蘋果iOS系統源碼思考：對象的引用計數存儲在哪裏？--從runtime源碼獲得的啓示

時間 2019-11-06

標籤蘋果 ios 系統源碼思考對象引用計數存儲哪裏 runtime 獲得啓示欄目 iOS 简体版

原文原文鏈接

引言：這篇文章旨在從runtime源碼中分析出 引用計數 值自己的保存位置，適合對底層原理有興趣的朋友，或者面試造火箭的同窗（好比百度的面試官很是喜歡問底層原理：好，我知道你說了深淺複製的區別一大堆，若是我讓你本身實現一個copy，你能實現嗎？若是我讓你實現引用計數的功能，你有思路嗎？）。於是本文並 不適用於 專一業務層快速開發的同窗，由於這裏將貼有大量的源碼。沒有耐心的同窗能夠先收藏暫時迴避一下，往後造火箭造飛機的時候再來。html

核心問題

iOS開發者都知道OC裏面的內存管理是經過對象的引用計數來管理的，或手動MRC，或自動ARC，有些操做可讓引用計數加1，有些能夠減1，一旦一個對象的引用計數爲0，就回收內存了。c++

但是，你僅僅知道這裏就好了嗎？期望你能造火箭造飛機的面試官可不這麼想了，好比問你一句，一個對象的 引用計數自己 保存在哪裏？？不關注底層的面試者，這時候可能會懵逼。git

研究方式

這篇文章不一樣於其它文章經過 clang編譯 一個類文件以查看它的實現原理（筆者曾用clang編譯分析Block的原理，傳送門），而是直接經過下載runtime的源碼來查看分析。github

依據版本

蘋果開源了runtime的代碼，查看的方式既能夠經過在線網頁版預覽，也能夠下載歸檔文件到本地查看。本篇文件討論的版本是 objc4-723。面試

1. 類與對象
- 1.1 對象 -- Object
- 1.2 對象 -- NSObject
- 1.3 對象 -- objc_object
- 1.4 類 -- objc_class
- 1.5 NSObject，objc_object，objc_class 三者的關係
1. 手動引用對引用計數的影響 -- retain操做
- 2.1 兩種對象：NSObject與Object的引用增長
- 2.2 歸根結底 -- NSObject對象的rootRetain()
1. isa與Tagged Pointer
- 3.1 NSObject的惟一成員變量 -- isa
- 3.2 isa_t聯合體裏面的數據含義
- 3.3 isa_t聯合體裏面的宏
- 3.4 是否Tagged Pointer的判斷
- 3.5 與isa類型有關的宏
- 3.6 怎麼判斷是否支持優化的isa指針？-- 看設備、本身設置。
- 3.7 怎麼判斷是否Tagged Pointer的對象？-- 看對象、本身設置
- 3.8 引用計數的存儲形式 -- 散列表
1. 散列表
4.1 增長引用計數 -- sidetable_retain()
4.2 增長引用計數 -- sidetable_tryRetain()
4.3 獲取散列表 -- SideTable()
1. 設置變量致使的引用計數變化 -- objc_retain操做
- 5.1 狀況1
- 5.2 狀況2
- 5.3 objc_storeStrong致使的retain
1. 新建對象（分配內存與初始化）致使的引用計數變化 -- alloc 和 init 操做
- 6.1 分配內存 -- alloc
- 6.2 初始化 -- init
1. 獲取引用計數
1. 結論
1. 拓展閱讀

1. 類與對象

下載完工程，打開查看緩存

module.modulemap 頭文件描述文件bash

module ObjectiveC [system] [extern_c] {
  umbrella "."
  export *
  module * { 
    export *
  }

  module NSObject {
    requires objc
    header "NSObject.h"
    export *
  }

#if defined(BUILD_FOR_OSX)
  module List {
    // Uses @defs, which does not work in ObjC++ or non-ARC.
    requires objc, !objc_arc, !cplusplus
    header "List.h"
    export *
  }

  module Object {
    requires objc
    header "Object.h"
    export *
  }

  module Protocol {
    requires objc
    header "Protocol.h"
    export *
  }
#endif

#if !defined(BUILD_FOR_OSX)
  // These file are not available outside macOS.
  exclude header "hashtable.h"
  exclude header "hashtable2.h"
#endif
}
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這裏的Module本質上是一個描述文件，用來描述Module中包涵的內容，每一個Module中必須包涵一個umbrella頭文件，這個文件用來#import全部這個Module下的文件，好比#import <UIKit/UIKit.h>這個UIKit.h就是一個umbrella文件。關於Module更多參考這篇文章。數據結構

從#if defined(BUILD_FOR_OSX)這句邏輯判斷可知， Object是針對macOS的，iOS開發暫時只關心NSObject便可。架構

1.1 對象 -- Object

Object.mm Objectapp

#include "objc-private.h"

#undef id
#undef Class

typedef struct objc_class *Class;
typedef struct objc_object *id;

#if __OBJC2__

__OSX_AVAILABLE(10.0) 
__IOS_UNAVAILABLE __TVOS_UNAVAILABLE
__WATCHOS_UNAVAILABLE __BRIDGEOS_UNAVAILABLE
OBJC_ROOT_CLASS
@interface Object { 
    Class isa; 
} 
@end

@implementation Object

+ (id)initialize
{
    return self; 
}

+ (id)class
{
    return self;
}

-(id) retain
{
    return _objc_rootRetain(self);
}

-(void) release
{
    _objc_rootRelease(self);
}

-(id) autorelease
{
    return _objc_rootAutorelease(self);
}

+(id) retain
{
    return self;
}

+(void) release
{
}

+(id) autorelease
{
    return self;
}


@end
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1.2 對象 -- NSObject

NSObject.h NSObject

#ifndef _OBJC_NSOBJECT_H_
#define _OBJC_NSOBJECT_H_

#if __OBJC__

#include <objc/objc.h>
#include <objc/NSObjCRuntime.h>

@class NSString, NSMethodSignature, NSInvocation;

@protocol NSObject

- (BOOL)isEqual:(id)object;
@property (readonly) NSUInteger hash;

@property (readonly) Class superclass;
- (Class)class OBJC_SWIFT_UNAVAILABLE("use 'type(of: anObject)' instead");
- (instancetype)self;

- (id)performSelector:(SEL)aSelector;
- (id)performSelector:(SEL)aSelector withObject:(id)object;
- (id)performSelector:(SEL)aSelector withObject:(id)object1 withObject:(id)object2;

- (BOOL)isProxy;

- (BOOL)isKindOfClass:(Class)aClass;
- (BOOL)isMemberOfClass:(Class)aClass;
- (BOOL)conformsToProtocol:(Protocol *)aProtocol;

- (BOOL)respondsToSelector:(SEL)aSelector;

- (instancetype)retain OBJC_ARC_UNAVAILABLE;
- (oneway void)release OBJC_ARC_UNAVAILABLE;
- (instancetype)autorelease OBJC_ARC_UNAVAILABLE;
- (NSUInteger)retainCount OBJC_ARC_UNAVAILABLE;

- (struct _NSZone *)zone OBJC_ARC_UNAVAILABLE;

@property (readonly, copy) NSString *description;
@optional
@property (readonly, copy) NSString *debugDescription;

@end


OBJC_AVAILABLE(10.0, 2.0, 9.0, 1.0, 2.0)
OBJC_ROOT_CLASS
OBJC_EXPORT
@interface NSObject <NSObject> {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wobjc-interface-ivars"
    Class isa  OBJC_ISA_AVAILABILITY;
#pragma clang diagnostic pop
}

+ (void)load;

