IOS底層探索Cache_t
Cache_t的數據結構
1.下載objc818可調試源碼c++
2.在main.m文件添加以下代碼：git
#import <Foundation/Foundation.h>
@interface ABPerson : NSObject
@end
@implementation ABPerson
@end int main(int argc, const char * argv[]) {
@autoreleasepool {
Class pClass = [ABPerson class];
NSLog(@"%@",pClass);
}
return 0;
}
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3.經過lldb調試Cache_t的數據結構github
p/x pClass獲取類對象的首地址p/x 0x0000000100008498 + 0x10首地址偏移16個字節拿到cache_tp (cache_t *)0x00000001000084a8打印cache_t地址p *$2查看cache_t結構
struct cache_t {
private:
explicit_atomic<uintptr_t> _bucketsAndMaybeMask; // 8
union {
struct {
explicit_atomic<mask_t> _maybeMask; // 4
#if __LP64__
uint16_t _flags; // 2
#endif
uint16_t _occupied; // 2
};
explicit_atomic<preopt_cache_t *> _originalPreoptCache; // 8
};
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可以發現打印出的結構與objc源碼中cache_t的結構相對應。緩存
void cache_t::insert(SEL sel, IMP imp, id receiver) {
省略部分代碼
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.
do {
if (fastpath(b[i].sel() == 0)) {
incrementOccupied();
b[i].set<Atomic, Encoded>(b, 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));
bad_cache(receiver, (SEL)sel);
#endif // !DEBUG_TASK_THREADS
}
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cache_t中的insert方法裏面是將SEL和IMP用bucket_t包裝起來的。markdown
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
省略部分代碼
}
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進一步說明了bucket_t是包裝SEL和IMP的。數據結構
Cache_t的數據結構圖 app
LLDB驗證方法的存儲
修改main.m文件代碼：less
@interface ABPerson : NSObject
-(void)saySomething;
@end
@implementation ABPerson
-(void)saySomething
{
NSLog(@"saySomething");
}
@end int main(int argc, const char * argv[]) {
@autoreleasepool {
ABPerson *p = [ABPerson alloc];
Class pClass = [ABPerson class];
NSLog(@"%@",pClass);
}
return 0;
}
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p/x pClass獲取類對象的首地址p/x 0x00000001000084b8 + 0x10首地址偏移16個字節拿到cache_tp (cache_t *)0x00000001000084c8打印cache_t地址p *$2查看cache_t結構p [p saySomething]調用一次方法，讓其有緩存p $3.buckets()[4]內存平移4個單位獲取值p $4.imp(nil,pClass)打印imp
小規模取樣
1.參照objc_class結構定義ab_objc_classoop
struct objc_class : objc_object {
省略部分代碼
// 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
省略部分代碼
}
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struct ab_objc_class {
Class isa;
Class superclass;
struct ab_cache_t cache;
struct ab_class_data_bits_t bits;
};
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2.參照cache_t定義ab_cache_t源碼分析
struct cache_t {
private:
explicit_atomic<uintptr_t> _bucketsAndMaybeMask; // 8
union {
struct {
explicit_atomic<mask_t> _maybeMask; // 4
#if __LP64__
uint16_t _flags; // 2
#endif
uint16_t _occupied; // 2
};
explicit_atomic<preopt_cache_t *> _originalPreoptCache; // 8
};
省略部分代碼
}
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typedef uint32_t mask_t;
struct ab_cache_t {
struct ab_bucket_t *_bukets; // 8
mask_t _maybeMask; // 4
uint16_t _flags; // 2
uint16_t _occupied; // 2
};
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3.參照class_data_bits_t定義ab_class_data_bits_t
struct class_data_bits_t {
friend objc_class;
// Values are the FAST_ flags above.
uintptr_t bits;
省略部分代碼
}
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struct ab_class_data_bits_t {
uintptr_t bits;
};
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4.參照bucket_t定義ab_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
省略部分代碼
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struct ab_bucket_t {
SEL _sel;
IMP _imp;
};
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測試類ABPerson的定義
@interface ABPerson : NSObject
- (void)say1;
- (void)say2;
- (void)say3;
- (void)say4;
- (void)say5;
- (void)say6;
- (void)say7;
+ (void)sayHappy;
@end
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@implementation ABPerson
- (void)say1{
NSLog(@"ABPerson say : %s",__func__);
}
- (void)say2{
NSLog(@"ABPerson say : %s",__func__);
}
- (void)say3{
NSLog(@"ABPerson say : %s",__func__);
}
- (void)say4{
NSLog(@"ABPerson say : %s",__func__);
}
- (void)say5{
NSLog(@"ABPerson say : %s",__func__);
}
- (void)say6{
NSLog(@"ABPerson say : %s",__func__);
}
- (void)say7{
NSLog(@"ABPerson say : %s",__func__);
}
+ (void)sayHappy{
NSLog(@"ABPerson say : %s",__func__);
}
@end
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測試即輸出：
int main(int argc, const char * argv[]) {
@autoreleasepool {
ABPerson *p = [ABPerson alloc];
Class pClass = p.class;
[p say1];
[p say2];
[p say3];
[pClass sayHappy];
struct ab_objc_class *ab_class = (__bridge struct ab_objc_class *)(pClass);
NSLog(@"%hu - %u",ab_class->cache._occupied,ab_class->cache._maybeMask);
for (mask_t i = 0; i<ab_class->cache._maybeMask; i++) {
struct ab_bucket_t bucket = ab_class->cache._bukets[i];
NSLog(@"%@ - %pf",NSStringFromSelector(bucket._sel),bucket._imp);
}
NSLog(@"Hello, World!");
}
return 0;
}
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沒有報錯，而且可以正常輸出（輸出結果後面還會分析到，這裏先略過），說明小規模取樣是可行的。
insert源碼分析
void insert(SEL sel, IMP imp, id receiver);
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void cache_t::insert(SEL sel, IMP imp, id receiver) {
省略部分代碼
mask_t newOccupied = occupied() + 1; // 1+1
unsigned oldCapacity = capacity(), capacity = oldCapacity;
//首次是空的cache
if (slowpath(isConstantEmptyCache())) {
// Cache is read-only. Replace it.
