iOS底层原理之cache_t分析

咱们在iOS底层原理之isa走位与类结构分析 中分析了objc_class结构体中的bits属性，今天咱们分析cache_t属性。

什么是cache_t

cache_t部分源码

struct cache_t {
#if CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_OUTLINED
    explicit_atomic<struct bucket_t *> _buckets;
    explicit_atomic<mask_t> _mask;
#elif CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_HIGH_16
    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.");
#elif CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_LOW_4
    // _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;

    static constexpr uintptr_t maskBits = 4;
    static constexpr uintptr_t maskMask = (1 << maskBits) - 1;
    static constexpr uintptr_t bucketsMask = ~maskMask;
#else
#error Unknown cache mask storage type.
#endif
    
#if __LP64__
    uint16_t _flags;
#endif
    uint16_t _occupied;

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();

#if __LP64__
    bool getBit(uint16_t flags) const {
        return _flags & flags;
    }
    void setBit(uint16_t set) {
        __c11_atomic_fetch_or((_Atomic(uint16_t) *)&_flags, set, __ATOMIC_RELAXED);
    }
    void clearBit(uint16_t clear) {
        __c11_atomic_fetch_and((_Atomic(uint16_t) *)&_flags, ~clear, __ATOMIC_RELAXED);
    }
#endif

#if FAST_CACHE_ALLOC_MASK
    bool hasFastInstanceSize(size_t extra) const
    {
        if (__builtin_constant_p(extra) && extra == 0) {
            return _flags & FAST_CACHE_ALLOC_MASK16;
        }
        return _flags & FAST_CACHE_ALLOC_MASK;
    }

    size_t fastInstanceSize(size_t extra) const
    {
        ASSERT(hasFastInstanceSize(extra));

        if (__builtin_constant_p(extra) && extra == 0) {
            return _flags & FAST_CACHE_ALLOC_MASK16;
        } else {
            size_t size = _flags & FAST_CACHE_ALLOC_MASK;
            // remove the FAST_CACHE_ALLOC_DELTA16 that was added
            // by setFastInstanceSize
            return align16(size + extra - FAST_CACHE_ALLOC_DELTA16);
        }
    }

    void setFastInstanceSize(size_t newSize)
    {
        // Set during realization or construction only. No locking needed.
        uint16_t newBits = _flags & ~FAST_CACHE_ALLOC_MASK;
        uint16_t sizeBits;

        // Adding FAST_CACHE_ALLOC_DELTA16 allows for FAST_CACHE_ALLOC_MASK16
        // to yield the proper 16byte aligned allocation size with a single mask
        sizeBits = word_align(newSize) + FAST_CACHE_ALLOC_DELTA16;
        sizeBits &= FAST_CACHE_ALLOC_MASK;
        if (newSize <= sizeBits) {
            newBits |= sizeBits;
        }
        _flags = newBits;
    }
#else
    bool hasFastInstanceSize(size_t extra) const {
        return false;
    }
    size_t fastInstanceSize(size_t extra) const {
        abort();
    }
    void setFastInstanceSize(size_t extra) {
        // nothing
    }
#endif

    static size_t bytesForCapacity(uint32_t cap);
    static struct bucket_t * endMarker(struct bucket_t *b, uint32_t cap);

    void reallocate(mask_t oldCapacity, mask_t newCapacity, bool freeOld);
    void insert(Class cls, SEL sel, IMP imp, id receiver);

    static void bad_cache(id receiver, SEL sel, Class isa) __attribute__((noreturn, cold));
};
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从上述源码中咱们能够看出，cache_t中存储了函数方法的集合以及声明的变量等。

_buckets、_mask、_occupied

• _buckets：buket_t类型的结构体数组，存储方法指针地址_IMP以及方法编号_SEL
• _mask：mask_t m = capacity - 1，能够理解为cache_t的缓存容量大小，因为(capacity = MAX_CACHE_SIZE;)中MAX_CACHE_SIZE是2整数次幂
• _occupied：记录缓存方法的个数

lldbcache_t代码跟踪

咱们在LGPerson类中声明一下方法，并实现

@interface LGPerson : NSObject

- (void)sayHello;

