Inside Bitcoin

1// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file. See the AUTHORS file for names of contributors.
4
5#include "util/arena.h"
6
7namespace leveldb {
8
9static const int kBlockSize = 4096;
10
11Arena::Arena()
12    : alloc_ptr_(nullptr), alloc_bytes_remaining_(0), memory_usage_(0) {}
13
14Arena::~Arena() {
15  for (size_t i = 0; i < blocks_.size(); i++) {
16    delete[] blocks_[i];
17  }
18}
19
20char* Arena::AllocateFallback(size_t bytes) {
21  if (bytes > kBlockSize / 4) {
22    // Object is more than a quarter of our block size.  Allocate it separately
23    // to avoid wasting too much space in leftover bytes.
24    char* result = AllocateNewBlock(bytes);
25    return result;
26  }
27
28  // We waste the remaining space in the current block.
29  alloc_ptr_ = AllocateNewBlock(kBlockSize);
30  alloc_bytes_remaining_ = kBlockSize;
31
32  char* result = alloc_ptr_;
33  alloc_ptr_ += bytes;
34  alloc_bytes_remaining_ -= bytes;
35  return result;
36}
37
38char* Arena::AllocateAligned(size_t bytes) {
39  const int align = (sizeof(void*) > 8) ? sizeof(void*) : 8;
40  static_assert((align & (align - 1)) == 0,
41                "Pointer size should be a power of 2");
42  size_t current_mod = reinterpret_cast<uintptr_t>(alloc_ptr_) & (align - 1);
43  size_t slop = (current_mod == 0 ? 0 : align - current_mod);
44  size_t needed = bytes + slop;
45  char* result;
46  if (needed <= alloc_bytes_remaining_) {
47    result = alloc_ptr_ + slop;
48    alloc_ptr_ += needed;
49    alloc_bytes_remaining_ -= needed;
50  } else {
51    // AllocateFallback always returned aligned memory
52    result = AllocateFallback(bytes);
53  }
54  assert((reinterpret_cast<uintptr_t>(result) & (align - 1)) == 0);
55  return result;
56}
57
58char* Arena::AllocateNewBlock(size_t block_bytes) {
59  char* result = new char[block_bytes];
60  blocks_.push_back(result);
61  memory_usage_.fetch_add(block_bytes + sizeof(char*),
62                          std::memory_order_relaxed);
63  return result;
64}
65
66}  // namespace leveldb
67