前一篇文章介紹了objc4 779.1的調試環(huán)境搭建，這里以alloc、init、new方法為例看一下詳細的執(zhí)行過程。
alloc流程圖
-
LYPerson alloc 流程
1.png -
NSObject alloc 流程
2.png
上圖是一步步斷點調試而來，調試方法請參考源碼探索方法。我們簡單分析一下，不管是NSObjec還是LYPerson，alloc都不是首先調用的方法，甚至NSObjec連alloc都沒有調用。
為什么首先調用的是objc_alloc方法？查找llvm源碼得知，在編譯時進行了優(yōu)化。當shouldUseRuntimeFunctionsForAlloc返回true時，對alloc進行優(yōu)化。
- 搜索
shouldUseRuntimeFunctionsForAlloc
image.png - 搜索
EmitObjCAlloc(
image.png
可以看到這兩個方法把alloc就轉換成了objc_alloc
1、objc_alloc
objc_alloc(Class cls)
{
return callAlloc(cls, true/*checkNil*/, false/*allocWithZone*/);
}
直接調用callAlloc方法。
2、callAlloc
static ALWAYS_INLINE id callAlloc(Class cls, bool checkNil, bool allocWithZone=false)
{
#if __OBJC2__
if (slowpath(checkNil && !cls)) return nil;
if (fastpath(!cls->ISA()->hasCustomAWZ())) {
return _objc_rootAllocWithZone(cls, nil);
}
#endif
// No shortcuts available.
if (allocWithZone) {//傳入參數為false
return ((id(*)(id, SEL, struct _NSZone *))objc_msgSend)(cls, @selector(allocWithZone:), nil);
}
return ((id(*)(id, SEL))objc_msgSend)(cls, @selector(alloc));
}
探索發(fā)現(xiàn)LYPerson走到最后一步，調用alloc方法；NSObjec調用的是_objc_rootAllocWithZone。hasCustomAWZ()判斷當前類是否有默認的allocWithZone方法。
bool hasCustomAWZ() const {
return !cache.getBit(FAST_CACHE_HAS_DEFAULT_AWZ);
}
bool getBit(uint16_t flags) const {
return _flags & flags;
}
#define fastpath(x) (__builtin_expect(bool(x), 1))
#define slowpath(x) (__builtin_expect(bool(x), 0))
簡述一下fastpath(x)和slowpath(x)，這兩個方法的返回值是一樣的。x 值為正返回為真，反之返回為假。引入這兩個宏目的是增加條件分支預測的準確性，cpu 會提前裝載后面的指令，遇到條件轉移指令時會提前預測并裝載某個分支的指令。slowpath 表示你可以確認該條件是極少發(fā)生的，相反 fastpath 表示該條件多數情況下會發(fā)生。編譯器會產生相應的代碼來優(yōu)化 cpu 執(zhí)行效率。
參考：
gcc 編譯器 ， __builtin_expect() 研究
iOS性能優(yōu)化系列之__builtin_expect分支預測優(yōu)化
3、alloc
+ (id)alloc {
return _objc_rootAlloc(self);
}
4、_objc_rootAlloc
id _objc_rootAlloc(Class cls)
{
return callAlloc(cls, false/*checkNil*/, true/*allocWithZone*/);
}
直接調用callAlloc，但是這次callAlloc中調用的是_objc_rootAllocWithZone
5、 _objc_rootAllocWithZone
id _objc_rootAllocWithZone(Class cls, malloc_zone_t *zone __unused)
{
// allocWithZone under __OBJC2__ ignores the zone parameter
return _class_createInstanceFromZone(cls, 0, nil,
OBJECT_CONSTRUCT_CALL_BADALLOC);
}
直接調用_class_createInstanceFromZone。
6、_class_createInstanceFromZone
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
bool hasCxxCtor = cxxConstruct && cls->hasCxxCtor();
bool hasCxxDtor = cls->hasCxxDtor();
bool fast = cls->canAllocNonpointer();
size_t size;
size = cls->instanceSize(extraBytes);
if (outAllocatedSize) *outAllocatedSize = size;
id obj;
if (zone) {
obj = (id)malloc_zone_calloc((malloc_zone_t *)zone, 1, size);
} else {
obj = (id)calloc(1, size);
}
if (slowpath(!obj)) {
if (construct_flags & OBJECT_CONSTRUCT_CALL_BADALLOC) {
return _objc_callBadAllocHandler(cls);
}
return nil;
}
if (!zone && fast) {
