objc源码解析 | 引用计数之retain release

本文基于objc4-750 点此下载

@interface NSObject  {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wobjc-interface-ivars"
    Class isa  OBJC_ISA_AVAILABILITY;
#pragma clang diagnostic pop
}

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

struct objc_object {
private:
    isa_t isa;
    ...
}

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
    ...
}

可以看出所有类、对象最终都由objc_object组成

来看下引用计数

- (NSUInteger)retainCount {
    return ((id)self)->rootRetainCount();
}

#if SUPPORT_NONPOINTER_ISA
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();
}

#else
uintptr_t objc_object::rootRetainCount()
{
    if (isTaggedPointer()) return (uintptr_t)this;
    return sidetable_retainCount();
}
#endif

可以看出普通的isa引用计数存储于sidetable
nonpointer的isa引用计数存储于extra_rc与sidetable

来看下SideTable部分的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;
}

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;
}

来看下isa_t


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

    Class cls;
    uintptr_t bits;
#if defined(ISA_BITFIELD)
    struct {
        ISA_BITFIELD;  // defined in isa.h
    };
#endif
};

isa_t为union结构, 来看下isa的位域表示

// 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

#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)

# if __arm64__
#   define ISA_MASK        0x0000000ffffffff8ULL
#   define ISA_MAGIC_MASK  0x000003f000000001ULL
#   define ISA_MAGIC_VALUE 0x000001a000000001ULL
#   define ISA_BITFIELD                                                      \
      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
#   define ISA_BITFIELD                                                        \
      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

64位的指针，在实际使用中，这些位数并不是都用上了
/MACH_VM_MAX_ADDRESS 0x7fffffe00000/
X86-64的Mac仅使用了其中的47位
/MACH_VM_MAX_ADDRESS 0x1000000000/
ARM-64的iOS仅使用了其中的36位

另外指针还需要对齐，释放了另外3位，所以shiftcls在x86_64和arm64分别为44和33

对于arm64下的nonpointer, 其余31位可另作他用
uintptr_t nonpointer : 1; // 0表示普通的isa指针
uintptr_t extra_rc : 19; // nonpointer时用于存储引用计数
uintptr_t has_sidetable_rc : 1; // extra_rc不够用，加上sidetable

RC_ONE (1ULL<<45) // 64-19=45 引用计数+1 = extra_rc + RC_ONE
RC_HALF (1ULL<<18) // extra_rc溢出时，delta = RC_HALF

extra_rc可以存储的数量为2^19 多数情况不需要用到sidetable

再来看下SideTable

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

来看下RefcountMap

// RefcountMap disguises its pointers because we 
// don't want the table to act as a root for `leaks`.
typedef objc::DenseMap,size_t,true> RefcountMap;

template
class DenseMapBase {
protected:
  typedef std::pair BucketT;
  ...
}

iterator find(const KeyT &Val) {
    BucketT *TheBucket;
    if (LookupBucketFor(Val, TheBucket))
      return iterator(TheBucket, getBucketsEnd(), true);
    return end();
}

伪装指针DisguisedPtr，这里暂不展开，存储引用计数类型为size_t

来看下SideTables()

// We cannot use a C++ static initializer to initialize SideTables because
// libc calls us before our C++ initializers run. We also don't want a global 
// pointer to this struct because of the extra indirection.
// Do it the hard way.
alignas(StripedMap) static uint8_t 
    SideTableBuf[sizeof(StripedMap)];

static StripedMap& SideTables() {
    return *reinterpret_cast*>(SideTableBuf);
}

class StripedMap {

    enum { CacheLineSize = 64 };

#if TARGET_OS_EMBEDDED
    enum { StripeCount = 8 };
#else
    enum { StripeCount = 64 };
#endif

    struct PaddedT {
        T value alignas(CacheLineSize);
    };

    PaddedT array[StripeCount];

    static unsigned int indexForPointer(const void *p) {
        uintptr_t addr = reinterpret_cast(p);
        return ((addr >> 4) ^ (addr >> 9)) % StripeCount;
    }

 public:
    T& operator[] (const void *p) { 
        return array[indexForPointer(p)].value; 
    }
    const T& operator[] (const void *p) const { 
        return const_cast>(this)[p]; 
    }
    ...
}

SideTable& table = SideTables()[this]
RefcountMap::iterator it = table.refcnts.find(this);

