isa_analysis.c

#import <Foundation/Foundation.h>
#import <objc/runtime.h>


int main() {



  // extra_rc must be the MSB-most field (so it matches carry/overflow flags)
  // indexed 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 isa_t {
    uintptr_t indexed           : 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)
  };

  NSLog(@"arm64");
  static void *someKey = &someKey;
  id object = [NSObject new];

  struct isa_t *isa = (__bridge struct isa_t *)object;

  NSLog(@"%d", isa->has_assoc);


  objc_setAssociatedObject(object, someKey, @"Hello World!", OBJC_ASSOCIATION_RETAIN);

  NSLog(@"%d", isa->has_assoc);


  NSLog(@"%p", *(void * *)(__bridge void *)object);


# elif __x86_64__
#   define ISA_MASK        0x00007ffffffffff8ULL
#   define ISA_MAGIC_MASK  0x001f800000000001ULL
#   define ISA_MAGIC_VALUE 0x001d800000000001ULL
  struct isa_t {
    uintptr_t indexed           : 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)
  };

  NSLog(@"x86_64");
  static void *someKey = &someKey;
  id object = [NSObject new];

  struct isa_t *isa = (__bridge struct isa_t *)object;

  NSLog(@"%d", isa->has_assoc);


  objc_setAssociatedObject(object, someKey, @"Hello World!", OBJC_ASSOCIATION_RETAIN);

  NSLog(@"%d", isa->has_assoc);


  NSLog(@"%p", *(void * *)(__bridge void *)object);
# else
  // Available bits in isa field are architecture-specific.
#   error unknown architecture
# endif


  return 0;
}

shiftcls must shift left 3 bit