+ (void)initialize;
- (instancetype)init
#if NS_ENFORCE_NSOBJECT_DESIGNATED_INITIALIZER
    NS_DESIGNATED_INITIALIZER
#endif
    ;

+ (instancetype)new OBJC_SWIFT_UNAVAILABLE("use object initializers instead");
+ (instancetype)allocWithZone:(struct _NSZone *)zone OBJC_SWIFT_UNAVAILABLE("use object initializers instead");
+ (instancetype)alloc OBJC_SWIFT_UNAVAILABLE("use object initializers instead");
- (void)dealloc OBJC_SWIFT_UNAVAILABLE("use 'deinit' to define a de-initializer");

- (void)finalize OBJC_DEPRECATED("Objective-C garbage collection is no longer supported");

- (id)copy;
- (id)mutableCopy;

+ (id)copyWithZone:(struct _NSZone *)zone OBJC_ARC_UNAVAILABLE;
+ (id)mutableCopyWithZone:(struct _NSZone *)zone OBJC_ARC_UNAVAILABLE;

+ (BOOL)instancesRespondToSelector:(SEL)aSelector;
+ (BOOL)conformsToProtocol:(Protocol *)protocol;
- (IMP)methodForSelector:(SEL)aSelector;
+ (IMP)instanceMethodForSelector:(SEL)aSelector;
- (void)doesNotRecognizeSelector:(SEL)aSelector;

- (id)forwardingTargetForSelector:(SEL)aSelector OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0);
- (void)forwardInvocation:(NSInvocation *)anInvocation OBJC_SWIFT_UNAVAILABLE("");
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector OBJC_SWIFT_UNAVAILABLE("");

+ (NSMethodSignature *)instanceMethodSignatureForSelector:(SEL)aSelector OBJC_SWIFT_UNAVAILABLE("");

- (BOOL)allowsWeakReference UNAVAILABLE_ATTRIBUTE;
- (BOOL)retainWeakReference UNAVAILABLE_ATTRIBUTE;

+ (BOOL)isSubclassOfClass:(Class)aClass;

+ (BOOL)resolveClassMethod:(SEL)sel OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0);
+ (BOOL)resolveInstanceMethod:(SEL)sel OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0);

+ (NSUInteger)hash;
+ (Class)superclass;
+ (Class)class OBJC_SWIFT_UNAVAILABLE("use 'aClass.self' instead");
+ (NSString *)description;
+ (NSString *)debugDescription;

@end

#endif

#endif
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1.3 對象 -- objc_object

關鍵信息：

isa: isa_t類型的指針，詳情可見下面3.2節。簡單的說，它是這樣的一個聯合體，包含了bits （是一個 uintptr_t 類型的值，做爲isa初始化列表中必初始化的值，能夠用來獲取isa結構體）和 cls （該變量會指向對象所屬的類的結構，在 64 位設備上會佔用 8byte）。

objc-private.h objc_object

struct objc_object {
private:
    isa_t isa;

public:

    // ISA() assumes this is NOT a tagged pointer object
    Class ISA();

    // getIsa() allows this to be a tagged pointer object
    Class getIsa();

    // initIsa() should be used to init the isa of new objects only.
    // If this object already has an isa, use changeIsa() for correctness.
    // initInstanceIsa(): objects with no custom RR/AWZ
    // initClassIsa(): class objects
    // initProtocolIsa(): protocol objects
    // initIsa(): other objects
    void initIsa(Class cls /*nonpointer=false*/);
    void initClassIsa(Class cls /*nonpointer=maybe*/);
    void initProtocolIsa(Class cls /*nonpointer=maybe*/);
    void initInstanceIsa(Class cls, bool hasCxxDtor);

    // changeIsa() should be used to change the isa of existing objects.
    // If this is a new object, use initIsa() for performance.
    Class changeIsa(Class newCls);

    bool hasNonpointerIsa();
    bool isTaggedPointer();
    bool isBasicTaggedPointer();
    bool isExtTaggedPointer();
    bool isClass();

    // object may have associated objects?
    bool hasAssociatedObjects();
    void setHasAssociatedObjects();

    // object may be weakly referenced?
    bool isWeaklyReferenced();
    void setWeaklyReferenced_nolock();

    // object may have -.cxx_destruct implementation?
    bool hasCxxDtor();

    // Optimized calls to retain/release methods
    id retain();
    void release();
    id autorelease();

    // Implementations of retain/release methods
    id rootRetain();
    bool rootRelease();
    id rootAutorelease();
    bool rootTryRetain();
    bool rootReleaseShouldDealloc();
    uintptr_t rootRetainCount();

    // Implementation of dealloc methods
    bool rootIsDeallocating();
    void clearDeallocating();
    void rootDealloc();

private:
    void initIsa(Class newCls, bool nonpointer, bool hasCxxDtor);

    // Slow paths for inline control
    id rootAutorelease2();
    bool overrelease_error();

#if SUPPORT_NONPOINTER_ISA
    // Unified retain count manipulation for nonpointer isa
    id rootRetain(bool tryRetain, bool handleOverflow);
    bool rootRelease(bool performDealloc, bool handleUnderflow);
    id rootRetain_overflow(bool tryRetain);
    bool rootRelease_underflow(bool performDealloc);

    void clearDeallocating_slow();

    // Side table retain count overflow for nonpointer isa
    void sidetable_lock();
    void sidetable_unlock();

    void sidetable_moveExtraRC_nolock(size_t extra_rc, bool isDeallocating, bool weaklyReferenced);
    bool sidetable_addExtraRC_nolock(size_t delta_rc);
    size_t sidetable_subExtraRC_nolock(size_t delta_rc);
    size_t sidetable_getExtraRC_nolock();
#endif

    // Side-table-only retain count
    bool sidetable_isDeallocating();
    void sidetable_clearDeallocating();

    bool sidetable_isWeaklyReferenced();
    void sidetable_setWeaklyReferenced_nolock();

    id sidetable_retain();
    id sidetable_retain_slow(SideTable& table);

    uintptr_t sidetable_release(bool performDealloc = true);
    uintptr_t sidetable_release_slow(SideTable& table, bool performDealloc = true);

    bool sidetable_tryRetain();

    uintptr_t sidetable_retainCount();
#if DEBUG
    bool sidetable_present();
#endif
};
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1.4 類 -- objc_class

關鍵信息：

isa: 繼承於objc_object
superclass: 指向本身父類的指針
cache: 方法緩存
bits: 它是一個class_data_bits_t類型的指針。做爲本類的實例方法鏈表。

注意區別：

這裏的bits是class_data_bits_t類型的，上一節objc_object的isa_t類型數據中也有一個uintptr_t類型的bits，可是這是兩種結構。

因而可知，objc_class 繼承於 objc_object，因此也是包含一個isa的類。在OC裏，不僅是對象的實例包含一個isa，這個對象的類自己也有這麼一個isa，類自己也是一個對象。

objc-runtime-new.h objc_class

struct objc_class : objc_object {
    // Class ISA;
    Class superclass;
    cache_t cache;             // formerly cache pointer and vtable
    class_data_bits_t bits;    // class_rw_t * plus custom rr/alloc flags

    class_rw_t *data() { 
        return bits.data();
    }
    void setData(class_rw_t *newData) {
        bits.setData(newData);
    }

    void setInfo(uint32_t set) {
        assert(isFuture()  ||  isRealized());
        data()->setFlags(set);
    }

    void clearInfo(uint32_t clear) {
        assert(isFuture()  ||  isRealized());
        data()->clearFlags(clear);
    }

