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0531ce1d XL |
1 | //! Caches run-time feature detection so that it only needs to be computed |
2 | //! once. | |
3 | ||
4 | #![allow(dead_code)] // not used on all platforms | |
5 | ||
532ac7d7 | 6 | use crate::sync::atomic::Ordering; |
0531ce1d | 7 | |
74b04a01 | 8 | use crate::sync::atomic::AtomicUsize; |
0531ce1d XL |
9 | |
10 | /// Sets the `bit` of `x`. | |
8faf50e0 XL |
11 | #[inline] |
12 | const fn set_bit(x: u64, bit: u32) -> u64 { | |
0531ce1d XL |
13 | x | 1 << bit |
14 | } | |
15 | ||
16 | /// Tests the `bit` of `x`. | |
8faf50e0 XL |
17 | #[inline] |
18 | const fn test_bit(x: u64, bit: u32) -> bool { | |
0531ce1d XL |
19 | x & (1 << bit) != 0 |
20 | } | |
21 | ||
74b04a01 XL |
22 | /// Unset the `bit of `x`. |
23 | #[inline] | |
24 | const fn unset_bit(x: u64, bit: u32) -> u64 { | |
25 | x & !(1 << bit) | |
26 | } | |
27 | ||
0531ce1d | 28 | /// Maximum number of features that can be cached. |
74b04a01 | 29 | const CACHE_CAPACITY: u32 = 62; |
0531ce1d XL |
30 | |
31 | /// This type is used to initialize the cache | |
32 | #[derive(Copy, Clone)] | |
8faf50e0 | 33 | pub(crate) struct Initializer(u64); |
0531ce1d | 34 | |
48663c56 | 35 | #[allow(clippy::use_self)] |
0531ce1d XL |
36 | impl Default for Initializer { |
37 | fn default() -> Self { | |
38 | Initializer(0) | |
39 | } | |
40 | } | |
41 | ||
74b04a01 XL |
42 | // NOTE: the `debug_assert!` would catch that we do not add more Features than |
43 | // the one fitting our cache. | |
0531ce1d XL |
44 | impl Initializer { |
45 | /// Tests the `bit` of the cache. | |
46 | #[allow(dead_code)] | |
8faf50e0 | 47 | #[inline] |
0731742a | 48 | pub(crate) fn test(self, bit: u32) -> bool { |
0531ce1d XL |
49 | debug_assert!( |
50 | bit < CACHE_CAPACITY, | |
51 | "too many features, time to increase the cache size!" | |
52 | ); | |
53 | test_bit(self.0, bit) | |
54 | } | |
55 | ||
56 | /// Sets the `bit` of the cache. | |
8faf50e0 XL |
57 | #[inline] |
58 | pub(crate) fn set(&mut self, bit: u32) { | |
0531ce1d XL |
59 | debug_assert!( |
60 | bit < CACHE_CAPACITY, | |
61 | "too many features, time to increase the cache size!" | |
62 | ); | |
63 | let v = self.0; | |
64 | self.0 = set_bit(v, bit); | |
65 | } | |
74b04a01 XL |
66 | |
67 | /// Unsets the `bit` of the cache. | |
68 | #[inline] | |
69 | pub(crate) fn unset(&mut self, bit: u32) { | |
70 | debug_assert!( | |
71 | bit < CACHE_CAPACITY, | |
72 | "too many features, time to increase the cache size!" | |
73 | ); | |
74 | let v = self.0; | |
75 | self.0 = unset_bit(v, bit); | |
76 | } | |
0531ce1d XL |
77 | } |
78 | ||
79 | /// This global variable is a cache of the features supported by the CPU. | |
74b04a01 XL |
80 | // Note: on x64, we only use the first slot |
81 | static CACHE: [Cache; 2] = [Cache::uninitialized(), Cache::uninitialized()]; | |
0531ce1d | 82 | |
74b04a01 | 83 | /// Feature cache with capacity for `usize::max_value() - 1` features. |
0531ce1d XL |
84 | /// |
85 | /// Note: the last feature bit is used to represent an | |
86 | /// uninitialized cache. | |
74b04a01 XL |
87 | /// |
88 | /// Note: we can use `Relaxed` atomic operations, because we are only interested | |
89 | /// in the effects of operations on a single memory location. That is, we only | |
90 | /// need "modification order", and not the full-blown "happens before". However, | |