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);
}
大致分為以下幾個步驟：
-
instanceSize計算出類的所需內存 -
calloc開辟內存 -
initInstanceIsa關聯(lián)Isa指針
1.instanceSize
#ifdef __LP64__
# define WORD_SHIFT 3UL
# define WORD_MASK 7UL
# define WORD_BITS 64
#else
# define WORD_SHIFT 2UL
# define WORD_MASK 3UL
# define WORD_BITS 32
#endif
/*
*字節(jié)對齊
*LP64機器下8字節(jié)對齊
*其他4字節(jié)對齊
*/
static inline uint32_t word_align(uint32_t x) {
return (x + WORD_MASK) & ~WORD_MASK;
}
/*
*實例大小
*實例大小instanceSize會存在ro中
*/
uint32_t unalignedInstanceSize() const {
ASSERT(isRealized());
return data()->ro->instanceSize;
}
/*
*返回字節(jié)對齊后的內存大小
*/
uint32_t alignedInstanceSize() const {
return word_align(unalignedInstanceSize());
}
size_t instanceSize(size_t extraBytes) const {
if (fastpath(cache.hasFastInstanceSize(extraBytes))) {
return cache.fastInstanceSize(extraBytes);
}
size_t size = alignedInstanceSize() + extraBytes;
// CF requires all objects be at least 16 bytes.
if (size < 16) size = 16;//當內存小于16字節(jié)時，返回16字節(jié)
return size;
}
然而現(xiàn)在objc源碼普遍走的是fastInstanceSize函數，快速計算內存大小。而當前版本內存對齊是16字節(jié)對齊，蘋果早期是8字節(jié)對齊。
static inline size_t align16(size_t x) {
return (x + size_t(15)) & ~size_t(15);
}
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);
}
}
獲取內存大小后，直接調用calloc函數為對象分配內存空間
- calloc
The calloc( ) function contiguously allocates enough space for count objects that are size bytes of memory each and returns a pointer to the allocated memory. The allocated memory is filled with bytes of value zero. // calloc（）函數連續(xù)地為count對象分配足夠的空間，這些對象是內存的大小字節(jié)，并返回一個指向所分配內存的指針。分配的內存充滿了值為零的字節(jié)。
申請完內存，還需要初始化Isa指針。
- initIsa
inline void objc_object::initInstanceIsa(Class cls, bool hasCxxDtor)
{
ASSERT(!cls->instancesRequireRawIsa());
ASSERT(hasCxxDtor == cls->hasCxxDtor());
initIsa(cls, true, hasCxxDtor);
}
inline void objc_object::initIsa(Class cls)
{
initIsa(cls, false, false);
}
initIsa和initInstanceIsa最后調用的都是initIsa方法，就是參數不同而已
inline void
objc_object::initIsa(Class cls, bool nonpointer, bool hasCxxDtor)
{
ASSERT(!isTaggedPointer());
if (!nonpointer) {
isa = isa_t((uintptr_t)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;
}
}
初始化的過程就是對isa_t結構體初始化的過程。
到此alloc流程就走完了，主要就是計算并分配內存，關聯(lián)Isa。
init分析
- (id)init {
return _objc_rootInit(self);
}
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;
}
init方法的僅僅就是返回了當前對象而已。
new分析
// Calls [cls new]
id objc_opt_new(Class cls)
{
#if __OBJC2__
if (fastpath(cls && !cls->ISA()->hasCustomCore())) {
return [callAlloc(cls, false/*checkNil*/, true/*allocWithZone*/) init];
}
#endif
return ((id(*)(id, SEL))objc_msgSend)(cls, @selector(new));
}
+ (id)new {
return [callAlloc(self, false/*checkNil*/) init];
}
new類方法里同樣調用callAlloc和init方法，相當于[[cls alloc] init].
最后我們來看一道面試題
結果打印如下：
你們答對了嗎？
對象的內存實際上是在alloc方法里面開辟的，故p1、p2、p3在內存中的地址一致，只是指針地址不同。
