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1 : : // Copyright (c) 2022 The Bitcoin Core 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 BITCOIN_SUPPORT_ALLOCATORS_POOL_H
6 : : #define BITCOIN_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 : 0 : 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 : 5 : 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 : 5 : 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 : 5 : 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 : 5 : 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 : 5 : [[nodiscard]] static constexpr std::size_t NumElemAlignBytes(std::size_t bytes)
127 : : {
128 : 5 : 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 : 0 : [[nodiscard]] static constexpr bool IsFreeListUsable(std::size_t bytes, std::size_t alignment)
135 : : {
136 [ # # # # : 0 : 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 : 0 : void PlacementAddToList(void* p, ListNode*& node)
143 : : {
144 : 0 : node = new (p) ListNode{node};
145 : 0 : }
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 : 5 : void AllocateChunk()
154 : : {
155 : : // if there is still any available memory left, put it into the freelist.
156 : 5 : size_t remaining_available_bytes = std::distance(m_available_memory_it, m_available_memory_end);
157 [ + - # # : 5 : if (0 != remaining_available_bytes) {
# # # # #
# # # # #
# # ]
158 : 0 : PlacementAddToList(m_available_memory_it, m_free_lists[remaining_available_bytes / ELEM_ALIGN_BYTES]);
159 : 0 : }
160 : :
161 : 5 : void* storage = ::operator new (m_chunk_size_bytes, std::align_val_t{ELEM_ALIGN_BYTES});
162 : 5 : m_available_memory_it = new (storage) std::byte[m_chunk_size_bytes];
163 : 5 : m_available_memory_end = m_available_memory_it + m_chunk_size_bytes;
164 : 5 : m_allocated_chunks.emplace_back(m_available_memory_it);
165 : 5 : }
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 : 5 : explicit PoolResource(std::size_t chunk_size_bytes)
178 : 5 : : m_chunk_size_bytes(NumElemAlignBytes(chunk_size_bytes) * ELEM_ALIGN_BYTES)
179 : : {
180 [ + - # # : 5 : assert(m_chunk_size_bytes >= MAX_BLOCK_SIZE_BYTES);
# # # # #
# # # # #
# # ]
181 [ - + # # : 5 : AllocateChunk();
# # # # #
# # # # #
# # ]
182 : 5 : }
183 : :
184 : : /**
185 : : * Construct a new Pool Resource object, defaults to 2^18=262144 chunk size.
186 : : */
187 : 5 : 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 : 5 : ~PoolResource()
201 : : {
202 [ + + # # : 10 : for (std::byte* chunk : m_allocated_chunks) {
# # # # #
# # # # #
# # ]
203 [ + - # # : 5 : std::destroy(chunk, chunk + m_chunk_size_bytes);
# # # # #
# # # # #
# # ]
204 : 5 : ::operator delete ((void*)chunk, std::align_val_t{ELEM_ALIGN_BYTES});
205 : : }
206 : 5 : }
207 : :
208 : : /**
209 : : * Allocates a block of bytes. If possible the freelist is used, otherwise allocation
210 : : * is forwarded to ::operator new().
211 : : */
212 : 0 : void* Allocate(std::size_t bytes, std::size_t alignment)
213 : : {
214 [ # # # # : 0 : if (IsFreeListUsable(bytes, alignment)) {
# # # # #
# # # # #
# # ]
215 : 0 : const std::size_t num_alignments = NumElemAlignBytes(bytes);
216 [ # # # # : 0 : 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 : 0 : 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 : 0 : const std::ptrdiff_t round_bytes = static_cast<std::ptrdiff_t>(num_alignments * ELEM_ALIGN_BYTES);
225 [ # # # # : 0 : 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 : 0 : AllocateChunk();
228 : 0 : }
229 : :
230 : : // Make sure we use the right amount of bytes for that freelist (might be rounded up),
231 : 0 : 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 : 0 : return ::operator new (bytes, std::align_val_t{alignment});
236 : 0 : }
237 : :
238 : : /**
239 : : * Returns a block to the freelists, or deletes the block when it did not come from the chunks.
240 : : */
241 : 0 : void Deallocate(void* p, std::size_t bytes, std::size_t alignment) noexcept
242 : : {
243 [ # # # # : 0 : if (IsFreeListUsable(bytes, alignment)) {
# # # # #
# # # # #
# # ]
244 [ # # ]: 0 : 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 [ # # # # : 0 : PlacementAddToList(p, m_free_lists[num_alignments]);
# # # # #
# # # # #
# # ]
248 : 0 : } else {
249 : : // Can't use the pool => forward deallocation to ::operator delete().
250 : 0 : ::operator delete (p, std::align_val_t{alignment});
251 : : }
252 : 0 : }
253 : :
254 : : /**
255 : : * Number of allocated chunks
256 : : */
257 : 14 : [[nodiscard]] std::size_t NumAllocatedChunks() const
258 : : {
259 : 14 : return m_allocated_chunks.size();
260 : : }
261 : :
262 : : /**
263 : : * Size in bytes to allocate per chunk, currently hardcoded to a fixed size.
264 : : */
265 : 7 : [[nodiscard]] size_t ChunkSizeBytes() const
266 : : {
267 : 7 : 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>
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 : 5 : PoolAllocator(ResourceType* resource) noexcept
291 : 5 : : m_resource(resource)
292 : : {
293 : 5 : }
294 : :
295 : : PoolAllocator(const PoolAllocator& other) noexcept = default;
296 : : PoolAllocator& operator=(const PoolAllocator& other) noexcept = default;
297 : :
298 : : template <class U>
299 : 12 : PoolAllocator(const PoolAllocator<U, MAX_BLOCK_SIZE_BYTES, ALIGN_BYTES>& other) noexcept
300 : 12 : : m_resource(other.resource())
301 : : {
302 : 12 : }
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 : 0 : T* allocate(size_t n)
317 : : {
318 : 0 : return static_cast<T*>(m_resource->Allocate(n * sizeof(T), alignof(T)));
319 : : }
320 : :
321 : : /**
322 : : * Forwards each call to the resource.
323 : : */
324 : 0 : void deallocate(T* p, size_t n) noexcept
325 : : {
326 : 0 : m_resource->Deallocate(p, n * sizeof(T), alignof(T));
327 : 0 : }
328 : :
329 : 19 : ResourceType* resource() const noexcept
330 : : {
331 : 19 : 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 // BITCOIN_SUPPORT_ALLOCATORS_POOL_H
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