iOS内存管理-alloc方法剖析

本文目标了解alloc内部做了什么

objc源码

实例化对象步骤

Person *p = [[Person alloc]init];

alloc方法执行之后，会向系统申请一块内存，内存多大取决于Persion属性。这块内存是堆内存。返回一个指针地址保存在p，p的值放在栈区

下面梳理底层代码

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

id
_objc_rootAlloc(Class cls)
{
    return callAlloc(cls, false/*checkNil*/, true/*allocWithZone*/);
}

下面看callAlloc

static ALWAYS_INLINE id
callAlloc(Class cls, bool checkNil, bool allocWithZone=false)
{
    if (slowpath(checkNil && !cls)) return nil;
    if (fastpath(!cls->ISA()->hasCustomAWZ())) {
        return _objc_rootAllocWithZone(cls, nil);
    }

    // No shortcuts available.
    if (allocWithZone) {
        return ((id(*)(id, SEL, struct _NSZone *))objc_msgSend)(cls, @selector(allocWithZone:), nil);
    }
    return ((id(*)(id, SEL))objc_msgSend)(cls, @selector(alloc));
}


id
_objc_rootAllocWithZone(Class cls, objc_zone_t zone __unused)
{
    // allocWithZone under __OBJC2__ ignores the zone parameter
    return _class_createInstanceFromZone(cls, 0, nil,
                                         OBJECT_CONSTRUCT_CALL_BADALLOC);
}

class_createInstanceFromZone创建实例

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
    //C++析构
    bool hasCxxCtor = cxxConstruct && cls->hasCxxCtor();
    bool hasCxxDtor = cls->hasCxxDtor();
    bool fast = cls->canAllocNonpointer();
    size_t size;

//计算对象占用内存大小
    size = cls->instanceSize(extraBytes);
    
    if (outAllocatedSize) *outAllocatedSize = size;

    id obj;
#if SUPPORT_ZONES
    if (zone) {
        obj = (id)malloc_zone_calloc((malloc_zone_t *)zone, 1, size);
    } else {
#endif
//开辟内存空间
        obj = (id)calloc(1, size);
#if SUPPORT_ZONES
    }
#endif
    if (slowpath(!obj)) {
        if (construct_flags & OBJECT_CONSTRUCT_CALL_BADALLOC) {
            return _objc_callBadAllocHandler(cls);
        }
        return nil;
    }

    if (!zone && fast) {
    //绑定isa和对象关系
        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);
}

主要三个方法：
计算需要内存大小；开辟堆空间；关联内存空间到相应的类

计算内存大小步骤

inline size_t instanceSize(size_t extraBytes) const {
        if (fastpath(cache.hasFastInstanceSize(extraBytes))) {
        //返回缓存
            return cache.fastInstanceSize(extraBytes);
        }

//否则计算，下面是计算
        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() const {
        ASSERT(isRealized());
        return data()->ro()->instanceSize;
    }

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

对象大小16字节对齐，小于16，返回16

1. cpu读取数据，以块为单位，减少读取频次，提升性能，8个字节没有多余空间，16字节正好。

2. 16字节原因是对象中，默认第一个属性isa，占据8个字节，后面8个字节预留，可作为isa优化。（内存管理方案中no_isapoint对isa做优化）

size_t fastInstanceSize(size_t extra) const
    {
        ASSERT(hasFastInstanceSize(extra));

        if (__builtin_constant_p(extra) && extra == 0) {
            return _flags & FAST_CACHE_ALLOC_MASK16;
        } else {
            size_t size = _flags & FAST_CACHE_ALLOC_MASK;
            // remove the FAST_CACHE_ALLOC_DELTA16 that was added
            // by setFastInstanceSize
            return align16(size + extra - FAST_CACHE_ALLOC_DELTA16);
        }
    }
    
//对齐    
static inline size_t align16(size_t x) {
    return (x + size_t(15)) & ~size_t(15);
}

uint32_t unalignedInstanceSize() const {
        ASSERT(isRealized());
        return data()->ro()->instanceSize;
    }

ro()->instanceSize从内存取出干净内存，占8个字节
对象中属性对齐是会根据8字节原则，成员变量越多，对象占用空间越大。

因此总结如下：
对象中属性对齐方式和对象申请大小对齐不一样。这个结论需要了解源码，认真推敲看看，不要盲目背。