内存布局

代码段:保存程序二进制。
bss:一般保存全局静态变量等。
data:保存初始化的全局变量，静态变量。
栈：保存函数，方法。iOS开发中一般为0x7段。
堆：通过alloc出来的对象，保存在堆里。 0x6段

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TaggedPointer

TaggedPointer为苹果为内存优化提供的一个方案。
主要存储一些小字节的数据。

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TaggesPointer被设计为2大部分，标志位区和数据区。
由特殊标志位来标志类型。使得读写更加效率。

SideTables

哈希表

先说说存储结构，实际上在我们学过的数据结构可以归结为两类：连续的的存储结构和不联系的存储结构，其代表分别为数组和链表。而我们学过的堆栈，队列，树，图，都可以用这两种结构来实现。连续的存储结构——数组，在数据的查找和修改上具有很好的优点，很方便，时间复杂度很小。但是在数据的增添和删除上则显得很麻烦，空间复杂度很大。而非连续，非顺序的存储结构——链表恰和数组相反，数据的增添和删除容易，空间复杂度很小，查找和修改复杂，时间复杂度很大。

那么有没有一种数据结构能折衷一下数组和链表的优缺点呢？那就是——哈希表，既满足了数据的查找和修改很容易，同时又不占用很多空间的特点。

哈希表是基于哈希函数的，哈希表中的元素是有哈希函数确定的，哈希表作为一种数据结构，我们用哈希表来存储数据，在保存的时候存入的是一个<key—value>的结构，value由哈希函数作用于key上得到。但是存在一个哈希冲突问题，那就是当你用hash函数作用在两个互不相同的key上，得到的value值相等。

在runtime中，维护着一个sideTables，他是一个哈希表。

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sideTables由多个SideTable组成。

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sideTable又由一个自旋锁slock，一个引用计数表refcnts，一个弱引用表weak_table组成。

Retain和Release

1. Retain

objc_object::rootRetain(bool tryRetain, bool handleOverflow)
{
    if (isTaggedPointer()) return (id)this;

    bool sideTableLocked = false;
    bool transcribeToSideTable = false;

    isa_t oldisa;
    isa_t newisa;

    do {
        transcribeToSideTable = false;
        oldisa = LoadExclusive(&isa.bits);
        newisa = oldisa;
        if (slowpath(!newisa.nonpointer)) {
            ClearExclusive(&isa.bits);
            if (!tryRetain && sideTableLocked) sidetable_unlock();
            if (tryRetain) return sidetable_tryRetain() ? (id)this : nil;
            else return sidetable_retain();
        }
        // don't check newisa.fast_rr; we already called any RR overrides
        if (slowpath(tryRetain && newisa.deallocating)) {
            ClearExclusive(&isa.bits);
            if (!tryRetain && sideTableLocked) sidetable_unlock();
            return nil;
        }
        uintptr_t carry;
        newisa.bits = addc(newisa.bits, RC_ONE, 0, &carry);  // extra_rc++

        if (slowpath(carry)) {
            // newisa.extra_rc++ overflowed
            if (!handleOverflow) {
                ClearExclusive(&isa.bits);
                return rootRetain_overflow(tryRetain);
            }
            // Leave half of the retain counts inline and 
            // prepare to copy the other half to the side table.
            if (!tryRetain && !sideTableLocked) sidetable_lock();
            sideTableLocked = true;
            transcribeToSideTable = true;
            newisa.extra_rc = RC_HALF;
            newisa.has_sidetable_rc = true;
        }
    } while (slowpath(!StoreExclusive(&isa.bits, oldisa.bits, newisa.bits)));

    if (slowpath(transcribeToSideTable)) {
        // Copy the other half of the retain counts to the side table.
        sidetable_addExtraRC_nolock(RC_HALF);
    }

    if (slowpath(!tryRetain && sideTableLocked)) sidetable_unlock();
    return (id)this;
}

当调用retain时，isa.bits中的extra_rc+1
当extra_rc溢出时，sideTable.refcnts+1。
而之前提到的TaggedPointer是不进行引用计数的。

2. retainCount方法

   
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   返回extra_rc+1的值，如果sidetable_rc有值再加上sideTable中的值。

