iOS-底层原理07-catch_t

《iOS底层原理文章汇总》

类的结构@2x.png

对象的内存结构

(lldb) x/4gx person
0x101025660: 0x001d8001000033dd 0x0000000100002080
0x101025670: 0x0000000100002060 0x0000000000000000
(lldb) po 0x001d8001000033dd
8303516107944925

(lldb) po 0x0000000100002080
fish

(lldb) po 0x0000000100002060
cloud

(lldb) po 0x001d8001000033dd & 0x00007ffffffffff8ULL
DCPerson

(lldb)

cache_t结构

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));
};
cache_t@2x.png

  • 1.第一次运行一个方法之前，cache_t缓存中的方法为0,之后缓存中的方法的数量为前面已经执行的方法的数量,由类的结构体得知


    cache_t偏移16字节@2x.png

可以明显发现编译一个类方法之后，cache_t中的_sel，_imp,_mask,_occupied的值发生明显变化


编译方法前后cache_t的变化@2x.png
(lldb) p/x DCPerson.class
(Class) $0 = 0x0000000100003428 DCPerson
(lldb) p (cache_t *)0x0000000100003438
(cache_t *) $1 = 0x0000000100003438
(lldb) p *$1
(cache_t) $2 = {
  _buckets = {
    std::__1::atomic<bucket_t *> = 0x00000001003323d0 {
      _sel = {
        std::__1::atomic<objc_selector *> = (null)
      }
      _imp = {
        std::__1::atomic<unsigned long> = 0
      }
    }
  }
  _mask = {
    std::__1::atomic<unsigned int> = 0
  }
  _flags = 32820
  _occupied = 0
}
2020-10-07 17:12:52.237343+0800 DCPerson[10411:5356955] sayHello
(lldb) p *$1
(cache_t) $3 = {
  _buckets = {
    std::__1::atomic<bucket_t *> = 0x00000001018a0000 {
      _sel = {
        std::__1::atomic<objc_selector *> = ""
      }
      _imp = {
        std::__1::atomic<unsigned long> = 11944
      }
    }
  }
  _mask = {
    std::__1::atomic<unsigned int> = 3
  }
  _flags = 32820
  _occupied = 1
}
21.gif
  • 2.打印出来并没有我们想看到的sayHello函数，在cache_t结构体中找到函数struct bucket_t *buckets();通过buckets()获取到结构体bucket_t，再到bucket_t查找到函数sel()和inline IMP imp(Class cls),找到sayHello方法如下:
(lldb) p/x DCPerson.class
(Class) $0 = 0x0000000100003428 DCPerson
(lldb) p (cache_t *)0x0000000100003438
(cache_t *) $1 = 0x0000000100003438
(lldb) p *$1
(cache_t) $2 = {
  _buckets = {
    std::__1::atomic<bucket_t *> = 0x00000001003323d0 {
      _sel = {
        std::__1::atomic<objc_selector *> = (null)
      }
      _imp = {
        std::__1::atomic<unsigned long> = 0
      }
    }
  }
  _mask = {
    std::__1::atomic<unsigned int> = 0
  }
  _flags = 32820
  _occupied = 0
}
2020-10-07 17:12:52.237343+0800 DCPerson[10411:5356955] sayHello
(lldb) p *$1
(cache_t) $3 = {
  _buckets = {
    std::__1::atomic<bucket_t *> = 0x00000001018a0000 {
      _sel = {
        std::__1::atomic<objc_selector *> = ""
      }
      _imp = {
        std::__1::atomic<unsigned long> = 11944
      }
    }
  }
  _mask = {
    std::__1::atomic<unsigned int> = 3
  }
  _flags = 32820
  _occupied = 1
}
(lldb) p $3.buckets()
(bucket_t *) $4 = 0x00000001018a0000
(lldb) p *$4
(bucket_t) $5 = {
  _sel = {
    std::__1::atomic<objc_selector *> = ""
  }
  _imp = {
    std::__1::atomic<unsigned long> = 11944
  }
}
(lldb) p $5.sel()
(SEL) $6 = "sayHello"
(lldb) p $5.imp(DCPerson.class)
(IMP) $7 = 0x0000000100001a80 (DCPerson`-[DCPerson sayHello])
22.gif

  • 3.通过MachOView查看最后的DCPerson sayHello的地址是否正确


    sayHello方法地址MachOView@2x.png
  • 4.查看第二个方法:通过指针偏移或数组取值两种方式获取
  • I指针偏移
-(void)sayHello;
-(void)sayCode;

