紧接着我们来分析类结构体中cache_t, 只从单词来看就能猜出来是与缓存有关. 下面我们先看cache_t的源码:
struct cache_t { #if 1 // Mac struct bucket_t * _buckets; mask_t _mask; #elif 1 // 真机 (尽量用真机调试, 因为真机更贴近日常使用) 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; #endif uint16_t _flags; // 标志位 uint16_t _occupied; // 被占用的 public: static bucket_t *emptyBuckets(); struct bucket_t *buckets(); mask_t mask(); mask_t occupied(); void incrementOccupied(); void setBucketsAndMask(struct bucket_t *newBuckets, mask_t newMask); void initializeToEmpty(); unsigned capacity(); bool isConstantEmptyCache(); bool canBeFreed(); void reallocate(mask_t oldCapacity, mask_t newCapacity, bool freeOld); void insert(Class cls, SEL sel, IMP imp, id receiver); };
在进行结构分析时, 我们先写了一个例子来分析cache_t的结构. 具体例子如下:
//LGPerson.h文件 @interface LGPerson : NSObject @property (nonatomic, copy) NSString *lgName; @property (nonatomic, strong) NSString *nickName; - (void)say1; - (void)say2; - (void)say3; - (void)say4; //LGPerson.m文件 #import "LGPerson.h" @implementation LGPerson - (void)say1{ NSLog(@"LGPerson say : %s",__func__); } - (void)say2{ NSLog(@"LGPerson say : %s",__func__); } - (void)say3{ NSLog(@"LGPerson say : %s",__func__); } - (void)say4{ NSLog(@"LGPerson say : %s",__func__); } @end
具体调用以及打印结果, 见下方代码:
#import <Foundation/Foundation.h> #import "LGPerson.h" #import <objc/runtime.h> typedef uint32_t mask_t; // x86_64 & arm64 asm are less efficient with 16-bits struct lg_bucket_t { SEL _sel; IMP _imp; }; struct lg_cache_t { struct lg_bucket_t * _buckets; mask_t _mask; uint16_t _flags; uint16_t _occupied; }; struct lg_class_data_bits_t { uintptr_t bits; }; struct lg_objc_class { Class ISA; Class superclass; struct lg_cache_t cache; // formerly cache pointer and vtable struct lg_class_data_bits_t bits; // class_rw_t * plus custom rr/alloc flags }; int main(int argc, const char * argv[]) { @autoreleasepool { LGPerson *p = [LGPerson alloc]; Class pClass = [LGPerson class]; // objc_clas [p say1]; [p say2]; // [p say3]; // [p say4]; struct lg_objc_class *lg_pClass = (__bridge struct lg_objc_class *)(pClass); NSLog(@"%hu - %u",lg_pClass->cache._occupied,lg_pClass->cache._mask); for (mask_t i = 0; i<lg_pClass->cache._mask; i++) { // 打印获取的 bucket struct lg_bucket_t bucket = lg_pClass->cache._buckets[i]; NSLog(@"%@ - %p",NSStringFromSelector(bucket._sel),bucket._imp); } NSLog(@"Hello, World!"); } return 0; } //当屏蔽 [p say3]; [p say4]; 方法时的打印结果: 2020-09-17 21:23:31.259617+0800 003-cache_t脱离源码环境分析[3956:50598] LGPerson say : -[LGPerson say1] 2020-09-17 21:23:31.259990+0800 003-cache_t脱离源码环境分析[3956:50598] LGPerson say : -[LGPerson say2] 2020-09-17 21:23:31.260029+0800 003-cache_t脱离源码环境分析[3956:50598] 2 - 3 2020-09-17 21:23:35.029171+0800 003-cache_t脱离源码环境分析[3956:50598] say1 - 0x2828 2020-09-17 21:23:35.029352+0800 003-cache_t脱离源码环境分析[3956:50598] say2 - 0x2818 2020-09-17 21:23:35.029409+0800 003-cache_t脱离源码环境分析[3956:50598] (null) - 