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use super::Collectible;
use std::mem::ManuallyDrop;
use std::ops::Deref;
use std::ptr::NonNull;
use std::sync::atomic::AtomicUsize;
use std::sync::atomic::Ordering::{self, Relaxed};
/// [`RefCounted`] stores an instance of type `T`, and a union of a link to the next
/// [`Collectible`] or the reference counter.
pub(super) struct RefCounted<T> {
instance: T,
next_or_refcnt: LinkOrRefCnt,
}
impl<T> RefCounted<T> {
/// Creates a new [`RefCounted`] that allows ownership sharing.
#[inline]
pub(super) const fn new_shared(t: T) -> Self {
Self {
instance: t,
next_or_refcnt: LinkOrRefCnt::new_shared(),
}
}
/// Creates a new [`RefCounted`] that disallows reference counting.
///
/// The reference counter field is never used until the instance is retired.
#[inline]
pub(super) const fn new_unique(t: T) -> Self {
Self {
instance: t,
next_or_refcnt: LinkOrRefCnt::new_unique(),
}
}
/// Tries to add a strong reference to the underlying instance.
///
/// `order` must be as strong as `Acquire` for the caller to correctly validate the newest
/// state of the pointer.
#[inline]
pub(super) fn try_add_ref(&self, order: Ordering) -> bool {
self.ref_cnt()
.fetch_update(
order,
order,
|r| {
if r % 2 == 1 {
Some(r + 2)
} else {
None
}
},
)
.is_ok()
}
/// Returns a mutable reference to the instance if the number of owners is `1`.
#[inline]
pub(super) fn get_mut_shared(&mut self) -> Option<&mut T> {
if self.ref_cnt().load(Relaxed) == 1 {
Some(&mut self.instance)
} else {
None
}
}
/// Returns a mutable reference to the instance if it is uniquely owned.
#[inline]
pub(super) fn get_mut_unique(&mut self) -> &mut T {
debug_assert_eq!(self.ref_cnt().load(Relaxed), 0);
&mut self.instance
}
/// Adds a strong reference to the underlying instance.
#[inline]
pub(super) fn add_ref(&self) {
let mut current = self.ref_cnt().load(Relaxed);
loop {
debug_assert_eq!(current % 2, 1);
debug_assert!(current <= usize::MAX - 2, "reference count overflow");
match self
.ref_cnt()
.compare_exchange_weak(current, current + 2, Relaxed, Relaxed)
{
Ok(_) => break,
Err(actual) => {
current = actual;
}
}
}
}
/// Drops a strong reference to the underlying instance.
///
/// Returns `true` if it the last reference was dropped.
#[inline]
pub(super) fn drop_ref(&self) -> bool {
// It does not have to be a load-acquire as everything's synchronized via the global
// epoch.
let mut current = self.ref_cnt().load(Relaxed);
loop {
debug_assert_ne!(current, 0);
let new = if current <= 1 { 0 } else { current - 2 };
match self
.ref_cnt()
.compare_exchange_weak(current, new, Relaxed, Relaxed)
{
Ok(_) => break,
Err(actual) => {
current = actual;
}
}
}
current == 1
}
/// Returns a reference to its reference count.
#[inline]
pub(super) fn ref_cnt(&self) -> &AtomicUsize {
unsafe { &self.next_or_refcnt.refcnt.0 }
}
/// Returns a `dyn Collectible` reference to `self`.
#[inline]
pub(super) fn as_collectible(&self) -> &dyn Collectible {
self
}
}
impl<T> Deref for RefCounted<T> {
type Target = T;
#[inline]
fn deref(&self) -> &Self::Target {
&self.instance
}
}
impl<T> Collectible for RefCounted<T> {
#[inline]
fn next_ptr_mut(&mut self) -> &mut Option<NonNull<dyn Collectible>> {
unsafe { &mut self.next_or_refcnt.next }
}
}
/// [`LinkOrRefCnt`] is a union of a dynamic pointer to [`Collectible`] and a reference count.
pub(super) union LinkOrRefCnt {
next: Option<NonNull<dyn Collectible>>,
refcnt: ManuallyDrop<(AtomicUsize, usize)>,
}
impl LinkOrRefCnt {
#[inline]
const fn new_shared() -> Self {
LinkOrRefCnt {
refcnt: ManuallyDrop::new((AtomicUsize::new(1), 0)),
}
}
#[inline]
const fn new_unique() -> Self {
LinkOrRefCnt {
refcnt: ManuallyDrop::new((AtomicUsize::new(0), 0)),
}
}
}