2020-10-07 19:24:19.931422+0800 DCPerson[84824:5536090] sayHello
DCPerson was compiled with optimization - stepping may behave oddly; variables may not be available.
2020-10-07 19:24:27.426872+0800 DCPerson[84824:5536090] -[DCPerson sayCode]
(lldb) p/x DCPerson.class
(Class) $0 = 0x0000000100003428 DCPerson
(lldb) p (cache_t *)0x0000000100003438
(cache_t *) $1 = 0x0000000100003438
(lldb) p *$1
(cache_t) $2 = {
  _buckets = {
    std::__1::atomic<bucket_t *> = 0x0000000100723b90 {
      _sel = {
        std::__1::atomic<objc_selector *> = ""
      }
      _imp = {
        std::__1::atomic<unsigned long> = 11944
      }
    }
  }
  _mask = {
    std::__1::atomic<unsigned int> = 3
  }
  _flags = 32820
  _occupied = 2
}
(lldb) p $2.buckets()
(bucket_t *) $3 = 0x0000000100723b90
(lldb) p *$3
(bucket_t) $4 = {
  _sel = {
    std::__1::atomic<objc_selector *> = ""
  }
  _imp = {
    std::__1::atomic<unsigned long> = 11944
  }
}
(lldb) p *($3+1)
(bucket_t) $5 = {
  _sel = {
    std::__1::atomic<objc_selector *> = ""
  }
  _imp = {
    std::__1::atomic<unsigned long> = 11928
  }
}
(lldb) p $5.sel()
(SEL) $6 = "sayCode"
(lldb) p $5.imp(DCPerson.class)
(IMP) $7 = 0x0000000100001ab0 (DCPerson`-[DCPerson sayCode])
  • II数组下标
2020-10-07 19:24:19.931422+0800 DCPerson[84824:5536090] sayHello
DCPerson was compiled with optimization - stepping may behave oddly; variables may not be available.
2020-10-07 19:24:27.426872+0800 DCPerson[84824:5536090] -[DCPerson sayCode]
(lldb) p/x DCPerson.class
(Class) $0 = 0x0000000100003428 DCPerson
(lldb) p (cache_t *)0x0000000100003438
(cache_t *) $1 = 0x0000000100003438
(lldb) p *$1
(cache_t) $2 = {
  _buckets = {
    std::__1::atomic<bucket_t *> = 0x0000000100723b90 {
      _sel = {
        std::__1::atomic<objc_selector *> = ""
      }
      _imp = {
        std::__1::atomic<unsigned long> = 11944
      }
    }
  }
  _mask = {
    std::__1::atomic<unsigned int> = 3
  }
  _flags = 32820
  _occupied = 2
}
(lldb) p $2.buckets()
(bucket_t *) $3 = 0x0000000100723b90
(lldb) p *$3
(bucket_t) $4 = {
  _sel = {
    std::__1::atomic<objc_selector *> = ""
  }
  _imp = {
    std::__1::atomic<unsigned long> = 11944
  }
}
(lldb) p $2.buckets()[1]
(bucket_t) $8 = {
  _sel = {
    std::__1::atomic<objc_selector *> = ""
  }
  _imp = {
    std::__1::atomic<unsigned long> = 11928
  }
}
(lldb) p $8.sel()
(SEL) $9 = "sayCode"
(lldb) p $8.imp(DCPerson.class)
(IMP) $10 = 0x0000000100001ab0 (DCPerson`-[DCPerson sayCode])

脱离源码环境调试buckets()

struct lg_bucket_t {
    // IMP-first is better for arm64e ptrauth and no worse for arm64.
    SEL _sel;
    IMP _imp;
};

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

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 {
        DCPerson *person = [DCPerson alloc];
        Class pClass = [DCPerson class];//objc_class
//        person.name = @"cloud";
//        person.nickName = @"fish";
        [person say1];
        [person say2];
//        [person say3];
//        [person 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++) {
            //打印获取到的bucket
            struct lg_bucket_t bucket = lg_pClass->cache._buckets[i];
            NSLog(@"%@ - %p",NSStringFromSelector(bucket._sel),bucket._imp);
        }

        NSLog(@"%@,%@",person,pClass);
    }
    return 0;
}
//输出
2020-10-07 20:44:00.694375+0800 iOS-脱离源码环境调试buckets()[34019:5667773] -[DCPerson say1]
2020-10-07 20:44:00.694750+0800 iOS-脱离源码环境调试buckets()[34019:5667773] -[DCPerson say2]
2020-10-07 20:44:00.694928+0800 iOS-脱离源码环境调试buckets()[34019:5667773] 2 - 3
2020-10-07 20:44:00.695075+0800 iOS-脱离源码环境调试buckets()[34019:5667773] say1 - 0x2970
2020-10-07 20:44:00.695144+0800 iOS-脱离源码环境调试buckets()[34019:5667773] say2 - 0x2ea0
2020-10-07 20:44:00.695188+0800 iOS-脱离源码环境调试buckets()[34019:5667773] (null) - 0x0
2020-10-07 20:44:00.695400+0800 iOS-脱离源码环境调试buckets()[34019:5667773] <DCPerson: 0x10052d320>,DCPerson