3.release

objc_object::rootRelease(bool performDealloc, bool handleUnderflow)
{
    if (isTaggedPointer()) return false;

    bool sideTableLocked = false;

    isa_t oldisa;
    isa_t newisa;

 retry:
    do {
        oldisa = LoadExclusive(&isa.bits);
        newisa = oldisa;
        if (slowpath(!newisa.nonpointer)) {
            ClearExclusive(&isa.bits);
            if (sideTableLocked) sidetable_unlock();
            return sidetable_release(performDealloc);
        }
        // don't check newisa.fast_rr; we already called any RR overrides
        uintptr_t carry;
        newisa.bits = subc(newisa.bits, RC_ONE, 0, &carry);  // extra_rc--
        if (slowpath(carry)) {
            // don't ClearExclusive()
            goto underflow;
        }
    } while (slowpath(!StoreReleaseExclusive(&isa.bits, 
                                             oldisa.bits, newisa.bits)));

    if (slowpath(sideTableLocked)) sidetable_unlock();
    return false;

 underflow:
    // newisa.extra_rc-- underflowed: borrow from side table or deallocate

    // abandon newisa to undo the decrement
    newisa = oldisa;

    if (slowpath(newisa.has_sidetable_rc)) {
        if (!handleUnderflow) {
            ClearExclusive(&isa.bits);
            return rootRelease_underflow(performDealloc);
        }

        // Transfer retain count from side table to inline storage.

        if (!sideTableLocked) {
            ClearExclusive(&isa.bits);
            sidetable_lock();
            sideTableLocked = true;
            // Need to start over to avoid a race against 
            // the nonpointer -> raw pointer transition.
            goto retry;
        }

        // Try to remove some retain counts from the side table.        
        size_t borrowed = sidetable_subExtraRC_nolock(RC_HALF);

        // To avoid races, has_sidetable_rc must remain set 
        // even if the side table count is now zero.

        if (borrowed > 0) {
            // Side table retain count decreased.
            // Try to add them to the inline count.
            newisa.extra_rc = borrowed - 1;  // redo the original decrement too
            bool stored = StoreReleaseExclusive(&isa.bits, 
                                                oldisa.bits, newisa.bits);
            if (!stored) {
                // Inline update failed. 
                // Try it again right now. This prevents livelock on LL/SC 
                // architectures where the side table access itself may have 
                // dropped the reservation.
                isa_t oldisa2 = LoadExclusive(&isa.bits);
                isa_t newisa2 = oldisa2;
                if (newisa2.nonpointer) {
                    uintptr_t overflow;
                    newisa2.bits = 
                        addc(newisa2.bits, RC_ONE * (borrowed-1), 0, &overflow);
                    if (!overflow) {
                        stored = StoreReleaseExclusive(&isa.bits, oldisa2.bits, 
                                                       newisa2.bits);
                    }
                }
            }

            if (!stored) {
                // Inline update failed.
                // Put the retains back in the side table.
                sidetable_addExtraRC_nolock(borrowed);
                goto retry;
            }

            // Decrement successful after borrowing from side table.
            // This decrement cannot be the deallocating decrement - the side 
            // table lock and has_sidetable_rc bit ensure that if everyone 
            // else tried to -release while we worked, the last one would block.
            sidetable_unlock();
            return false;
        }
        else {
            // Side table is empty after all. Fall-through to the dealloc path.
        }
    }

    // Really deallocate.

    if (slowpath(newisa.deallocating)) {
        ClearExclusive(&isa.bits);
        if (sideTableLocked) sidetable_unlock();
        return overrelease_error();
        // does not actually return
    }
    newisa.deallocating = true;
    if (!StoreExclusive(&isa.bits, oldisa.bits, newisa.bits)) goto retry;

    if (slowpath(sideTableLocked)) sidetable_unlock();

    __sync_synchronize();
    if (performDealloc) {
        ((void(*)(objc_object *, SEL))objc_msgSend)(this, SEL_dealloc);
    }
    return true;
}

extra_rc - 1
当extra_rc下溢出时，查找sideTable。如果sideTable也没有时，对this发送SEL_dealloc方法进行析构。