Objective-C同时为我们提供了增加引用计数的 retain 和减少引用计数的 release 方法。
这篇文章会在源码层面介绍 Objective-C 中 retain 和 release 的实现。
从retain开始
如今我们已经进入全面使用ARC的时代,几年前还经常使用 retain 和 release 方法已经很难出现在我们的视野中了,绝大多数内存管理的实现细节都由编译器代劳了。
- (id)retain {
return ((id)self)->rootRetain();
}
ALWAYS_INLINE id
objc_object::rootRetain()
{
return rootRetain(false, false);
}
这个id objc_object::rootRetain(bool tryRetain, bool handleOverflow) 方法是最重要的方法,其原理就是将 isa 结构体中的 extra_rc 的值加一。
extra_rc 就是用于保存自动引用计数的标志位。
下面是ARM64时 isa 结构体的结构
union isa_t
{
isa_t() { }
isa_t(uintptr_t value) : bits(value) { }
Class cls;
uintptr_t bits;
struct {
uintptr_t nonpointer : 1;
uintptr_t has_assoc : 1;
uintptr_t has_cxx_dtor : 1;
uintptr_t shiftcls : 33;
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)
};
};
接下来分三种情况对 rootRetain(bool tryRetain, bool handleOverflow) 方法进行分析。
正常的rootRetain
下面是简化后的方法实现,只处理一般情况的代码:
ALWAYS_INLINE id
objc_object::rootRetain(bool tryRetain, bool handleOverflow)
{
bool sideTableLocked = false;
bool transcribeToSideTable = false;
isa_t oldisa;
isa_t newisa;
do {
oldisa = LoadExclusive(&isa.bits);
newisa = oldisa;
uintptr_t carry;
newisa.bits = addc(newisa.bits, RC_ONE, 0, &carry); // extra_rc++
} while (slowpath(!StoreExclusive(&isa.bits, oldisa.bits, newisa.bits)));
return (id)this;
}
这里假设条件是 isa 中的 extra_rc 的位数足以存储引用计数
- 1、使用 LoadExclusive 加载 isa 的值
- 2、调用 addc(newisa.bits, RC_ONE, 0, &carry) 方法将 isa 中 extra_rc 的值加一
- 3、调用 StoreExclusive(&isa.bits, oldisa.bits, newisa.bits) 更新 isa 的值
- 4、返回当前对象
进位版本的rootRetain
这里调用 addc 方法将 isa 中的 extra_rc 值加一时, extra_rc 不足以保存引用计数,引起进位时。
ALWAYS_INLINE id
objc_object::rootRetain(bool tryRetain, bool handleOverflow)
{
isa_t oldisa;
isa_t newisa;
oldisa = LoadExclusive(&isa.bits);
newisa = oldisa;
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);
}
}
}
extra_rc 不足以保存引用计数,并且 handleOverflow 为 false。
当传入的 handleOverflow 为 false 时,我们会调用 rootRetain_overflow 方法
NEVER_INLINE id
objc_object::rootRetain_overflow(bool tryRetain)
{
return rootRetain(tryRetain, true);
}
这个方法就是重新执行 rootRetain 方法,并传入 handleOverflow 为 true。
进位版本的rootRetain 处理溢出
当传入的 handleOverflow = true 时, 会在 rootRetain 方法中处理引用计数的溢出。
ALWAYS_INLINE id
objc_object::rootRetain(bool tryRetain, bool handleOverflow)
{
isa_t oldisa;
isa_t newisa;
do {
transcribeToSideTable = false;
oldisa = LoadExclusive(&isa.bits);
newisa = oldisa;
uintptr_t carry;
newisa.bits = addc(newisa.bits, RC_ONE, 0, &carry); // extra_rc++
if (slowpath(carry)) {
// newisa.extra_rc++ overflowed
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);
}
return (id)this;
}
因为 extra_rc 已经溢出了,更新它的值为 RC_HALF:
define RC_HALF (1ULL<<18)
extra_rc 为19位, RC_HALF 为 1 左移18位
之后设置 has_sidetable_rc 为 true,存储新的 isa 值之后,调用 sidetable_addExtraRC_nolock 方法
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;
}
}
这里将溢出的 RC_HALF 存储到 SideTable
在iOS的内存管理中,使用了 isa 结构体中的 extra_rc 和 SideTable 来存储对象的自动引用计数.