    // set and clear must not overlap
    void changeInfo(uint32_t set, uint32_t clear) {
        assert(isFuture()  ||  isRealized());
        assert((set & clear) == 0);
        data()->changeFlags(set, clear);
    }

    bool hasCustomRR() {
        return ! bits.hasDefaultRR();
    }
    void setHasDefaultRR() {
        assert(isInitializing());
        bits.setHasDefaultRR();
    }
    void setHasCustomRR(bool inherited = false);
    void printCustomRR(bool inherited);

    bool hasCustomAWZ() {
        return ! bits.hasDefaultAWZ();
    }
    void setHasDefaultAWZ() {
        assert(isInitializing());
        bits.setHasDefaultAWZ();
    }
    void setHasCustomAWZ(bool inherited = false);
    void printCustomAWZ(bool inherited);

    bool instancesRequireRawIsa() {
        return bits.instancesRequireRawIsa();
    }
    void setInstancesRequireRawIsa(bool inherited = false);
    void printInstancesRequireRawIsa(bool inherited);

    bool canAllocNonpointer() {
        assert(!isFuture());
        return !instancesRequireRawIsa();
    }
    bool canAllocFast() {
        assert(!isFuture());
        return bits.canAllocFast();
    }


    bool hasCxxCtor() {
        // addSubclass() propagates this flag from the superclass.
        assert(isRealized());
        return bits.hasCxxCtor();
    }
    void setHasCxxCtor() { 
        bits.setHasCxxCtor();
    }

    bool hasCxxDtor() {
        // addSubclass() propagates this flag from the superclass.
        assert(isRealized());
        return bits.hasCxxDtor();
    }
    void setHasCxxDtor() { 
        bits.setHasCxxDtor();
    }


    bool isSwift() {
        return bits.isSwift();
    }


    // Return YES if the class's ivars are managed by ARC, // or the class is MRC but has ARC-style weak ivars. bool hasAutomaticIvars() { return data()->ro->flags & (RO_IS_ARC | RO_HAS_WEAK_WITHOUT_ARC); } // Return YES if the class's ivars are managed by ARC.
    bool isARC() {
        return data()->ro->flags & RO_IS_ARC;
    }


#if SUPPORT_NONPOINTER_ISA
    // Tracked in non-pointer isas; not tracked otherwise
#else
    bool instancesHaveAssociatedObjects() {
        // this may be an unrealized future class in the CF-bridged case
        assert(isFuture()  ||  isRealized());
        return data()->flags & RW_INSTANCES_HAVE_ASSOCIATED_OBJECTS;
    }

    void setInstancesHaveAssociatedObjects() {
        // this may be an unrealized future class in the CF-bridged case
        assert(isFuture()  ||  isRealized());
        setInfo(RW_INSTANCES_HAVE_ASSOCIATED_OBJECTS);
    }
#endif

    bool shouldGrowCache() {
        return true;
    }

    void setShouldGrowCache(bool) {
        // fixme good or bad for memory use?
    }

    bool isInitializing() {
        return getMeta()->data()->flags & RW_INITIALIZING;
    }

    void setInitializing() {
        assert(!isMetaClass());
        ISA()->setInfo(RW_INITIALIZING);
    }

    bool isInitialized() {
        return getMeta()->data()->flags & RW_INITIALIZED;
    }

    void setInitialized();

    bool isLoadable() {
        assert(isRealized());
        return true;  // any class registered for +load is definitely loadable
    }

    IMP getLoadMethod();

    // Locking: To prevent concurrent realization, hold runtimeLock.
    bool isRealized() {
        return data()->flags & RW_REALIZED;
    }

    // Returns true if this is an unrealized future class.
    // Locking: To prevent concurrent realization, hold runtimeLock.
    bool isFuture() { 
        return data()->flags & RW_FUTURE;
    }

    bool isMetaClass() {
        assert(this);
        assert(isRealized());
        return data()->ro->flags & RO_META;
    }

    // NOT identical to this->ISA when this is a metaclass
    Class getMeta() {
        if (isMetaClass()) return (Class)this;
        else return this->ISA();
    }

    bool isRootClass() {
        return superclass == nil;
    }
    bool isRootMetaclass() {
        return ISA() == (Class)this;
    }

    const char *mangledName() { 
        // fixme can't assert locks here assert(this); if (isRealized() || isFuture()) { return data()->ro->name; } else { return ((const class_ro_t *)data())->name; } } const char *demangledName(bool realize = false); const char *nameForLogging(); // May be unaligned depending on class's ivars.
    uint32_t unalignedInstanceStart() {
        assert(isRealized());
        return data()->ro->instanceStart;
    }

    // Class's instance start rounded up to a pointer-size boundary. // This is used for ARC layout bitmaps. uint32_t alignedInstanceStart() { return word_align(unalignedInstanceStart()); } // 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()); } 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; } void setInstanceSize(uint32_t newSize) { assert(isRealized()); if (newSize != data()->ro->instanceSize) { assert(data()->flags & RW_COPIED_RO); *const_cast<uint32_t *>(&data()->ro->instanceSize) = newSize; } bits.setFastInstanceSize(newSize); } void chooseClassArrayIndex(); void setClassArrayIndex(unsigned Idx) { bits.setClassArrayIndex(Idx); } unsigned classArrayIndex() { return bits.classArrayIndex(); } }; 複製代碼

1.5 NSObject，objc_object，objc_class 三者的關係

1）NSObject與objc_class

NSObject有一個Class類型，名爲isa成員變量

繼續查看Class的本質，能夠發現Class 其實就是 C 語言定義的結構體類型(struct objc_class)的指針，這個聲明說明 Objective-C 的類實際上就是 struct objc_class。

另外，第二個定義是常常遇到的 id 類型，這裏能夠看出 id 類型是 C 語言定義的結構體類型(struct objc_object)的指針，咱們知道咱們能夠用 id 來聲明一個對象，因此這也說明了 Objective-C 的對象實際上就是 struct objc_object。

2）objc_object與objc_class

繼續查看objc_class的本質，能夠發現objc_class是一個繼承自objc_object的結構體。因此 Objective-C 中的類自身也是一個對象，只是除了 objc_object 中定義的成員變量外，還有另外三個成員變量：superclass、cache 和 bits。

注意，這裏面的 「結構體」 並不是 C語言 裏面的結構體，而是 C++語言 裏面的結構體，並且這個概念僅限字面意思的結構體。嚴格來說，其實struct關鍵字定義的是類，跟class關鍵字定義的類除了默認訪問權限的區別，沒有區別。這一點，國內人寫的C++書籍卻不多有注意到。下面是比較權威的《C++ Primer》（第546頁）一書關於這點的說明。

3）知識補課

C++中的struct對C中的struct進行了擴充，它已經再也不只是一個包含不一樣數據類型的數據結構了，它已經獲取了太多的功能。下面簡單列一下C++的struct跟C中的struct不同的地方：

struct能包含成員函數
struct能繼承
struct能實現多態

2. 手動引用對引用計數的影響 -- retain操做

2.1 兩種對象：NSObject與Object的引用增長

① NSObject的retain

NSObject.mm retain

+ (id)retain {
    return (id)self;
}

// Replaced by ObjectAlloc
- (id)retain {
    return ((id)self)->rootRetain();
}
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② Object的retain

Object.mm retain

+(id) retain
{
    return self;
}

-(id) retain
{
    return _objc_rootRetain(self);
}
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NSObject.mm _objc_rootRetain(id obj)

id
_objc_rootRetain(id obj)
{
    assert(obj);

    return obj->rootRetain();
}
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可見，不管是NSObject仍是Object的 retain，歸根結底，調用的都是 objc_object 的 rootRetain()。

2.2 歸根結底 -- NSObject對象的rootRetain()

objc4/objc4-723/runtime/objc-object.h objc_object::rootRetain()