91 | /// we use `SeqCst` just to be on the safe side. | |
92 | struct Cache(AtomicUsize); | |
0531ce1d | 93 | |
0531ce1d | 94 | impl Cache { |
74b04a01 XL |
95 | const CAPACITY: u32 = (core::mem::size_of::<usize>() * 8 - 1) as u32; |
96 | const MASK: usize = (1 << Cache::CAPACITY) - 1; | |
97 | ||
0531ce1d | 98 | /// Creates an uninitialized cache. |
48663c56 | 99 | #[allow(clippy::declare_interior_mutable_const)] |
0531ce1d | 100 | const fn uninitialized() -> Self { |
74b04a01 | 101 | Cache(AtomicUsize::new(usize::max_value())) |
0531ce1d XL |
102 | } |
103 | /// Is the cache uninitialized? | |
8faf50e0 XL |
104 | #[inline] |
105 | pub(crate) fn is_uninitialized(&self) -> bool { | |
74b04a01 | 106 | self.0.load(Ordering::SeqCst) == usize::max_value() |
0531ce1d XL |
107 | } |
108 | ||
109 | /// Is the `bit` in the cache set? | |
8faf50e0 XL |
110 | #[inline] |
111 | pub(crate) fn test(&self, bit: u32) -> bool { | |
74b04a01 | 112 | test_bit(self.0.load(Ordering::SeqCst) as u64, bit) |
0531ce1d XL |
113 | } |
114 | ||
115 | /// Initializes the cache. | |
8faf50e0 | 116 | #[inline] |
74b04a01 XL |
117 | fn initialize(&self, value: usize) { |
118 | self.0.store(value, Ordering::SeqCst); | |
0531ce1d XL |
119 | } |
120 | } | |
121 | ||
74b04a01 XL |
122 | cfg_if::cfg_if! { |
123 | if #[cfg(feature = "std_detect_env_override")] { | |
124 | #[inline(never)] | |
125 | fn initialize(mut value: Initializer) { | |
126 | if let Ok(disable) = crate::env::var("RUST_STD_DETECT_UNSTABLE") { | |
127 | for v in disable.split(" ") { | |
128 | let _ = super::Feature::from_str(v).map(|v| value.unset(v as u32)); | |
129 | } | |
130 | } | |
131 | do_initialize(value); | |
132 | } | |
133 | } else { | |
134 | #[inline] | |
135 | fn initialize(value: Initializer) { | |
136 | do_initialize(value); | |
0531ce1d XL |
137 | } |
138 | } | |
74b04a01 | 139 | } |
0531ce1d | 140 | |
74b04a01 XL |
141 | #[inline] |
142 | fn do_initialize(value: Initializer) { | |
143 | CACHE[0].initialize((value.0) as usize & Cache::MASK); | |
144 | CACHE[1].initialize((value.0 >> Cache::CAPACITY) as usize & Cache::MASK); | |
0531ce1d XL |
145 | } |
146 | ||
532ac7d7 | 147 | /// Tests the `bit` of the storage. If the storage has not been initialized, |
0531ce1d XL |
148 | /// initializes it with the result of `f()`. |
149 | /// | |
150 | /// On its first invocation, it detects the CPU features and caches them in the | |
83c7162d | 151 | /// `CACHE` global variable as an `AtomicU64`. |
0531ce1d XL |
152 | /// |
153 | /// It uses the `Feature` variant to index into this variable as a bitset. If | |
154 | /// the bit is set, the feature is enabled, and otherwise it is disabled. | |
74b04a01 XL |
155 | /// |
156 | /// If the feature `std_detect_env_override` is enabled looks for the env | |
157 | /// variable `RUST_STD_DETECT_UNSTABLE` and uses its its content to disable | |
158 | /// Features that would had been otherwise detected. | |
8faf50e0 XL |
159 | #[inline] |
160 | pub(crate) fn test<F>(bit: u32, f: F) -> bool | |
0531ce1d XL |
161 | where |
162 | F: FnOnce() -> Initializer, | |
163 | { | |
74b04a01 XL |
164 | let (bit, idx) = if bit < Cache::CAPACITY { |
165 | (bit, 0) | |
166 | } else { | |
167 | (bit - Cache::CAPACITY, 1) | |
168 | }; | |
169 | ||
170 | if CACHE[idx].is_uninitialized() { | |
171 | initialize(f()) | |
0531ce1d | 172 | } |
74b04a01 | 173 | CACHE[idx].test(bit) |
0531ce1d | 174 | } |