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 
7 namespace leveldb {
8 
9 static const int kBlockSize = 4096;
10 
Arena()11 Arena::Arena()
12     : alloc_ptr_(nullptr), alloc_bytes_remaining_(0), memory_usage_(0) {}
13 
~Arena()14 Arena::~Arena() {
15   for (size_t i = 0; i < blocks_.size(); i++) {
16     delete[] blocks_[i];
17   }
18 }
19 
AllocateFallback(size_t bytes)20 char* 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 
AllocateAligned(size_t bytes)38 char* 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 
AllocateNewBlock(size_t block_bytes)58 char* 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