SideTables()返回全局变量SideTableBuf，StripedMap重载了operator[]，通过this索引至找到SideTable
再通过this在table.refcnts查找对应的iterator，return end()表示未找到

看下site_t具体存储

#ifdef __LP64__
    #define WORD_BITS 64
#else
    #define WORD_BITS 32
#endif

// 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)

前2位用作标识
引用计数+1即加上SIDE_TABLE_RC_ONE

接着看retain与release

__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();
}

autorelease的先不展开，先看下retain

id objc_object::retain()
{
    assert(!isTaggedPointer());

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

    return ((id(*)(objc_object *, SEL))objc_msgSend)(this, SEL_retain);
}

id objc_object::rootRetain()
{
    return rootRetain(false, false);
}

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;
}

!nonpointer 走sidetable_retain()
否则extra_rc++ 溢出走sidetable_addExtraRC_nolock(RC_HALF)
溢出时, extra_rc减半, 剩下的RC_HALF存至sidetable

来看下sidetable retain部分

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;
}

// Move some retain counts to the side table from the isa field.
// Returns true if the object is now pinned.
bool objc_object::sidetable_addExtraRC_nolock(size_t delta_rc)
{
    assert(isa.nonpointer);
    SideTable& table = SideTables()[this];

    size_t& refcntStorage = table.refcnts[this];
    size_t oldRefcnt = refcntStorage;
    // isa-side bits should not be set here
    assert((oldRefcnt & SIDE_TABLE_DEALLOCATING) == 0);
    assert((oldRefcnt & SIDE_TABLE_WEAKLY_REFERENCED) == 0);

    if (oldRefcnt & SIDE_TABLE_RC_PINNED) return true;

    uintptr_t carry;
    size_t newRefcnt = 
        addc(oldRefcnt, delta_rc << SIDE_TABLE_RC_SHIFT, 0, &carry);
    if (carry) {
        refcntStorage =
            SIDE_TABLE_RC_PINNED | (oldRefcnt & SIDE_TABLE_FLAG_MASK);
        return true;
    }
    else {
        refcntStorage = newRefcnt;
        return false;
    }
}

release基本与retain相反
!nonpointer 走sidetable_release()
否则extra_rc-- 溢出检查sidetable, borrowed走sidetable_subExtraRC_nolock(RC_HALF)

看下具体实现

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

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

    ((void(*)(objc_object *, SEL))objc_msgSend)(this, SEL_release);
}

bool 
objc_object::rootRelease(bool performDealloc, bool handleUnderflow)
{
    if (isTaggedPointer()) return false;

    bool sideTableLocked = false;

    isa_t oldisa;
    isa_t newisa;

 retry:
    do {
        oldisa = LoadExclusive(&isa.bits);
        newisa = oldisa;
        if (slowpath(!newisa.nonpointer)) {
            ClearExclusive(&isa.bits);
            if (sideTableLocked) sidetable_unlock();
            return sidetable_release(performDealloc);
        }
        // don't check newisa.fast_rr; we already called any RR overrides
        uintptr_t carry;
        newisa.bits = subc(newisa.bits, RC_ONE, 0, &carry);  // extra_rc--
        if (slowpath(carry)) {
            // don't ClearExclusive()
            goto underflow;
        }
    } while (slowpath(!StoreReleaseExclusive(&isa.bits, 
                                             oldisa.bits, newisa.bits)));

    if (slowpath(sideTableLocked)) sidetable_unlock();
    return false;

 underflow:
    // newisa.extra_rc-- underflowed: borrow from side table or deallocate

    // abandon newisa to undo the decrement
    newisa = oldisa;

    if (slowpath(newisa.has_sidetable_rc)) {
        if (!handleUnderflow) {
            ClearExclusive(&isa.bits);
            return rootRelease_underflow(performDealloc);
        }

        // Transfer retain count from side table to inline storage.

        if (!sideTableLocked) {
            ClearExclusive(&isa.bits);
            sidetable_lock();
            sideTableLocked = true;
            // Need to start over to avoid a race against 
            // the nonpointer -> raw pointer transition.
            goto retry;
        }

        // Try to remove some retain counts from the side table.        
        size_t borrowed = sidetable_subExtraRC_nolock(RC_HALF);

        // To avoid races, has_sidetable_rc must remain set 
        // even if the side table count is now zero.

