iOS weak 的实现

作者: 哦呵呵y | 来源:发表于2018-10-15 14:56 被阅读117次

iOS ARC中引用计数的实现
 iOS weak 的实现
 ARC中的数据结构以及寻址方式

SideTables 是一个全局的 hash 表,用来存储对象多余的引用计数，以及弱引用表

一、 SideTables

image.png

SideTables 是一个全局的 hash 表, 其中包含了一个数量为64的数组，数组中的存储的为SideTable结构体，其中通过对象内存地址作为hash表的key

    enum { StripeCount = 64 };

    PaddedT array[StripeCount];

    static unsigned int indexForPointer(const void *p) {
        uintptr_t addr = reinterpret_cast<uintptr_t>(p);
        return ((addr >> 4) ^ (addr >> 9)) % StripeCount;
    }

SideTable中包含一个 c++ Map RefcountMap refcnts 用来对象存储额外的引用计数，一个结构体weak_table_t weak_table 用来存储对象的弱引用数据

image.png

RefcountMap refcnts 中通过一个size_t（64位系统中占用64位）来保存引用计数，其中1位用来存储固定标志位，在溢出的时候使用，一位表示正在释放中，一位表示是否有弱引用，其余位表示实际的引用计数

RefcountMap refcnts 是一个C++的对象，内部包含了一个迭代器
其中以DisguisedPtr<objc_object> 对象指针为key，size_t 为value保存对象引用计数
将key、value通过std::pair打包以后，放入迭代器中，所以取出值之后，.first代表key，.second代表value
在上一篇中、retain release方法会使用到SideTables来存放引用计数

// Move some retain counts to the side table from the isa field.
// Returns true if the object is now pinned.
bool 
objc_object::sidetable_addExtraRC_nolock(size_t delta_rc)
{
    assert(isa.nonpointer);
    SideTable& table = SideTables()[this];

    size_t& refcntStorage = table.refcnts[this];
    size_t oldRefcnt = refcntStorage;
    // isa-side bits should not be set here
    assert((oldRefcnt & SIDE_TABLE_DEALLOCATING) == 0);
    assert((oldRefcnt & SIDE_TABLE_WEAKLY_REFERENCED) == 0);

    if (oldRefcnt & SIDE_TABLE_RC_PINNED) return true;

    uintptr_t carry;
    size_t newRefcnt = 
        addc(oldRefcnt, delta_rc << SIDE_TABLE_RC_SHIFT, 0, &carry);
    if (carry) {
        refcntStorage =
            SIDE_TABLE_RC_PINNED | (oldRefcnt & SIDE_TABLE_FLAG_MASK);
        return true;
    }
    else {
        refcntStorage = newRefcnt;
        return false;
    }
}


// Move some retain counts from the side table to the isa field.
// Returns the actual count subtracted, which may be less than the request.
size_t 
objc_object::sidetable_subExtraRC_nolock(size_t delta_rc)
{
    assert(isa.nonpointer);
    SideTable& table = SideTables()[this];

    RefcountMap::iterator it = table.refcnts.find(this);
    if (it == table.refcnts.end()  ||  it->second == 0) {
        // Side table retain count is zero. Can't borrow.
        return 0;
    }
    size_t oldRefcnt = it->second;

    // isa-side bits should not be set here
    assert((oldRefcnt & SIDE_TABLE_DEALLOCATING) == 0);
    assert((oldRefcnt & SIDE_TABLE_WEAKLY_REFERENCED) == 0);

    size_t newRefcnt = oldRefcnt - (delta_rc << SIDE_TABLE_RC_SHIFT);
    assert(oldRefcnt > newRefcnt);  // shouldn't underflow
    it->second = newRefcnt;
    return delta_rc;
}

第一个方法用来将多余的引用计数保存到SideTables中, 第二个方法用来取出引用计数
其中都用到了SideTables中的RefcountMap refcnts，但是两个方法的实现并不相同

// 第一种方式
    SideTable& table = SideTables()[this];

    size_t& refcntStorage = table.refcnts[this];
    size_t oldRefcnt = refcntStorage;

 // 第二种方式
    SideTable& table = SideTables()[this];

    RefcountMap::iterator it = table.refcnts.find(this);
    if (it == table.refcnts.end()  ||  it->second == 0) {
        // Side table retain count is zero. Can't borrow.
        return 0;
    }
    size_t oldRefcnt = it->second;

