《iOS底层原理文章汇总》
运行时感受，通过clang底层编译
编译前的代码
#import
@interface DCPerson :NSObject
-(void)sayNB;
-(void)sayHello;
@end
@implementation DCPerson
-(void)sayNB{
NSLog(@"%s",__func__);
}
-(void)sayHello{
NSLog(@"%s",__func__);
}
@end
int main(int argc, const char * argv[]) {
@autoreleasepool {
DCPerson *person = [DCPerson alloc];
[person sayNB];
[person sayHello];
NSLog(@"Hello, World!");
}
return 0;
}
编译后的代码
int main(int argc, const char * argv[]) {
/* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;
DCPerson *person = ((DCPerson *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("DCPerson"), sel_registerName("alloc"));
((void (*)(id, SEL))(void *)objc_msgSend)((id)person, sel_registerName("sayNB"));
((void (*)(id, SEL))(void *)objc_msgSend)((id)person, sel_registerName("sayHello"));
NSLog((NSString *)&__NSConstantStringImpl__var_folders_0d_yy9dm1g92sjdnrzk1pn6cgf80000gn_T_main_9aaf89_mi_2);
}
return 0;
}
- 1.调用sayNB方法进行等价互换objc_msgSend(person,sel_registerName("sayNB"));
objc_msgSend(person,sel_registerName("sayNB"));
- 2.使用objc_msgSendSuper
@interface Anthropoid : NSObject
-(void)sayHello;
@end
@implementation Anthropoid
-(void)sayHello{
NSLog(@"%s",__func__);
}
@end
@interface DCPerson :Anthropoid
-(void)sayNB;
@end
@implementation DCPerson
-(void)sayNB{
NSLog(@"%s",__func__);
}
@end
//消息的接受者还是自己 - 父类 - 直接找父类
struct objc_super lgsuper;
lgsuper.receiver = person;
lgsuper.super_class = [Anthropoid class];
objc_msgSendSuper(&lgsuper, sel_registerName("sayHello"));
//输出
-[Anthropoid sayHello]
OC 方法 - 消息 (sel imp) sel -> imp -> 内容
isa流程图.png
objc_msgSend流程分析.png
对象 - ISA - 方法(类) - cache_t - method_list(bits)
伪代码实现
[person sayHello] -> imp (cache -> bucket (sel imp))
//获取当前的对象
id person = 0x100000
//获取isa
isa_t isa = 0x000000
//isa -> class -> cache
cache_t cache = isa + 16字节
//arm64
//mask|buckets 在一起的
buckets = cache & 0x0000ffffffffffff
//获取mask
mask = cache LSR #48
//下标 = mask * & sel
index = mask & p1
//bucket 从buckets遍历的开始(起始查询的bucket)
bucket = buckets + index * 16 (sel imp = 16)
int count = 0;
//CheckMiss $0
do{
if(count == 2) goto CheckMiss
if(bucket == buckets){//进入第二层判断
//bucket == 第一个元素
//bucket人为设置到最后一个元素
bucket = buckets + mask * 16;
count++;
}
// {imp,sel} = * --bucket
//缓存的查找的顺序是:向前查找
bucket --;
imp = bucket.imp;
sel = bucket.sel;
}while(bucket.sel != _cmd)//bucket里面的sel是否匹配_cmd
//CacheHit $0
return imp;
CheckMiss:
CheckMiss(normal)
伪代码自己完善下？？？
一个bucket中存储了16个字节的数据，因为sel和imp分别占用8字节
p12 = buckets + (mask << 1 + PTRSHIFT)表示直接索引到最后一个元素
/*
//伪代码实现
LLookupStart$1:
// p1 = SEL, p16 = isa
ldr p11, [x16, #CACHE] // p11 = mask|buckets
#if CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_HIGH_16
and p10, p11, #0x0000ffffffffffff // p10 = buckets
and p12, p1, p11, LSR #48 // x12 = _cmd & mask
#elif CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_LOW_4
and p10, p11, #~0xf // p10 = buckets
and p11, p11, #0xf // p11 = maskShift
mov p12, #0xffff
lsr p11, p12, p11 // p11 = mask = 0xffff >> p11
and p12, p1, p11 // x12 = _cmd & mask
#else
#error Unsupported cache mask storage for ARM64.
#endif
add p12, p10, p12, LSL #(1+PTRSHIFT)
// p12 = buckets + ((_cmd & mask) << (1+PTRSHIFT))
ldp p17, p9, [x12] // {imp, sel} = *bucket
1: cmp p9, p1 // if (bucket->sel != _cmd)
b.ne 2f // scan more
CacheHit $0 // call or return imp
2: // not hit: p12 = not-hit bucket
CheckMiss $0 // miss if bucket->sel == 0
cmp p12, p10 // wrap if bucket == buckets
b.eq 3f
ldp p17, p9, [x12, #-BUCKET_SIZE]! // {imp, sel} = *--bucket
b 1b // loop
3: // wrap: p12 = first bucket, w11 = mask
#if CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_HIGH_16
add p12, p12, p11, LSR #(48 - (1+PTRSHIFT))
// p12 = buckets + (mask << 1+PTRSHIFT)
#elif CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_LOW_4
add p12, p12, p11, LSL #(1+PTRSHIFT)
// p12 = buckets + (mask << 1+PTRSHIFT)
#else
#error Unsupported cache mask storage for ARM64.
#endif
// Clone scanning loop to miss instead of hang when cache is corrupt.
