arena.cc raw

   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  
  11  Arena::Arena()
  12      : alloc_ptr_(nullptr), alloc_bytes_remaining_(0), memory_usage_(0) {}
  13  
  14  Arena::~Arena() {
  15    for (size_t i = 0; i < blocks_.size(); i++) {
  16      delete[] blocks_[i];
  17    }
  18  }
  19  
  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  
  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  
  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