其实第一种方式是通过重载[]操作符来实现查找的，但是添加的时候需要判断如果不存在记录的话，需要新增一条数据保存，所以使用了两种不同的方法

 // 通过操作符重载实现 查找、新增插入
  value_type& FindAndConstruct(const KeyT &Key) {
    BucketT *TheBucket;
    if (LookupBucketFor(Key, TheBucket))
      return *TheBucket;

    return *InsertIntoBucket(Key, ValueT(), TheBucket);
  }

  ValueT &operator[](const KeyT &Key) {
    return FindAndConstruct(Key).second;
  }

两个方法都是获取到RefcountMap refcnts 进行引用计数的增加和减少

weak_table_t 弱引用表

全局弱引用表 weak_table_t

struct weak_table_t {
    weak_entry_t *weak_entries;
    size_t    num_entries;
    uintptr_t mask;
    uintptr_t max_hash_displacement;
};

弱引用表的内部结构

struct weak_entry_t {
    DisguisedPtr<objc_object> referent;
    union {
        struct {
            weak_referrer_t *referrers;
            uintptr_t        out_of_line_ness : 2;
            uintptr_t        num_refs : PTR_MINUS_2;
            uintptr_t        mask;
            uintptr_t        max_hash_displacement;
        };
        struct {
            // out_of_line_ness field is low bits of inline_referrers[1]
            weak_referrer_t  inline_referrers[WEAK_INLINE_COUNT];
        };
    };
};

1️⃣. 其中主要包含了两个属性DisguisedPtr<objc_object> referent 对象指针，还有一个容器类保存所以只需这个对象的弱引用
2️⃣. 共用体中包含两种结构体，当弱引用数量少于4的时候，使用数据结构来存储，当超过4个的时候使用hash表进行存储，out_of_line_ness 默认为 ob00，当弱引用数量大于4的时候，设置为 REFERRERS_OUT_OF_LINE ob10，通过判断out_of_line_ness来决定用什么方式存储
3️⃣. weak_referrer_t *referrers 是一个二级指针实现的hash表

二、 weak 的实现原理

{
  id obj1 = [[NSObject alloc] init];
  id __weak obj2 = obj1;
}

当我们创建弱引用对象时，系统会编译成以下代码

id obj2;
objc_initWeak(&obj2, obj1);
objc_destroyWeak(&obj2);

通过 objc_initWeak(&obj2, obj1);创建weak引用，在对象作用域结束时，使用objc_destroyWeak(&obj2);来释放引用

创建弱引用

id
objc_initWeak(id *location, id newObj)
{
    if (!newObj) {
        *location = nil;
        return nil;
    }

    return storeWeak<false/*old*/, true/*new*/, true/*crash*/>
        (location, (objc_object*)newObj);
}

void
objc_destroyWeak(id *location)
{
    (void)storeWeak<true/*old*/, false/*new*/, false/*crash*/>
        (location, nil);
}

两个方法最终都调用了storeWeak，但是两个方法调用参数不同。
init方法中，将要引用的对象作为参数传递，并且HaveOld参数为false，HaveNew参数为true
而destroy方法中，将nil作为新对象传入，并且HaveOld参数为true，HaveNew参数为false

HaveOld 代表是否有旧的引用，如果为true，则代表有旧的引用需要释放
HaveNew 代表是否有新的引用，如果为true，则代表要存储新的引用

template <bool HaveOld, bool HaveNew, bool CrashIfDeallocating>
static id 
storeWeak(id *location, objc_object *newObj)
{
    assert(HaveOld  ||  HaveNew);
    if (!HaveNew) assert(newObj == nil);

    Class previouslyInitializedClass = nil;
    id oldObj;
    SideTable *oldTable;
    SideTable *newTable;

    // Acquire locks for old and new values.
    // Order by lock address to prevent lock ordering problems. 
    // Retry if the old value changes underneath us.