0x0 //当打开屏幕 [p say3]; [p say4]; 的打印结果为: 2020-09-17 21:37:09.315693+0800 003-cache_t脱离源码环境分析[4296:55622] LGPerson say : -[LGPerson say1] 2020-09-17 21:37:09.315964+0800 003-cache_t脱离源码环境分析[4296:55622] LGPerson say : -[LGPerson say2] 2020-09-17 21:37:09.316001+0800 003-cache_t脱离源码环境分析[4296:55622] LGPerson say : -[LGPerson say3] 2020-09-17 21:37:09.316076+0800 003-cache_t脱离源码环境分析[4296:55622] LGPerson say : -[LGPerson say4] 2020-09-17 21:37:09.316099+0800 003-cache_t脱离源码环境分析[4296:55622] 2 - 7 2020-09-17 21:37:09.316196+0800 003-cache_t脱离源码环境分析[4296:55622] say4 - 0x29a8 2020-09-17 21:37:09.316217+0800 003-cache_t脱离源码环境分析[4296:55622] (null) - 0x0 2020-09-17 21:37:09.316283+0800 003-cache_t脱离源码环境分析[4296:55622] say3 - 0x29d8 2020-09-17 21:37:09.316315+0800 003-cache_t脱离源码环境分析[4296:55622] (null) - 0x0 2020-09-17 21:37:09.316330+0800 003-cache_t脱离源码环境分析[4296:55622] (null) - 0x0 2020-09-17 21:37:09.316366+0800 003-cache_t脱离源码环境分析[4296:55622] (null) - 0x0 2020-09-17 21:37:09.316382+0800 003-cache_t脱离源码环境分析[4296:55622] (null) - 0x0 我们先抛出问题:以及最终的得出的结论 // _occupied _mask 是什么 cup - 1 // 会变化 2-3 -> 2-7 (说明有做扩容操作) // bucket 会有丢失 重新申请 // 顺序有点问题 哈希 // 当打开屏蔽方法后, 没有打印出 say1 say2, 证明有做刷新释放操作
然后我们从cache_t的源码来分析: 每当对象调用一个方法时, 如果在cache里面没有找到, 就会insert一条缓存:
void cache_t::insert(Class cls, SEL sel, IMP imp, id receiver) { // Use the cache as-is if it is less than 3/4 full mask_t newOccupied = occupied() + 1; unsigned oldCapacity = capacity(), capacity = oldCapacity; // 1. 如果Cache 是空的话，会初始化一个 4 个字节的空间 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. // 2. newOccupied + CACHE_END_MARKER <= capacity / 4 * 3 ，直接插入 } else { // 3. 否则会扩容, 扩容为原空间的 2倍大小 capacity = capacity ? capacity * 2 : INIT_CACHE_SIZE; if (capacity > MAX_CACHE_SIZE) { capacity = MAX_CACHE_SIZE; } reallocate(oldCapacity, capacity, true); // // 重新分配空间 存储新的数据，抹除已有缓存 } // 4. 初始化一个指针数组 bucket_t *b = buckets(); // 5. 设置掩码为 capacity - 1 mask_t m = capacity - 1; // 6. 根据sel 计算 hash 值 mask_t begin = cache_hash(sel, m); mask_t i = begin; /* * 扫描第一个未使用的插槽，并且插入 */ // 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 { // 7. 当前 插槽 取到 的sel 地址为0， 那么插入新的值 if (fastpath(b[i].sel() == 0)) { // 8. 增加占用字段并且插入 incrementOccupied(); b[i].set<Atomic, Encoded>(sel, imp, cls); return; } // 8. 多线程做的判断 if (b[i].sel() == sel) { // 如果找到需要缓存的方法，什么都不做，并退出循环 // The entry was added to the cache by some other thread // before we grabbed the cacheUpdateLock. return; } // 9. 如果当前位置已有值，那么就找下一个位置 判断不等于初始下标值 begin 是为了将散列表中的数据全部遍历结束，而cache_next( ) 是为了解决哈希冲突而进行的二次哈希. } while (fastpath((i = cache_next(i, m)) != begin)); } // 10. hash 算法, 保证不会越界 static inline mask_t cache_hash(SEL sel, mask_t mask) { return (mask_t)(uintptr_t)sel & mask; }
cache_t缓存流程图
总结一下cache_t的结构:
cache_t结构
- _buckets ：是一个散列表，用来存储 缓存方法的 sel 和 imp.
- _mask ： 有2个作用，1: 作为当前可存储的最大容量；2: 作为掩码，取已缓存方法在 _buckets 中的下标.
- _occupied : _buckets 中 已缓存的方法数量.
