Make Rc<T>::deref and Arc<T>::deref zero-cost by EFanZh · Pull Request #132553 · rust-lang/rust (original) (raw)

Currently, Rc<T> and Arc<T> store pointers to RcInner<T> and ArcInner<T>. This PR changes the pointers so that they point to T directly instead.

This is based on the assumption that we access the T value more frequently than accessing reference counts. With this change, accessing the data can be done without offsetting pointers from RcInner<T> and ArcInner<T> to their contained data. This change might also enables some possibly useful future optimizations, such as:

• Convert &[Rc<T>] into &[&T] within O(1) time.
• Convert &[Rc<T>] into Vec<&T> utilizing memcpy.
• Convert &Option<Rc<T>> into Option<&T> without branching.
• Make Rc<T> and Arc<T> FFI compatible types where T: Sized.