以release结束
- (oneway void)release {
((id)self)->rootRelease();
}
ALWAYS_INLINE bool
objc_object::rootRelease()
{
return rootRelease(true, false);
}
在分析 release 方法时,根据上下文的不同,将 release 方法的实现拆分为三部分。
正常的 release
ALWAYS_INLINE bool
objc_object::rootRelease(bool performDealloc, bool handleUnderflow)
{
isa_t oldisa;
isa_t newisa;
do {
oldisa = LoadExclusive(&isa.bits);
newisa = oldisa;
uintptr_t carry;
newisa.bits = subc(newisa.bits, RC_ONE, 0, &carry); // extra_rc--
} while (slowpath(!StoreReleaseExclusive(&isa.bits,
oldisa.bits, newisa.bits)));
return false;
}
- 1、使用 LoadExclusive 获取 isa 内容
- 2、调用 subc 将 isa 中 extra_rc 引用计数减一
- 3、调用 StoreReleaseExclusive 方法保存新的 isa
从 SideTable 借位
ALWAYS_INLINE bool
objc_object::rootRelease(bool performDealloc, bool handleUnderflow)
{
isa_t oldisa;
isa_t newisa;
do {
oldisa = LoadExclusive(&isa.bits);
newisa = oldisa;
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);
}
}
}
需要借位,并且 handleUnderflow 为 false 时。
当传入的 handleUnderflow 为 false 时,会调用 rootRelease_underflow 方法
NEVER_INLINE bool
objc_object::rootRelease_underflow(bool performDealloc)
{
return rootRelease(performDealloc, true);
}
借位版本的 rootRelease 处理借位
当传入的 handleUnderflow 为 true时, 会在 rootRelease 中处理借位
ALWAYS_INLINE bool
objc_object::rootRelease(bool performDealloc, bool handleUnderflow)
{
bool sideTableLocked = false;
isa_t oldisa;
isa_t newisa;
do {
oldisa = LoadExclusive(&isa.bits);
newisa = oldisa;
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);
}
// 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);
return false;
}
else {
// Side table is empty after all. Fall-through to the dealloc path.
}
}
}
- 借位时会调用sidetable_subExtraRC_nolock函数
- 借位成功时函数返回 RC_HALF,更新 extra_rc 的值为 RC_HALF - 1 ,调用 StoreReleaseExclusive 更新 isa的值
- 借位失败时函数返回 0, 也就是对象需要销毁
函数sidetable_subExtraRC_nolock如下,传入参数为RC_HALF:
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;
}
- 从SideTable 中借位时
-
SideTable 中 retain count 为0 时,借位失败函数返回0
-
SideTable 中 retain count 不为0 时,借位成功函数返回RC_HALF
-
rootRelease 中调用 dealloc
// 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;
这段代码是 处理借位时 borrowed 为0 时,对象的 dealloc 处理
直接调用objc_msgSend向当前对象发送 SEL_dealloc 消息, 为了确保消息只发送一次,使用了 deallocating 标记
获取 retainCount
- (NSUInteger)retainCount {
return ((id)self)->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();
}
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;
}
retainCount由三部分组成:
- 数值1
- extra_rc 中存储的值
- sidetable_getExtraRC_nolock 从sideTable中得到的值
小结
- Objective-C 使用 isa 中的 extra_rc 和 SideTable 来存储对象的引用计数
- 对象实际的引用计数会在 extra_rc 和 SideTable 的基础上加一
- 在对象的引用计数归零时,会通过 objc_msgSend 调用 SEL_dealloc 回收对象
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