1 // Copyright (c) 2022 The Limenka developers
2 // Distributed under the MIT software license, see the accompanying
3 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
4 5 #ifndef LIMENKA_SUPPORT_ALLOCATORS_POOL_H
6 #define LIMENKA_SUPPORT_ALLOCATORS_POOL_H
7 8 #include <array>
9 #include <cassert>
10 #include <cstddef>
11 #include <list>
12 #include <memory>
13 #include <new>
14 #include <type_traits>
15 #include <utility>
16 17 /**
18 * A memory resource similar to std::pmr::unsynchronized_pool_resource, but
19 * optimized for node-based containers. It has the following properties:
20 *
21 * * Owns the allocated memory and frees it on destruction, even when deallocate
22 * has not been called on the allocated blocks.
23 *
24 * * Consists of a number of pools, each one for a different block size.
25 * Each pool holds blocks of uniform size in a freelist.
26 *
27 * * Exhausting memory in a freelist causes a new allocation of a fixed size chunk.
28 * This chunk is used to carve out blocks.
29 *
30 * * Block sizes or alignments that can not be served by the pools are allocated
31 * and deallocated by operator new().
32 *
33 * PoolResource is not thread-safe. It is intended to be used by PoolAllocator.
34 *
35 * @tparam MAX_BLOCK_SIZE_BYTES Maximum size to allocate with the pool. If larger
36 * sizes are requested, allocation falls back to new().
37 *
38 * @tparam ALIGN_BYTES Required alignment for the allocations.
39 *
40 * An example: If you create a PoolResource<128, 8>(262144) and perform a bunch of
41 * allocations and deallocate 2 blocks with size 8 bytes, and 3 blocks with size 16,
42 * the members will look like this:
43 *
44 * m_free_lists m_allocated_chunks
45 * ┌───┐ ┌───┐ ┌────────────-------──────┐
46 * │ │ blocks │ ├─►│ 262144 B │
47 * │ │ ┌─────┐ ┌─────┐ └─┬─┘ └────────────-------──────┘
48 * │ 1 ├─►│ 8 B ├─►│ 8 B │ │
49 * │ │ └─────┘ └─────┘ :
50 * │ │ │
51 * │ │ ┌─────┐ ┌─────┐ ┌─────┐ ▼
52 * │ 2 ├─►│16 B ├─►│16 B ├─►│16 B │ ┌───┐ ┌─────────────────────────┐
53 * │ │ └─────┘ └─────┘ └─────┘ │ ├─►│ ▲ │ ▲
54 * │ │ └───┘ └──────────┬──────────────┘ │
55 * │ . │ │ m_available_memory_end
56 * │ . │ m_available_memory_it
57 * │ . │
58 * │ │
59 * │ │
60 * │16 │
61 * └───┘
62 *
63 * Here m_free_lists[1] holds the 2 blocks of size 8 bytes, and m_free_lists[2]
64 * holds the 3 blocks of size 16. The blocks came from the data stored in the
65 * m_allocated_chunks list. Each chunk has bytes 262144. The last chunk has still
66 * some memory available for the blocks, and when m_available_memory_it is at the
67 * end, a new chunk will be allocated and added to the list.
68 */
69 template <std::size_t MAX_BLOCK_SIZE_BYTES, std::size_t ALIGN_BYTES>
70 class PoolResource final
71 {
72 static_assert(ALIGN_BYTES > 0, "ALIGN_BYTES must be nonzero");
73 static_assert((ALIGN_BYTES & (ALIGN_BYTES - 1)) == 0, "ALIGN_BYTES must be a power of two");
74 75 /**
76 * In-place linked list of the allocations, used for the freelist.
77 */
78 struct ListNode {
79 ListNode* m_next;
80 81 explicit ListNode(ListNode* next) : m_next(next) {}
82 };
83 static_assert(std::is_trivially_destructible_v<ListNode>, "Make sure we don't need to manually call a destructor");
84 85 /**
86 * Internal alignment value. The larger of the requested ALIGN_BYTES and alignof(FreeList).
87 */
88 static constexpr std::size_t ELEM_ALIGN_BYTES = std::max(alignof(ListNode), ALIGN_BYTES);
89 static_assert((ELEM_ALIGN_BYTES & (ELEM_ALIGN_BYTES - 1)) == 0, "ELEM_ALIGN_BYTES must be a power of two");
90 static_assert(sizeof(ListNode) <= ELEM_ALIGN_BYTES, "Units of size ELEM_SIZE_ALIGN need to be able to store a ListNode");
91 static_assert((MAX_BLOCK_SIZE_BYTES & (ELEM_ALIGN_BYTES - 1)) == 0, "MAX_BLOCK_SIZE_BYTES needs to be a multiple of the alignment.");
92 93 /**
94 * Size in bytes to allocate per chunk
95 */
96 const size_t m_chunk_size_bytes;
97 98 /**
99 * Contains all allocated pools of memory, used to free the data in the destructor.
100 */
101 std::list<std::byte*> m_allocated_chunks{};
102 103 /**
104 * Single linked lists of all data that came from deallocating.