ALWAYS_INLINE id 
objc_object::rootRetain()
{
    return rootRetain(false, false);
}
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objc4/objc4-723/runtime/objc-object.h objc_object::rootRetain(bool tryRetain, bool handleOverflow)

ALWAYS_INLINE id 
objc_object::rootRetain(bool tryRetain, bool handleOverflow)
{
    if (isTaggedPointer()) return (id)this;

    bool sideTableLocked = false;
    bool transcribeToSideTable = false;

    isa_t oldisa;
    isa_t newisa;

    do {
        transcribeToSideTable = false;
        oldisa = LoadExclusive(&isa.bits);
        newisa = oldisa;
        if (slowpath(!newisa.nonpointer)) {
            ClearExclusive(&isa.bits);
            if (!tryRetain && sideTableLocked) sidetable_unlock();
            if (tryRetain) return sidetable_tryRetain() ? (id)this : nil;
            else return sidetable_retain();
        }
        // don't check newisa.fast_rr; we already called any RR overrides if (slowpath(tryRetain && newisa.deallocating)) { ClearExclusive(&isa.bits); if (!tryRetain && sideTableLocked) sidetable_unlock(); return nil; } uintptr_t carry; newisa.bits = addc(newisa.bits, RC_ONE, 0, &carry); // extra_rc++ if (slowpath(carry)) { // newisa.extra_rc++ overflowed if (!handleOverflow) { ClearExclusive(&isa.bits); return rootRetain_overflow(tryRetain); } // Leave half of the retain counts inline and // prepare to copy the other half to the side table. if (!tryRetain && !sideTableLocked) sidetable_lock(); sideTableLocked = true; transcribeToSideTable = true; newisa.extra_rc = RC_HALF; newisa.has_sidetable_rc = true; } } while (slowpath(!StoreExclusive(&isa.bits, oldisa.bits, newisa.bits))); if (slowpath(transcribeToSideTable)) { // Copy the other half of the retain counts to the side table. sidetable_addExtraRC_nolock(RC_HALF); } if (slowpath(!tryRetain && sideTableLocked)) sidetable_unlock(); return (id)this; } 複製代碼

其中，手動retain對引用計數的影響關鍵在這麼一句話：

newisa.bits = addc(newisa.bits, RC_ONE, 0, &carry);  // extra_rc++
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對isa的 extra_rc 變量進行+1，前面說到isa會存不少東西。

3. isa與Tagged Pointer

3.1 NSObject的惟一成員變量 -- isa

NSObject.h NSObject的isa

OBJC_AVAILABLE(10.0, 2.0, 9.0, 1.0, 2.0)
OBJC_ROOT_CLASS
OBJC_EXPORT
@interface NSObject <NSObject> {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wobjc-interface-ivars"
    Class isa  OBJC_ISA_AVAILABILITY;
#pragma clang diagnostic pop
}
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其中，Class isa繼續查看Class的定義：

objc-private.h Class

typedef struct objc_class *Class;
typedef struct objc_object *id;
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其中，objc_object類內部結構：

其中，私有的成員數據isa爲isa_t類型的聯合體：

objc-private.h isa_t

union isa_t 
{
    isa_t() { }
    isa_t(uintptr_t value) : bits(value) { }

    Class cls;
    uintptr_t bits;

#if SUPPORT_PACKED_ISA

    // extra_rc must be the MSB-most field (so it matches carry/overflow flags)
    // nonpointer must be the LSB (fixme or get rid of it)
    // shiftcls must occupy the same bits that a real class pointer would
    // bits + RC_ONE is equivalent to extra_rc + 1
    // RC_HALF is the high bit of extra_rc (i.e. half of its range)

    // future expansion:
    // uintptr_t fast_rr : 1;     // no r/r overrides
    // uintptr_t lock : 2;        // lock for atomic property, @synch
    // uintptr_t extraBytes : 1;  // allocated with extra bytes

# if __arm64__
# define ISA_MASK 0x0000000ffffffff8ULL
# define ISA_MAGIC_MASK 0x000003f000000001ULL
# define ISA_MAGIC_VALUE 0x000001a000000001ULL
    struct {
        uintptr_t nonpointer        : 1;
        uintptr_t has_assoc         : 1;
        uintptr_t has_cxx_dtor      : 1;
        uintptr_t shiftcls          : 33; // MACH_VM_MAX_ADDRESS 0x1000000000
        uintptr_t magic             : 6;
        uintptr_t weakly_referenced : 1;
        uintptr_t deallocating      : 1;
        uintptr_t has_sidetable_rc  : 1;
        uintptr_t extra_rc          : 19;
# define RC_ONE (1ULL<<45)
# define RC_HALF (1ULL<<18)
    };

# elif __x86_64__
# define ISA_MASK 0x00007ffffffffff8ULL
# define ISA_MAGIC_MASK 0x001f800000000001ULL
# define ISA_MAGIC_VALUE 0x001d800000000001ULL
    struct {
        uintptr_t nonpointer        : 1;
        uintptr_t has_assoc         : 1;
        uintptr_t has_cxx_dtor      : 1;
        uintptr_t shiftcls          : 44; // MACH_VM_MAX_ADDRESS 0x7fffffe00000
        uintptr_t magic             : 6;
        uintptr_t weakly_referenced : 1;
        uintptr_t deallocating      : 1;
        uintptr_t has_sidetable_rc  : 1;
        uintptr_t extra_rc          : 8;
# define RC_ONE (1ULL<<56)
# define RC_HALF (1ULL<<7)
    };

# else
# error unknown architecture for packed isa
# endif

// SUPPORT_PACKED_ISA
#endif


#if SUPPORT_INDEXED_ISA

# if __ARM_ARCH_7K__ >= 2

# define ISA_INDEX_IS_NPI 1
# define ISA_INDEX_MASK 0x0001FFFC
# define ISA_INDEX_SHIFT 2
# define ISA_INDEX_BITS 15
# define ISA_INDEX_COUNT (1 << ISA_INDEX_BITS)
# define ISA_INDEX_MAGIC_MASK 0x001E0001
# define ISA_INDEX_MAGIC_VALUE 0x001C0001
    struct {
        uintptr_t nonpointer        : 1;
        uintptr_t has_assoc         : 1;
        uintptr_t indexcls          : 15;
        uintptr_t magic             : 4;
        uintptr_t has_cxx_dtor      : 1;
        uintptr_t weakly_referenced : 1;
        uintptr_t deallocating      : 1;
        uintptr_t has_sidetable_rc  : 1;
        uintptr_t extra_rc          : 7;
# define RC_ONE (1ULL<<25)
# define RC_HALF (1ULL<<6)
    };

# else
# error unknown architecture for indexed isa
# endif

// SUPPORT_INDEXED_ISA
#endif

};
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其中，cls 變量會指向對象所屬的類的結構，在 64 位設備上會佔用 8byte。

另外，bits 變量保存着isa的惟一標誌（能夠根據bits獲取isa），是一個類型爲 uintptr_t 的數據， uintptr_t的定義：

typedef unsigned long		uintptr_t;
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知識回顧

不熟悉C++的朋友可能很難看出來bits會是如何初始化的，其實，這是一種與構造函數並列的初始化辦法 -- 初始化列表。關於初始化列表的定義，截取百度百科的一段話：

因此，再回過來看bits，bits以isa_t(uintptr_t value)中的value爲初始化的值:

例如isa初始化的API objc_object::initIsa(Class cls)`中，有這樣一句：

isa_t newisa(0);
newisa.bits = ISA_INDEX_MAGIC_VALUE;
//...
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而這個bits值能夠用來獲取isa（注意區分左右兩邊的bits分別是兩個東西）:

isa_t bits = LoadExclusive(&isa.bits);
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其中，LoadExclusive根據平臺的不一樣，實現體並不同，這是__arm64__平臺的實現體：

