Objective-C 内部机制：Tagged Pointer 对象

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#if __arm64__
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  // ARM64 uses a new tagged pointer scheme...
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# define _OBJC_TAG_MASK (1UL<<63)
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#elif (TARGET_OS_OSX || TARGET_OS_MACCATALYST) && __x86_64__
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  // 64-bit Mac - tag bit is LSB
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# define _OBJC_TAG_MASK 1UL
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#else
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  // Everything else - tag bit is MSB
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# define _OBJC_TAG_MASK (1UL<<63)
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#endif
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static inline bool
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_objc_isTaggedPointer(const void *ptr) {
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  return ((uintptr_t)ptr & _OBJC_TAG_MASK) == _OBJC_TAG_MASK;
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}

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inline Class
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objc_object::getIsa() {
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  if (fastpath(!isTaggedPointer())) return ISA(/*authenticated*/true);
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  extern objc_class OBJC_CLASS_$___NSUnrecognizedTaggedPointer;
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  uintptr_t slot, ptr = (uintptr_t)this;
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  Class cls;
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  slot = (ptr >> _OBJC_TAG_SLOT_SHIFT) & _OBJC_TAG_SLOT_MASK;
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  cls = objc_tag_classes[slot];
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  if (slowpath(cls == (Class)&OBJC_CLASS_$___NSUnrecognizedTaggedPointer)) {
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      slot = (ptr >> _OBJC_TAG_EXT_SLOT_SHIFT) & _OBJC_TAG_EXT_SLOT_MASK;
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      cls = objc_tag_ext_classes[slot];
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  }
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  return cls;
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}

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(lldb) reg re x0 x1
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      x0 = 0x0000000000000003
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      x1 = 0x0000000203a994f0  (void *)0x0000000203a99518: __NSCFNumber

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@implementation __NSCFNumber
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- (NSInteger)integerValue {
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  return [self longValue];
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}
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- (long)longValue {
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  long longValue;
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  CFNumberGetValue((__bridge CFNumberRef)self, kCFNumberSInt64Type, &longValue);
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  return longValue;
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}
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@end
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Boolean CFNumberGetValue(CFNumberRef number, CFNumberType theType, void *valuePtr) {
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  if (_objc_isTaggedPointer(number)) {
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    if (_objc_getTaggedPointerTag(number) == OBJC_TAG_NSNumber) {
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      long localMemory;
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      valuePtr = valuePtr ?: (void *)&localMemory;
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      uintptr_t value = _objc_getTaggedPointerValue(number);
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      uintptr_t shift = (value & 0x08) ? 0x4 : 0x6;
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      CFNumberType type = __CFNumberTypeTable[theType].canonicalType;
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      if (type <= kCFNumberFloat64Type) {
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        value = value >> shift;
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        switch (type) {
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        case kCFNumberSInt8Type:
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          *(uint8_t *)valuePtr = (uint8_t)value;
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          break;
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        case kCFNumberSInt16Type:
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          *(uint16_t *)valuePtr = (uint16_t)value;
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          break;
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        case kCFNumberSInt32Type:
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          *(uint32_t *)valuePtr = (uint32_t)value;
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          break;
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        case kCFNumberSInt64Type:
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          *(uint64_t *)valuePtr = (uint64_t)value;
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          break;
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        case kCFNumberFloat32Type:
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          *(float *)valuePtr = (float)value;
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          break;
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        case kCFNumberFloat64Type:
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          *(double *)valuePtr = (double)value;
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          break;
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        }
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        return true;
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      } else {
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        return __CFNumberGetValueCompat(number, theType, valuePtr);
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      }
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    } else {
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      theType = __CFNumberTypeTable[theType].canonicalType;
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      return [(__bridge id)number _getValue:valuePtr forType:theType];
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    }
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  } else {
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    // ...
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  }
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}