1use core::sync::atomic::{AtomicUsize, Ordering::Relaxed};
2
3// This structure represents a lazily initialized static usize value. Useful
4// when it is preferable to just rerun initialization instead of locking.
5// unsync_init will invoke an init() function until it succeeds, then return the
6// cached value for future calls.
7//
8// unsync_init supports init() "failing". If the init() method returns UNINIT,
9// that value will be returned as normal, but will not be cached.
10//
11// Users should only depend on the _value_ returned by init() functions.
12// Specifically, for the following init() function:
13// fn init() -> usize {
14// a();
15// let v = b();
16// c();
17// v
18// }
19// the effects of c() or writes to shared memory will not necessarily be
20// observed and additional synchronization methods may be needed.
21pub(crate) struct LazyUsize(AtomicUsize);
22
23impl LazyUsize {
24 pub const fn new() -> Self {
25 Self(AtomicUsize::new(Self::UNINIT))
26 }
27
28 // The initialization is not completed.
29 pub const UNINIT: usize = usize::max_value();
30
31 // Runs the init() function at most once, returning the value of some run of
32 // init(). Multiple callers can run their init() functions in parallel.
33 // init() should always return the same value, if it succeeds.
34 pub fn unsync_init(&self, init: impl FnOnce() -> usize) -> usize {
35 // Relaxed ordering is fine, as we only have a single atomic variable.
36 let mut val: usize = self.0.load(order:Relaxed);
37 if val == Self::UNINIT {
38 val = init();
39 self.0.store(val, order:Relaxed);
40 }
41 val
42 }
43}
44
45// Identical to LazyUsize except with bool instead of usize.
46pub(crate) struct LazyBool(LazyUsize);
47
48impl LazyBool {
49 pub const fn new() -> Self {
50 Self(LazyUsize::new())
51 }
52
53 pub fn unsync_init(&self, init: impl FnOnce() -> bool) -> bool {
54 self.0.unsync_init(|| init() as usize) != 0
55 }
56}
57