crossbeam_epoch/
deferred.rs

1use alloc::boxed::Box;
2use core::fmt;
3use core::marker::PhantomData;
4use core::mem::{self, MaybeUninit};
5use core::ptr;
6
7/// Number of words a piece of `Data` can hold.
8///
9/// Three words should be enough for the majority of cases. For example, you can fit inside it the
10/// function pointer together with a fat pointer representing an object that needs to be destroyed.
11const DATA_WORDS: usize = 3;
12
13/// Some space to keep a `FnOnce()` object on the stack.
14type Data = [usize; DATA_WORDS];
15
16/// A `FnOnce()` that is stored inline if small, or otherwise boxed on the heap.
17///
18/// This is a handy way of keeping an unsized `FnOnce()` within a sized structure.
19pub(crate) struct Deferred {
20    call: unsafe fn(*mut u8),
21    data: MaybeUninit<Data>,
22    _marker: PhantomData<*mut ()>, // !Send + !Sync
23}
24
25impl fmt::Debug for Deferred {
26    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error> {
27        f.pad("Deferred { .. }")
28    }
29}
30
31impl Deferred {
32    pub(crate) const NO_OP: Self = {
33        fn no_op_call(_raw: *mut u8) {}
34        Self {
35            call: no_op_call,
36            data: MaybeUninit::uninit(),
37            _marker: PhantomData,
38        }
39    };
40
41    /// Constructs a new `Deferred` from a `FnOnce()`.
42    pub(crate) fn new<F: FnOnce()>(f: F) -> Self {
43        let size = mem::size_of::<F>();
44        let align = mem::align_of::<F>();
45
46        unsafe {
47            if size <= mem::size_of::<Data>() && align <= mem::align_of::<Data>() {
48                let mut data = MaybeUninit::<Data>::uninit();
49                ptr::write(data.as_mut_ptr().cast::<F>(), f);
50
51                unsafe fn call<F: FnOnce()>(raw: *mut u8) {
52                    let f: F = ptr::read(raw.cast::<F>());
53                    f();
54                }
55
56                Deferred {
57                    call: call::<F>,
58                    data,
59                    _marker: PhantomData,
60                }
61            } else {
62                let b: Box<F> = Box::new(f);
63                let mut data = MaybeUninit::<Data>::uninit();
64                ptr::write(data.as_mut_ptr().cast::<Box<F>>(), b);
65
66                unsafe fn call<F: FnOnce()>(raw: *mut u8) {
67                    // It's safe to cast `raw` from `*mut u8` to `*mut Box<F>`, because `raw` is
68                    // originally derived from `*mut Box<F>`.
69                    let b: Box<F> = ptr::read(raw.cast::<Box<F>>());
70                    (*b)();
71                }
72
73                Deferred {
74                    call: call::<F>,
75                    data,
76                    _marker: PhantomData,
77                }
78            }
79        }
80    }
81
82    /// Calls the function.
83    #[inline]
84    pub(crate) fn call(mut self) {
85        let call = self.call;
86        unsafe { call(self.data.as_mut_ptr().cast::<u8>()) };
87    }
88}
89
90#[cfg(all(test, not(crossbeam_loom)))]
91mod tests {
92    use super::Deferred;
93    use std::cell::Cell;
94    use std::convert::identity;
95
96    #[test]
97    fn on_stack() {
98        let fired = &Cell::new(false);
99        let a = [0usize; 1];
100
101        let d = Deferred::new(move || {
102            let _ = identity(a);
103            fired.set(true);
104        });
105
106        assert!(!fired.get());
107        d.call();
108        assert!(fired.get());
109    }
110
111    #[test]
112    fn on_heap() {
113        let fired = &Cell::new(false);
114        let a = [0usize; 10];
115
116        let d = Deferred::new(move || {
117            let _ = identity(a);
118            fired.set(true);
119        });
120
121        assert!(!fired.get());
122        d.call();
123        assert!(fired.get());
124    }
125
126    #[test]
127    fn string() {
128        let a = "hello".to_string();
129        let d = Deferred::new(move || assert_eq!(a, "hello"));
130        d.call();
131    }
132
133    #[test]
134    fn boxed_slice_i32() {
135        let a: Box<[i32]> = vec![2, 3, 5, 7].into_boxed_slice();
136        let d = Deferred::new(move || assert_eq!(*a, [2, 3, 5, 7]));
137        d.call();
138    }
139
140    #[test]
141    fn long_slice_usize() {
142        let a: [usize; 5] = [2, 3, 5, 7, 11];
143        let d = Deferred::new(move || assert_eq!(a, [2, 3, 5, 7, 11]));
144        d.call();
145    }
146}
crossbeam_epoch/deferred.rs

crossbeam_epoch/
deferred.rs
