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
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
|
//! Caches run-time feature detection so that it only needs to be computed
//! once.
#![allow(dead_code)] // not used on all platforms
use core::sync::atomic::Ordering;
use core::sync::atomic::AtomicUsize;
/// Sets the `bit` of `x`.
#[inline]
const fn set_bit(x: u64, bit: u32) -> u64 {
x | 1 << bit
}
/// Tests the `bit` of `x`.
#[inline]
const fn test_bit(x: u64, bit: u32) -> bool {
x & (1 << bit) != 0
}
/// Unset the `bit of `x`.
#[inline]
const fn unset_bit(x: u64, bit: u32) -> u64 {
x & !(1 << bit)
}
/// Maximum number of features that can be cached.
const CACHE_CAPACITY: u32 = 62;
/// This type is used to initialize the cache
#[derive(Copy, Clone)]
pub(crate) struct Initializer(u64);
#[allow(clippy::use_self)]
impl Default for Initializer {
fn default() -> Self {
Initializer(0)
}
}
// NOTE: the `debug_assert!` would catch that we do not add more Features than
// the one fitting our cache.
impl Initializer {
/// Tests the `bit` of the cache.
#[inline]
pub(crate) fn test(self, bit: u32) -> bool {
debug_assert!(
bit < CACHE_CAPACITY,
"too many features, time to increase the cache size!"
);
test_bit(self.0, bit)
}
/// Sets the `bit` of the cache.
#[inline]
pub(crate) fn set(&mut self, bit: u32) {
debug_assert!(
bit < CACHE_CAPACITY,
"too many features, time to increase the cache size!"
);
let v = self.0;
self.0 = set_bit(v, bit);
}
/// Unsets the `bit` of the cache.
#[inline]
pub(crate) fn unset(&mut self, bit: u32) {
debug_assert!(
bit < CACHE_CAPACITY,
"too many features, time to increase the cache size!"
);
let v = self.0;
self.0 = unset_bit(v, bit);
}
}
/// This global variable is a cache of the features supported by the CPU.
// Note: on x64, we only use the first slot
static CACHE: [Cache; 2] = [Cache::uninitialized(), Cache::uninitialized()];
/// Feature cache with capacity for `size_of::<usize::MAX>() * 8 - 1` features.
///
/// Note: 0 is used to represent an uninitialized cache, and (at least) the most
/// significant bit is set on any cache which has been initialized.
///
/// Note: we use `Relaxed` atomic operations, because we are only interested in
/// the effects of operations on a single memory location. That is, we only need
/// "modification order", and not the full-blown "happens before".
struct Cache(AtomicUsize);
impl Cache {
const CAPACITY: u32 = (core::mem::size_of::<usize>() * 8 - 1) as u32;
const MASK: usize = (1 << Cache::CAPACITY) - 1;
const INITIALIZED_BIT: usize = 1usize << Cache::CAPACITY;
/// Creates an uninitialized cache.
#[allow(clippy::declare_interior_mutable_const)]
const fn uninitialized() -> Self {
Cache(AtomicUsize::new(0))
}
/// Is the `bit` in the cache set? Returns `None` if the cache has not been initialized.
#[inline]
pub(crate) fn test(&self, bit: u32) -> Option<bool> {
let cached = self.0.load(Ordering::Relaxed);
if cached == 0 {
None
} else {
Some(test_bit(cached as u64, bit))
}
}
/// Initializes the cache.
#[inline]
fn initialize(&self, value: usize) -> usize {
debug_assert_eq!((value & !Cache::MASK), 0);
self.0
.store(value | Cache::INITIALIZED_BIT, Ordering::Relaxed);
value
}
}
cfg_if::cfg_if! {
if #[cfg(feature = "std_detect_env_override")] {
#[inline]
fn initialize(mut value: Initializer) -> Initializer {
let env = unsafe {
libc::getenv(b"RUST_STD_DETECT_UNSTABLE\0".as_ptr() as *const libc::c_char)
};
if !env.is_null() {
let len = unsafe { libc::strlen(env) };
let env = unsafe { core::slice::from_raw_parts(env as *const u8, len) };
if let Ok(disable) = core::str::from_utf8(env) {
for v in disable.split(" ") {
let _ = super::Feature::from_str(v).map(|v| value.unset(v as u32));
}
}
}
do_initialize(value);
value
}
} else {
#[inline]
fn initialize(value: Initializer) -> Initializer {
do_initialize(value);
value
}
}
}
#[inline]
fn do_initialize(value: Initializer) {
CACHE[0].initialize((value.0) as usize & Cache::MASK);
CACHE[1].initialize((value.0 >> Cache::CAPACITY) as usize & Cache::MASK);
}
// We only have to detect features once, and it's fairly costly, so hint to LLVM
// that it should assume that cache hits are more common than misses (which is
// the point of caching). It's possibly unfortunate that this function needs to
// reach across modules like this to call `os::detect_features`, but it produces
// the best code out of several attempted variants.
//
// The `Initializer` that the cache was initialized with is returned, so that
// the caller can call `test()` on it without having to load the value from the
// cache again.
#[cold]
fn detect_and_initialize() -> Initializer {
initialize(super::os::detect_features())
}
/// Tests the `bit` of the storage. If the storage has not been initialized,
/// initializes it with the result of `os::detect_features()`.
///
/// On its first invocation, it detects the CPU features and caches them in the
/// `CACHE` global variable as an `AtomicU64`.
///
/// It uses the `Feature` variant to index into this variable as a bitset. If
/// the bit is set, the feature is enabled, and otherwise it is disabled.
///
/// If the feature `std_detect_env_override` is enabled looks for the env
/// variable `RUST_STD_DETECT_UNSTABLE` and uses its content to disable
/// Features that would had been otherwise detected.
#[inline]
pub(crate) fn test(bit: u32) -> bool {
let (relative_bit, idx) = if bit < Cache::CAPACITY {
(bit, 0)
} else {
(bit - Cache::CAPACITY, 1)
};
CACHE[idx]
.test(relative_bit)
.unwrap_or_else(|| detect_and_initialize().test(bit))
}
|
