class结构
struct objc_class : objc_object {
// Class ISA;
Class superclass;
cache_t cache; // formerly cache pointer and vtable
class_data_bits_t bits; // class_rw_t * plus custom rr/alloc flags
class_rw_t *data() const {
return bits.data();
}
之前我们已经看了data了放了什么,现在我们来看看cache_t
cache_t
补充
系统架构
macOS i386
模拟器: x86
真机:arm64
#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
#endif
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
...
#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();
explicit_atomic 是为了安全操作,暂不细看
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存着imp和sel
详看CACHE_MASK_STORAGE_HIGH_16 真机进行了什么操作
cache.png
测试代码:
LGPerson *p = [LGPerson alloc];
Class pClass = [LGPerson class];
// p.lgName = @"Cooci";
// p.nickName = @"KC";
// 缓存一次方法 sayHello
// 4
[p sayHello];
通过lldb打印
(lldb) p/x pClass
(Class) $0 = 0x0000000100002288 LGPerson
(lldb) p (cache_t*)0x0000000100002298
(cache_t *) $1 = 0x0000000100002298
(lldb) p *$1
(cache_t) $2 = {
_buckets = {
std::__1::atomic<bucket_t *> = 0x000000010101fe00 {
_sel = {
std::__1::atomic<objc_selector *> = ""
}
_imp = {
std::__1::atomic<unsigned long> = 11912
}
}
}
_mask = {
std::__1::atomic<unsigned int> = 3
}
_flags = 32804
_occupied = 1
}
(lldb) p $2.buckets()
(bucket_t *) $3 = 0x000000010101fe00
(lldb) p *$3
(bucket_t) $4 = {
_sel = {
std::__1::atomic<objc_selector *> = ""
}
_imp = {
std::__1::atomic<unsigned long> = 11912
}
}
(lldb) p $4.sel()
(SEL) $5 = "sayHello"
(lldb)
或者我们在没有源码的环境中添加结构体,进行强转测试
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);
}
}
return 0;
}
打印结果
LGPerson say : -[LGPerson say1]
LGPerson say : -[LGPerson say2]
LGPerson say : -[LGPerson say3]
LGPerson say : -[LGPerson say4]
2 - 7
say4 - 0x29b8
(null) - 0x0
say3 - 0x29e8
(null) - 0x0
(null) - 0x0
(null) - 0x0
(null) - 0x0
疑问
为什么_occupied,_mask是什么,为什么是2-7?
我们查看源码,只有方法中才会对数据进行操作,我们在源码中看到incrementOccupied()
{
_occupied++;
}
全局搜索,在cache_t::insert(Class cls, SEL sel, IMP imp, id receiver)找到使用这个方法的地方
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;
if (slowpath(isConstantEmptyCache())) {
// Cache is read-only. Replace it.
if (!capacity) capacity = INIT_CACHE_SIZE;
reallocate(oldCapacity, capacity, /* freeOld */false);///第一次创建,总容量为4
}
else if (fastpath(newOccupied + CACHE_END_MARKER <= capacity / 4 * 3)) { // 4 3 + 1 bucket cache_t
// Cache is less than 3/4 full. Use it as-is.
}
else {
///不少于3/4的时候进行扩容
capacity = capacity ? capacity * 2 : INIT_CACHE_SIZE; // 扩容两倍 4
if (capacity > MAX_CACHE_SIZE) {
capacity = MAX_CACHE_SIZE;
}
reallocate(oldCapacity, capacity, true); // 内存 库容完毕
}
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)) {//如果没找到方法就存值
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_next算法进行打乱了顺序
cache_t::bad_cache(receiver, (SEL)sel, cls);
}
总结
-
_occupied就是实际的大小,_mask就是capacity - 1
bucket为什么会丢失
- 在重新申请内存时被清除掉了原有的
为什么会乱序
- 里面有哈希
分析图
Cooci 关于Cache_t原理分析图.png
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