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use std::alloc::{Layout, alloc, dealloc};
use std::mem::offset_of;
use std::ops::Deref;
use std::ptr::{NonNull, null};
use std::sync::atomic::Ordering::{self, AcqRel, Acquire, Relaxed, Release};
use super::collector::Collector;
use super::link::Link;
/// [`RefCounted`] stores an instance of type `T`, and a union of a link to the next
/// garbage instance and the reference counter.
#[repr(C)] // Prevent `link` from being reordered before `instance`.
pub(super) struct RefCounted<T> {
/// Instance of type `T`.
instance: T,
/// Link to the next garbage instance.
link: Link,
}
impl<T> RefCounted<T> {
/// Creates a new [`RefCounted`] that allows ownership sharing.
#[inline]
pub(super) fn new_shared<F: FnOnce() -> T>(f: F) -> NonNull<RefCounted<T>> {
unsafe {
let ptr = alloc(Layout::new::<Self>()).cast::<Self>();
ptr.write(Self {
instance: f(),
link: Link::new_shared(|ptr: *mut Link| {
Self::dealloc(Link::cast::<Self>(ptr, offset_of!(Self, link)));
}),
});
NonNull::new_unchecked(ptr)
}
}
/// 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<F: FnOnce() -> T>(f: F) -> NonNull<RefCounted<T>> {
unsafe {
let ptr = alloc(Layout::new::<Self>()).cast::<Self>();
ptr.write(Self {
instance: f(),
link: Link::new_unique(|ptr: *mut Link| {
Self::dealloc(Link::cast::<Self>(ptr, offset_of!(Self, link)));
}),
});
NonNull::new_unchecked(ptr)
}
}
/// 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.link
.ref_cnt()
.fetch_update(
order,
order,
#[inline]
|r| {
if r.addr() & 1 == 1 {
Some(r.map_addr(|addr| addr + 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.link.ref_cnt().load(Relaxed).addr() == 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
}
/// Drops and deallocates itself.
#[inline]
pub(super) fn dealloc(ptr: *mut Self) {
unsafe {
ptr.map_addr(|addr| addr + offset_of!(Self, instance))
.cast::<T>()
.drop_in_place();
dealloc(ptr.cast::<u8>(), Layout::new::<Self>());
}
}
/// Adds a strong reference to the underlying instance.
#[inline]
pub(super) fn add_ref(&self) {
let mut current = self.link.ref_cnt().load(Relaxed);
loop {
debug_assert_eq!(current.addr() & 1, 1);
debug_assert!(current.addr() <= usize::MAX - 2, "reference count overflow");
match self.link.ref_cnt().compare_exchange_weak(
current,
current.map_addr(|addr| addr + 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.link.ref_cnt().load(Relaxed);
loop {
debug_assert_ne!(current.addr(), 0);
match self.link.ref_cnt().compare_exchange_weak(
current,
current.map_addr(|addr| addr.saturating_sub(2)),
Relaxed,
Relaxed,
) {
Ok(_) => break,
Err(actual) => {
current = actual;
}
}
}
current.addr() == 1
}
/// Returns a pointer to the instance.
#[inline]
pub(super) const fn inst_ptr(self_ptr: *const Self) -> *const T {
self_ptr.cast::<T>()
}
/// Returns a pointer to the [`Link`].
#[inline]
pub(super) fn link_ptr(self_ptr: *const Self) -> *const Link {
self_ptr
.map_addr(|addr| addr + offset_of!(Self, link))
.cast::<Link>()
}
/// Returns a non-null pointer to the instance.
#[inline]
pub(super) const fn inst_non_null_ptr(self_ptr: NonNull<Self>) -> NonNull<T> {
self_ptr.cast::<T>()
}
/// Passes a pointer to [`RefCounted`] to the garbage collector.
#[inline]
pub(super) fn pass_to_collector(ptr: *mut Self) {
Collector::collect(Collector::current(), Self::link_ptr(ptr).cast_mut(), null());
}
}
impl<T> Deref for RefCounted<T> {
type Target = T;
#[inline]
fn deref(&self) -> &Self::Target {
&self.instance
}
}
/// Returns a safe memory [`Ordering`] for loading a [`RefCounted`].
#[inline]
pub(super) const fn safe_load_ordering(mo: Ordering) -> Ordering {
if cfg!(all(
any(target_arch = "arm", target_arch = "aarch64"),
not(any(miri, feature = "loom")),
)) {
// The condition was copied from `crossbeam-utils`:
// https://github.com/crossbeam-rs/crossbeam-utils/blob/master/src/atomic/consume.rs#L28
mo
} else {
// Dependent loads are never reordered in all the supported platforms, but to be on the
// safe side, `Relaxed` and `Release` are upgraded to `Acquire` and `AcqRel`.
match mo {
Relaxed => Acquire,
Release => AcqRel,
_ => mo,
}
}
}
/// Puts an appropriate memory fence after a load operation.
#[inline]
pub(super) fn post_load_fence(mo: Ordering) {
if cfg!(all(
any(target_arch = "arm", target_arch = "aarch64"),
not(any(miri, feature = "loom")),
)) && mo == Relaxed
{
// See `safe_load_ordering`.
std::sync::atomic::compiler_fence(Acquire);
}
}
/// Returns a safe memory [`Ordering`] for storing a [`RefCounted`].
#[inline]
pub(super) const fn safe_store_ordering(mo: Ordering) -> Ordering {
match mo {
Relaxed => Release,
Acquire => AcqRel,
_ => mo,
}
}