增加调用两个方法查看bucket的值

        [person say1];
        [person say2];
        [person say3];
        [person say4];
//输出
2020-10-07 20:46:46.509218+0800 iOS-脱离源码环境调试buckets()[35630:5672751] -[DCPerson say1]
2020-10-07 20:46:46.509623+0800 iOS-脱离源码环境调试buckets()[35630:5672751] -[DCPerson say2]
2020-10-07 20:46:46.509829+0800 iOS-脱离源码环境调试buckets()[35630:5672751] -[DCPerson say3]
2020-10-07 20:46:46.509870+0800 iOS-脱离源码环境调试buckets()[35630:5672751] -[DCPerson say4]
2020-10-07 20:46:46.509906+0800 iOS-脱离源码环境调试buckets()[35630:5672751] 2 - 7
2020-10-07 20:46:46.510002+0800 iOS-脱离源码环境调试buckets()[35630:5672751] say4 - 0x2e10
2020-10-07 20:46:46.510045+0800 iOS-脱离源码环境调试buckets()[35630:5672751] (null) - 0x0
2020-10-07 20:46:46.510102+0800 iOS-脱离源码环境调试buckets()[35630:5672751] say3 - 0x2ec0
2020-10-07 20:46:46.510139+0800 iOS-脱离源码环境调试buckets()[35630:5672751] (null) - 0x0
2020-10-07 20:46:46.510172+0800 iOS-脱离源码环境调试buckets()[35630:5672751] (null) - 0x0
2020-10-07 20:46:46.510204+0800 iOS-脱离源码环境调试buckets()[35630:5672751] (null) - 0x0
2020-10-07 20:46:46.510235+0800 iOS-脱离源码环境调试buckets()[35630:5672751] (null) - 0x0
2020-10-07 20:46:46.510436+0800 iOS-脱离源码环境调试buckets()[35630:5672751] <DCPerson: 0x100496cc0>,DCPerson
occupied&mask@2x.png
  • 1._occupied,_mask是什么？
    当前存在几个函数在缓存中，_mask掩码数据为capacity-1
  • 2.会变化2-3 -> 2-7?
    4-1=3,8-1=7
  • 3.bucket会有丢失，打印say3和say4,没有打印say1和say2?
    超过两个之后会重新梳理，原来的干掉，重新申请内存，扩容
  • 4.顺序有点问题，先打印say4后打印say3?
    通过哈希存储，无序

cache_t底层原理(属性代表存储，函数代表变化)

源码lookUpImpOrForward-->log_and_fill_cache-->cache_fill-->cache_t::insert


IMP lookUpImpOrForward(id inst, SEL sel, Class cls, int behavior)
{
    const IMP forward_imp = (IMP)_objc_msgForward_impcache;
    IMP imp = nil;
    Class curClass;

    runtimeLock.assertUnlocked();

    // Optimistic cache lookup
    if (fastpath(behavior & LOOKUP_CACHE)) {
        imp = cache_getImp(cls, sel);
        if (imp) goto done_nolock;
    }

    // runtimeLock is held during isRealized and isInitialized checking
    // to prevent races against concurrent realization.

    // runtimeLock is held during method search to make
    // method-lookup + cache-fill atomic with respect to method addition.
    // Otherwise, a category could be added but ignored indefinitely because
    // the cache was re-filled with the old value after the cache flush on
    // behalf of the category.

    runtimeLock.lock();

    // We don't want people to be able to craft a binary blob that looks like
    // a class but really isn't one and do a CFI attack.
    //
    // To make these harder we want to make sure this is a class that was
    // either built into the binary or legitimately registered through
    // objc_duplicateClass, objc_initializeClassPair or objc_allocateClassPair.
    checkIsKnownClass(cls);

    if (slowpath(!cls->isRealized())) {
        cls = realizeClassMaybeSwiftAndLeaveLocked(cls, runtimeLock);
        // runtimeLock may have been dropped but is now locked again
    }

    if (slowpath((behavior & LOOKUP_INITIALIZE) && !cls->isInitialized())) {
        cls = initializeAndLeaveLocked(cls, inst, runtimeLock);
        // runtimeLock may have been dropped but is now locked again

        // If sel == initialize, class_initialize will send +initialize and 
        // then the messenger will send +initialize again after this 
        // procedure finishes. Of course, if this is not being called 
        // from the messenger then it won't happen. 2778172
    }

    runtimeLock.assertLocked();
    curClass = cls;

