src/leveldb/util/arena.cc

   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 #include <assert.h>
   7
   8 namespace leveldb {
   9
  10 static const int kBlockSize = 4096;
  11
  12 Arena::Arena() : memory_usage_(0) {
  13   alloc_ptr_ = NULL;  // First allocation will allocate a block
  14   alloc_bytes_remaining_ = 0;
  15 }
  16
  17 Arena::~Arena() {
  18   for (size_t i = 0; i < blocks_.size(); i++) {
  19     delete[] blocks_[i];
  20   }
  21 }
  22
  23 char* Arena::AllocateFallback(size_t bytes) {
  24   if (bytes > kBlockSize / 4) {
  25     // Object is more than a quarter of our block size.  Allocate it separately
  26     // to avoid wasting too much space in leftover bytes.
  27     char* result = AllocateNewBlock(bytes);
  28     return result;
  29   }
  30
  31   // We waste the remaining space in the current block.
  32   alloc_ptr_ = AllocateNewBlock(kBlockSize);
  33   alloc_bytes_remaining_ = kBlockSize;
  34
  35   char* result = alloc_ptr_;
  36   alloc_ptr_ += bytes;
  37   alloc_bytes_remaining_ -= bytes;
  38   return result;
  39 }
  40
  41 char* Arena::AllocateAligned(size_t bytes) {
  42   const int align = (sizeof(void*) > 8) ? sizeof(void*) : 8;
  43   assert((align & (align-1)) == 0);   // Pointer size should be a power of 2
  44   size_t current_mod = reinterpret_cast<uintptr_t>(alloc_ptr_) & (align-1);
  45   size_t slop = (current_mod == 0 ? 0 : align - current_mod);
  46   size_t needed = bytes + slop;
  47   char* result;
  48   if (needed <= alloc_bytes_remaining_) {
  49     result = alloc_ptr_ + slop;
  50     alloc_ptr_ += needed;
  51     alloc_bytes_remaining_ -= needed;
  52   } else {
  53     // AllocateFallback always returned aligned memory
  54     result = AllocateFallback(bytes);
  55   }
  56   assert((reinterpret_cast<uintptr_t>(result) & (align-1)) == 0);
  57   return result;
  58 }
  59
  60 char* Arena::AllocateNewBlock(size_t block_bytes) {
  61   char* result = new char[block_bytes];
  62   blocks_.push_back(result);
  63   memory_usage_.NoBarrier_Store(
  64       reinterpret_cast<void*>(MemoryUsage() + block_bytes + sizeof(char*)));
  65   return result;
  66 }
  67
  68 }  // namespace leveldb