- (void)sayCode;

- (void)sayMaster;

- (void)sayNB;

+ (void)sayHappy;

@end

@implementation LGPerson
- (void)sayHello{
    NSLog(@"LGPerson say : %s",__func__);
}

- (void)sayCode{
    NSLog(@"LGPerson say : %s",__func__);
}

- (void)sayMaster{
    NSLog(@"LGPerson say : %s",__func__);
}

- (void)sayNB{
    NSLog(@"LGPerson say : %s",__func__);
}

+ (void)sayHappy{
    NSLog(@"LGPerson say : %s",__func__);
}
@end
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在main()函数中代码实现

LGPerson *p  = [LGPerson alloc];
Class pClass = [LGPerson class];
[p sayHello];
[p sayCode];
[p sayMaster];
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• 先查看未调用任何实例方法时候状况

此时 咱们能够看到cache尚未存储方法

• 再看调用一个实例方法后状况

此时_buckets、_mask、_occupied均发生了变化

• 调用两个实例方法后状况此时_mask = 3，_occupied = 2
• 当调用三个实例方法后状况此时_mask = 7，_occupied = 1。为何_mask变换这么大以及_occupied为何不是3？想要解决这个问题咱们要看它们是怎么实现的。

cache_t 底层实现

探究cache_t初始化

咱们从cache_t结构体中发现上述方法，咱们在源码中搜索incrementOccupied中发现了初始化cache_t，发现咱们在方法中发现了调用的地方，咱们探究下该方法咱们先看须要注意的两处代码

• INIT_CACHE_SIZE

enum {
    INIT_CACHE_SIZE_LOG2 = 2,
    INIT_CACHE_SIZE      = (1 << INIT_CACHE_SIZE_LOG2),
    MAX_CACHE_SIZE_LOG2  = 16,
    MAX_CACHE_SIZE       = (1 << MAX_CACHE_SIZE_LOG2),
};
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经过INIT_CACHE_SIZE，咱们知道每次开辟的大小是2的幂，第一次是4，因此当咱们的调用一个或两个实例方法的时候_mask = 3

• reallocate方法中

void cache_t::reallocate(mask_t oldCapacity, mask_t newCapacity, bool freeOld)
{
    bucket_t *oldBuckets = buckets();
    bucket_t *newBuckets = allocateBuckets(newCapacity);

    // Cache's old contents are not propagated. 
    // This is thought to save cache memory at the cost of extra cache fills.
    // fixme re-measure this

    ASSERT(newCapacity > 0);
    ASSERT((uintptr_t)(mask_t)(newCapacity-1) == newCapacity-1);

    setBucketsAndMask(newBuckets, newCapacity - 1);
    
    if (freeOld) {
        cache_collect_free(oldBuckets, oldCapacity);
    }
}
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咱们发现了setBucketsAndMask方法，setBucketsAndMask代码为

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
}
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在setBucketsAndMask方法中将_occupied = 0初始化为0了，所以咱们获得每次扩容后，_occupied都从0开始再次计数，估调用三个实例方法后_occupied = 1

cache_t总结

• cache经过void cache_t::insert(Class cls, SEL sel, IMP imp, id receiver)方法缓存调用的方法
• ① 若是第一次添加，则经过reallocate方法开辟空间，在reallocate方法中通setBucketsAndMask方法将_occupied初始化为0，开辟大小为4，由于_mask = capacity - 1，其中capacity即为开辟的大小，因此 _mask = 3;
• ②若是insert的方法个数小于等于已经开辟个数的3/4则不进行处理
• ③若是大于3/4，则进行扩容处理，此时经过reallocate扩容，每次扩容按照原先的两倍来扩容，将_occupied初始化为0，

cache_t原理图