iOS 引用計數 retainCount、retain、release 原始碼分析+註釋+實驗
這篇文章與上一篇有較大的關聯,沒看過的可以先去看看 ^ _ ^
物件alloc後retainCount為什麼引用計數為1
Person *p = [Person alloc]; // extrac = 0
// alloc出來的引用計數為多少 -- 0 -- 1
NSLog(@"%lu",(unsigned long)[p retainCount]); // 1
[p retain]; // extrac = 0 - 1
NSLog(@"%lu",(unsigned long)[p retainCount]); // extrac+1 = 2
[p release];// -1
NSLog(@"1 == %lu" ,(unsigned long)[p retainCount]); // 1
[p release];// 1-1 -- 引用計數位0的時候 我就析構 ? -- 響應 訊息
NSLog(@" 0 == %lu",(unsigned long)[p retainCount]); // 0
[p release];// -1
NSLog(@"-1 == %lu",(unsigned long)[p retainCount]); // -1
NSLog(@"完了");
複製程式碼
物件alloc的時候,最終會走向建立isa
並沒有進行retainCount,引用計數為0,進行列印retainCount的時候由於當前引用計數為0,如果一直為0,那麼物件就會被銷燬,導致我們現在在做無用功,所以在objc_object::rootRetainCount()
if (bits.nonpointer) {}
,isa初始化的時候,nonpointer為1,所以在呼叫retainCount時,會預設給該物件的引用計數+1。
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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retain & release
retain
執行順序 <1> - (id)retain {} <2> objc_object::rootRetain()引數分別:false,false <3> objc_object::rootRetain(bool tryRetain,bool handleOverflow) <4> 判斷新舊isa是否一致迴圈,一致就執行<9>,否則執行<5> <5> 迴圈獲取舊值,並賦給新值,為新值進行extra_rc+1 <6> 判斷是否溢位(x86_64 256),沒溢位就執行<9>,溢位走<7> <7> 執行rootRetain_overflow,回到<3>,handleOverflow為true,下次過來時執行<8> <8> x86_64留下引用計數的一半128,複製另一半存進去散列表 <9> return
// 並且呼叫retain的時候,傳入的兩個引數均為false
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;
// 迴圈條件:判斷是否獨一份儲存,對比新舊isa,如果不是,就迴圈
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
// 如果當前物件的isa 正在銷燬
if (slowpath(tryRetain && newisa.deallocating)) {
ClearExclusive(&isa.bits);
if (!tryRetain && sideTableLocked) sidetable_unlock();
return nil;
}
//是否溢位,
//經過實驗:在x86_64架構下,當newisa.extra_rc為255時,在進行addc,就會發生溢位
//溢位之後,將會拿2的7次方的extra_rc 存到散列表中,newisa.extra_rc回到128
uintptr_t carry;
//這裡newisa.extra_rc 會+1 RC_ONE
newisa.bits = addc(newisa.bits,RC_ONE,&carry); // extra_rc++
printf("%lu,",newisa.extra_rc);
//newisa.extra_rc++如果溢位
if (slowpath(carry)) {
// newisa.extra_rc++ overflowed
//第一次來的話,handleOverflow是false,會進判斷語句
if (!handleOverflow) {
ClearExclusive(&isa.bits);
//這裡重新呼叫了當前方法rootRetain,但是handleOverflow = true
return rootRetain_overflow(tryRetain);
}
// Leave half of the retain counts inline and
// prepare to copy the other half to the side table.
// retry之後會來到這裡
// 翻譯:留下內部關聯物件的一半,準備複製另一半存進去散列表
if (!tryRetain && !sideTableLocked) {
sidetable_lock();
}
sideTableLocked = true;
transcribeToSideTable = true;
newisa.extra_rc = RC_HALF;
newisa.has_sidetable_rc = true;
}
//當且僅當舊值與儲存中的當前值一致時,才把新值寫入儲存。
} while (slowpath(!StoreExclusive(&isa.bits,oldisa.bits,newisa.bits)));
if (slowpath(transcribeToSideTable)) {
// Copy the other half of the retain counts to the side table.