#if __arm64__

static ALWAYS_INLINE
uintptr_t 
LoadExclusive(uintptr_t *src)
{
    uintptr_t result;
    asm("ldxr %x0, [%x1]" 
        : "=r" (result) 
        : "r" (src), "m" (*src));
    return result;
}
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對這個isa (這裏是左邊的bits，它是個isa，而非右邊的uintptr_t) 的調用，好比獲取引用計數的源代碼中就有幾處：

inline uintptr_t 
objc_object::rootRetainCount()
{
    if (isTaggedPointer()) return (uintptr_t)this;

    sidetable_lock();
    isa_t bits = LoadExclusive(&isa.bits);
    ClearExclusive(&isa.bits);
    if (bits.nonpointer) {
        uintptr_t rc = 1 + bits.extra_rc;
        if (bits.has_sidetable_rc) {
            rc += sidetable_getExtraRC_nolock();
        }
        sidetable_unlock();
        return rc;
    }

    sidetable_unlock();
    return sidetable_retainCount();
}
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調用的有： bits.extra_rc bits.nonpointer bits.has_sidetable_rc

3.2 isa_t聯合體裏面struct的數據含義

`nonpointer`

該變量佔用 1bit 內存空間，能夠有兩個值：0 和 1，分別表明不一樣的 isa_t 的類型：

0 表示 isa_t 沒有開啓指針優化，不使用 isa_t 中定義的結構體。訪問 objc_object 的 isa 會直接返回 isa_t 結構中的 cls 變量，cls 變量會指向對象所屬的類的結構，在 64 位設備上會佔用 8byte。
1 表示 isa_t 開啓了指針優化，不能直接訪問 objc_object 的 isa 成員變量 (由於 isa 已經不是一個合法的內存指針了，而是一個 Tagged Pointer )，從其名字 nonpointer 也可獲知這個 isa 已經不是一個指針了。可是 isa 中包含了類信息、對象的引用計數等信息，在 64 位設備上充分利用了內存空間。

`shiftcls`

存儲類指針的值。開啓指針優化的狀況下，在 arm64 架構中有 33 位用來存儲類指針。

`has_assoc`

該變量與對象的關聯引用有關，當對象有關聯引用時，釋放對象時須要作額外的邏輯。關聯引用就是咱們一般用 objc_setAssociatedObject 方法設置給對象的，這裏對於關聯引用不作過多分析，若是後續有時間寫關聯引用實現時再深刻分析關聯引用有關的代碼。

`has_cxx_dtor`

表示該對象是否有 C++ 或者 Objc 的析構器，若是有析構函數，則須要作析構邏輯，若是沒有，則能夠更快的釋放對象。

`magic`

用於判斷對象是否已經完成了初始化，在 arm64 中 0x16 是調試器判斷當前對象是真的對象仍是沒有初始化的空間(在 x86_64 中該值爲 0x3b)。

`weakly_referenced`

標誌對象是否被指向或者曾經指向一個 ARC 的弱變量，沒有弱引用的對象能夠更快釋放。

`deallocating`

標誌對象是否正在釋放內存。

`extra_rc`

表示該對象的引用計數值，其實是引用計數值減 1，例如，若是對象的引用計數爲 10，那麼 extra_rc 爲 9。若是引用計數大於 10，則須要使用到下面的 has_sidetable_rc。

`has_sidetable_rc`

當對象引用計數大於 10 時，則has_sidetable_rc 的值爲 1，那麼引用計數會存儲在一個叫 SideTable 的類的屬性中，這是一個散列表。

`ISA_MAGIC_MASK`

經過掩碼方式獲取 magic 值。

`ISA_MASK`

經過掩碼方式獲取 isa 的類指針值。

`RC_ONE` 和 `RC_HALF`

用於引用計數的相關計算。

3.3 isa_t聯合體裏面的宏

SUPPORT_PACKED_ISA

表示平臺是否支持在 isa 指針中插入除 Class 以外的信息。

若是支持就會將 Class 信息放入 isa_t 定義的 struct 內，並附上一些其餘信息，例如上面的 nonpointer 等等；
若是不支持，那麼不會使用 isa_t 內定義的 struct，這時 isa_t 只使用 cls(Class 指針)。

小結：在 iOS 以及 MacOSX 設備上，SUPPORT_PACKED_ISA 定義爲 1。

__arm64__、__x86_64__

表示 CPU 架構，例如電腦通常是 __x86_64__ 架構，手機通常是 arm 結構，這裏 64 表明是 64 位 CPU。上面只列出了 __arm64__ 架構的定義。

小結：iOS 設備上 __arm64__ 是 1。

SUPPORT_INDEXED_ISA

SUPPORT_INDEXED_ISA 表示 isa_t 中存放的 Class 信息是 Class 的地址，仍是一個索引(根據該索引可在類信息表中查找該類結構地址)。能夠看出，多了一個 uintptr_t indexcls : 15;。

小結：iOS 設備上 SUPPORT_INDEXED_ISA 是 0。

3.4 是否Tagged Pointer的判斷

objc-object.h objc_object::isTaggedPointer()

inline bool 
objc_object::isTaggedPointer() 
{
    return _objc_isTaggedPointer(this);
}
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objc-internal.h _objc_isTaggedPointer(const void * _Nullable ptr)

static inline bool 
_objc_isTaggedPointer(const void * _Nullable ptr) 
{
    return ((uintptr_t)ptr & _OBJC_TAG_MASK) == _OBJC_TAG_MASK;
}
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3.5 與isa類型有關的宏

SUPPORT_NONPOINTER_ISA

用於標記是否支持優化的 isa 指針，其字面含義意思是 isa 的內容再也不是類的指針了，而是包含了更多信息，好比引用計數，析構狀態，被其餘 weak 變量引用狀況。下面看看SUPPORT_NONPOINTER_ISA及其相關宏的定義：

objc-config.h SUPPORT_TAGGED_POINTERS

// Define SUPPORT_TAGGED_POINTERS=1 to enable tagged pointer objects
// Be sure to edit tagged pointer SPI in objc-internal.h as well.
#if !(__OBJC2__ && __LP64__)
# define SUPPORT_TAGGED_POINTERS 0
#else
# define SUPPORT_TAGGED_POINTERS 1
#endif

// Define SUPPORT_MSB_TAGGED_POINTERS to use the MSB 
// as the tagged pointer marker instead of the LSB.
// Be sure to edit tagged pointer SPI in objc-internal.h as well.
#if !SUPPORT_TAGGED_POINTERS || !TARGET_OS_IPHONE
# define SUPPORT_MSB_TAGGED_POINTERS 0
#else
# define SUPPORT_MSB_TAGGED_POINTERS 1
#endif

// Define SUPPORT_INDEXED_ISA=1 on platforms that store the class in the isa 
// field as an index into a class table.
// Note, keep this in sync with any .s files which also define it.
// Be sure to edit objc-abi.h as well.
#if __ARM_ARCH_7K__ >= 2
# define SUPPORT_INDEXED_ISA 1
#else
# define SUPPORT_INDEXED_ISA 0
#endif

// Define SUPPORT_PACKED_ISA=1 on platforms that store the class in the isa 
// field as a maskable pointer with other data around it.
#if (!__LP64__ || TARGET_OS_WIN32 || TARGET_OS_SIMULATOR)
# define SUPPORT_PACKED_ISA 0
#else
# define SUPPORT_PACKED_ISA 1
#endif

