Swift引用计数

• Swift引用计数官方文档描述如下

  Strong and unowned variables point at the object.
   Weak variables point at the object's side table.
  
  
   Storage layout:
  
   HeapObject {
     isa
  /// 只存储 strong unowned 引用计数
     InlineRefCounts {
       atomic<InlineRefCountBits> {
         strong RC + unowned RC + flags
         OR
         HeapObjectSideTableEntry*
       }
     }
   }
  /// 存储 weak 以及 strong unowned的引用计数
   HeapObjectSideTableEntry {
     SideTableRefCounts {
       object pointer
       atomic<SideTableRefCountBits> {
         strong RC + unowned RC + weak RC + flags
       }
     }   
   }
  

• 获取class-refcount的swift源码提供的函数如下,直接粘贴进项目可以直接打印:(纯Swift-class)

  /// 获取强引用计数
  @_silgen_name("swift_retainCount")
  public func _getRetainCount(_ Value: AnyObject) -> UInt
    
  /// 获取unowned引用计数
  @_silgen_name("swift_unownedRetainCount")
  public func _getUnownedRetainCount(_ Value : AnyObject) -> UInt
    
  /// 获取weak引用计数
  @_silgen_name("swift_weakRetainCount")
  public func _getWeakRetainCount(_ Value : AnyObject) -> UInt
  

• 一个swift-class初始化的时候weak-refcount和unowned-refcount以及strong-refcount默认都是1

• 一个Class引用计数存储位置的

  /// RefCountNotInline 当使用weak时的标识
  /// RefCountIsInline  当前未使用weak
  enum RefCountInlinedness { RefCountNotInline = false, RefCountIsInline = true };
  

• 如果当前class没有使用weak引用计数,存储的bits标识是RefCountBitsT<RefCountIsInline> InlineRefCountBits

  typedef RefCountBitsT<RefCountIsInline> InlineRefCountBits;
  

• 如果当前class中使用的weak引用计数, 存储的bits标识是class SideTableRefCountBits : public RefCountBitsT<RefCountNotInline>

  /// sideTable  主要记录weak的引用及时
  class SideTableRefCountBits : public RefCountBitsT<RefCountNotInline>
  {
    /// weak 引用计数
    uint32_t weakBits;
  
    public:
    SideTableRefCountBits() = default;
    
    constexpr
    SideTableRefCountBits(uint32_t strongExtraCount, uint32_t unownedCount)
      : RefCountBitsT<RefCountNotInline>(strongExtraCount, unownedCount)
      // weak refcount starts at 1 on behalf of the unowned count
        /// Weak 引用计数 如果 SideTableRefCountBits初始化 默认是1
      , weakBits(1)
    { }
  
    LLVM_ATTRIBUTE_ALWAYS_INLINE
    SideTableRefCountBits(HeapObjectSideTableEntry* side) = delete;
  
    LLVM_ATTRIBUTE_ALWAYS_INLINE
    SideTableRefCountBits(InlineRefCountBits newbits)
      : RefCountBitsT<RefCountNotInline>(&newbits), weakBits(1)
    { }
  
    
    LLVM_ATTRIBUTE_ALWAYS_INLINE
    void incrementWeakRefCount() {
      weakBits++;
    }
  
    LLVM_ATTRIBUTE_ALWAYS_INLINE
      /// 标识是否需要释放
    bool decrementWeakRefCount() {
      assert(weakBits > 0);
      weakBits--;
      return weakBits == 0;
    }
  
    LLVM_ATTRIBUTE_ALWAYS_INLINE
    uint32_t getWeakRefCount() {
      return weakBits;
    }
  
    // Side table ref count never has a side table of its own.
    LLVM_ATTRIBUTE_ALWAYS_INLINE
    bool hasSideTable() {
      return false;
    }
  };
  

• bits的的模板类,主要记录class的一些标识,包括是否存在side-table,是否是静态变量 ,还包含好引用计数的增加和减少

  // Basic encoding of refcount and flag data into the object's header.
  ///  RefCountBitsT
  template <RefCountInlinedness refcountIsInline>
  class RefCountBitsT {
  
    friend class RefCountBitsT<RefCountIsInline>;
    friend class RefCountBitsT<RefCountNotInline>;
    
    static const RefCountInlinedness Inlinedness = refcountIsInline;
  
     /// 萃取 type的 bits位数
    typedef typename RefCountBitsInt<refcountIsInline, sizeof(void*)>::Type
      BitsType;
    typedef typename RefCountBitsInt<refcountIsInline, sizeof(void*)>::SignedType
      SignedBitsType;
  
      ///  根据 type 萃取 偏移量
    typedef RefCountBitOffsets<sizeof(BitsType)>
      Offsets;
  
    BitsType bits;
    
    /// 获取side-table
    HeapObjectSideTableEntry *getSideTable() const {
      assert(hasSideTable());
  
      // Stored value is a shifted pointer.
      return reinterpret_cast<HeapObjectSideTableEntry *>
        (uintptr_t(getField(SideTable)) << Offsets::SideTableUnusedLowBits);
    }
    
