底层探寻alloc都发生了什么

static ALWAYS_INLINE id  
_class_createInstanceFromZone(Class cls, size_t extraBytes, void *zone,
                              int construct_flags = OBJECT_CONSTRUCT_NONE,
                              bool cxxConstruct = true,
                              size_t *outAllocatedSize = nil)  
{  
    ASSERT(cls->isRealized());  
    // Read class's info bits all at once for performance  
    bool hasCxxCtor = cxxConstruct && cls->hasCxxCtor();  
    bool hasCxxDtor = cls->hasCxxDtor();  
    bool fast = cls->canAllocNonpointer();  
    size_t size;  
    // 1:要开辟多少内存  
    size = cls->instanceSize(extraBytes);  
    if (outAllocatedSize) *outAllocatedSize = size;  
    id obj;  
    if (zone) {  
        obj = (id)malloc_zone_calloc((malloc_zone_t *)zone, 1, size);  
    } else {  
        // 2;怎么去申请内存  
        obj = (id)calloc(1, size);  
    }  
    if (slowpath(!obj)) {  
        if (construct_flags & OBJECT_CONSTRUCT_CALL_BADALLOC) {  
            return _objc_callBadAllocHandler(cls);  
        }  
        return nil;  
    }  
    // 3: ?
    if (!zone && fast) {  
        obj->initInstanceIsa(cls, hasCxxDtor);  
    } else {  
        // Use raw pointer isa on the assumption that they might be  
        // doing something weird with the zone or RR.  
        obj->initIsa(cls);  
    }  
    if (fastpath(!hasCxxCtor)) {  
        return obj;  
    }  
    construct_flags |= OBJECT_CONSTRUCT_FREE_ONFAILURE;  
    return object_cxxConstructFromClass(obj, cls, construct_flags);  
}

//x很可能为真， fastpath 可以简称为 真值判断  
#define fastpath(x) (__builtin_expect(bool(x), 1))   
//x很可能为假，slowpath 可以简称为 假值判断  
#define slowpath(x) (__builtin_expect(bool(x), 0))