cache_t的结构

struct cache_t {
#if CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_OUTLINED // mac环境、模拟器
    explicit_atomic<struct bucket_t *> _buckets;
    explicit_atomic<mask_t> _mask;
#elif CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_HIGH_16 // 真机环境，64位系统
    explicit_atomic<uintptr_t> _maskAndBuckets;
    mask_t _mask_unused;    
// How much the mask is shifted by.
    static constexpr uintptr_t maskShift = 48;
    
    // Additional bits after the mask which must be zero. msgSend
    // takes advantage of these additional bits to construct the value
    // `mask << 4` from `_maskAndBuckets` in a single instruction.
    static constexpr uintptr_t maskZeroBits = 4;
    
    // The largest mask value we can store.
    static constexpr uintptr_t maxMask = ((uintptr_t)1 << (64 - maskShift)) - 1;
    
    // The mask applied to `_maskAndBuckets` to retrieve the buckets pointer.
    static constexpr uintptr_t bucketsMask = ((uintptr_t)1 << (maskShift - maskZeroBits)) - 1;
#elif CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_LOW_4 // 真机环境，低于64位系统
    // _maskAndBuckets stores the mask shift in the low 4 bits, and
    // the buckets pointer in the remainder of the value. The mask
    // shift is the value where (0xffff >> shift) produces the correct
    // mask. This is equal to 16 - log2(cache_size).
    explicit_atomic<uintptr_t> _maskAndBuckets;
    mask_t _mask_unused;
#if __LP64__
    uint16_t _flags;
#endif
    uint16_t _occupied;
    ......
};

• 主要包含4个成员_buckets、_mask、_flags、_occupied
• 真机环境下对_buckets和_mask进行了优化，从成员名字也可以看出_maskAndBuckets，是将两个成员放到一起了，从下面几个static成员猜测_mask应该是占16位，_buckets占48
• _buckets是一个bucket_t类型的散列表，bucket_t里面有方法名，及其地址,bucket_t的结构如下

struct bucket_t {
private:
    // IMP-first is better for arm64e ptrauth and no worse for arm64.
    // SEL-first is better for armv7* and i386 and x86_64.
#if __arm64__
    explicit_atomic<uintptr_t> _imp;
    explicit_atomic<SEL> _sel;
#else
    explicit_atomic<SEL> _sel;
    explicit_atomic<uintptr_t> _imp;
#endif
}

• _mask表示散列表的长度-1
• _flags
• _occupied表示缓存的方法数

cache的插入流程

• 方法查找lookUpImpOrForward->log_and_fill_cache->cache_fill->cache->insert->reallocate
• 我们着重看下ache->insert的实现

void cache_t::insert(Class cls, SEL sel, IMP imp, id receiver)
{
#if CONFIG_USE_CACHE_LOCK
    cacheUpdateLock.assertLocked();
#else
    runtimeLock.assertLocked();
#endif

    ASSERT(sel != 0 && cls->isInitialized());

    // Use the cache as-is if it is less than 3/4 full
    mask_t newOccupied = occupied() + 1;// 已有缓存数+1
    unsigned oldCapacity = capacity(), capacity = oldCapacity;
    if (slowpath(isConstantEmptyCache())) {// 当缓存为空的时候，创建缓存，小概率事件
        // Cache is read-only. Replace it.
        if (!capacity) capacity = INIT_CACHE_SIZE; // 缓存空间初始值为4
        reallocate(oldCapacity, capacity, /* freeOld */false);// 开辟缓存空间，最后一个参数传false表示新创建的不用释放原来的空间，后面的扩容需要释放旧的缓存
    }
    else if (fastpath(newOccupied + CACHE_END_MARKER <= capacity / 4 * 3)) {
        // Cache is less than 3/4 full. Use it as-is.
        // 如果小于等于缓存空间大小的3/4直接往下执行
    }
    else {// 如果超过了空间的3/4进行2倍扩容
        capacity = capacity ? capacity * 2 : INIT_CACHE_SIZE;
        if (capacity > MAX_CACHE_SIZE) {// 判断是否超出最大缓存空间
            capacity = MAX_CACHE_SIZE;
        }
        reallocate(oldCapacity, capacity, true);// 开辟空间，并释放旧的缓存
    }

    bucket_t *b = buckets();// 缓存方法表
    mask_t m = capacity - 1; // mask 为最大空间数-1
    mask_t begin = cache_hash(sel, m);// 获取哈希下标
    mask_t i = begin;

    // 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.
    do {
        if (fastpath(b[i].sel() == 0)) {// 如果i下标对应的数据为空则存入到i位置
            incrementOccupied();
            b[i].set<Atomic, Encoded>(sel, imp, cls);
            return;
        }
        if (b[i].sel() == sel) {
            // The entry was added to the cache by some other thread
            // before we grabbed the cacheUpdateLock.
            return;
        }
    } while (fastpath((i = cache_next(i, m)) != begin));// 在_arm64架构下cache_next就是返回`i ? i-1 : mask;`

    cache_t::bad_cache(receiver, (SEL)sel, cls);
}