objc_msgSend 消息快速查找 - cache 查找

//NORMAL, _objc_msgSend, __objc_msgSend_uncached
.macro CacheLookup Mode, Function, MissLabelDynamic, MissLabelConstant
    //   requirements:
    //   //缓存不存在返回NULL，x0设置为0
    //   GETIMP:
    //     The cache-miss is just returning NULL (setting x0 to 0)
    //   参数说明
    //   NORMAL and LOOKUP:
    //   - x0 contains the receiver
    //   - x1 contains the selector
    //   - x16 contains the isa
    //   - other registers are set as per calling conventions
    //
//调用过来的p16存储的是cls，将cls存储在x15.
    mov x15, x16            // stash the original isa
//_objc_msgSend
LLookupStart\Function:
    // p1 = SEL, p16 = isa
//arm64 64 OSX/SIMULATOR
#if CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_HIGH_16_BIG_ADDRS
    //isa->cache，首地址也就是_bucketsAndMaybeMask
    ldr p10, [x16, #CACHE]              // p10 = mask|buckets
    //lsr逻辑右移 p11 = _bucketsAndMaybeMask >> 48 也就是 mask
    lsr p11, p10, #48           // p11 = mask
    //p10 = _bucketsAndMaybeMask & 0xffffffffffff = buckets（保留后48位）
    and p10, p10, #0xffffffffffff   // p10 = buckets
    //x12 = cmd & mask   w1为第二个参数cmd（self，cmd...），w11也就是p11 也就是执行cache_hash。这里没有>>7位的操作
    and w12, w1, w11            // x12 = _cmd & mask
//arm64 64 真机这里p11计算后是_bucketsAndMaybeMask
#elif CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_HIGH_16
    ldr p11, [x16, #CACHE]          // p11 = mask|buckets
//arm64 + iOS + !模拟器 + 非mac应用
#if CONFIG_USE_PREOPT_CACHES
//iphone 12以后指针验证
#if __has_feature(ptrauth_calls)
    //tbnz 测试位不为0则跳转。与tbz对应。p11 第0位不为0则跳转 LLookupPreopt\Function。
    tbnz    p11, #0, LLookupPreopt\Function
    //p10 = _bucketsAndMaybeMask & 0x0000ffffffffffff = buckets
    and p10, p11, #0x0000ffffffffffff   // p10 = buckets
#else
    //p10 = _bucketsAndMaybeMask & 0x0000fffffffffffe = buckets
    and p10, p11, #0x0000fffffffffffe   // p10 = buckets
    //p11 第0位不为0则跳转 LLookupPreopt\Function。
    tbnz    p11, #0, LLookupPreopt\Function
#endif
    //eor 逻辑异或(^) 格式为：EOR{S}{cond} Rd, Rn, Operand2
    //p12 = selector ^ (selector >> 7) select 右移7位&自己给到p12
    eor p12, p1, p1, LSR #7
    //p12 = p12 & (_bucketsAndMaybeMask >> 48) = index & mask值 = buckets中的下标
    and p12, p12, p11, LSR #48      // x12 = (_cmd ^ (_cmd >> 7)) & mask
#else
    //p10 = _bucketsAndMaybeMask & 0x0000ffffffffffff = buckets
    and p10, p11, #0x0000ffffffffffff   // p10 = buckets
    //p12 = selector & (_bucketsAndMaybeMask >>48) = sel & mask = buckets中的下标
    and p12, p1, p11, LSR #48       // x12 = _cmd & mask
#endif // CONFIG_USE_PREOPT_CACHES
//arm64 32
#elif CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_LOW_4
    //后4位为mask前置0的个数的case
    ldr p11, [x16, #CACHE]              // p11 = mask|buckets
    and p10, p11, #~0xf         // p10 = buckets 相当于后4位置为0，取前32位
    and p11, p11, #0xf          // p11 = maskShift 取的是后4位，为mask前置位的0的个数
    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
    //通过上面的计算 p10 = buckets，p11 = mask（arm64真机是_bucketsAndMaybeMask）， p12 = index
    // p13（bucket_t） = buckets + 下标 << 4   PTRSHIFT arm64 为3.  <<4 位为16字节 buckets + 下标 *16 = buckets + index *16 也就是直接平移到了第几个元素的地址。
    add p13, p10, p12, LSL #(1+PTRSHIFT)
                        // p13 = buckets + ((_cmd & mask) << (1+PTRSHIFT))
    //这里就直接遍历查找了，因为arm64下cache_next相当于遍历（这里只扫描了前面）
                        // do {
    //p17 = imp, p9 = sel
1:  ldp p17, p9, [x13], #-BUCKET_SIZE   //     {imp, sel} = *bucket--
    //sel - _cmd != 0 则跳转 3:，也就意味着没有找到就跳转到__objc_msgSend_uncached
    cmp p9, p1              //     if (sel != _cmd) {
    b.ne    3f              //         scan more
                        //     } else {
    //找到则调用或者返回imp，Mode为 NORMAL
2:  CacheHit \Mode              // hit:    call or return imp  命中
                        //     }
//__objc_msgSend_uncached
//缓存中找不到方法就走__objc_msgSend_uncached逻辑了。
    //cbz 为0跳转 sel == nil 跳转 \MissLabelDynamic
3:  cbz p9, \MissLabelDynamic       //     if (sel == 0) goto Miss; 有空位没有找到说明没有缓存
    //bucket_t - buckets 由于是递减操作
    cmp p13, p10            // } while (bucket >= buckets) //⚠️ 这里一直是往前找，后面的元素在后面还有一次循环。
    //无符号大于等于 则跳转1:f b 分别代表front与back
    b.hs    1b

