objc4-750源码解析 alloc

从苹果开源源码库objc/libsystem/dyld/coreFoundation(CF) 下一份源码

• 可以看到runtime的两个版本：objc-runtime-old; objc-2（objc-runtime-new）

• NSObject.mm 混编

• alloc 调用流程

不管是直接看alloc，还是 objc_alloc

// Calls [cls alloc].
id
objc_alloc(Class cls)
{
    return callAlloc(cls, true/*checkNil*/, false/*allocWithZone*/);
}

+ (id)alloc {
    return _objc_rootAlloc(self);
}

// Base class implementation of +alloc. cls is not nil.
// Calls [cls allocWithZone:nil].
id
_objc_rootAlloc(Class cls)
{
    return callAlloc(cls, false/*checkNil*/, true/*allocWithZone*/);
}

最后都是调用 callAlloc

// Call [cls alloc] or [cls allocWithZone:nil], with appropriate 
// shortcutting optimizations.
static ALWAYS_INLINE id
callAlloc(Class cls, bool checkNil, bool allocWithZone=false)
{
    if (slowpath(checkNil && !cls)) return nil;

#if __OBJC2__
    if (fastpath(!cls->ISA()->hasCustomAWZ())) {
        // No alloc/allocWithZone implementation. Go straight to the allocator.
        // fixme store hasCustomAWZ in the non-meta class and 
        // add it to canAllocFast's summary
        if (fastpath(cls->canAllocFast())) {
            // No ctors, raw isa, etc. Go straight to the metal.
            bool dtor = cls->hasCxxDtor();
            id obj = (id)calloc(1, cls->bits.fastInstanceSize());
            if (slowpath(!obj)) return callBadAllocHandler(cls);
            obj->initInstanceIsa(cls, dtor);
            return obj;
        }
        else {
            // Has ctor or raw isa or something. Use the slower path.
            id obj = class_createInstance(cls, 0);
            if (slowpath(!obj)) return callBadAllocHandler(cls);
            return obj;
        }
    }
#endif

    // No shortcuts available.
    if (allocWithZone) return [cls allocWithZone:nil];
    return [cls alloc];
}

然后真正创建的方法 class_createInstance

id 
class_createInstance(Class cls, size_t extraBytes)
{
    return _class_createInstanceFromZone(cls, extraBytes, nil);
}


static __attribute__((always_inline)) 
id
_class_createInstanceFromZone(Class cls, size_t extraBytes, void *zone, 
                              bool cxxConstruct = true, 
                              size_t *outAllocatedSize = nil)
{
    if (!cls) return nil;

    assert(cls->isRealized());

    // Read class's info bits all at once for performance
    bool hasCxxCtor = cls->hasCxxCtor();
    bool hasCxxDtor = cls->hasCxxDtor();
    bool fast = cls->canAllocNonpointer();

    size_t size = cls->instanceSize(extraBytes);
    if (outAllocatedSize) *outAllocatedSize = size;

    id obj;
    if (!zone  &&  fast) {
        obj = (id)calloc(1, size);
        if (!obj) return nil;
        obj->initInstanceIsa(cls, hasCxxDtor);
    } 
    else {
        if (zone) {
            obj = (id)malloc_zone_calloc ((malloc_zone_t *)zone, 1, size);
        } else {
            obj = (id)calloc(1, size);
        }
        if (!obj) return nil;

        // Use raw pointer isa on the assumption that they might be 
        // doing something weird with the zone or RR.
        obj->initIsa(cls);
    }

    if (cxxConstruct && hasCxxCtor) {
        obj = _objc_constructOrFree(obj, cls);
    }

    return obj;
}

核心：cls->instanceSize ；calloc；initInstanceIsa

calloc 在 malloc中无法查看更多细节

截屏2019-12-24下午5.07.30.png

cls->instanceSize :计算对象需要占用的大小（系统分配），8字节对齐，最少16字节。

size_t instanceSize(size_t extraBytes) {
    size_t size = alignedInstanceSize() + extraBytes;
    // CF requires all objects be at least 16 bytes.
    if (size < 16) size = 16;
    return size;
}
// May be unaligned depending on class's ivars.
uint32_t unalignedInstanceSize() {
    assert(isRealized());
    return data()->ro->instanceSize;
}

// Class's ivar size rounded up to a pointer-size boundary.
uint32_t alignedInstanceSize() {
    return word_align(unalignedInstanceSize());
}

//define WORD_MASK 7UL
static inline uint32_t word_align(uint32_t x) {
    return (x + WORD_MASK) & ~WORD_MASK;
}

空对象默认有isa属性，isa 是个 typedef struct objc_class *Class（结构体指针），需要：8字节;

内存对齐截图

size：

1. 对象需要的内存空间，8的倍数-8字节对齐
2. 最少16字节，预留，防止意外

需要说明一下，我们常用的获取对象size的两个方法： malloc_size(object)返回是系统分配的内存大小，16字节对齐；而class_getInstanceSize([object class])是对象内存占用的大小，8字节对齐，并且没有最少16字节的逻辑，代码如下：

size_t class_getInstanceSize(Class cls)
{
    if (!cls) return 0;
    return cls->alignedInstanceSize();
}

// Class's ivar size rounded up to a pointer-size boundary.
uint32_t alignedInstanceSize() {
    return word_align(unalignedInstanceSize());
}

代码跟踪

alloc 创建对象跟踪
calloc：申请内存空间
initInstanceIsa：关联class(isa)
initInstanceIsa 详解