iOS-類（NSObject）的方法緩存

時間 2020-01-06

標籤 ios nsobject 方法緩存欄目 iOS 简体版

原文原文鏈接

在上一篇文章中大概的分析了類的結構、以及類的屬性與方法的存儲，接下來咱們分析類結構中的方法緩存：cache。數組

類的結構：緩存

truct objc_class : objc_object {
    // Class ISA;
    Class superclass;
    cache_t cache;             // formerly cache pointer and vtable
    class_data_bits_t bits;    // class_rw_t * plus custom rr/alloc flags

。。。
}
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1、cache介紹

一、cache的做用

類的方法緩存，增長方法查找效率。bash

二、cache_t內部結構：

struct cache_t {
    struct bucket_t *_buckets;
    mask_t _mask;
    mask_t _occupied;

public:
    struct bucket_t *buckets();
    mask_t mask();
    mask_t occupied();
    void incrementOccupied();
    void setBucketsAndMask(struct bucket_t *newBuckets, mask_t newMask);
    void initializeToEmpty();

    mask_t capacity();
    bool isConstantEmptyCache();
    bool canBeFreed();

    static size_t bytesForCapacity(uint32_t cap);
    static struct bucket_t * endMarker(struct bucket_t *b, uint32_t cap);

    void expand();
    void reallocate(mask_t oldCapacity, mask_t newCapacity);
    struct bucket_t * find(cache_key_t key, id receiver);

    static void bad_cache(id receiver, SEL sel, Class isa) __attribute__((noreturn));
};
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_buckets：bucket_t結構體的數組，bucket_t是用來存放方法的SEL內存地址和IMP的
_mask：數組大小 - 1，用做掩碼。
_occupied：當前已緩存的方法數。

struct bucket_t {
    cache_key_t _key; // typedef unsigned long
    IMP _imp;
...
};
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_key：cache_key_t 就是 unsigned long類型，用來存儲SEL的內存地址。SEL應該是char類型的字符串，char強轉unsigned long，其實就是SEL的內存地址。
_imp：方法對應的函數內存地址。

2、cache_t結構中的主要函數

一、緩存查找

bucket_t * cache_t::find(cache_key_t k, id receiver)
{
    assert(k != 0);

    bucket_t *b = buckets();
    mask_t m = mask();
    mask_t begin = cache_hash(k, m);
    mask_t i = begin;
    do {
        if (b[i].key() == 0  ||  b[i].key() == k) {
            return &b[i];
        }
    } while ((i = cache_next(i, m)) != begin);

    Class cls = (Class)((uintptr_t)this - offsetof(objc_class, cache));
    cache_t::bad_cache(receiver, (SEL)k, cls);
}

// --------------------find內部調用的cache_hash-------------------------
static inline mask_t cache_hash(cache_key_t key, mask_t mask) 
{
    return (mask_t)(key & mask);
}
// --------------------find內部調用的cache_next（arm64）-----------------
static inline mask_t cache_next(mask_t i, mask_t mask) {
    return i ? i-1 : mask;
}
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分析：less

獲取到buckets散列表與掩碼mask，經過cache_hash(key & mask)獲得key值對應的索引begin。函數
將begin賦值給i，方便切換索引。post
進行do-while循環查找ui
```
(i = cache_next(i, m)) != begin
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```
好比起始下標是4，總長度是8，依次遞減，當i = 0時，返回mask，則繼續從總長度 = 8開始查找，直到index 再次等於 4，說明已經遍歷一圈了，仍是沒找到，方法緩存查找結束。this
```
if (b[i].key() == 0  ||  b[i].key() == k)
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```
用i做爲索引從散列表中取值，若是取出來的bucket_t的 key = k，則查詢成功，返回該bucket_t。spa

若是key = 0，說明在索引i的位置上尚未緩存過方法，一樣須要返回該bucket_t，用於終止緩存查詢。線程
若是此時尚未找到key對應的bucket_t，或者是空的bucket_t，則循環結束，說明查找失敗，調用bad_cache方法。

二、緩存填充插入

緩存填充插入函數中調用了查找方法，這個方法也是咱們須要關注的重點

static void cache_fill_nolock(Class cls, SEL sel, IMP imp, id receiver)
{
    cacheUpdateLock.assertLocked();

