Ownership tag pointers

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Hi, I would like to share a concept that I implemented and works, maybe could work in swift since it is about ref-counting objects.

The idea is every ref object pointer carries a one‑bit tag in its lowest bit:

  • Own_Ref (bit = 1) – owns the object; responsible for decrementing the reference count on release.

  • Borrow_Ref (bit = 0) – a non‑owning view; release is a no‑op, meaning after checking the bit.

The api is

Operation Meaning Input → Output RC Change
borrow(x) Create a borrowed view. Any → Borrow_Ref None
transfer(x) Move a reference unchanged. Own → Own, Borrow → Borrow None
own(x) Promote to Own_Ref if needed,
ex. when the pointer is returned.		 Borrow → Own (RC++)
Own → Own (no‑op)		 Only if Borrowed
share(x) Create an independent owner. Any → Own_Ref Always RC++
release(x) Drop this reference. Own → RC-- (free if 0)
Borrow → nothing		 Only if Owned

Here using liveness for insertion on a single thread

Function call

x liveness Operation Release x?
Live after call callee(borrow(x)) -
Dead after call callee(transfer(x)) No, responsibility moved

v = get(obj, f) operartion

v liveness Operation
Live after v = share(raw_get(obj, f))
Dead after v = borrow(raw_get(obj, f))

set(obj, f, val) operation

  1. release(transfer(raw_get(obj, f))) – always release the old field value.
  2. Then:
val liveness Operation Release val ?
Dead after raw_set(obj, f, own(val)) No,responsability moved
Live after raw_set(obj, f, share(val)) -

Example of optimization:

  1. share -> borrow when the pointer is does not escape
  2. own -> transfer when the previous pointer is dead.

More optimization could be done, whenever it is safe to do.

This is not a complete description, it is about summarizing the idea.