此文实际成于 2015/07/29

objc_retain

__attribute__((aligned(16)))
id 
objc_retain(id obj)
{
    if (!obj) return obj;
    if (obj->isTaggedPointer()) return obj;
    return obj->retain();
}

主要调用以下其类方法：

// Equivalent to calling [this retain], with shortcuts if there is no override
inline id 
objc_object::retain()
{
    // UseGC is allowed here, but requires hasCustomRR.
    assert(!UseGC  ||  ISA()->hasCustomRR());
    assert(!isTaggedPointer());

    if (! ISA()->hasCustomRR()) {
        return sidetable_retain();
    }

    return ((id(*)(objc_object *, SEL))objc_msgSend)(this, SEL_retain);
}

因此主要就是这一句代码在起 retian的作用：

return ((id(*)(objc_object *, SEL))objc_msgSend)(this, SEL_retain);
它向对象发送retain的消息。

即调用：

// Replaced by ObjectAlloc
- (id)retain {
    return ((id)self)->rootRetain();
}

调用：

// Base retain implementation, ignoring overrides.
// This does not check isa.fast_rr; if there is an RR override then 
// it was already called and it chose to call [super retain].
inline id 
objc_object::rootRetain()
{
    assert(!UseGC);

    if (isTaggedPointer()) return (id)this;
    return sidetable_retain();
}

然后到：

id
objc_object::sidetable_retain()
{
#if SUPPORT_NONPOINTER_ISA
    assert(!isa.indexed);
#endif
    SideTable *table = SideTable::tableForPointer(this);

    if (spinlock_trylock(&table->slock)) {
        size_t& refcntStorage = table->refcnts[this];
        if (! (refcntStorage & SIDE_TABLE_RC_PINNED)) {
            refcntStorage += SIDE_TABLE_RC_ONE;
        }
        spinlock_unlock(&table->slock);
        return (id)this;
    }
    return sidetable_retain_slow(table);
}

主要就是引用计数加一： refcntStorage += SIDE_TABLE_RC_ONE;
不过有点费解的是，SIDE_TABLE_RC_ONE的宏定义如下：
#define SIDE_TABLE_RC_ONE (1UL<<2) // MSB-ward of deallocating bit

这样每次加的值就是 1 << 2 = 4了。

因为我需要更清楚其retainCount 的对应方法。所以：

objc_object::rootRetainCount()

- (NSUInteger)retainCount {
    return ((id)self)->rootRetainCount();
}

接着：

inline uintptr_t 
objc_object::rootRetainCount()
{
    assert(!UseGC);

    if (isTaggedPointer()) return (uintptr_t)this;
    return sidetable_retainCount();
}

然后：

uintptr_t
objc_object::sidetable_retainCount()
{
    SideTable *table = SideTable::tableForPointer(this);

    size_t refcnt_result = 1;
    
    spinlock_lock(&table->slock);
    RefcountMap::iterator it = table->refcnts.find(this);
    if (it != table->refcnts.end()) {
        // this is valid for SIDE_TABLE_RC_PINNED too
        refcnt_result += it->second >> SIDE_TABLE_RC_SHIFT;
    }
    spinlock_unlock(&table->slock);
    return refcnt_result;
}

先看下这个SHIFT
#define SIDE_TABLE_RC_SHIFT 2

int >> 2 相当于是除4的效果。之前一是一次加4，现在的引用计数的结果是以除4的结果计算。
所以也还是相对应的。
因为假充引用计数为 2 4 + 4 = 8 然后 8 >> 2 = 2

objc_release

跟retain它后面调用如下：

// Base release implementation, ignoring overrides.
// Does not call -dealloc.
// Returns true if the object should now be deallocated.
// This does not check isa.fast_rr; if there is an RR override then 
// it was already called and it chose to call [super release].
inline bool 
objc_object::rootRelease()
{
    assert(!UseGC);

    if (isTaggedPointer()) return false;
    return sidetable_release(true);
}

最后都转变成操作 SideTable 的相关操作：

bool 
objc_object::sidetable_release(bool performDealloc)
{
#if SUPPORT_NONPOINTER_ISA
    assert(!isa.indexed);
#endif
    SideTable *table = SideTable::tableForPointer(this);

    bool do_dealloc = false;

    if (spinlock_trylock(&table->slock)) {
        RefcountMap::iterator it = table->refcnts.find(this);
        if (it == table->refcnts.end()) {
            do_dealloc = true;
            table->refcnts[this] = SIDE_TABLE_DEALLOCATING;
        } else if (it->second < SIDE_TABLE_DEALLOCATING) {
            // SIDE_TABLE_WEAKLY_REFERENCED may be set. Don't change it.
            do_dealloc = true;
            it->second |= SIDE_TABLE_DEALLOCATING;
        } else if (! (it->second & SIDE_TABLE_RC_PINNED)) {
            it->second -= SIDE_TABLE_RC_ONE;
        }
        spinlock_unlock(&table->slock);
        if (do_dealloc  &&  performDealloc) {
            ((void(*)(objc_object *, SEL))objc_msgSend)(this, SEL_dealloc);
        }
        return do_dealloc;
    }

    return sidetable_release_slow(table, performDealloc);
}

看到上面判断引用计数是否为 0，总算是明白了为什么引用计数加1要加4了。
当值为 1，和 2 是用作特殊的标志的。

#define SIDE_TABLE_WEAKLY_REFERENCED (1UL<<0)
#define SIDE_TABLE_DEALLOCATING      (1UL<<1)  // MSB-ward of weak bit

附录

SUPPORT_NONPOINTER_ISA

上面的宏判断
对于 基于 arm64的 iOS 上 Objective-C 的对象的isa字段不再是一个指针。
对于此转变的解释见： http://www.sealiesoftware.com/blog/archive/2013/09/24/objc_explain_Non-pointer_isa.html
宏声明如下：

// Define SUPPORT_NONPOINTER_ISA=1 to enable extra data in the isa field.
#if !__LP64__  ||  TARGET_OS_WIN32  ||  TARGET_IPHONE_SIMULATOR  ||  __x86_64__
#   define SUPPORT_NONPOINTER_ISA 0
#else
#   define SUPPORT_NONPOINTER_ISA 1
#endif