    // The code used to lookpu the class's cache again right after
    // we take the lock but for the vast majority of the cases
    // evidence shows this is a miss most of the time, hence a time loss.
    //
    // The only codepath calling into this without having performed some
    // kind of cache lookup is class_getInstanceMethod().

    for (unsigned attempts = unreasonableClassCount();;) {
        // curClass method list.
        Method meth = getMethodNoSuper_nolock(curClass, sel);
        if (meth) {
            imp = meth->imp(false);
            goto done;
        }

        if (slowpath((curClass = curClass->superclass) == nil)) {
            // No implementation found, and method resolver didn't help.
            // Use forwarding.
            imp = forward_imp;
            break;
        }

        // Halt if there is a cycle in the superclass chain.
        if (slowpath(--attempts == 0)) {
            _objc_fatal("Memory corruption in class list.");
        }

        // Superclass cache.
        imp = cache_getImp(curClass, sel);
        if (slowpath(imp == forward_imp)) {
            // Found a forward:: entry in a superclass.
            // Stop searching, but don't cache yet; call method
            // resolver for this class first.
            break;
        }
        if (fastpath(imp)) {
            // Found the method in a superclass. Cache it in this class.
            goto done;
        }
    }

    // No implementation found. Try method resolver once.

    if (slowpath(behavior & LOOKUP_RESOLVER)) {
        behavior ^= LOOKUP_RESOLVER;
        return resolveMethod_locked(inst, sel, cls, behavior);
    }

 done:
    log_and_fill_cache(cls, imp, sel, inst, curClass);
    runtimeLock.unlock();
 done_nolock:
    if (slowpath((behavior & LOOKUP_NIL) && imp == forward_imp)) {
        return nil;
    }
    return imp;
}

static void
log_and_fill_cache(Class cls, IMP imp, SEL sel, id receiver, Class implementer)
{
#if SUPPORT_MESSAGE_LOGGING
    if (slowpath(objcMsgLogEnabled && implementer)) {
        bool cacheIt = logMessageSend(implementer->isMetaClass(), 
                                      cls->nameForLogging(),
                                      implementer->nameForLogging(), 
                                      sel);
        if (!cacheIt) return;
    }
#endif
    cache_fill(cls, sel, imp, receiver);
}

void cache_fill(Class cls, SEL sel, IMP imp, id receiver)
{
    runtimeLock.assertLocked();

#if !DEBUG_TASK_THREADS
    // Never cache before +initialize is done
    if (cls->isInitialized()) {
        cache_t *cache = getCache(cls);
#if CONFIG_USE_CACHE_LOCK
        mutex_locker_t lock(cacheUpdateLock);
#endif
        cache->insert(cls, sel, imp, receiver);
    }
#else
    _collecting_in_critical();
#endif
}


ALWAYS_INLINE
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<Atomic, Encoded>(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);
}

ALWAYS_INLINE
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);
    }
}

bucket_t *allocateBuckets(mask_t newCapacity)
{
    // Allocate one extra bucket to mark the end of the list.
    // This can't overflow mask_t because newCapacity is a power of 2.
    bucket_t *newBuckets = (bucket_t *)
        calloc(cache_t::bytesForCapacity(newCapacity), 1);

    bucket_t *end = cache_t::endMarker(newBuckets, newCapacity);

#if __arm__
    // End marker's sel is 1 and imp points BEFORE the first bucket.
    // This saves an instruction in objc_msgSend.
    end->set<NotAtomic, Raw>((SEL)(uintptr_t)1, (IMP)(newBuckets - 1), nil);
#else
    // End marker's sel is 1 and imp points to the first bucket.
    end->set<NotAtomic, Raw>((SEL)(uintptr_t)1, (IMP)newBuckets, nil);
#endif
    
    if (PrintCaches) recordNewCache(newCapacity);

    return newBuckets;
}

static void cache_collect_free(bucket_t *data, mask_t capacity)
{
#if CONFIG_USE_CACHE_LOCK
    cacheUpdateLock.assertLocked();
#else
    runtimeLock.assertLocked();
#endif

    if (PrintCaches) recordDeadCache(capacity);

    _garbage_make_room ();
    garbage_byte_size += cache_t::bytesForCapacity(capacity);
    garbage_refs[garbage_count++] = data;
    cache_collect(false);
}

查看哪儿有调用void incrementOccupied();方法,发现cache_t::insert方法用有调用


incrementOccupied@2x.png

cache_t_insert@2x.png

分析流程如下图，函数sel()和imp通过哈希算法存储，若有重复的，就会哈希再哈希，哈希存储是无序的，先打印say4,后打印say3。


Cooci 关于Cache_t原理分析图.png
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