1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
use super::collectible::{Collectible, Link};
use super::collector::Collector;
use std::ops::Deref;
use std::ptr::{self, addr_of, 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: Link,
}
impl<T> RefCounted<T> {
/// Creates a new [`RefCounted`] that allows ownership sharing.
#[inline]
pub(super) fn new_shared(instance: T) -> *const RefCounted<T> {
let boxed = Box::new(Self {
instance,
next_or_refcnt: Link::new_shared(),
});
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) -> *const RefCounted<T> {
let boxed = Box::new(Self {
instance,
next_or_refcnt: Link::new_unique(),
});
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 % 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 pointer to the instance.
#[inline]
pub(super) 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 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) {
let dyn_mut_ptr: *mut dyn Collectible = ptr;
#[allow(clippy::transmute_ptr_to_ptr)]
let dyn_mut_ptr: *mut dyn Collectible = unsafe { std::mem::transmute(dyn_mut_ptr) };
unsafe {
Collector::collect(Collector::current(), dyn_mut_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);
}
}