105 * m_free_lists[n] will serve blocks of size n*ELEM_ALIGN_BYTES.
106 */
107 std::array<ListNode*, MAX_BLOCK_SIZE_BYTES / ELEM_ALIGN_BYTES + 1> m_free_lists{};
108 109 /**
110 * Points to the beginning of available memory for carving out allocations.
111 */
112 std::byte* m_available_memory_it = nullptr;
113 114 /**
115 * Points to the end of available memory for carving out allocations.
116 *
117 * That member variable is redundant, and is always equal to `m_allocated_chunks.back() + m_chunk_size_bytes`
118 * whenever it is accessed, but `m_available_memory_end` caches this for clarity and efficiency.
119 */
120 std::byte* m_available_memory_end = nullptr;
121 122 /**
123 * How many multiple of ELEM_ALIGN_BYTES are necessary to fit bytes. We use that result directly as an index
124 * into m_free_lists. Round up for the special case when bytes==0.
125 */
126 [[nodiscard]] static constexpr std::size_t NumElemAlignBytes(std::size_t bytes)
127 {
128 return (bytes + ELEM_ALIGN_BYTES - 1) / ELEM_ALIGN_BYTES + (bytes == 0);
129 }
130 131 /**
132 * True when it is possible to make use of the freelist
133 */
134 [[nodiscard]] static constexpr bool IsFreeListUsable(std::size_t bytes, std::size_t alignment)
135 {
136 return alignment <= ELEM_ALIGN_BYTES && bytes <= MAX_BLOCK_SIZE_BYTES;
137 }
138 139 /**
140 * Replaces node with placement constructed ListNode that points to the previous node
141 */
142 void PlacementAddToList(void* p, ListNode*& node)
143 {
144 node = new (p) ListNode{node};
145 }
146 147 /**
148 * Allocate one full memory chunk which will be used to carve out allocations.
149 * Also puts any leftover bytes into the freelist.
150 *
151 * Precondition: leftover bytes are either 0 or few enough to fit into a place in the freelist
152 */
153 void AllocateChunk()
154 {
155 // if there is still any available memory left, put it into the freelist.
156 size_t remaining_available_bytes = m_available_memory_end - m_available_memory_it;
157 if (0 != remaining_available_bytes) {
158 PlacementAddToList(m_available_memory_it, m_free_lists[remaining_available_bytes / ELEM_ALIGN_BYTES]);
159 }
160 161 void* storage = ::operator new (m_chunk_size_bytes, std::align_val_t{ELEM_ALIGN_BYTES});
162 m_available_memory_it = new (storage) std::byte[m_chunk_size_bytes];
163 m_available_memory_end = m_available_memory_it + m_chunk_size_bytes;
164 m_allocated_chunks.emplace_back(m_available_memory_it);
165 }
166 167 /**
168 * Access to internals for testing purpose only
169 */
170 friend class PoolResourceTester;
171 172 public:
173 /**
174 * Construct a new PoolResource object which allocates the first chunk.
175 * chunk_size_bytes will be rounded up to next multiple of ELEM_ALIGN_BYTES.
176 */
177 explicit PoolResource(std::size_t chunk_size_bytes)
178 : m_chunk_size_bytes(NumElemAlignBytes(chunk_size_bytes) * ELEM_ALIGN_BYTES)
179 {
180 assert(m_chunk_size_bytes >= MAX_BLOCK_SIZE_BYTES);
181 AllocateChunk();
182 }
183 184 /**
185 * Construct a new Pool Resource object, defaults to 2^18=262144 chunk size.
186 */
187 PoolResource() : PoolResource(262144) {}
188 189 /**
190 * Disable copy & move semantics, these are not supported for the resource.
191 */
192 PoolResource(const PoolResource&) = delete;
193 PoolResource& operator=(const PoolResource&) = delete;
194 PoolResource(PoolResource&&) = delete;
195 PoolResource& operator=(PoolResource&&) = delete;
196 197 /**
198 * Deallocates all memory allocated associated with the memory resource.
199 */
200 ~PoolResource()
201 {
202 for (std::byte* chunk : m_allocated_chunks) {
203 std::destroy(chunk, chunk + m_chunk_size_bytes);
204 ::operator delete ((void*)chunk, std::align_val_t{ELEM_ALIGN_BYTES});
205 }
206 }
207 208 /**
209 * Allocates a block of bytes. If possible the freelist is used, otherwise allocation
210 * is forwarded to ::operator new().
211 */
212 void* Allocate(std::size_t bytes, std::size_t alignment)
213 {
214 if (IsFreeListUsable(bytes, alignment)) {
215 const std::size_t num_alignments = NumElemAlignBytes(bytes);
216 if (nullptr != m_free_lists[num_alignments]) {
217 // we've already got data in the pool's freelist, unlink one element and return the pointer
218 // to the unlinked memory. Since FreeList is trivially destructible we can just treat it as
219 // uninitialized memory.