        if (borrowed > 0) {
            // Side table retain count decreased.
            // Try to add them to the inline count.
            newisa.extra_rc = borrowed - 1;  // redo the original decrement too
            bool stored = StoreReleaseExclusive(&isa.bits, 
                                                oldisa.bits, newisa.bits);
            if (!stored) {
                // Inline update failed. 
                // Try it again right now. This prevents livelock on LL/SC 
                // architectures where the side table access itself may have 
                // dropped the reservation.
                isa_t oldisa2 = LoadExclusive(&isa.bits);
                isa_t newisa2 = oldisa2;
                if (newisa2.nonpointer) {
                    uintptr_t overflow;
                    newisa2.bits = 
                        addc(newisa2.bits, RC_ONE * (borrowed-1), 0, &overflow);
                    if (!overflow) {
                        stored = StoreReleaseExclusive(&isa.bits, oldisa2.bits, 
                                                       newisa2.bits);
                    }
                }
            }

            if (!stored) {
                // Inline update failed.
                // Put the retains back in the side table.
                sidetable_addExtraRC_nolock(borrowed);
                goto retry;
            }

            // Decrement successful after borrowing from side table.
            // This decrement cannot be the deallocating decrement - the side 
            // table lock and has_sidetable_rc bit ensure that if everyone 
            // else tried to -release while we worked, the last one would block.
            sidetable_unlock();
            return false;
        }
        else {
            // Side table is empty after all. Fall-through to the dealloc path.
        }
    }

    // Really deallocate.

    if (slowpath(newisa.deallocating)) {
        ClearExclusive(&isa.bits);
        if (sideTableLocked) sidetable_unlock();
        return overrelease_error();
        // does not actually return
    }
    newisa.deallocating = true;
    if (!StoreExclusive(&isa.bits, oldisa.bits, newisa.bits)) goto retry;

    if (slowpath(sideTableLocked)) sidetable_unlock();

    __sync_synchronize();
    if (performDealloc) {
        ((void(*)(objc_object *, SEL))objc_msgSend)(this, SEL_dealloc);
    }
    return true;
}

sidetable release部分

// return uintptr_t instead of bool so that the various raw-isa 
// -release paths all return zero in eax
uintptr_t objc_object::sidetable_release(bool performDealloc)
{
#if SUPPORT_NONPOINTER_ISA
    assert(!isa.nonpointer);
#endif
    SideTable& table = SideTables()[this];

    bool do_dealloc = false;

    table.lock();
    RefcountMap::iterator it = table.refcnts.find(this);
    if (it == table.refcnts.end()) {
        do_dealloc = true;
        table.refcnts[this] = SIDE_TABLE_DEALLOCATING; //没有
    } else if (it->second < SIDE_TABLE_DEALLOCATING) {
        // SIDE_TABLE_WEAKLY_REFERENCED may be set. Don't change it.
        do_dealloc = true;
        it->second |= SIDE_TABLE_DEALLOCATING;
    } else if (! (it->second & SIDE_TABLE_RC_PINNED)) {
        it->second -= SIDE_TABLE_RC_ONE;
    }
    table.unlock();
    if (do_dealloc  &&  performDealloc) {
        ((void(*)(objc_object *, SEL))objc_msgSend)(this, SEL_dealloc);
    }
    return do_dealloc;
}

// Move some retain counts from the side table to the isa field.
// Returns the actual count subtracted, which may be less than the request.
size_t objc_object::sidetable_subExtraRC_nolock(size_t delta_rc)
{
    assert(isa.nonpointer);
    SideTable& table = SideTables()[this];

    RefcountMap::iterator it = table.refcnts.find(this);
    if (it == table.refcnts.end()  ||  it->second == 0) {
        // Side table retain count is zero. Can't borrow.
        return 0;
    }
    size_t oldRefcnt = it->second;

    // isa-side bits should not be set here
    assert((oldRefcnt & SIDE_TABLE_DEALLOCATING) == 0);
    assert((oldRefcnt & SIDE_TABLE_WEAKLY_REFERENCED) == 0);

    size_t newRefcnt = oldRefcnt - (delta_rc << SIDE_TABLE_RC_SHIFT);
    assert(oldRefcnt > newRefcnt);  // shouldn't underflow
    it->second = newRefcnt;
    return delta_rc;
}

防止篇幅过长，weak & autorelease部分放到下篇

objc源码解析 | 引用计数之retain release

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