上篇文章我们分析了objc_msgSend在缓存里查找imp的流程，我们紧接着上一篇分析__objc_msgSend_uncached。

__objc_msgSend_uncached

.endmacro

    STATIC_ENTRY __objc_msgSend_uncached
    UNWIND __objc_msgSend_uncached, FrameWithNoSaves

    // THIS IS NOT A CALLABLE C FUNCTION
    // Out-of-band p16 is the class to search
    
    MethodTableLookup
    TailCallFunctionPointer x17

    END_ENTRY __objc_msgSend_uncached

主要是走两个方法，MethodTableLookup和TailCallFunctionPointer x17。

MethodTableLookup

.macro MethodTableLookup
    
    // push frame
    SignLR
    stp fp, lr, [sp, #-16]!
    mov fp, sp

    // save parameter registers: x0..x8, q0..q7
    //把参数写入到寄存器
    sub sp, sp, #(10*8 + 8*16)
    stp q0, q1, [sp, #(0*16)]
    stp q2, q3, [sp, #(2*16)]
    stp q4, q5, [sp, #(4*16)]
    stp q6, q7, [sp, #(6*16)]
    stp x0, x1, [sp, #(8*16+0*8)]
    stp x2, x3, [sp, #(8*16+2*8)]
    stp x4, x5, [sp, #(8*16+4*8)]
    stp x6, x7, [sp, #(8*16+6*8)]
    str x8,     [sp, #(8*16+8*8)]

    // lookUpImpOrForward(obj, sel, cls, LOOKUP_INITIALIZE | LOOKUP_RESOLVER)
    // receiver and selector already in x0 and x1
    mov x2, x16
    mov x3, #3
    bl  _lookUpImpOrForward

    // IMP in x0
    mov x17, x0
    
    // restore registers and return
    //读取寄存器里的参数
    ldp q0, q1, [sp, #(0*16)]
    ldp q2, q3, [sp, #(2*16)]
    ldp q4, q5, [sp, #(4*16)]
    ldp q6, q7, [sp, #(6*16)]
    ldp x0, x1, [sp, #(8*16+0*8)]
    ldp x2, x3, [sp, #(8*16+2*8)]
    ldp x4, x5, [sp, #(8*16+4*8)]
    ldp x6, x7, [sp, #(8*16+6*8)]
    ldr x8,     [sp, #(8*16+8*8)]

    mov sp, fp
    ldp fp, lr, [sp], #16
    AuthenticateLR

.endmacro

主要代码是跳转到_lookUpImpOrForward。

ldp/stp是 ldr/str 的衍生, 可以同时读/写两个寄存器, ldr/str只能读写一个
sub sp, sp, #0x20 ; 拉伸栈空间32(20 = 2*16)个字节
stp x0 , x1, [sp, #0x10] ; sp往上加16(10 = 1 * 16)个字节,存放x0 和 x1
ldp x1 , x0, [sp, #0x10] ; 将sp偏移16个字节的值取出来,放入x1 和 x0

_lookUpImpOrForward

直接搜索_lookUpImpOrForward发现找不到源码，这里拓展一个知识点。

在objc_msgSend处打上断点，按住control + step into，进入libobjc.A.dylib`objc_msgSend:找到_objc_msgSend_uncached

• lookUpImpOrForward 源码

IMP lookUpImpOrForward(id inst, SEL sel, Class cls, int behavior)
{
    const IMP forward_imp = (IMP)_objc_msgForward_impcache;
    IMP imp = nil;
    Class curClass;

    runtimeLock.assertUnlocked();

    // 重新查找一次缓存，避免多线程的影响
    if (fastpath(behavior & LOOKUP_CACHE)) {
        imp = cache_getImp(cls, sel);
        if (imp) goto done_nolock;
    }

    runtimeLock.lock();

   //检查类是否是已知的类
    checkIsKnownClass(cls);

    if (slowpath(!cls->isRealized())) {
        cls = realizeClassMaybeSwiftAndLeaveLocked(cls, runtimeLock);
        // runtimeLock may have been dropped but is now locked again
    }

    if (slowpath((behavior & LOOKUP_INITIALIZE) && !cls->isInitialized())) {
        cls = initializeAndLeaveLocked(cls, inst, runtimeLock);
        // runtimeLock may have been dropped but is now locked again

        // If sel == initialize, class_initialize will send +initialize and 
        // then the messenger will send +initialize again after this 
        // procedure finishes. Of course, if this is not being called 
        // from the messenger then it won't happen. 2778172
    }

    runtimeLock.assertLocked();
    curClass = cls;

    // The code used to lookpu the class's cache again right after
    // we take the lock but for the vast majority of the cases
    // evidence shows this is a miss most of the time, hence a time loss.
    //
    // The only codepath calling into this without having performed some
    // kind of cache lookup is class_getInstanceMethod().

    for (unsigned attempts = unreasonableClassCount();;) {
        // 在类中查找imp，没有返回nil，不找父类
        Method meth = getMethodNoSuper_nolock(curClass, sel);
        if (meth) {
            imp = meth->imp;
            goto done;
        }
        //curClass = curClass->superclass
        //查找继承链，直到nil；没有找到的话赋值forward_imp
        if (slowpath((curClass = curClass->superclass) == nil)) {
            // No implementation found, and method resolver didn't help.
            // Use forwarding.
            imp = forward_imp;
            break;
        }

        // Halt if there is a cycle in the superclass chain.
        if (slowpath(--attempts == 0)) {
            _objc_fatal("Memory corruption in class list.");
        }

        // curClass指向父类时，从缓存中查找一次imp
        imp = cache_getImp(curClass, sel);
        //imp == forward_imp 跳出循环
        if (slowpath(imp == forward_imp)) {
            break;
        }
        //查找到了 imp 跳转到done
        if (fastpath(imp)) {
            // Found the method in a superclass. Cache it in this class.
            goto done;
        }
    }