// 拷貝一半(128)進散列表
sidetable_addExtraRC_nolock(RC_HALF);
}
if (slowpath(!tryRetain && sideTableLocked)) sidetable_unlock();
return (id)this;
}
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sidetable_addExtraRC_nolock散列表新增引用計數
這個在上一篇文章,記憶體管理方案中已經有提到過了,這裡在發一次,加點印象。
bool
objc_object::sidetable_addExtraRC_nolock(size_t delta_rc)
{
assert(isa.nonpointer);
// 通過SideTables() 獲取SideTable
SideTable& table = SideTables()[this];
//獲取引用計數的size
size_t& refcntStorage = table.refcnts[this];
// 賦值給oldRefcnt
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);
// 如果oldRefcnt & SIDE_TABLE_RC_PINNED = 1
// 就是 oldRefcnt = 2147483648 (32位情況)
if (oldRefcnt & SIDE_TABLE_RC_PINNED) return true;
//引用計數也溢位判斷引數
uintptr_t carry;
// 引用計數 add
//delta_rc左移兩位,右邊的兩位分別是DEALLOCATING(銷燬ing) 跟WEAKLY_REFERENCED(弱引用計數)
size_t newRefcnt =
addc(oldRefcnt,delta_rc << SIDE_TABLE_RC_SHIFT,&carry);
//如果sidetable也溢位了。
//這裡我for了幾百萬次,也沒有溢位,可見sidetable能容納很多的引用計數
if (carry) {
// 如果是32位的情況 SIDE_TABLE_RC_PINNED = 1<< (32-1)
// int的最大值 SIDE_TABLE_RC_PINNED = 2147483648
// SIDE_TABLE_FLAG_MASK = 3
// refcntStorage = 2147483648 | (oldRefcnt & 3)
// 如果溢位,直接把refcntStorage 設定成最大值
refcntStorage =
SIDE_TABLE_RC_PINNED | (oldRefcnt & SIDE_TABLE_FLAG_MASK);
return true;
}
else {
refcntStorage = newRefcnt;
return false;
}
}
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release
執行順序 <1> - (oneway void)release {} <2> objc_object::rootRelease() 引數分別:true,false <3> objc_object::rootRelease(bool performDealloc,bool handleUnderflow) <4> 判斷新舊isa是否一致迴圈,一致就執行return,否則執行<5> <5> 迴圈獲取舊值,並賦給新值,為新值進行extra_rc-1 <6> 判斷是否溢位,沒溢位就執行return,溢位走<7> underflow <7> 判斷是否有用到散列表 <8> 從散列表中拿出RC_HALF,將這部分存進newisa <9> 存成功就return,不成功就重試,再不行就把拿出來的放回去,然後goto retry; <10> dealloc
ALWAYS_INLINE bool
objc_object::rootRelease(bool performDealloc,bool handleUnderflow)
{
if (isTaggedPointer()) return false;
bool sideTableLocked = false;
//新舊isa
isa_t oldisa;
isa_t newisa;
retry:
//跟retain一樣的判斷條件
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.extra_rc-1
//如果溢位的時候, newisa.extra_rc = 255
newisa.bits = subc(newisa.bits,&carry); // extra_rc--
if (slowpath(carry)) {
// don't ClearExclusive()
//如果溢位走這
printf("釋放溢位了,underflow\n");
goto underflow;
}
} while (slowpath(!StoreReleaseExclusive(&isa.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
// 重新把舊isa給新isa,意思是把引用計數-1操作還原
// 這時候的 newisa.extra_rc = 0
newisa = oldisa;
// retain的時候。如果有用到散列表,會 newisa.has_sidetable_rc = true;
if (slowpath(newisa.has_sidetable_rc)) {
printf("發現has_sidetable_rc = true \n");
// 呼叫release的時候handleUnderflow = false
if (!handleUnderflow) {
ClearExclusive(&isa.bits);
//類似retain時候retry,重新來一次,但是handleUnderflow為true
return rootRelease_underflow(performDealloc);
}
// Transfer retain count from side table to inline storage.
// 進判斷前 sideTableLocked 沒有重新賦值,所以一直是false
if (!sideTableLocked) {
ClearExclusive(&isa.bits);
sidetable_lock();
sideTableLocked = true;
// Need to start over to avoid a race against
// the nonpointer -> raw pointer transition.
// 去retry,重新回到上面,重複走一遍
goto retry;
}
// Try to remove some retain counts from the side table.
// 從散列表中拿出RC_HALF的引用計數
size_t borrowed = sidetable_subExtraRC_nolock(RC_HALF);
printf("借出來的 size === %lu \n",borrowed);
// 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
// 把拿出來的引用計數存到newisa
bool stored = StoreReleaseExclusive(&isa.bits,newisa.bits);
if (!stored) {
//如果沒存成功,就換個姿勢再試試
// 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),&overflow);
if (!overflow) {
stored = StoreReleaseExclusive(&isa.bits,oldisa2.bits,newisa2.bits);
}
}
}
if (!stored) {
// 如果還是沒成功,把拿出來的放回去
// 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.
// 如果newisa.has_sidetable_rc != true;
// 就拋錯,release太多
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,newisa.bits)) goto retry;
if (slowpath(sideTableLocked)) sidetable_unlock();
__sync_synchronize();
if (performDealloc) {
((void(*)(objc_object *,SEL))objc_msgSend)(this,SEL_dealloc);
}
return true;
}
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###實驗: 在這個實驗中
- 我先對p retain了257次,256次保證溢位,在第257次時觀察在已經使用到了sidetable的情況下的引用計數。
- 然後再對p release了260次,保證能release次數大於retain次數,保證p dealloc,並且觀察在dealloc後,繼續release的情況。
Person *p =[Person alloc]; // extrac = 0
NSLog(@"開始retain\n");
// 在Mac下 保證能retain溢位,並多retain一次
for (int i = 0 ; i<257; i++) {
[p retain];
}
NSLog(@"開始release\n");
// 在Mac下 保證能release溢位,並且多釋放幾次
for (int i = 0 ; i<260; i++) {
[p release];
}
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-
然後我在原始碼中各個位置都做了log處理,觀察進行lldb除錯
-
先看retain的log
-
retain結論:在retain發生溢位後,會存入128到散列表,newisa的當前引用計數為128,再繼續retain就在128的基礎上+1。
-
看release的log ,此時p的引用計數為129(但是如果呼叫retainCount就會是130)
-
release總結:
<1> 在release發生溢位,且當前newisa的
has_sidetable_rc
為true後> <2> 走performDealloc
,將handleUnderflow
設定成true,然後再遞迴一次 <3> 過了handleUnderflow
這關之後,繼續遇到了sideTableLocked
<4> release的時候sideTableLocked
預設為false,把sideTableLocked
設定為true後,就又要回到retry(這裡應該不算遞迴),又走了一遍上面的一大串程式碼。 <5> 可謂是過關斬將遇到兩個攔路虎handleUnderflow 、sideTableLocked
,過了兩關後,從sidetable中拿出RC_HALF(2^7)的引用計數,-1 之後交給當前newisa.extra_rc。
TO BE CONTINUE ~