = ::operator new (m_chunk_size_bytes, std::align_val_t{ELEM_ALIGN_BYTES});
m_available_memory_it = new (storage) std::byte[m_chunk_size_bytes];
m_available_memory_end = m_available_memory_it + m_chunk_size_bytes;
m_allocated_chunks.emplace_back(m_available_memory_it);
}
/**
* Access to internals for testing purpose only
*/
friend class PoolResourceTester;
public:
/**
* Construct a new PoolResource object which allocates the first chunk.
* chunk_size_bytes will be rounded up to next multiple of ELEM_ALIGN_BYTES.
*/
explicit PoolResource(std::size_t chunk_size_bytes)
: m_chunk_size_bytes(NumElemAlignBytes(chunk_size_bytes) * ELEM_ALIGN_BYTES)
{
assert(m_chunk_size_bytes >= MAX_BLOCK_SIZE_BYTES);
AllocateChunk();
}
/**
* Construct a new Pool Resource object, defaults to 2^18=262144 chunk size.
*/
PoolResource() : PoolResource(262144) {}
/**
* Disable copy & move semantics, these are not supported for the resource.
*/
PoolResource(const PoolResource&) = delete;
PoolResource& operator=(const PoolResource&) = delete;
PoolResource(PoolResource&&) = delete;
PoolResource& operator=(PoolResource&&) = delete;
/**
* Deallocates all memory allocated associated with the memory resource.
*/
~PoolResource()
{
for (std::byte* chunk : m_allocated_chunks) {
std::destroy(chunk, chunk + m_chunk_size_bytes);
::operator delete ((void*)chunk, std::align_val_t{ELEM_ALIGN_BYTES});
}
}
/**
* Allocates a block of bytes. If possible the freelist is used, otherwise allocation
* is forwarded to ::operator new().
*/
void* Allocate(std::size_t bytes, std::size_t alignment)
{
if (IsFreeListUsable(bytes, alignment)) {
const std::size_t num_alignments = NumElemAlignBytes(bytes);
if (nullptr != m_free_lists[num_alignments]) {
// we've already got data in the pool's freelist, unlink one element and return the pointer
// to the unlinked memory. Since FreeList is trivially destructible we can just treat it as
// uninitialized memory.
return std::exchange(m_free_lists[num_alignments], m_free_lists[num_alignments]->m_next);
}
// freelist is empty: get one allocation from allocated chunk memory.
const std::ptrdiff_t round_bytes = static_cast<std::ptrdiff_t>(num_alignments * ELEM_ALIGN_BYTES);
if (round_bytes > m_available_memory_end - m_available_memory_it) {
// slow path, only happens when a new chunk needs to be allocated
AllocateChunk();
}
// Make sure we use the right amount of bytes for that freelist (might be rounded up),
return std::exchange(m_available_memory_it, m_available_memory_it + round_bytes);
}
// Can't use the pool => use operator new()
return ::operator new (bytes, std::align_val_t{alignment});
}
/**
* Returns a block to the freelists, or deletes the block when it did not come from the chunks.
*/
void Deallocate(void* p, std::size_t bytes, std::size_t alignment) noexcept
{
if (IsFreeListUsable(bytes, alignment)) {
const std::size_t num_alignments = NumElemAlignBytes(bytes);
// put the memory block into the linked list. We can placement construct the FreeList
// into the memory since we can be sure the alignment is correct.
PlacementAddToList(p, m_free_lists[num_alignments]);
} else {
// Can't use the pool => forward deallocation to ::operator delete().
::operator delete (p, std::align_val_t{alignment});
}
}
/**
* Number of allocated chunks
*/
[[nodiscard]] std::size_t NumAllocatedChunks() const
{
return m_allocated_chunks.size();
}
/**
* Size in bytes to allocate per chunk, currently hardcoded to a fixed size.
*/
[[nodiscard]] size_t ChunkSizeBytes() const
{
return m_chunk_size_bytes;
}
};
/**
* Forwards all allocations/deallocations to the PoolResource.
*/
template <class T, std::size_t MAX_BLOCK_SIZE_BYTES, std::size_t ALIGN_BYTES = alignof(T)>
class PoolAllocator
{
PoolResource<MAX_BLOCK_SIZE_BYTES, ALIGN_BYTES>* m_resource;
template <typename U, std::size_t M, std::size_t A>
friend class PoolAllocator;
public:
using value_type = T;
using ResourceType = PoolResource<MAX_BLOCK_SIZE_BYTES, ALIGN_BYTES>;
/**
* Not explicit so we can easily construct it with the correct resource
*/
PoolAllocator(ResourceType* resource) noexcept
: m_resource(resource)
{
}
PoolAllocator(const PoolAllocator& other) noexcept = default;
PoolAllocator& operator=(const PoolAllocator& other) noexcept = default;
template <class U>
PoolAllocator(const PoolAllocator<U, MAX_BLOCK_SIZE_BYTES, ALIGN_BYTES>& other) noexcept
: m_resource(other.resource())
{
}
/**
* The rebind struct here is mandatory because we use non type template arguments for
* PoolAllocator. See https://en.cppreference.com/w/cpp/named_req/Allocator#cite_note-2
*/
template <typename U>
struct rebind {
using other = PoolAllocator<U, MAX_BLOCK_SIZE_BYTES, ALIGN_BYTES>;
};
/**
* Forwards each call to the resource.
*/
T* allocate(size_t n)
{
return static_cast<T*>(m_resource->Allocate(n * sizeof(T), alignof(T)));
}
/**
* Forwards each call to the resource.
*/
void deallocate(T* p, size_t n) noexcept
{
m_resource->Deallocate(p, n * sizeof(T), alignof(T));
}
ResourceType* resource() const noexcept
{
return m_resource;
}
};
template <class T1, class T2, std::size_t MAX_BLOCK_SIZE_BYTES, std::size_t ALIGN_BYTES>
bool operator==(const PoolAllocator<T1, MAX_BLOCK_SIZE_BYTES, ALIGN_BYTES>& a,
const PoolAllocator<T2, MAX_BLOCK_SIZE_BYTES, ALIGN_BYTES>& b) noexcept
{
return a.resource() == b.resource();
}
template <class T1, class T2, std::size_t MAX_BLOCK_SIZE_BYTES, std::size_t ALIGN_BYTES>
bool operator!=(const PoolAllocator<T1, MAX_BLOCK_SIZE_BYTES, ALIGN_BYTES>& a,
const PoolAllocator<T2, MAX_BLOCK_SIZE_BYTES, ALIGN_BYTES>& b) noexcept
{
return !(a == b);
}
#endif // BITCOIN_SUPPORT_ALLOCATORS_POOL_H