    /// 增加strong引用计数
    void setStrongExtraRefCount(uint32_t value) {
      assert(!hasSideTable());
      setField(StrongExtraRefCount, value);
    }
    
    /// 增加strong引用计数
    bool incrementStrongExtraRefCount(uint32_t inc) {
      // This deliberately overflows into the UseSlowRC field.
      bits += BitsType(inc) << Offsets::StrongExtraRefCountShift;
      return (SignedBitsType(bits) >= 0);
    }
  }
  

• 其中RefCountBitsInt模板类负责适配不同的机型,如果32位机型overflow就是用64位

  template <RefCountInlinedness refcountIsInline, size_t sizeofPointer>
  struct RefCountBitsInt;
  
  /// 64 位
  template <RefCountInlinedness refcountIsInline>
  struct RefCountBitsInt<refcountIsInline, 8> {
    typedef uint64_t Type;
    typedef int64_t SignedType;
  };
  
  // 32-bit out of line
  // 32 位
  template <>
  struct RefCountBitsInt<RefCountNotInline, 4> {
    typedef uint64_t Type;
    typedef int64_t SignedType;
  };
  
  // 32-bit inline
  template <>
  struct RefCountBitsInt<RefCountIsInline, 4> {
    typedef uint32_t Type;
    typedef int32_t SignedType;  
  };
  

• RefCounts中RefCountBits使用的模板实体类

/// strong unowned 引用计数
typedef RefCounts<InlineRefCountBits> InlineRefCounts;

/// weak strong unowned 引用计数
typedef RefCounts<SideTableRefCountBits> SideTableRefCounts;

• RefCounts内存模型

  ///模板类
  template <typename RefCountBits>
  class RefCounts {
    /// strong unknown 引用计数
    std::atomic<RefCountBits> refCounts;
    
    public:
    /// 是否初始化
    enum Initialized_t { Initialized };
    /// 是否是常量, 不需要使用内存管理
    enum Immortal_t { Immortal };
    
    /// Return true if the object can be freed directly right now.
    /// (transition DEINITING -> DEAD)
    /// This is used in swift_deallocObject().
    /// Can be freed now means:  
    ///   no side table
    ///   unowned reference count is 1
    /// The object is assumed to be deiniting with no strong references already.
    
    /// 判断当前对象是否需要释放
    bool canBeFreedNow() const {
      auto bits = refCounts.load(SWIFT_MEMORY_ORDER_CONSUME);
      return (!bits.hasSideTable() &&
              /// 释放可以被释放
              bits.getIsDeiniting() &&
              /// 额外的引用计数
              bits.getStrongExtraRefCount() == 0 &&
              /// 这里的unknown的引用计数为1也会被释放
              bits.getUnownedRefCount() == 1);
    }
    
    /// Weak 存储的位置
    // WEAK
    
    public:
    // Returns the object's side table entry (creating it if necessary) with
    // its weak ref count incremented.
    // Returns nullptr if the object is already deiniting.
    // Use this when creating a new weak reference to an object.
    
    /// weak 引用计数管理
    HeapObjectSideTableEntry* formWeakReference();
    
    /// 判断对象是否需要释放
    template <PerformDeinit performDeinit>
    bool doDecrementSlow(RefCountBits oldbits, uint32_t dec) {
      RefCountBits newbits;
      
      bool deinitNow;
      do {
        ///记录
        newbits = oldbits;
        
        /// 获取有没有使用引用计数的地方
        bool fast =
          newbits.decrementStrongExtraRefCount(dec);
        if (fast) {
          // Decrement completed normally. New refcount is not zero.
          deinitNow = false;
        }
        /// 判断是否是常量
        else if (oldbits.isImmortal()) {
          return false;
          /// 判断是否存在side-Table
        } else if (oldbits.hasSideTable()) {
          // Decrement failed because we're on some other slow path.
          return doDecrementSideTable<performDeinit>(oldbits, dec);
        }
        else {
          // Decrement underflowed. Begin deinit.
          // LIVE -> DEINITING
          deinitNow = true;
          assert(!oldbits.getIsDeiniting());  // FIXME: make this an error?
          newbits = oldbits;  // Undo failed decrement of newbits.
          newbits.setStrongExtraRefCount(0);
          newbits.setIsDeiniting(true);
        }
      } while (!refCounts.compare_exchange_weak(oldbits, newbits,
                                                std::memory_order_release,
                                                std::memory_order_relaxed));
      if (performDeinit && deinitNow) {
        /// 原子性的加锁----非原子性释放 不用使用栅栏函数
        std::atomic_thread_fence(std::memory_order_acquire);
        
        /// 调用swift对象释放
        ///getHeapObject() 获取当前对象的内存地址
        _swift_release_dealloc(getHeapObject());
      }
  
      return deinitNow;
    }
  }
  

• 关于HeapObjectSideTableEntry的定义

  class HeapObjectSideTableEntry {
    // FIXME: does object need to be atomic?
    
    /// 存储的对象
    std::atomic<HeapObject*> object;
    
    /// 引用计数
    SideTableRefCounts refCounts;
  
    public:
    HeapObjectSideTableEntry(HeapObject *newObject)
      : object(newObject), refCounts()
    { }
  }