    // No implementation found. Try method resolver once.

    if (slowpath(behavior & LOOKUP_RESOLVER)) {
        behavior ^= LOOKUP_RESOLVER;
        return resolveMethod_locked(inst, sel, cls, behavior);
    }

 done:
    log_and_fill_cache(cls, imp, sel, inst, curClass);
    runtimeLock.unlock();
 done_nolock:
    if (slowpath((behavior & LOOKUP_NIL) && imp == forward_imp)) {
        return nil;
    }
    return imp;
}

主要流程：
1.为避免多线程影响，进来先cache_getImp从缓存中取一次，找到了直接返回imp
2.检查cls是否是IsKnownClass
3.从当前类开始在继承链中查找imp，找到就返回imp；没找到就返回forward_imp；

• getMethodNoSuper_nolock源码

static method_t *
getMethodNoSuper_nolock(Class cls, SEL sel)
{
    runtimeLock.assertLocked();

    ASSERT(cls->isRealized());
    //获取类的method_list
    auto const methods = cls->data()->methods();
    //mlists是数组首元素 end是第二个元素
    for (auto mlists = methods.beginLists(),
              end = methods.endLists();
         mlists != end;
         ++mlists)
    {
        method_t *m = search_method_list_inline(*mlists, sel);
        if (m) return m;
    }
    return nil;
}

• search_method_list_inline

ALWAYS_INLINE static method_t *
search_method_list_inline(const method_list_t *mlist, SEL sel)
{
    int methodListIsFixedUp = mlist->isFixedUp();
    int methodListHasExpectedSize = mlist->entsize() == sizeof(method_t);
    
    if (fastpath(methodListIsFixedUp && methodListHasExpectedSize)) {
        //在有序MethodList查找
        return findMethodInSortedMethodList(sel, mlist);
    } else {
        // 遍历mlist 从第一个到最后一个，如果sel相等，返回元素地址
        for (auto& meth : *mlist) {
            if (meth.name == sel) return &meth;
        }
    }
#if DEBUG
    // sanity-check negative results
    if (mlist->isFixedUp()) {
        for (auto& meth : *mlist) {
            if (meth.name == sel) {
                _objc_fatal("linear search worked when binary search did not");
            }
        }
    }
#endif
    return nil;
}

• findMethodInSortedMethodList

ALWAYS_INLINE static method_t *
findMethodInSortedMethodList(SEL key, const method_list_t *list)
{
    ASSERT(list);

    const method_t * const first = &list->first;
    const method_t *base = first;
    const method_t *probe;
    uintptr_t keyValue = (uintptr_t)key;
    uint32_t count;
    
    for (count = list->count; count != 0; count >>= 1) {
        probe = base + (count >> 1);
        
        uintptr_t probeValue = (uintptr_t)probe->name;
        
        if (keyValue == probeValue) {
            // `probe` is a match.
            // Rewind looking for the *first* occurrence of this value.
            // This is required for correct category overrides.
            //找到的是category加进来的方法时，将指针向前移动一位
            //默认category写的方法存在前面，类本身在后面一位
            while (probe > first && keyValue == (uintptr_t)probe[-1].name) {
                probe--;
            }
            return (method_t *)probe;
        }
        
        if (keyValue > probeValue) {
            base = probe + 1;
            count--;
        }
    }
    
    return nil;
}

findMethodInSortedMethodList是一个二分法查找过程，找到返回method_t，没有找到返回nil。到这里查找的流程就结束了，我们接着看一下找到imp之后的处理。

注意：有category添加方法时，要指针向前平移一位，获得返回值

• 回到lookUpImpOrForward里的done

done:
    log_and_fill_cache(cls, imp, sel, inst, curClass);
    runtimeLock.unlock();
done_nolock:
    if (slowpath((behavior & LOOKUP_NIL) && imp == forward_imp)) {
        return nil;
    }
    return imp;

查找到imp时，调用log_and_fill_cache然后调用cache_fill把方法存入cache_t中。与前面内容呼应上了。

static void
log_and_fill_cache(Class cls, IMP imp, SEL sel, id receiver, Class implementer)
{
#if SUPPORT_MESSAGE_LOGGING
    if (slowpath(objcMsgLogEnabled && implementer)) {
        bool cacheIt = logMessageSend(implementer->isMetaClass(), 
                                      cls->nameForLogging(),
                                      implementer->nameForLogging(), 
                                      sel);
        if (!cacheIt) return;
    }
#endif
    cache_fill(cls, sel, imp, receiver);
}

消息转发

没有查找到imp的时候，并不是返回nil，而是返回forward_imp。

  const IMP forward_imp = (IMP)_objc_msgForward_impcache;

全局搜索_objc_msgForward_impcache，在objc-msg-arm64.s中找到源码

__objc_msgForward_impcache只是简单的跳转到__objc_msgForward

• __objc_msgForward

ENTRY __objc_msgForward
    //将__objc_forward_handler@PAGE地址存到 x17
    adrp    x17, __objc_forward_handler@PAGE
    //x17偏移__objc_forward_handler@PAGEOFF加载到p17
    ldr p17, [x17, __objc_forward_handler@PAGEOFF]
    TailCallFunctionPointer x17
    
    END_ENTRY __objc_msgForward

主要研究的还是__objc_forward_handler方法。全局搜索__objc_forward_handler发现找不到源码，__objc_forward_handler其实是用C写的，去掉一个下划线，全局搜索。

unrecognized selector sent to instance 0x101484880

动态方法决议

没有找到imp时，并不是直接报错，还做了其他处理。