iOS isa

一般都说isa指针，那么要了解isa，就得先搞清楚何为指针 https://zhidao.baidu.com/question/1372805343211755539.html

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objc-private.h中，搜索isa，即可查询到如下信息

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

    Class cls;
    uintptr_t bits;

#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)

    // future expansion:
    // uintptr_t fast_rr : 1;     // no r/r overrides
    // uintptr_t lock : 2;        // lock for atomic property, @synch
    // uintptr_t extraBytes : 1;  // allocated with extra bytes

# if __arm64__
#   define ISA_MASK        0x0000000ffffffff8ULL
#   define ISA_MAGIC_MASK  0x000003f000000001ULL
#   define ISA_MAGIC_VALUE 0x000001a000000001ULL
    struct {
        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
    struct {
        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


#if SUPPORT_INDEXED_ISA

# if  __ARM_ARCH_7K__ >= 2

#   define ISA_INDEX_IS_NPI      1
#   define ISA_INDEX_MASK        0x0001FFFC
#   define ISA_INDEX_SHIFT       2
#   define ISA_INDEX_BITS        15
#   define ISA_INDEX_COUNT       (1 << ISA_INDEX_BITS)
#   define ISA_INDEX_MAGIC_MASK  0x001E0001
#   define ISA_INDEX_MAGIC_VALUE 0x001C0001
    struct {
        uintptr_t nonpointer        : 1;
        uintptr_t has_assoc         : 1;
        uintptr_t indexcls          : 15;
        uintptr_t magic             : 4;
        uintptr_t has_cxx_dtor      : 1;
        uintptr_t weakly_referenced : 1;
        uintptr_t deallocating      : 1;
        uintptr_t has_sidetable_rc  : 1;
        uintptr_t extra_rc          : 7;
#       define RC_ONE   (1ULL<<25)
#       define RC_HALF  (1ULL<<6)
    };

# else
#   error unknown architecture for indexed isa
# endif

// SUPPORT_INDEXED_ISA
#endif

};

isa属于C语言共用体