// INIT_CACHE_SIZE_LOG2 = 2,
//INIT_CACHE_SIZE = (1 << INIT_CACHE_SIZE_LOG2),
//1左移兩位就是4，INIT_CACHE_SIZE= 4
if (!capacity) capacity = INIT_CACHE_SIZE;//初始化容量爲4
reallocate(oldCapacity, capacity, /* freeOld */false);
}
//cache_fill_ratio:capacity * 3 / 4; 若是小於等於當前容積的3/4，正常插入數據
else if (fastpath(newOccupied + CACHE_END_MARKER <= cache_fill_ratio(capacity))) {
// Cache is less than 3/4 or 7/8 full. Use it as-is.
}
#if CACHE_ALLOW_FULL_UTILIZATION
else if (capacity <= FULL_UTILIZATION_CACHE_SIZE && newOccupied + CACHE_END_MARKER <= capacity) {
// Allow 100% cache utilization for small buckets. Use it as-is.
}
#endif
else {
//若是大於當前容積的3/4，就進行容積的2倍擴容 ：4*2 = 8
capacity = capacity ? capacity * 2 : INIT_CACHE_SIZE;
if (capacity > MAX_CACHE_SIZE) {
capacity = MAX_CACHE_SIZE;
}
//分配內存
reallocate(oldCapacity, capacity, true);
}
bucket_t *b = buckets();
//capacity首次容量爲4
mask_t m = capacity - 1; // 4-1=3 8-1 = 7
//經過cache_hash獲得哈希地址
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.
do {
//sel首次加入進來
if (fastpath(b[i].sel() == 0)) {
incrementOccupied(); //occupied++;
//bucket插入sel和imp
b[i].set<Atomic, Encoded>(b, sel, imp, cls());
return;
}
//已存在sel就跳過
if (b[i].sel() == sel) {
// The entry was added to the cache by some other thread
// before we grabbed the cacheUpdateLock.
return;
}
//cache_next: (i+1) & mask
} while (fastpath((i = cache_next(i, m)) != begin));
bad_cache(receiver, (SEL)sel);
#endif // !DEBUG_TASK_THREADS
}
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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);
//存儲newBuckets指針
setBucketsAndMask(newBuckets, newCapacity - 1);
if (freeOld) {
//回收舊內存
collect_free(oldBuckets, oldCapacity);
}
}
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void cache_t::setBucketsAndMask(struct bucket_t *newBuckets, mask_t newMask) {
#ifdef __arm__
mega_barrier();
//存儲newBuckets指針
_bucketsAndMaybeMask.store((uintptr_t)newBuckets, memory_order_relaxed);
mega_barrier();
//存儲newMask值
_maybeMask.store(newMask, memory_order_relaxed);
_occupied = 0;
#elif __x86_64__ || i386
// ensure other threads see buckets contents before buckets pointer
_bucketsAndMaybeMask.store((uintptr_t)newBuckets, memory_order_release);
// ensure other threads see new buckets before new mask
_maybeMask.store(newMask, memory_order_release);
_occupied = 0;
#else
#error Don't know how to do setBucketsAndMask on this architecture. #endif } 複製代碼
小規模取樣中調用了say1、say2、say3，輸出cache._occupied = 1，cache._maybeMask = 7。方法存儲也不是按順序存儲的。 如今去掉一個，只調用say1、say2：
輸出cache._occupied = 2，cache._maybeMask = 3。
根據上面分析的原理：
- 初始化容量爲
4，mask = capacity - 1因此爲cache._maybeMask = 3，當前調用了2個方法因此cache._occupied = 2 - 調用
3個方法後：newOccupied = occupied() + 1，即newOccupied = 3
newOccupied + CACHE_END_MARKER = 3 + 1 = 4 > capacity * 3 / 4 = 3，
擴容爲2倍的capacity，即capacity = 2 * 4 = 8，mask = capacity - 1，
因此cache._maybeMask = 8 - 1 = 7，又由於分配空間的時候，collect_free回收舊的內存，以前的被清空，只有新的方法say3加進去，因此cache._occupied = 1。由於是哈希表存儲的說以並非從頭開始純粹的。
最後，附上插入分析圖：
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相關文章
- 1. iOS 底層探索 - cache_t
- 2. cache_t底層探索
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