#elif CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_HIGH_16  // 真机64
    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;
    
    // Ensure we have enough bits for the buckets pointer.
    static_assert(bucketsMask >= MACH_VM_MAX_ADDRESS, "Bucket field doesn‘t have enough bits for arbitrary pointers.");

 1 ALWAYS_INLINE
 2 void cache_t::insert(Class cls, SEL sel, IMP imp, id receiver)
 3 {
 4 #if CONFIG_USE_CACHE_LOCK
 5     cacheUpdateLock.assertLocked();
 6 #else
 7     runtimeLock.assertLocked();
 8 #endif
 9 
10     ASSERT(sel != 0 && cls->isInitialized());
11 
12     // Use the cache as-is if it is less than 3/4 full
13     mask_t newOccupied = occupied() + 1;// occupied():return _occupied; --> _occupied + 1
14     unsigned oldCapacity = capacity(), capacity = oldCapacity;  // _mask.load() --> capacity 空间
15     if (slowpath(isConstantEmptyCache())) {
16         // 初始化，occupied=0，buckets()是空
17         // Cache is read-only. Replace it.
18         if (!capacity) capacity = INIT_CACHE_SIZE;// capacity 给1<<2的空间 4
19         // 真正去向系统开辟内存
20         reallocate(oldCapacity, capacity, /* freeOld */false);
21     }
22     else if (fastpath(newOccupied + CACHE_END_MARKER <= capacity / 4 * 3)) {
23         // Cache is less than 3/4 full. Use it as-is.
24         // cache < 3/4 capacity
25     }
26     else {
27         capacity = capacity ? capacity * 2 : INIT_CACHE_SIZE;   // 扩容 如果capacity 不为空 扩容为当前的2倍；为空则去开辟 4
28         if (capacity > MAX_CACHE_SIZE) {// 空间最大 1<<16 = 2^16
29             capacity = MAX_CACHE_SIZE;
30         }
31         reallocate(oldCapacity, capacity, true);// 重新开辟空间，true：旧的free
32     }
33 
34     bucket_t *b = buckets();
35     mask_t m = capacity - 1;// 2^2-1=3  2^3-1=7
36     mask_t begin = cache_hash(sel, m);// sel & mask
37     mask_t i = begin;
38 
39     // Scan for the first unused slot and insert there.
40     // There is guaranteed to be an empty slot because the
41     // minimum size is 4 and we resized at 3/4 full.
42     do {
43         // 位置是空的可以放
44         if (fastpath(b[i].sel() == 0)) {
45             incrementOccupied();
46             b[i].set<Atomic, Encoded>(sel, imp, cls);
47             return;
48         }
49         // 此位置已经存值,且 .sel 就是传来的这个 sel 了
50         if (b[i].sel() == sel) {
51             // The entry was added to the cache by some other thread
52             // before we grabbed the cacheUpdateLock.
53             return;
54         }
55     } while (fastpath((i = cache_next(i, m)) != begin));// 再次哈希 --> (i+1)&mask != begin
56 
57     cache_t::bad_cache(receiver, (SEL)sel, cls);
58 }