//没有命中cache  查找 p13 = mask对应的元素，也就是倒数第二个
#if CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_HIGH_16_BIG_ADDRS
    //p13 = buckets + (mask << 4) 平移找到对应mask的bucket_t。UXTW 将w11扩展为64位后左移4
    add p13, p10, w11, UXTW #(1+PTRSHIFT)
                        // p13 = buckets + (mask << 1+PTRSHIFT)
#elif CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_HIGH_16
    //p13 = buckets + (mask >> 44) 这里右移44位，少移动4位就不用再左移了。因为maskZeroBits的存在 就找到了mask对应元素的地址
    add p13, p10, p11, LSR #(48 - (1+PTRSHIFT))
                        // p13 = buckets + (mask << 1+PTRSHIFT)
                        // see comment about maskZeroBits
#elif CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_LOW_4
    //p13 = buckets + (mask << 4) 找到对应mask的bucket_t。
    add p13, p10, p11, LSL #(1+PTRSHIFT)
                        // p13 = buckets + (mask << 1+PTRSHIFT)
#else
#error Unsupported cache mask storage for ARM64.
#endif
    //p12 = buckets + (p12<<4) index对应的bucket_t
    add p12, p10, p12, LSL #(1+PTRSHIFT)
                        // p12 = first probed bucket

    //之前已经往前查找过了，这里从后往index查找
                        // do {
    //p17 = imp p9 = sel
4:  ldp p17, p9, [x13], #-BUCKET_SIZE   //     {imp, sel} = *bucket--
    //sel - _cmd
    cmp p9, p1              //     if (sel == _cmd)
    //sel == _cmd跳转CacheHit
    b.eq    2b              //         goto hit
    //sel ！= nil
    cmp p9, #0              // } while (sel != 0 &&
    //
    ccmp    p13, p12, #0, ne        //     bucket > first_probed)
    //有值跳转4:
    b.hi    4b

LLookupEnd\Function:
LLookupRecover\Function:
//仍然没有找到缓存，缓存彻底不存在 __objc_msgSend_uncached()
    b   \MissLabelDynamic

//NORMAL, _objc_msgSend, __objc_msgSend_uncached
void CacheLookup(Mode,Function,MissLabelDynamic,MissLabelConstant) {
    //1. 根据架构不同集算sel在buckets中的index
    if (arm64_64 && OSX/SIMULATOR) {
        p10 = isa->cache //_bucketsAndMaybeMask
        p11 = _bucketsAndMaybeMask >> 48//mask
        p10 = _bucketsAndMaybeMask & 0xffffffffffff//buckets
        x12 = sel & mask //index 也就是执行cache_hash
    } else if (arm64_64) {//真机 //这个分支下没有计算mask
        p11 = isa->cache //_bucketsAndMaybeMask
        if (arm64 + iOS + !模拟器 + 非mac应用) {
            if (开启指针验证 ) {
                if (_bucketsAndMaybeMask 第0位 ！= 0) {
                    goto LLookupPreopt\Function
                } else {
                    p10 = _bucketsAndMaybeMask & 0x0000ffffffffffff//buckets
                }
            } else {
                p10 = _bucketsAndMaybeMask & 0x0000fffffffffffe //buckets
                if (_bucketsAndMaybeMask 第0位 ！= 0) {
                    goto LLookupPreopt\Function
                }
            }
            //计算index
            p12 = selector ^ (selector >> 7)
            p12 = p12 & (_bucketsAndMaybeMask & 48) = p12 & mask//index
        } else {
            p10 = _bucketsAndMaybeMask & 0x0000ffffffffffff //buckets
            p12 = selector & (_bucketsAndMaybeMask >>48) //index
        }
    } else if (arm64_32) {
        p11 = _bucketsAndMaybeMask
        p10 =  _bucketsAndMaybeMask &（~0xf）//buckets 相当于后4位置为0，取前32位
        p11 = _bucketsAndMaybeMask & 0xf //mask前置位0的个数
        p11 =  0xffff >> p11 //获取到mask的值
        x12 = selector & mask //index
    } else {
        #error Unsupported cache mask storage for ARM64.
    }
    
