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
use std::cmp::PartialEq;
/// [`Tag`] is a four-state `Enum` that can be embedded in a pointer as the two least
/// significant bits of the pointer value.
#[derive(Clone, Copy, Debug, Eq, Ord, PartialEq, PartialOrd)]
pub enum Tag {
/// None tagged.
None,
/// The first bit is tagged.
First,
/// The second bit is tagged.
Second,
/// Both bits are tagged.
Both,
}
impl Tag {
/// Interprets the [`Tag`] as an integer.
#[inline]
pub(super) const fn value(self) -> usize {
match self {
Self::None => 0,
Self::First => 1,
Self::Second => 2,
Self::Both => 3,
}
}
/// Returns the tag embedded in the pointer.
#[inline]
pub(super) fn into_tag<P>(ptr: *const P) -> Self {
match ptr.addr() & 3 {
0 => Tag::None,
1 => Tag::First,
2 => Tag::Second,
_ => Tag::Both,
}
}
/// Sets a tag, overwriting any existing tag in the pointer.
#[inline]
pub(super) fn update_tag<P>(ptr: *const P, tag: Tag) -> *const P {
ptr.map_addr(|addr| (addr & (!3)) | tag.value())
}
/// Returns the pointer with the tag bits erased.
#[inline]
pub(super) fn unset_tag<P>(ptr: *const P) -> *const P {
ptr.map_addr(|addr| addr & (!3))
}
}
impl TryFrom<u8> for Tag {
type Error = u8;
#[inline]
fn try_from(val: u8) -> Result<Self, Self::Error> {
match val {
0 => Ok(Tag::None),
1 => Ok(Tag::First),
2 => Ok(Tag::Second),
3 => Ok(Tag::Both),
_ => Err(val),
}
}
}
impl From<Tag> for u8 {
#[inline]
fn from(t: Tag) -> Self {
match t {
Tag::None => 0,
Tag::First => 1,
Tag::Second => 2,
Tag::Both => 3,
}
}
}