// 分别获取新旧值相关联的引用表
 retry:
    if (HaveOld) {
        oldObj = *location;
        oldTable = &SideTables()[oldObj];
    } else {
        oldTable = nil;
    }
    if (HaveNew) {
        newTable = &SideTables()[newObj];
    } else {
        newTable = nil;
    }
// 加锁防止多线程资源竞争
    SideTable::lockTwo<HaveOld, HaveNew>(oldTable, newTable);
// 如果旧值改变就重新获取旧值相关联的表
    if (HaveOld  &&  *location != oldObj) {
        SideTable::unlockTwo<HaveOld, HaveNew>(oldTable, newTable);
        goto retry;
    }

    // Prevent a deadlock between the weak reference machinery
    // and the +initialize machinery by ensuring that no 
    // weakly-referenced object has an un-+initialized isa.
// 如果有新值，判断新值所属的类是否已经初始化，如果没有初始化，则先执行初始化，防止+initialize内部调用storeWeak产生死锁
    if (HaveNew  &&  newObj) {
        Class cls = newObj->getIsa();
        if (cls != previouslyInitializedClass  &&  
            !((objc_class *)cls)->isInitialized()) 
        {
            SideTable::unlockTwo<HaveOld, HaveNew>(oldTable, newTable);
            _class_initialize(_class_getNonMetaClass(cls, (id)newObj));

            // If this class is finished with +initialize then we're good.
            // If this class is still running +initialize on this thread 
            // (i.e. +initialize called storeWeak on an instance of itself)
            // then we may proceed but it will appear initializing and 
            // not yet initialized to the check above.
            // Instead set previouslyInitializedClass to recognize it on retry.
            previouslyInitializedClass = cls;

            goto retry;
        }
    }

    // Clean up old value, if any.
// 清空旧值
    if (HaveOld) {
        weak_unregister_no_lock(&oldTable->weak_table, oldObj, location);
    }

    // Assign new value, if any.
// 
    if (HaveNew) {
        newObj = (objc_object *)weak_register_no_lock(&newTable->weak_table, 
                                                      (id)newObj, location, 
                                                      CrashIfDeallocating);
        // weak_register_no_lock returns nil if weak store should be rejected

        // Set is-weakly-referenced bit in refcount table.
// 如果存储成功则设置SideTable中弱引用标志位
        if (newObj  &&  !newObj->isTaggedPointer()) {
            newObj->setWeaklyReferenced_nolock();
        }

        // Do not set *location anywhere else. That would introduce a race.
        *location = (id)newObj;
    }
    else {
        // No new value. The storage is not changed.
    }
    
    SideTable::unlockTwo<HaveOld, HaveNew>(oldTable, newTable);

    return (id)newObj;
}

以上就是 store_weak 这个函数的实现，它主要做了以下几件事：

分别获取新旧值的散列表指针
如果有旧值就调用 weak_unregister_no_lock 函数清除旧值
如果有新值就调用 weak_register_no_lock 函数分配新值

weak_register_no_lock 实现：

id 
weak_register_no_lock(weak_table_t *weak_table, id referent_id, 
                      id *referrer_id, bool crashIfDeallocating)
{
    objc_object *referent = (objc_object *)referent_id;
    objc_object **referrer = (objc_object **)referrer_id;

    if (!referent  ||  referent->isTaggedPointer()) return referent_id;

    // ensure that the referenced object is viable
    bool deallocating;
    if (!referent->ISA()->hasCustomRR()) {
        deallocating = referent->rootIsDeallocating();
    }
    else {
        BOOL (*allowsWeakReference)(objc_object *, SEL) = 
            (BOOL(*)(objc_object *, SEL))
            object_getMethodImplementation((id)referent, 
                                           SEL_allowsWeakReference);
        if ((IMP)allowsWeakReference == _objc_msgForward) {
            return nil;
        }
        deallocating =
            ! (*allowsWeakReference)(referent, SEL_allowsWeakReference);
    }

    if (deallocating) {
        if (crashIfDeallocating) {
            _objc_fatal("Cannot form weak reference to instance (%p) of "
                        "class %s. It is possible that this object was "
                        "over-released, or is in the process of deallocation.",
                        (void*)referent, object_getClassName((id)referent));
        } else {
            return nil;
        }
    }

    // now remember it and where it is being stored
    weak_entry_t *entry;
    if ((entry = weak_entry_for_referent(weak_table, referent))) {
        append_referrer(entry, referrer);
    } 
    else {
        weak_entry_t new_entry(referent, referrer);
        weak_grow_maybe(weak_table);
        weak_entry_insert(weak_table, &new_entry);
    }

    // Do not set *referrer. objc_storeWeak() requires that the 
    // value not change.

