在消息机制概括中我们讲了Objective-C是怎么寻找方法的，我也知道iOS存储方法是一个二维数组(methods)，如果每次查找方法都要去遍历methods，是一件非常耗时的操作，那么苹果有没有实现一种快捷的方式呢？有的，那就是缓存技术（PS好像说到耗时操作，我们都会想到缓存，比如我们常用的SDWebimage他就有一道缓存机制）。

在前面我们学习Class结构的时候还有一个chache变量我们没有讲，他就是iOS缓存方法的变量？今天我们就来聊聊他。

cache_t & bucket_t 结构与关系

在查看查看cache_t源码之前，我们先学习两个与机型有关的宏CACHE_MASK_STORAGE和CONFIG_USE_PREOPT_CACHES。

源码分析中用到的环境宏

CACHE_MASK_STORAGE

#define CACHE_MASK_STORAGE_OUTLINED 1
#define CACHE_MASK_STORAGE_HIGH_16 2
#define CACHE_MASK_STORAGE_LOW_4 3
#define CACHE_MASK_STORAGE_HIGH_16_BIG_ADDRS 4
   
#if defined(__arm64__) && __LP64__ // arm64 64位指针
    #if TARGET_OS_OSX || TARGET_OS_SIMULATOR //模拟器
        #define CACHE_MASK_STORAGE CACHE_MASK_STORAGE_HIGH_16_BIG_ADDRS
    #else
        #define CACHE_MASK_STORAGE CACHE_MASK_STORAGE_HIGH_16 // 真机iPhone是这个
    #endif
#elif defined(__arm64__) && !__LP64__ // // arm64 非64位指针
    #define CACHE_MASK_STORAGE CACHE_MASK_STORAGE_LOW_4
#else // 
    #define CACHE_MASK_STORAGE CACHE_MASK_STORAGE_OUTLINED
#endif

CONFIG_USE_PREOPT_CACHES

#if defined(__arm64__) && TARGET_OS_IOS && !TARGET_OS_SIMULATOR && !TARGET_OS_MACCATALYST
#define CONFIG_USE_PREOPT_CACHES 1 // arm64 真机iOS
#else
#define CONFIG_USE_PREOPT_CACHES 0 // 其他：模拟器、macos……
#endif

cache_t & bucket_t

cache_t

struct cache_t {
private:
    explicit_atomic<uintptr_t> _bucketsAndMaybeMask;
    union {
        struct {
            explicit_atomic<mask_t>    _maybeMask;
#if __LP64__
            uint16_t                   _flags;
#endif
            uint16_t                   _occupied;
        };
        explicit_atomic<preopt_cache_t *> _originalPreoptCache;
    };

#if CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_OUTLINED
    // _bucketsAndMaybeMask is a buckets_t pointer
    // _maybeMask is the buckets mask

    static constexpr uintptr_t bucketsMask = ~0ul;
    static_assert(!CONFIG_USE_PREOPT_CACHES, "preoptimized caches not supported");
#elif CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_HIGH_16_BIG_ADDRS
    static constexpr uintptr_t maskShift = 48;
    static constexpr uintptr_t maxMask = ((uintptr_t)1 << (64 - maskShift)) - 1;
    static constexpr uintptr_t bucketsMask = ((uintptr_t)1 << maskShift) - 1;
    
    static_assert(bucketsMask >= MACH_VM_MAX_ADDRESS, "Bucket field doesn't have enough bits for arbitrary pointers.");
#if CONFIG_USE_PREOPT_CACHES
    static constexpr uintptr_t preoptBucketsMarker = 1ul;
    static constexpr uintptr_t preoptBucketsMask = bucketsMask & ~preoptBucketsMarker;
#endif
#elif CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_HIGH_16
    // _bucketsAndMaybeMask is a buckets_t pointer in the low 48 bits
    // _maybeMask is unused, the mask is stored in the top 16 bits.

    // 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.");

#if CONFIG_USE_PREOPT_CACHES
    static constexpr uintptr_t preoptBucketsMarker = 1ul;
#if __has_feature(ptrauth_calls)
    // 63..60: hash_mask_shift
    // 59..55: hash_shift
    // 54.. 1: buckets ptr + auth
    //      0: always 1
    static constexpr uintptr_t preoptBucketsMask = 0x007ffffffffffffe;
    static inline uintptr_t preoptBucketsHashParams(const preopt_cache_t *cache) {
        uintptr_t value = (uintptr_t)cache->shift << 55;
        // masks have 11 bits but can be 0, so we compute
        // the right shift for 0x7fff rather than 0xffff
        return value | ((objc::mask16ShiftBits(cache->mask) - 1) << 60);
    }
#else
    // 63..53: hash_mask
    // 52..48: hash_shift
    // 47.. 1: buckets ptr
    //      0: always 1
    static constexpr uintptr_t preoptBucketsMask = 0x0000fffffffffffe;
    static inline uintptr_t preoptBucketsHashParams(const preopt_cache_t *cache) {
        return (uintptr_t)cache->hash_params << 48;
    }
#endif
#endif // CONFIG_USE_PREOPT_CACHES
#elif CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_LOW_4
    // _bucketsAndMaybeMask is a buckets_t pointer in the top 28 bits
    // _maybeMask is unused, the mask length is stored in the low 4 bits

    static constexpr uintptr_t maskBits = 4;
    static constexpr uintptr_t maskMask = (1 << maskBits) - 1;
    static constexpr uintptr_t bucketsMask = ~maskMask;
    static_assert(!CONFIG_USE_PREOPT_CACHES, "preoptimized caches not supported");
#else
#error Unknown cache mask storage type.
#endif

    bool isConstantEmptyCache() const;
    bool canBeFreed() const;
    mask_t mask() const;

#if CONFIG_USE_PREOPT_CACHES
    void initializeToPreoptCacheInDisguise(const preopt_cache_t *cache);
    const preopt_cache_t *disguised_preopt_cache() const;
#endif
	......
}

_bucketsAndMaybeMask：存放着buckets的地址，可以通过**_bucketsAndMaybeMask** & bucketsMask（bucketsMask的值在不同环境是不同）得到buckets地址，也可以使用方法buckets()拿到地址。
_maybeMask：散列表长度-1
_occupied：已经缓存方法的数量

cache_t分析如下图所示

未命名文件 (1).jpg

bucket_t

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
}

bucket_t中存放着方法的方法指针imp和方法选择器sel