    // Never cache before +initialize is done
    if (!cls->isInitialized()) return;

    // Make sure the entry was not added to the cache by some other thread 
    // before we grabbed the cacheUpdateLock.
    if (cache_getImp(cls, sel)) return;

    cache_t *cache = getCache(cls);
    cache_key_t key = getKey(sel);

    // Use the cache as-is if it is less than 3/4 full
    mask_t newOccupied = cache->occupied() + 1;
    mask_t capacity = cache->capacity();
    if (cache->isConstantEmptyCache()) {
        // Cache is read-only. Replace it.
        cache->reallocate(capacity, capacity ?: INIT_CACHE_SIZE);
    }
    else if (newOccupied <= capacity / 4 * 3) {
        // Cache is less than 3/4 full. Use it as-is.
    }
    else {
        // Cache is too full. Expand it.
        cache->expand();
    }

    // Scan for the first unused slot and insert there.
    // There is guaranteed to be an empty slot because the 
    // minimum size is 4 and we resized at 3/4 full.
    bucket_t *bucket = cache->find(key, receiver);
    if (bucket->key() == 0) cache->incrementOccupied();
    bucket->set(key, imp);
}
//-------------------------capacity() -----------------------------
mask_t cache_t::capacity() 
{
    return mask() ? mask()+1 : 0; 
}
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在確保緩存沒有被其餘線程添加
記錄對當前將要緩存的方法數mask_t newOccupied = cache->occupied() + 1;
獲取當前散列表的容量
判斷當前cache是否爲只讀的，也就是還未初始化，則從新分配緩存容量
若是newOccupied大於容量的3/4,就要進行擴容
經過find函數獲取到對應的bucket_t。
判斷返回的bucket->key() 是否爲0

若是爲0，就說明該位置上是空的，沒有緩存過方法，所以須要添加一個新的緩存。
```
void cache_t::incrementOccupied() 
{
    _occupied++;
}
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```
將key與imp保存在緩存中

三、緩存擴容

void cache_t::expand()
{
    cacheUpdateLock.assertLocked();
    
    uint32_t oldCapacity = capacity();
    uint32_t newCapacity = oldCapacity ? oldCapacity*2 : INIT_CACHE_SIZE;

    if ((uint32_t)(mask_t)newCapacity != newCapacity) {
        // mask overflow - can not grow further
        // fixme this wastes one bit of mask
        newCapacity = oldCapacity;
    }

    reallocate(oldCapacity, newCapacity);
}

//--------------------------------------------------------
INIT_CACHE_SIZE      = (1 << INIT_CACHE_SIZE_LOG2) // 4
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當前緩存容量不存在時，開闢一個INIT_CACHE_SIZE（4）大小的緩存容量，存在則*2翻倍。

四、緩存分配

能夠看到，緩存填充插入與擴容都須要用到緩存分配函數reallocate：

void cache_t::reallocate(mask_t oldCapacity, mask_t newCapacity)
{
    bool freeOld = canBeFreed();

    bucket_t *oldBuckets = buckets();
    bucket_t *newBuckets = allocateBuckets(newCapacity);

    // Cache's old contents are not propagated. 
    // This is thought to save cache memory at the cost of extra cache fills.
    // fixme re-measure this

    assert(newCapacity > 0);
    assert((uintptr_t)(mask_t)(newCapacity-1) == newCapacity-1);

    setBucketsAndMask(newBuckets, newCapacity - 1);
    
    if (freeOld) {
        cache_collect_free(oldBuckets, oldCapacity);
        cache_collect(false);
    }
}
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用到的函數：

// 釋放判斷
bool cache_t::canBeFreed()
{
    return !isConstantEmptyCache();
}

bool cache_t::isConstantEmptyCache()
{
    return 
        occupied() == 0  &&  
        buckets() == emptyBucketsForCapacity(capacity(), false);
}

//---------------------------setBucketsAndMask-------------------------------
void cache_t::setBucketsAndMask(struct bucket_t *newBuckets, mask_t newMask)
{
    mega_barrier();

    _buckets = newBuckets;
    _mask = newMask;
    _occupied = 0;
}
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