    //通过上面的计算 p10 = buckets，p11 = mask/_bucketsAndMaybeMask， p12 = index
    p13 = buckets + index << 4 //找到cls对应的buckets地址。地址平移找到对应bucket_t。
    
    //2.找缓存（这里只扫描了前面）
    do {
        p13 = *bucket-- //赋值后指向前一个bucket
        p17 = bucket.imp
        p9 = bucket.sel
        if (p9 != selector) {
            if (p9 == 0) {//说明没有缓存
                __objc_msgSend_uncached()
            }
        } else {//缓存命中，走命中逻辑 call or return imp
            CacheHit \Mode
        }
    } while(bucket >= buckets) //buckets是首地址，bucket是index对应的buckct往前移动
  
    //查找完后还没有缓存？
    //查找 p13 = mask对应的元素，也就是最后一个元素
    if (arm64_64 && OSX/SIMULATOR) {
        p13 = buckets + (mask << 4)
    } else if (arm64_64) {//真机
        p13 = buckets + (_bucketsAndMaybeMask >> 44)//这里右移44位，少移动4位就不用再左移了。这里就找到了对应index的bucket_t。
    } else if (arm64_32) {
        p13 = buckets + (mask << 4)
    } else {
        #error Unsupported cache mask storage for ARM64.
    }
    
    //index的bucket_t 从mask对应的buckets开始再往前找
    p12 = buckets + (index<<4)
    do {
        p17 = imp;
        p9 = sel;
        *p13--;
        if (p9 == selector) {//命中
            CacheHit \Mode
        }
    } while (p9 != nil && bucket > p12)//从后往前 p9位nil则证明没有存，也就不存在缓存了。
    
    //仍然没有找到缓存，缓存彻底不存在。
    __objc_msgSend_uncached()
}

LLookupPreopt\Function:
#if __has_feature(ptrauth_calls)
    //p10 = _bucketsAndMaybeMask & 0x007ffffffffffffe = buckets
    and p10, p11, #0x007ffffffffffffe   // p10 = x
    //buckets x16为cls 验证
    autdb   x10, x16            // auth as early as possible
#endif

    // x12 = (_cmd - first_shared_cache_sel)
    //(_cmd >> 12 + PAGE) << 12 + PAGEOFF 第一个sel
    adrp    x9, _MagicSelRef@PAGE
    ldr p9, [x9, _MagicSelRef@PAGEOFF]
    //差值index
    sub p12, p1, p9

    // w9  = ((_cmd - first_shared_cache_sel) >> hash_shift & hash_mask)
#if __has_feature(ptrauth_calls)
    // bits 63..60 of x11 are the number of bits in hash_mask
    // bits 59..55 of x11 is hash_shift

    // 取到 hash_shift...
    lsr x17, x11, #55           // w17 = (hash_shift, ...)
    //w9 = index >> hash_shift
    lsr w9, w12, w17            // >>= shift
    //x17 = _bucketsAndMaybeMask >>60 //mask_bits
    lsr x17, x11, #60           // w17 = mask_bits
    mov x11, #0x7fff
    //x11 = 0x7fff >> mask_bits //mask
    lsr x11, x11, x17           // p11 = mask (0x7fff >> mask_bits)
    //x9 = x9 & mask
    and x9, x9, x11         // &= mask
#else
    // bits 63..53 of x11 is hash_mask
    // bits 52..48 of x11 is hash_shift
    lsr x17, x11, #48           // w17 = (hash_shift, hash_mask)
    lsr w9, w12, w17            // >>= shift
    and x9, x9, x11, LSR #53        // &=  mask
#endif
    //x17 = el_offs | (imp_offs << 32)
    ldr x17, [x10, x9, LSL #3]      // x17 == sel_offs | (imp_offs << 32)
    // cmp x12  x17 是否找到sel
    cmp x12, w17, uxtw