// The slow path may detect any corruption and halt later.
ldp p17, p9, [x12] // {imp, sel} = *bucket
1: cmp p9, p1 // if (bucket->sel != _cmd)
b.ne 2f // scan more
CacheHit $0 // call or return imp
2: // not hit: p12 = not-hit bucket
CheckMiss $0 // miss if bucket->sel == 0
cmp p12, p10 // wrap if bucket == buckets
b.eq 3f
ldp p17, p9, [x12, #-BUCKET_SIZE]! // {imp, sel} = *--bucket
b 1b // loop
LLookupEnd$1:
LLookupRecover$1:
3: // double wrap
JumpMiss $0
*/
[person sayHello] -> imp(cache -> bucket(sel,imp))
//获取当前对象
id person = 0x1000000
//获取isa
isa = 0x00000001
//isa -> class -> cache
cache_t cache = isa + 16字节
//arm64
//mask|buckets在一起的
buckets = cache & 0x0000ffffffffffff
//获取mask
mask = cache LSR(逻辑右移) #48位
//下标 = mask & sel
index = mask & p1
//获取单个的bucket，从buckets,遍历的开始(起始查询的bucket)
bucket = buckets + ((_cmd & mask) << (1+PTRSHIFT))
bucket = buckets + index * 16(sel,imp分别8字节)
while(bucket.sel != _cmd){//bucket里面的sel是否匹配_cmd
//不满足，没有找到_cmd进入第二层判断
if(bucket == buckets){
//bucket == 第一个元素
//bucket 人为设置到最后一个元素
p12 = buckets + (mask << 1+PTRSHIFT)
bucket = buckets + mask * 16(从后往前查找，减减(--操作))
}//{imp,sel} = *--bucket,缓存查找的顺序:向前查找
bucket--;
imp = bucket.imp;
sel = bucket.sel;
}
return bucket.imp;//跳出循环即找到缓存中的方法，返回bucket.imp
goto CheckMiss:CheckMiss(normal)
消息发送到汇编之后，进入入口函数ENTRY,之后如何找到当前的类？
ENTRY _objc_msgSend,之后通过ISA找到类
ldr p13, [x0] // p13 = isa
GetClassFromIsa_p16 p13 // p16 = class
isa得到类@2x.png
查看GetClassFromIsa_p16,$0表示当前对象传过来的第一个参数为isa,和ISA_MASK(掩码)进行与运算，放到P16的位置
.macro GetClassFromIsa_p16 /* src */
#if SUPPORT_INDEXED_ISA
// Indexed isa
mov p16, $0 // optimistically set dst = src
tbz p16, #ISA_INDEX_IS_NPI_BIT, 1f // done if not non-pointer isa
// isa in p16 is indexed
adrp x10, _objc_indexed_classes@PAGE
add x10, x10, _objc_indexed_classes@PAGEOFF
ubfx p16, p16, #ISA_INDEX_SHIFT, #ISA_INDEX_BITS // extract index
ldr p16, [x10, p16, UXTP #PTRSHIFT] // load class from array
1:
#elif __LP64__
// 64-bit packed isa
and p16, $0, #ISA_MASK
#else
// 32-bit raw isa
mov p16, $0
#endif
CacheLookup:若bucket==buckets，表示是第一个元素，则到3f,将bucket移到最后一个元素，比较当前是否是要查找的_cmd == bucket.imp，若是，缓存命中直接返回，若不是，则跳2f,先判断是否遗失(是否为空，sel是否存在(bucket.sel == 0))，若不存在且bucket == buckets，即当前bucket为第一个元素(顶层),则跳JumpMiss $0，表示遗失了对象的方法即没找到，若bucket != buckets，则向前继续查找，进行bucket--，往前查找。
add p12, p10, p12, LSL #(1+PTRSHIFT)
// p12 = buckets + ((_cmd & mask) << (1+PTRSHIFT))
ldp p17, p9, [x12] // {imp, sel} = *bucket
1: cmp p9, p1 // if (bucket->sel != _cmd)
b.ne 2f // scan more
CacheHit $0 // call or return imp
2: // not hit: p12 = not-hit bucket
CheckMiss $0 // miss if bucket->sel == 0
cmp p12, p10 // wrap if bucket == buckets
b.eq 3f
ldp p17, p9, [x12, #-BUCKET_SIZE]! // {imp, sel} = *--bucket
b 1b // loop
3: // wrap: p12 = first bucket, w11 = mask
#if CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_HIGH_16
add p12, p12, p11, LSR #(48 - (1+PTRSHIFT))
// p12 = buckets + (mask << 1+PTRSHIFT)
#elif CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_LOW_4
add p12, p12, p11, LSL #(1+PTRSHIFT)
// p12 = buckets + (mask << 1+PTRSHIFT)
#else
#error Unsupported cache mask storage for ARM64.
#endif
// Clone scanning loop to miss instead of hang when cache is corrupt.
// The slow path may detect any corruption and halt later.
ldp p17, p9, [x12] // {imp, sel} = *bucket
1: cmp p9, p1 // if (bucket->sel != _cmd)
b.ne 2f // scan more
CacheHit $0 // call or return imp
2: // not hit: p12 = not-hit bucket
CheckMiss $0 // miss if bucket->sel == 0
cmp p12, p10 // wrap if bucket == buckets
b.eq 3f
ldp p17, p9, [x12, #-BUCKET_SIZE]! // {imp, sel} = *--bucket
b 1b // loop
LLookupEnd$1:
LLookupRecover$1:
3: // double wrap
JumpMiss $0
.endmacro