// Define SUPPORT_NONPOINTER_ISA=1 on any platform that may store something
// in the isa field that is not a raw pointer.
#if !SUPPORT_INDEXED_ISA && !SUPPORT_PACKED_ISA
# define SUPPORT_NONPOINTER_ISA 0
#else
# define SUPPORT_NONPOINTER_ISA 1
#endif
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3.6 怎麼判斷是否支持優化的isa指針？-- 看設備、本身設置。

已知iOS系統的SUPPORT_PACKED_ISA爲1，SUPPORT_INDEXED_ISA爲0，根據4.5節中源代碼的定義可知，iOS系統的SUPPORT_NONPOINTER_ISA爲1。
在環境變量中設置OBJC_DISABLE_NONPOINTER_ISA。

即，iOS系統 支持 優化的isa指針。

在 64 位環境下，優化的 isa 指針並非就必定會存儲引用計數，畢竟用 19bit （iOS 系統）保存引用計數不必定夠。須要注意的是這 19 位保存的是引用計數的值減一。

3.7 怎麼判斷是否Tagged Pointer的對象？-- 看對象、本身設置

能夠啓用Tagged Pointer的類對象有：NSDate、NSNumber、NSString。Tagged Pointer專門用來存儲小的對象。
在環境變量中設置OBJC_DISABLE_TAGGED_POINTERS=YES強制不啓用Tagged Pointer。

3.8 引用計數的存儲形式 -- 散列表

下面對sidetable_retain進行分析。

4. 散列表

4.1 增長引用計數 -- sidetable_retain()

第2節的增長引用假設，以及後面第8節的獲取引用計數會用到下面的API:

NSObject.mm objc_object::sidetable_retain()

id
objc_object::sidetable_retain()
{
#if SUPPORT_NONPOINTER_ISA
    assert(!isa.nonpointer);
#endif
    SideTable& table = SideTables()[this];
    
    table.lock();
    size_t& refcntStorage = table.refcnts[this];
    if (! (refcntStorage & SIDE_TABLE_RC_PINNED)) {
        refcntStorage += SIDE_TABLE_RC_ONE;
    }
    table.unlock();

    return (id)this;
}
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4.2 增長引用計數 -- sidetable_tryRetain()

NSObject.mm objc_object::sidetable_tryRetain()

bool
objc_object::sidetable_tryRetain()
{
#if SUPPORT_NONPOINTER_ISA
    assert(!isa.nonpointer);
#endif
    SideTable& table = SideTables()[this];

    // NO SPINLOCK HERE
    // _objc_rootTryRetain() is called exclusively by _objc_loadWeak(), 
    // which already acquired the lock on our behalf.

    // fixme can't do this efficiently with os_lock_handoff_s // if (table.slock == 0) { // _objc_fatal("Do not call -_tryRetain."); // } bool result = true; RefcountMap::iterator it = table.refcnts.find(this); if (it == table.refcnts.end()) { table.refcnts[this] = SIDE_TABLE_RC_ONE; } else if (it->second & SIDE_TABLE_DEALLOCATING) { result = false; } else if (! (it->second & SIDE_TABLE_RC_PINNED)) { it->second += SIDE_TABLE_RC_ONE; } return result; } 複製代碼

4.3 獲取散列表 -- SideTable()

NSObject.mm SideTable

struct SideTable {
    spinlock_t slock;
    RefcountMap refcnts;
    weak_table_t weak_table;

    SideTable() {
        memset(&weak_table, 0, sizeof(weak_table));
    }

    ~SideTable() {
        _objc_fatal("Do not delete SideTable.");
    }

    void lock() { slock.lock(); }
    void unlock() { slock.unlock(); }
    void forceReset() { slock.forceReset(); }

    // Address-ordered lock discipline for a pair of side tables.

    template<HaveOld, HaveNew>
    static void lockTwo(SideTable *lock1, SideTable *lock2);
    template<HaveOld, HaveNew>
    static void unlockTwo(SideTable *lock1, SideTable *lock2);
};
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其中，RefcountMap以及HaveOld，HaveNew的定義爲：

// RefcountMap disguises its pointers because we 
// don't want the table to act as a root for `leaks`. typedef objc::DenseMap<DisguisedPtr<objc_object>,size_t,true> RefcountMap; // Template parameters. enum HaveOld { DontHaveOld = false, DoHaveOld = true }; enum HaveNew { DontHaveNew = false, DoHaveNew = true }; 複製代碼

llvm-DenseMap.h DenseMap/DenseMapBase

DenseMapBase

DenseMap

5. 設置變量致使的引用計數變化 -- objc_retain操做

5.1 狀況1 -- strong

runtime.h 設置strong變量

/** 
 * Sets the value of an instance variable in an object.
 * 
 * @param obj The object containing the instance variable whose value you want to set.
 * @param ivar The Ivar describing the instance variable whose value you want to set.
 * @param value The new value for the instance variable.
 * 
 * @note Instance variables with known memory management (such as ARC strong and weak)
 *  use that memory management. Instance variables with unknown memory management 
 *  are assigned as if they were strong.
 * @note \c object_setIvar is faster than \c object_setInstanceVariable if the Ivar
 *  for the instance variable is already known.
 */
OBJC_EXPORT void
object_setIvarWithStrongDefault(id _Nullable obj, Ivar _Nonnull ivar,
                                id _Nullable value) 
    OBJC_AVAILABLE(10.12, 10.0, 10.0, 3.0, 2.0);
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objc-class.mm object_setIvarWithStrongDefault

void object_setIvarWithStrongDefault(id obj, Ivar ivar, id value)
{
    return _object_setIvar(obj, ivar, value, true /*strong default*/);
}
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objc-class.mm _object_setIvar

static ALWAYS_INLINE 
void _object_setIvar(id obj, Ivar ivar, id value, bool assumeStrong)
{
    if (!obj  ||  !ivar  ||  obj->isTaggedPointer()) return;

    ptrdiff_t offset;
    objc_ivar_memory_management_t memoryManagement;
    _class_lookUpIvar(obj->ISA(), ivar, offset, memoryManagement);

    if (memoryManagement == objc_ivar_memoryUnknown) {
        if (assumeStrong) memoryManagement = objc_ivar_memoryStrong;
        else memoryManagement = objc_ivar_memoryUnretained;
    }

    id *location = (id *)((char *)obj + offset);

    switch (memoryManagement) {
    case objc_ivar_memoryWeak:       objc_storeWeak(location, value); break;
    case objc_ivar_memoryStrong:     objc_storeStrong(location, value); break;
    case objc_ivar_memoryUnretained: *location = value; break;
    case objc_ivar_memoryUnknown:    _objc_fatal("impossible");
    }
}
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NSObject.mm objc_storeStrong

void
objc_storeStrong(id *location, id obj)
{
    id prev = *location;
    if (obj == prev) {
        return;
    }
    objc_retain(obj);
    *location = obj;
    objc_release(prev);
}
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5.2 狀況2 -- weak

objc-class.mm 設置weak變量

object_setIvar(id _Nullable obj, Ivar _Nonnull ivar, id _Nullable value) 
    OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0);
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objc-class.mm object_setIvar

void object_setIvar(id obj, Ivar ivar, id value)
{
    return _object_setIvar(obj, ivar, value, false /*not strong default*/);
}
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可見，這裏一樣調用了 _object_setIvar，代碼狀況1，是同一個API。其中，不一樣於objc_storeStrong，走的是objc_storeWeak，下面分析一下：