    return referent_id;
}

weak_register_no_lock 用来保存弱引用信息，具体实现如下：

判断对象是否可以弱引用，是否正在释放。
查询weak_table_t, 判断对象是否已经保存有相关联的弱引用信息
如果已经有相关弱引用信息，则调用append_referrer方法添加进现在的weak_entry_t结构中，如果没有相关联信息，则创建weak_entry_t 节点，并且插入到weak_table_t弱引用表中。

weak_unregister_no_lock 实现：

void
weak_unregister_no_lock(weak_table_t *weak_table, id referent_id, 
                        id *referrer_id)
{
    objc_object *referent = (objc_object *)referent_id;
    objc_object **referrer = (objc_object **)referrer_id;

    weak_entry_t *entry;

    if (!referent) return;

    if ((entry = weak_entry_for_referent(weak_table, referent))) {
        remove_referrer(entry, referrer);
        bool empty = true;
        if (entry->out_of_line()  &&  entry->num_refs != 0) {
            empty = false;
        }
        else {
            for (size_t i = 0; i < WEAK_INLINE_COUNT; i++) {
                if (entry->inline_referrers[i]) {
                    empty = false; 
                    break;
                }
            }
        }

        if (empty) {
            weak_entry_remove(weak_table, entry);
        }
    }

    // Do not set *referrer = nil. objc_storeWeak() requires that the 
    // value not change.
}

weak_unregister_no_lock 用来移除弱引用信息，具体实现如下：

查询弱引用表中是否保存有相关联的弱引用信息
如果有，则调用remove_referrer方法移除相关联的弱引用信息
移除相关联弱引用信息之后，判断存储数组是否为空，如果为空，则调用weak_entry_remove移除weak_entry_t节点

在dealloc中，释放对象时会调用一下方法

NEVER_INLINE void
objc_object::clearDeallocating_slow()
{
    assert(isa.nonpointer  &&  (isa.weakly_referenced || isa.has_sidetable_rc));

    SideTable& table = SideTables()[this];
    table.lock();
    if (isa.weakly_referenced) {
        weak_clear_no_lock(&table.weak_table, (id)this);
    }
    if (isa.has_sidetable_rc) {
        table.refcnts.erase(this);
    }
    table.unlock();
}

此方法用来移除SideTables中对象相关联的数据

判断是否有弱引用，如果有调用weak_clear_no_lock方法移除所有相关联弱引用
判断是否有额外的引用计数存储在SideTables中，如果有则调用table.refcnts.erase(this);清除

void 
weak_clear_no_lock(weak_table_t *weak_table, id referent_id) 
{
    objc_object *referent = (objc_object *)referent_id;

    weak_entry_t *entry = weak_entry_for_referent(weak_table, referent);
    if (entry == nil) {
        /// XXX shouldn't happen, but does with mismatched CF/objc
        //printf("XXX no entry for clear deallocating %p\n", referent);
        return;
    }

    // zero out references
    weak_referrer_t *referrers;
    size_t count;
    
    if (entry->out_of_line()) {
        referrers = entry->referrers;
        count = TABLE_SIZE(entry);
    } 
    else {
        referrers = entry->inline_referrers;
        count = WEAK_INLINE_COUNT;
    }
    
    for (size_t i = 0; i < count; ++i) {
        objc_object **referrer = referrers[i];
        if (referrer) {
            if (*referrer == referent) {
                *referrer = nil;
            }
            else if (*referrer) {
                _objc_inform("__weak variable at %p holds %p instead of %p. "
                             "This is probably incorrect use of "
                             "objc_storeWeak() and objc_loadWeak(). "
                             "Break on objc_weak_error to debug.\n", 
                             referrer, (void*)*referrer, (void*)referent);
                objc_weak_error();
            }
        }
    }
    
    weak_entry_remove(weak_table, entry);
}

weak_clear_no_lock 方法用来清空对象相关联的所有弱引用数据，具体实现如下：

判断对象是否有相关联的弱引用数据，如果没有则直接return
通过out_of_line判断实际存储结构，如果为0b10则用的是指针数组，否则是长度为4的数组结构
遍历数组，将所有对象设置为nil
调用weak_entry_remove(weak_table, entry);移除节点

iOS weak 的实现

一、 SideTables

weak_table_t 弱引用表

二、 weak 的实现原理

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