OC 底层探索 10、objc_msgSend 流程 2 - 慢速查找

我们已经知道消息发送流程首先会走到缓存 cache 里面，那么当缓存中没有查询到消息时 __objc_msgSend_uncached，后续怎么继续执行呢？

一、切入口

__objc_msgSend_uncached --> MethodTableLookup --> _lookUpImpOrForward.

我们通过源码可以看到缓存中找不到会进入 _lookUpImpOrForward 的查找过程，汇编中搜索并未找到此方法：

1、全局搜索‘lookUpImpOrForward’，可以发现此方法是在objc-runtime-new.mm文件中C++实现的；

2、同样也可以通过 show 反汇编方式找到：Debug --> Debug Workflow --> Always show disassembly 勾上

control + stepinto 进入 objc_msgSend：

control + stepinto 进入 _objc_msgSend_uncached：

_lookUpImpOrForward -->  objc-runtime-new.mm 的 6116 行。

下面将从 lookUpImpOrForward 为入口进行消息查找流程的探索。

二、慢速查找分析

1、慢速查找流程图：

 

forward_imp：--> const IMP forward_imp = (IMP)_objc_msgForward_impcache;

这里代码会继续找到汇编里面：--> __objc_msgForward --> _objc_forward_handler ：

经典的报错信息！“+ -” 的打印时苹果人为手动添加的，这里其实也可说明在底层并不存在所谓的 +- 方法，都是函数而已，实例方法是类的实例方法，类方法也是元类的实例方法。

2、主要源码

1. 查找的主流程

IMP lookUpImpOrForward(id inst, SEL sel, Class cls, int behavior)
{// cache 中没有找到方法，开始走 lookUpImpOrForward 查找流程
    // 汇编有调 --> objc_msgForward --> 找不到方法报错信息的处理
    const IMP forward_imp = (IMP)_objc_msgForward_impcache;
    IMP imp = nil;
    Class curClass;

    runtimeLock.assertUnlocked();

    // Optimistic cache lookup
    if (fastpath(behavior & LOOKUP_CACHE)) {
        // 找缓存 - 是为了出现在此过程中方法又被人调用加进缓存了，有缓存了就不必继续慢速找了
        imp = cache_getImp(cls, sel);
        if (imp) goto done_nolock;// 找到了，去 done_nolock
    }

    // runtimeLock is held during isRealized and isInitialized checking
    // to prevent races against concurrent realization.

    // runtimeLock is held during method search to make
    // method-lookup + cache-fill atomic with respect to method addition.
    // Otherwise, a category could be added but ignored indefinitely because
    // the cache was re-filled with the old value after the cache flush on
    // behalf of the category.
    // 注释翻译不如英文准确不翻了
    runtimeLock.lock();

    // We don't want people to be able to craft a binary blob that looks like
    // a class but really isn't one and do a CFI attack.
    //
    // To make these harder we want to make sure this is a class that was
    // either built into the binary or legitimately registered through
    // objc_duplicateClass, objc_initializeClassPair or objc_allocateClassPair.
    //
    // TODO: this check is quite costly during process startup.
    checkIsKnownClass(cls);

    if (slowpath(!cls->isRealized())) {// cls 是否已实现，否则去将类信息进行处理 类元类方法全部要处理好的 --> 为了后面的方法查找
        cls = realizeClassMaybeSwiftAndLeaveLocked(cls, runtimeLock);
        // runtimeLock may have been dropped but is now locked again
    }

    // initialize 初始化
    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();;) {// 死循环，没有出口条件，跳出逻辑在循环内部
        // curClass method list.
        Method meth = getMethodNoSuper_nolock(curClass, sel);// 查找方法
        if (meth) {// 找着了
            imp = meth->imp;
            goto done;
        }
        // 自己没找着
        // curClass = superClass
        // 是nil 则直接 没找着方法把nil的forward_imp赋给imp，并跳出循环
        if (slowpath((curClass = curClass->superclass) == nil)) {
            // No implementation found, and method resolver didn't help.
            // Use forwarding.
            imp = forward_imp;
            break;
        }
// superclass 不是 nil 继续向下走

