在我们探索class的底层时,我们追踪到objc_class的源码,其中重要结构为
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
}
可以看出四个最重要的模块
- isa (注释掉并不是说没有,只是提醒这里继承了objc_object的isa属性)
- superclass (父类)
- cache (缓存)
- bits (方法变量等数据)
当研究节点到今天时,我们已经研究了isa 和bits 的结构 而superclass 依旧是一个class的属性 so我们还剩下一个cache_t 类型的cache还没有分析。
所以,今天的任务,就是分析cache的结构
cache_t lldb 分析
在我们之前的研究过程中,lldb都是我们的三板斧之一,简单,暴力,直观。所以今天我们继续用lldb分析
(项目基于objc的公开源码 781版本 同时项目直接在mac上运行)
@interface FQPerson : NSObject
@property (nonatomic, strong) NSString * name;
@property (nonatomic, strong) NSString * nikeName;
-(void)sayHelloWorld;
-(void)eat1;
-(void)eat2;
-(void)eat3;
-(void)eat4;
-(void)eat5;
-(void)eat6;
+(void)cry;
@end
测试的类 在.m文件中实现这三个方法。
int main(int argc, const char * argv[]) {
@autoreleasepool {
// insert code here...
FQPerson *person = [FQPerson alloc];
Class personClass = [FQPerson class];
[person eat1];
[person sayHelloWorld];
NSLog(@"%@",personClass);
}
return 0;
}
测试的入口
我们在[person eat];
前下一个断点
开始我们的lldb尝试
通过我们之前的内存地址平移的方式,我们可以获取到cache的指针地址,并打印其中内容
从打印结果,我们可以看出cache_t的主要结构为
_buckets,
_mask,
_flags,
-
_occupied
从bucket_t的内容中,我们看到了sel和imp
而我们知道Sel和Imp和方法有关。
所以我们猜测cache缓存了方法相关的数据
于是,我们让运行[person eat1];
随后,我们继续打印cache_t
(lldb) p * $1
(cache_t) $3 = {
_buckets = {
std::__1::atomic<bucket_t *> = 0x0000000101906470 {
_sel = {
std::__1::atomic<objc_selector *> = ""
}
_imp = {
std::__1::atomic<unsigned long> = 8560
}
}
}
_mask = {
std::__1::atomic<unsigned int> = 3
}
_flags = 32804
_occupied = 1
}
此时_sel由Null变为了""
_mask变为了3
_occupied增加了1
可见确实在执行方法的过程中,在cache中存储了数据
现在,我们尝试打印其中可能储存的方法信息
可见cache_t中确实储存了调用过的方法信息
同时,我们使用machOView也可以验证我们存储的方法
cache_t代码分析
我们在lldb的分析中得到了一些成果
- cache_t中确实储存了方法信息
- 方法信息以Sel和Imp对的方式存在_buckets中。
但也存在很多问题,
- 缓存的存储伴随增删改查,这些是如何实现的?
- _mask,_occupied,_flags这些参数有什么作用?
现在,源码在手的优势就来了,让我们分析一下源码
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));
};
其中相关宏定义
于是,我们可以首先得到不同框架环境下cacha_t的中的属性并不相同,最大区别为真机中_maskAndBuckets
mask和buckets 存在同一个地方而非真机中是分开存储的
同时,我们也看到了一些值得我们研究的方法
void reallocate(mask_t oldCapacity, mask_t newCapacity, bool freeOld);
void insert(Class cls, SEL sel, IMP imp, id receiver);
由此,我们来分别研究一下。
void reallocate(mask_t oldCapacity, mask_t newCapacity, bool freeOld);
显然,这个是向系统申请开辟内存空间的过程
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);
}
}
简化成流程图为
中间_occupied 会被赋值为0 ,这就是为什么扩容后,_occupied 的值不会等于调用的方法数。
解释一下这里为什么不保留原先的数据。
举个例子,你买了一个小户型的房子,你住了一段时间家里人增加了,想换个大点的房子,这时候你并不是把墙敲了直接再盖两间就行了,因为你隔壁可能已经被分配给别人了,只能在别的空地上再给你建一栋足够大的房子,那这样,你之前的房子其实跟现在的房子并没有关系,如果数据全部迁移也会麻烦很多。因为这里的数据是缓存数据,并不是不能丢失的,所以直接丢弃,只开辟新空间。
void insert(Class cls, SEL sel, IMP imp, id receiver);
这个是向cache中存储的方法,也是我们最需要研究的方法
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);
}
我们再次简化成流程图
先判断是否有空间,如果没有直接默认申请4个空间
如果本身已有空间,判断newOccupied + CACHE_END_MARKER <= capacity / 4 * 3
如果满足,直接对bucket赋值
如果不满足,则2倍扩容。 然后清理空间
然后存储bucket。
bucket存储 流程
至此,我们大概分析了cache_t的结构和 数据存储的流程总图为
以及总结我们之前的问题 _occupied 为当前缓存中的计数 _mask 为当前申请的空间数-1.