220 return std::exchange(m_free_lists[num_alignments], m_free_lists[num_alignments]->m_next);
221 }
222 223 // freelist is empty: get one allocation from allocated chunk memory.
224 const std::ptrdiff_t round_bytes = static_cast<std::ptrdiff_t>(num_alignments * ELEM_ALIGN_BYTES);
225 if (round_bytes > m_available_memory_end - m_available_memory_it) {
226 // slow path, only happens when a new chunk needs to be allocated
227 AllocateChunk();
228 }
229 230 // Make sure we use the right amount of bytes for that freelist (might be rounded up),
231 return std::exchange(m_available_memory_it, m_available_memory_it + round_bytes);
232 }
233 234 // Can't use the pool => use operator new()
235 return ::operator new (bytes, std::align_val_t{alignment});
236 }
237 238 /**
239 * Returns a block to the freelists, or deletes the block when it did not come from the chunks.
240 */
241 void Deallocate(void* p, std::size_t bytes, std::size_t alignment) noexcept
242 {
243 if (IsFreeListUsable(bytes, alignment)) {
244 const std::size_t num_alignments = NumElemAlignBytes(bytes);
245 // put the memory block into the linked list. We can placement construct the FreeList
246 // into the memory since we can be sure the alignment is correct.
247 PlacementAddToList(p, m_free_lists[num_alignments]);
248 } else {
249 // Can't use the pool => forward deallocation to ::operator delete().
250 ::operator delete (p, std::align_val_t{alignment});
251 }
252 }
253 254 /**
255 * Number of allocated chunks
256 */
257 [[nodiscard]] std::size_t NumAllocatedChunks() const
258 {
259 return m_allocated_chunks.size();
260 }
261 262 /**
263 * Size in bytes to allocate per chunk, currently hardcoded to a fixed size.
264 */
265 [[nodiscard]] size_t ChunkSizeBytes() const
266 {
267 return m_chunk_size_bytes;
268 }
269 };
270 271 272 /**
273 * Forwards all allocations/deallocations to the PoolResource.
274 */
275 template <class T, std::size_t MAX_BLOCK_SIZE_BYTES, std::size_t ALIGN_BYTES = alignof(T)>
276 class PoolAllocator
277 {
278 PoolResource<MAX_BLOCK_SIZE_BYTES, ALIGN_BYTES>* m_resource;
279 280 template <typename U, std::size_t M, std::size_t A>
281 friend class PoolAllocator;
282 283 public:
284 using value_type = T;
285 using ResourceType = PoolResource<MAX_BLOCK_SIZE_BYTES, ALIGN_BYTES>;
286 287 /**
288 * Not explicit so we can easily construct it with the correct resource
289 */
290 PoolAllocator(ResourceType* resource) noexcept
291 : m_resource(resource)
292 {
293 }
294 295 PoolAllocator(const PoolAllocator& other) noexcept = default;
296 PoolAllocator& operator=(const PoolAllocator& other) noexcept = default;
297 298 template <class U>
299 PoolAllocator(const PoolAllocator<U, MAX_BLOCK_SIZE_BYTES, ALIGN_BYTES>& other) noexcept
300 : m_resource(other.resource())
301 {
302 }
303 304 /**
305 * The rebind struct here is mandatory because we use non type template arguments for
306 * PoolAllocator. See https://en.cppreference.com/w/cpp/named_req/Allocator#cite_note-2
307 */
308 template <typename U>
309 struct rebind {
310 using other = PoolAllocator<U, MAX_BLOCK_SIZE_BYTES, ALIGN_BYTES>;
311 };
312 313 /**
314 * Forwards each call to the resource.
315 */
316 T* allocate(size_t n)
317 {
318 return static_cast<T*>(m_resource->Allocate(n * sizeof(T), alignof(T)));
319 }
320 321 /**
322 * Forwards each call to the resource.
323 */
324 void deallocate(T* p, size_t n) noexcept
325 {
326 m_resource->Deallocate(p, n * sizeof(T), alignof(T));
327 }
328 329 ResourceType* resource() const noexcept
330 {
331 return m_resource;
332 }
333 };
334 335 template <class T1, class T2, std::size_t MAX_BLOCK_SIZE_BYTES, std::size_t ALIGN_BYTES>
336 bool operator==(const PoolAllocator<T1, MAX_BLOCK_SIZE_BYTES, ALIGN_BYTES>& a,
337 const PoolAllocator<T2, MAX_BLOCK_SIZE_BYTES, ALIGN_BYTES>& b) noexcept
338 {
339 return a.resource() == b.resource();
340 }
341 342 template <class T1, class T2, std::size_t MAX_BLOCK_SIZE_BYTES, std::size_t ALIGN_BYTES>
343 bool operator!=(const PoolAllocator<T1, MAX_BLOCK_SIZE_BYTES, ALIGN_BYTES>& a,
344 const PoolAllocator<T2, MAX_BLOCK_SIZE_BYTES, ALIGN_BYTES>& b) noexcept
345 {
346 return !(a == b);
347 }
348 349 #endif // LIMENKA_SUPPORT_ALLOCATORS_POOL_H
350