        // Halt if there is a cycle in the superclass chain.
        // 如果超类链中存在循环，则停止
        if (slowpath(--attempts == 0)) {
            _objc_fatal("Memory corruption in class list.");// 类列表的内存污染了
        }

        // Superclass cache.
        // 找父类的缓存
        /**
         CacheLookup  GETIMP, _cache_getImp
         */
        imp = cache_getImp(curClass, sel);
        /*
         STATIC_ENTRY _cache_getImp

         GetClassFromIsa_p16 p0
         CacheLookup GETIMP, _cache_getImp // GETIMP，cache查找的参数是GETIMP，checkMiss
         
    LGetImpMiss:// cache 中没找到直接返回0
         mov    p0, #0
         ret

         END_ENTRY _cache_getImp
         */
        
        
        if (slowpath(imp == forward_imp)) {//
            // Found a forward:: entry in a superclass.
            // Stop searching, but don't cache yet; call method
            // resolver for this class first.
            break;
        }
        if (fastpath(imp)) {// 父类中找到了 goto done --> 对此方法进行缓存
            // Found the method in a superclass. Cache it in this class.
            goto done;
        }
    }

    // No implementation found. Try method resolver once.
　　// 上面找完了没找着方法 动态方法解析 resolver 一次 --> 此方法只会走一次once
    if (slowpath(behavior & LOOKUP_RESOLVER)) {/*
　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　&：3 & 2 = 0011 & 0010 = 0010
　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　第二次(方法动态处理中会再回来查一遍)再来条件就为false了: 0001 & 0010 = 0000
　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　*/
        behavior ^= LOOKUP_RESOLVER;// 异或操作 behavior = 0011^0010 = 0001
        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;
}

2. 二分查找方法 list 源码与注解

/***********************************************************************
 * search_method_list_inline
 **********************************************************************/
ALWAYS_INLINE static method_t *
findMethodInSortedMethodList(SEL key, const method_list_t *list)
{
    ASSERT(list);
    // list: 方法list是递增排序(类加载时完成的排序)的 即：name 转 unsigned long 类型 对应的数值 是递增的,例:0 1 2 3 4 5 ......
    const method_t * const first = &list->first;
    const method_t *base = first;// 方法list中第一个方法
    const method_t *probe;
    uintptr_t keyValue = (uintptr_t)key;// 要查找的方法  sel强转uintptr_t
    uint32_t count;// 方法数
    // 二分查找
    /**
     找 03   -    01 02 03 04 05 06 07 08
     probe = 1 +  8>>1 = 1+ 4 = 5
     判断第5个方法是否是要找的方法
        不是：对比 要找的方法 和 当前方法 位置谁大 03>05？false
     
     开始回第一步
     probe = 1+ count>>1 = 1 +  4>>1 = 1+2 = 3
     继续判断...
     02 == 03 false
     02 > 03 false
     
     继续
     probe = 1+ 2>>1 = 1+1 = 2
     02 == 02 ture
        while 判断
            判断找有重名的分类方法
     */
    for (count = list->count; count != 0; count >>= 1) {// count = 8>>1  4>>1  2>>1
        probe = base + (count >> 1);
        
        uintptr_t probeValue = (uintptr_t)probe->name;
        
        if (keyValue == probeValue) {// 判断这个方法是否是要找的方法
            // `probe` is a match.
            // Rewind looking for the *first* occurrence of this value.
            // This is required for correct category overrides.
            while (probe > first && keyValue == (uintptr_t)probe[-1].name) {// 判断分类方法 是否有重名的方法，有则往前找取分类方法 --> 分类排在前面
                probe--;
            }
            return (method_t *)probe;
        }
        
        if (keyValue > probeValue) {
            base = probe + 1;
            count--;
        }
    }
    
    return nil;
}