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use std::mem::offset_of;
use std::ops::Deref;
use std::ptr::{self, NonNull, addr_of};
use std::sync::atomic::AtomicUsize;
use std::sync::atomic::Ordering::{self, Relaxed};
use super::collectible::{Collectible, Link};
use super::collector::Collector;
/// [`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: Link,
}
impl<T> RefCounted<T> {
/// Creates a new [`RefCounted`] that allows ownership sharing.
#[inline]
pub(super) fn new_shared(instance: T) -> NonNull<RefCounted<T>> {
let boxed = Box::new(Self {
instance,
next_or_refcnt: Link::new_shared(),
});
unsafe { NonNull::new_unchecked(Box::into_raw(boxed)) }
}
/// Creates a new [`RefCounted`] that disallows reference counting.
///
/// The reference counter field is never used until the instance is retired.
#[inline]
pub(super) fn new_unique(instance: T) -> NonNull<RefCounted<T>> {
let boxed = Box::new(Self {
instance,
next_or_refcnt: Link::new_unique(),
});
unsafe { NonNull::new_unchecked(Box::into_raw(boxed)) }
}
/// 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 & 1 == 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) const fn get_mut_unique(&mut self) -> &mut T {
&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 & 1, 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 = current.saturating_sub(2);
match self
.ref_cnt()
.compare_exchange_weak(current, new, Relaxed, Relaxed)
{
Ok(_) => break,
Err(actual) => {
current = actual;
}
}
}
current == 1
}
/// Returns a pointer to the instance.
#[inline]
pub(super) const fn inst_ptr(self_ptr: *const Self) -> *const T {
if self_ptr.is_null() {
ptr::null()
} else {
unsafe { addr_of!((*self_ptr).instance) }
}
}
/// Returns a non-null pointer to the instance.
#[inline]
pub(super) const fn inst_non_null_ptr(self_ptr: NonNull<Self>) -> NonNull<T> {
let offset = offset_of!(Self, instance);
unsafe { self_ptr.cast::<u8>().add(offset).cast::<T>() }
}
/// Returns a reference to its reference count.
#[inline]
pub(super) fn ref_cnt(&self) -> &AtomicUsize {
self.next_or_refcnt.ref_cnt()
}
/// Passes a pointer to [`RefCounted`] to the garbage collector.
#[inline]
pub(super) fn pass_to_collector(ptr: *mut Self) {
// The lifetime of the pointer is extended to `'static`; it is not illegal since the safe
// public API does not permit to create a `RefCounted` of a non `'static` type, and those
// that allow to do so are all unsafe methods (e.g., `{Owned, Shard}::new_unchecked`) where
// the doc explicitly states that the pointed-to value may be dropped at an arbitrary time.
#[allow(clippy::transmute_ptr_to_ptr)]
let ptr = unsafe {
std::mem::transmute::<*mut (dyn Collectible + '_), *mut (dyn Collectible + 'static)>(
ptr,
)
};
Collector::collect(Collector::current(), ptr);
}
}
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(&self) -> Option<NonNull<dyn Collectible>> {
self.next_or_refcnt.next_ptr()
}
#[inline]
fn set_next_ptr(&self, next_ptr: Option<NonNull<dyn Collectible>>) {
self.next_or_refcnt.set_next_ptr(next_ptr);
}
}