1.为什么要用alloc?
We often use alloc in OC development to allocate memory for objects for example
School *s1 = [School alloc]
2. Alloc process
The alloc process can easily be found in Apple’s open source file nsobject. mm. The following
objetc_alloc->alloc->_objc_rootAlloc->callAlloc->_objc_rootAllocWithZone->_class_createInstanceFromZone
You can see that there are three conditional judgments after **_class_createInstanceFromZone**. These three conditional judgments are the essence of alloc and what we need to study
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1.size= cls->instanceSize
This line of code makes it clear that when we alloc we first need to know how much memory is needed to instantiate the class of the object.
instanceSize->cache.fastInstanceSize
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The setFastInstanceSize function is used to remove the MASK that has been added.
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If we go further, we will find three questions: 1. What is flags? 2. Why do logic and operations
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1. Size =_flags & MASK (MASK is just a constant defined in the initialized area =8)
Size =16 indicates that an empty object can take up 16 space, depending on how many attributes the class has
2. Align16 (size + Extra-Macro 8byte)
return (x+15) & ~15
This code is a classic 16-byte alignment algorithm, please refer to the specific process of C language – logical operation.
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2.obj=(id)calloc(1,size)
calloc->malloc_zone_calloc->zone->calloc->nano_malloc_check_clear->segregated_size_to_fit
It took a long time to find the key function
segregated_size_to_fit()
k=(size+nano_size -1 ) >> shift_nano_quantum
k=(x+15)>>4
slot_bytes=k<<4
3.obc->initInstanceIsa(cls,hasCxxDtor)
The object becomes a c++ structure with a size of 8 bytes x 8=64 bits when compiled by clang
initIsa->isa_t->newisa
Features: The structure of ISA_T is the union bit field containing CLS bits;
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ISA_BITFIELD here is what differentiates arm64 architecture from X86_64 architecture
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Nonpointer :1 Whether to enable pointer optimization for ISA. 0: Indicates pure ISA. 1: indicates that the ISA contains class information and object reference counting
Has_assoc :1 Whether there are associated objects
Has_cxx_dtor :1 whether c++ extensions are available
Uintptr_t shiftcls :33/44 ARM /x86 store class information
Magic :6 Specifies whether to initialize space
Weakly_referenced :1 is referred to an ARC weak reference variable that can not be released faster
Deallocating: 1 destructor
Has_sidetable_rc :1 Hash table When the reference count is greater than 10, you need to borrow this variable to store carry
Extra_rc :19 application counts
The newisa function does several things
1. Assign the value to bits cxx_dtor shiftcls
newisa.bits=ISA_MAGIC_VALUE; 0x001d800000000001ULL =59
newisa.has_cxx_dtor=hasCxxDtor
Newisa.shiftcls = CLS >>3 Move noptr ASOC CXX three places to the left
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Shows that ISA associates Pointers with classes
isa & 0x00007ffffffffff8ULL = class
The initIsa(CLS, True,hasCxxDtor) correlation class and extension completes the three steps of class_createInstanceFromZone under alloc.
We can find knowledge involved: ISA structure class algorithm cache logic operation and so on.