NSObject.mm objc_storeWeak

/** 
 * This function stores a new value into a __weak variable. It would
 * be used anywhere a __weak variable is the target of an assignment.
 * 
 * @param location The address of the weak pointer itself
 * @param newObj The new object this weak ptr should now point to
 * 
 * @return \e newObj
 */
id
objc_storeWeak(id *location, id newObj)
{
    return storeWeak<DoHaveOld, DoHaveNew, DoCrashIfDeallocating>
        (location, (objc_object *)newObj);
}
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上面有一個storeWeak<DoHaveOld, DoHaveNew, DoCrashIfDeallocating> (location, (objc_object *)newObj)，它的代碼有點長，核心的關鍵是更新了weak哈希表：->weak_table。讀者能夠從下面搜索一下這個關鍵詞的位置。

// Update a weak variable.
// If HaveOld is true, the variable has an existing value 
//   that needs to be cleaned up. This value might be nil.
// If HaveNew is true, there is a new value that needs to be 
//   assigned into the variable. This value might be nil.
// If CrashIfDeallocating is true, the process is halted if newObj is 
//   deallocating or newObj's class does not support weak references. // If CrashIfDeallocating is false, nil is stored instead. enum CrashIfDeallocating { DontCrashIfDeallocating = false, DoCrashIfDeallocating = true }; template <HaveOld haveOld, HaveNew haveNew, CrashIfDeallocating crashIfDeallocating> static id storeWeak(id *location, objc_object *newObj) { assert(haveOld || haveNew); if (!haveNew) assert(newObj == nil); Class previouslyInitializedClass = nil; id oldObj; SideTable *oldTable; SideTable *newTable; // Acquire locks for old and new values. // Order by lock address to prevent lock ordering problems. // Retry if the old value changes underneath us. retry: if (haveOld) { oldObj = *location; oldTable = &SideTables()[oldObj]; } else { oldTable = nil; } if (haveNew) { newTable = &SideTables()[newObj]; } else { newTable = nil; } SideTable::lockTwo<haveOld, haveNew>(oldTable, newTable); if (haveOld && *location != oldObj) { SideTable::unlockTwo<haveOld, haveNew>(oldTable, newTable); goto retry; } // Prevent a deadlock between the weak reference machinery // and the +initialize machinery by ensuring that no // weakly-referenced object has an un-+initialized isa. if (haveNew && newObj) { Class cls = newObj->getIsa(); if (cls != previouslyInitializedClass && !((objc_class *)cls)->isInitialized()) { SideTable::unlockTwo<haveOld, haveNew>(oldTable, newTable); _class_initialize(_class_getNonMetaClass(cls, (id)newObj)); // If this class is finished with +initialize then we're good.
            // If this class is still running +initialize on this thread 
            // (i.e. +initialize called storeWeak on an instance of itself)
            // then we may proceed but it will appear initializing and 
            // not yet initialized to the check above.
            // Instead set previouslyInitializedClass to recognize it on retry.
            previouslyInitializedClass = cls;

            goto retry;
        }
    }

    // Clean up old value, if any.
    if (haveOld) {
        weak_unregister_no_lock(&oldTable->weak_table, oldObj, location);
    }

    // Assign new value, if any.
    if (haveNew) {
        newObj = (objc_object *)
            weak_register_no_lock(&newTable->weak_table, (id)newObj, location, 
                                  crashIfDeallocating);
        // weak_register_no_lock returns nil if weak store should be rejected

        // Set is-weakly-referenced bit in refcount table.
        if (newObj  &&  !newObj->isTaggedPointer()) {
            newObj->setWeaklyReferenced_nolock();
        }

        // Do not set *location anywhere else. That would introduce a race.
        *location = (id)newObj;
    }
    else {
        // No new value. The storage is not changed.
    }
    
    SideTable::unlockTwo<haveOld, haveNew>(oldTable, newTable);

    return (id)newObj;
}
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5.3 objc_storeStrong致使的retain

上面第5.1節中有一個objc_storeStrong，這裏繼續分析它的原理。

NSObject.mm objc_storeStrong(id *location, id obj)

void
objc_storeStrong(id *location, id obj)
{
    id prev = *location;
    if (obj == prev) {
        return;
    }
    objc_retain(obj);
    *location = obj;
    objc_release(prev);
}
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NSObject.mm objc_retain(id obj)

/***********************************************************************
* Optimized retain/release/autorelease entrypoints
**********************************************************************/


#if __OBJC2__

__attribute__((aligned(16)))
id 
objc_retain(id obj)
{
    if (!obj) return obj;
    if (obj->isTaggedPointer()) return obj;
    return obj->retain();
}


__attribute__((aligned(16)))
void 
objc_release(id obj)
{
    if (!obj) return;
    if (obj->isTaggedPointer()) return;
    return obj->release();
}


__attribute__((aligned(16)))
id
objc_autorelease(id obj)
{
    if (!obj) return obj;
    if (obj->isTaggedPointer()) return obj;
    return obj->autorelease();
}


// OBJC2
#else
// not OBJC2


id objc_retain(id obj) { return [obj retain]; }
void objc_release(id obj) { [obj release]; }
id objc_autorelease(id obj) { return [obj autorelease]; }


#endif
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可知： 1）若是TaggedPointer，則返回自己。 2）若是非TaggedPointer，則由對象的retain()返回。

objc-object.h objc_object::retain()

// Equivalent to calling [this retain], with shortcuts if there is no override
inline id 
objc_object::retain()
{
    assert(!isTaggedPointer());

    if (fastpath(!ISA()->hasCustomRR())) {
        return rootRetain();
    }

    return ((id(*)(objc_object *, SEL))objc_msgSend)(this, SEL_retain);
}
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objc-object.h objc_object::rootRetain()

// Base retain implementation, ignoring overrides.
// This does not check isa.fast_rr; if there is an RR override then 
// it was already called and it chose to call [super retain].
//
// tryRetain=true is the -_tryRetain path.
// handleOverflow=false is the frameless fast path.
// handleOverflow=true is the framed slow path including overflow to side table
// The code is structured this way to prevent duplication.

ALWAYS_INLINE id 
objc_object::rootRetain()
{
    return rootRetain(false, false);
}
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這裏的rootRetain(false, false);正是上文第2.2節中介紹的，再也不贅述。

6. 新建對象（分配內存與初始化）致使的引用計數變化 -- alloc 和 init 操做

首先，新建一個對象的典型寫法：

NSObject *obj = [NSObject alloc] init];
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6.1 分配內存 -- alloc

+ (id)alloc {
    return _objc_rootAlloc(self);
}
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// 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*/);
}
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// 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]; } 複製代碼

分支1 -- `obj->initInstanceIsa(cls, dtor);`

inline void 
objc_object::initInstanceIsa(Class cls, bool hasCxxDtor)
{
    assert(!cls->instancesRequireRawIsa());
    assert(hasCxxDtor == cls->hasCxxDtor());

    initIsa(cls, true, hasCxxDtor);
}
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inline void 
objc_object::initIsa(Class cls, bool nonpointer, bool hasCxxDtor) 
{ 
    assert(!isTaggedPointer()); 
    
    if (!nonpointer) {
        isa.cls = cls;
    } else {
        assert(!DisableNonpointerIsa);
        assert(!cls->instancesRequireRawIsa());

        isa_t newisa(0);

#if SUPPORT_INDEXED_ISA
        assert(cls->classArrayIndex() > 0);
        newisa.bits = ISA_INDEX_MAGIC_VALUE;
        // isa.magic is part of ISA_MAGIC_VALUE
        // isa.nonpointer is part of ISA_MAGIC_VALUE
        newisa.has_cxx_dtor = hasCxxDtor;
        newisa.indexcls = (uintptr_t)cls->classArrayIndex();
#else
        newisa.bits = ISA_MAGIC_VALUE;
        // isa.magic is part of ISA_MAGIC_VALUE
        // isa.nonpointer is part of ISA_MAGIC_VALUE
        newisa.has_cxx_dtor = hasCxxDtor;
        newisa.shiftcls = (uintptr_t)cls >> 3;
#endif

