js引擎v8源码分析之SemiSpace（基于v8 0.1.5）

class SemiSpace  BASE_EMBEDDED {
 public:
  SemiSpace(int initial_capacity, int maximum_capacity);
  bool Setup(Address start, int size);
  void TearDown();
  bool HasBeenSetup() { return start_ != NULL; }
  bool Double();
  Address low() { return start_; }
  Address high() { return low() + capacity_; }
  Address age_mark() { return age_mark_; }
  void set_age_mark(Address mark) { age_mark_ = mark; }
  // 判断地址a是否在该对象管理的内存中，&address_mask即让a减去size-1的大小。如果等于start说明在管理范围内
  bool Contains(Address a) {
    return (reinterpret_cast<uint32_t>(a) & address_mask_) == reinterpret_cast<uint32_t>(start_);
  }
  // 类似上面的逻辑，但是堆对象低位是标记，判断时候需要处理一下，加SetUp
  bool Contains(Object* o) {
    return (reinterpret_cast<uint32_t>(o) & object_mask_) == object_expected_;
  }
  // 距离开始地址的p
  int SpaceOffsetForAddress(Address addr) { return addr - low(); }

 private:
  int capacity_;
  int maximum_capacity_;
  Address start_;
  Address age_mark_;
  // 见SetUp函数
  uint32_t address_mask_;
  uint32_t object_mask_;
  uint32_t object_expected_;

 public:
  TRACK_MEMORY("SemiSpace")
};

SemiSpace::SemiSpace(int initial_capacity, int maximum_capacity)
    : capacity_(initial_capacity), 
      maximum_capacity_(maximum_capacity),
      start_(NULL), 
      age_mark_(NULL) {
}

// 设置管理的地址范围
bool SemiSpace::Setup(Address start, int size) {
  ASSERT(size == maximum_capacity_);
  // 判断地址的有效性
  if (!MemoryAllocator::CommitBlock(start, capacity_)) return false;
  // 管理地址空间的首地址
  start_ = start;
  // 低于有效范围的掩码，即保证相与后的值小于等于管理的地址范围
  address_mask_ = ~(size - 1);
  // 计算堆对象地址掩码，低位是标记位，判断的时候需要保留,kHeapObjectTag是堆对象的标记
  object_mask_ = address_mask_ | kHeapObjectTag;
  // 见contains函数，对象地址里低位是标记位，判断的时候需要带上
  object_expected_ = reinterpret_cast<uint32_t>(start) | kHeapObjectTag;
  // gc相关
  age_mark_ = start_;
  return true;
}

void SemiSpace::TearDown() {
  start_ = NULL;
  capacity_ = 0;
}

bool SemiSpace::Double() {
  // 内存在其他地方分配了，这里校验地址是否合法，即是否分配了
  if (!MemoryAllocator::CommitBlock(high(), capacity_)) return false;
  capacity_ *= 2;
  return true;
}