方法本质和objc_msgSend 汇编源码分析
- objc_msgSend(recevier, _cmd)
- 1:判断 recevier(消息接收者) 是否存在
- recevier -> class ->isa
CacheLookup LOOKUP, _objc_msgLookup, __objc_msgLookup_uncached
// NORMAL, _objc_msgSend, __objc_msgSend_uncached , MissLabelConstant
.macro CacheLookup Mode, Function, MissLabelDynamic, MissLabelConstant
//
// Restart protocol:
//
// As soon as we're past the LLookupStart\Function label we may have
// loaded an invalid cache pointer or mask.
//
// When task_restartable_ranges_synchronize() is called,
// (or when a signal hits us) before we're past LLookupEnd\Function,
// then our PC will be reset to LLookupRecover\Function which forcefully
// jumps to the cache-miss codepath which have the following
// requirements:
//
// GETIMP:
// The cache-miss is just returning NULL (setting x0 to 0)
//
// NORMAL and LOOKUP:
// - x0 contains the receiver
// - x1 contains the selector
// - x16 contains the isa
// - other registers are set as per calling conventions
//
mov x15, x16 // stash the original isa
LLookupStart\Function:
// p1 = SEL, p16 = isa
#if CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_HIGH_16_BIG_ADDRS
ldr p10, [x16, #CACHE] // p10 = mask|buckets
lsr p11, p10, #48 // p11 = mask
and p10, p10, #0xffffffffffff // p10 = buckets
and w12, w1, w11 // x12 = _cmd & mask
#elif CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_HIGH_16
ldr p11, [x16, #CACHE] // p11 = mask|buckets
#if CONFIG_USE_PREOPT_CACHES
#if __has_feature(ptrauth_calls)
tbnz p11, #0, LLookupPreopt\Function
and p10, p11, #0x0000ffffffffffff // p10 = buckets
#else
and p10, p11, #0x0000fffffffffffe // p10 = buckets
tbnz p11, #0, LLookupPreopt\Function
#endif
eor p12, p1, p1, LSR #7
and p12, p12, p11, LSR #48 // x12 = (_cmd ^ (_cmd >> 7)) & mask
#else
// p11 cache -> p10 = buckets
// p11, LSR #48 -> mask
// p1(_cmd) & mask = index -> p12
and p10, p11, #0x0000ffffffffffff // p10 = buckets
and p12, p1, p11, LSR #48 // x12 = _cmd & mask
#endif // CONFIG_USE_PREOPT_CACHES
#elif CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_LOW_4
ldr p11, [x16, #CACHE] // p11 = mask|buckets
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
- ldr p11, [x16, #CACHE] 找到CACHE定义 #define CACHE (2 * SIZEOF_POINTER) 得出CACHE是16， x16是class 平移16 得到cache_t，所以 P11 = cache_t
-
and p10, p11, #0x0000fffffffffffe tbnz p11, #0, LLookupPreopt\Function 这两句得出cache_t&0x0000fffffffffffe（mask）的操作 对应如下图
image.png
得到 bucket_t 所有P10 = bucket_t
eor p12, p1, p1, LSR #7
and p12, p12, p11, LSR #48 // x12 = (_cmd ^ (_cmd >> 7)) & mask 这2句得出如
image.png
找到cache_hash的 index 进行递归缓存命中查找 如下
1: ldp p17, p9, [x13], #-BUCKET_SIZE // {imp, sel} = *bucket--
cmp p9, p1 // if (sel != _cmd) {
b.ne 3f // scan more
// } else {
2: CacheHit \Mode // hit: call or return imp
// }
3: cbz p9, \MissLabelDynamic // if (sel == 0) goto Miss;
cmp p13, p10 // } while (bucket >= buckets)
b.hs 1b
总结: objc_msgSend 在底层汇编就是快速（cmd -> imp）的过程，通过找到缓存池里的buckets 然后找到对应的index 进行地址平移查找，如果一直都找不到，直接跳转至CheckMiss，因为$0是normal，会跳转至__objc_msgSend_uncached，即进入慢速查找流程
image.png
