探索Cache_t

Cache_t 的整体分析

Cache_t的源码

在objc/objc-runtime-new源码下查找结构体cache_t源码。

struct cache_t {
#if CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_OUTLINED
    explicit_atomic _buckets;
    explicit_atomic _mask;
#elif CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_HIGH_16
    explicit_atomic _maskAndBuckets;
    ...//省略，掩码用处
#elif CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_LOW_4
    explicit_atomic _maskAndBuckets;
    ...//省略
#else
#error Unknown cache mask storage type.
#endif

#if __LP64__
    uint16_t _flags;
#endif
    uint16_t _occupied;
}

那么CACHE_MASK_STORAGE的判断是什么意义呢?查看CACHE_MASK_STORAGE宏。

arm64表示真机，LP64表示64位结构

#if defined(__arm64__) && __LP64__
#define CACHE_MASK_STORAGE CACHE_MASK_STORAGE_HIGH_16
#elif defined(__arm64__) && !__LP64__
#define CACHE_MASK_STORAGE CACHE_MASK_STORAGE_LOW_4  
#else
#define CACHE_MASK_STORAGE CACHE_MASK_STORAGE_OUTLINED//模拟器,MacOS
#endif

lldb获取cache_t

cache_t 缓存buckets集合

cache_t下有一个struct bucket_t *buckets();，这是一个bucket_t结构体集合的指针，我们可以像访问数组的方式一下，访问buckets内的内每一个bucket_t。

下面是bucket_t的部分源码

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 _imp;
    explicit_atomic _sel;
#else
    explicit_atomic _sel;
    explicit_atomic _imp;
#endif
...//省略
public:
    inline SEL sel() const { return _sel.load(memory_order::memory_order_relaxed); }

    inline IMP imp(Class cls) const {
    ...//省略
    }
    ...//省略
}

Cache_t的结构图

Cache_t结构.png

脱离源码环境调试分析

完整代码

typedef uint32_t mask_t;  // x86_64 & arm64 asm are less efficient with 16-bits

struct oc_bucket_t {
    SEL _sel;
    IMP _imp;
};

struct oc_cache_t {
    struct oc_bucket_t * _buckets;
    mask_t _mask;
    uint16_t _flags;
    uint16_t _occupied;
};

struct oc_class_data_bits_t {
    uintptr_t bits;
};

struct oc_objc_class {
    Class ISA;
    Class superclass;
    struct oc_cache_t cache;             // formerly cache pointer and vtable
    struct oc_class_data_bits_t bits;    // class_rw_t * plus custom rr/alloc flags
};

void logClass(Class class){
        struct oc_objc_class *lg_pClass = (__bridge struct oc_objc_class *)(class);
        NSLog(@"%hu - %u",lg_pClass->cache._occupied,lg_pClass->cache._mask);
        for (mask_t i = 0; icache._mask; i++) {
            // 打印获取的 bucket
            struct oc_bucket_t bucket = lg_pClass->cache._buckets[I];
            NSLog(@"%@ - %p",NSStringFromSelector(bucket._sel),bucket._imp);
        }

}

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        Person *p  = [Person alloc];
        Class pClass = [Person class];  // objc_clas
        [p say1];
        [p say2];
       
        logClass(pClass);
        [p say3];
        [p say4];
        logClass(pClass);
        
        [p say5];
        [p say6];
        logClass(pClass);
     
   
    }
    return 0;
}

输出结果

• 调用两个方法时：

  两种不同的方法.png

• 调用四个方法时：

四种不同的方法.png

Cache_t原理分析

insert方法分析

1. 在Cache_t中找到了 void incrementOccupied();函数，对_occupied进行自增

2. 全局搜索incrementOccupied()函数，发现只在objc-cache文件下cache_t的insert方法有调用，

incrementOccupied调用.png

3. 查看insert方法，我们发现进入该方法后，就会对当前的occupied进行+1赋值给新的变量 newOccupied，

   • 如果当前buckets为空，则进行重新开辟空间reallocate，reallocate中调用setBucketsAndMask进行初始化,_occupied等于0，INIT_CACHE_SIZE为4.

     void cache_t::setBucketsAndMask(struct bucket_t *newBuckets, mask_t newMask)
     {
     
     #ifdef __arm__
         // ensure other threads see buckets contents before buckets pointer
         mega_barrier();
         _buckets.store(newBuckets, memory_order::memory_order_relaxed);
         
         // ensure other threads see new buckets before new mask
         mega_barrier();
         
         _mask.store(newMask, memory_order::memory_order_relaxed);
         _occupied = 0;
     #elif __x86_64__ || i386
         // ensure other threads see buckets contents before buckets pointer
         _buckets.store(newBuckets, memory_order::memory_order_release);
         
         // ensure other threads see new buckets before new mask
         _mask.store(newMask, memory_order::memory_order_release);
         _occupied = 0;
     #else
     #error Don't know how to do setBucketsAndMask on this architecture.
     #endif
     }
     
     

   • 如果新增的值 小于容量的 ，则什么也不做

   • 否则进行扩容，重新开辟空间，_occupied等于0。

   • 最后缓存方法，并对_occupied进行自增。

   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;
       unsigned oldCapacity = capacity(), capacity = oldCapacity;
       if (slowpath(isConstantEmptyCache())) {
           // Cache is read-only. Replace it.
           if (!capacity) capacity = INIT_CACHE_SIZE;
           reallocate(oldCapacity, capacity, /* freeOld */false);
       }
       else if (fastpath(newOccupied + CACHE_END_MARKER <= capacity / 4 * 3)) {
           // Cache is less than 3/4 full. Use it as-is.
       }
       else {
           capacity = capacity ? capacity * 2 : INIT_CACHE_SIZE;
           if (capacity > MAX_CACHE_SIZE) {
               capacity = MAX_CACHE_SIZE;
           }
           reallocate(oldCapacity, capacity, true);
       }
   
       bucket_t *b = buckets();
       mask_t m = capacity - 1;
       mask_t begin = cache_hash(sel, m);
       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)) {
               incrementOccupied();
               b[i].set(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));
   
       cache_t::bad_cache(receiver, (SEL)sel, cls);
   }
   
   

Cache_t流程图

附上一个流程图

Cooci 关于Cache_t原理分析图.png

_occupied是什么?

_occupied为分配的内存中已经存储了sel-imp的的个数

_mask是什么?

_mask是指掩码数据，mask 等于capacity - 1

为什么buckets会有丢失？

在reallocate扩容时，是将原有的内存全部清除了，再重新申请了内存导致

为什么顺序有问题？

哈希算法计算下标，下标是随机的，并不是固定的.