rocksdb/util/arena_impl.cc

// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.

#include "util/arena_impl.h"

namespace leveldb {

ArenaImpl::ArenaImpl(size_t block_size) {
  if (block_size < kMinBlockSize) {
    block_size_ = kMinBlockSize;
  } else if (block_size > kMaxBlockSize) {
    block_size_ = kMaxBlockSize;
  } else {
    block_size_ = block_size;
  }

  blocks_memory_ = 0;
  alloc_ptr_ = nullptr;  // First allocation will allocate a block
  alloc_bytes_remaining_ = 0;
}

ArenaImpl::~ArenaImpl() {
  for (size_t i = 0; i < blocks_.size(); i++) {
    delete[] blocks_[i];
  }
}

char* ArenaImpl::AllocateFallback(size_t bytes) {
  if (bytes > block_size_ / 4) {
    // Object is more than a quarter of our block size.  Allocate it separately
    // to avoid wasting too much space in leftover bytes.
    char* result = AllocateNewBlock(bytes);
    return result;
  }

  // We waste the remaining space in the current block.
  alloc_ptr_ = AllocateNewBlock(block_size_);
  alloc_bytes_remaining_ = block_size_;

  char* result = alloc_ptr_;
  alloc_ptr_ += bytes;
  alloc_bytes_remaining_ -= bytes;
  return result;
}

char* ArenaImpl::AllocateAligned(size_t bytes) {
  const int align = sizeof(void*);    // We'll align to pointer size
  assert((align & (align-1)) == 0);   // Pointer size should be a power of 2
  size_t current_mod = reinterpret_cast<uintptr_t>(alloc_ptr_) & (align-1);
  size_t slop = (current_mod == 0 ? 0 : align - current_mod);
  size_t needed = bytes + slop;
  char* result;
  if (needed <= alloc_bytes_remaining_) {
    result = alloc_ptr_ + slop;
    alloc_ptr_ += needed;
    alloc_bytes_remaining_ -= needed;
  } else {
    // AllocateFallback always returned aligned memory
    result = AllocateFallback(bytes);
  }
  assert((reinterpret_cast<uintptr_t>(result) & (align-1)) == 0);
  return result;
}

char* ArenaImpl::AllocateNewBlock(size_t block_bytes) {
  char* result = new char[block_bytes];
  blocks_memory_ += block_bytes;
  blocks_.push_back(result);
  return result;
}

}  // namespace leveldb