        // This write must be performed in a single store in some cases
        // (for example when realizing a class because other threads
        // may simultaneously try to use the class).
        // fixme use atomics here to guarantee single-store and to
        // guarantee memory order w.r.t. the class index table
        // ...but not too atomic because we don't want to hurt instantiation isa = newisa; } } 複製代碼

上述代碼中，newisa.bits = ISA_MAGIC_VALUE; 是爲了對 isa 結構賦值一個初始值，經過對 isa_t 的結構分析，咱們能夠知道這次賦值只是對 nonpointer 和 magic 部分進行了賦值。

newisa.shiftcls = (uintptr_t)cls >> 3; 是將類的地址存儲在對象的 isa 結構中。這裏右移三位的主要緣由是用於將 Class 指針中無用的後三位清除減少內存的消耗，由於類的指針要按照字節（8 bits）對齊內存，其指針後三位都是沒有意義的 0。關於類指針對齊的詳細解析可參考：從 NSObject 的初始化了解 isa 。

分支2 -- `id obj = class_createInstance(cls, 0);`

id 
class_createInstance(Class cls, size_t extraBytes)
{
    return _class_createInstanceFromZone(cls, extraBytes, nil);
}
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/***********************************************************************
* class_createInstance
* fixme
* Locking: none
**********************************************************************/

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; } 複製代碼

其中，有個 obj->initIsa(cls);，初始化isa的操做：

inline void 
objc_object::initIsa(Class cls, bool nonpointer, bool hasCxxDtor) 
{ 
    assert(!isTaggedPointer()); 
    
    if (!nonpointer) {
        isa.cls = cls;
    } else {
        assert(!DisableNonpointerIsa);
        assert(!cls->instancesRequireRawIsa());

        isa_t newisa(0);

#if SUPPORT_INDEXED_ISA
        assert(cls->classArrayIndex() > 0);
        newisa.bits = ISA_INDEX_MAGIC_VALUE;
        // isa.magic is part of ISA_MAGIC_VALUE
        // isa.nonpointer is part of ISA_MAGIC_VALUE
        newisa.has_cxx_dtor = hasCxxDtor;
        newisa.indexcls = (uintptr_t)cls->classArrayIndex();
#else
        newisa.bits = ISA_MAGIC_VALUE;
        // isa.magic is part of ISA_MAGIC_VALUE
        // isa.nonpointer is part of ISA_MAGIC_VALUE
        newisa.has_cxx_dtor = hasCxxDtor;
        newisa.shiftcls = (uintptr_t)cls >> 3;
#endif

        // This write must be performed in a single store in some cases
        // (for example when realizing a class because other threads
        // may simultaneously try to use the class).
        // fixme use atomics here to guarantee single-store and to
        // guarantee memory order w.r.t. the class index table
        // ...but not too atomic because we don't want to hurt instantiation isa = newisa; } } 複製代碼

可見，alloc的時候會初始化isa，並經過newisa(0)的初始化列表辦法生成一個isa，並根據是否支持indexed isa分別設置.bits的值。

6.2 初始化 -- init

- (id)init {
    return _objc_rootInit(self);
}
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id
_objc_rootInit(id obj)
{
    // In practice, it will be hard to rely on this function.
    // Many classes do not properly chain -init calls.
    return obj;
}
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7. 獲取引用計數

NSObject.mm retainCount

- (NSUInteger)retainCount {
    return ((id)self)->rootRetainCount();
}
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objc-object.h objc_object::rootRetainCount()

inline uintptr_t 
objc_object::rootRetainCount()
{
    if (isTaggedPointer()) return (uintptr_t)this;

    sidetable_lock();
    isa_t bits = LoadExclusive(&isa.bits);
    ClearExclusive(&isa.bits);
    if (bits.nonpointer) {
        uintptr_t rc = 1 + bits.extra_rc;
        if (bits.has_sidetable_rc) {
            rc += sidetable_getExtraRC_nolock();
        }
        sidetable_unlock();
        return rc;
    }

    sidetable_unlock();
    return sidetable_retainCount();
}
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可見，獲取引用計數的關鍵在這麼一句話：

uintptr_t rc = 1 + bits.extra_rc;
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bits.extra_rc表示引用計數減1。固然，這隻針對狀況1，即bits.nonpointer爲1（開啓了指針優化），且bits.has_sidetable_rc爲0（表示不存儲在散列表Side Table中，而存儲在extra_rc中）。

狀況0 -- TaggedPointer

直接返回isa值自己。

狀況1 -- 非TaggedPointer，且開啓了指針優化，且存儲在extra_rc中

objc-os.h LoadExclusive(uintptr_t *src)

static ALWAYS_INLINE
uintptr_t 
LoadExclusive(uintptr_t *src)
{
    return *src;
}
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狀況2 -- 非TaggedPointer，且沒有開啓指針優化，且存儲在散列表中

NSObject.mm objc_object::sidetable_getExtraRC_nolock()

size_t 
objc_object::sidetable_getExtraRC_nolock()
{
    assert(isa.nonpointer);
    SideTable& table = SideTables()[this];
    RefcountMap::iterator it = table.refcnts.find(this);
    if (it == table.refcnts.end()) return 0;
    else return it->second >> SIDE_TABLE_RC_SHIFT;
}
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可見，其邏輯就是先從 SideTable 的靜態方法獲取當前實例對應的 SideTable 對象，其 refcnts 屬性就是以前說的存儲引用計數的散列表，這裏將其類型簡寫爲 RefcountMap：

typedef objc::DenseMap RefcountMap;
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而後在引用計數表中用迭代器查找當前實例對應的鍵值對，獲取引用計數值，並在此基礎上 +1 並將結果返回。這也就是爲何以前說引用計數表存儲的值爲實際引用計數減一。

須要注意的是爲何這裏把鍵值對的值作了向右移位操做（it->second >> SIDE_TABLE_RC_SHIFT）:

// The order of these bits is important.
#define SIDE_TABLE_WEAKLY_REFERENCED (1UL<<0)
#define SIDE_TABLE_DEALLOCATING (1UL<<1) // MSB-ward of weak bit
#define SIDE_TABLE_RC_ONE (1UL<<2) // MSB-ward of deallocating bit
#define SIDE_TABLE_RC_PINNED (1UL<<(WORD_BITS-1))

#define SIDE_TABLE_RC_SHIFT 2
#define SIDE_TABLE_FLAG_MASK (SIDE_TABLE_RC_ONE-1)
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能夠看出值的第一個 bit 表示該對象是否有過 weak 對象，若是沒有，在析構釋放內存時能夠更快；第二個 bit 表示該對象是否正在析構。從第三個 bit 開始纔是存儲引用計數數值的地方。因此這裏要作向右移兩位的操做，而對引用計數的 +1 和 -1 可使用 SIDE_TABLE_RC_ONE，還能夠用 SIDE_TABLE_RC_PINNED 來判斷是否引用計數值有可能溢出。

狀況3 -- 默認值

NSObject.mm objc_object::sidetable_retainCount()

uintptr_t
objc_object::sidetable_retainCount()
{
    SideTable& table = SideTables()[this];

    size_t refcnt_result = 1;
    
    table.lock();
    RefcountMap::iterator it = table.refcnts.find(this);
    if (it != table.refcnts.end()) {
        // this is valid for SIDE_TABLE_RC_PINNED too
        refcnt_result += it->second >> SIDE_TABLE_RC_SHIFT;
    }
    table.unlock();
    return refcnt_result;
}
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