.if \Mode == GETIMP
    b.ne    \MissLabelConstant      // cache miss
    //imp = isa - (sel_offs >> 32)
    sub x0, x16, x17, LSR #32       // imp = isa - imp_offs
    //注册imp
    SignAsImp x0
    ret
.else
    b.ne    5f              // cache miss
    //imp（x17） =  (isa - sel_offs>> 32)
    sub x17, x16, x17, LSR #32      // imp = isa - imp_offs
.if \Mode == NORMAL
    //跳转imp
    br  x17
.elseif \Mode == LOOKUP
    //x16 = isa | 3 //这里为或的意思
    orr x16, x16, #3 // for instrumentation, note that we hit a constant cache
    //注册imp
    SignAsImp x17
    ret
.else
.abort  unhandled mode \Mode
.endif
    //x9 = buckets-1
5:  ldursw  x9, [x10, #-8]          // offset -8 is the fallback offset
    //计算回调isa  x16 = x16 + x9
    add x16, x16, x9            // compute the fallback isa
    //使用新isa重新查找缓存
    b   LLookupStart\Function       // lookup again with a new isa
.endif

#define NORMAL 0
#define GETIMP 1
#define LOOKUP 2

// CacheHit: x17 = cached IMP, x10 = address of buckets, x1 = SEL, x16 = isa
.macro CacheHit
//这里传入的为NORMAL
.if $0 == NORMAL
    //调用imp TailCallCachedImp(imp,buckets,sel,isa)
    TailCallCachedImp x17, x10, x1, x16 // authenticate and call imp
.elseif $0 == GETIMP
    //返回imp
    mov p0, p17
    //imp == nil跳转9:
    cbz p0, 9f          // don't ptrauth a nil imp
    //有imp执行AuthAndResignAsIMP(imp,buckets,sel,isa)最后给到x0返回。
    AuthAndResignAsIMP x0, x10, x1, x16 // authenticate imp and re-sign as IMP
9:  ret             // return IMP
.elseif $0 == LOOKUP
    // No nil check for ptrauth: the caller would crash anyway when they
    // jump to a nil IMP. We don't care if that jump also fails ptrauth.
    //找imp(imp,buckets,sel,isa)
    AuthAndResignAsIMP x17, x10, x1, x16    // authenticate imp and re-sign as IMP
    //isa与x15比较
    cmp x16, x15
    //cinc如果相等 就将x16+1，否则就设成0.
    cinc    x16, x16, ne            // x16 += 1 when x15 != x16 (for instrumentation ; fallback to the parent class)
    ret             // return imp via x17
.else
.abort oops
.endif
.endmacro

//x17 = cached IMP, x10 = address of buckets, x1 = SEL, x16 = isa
void CacheHit(Mode) {
    if (Mode == NORMAL) {
        //imp = imp^cls 解码
        TailCallCachedImp x17, x10, x1, x16    // 解码跳转imp
    } else if (Mode == GETIMP) {
        p0 = IMP
        if (p0 == nil) {
            return
        } else {
            AuthAndResignAsIMP(imp,buckets,sel,isa)//resign cached imp as IMP
        }
    } else if (Mode == LOOKUP) {
        AuthAndResignAsIMP(x17, buckets, sel, isa)//resign cached imp as IMP
        if (isa == x15) {
            x16 += 1
        } else {
            x16 = 0
        }
    } else {
        .abort oops//报错
    }
}

STATIC_ENTRY __objc_msgSend_uncached
UNWIND __objc_msgSend_uncached, FrameWithNoSaves

// THIS IS NOT A CALLABLE C FUNCTION
// Out-of-band p15 is the class to search
//查找imp
MethodTableLookup
//跳转imp
TailCallFunctionPointer x17

END_ENTRY __objc_msgSend_uncached

.macro MethodTableLookup
    
    SAVE_REGS MSGSEND

    // lookUpImpOrForward(obj, sel, cls, LOOKUP_INITIALIZE | LOOKUP_RESOLVER)
    // receiver and selector already in x0 and x1
    //x2 = cls
    mov x2, x16
    //x3 = LOOKUP_INITIALIZE|LOOKUP_RESOLVER //是否初始化,imp没有实现尝试resolver
//_lookUpImpOrForward(receiver,selector,cls,LOOKUP_INITIALIZE | LOOKUP_RESOLVER)
    mov x3, #3
    bl  _lookUpImpOrForward

    // IMP in x0
    mov x17, x0

    RESTORE_REGS MSGSEND

.endmacro

IMP lookUpImpOrForward(id inst, SEL sel, Class cls, int behavior)