Source code
Revision control
Copy as Markdown
Other Tools
// Necessary for implementing atomic methods for `AtomicUnit`
#![allow(clippy::unit_arg)]
use crate::primitive::sync::atomic::{self, AtomicBool};
use core::cell::UnsafeCell;
use core::cmp;
use core::fmt;
use core::mem::{self, ManuallyDrop, MaybeUninit};
use core::sync::atomic::Ordering;
use core::ptr;
#[cfg(feature = "std")]
use std::panic::{RefUnwindSafe, UnwindSafe};
use super::seq_lock::SeqLock;
/// A thread-safe mutable memory location.
///
/// This type is equivalent to [`Cell`], except it can also be shared among multiple threads.
///
/// Operations on `AtomicCell`s use atomic instructions whenever possible, and synchronize using
/// global locks otherwise. You can call [`AtomicCell::<T>::is_lock_free()`] to check whether
/// atomic instructions or locks will be used.
///
/// Atomic loads use the [`Acquire`] ordering and atomic stores use the [`Release`] ordering.
///
/// [`Cell`]: std::cell::Cell
/// [`AtomicCell::<T>::is_lock_free()`]: AtomicCell::is_lock_free
/// [`Acquire`]: std::sync::atomic::Ordering::Acquire
/// [`Release`]: std::sync::atomic::Ordering::Release
#[repr(transparent)]
pub struct AtomicCell<T> {
/// The inner value.
///
/// If this value can be transmuted into a primitive atomic type, it will be treated as such.
/// Otherwise, all potentially concurrent operations on this data will be protected by a global
/// lock.
///
/// Using MaybeUninit to prevent code outside the cell from observing partially initialized state:
///
/// Note:
/// - we'll never store uninitialized `T` due to our API only using initialized `T`.
value: UnsafeCell<MaybeUninit<T>>,
}
unsafe impl<T: Send> Send for AtomicCell<T> {}
unsafe impl<T: Send> Sync for AtomicCell<T> {}
#[cfg(feature = "std")]
impl<T> UnwindSafe for AtomicCell<T> {}
#[cfg(feature = "std")]
impl<T> RefUnwindSafe for AtomicCell<T> {}
impl<T> AtomicCell<T> {
/// Creates a new atomic cell initialized with `val`.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::atomic::AtomicCell;
///
/// let a = AtomicCell::new(7);
/// ```
pub const fn new(val: T) -> AtomicCell<T> {
AtomicCell {
value: UnsafeCell::new(MaybeUninit::new(val)),
}
}
/// Consumes the atomic and returns the contained value.
///
/// This is safe because passing `self` by value guarantees that no other threads are
/// concurrently accessing the atomic data.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::atomic::AtomicCell;
///
/// let a = AtomicCell::new(7);
/// let v = a.into_inner();
///
/// assert_eq!(v, 7);
/// ```
pub fn into_inner(self) -> T {
let this = ManuallyDrop::new(self);
// SAFETY:
// - passing `self` by value guarantees that no other threads are concurrently
// accessing the atomic data
// - the raw pointer passed in is valid because we got it from an owned value.
// - `ManuallyDrop` prevents double dropping `T`
unsafe { this.as_ptr().read() }
}
/// Returns `true` if operations on values of this type are lock-free.
///
/// If the compiler or the platform doesn't support the necessary atomic instructions,
/// `AtomicCell<T>` will use global locks for every potentially concurrent atomic operation.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::atomic::AtomicCell;
///
/// // This type is internally represented as `AtomicUsize` so we can just use atomic
/// // operations provided by it.
/// assert_eq!(AtomicCell::<usize>::is_lock_free(), true);
///
/// // A wrapper struct around `isize`.
/// struct Foo {
/// bar: isize,
/// }
/// // `AtomicCell<Foo>` will be internally represented as `AtomicIsize`.
/// assert_eq!(AtomicCell::<Foo>::is_lock_free(), true);
///
/// // Operations on zero-sized types are always lock-free.
/// assert_eq!(AtomicCell::<()>::is_lock_free(), true);
///
/// // Very large types cannot be represented as any of the standard atomic types, so atomic
/// // operations on them will have to use global locks for synchronization.
/// assert_eq!(AtomicCell::<[u8; 1000]>::is_lock_free(), false);
/// ```
pub const fn is_lock_free() -> bool {
atomic_is_lock_free::<T>()
}
/// Stores `val` into the atomic cell.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::atomic::AtomicCell;
///
/// let a = AtomicCell::new(7);
///
/// assert_eq!(a.load(), 7);
/// a.store(8);
/// assert_eq!(a.load(), 8);
/// ```
pub fn store(&self, val: T) {
if mem::needs_drop::<T>() {
drop(self.swap(val));
} else {
unsafe {
atomic_store(self.as_ptr(), val);
}
}
}
/// Stores `val` into the atomic cell and returns the previous value.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::atomic::AtomicCell;
///
/// let a = AtomicCell::new(7);
///
/// assert_eq!(a.load(), 7);
/// assert_eq!(a.swap(8), 7);
/// assert_eq!(a.load(), 8);
/// ```
pub fn swap(&self, val: T) -> T {
unsafe { atomic_swap(self.as_ptr(), val) }
}
/// Returns a raw pointer to the underlying data in this atomic cell.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::atomic::AtomicCell;
///
/// let a = AtomicCell::new(5);
///
/// let ptr = a.as_ptr();
/// ```
#[inline]
pub fn as_ptr(&self) -> *mut T {
self.value.get().cast::<T>()
}
}
impl<T: Default> AtomicCell<T> {
/// Takes the value of the atomic cell, leaving `Default::default()` in its place.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::atomic::AtomicCell;
///
/// let a = AtomicCell::new(5);
/// let five = a.take();
///
/// assert_eq!(five, 5);
/// assert_eq!(a.into_inner(), 0);
/// ```
pub fn take(&self) -> T {
self.swap(Default::default())
}
}
impl<T: Copy> AtomicCell<T> {
/// Loads a value from the atomic cell.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::atomic::AtomicCell;
///
/// let a = AtomicCell::new(7);
///
/// assert_eq!(a.load(), 7);
/// ```
pub fn load(&self) -> T {
unsafe { atomic_load(self.as_ptr()) }
}
}
impl<T: Copy + Eq> AtomicCell<T> {
/// If the current value equals `current`, stores `new` into the atomic cell.
///
/// The return value is always the previous value. If it is equal to `current`, then the value
/// was updated.
///
/// # Examples
///
/// ```
/// # #![allow(deprecated)]
/// use crossbeam_utils::atomic::AtomicCell;
///
/// let a = AtomicCell::new(1);
///
/// assert_eq!(a.compare_and_swap(2, 3), 1);
/// assert_eq!(a.load(), 1);
///
/// assert_eq!(a.compare_and_swap(1, 2), 1);
/// assert_eq!(a.load(), 2);
/// ```
// TODO: remove in the next major version.
#[deprecated(note = "Use `compare_exchange` instead")]
pub fn compare_and_swap(&self, current: T, new: T) -> T {
match self.compare_exchange(current, new) {
Ok(v) => v,
Err(v) => v,
}
}
/// If the current value equals `current`, stores `new` into the atomic cell.
///
/// The return value is a result indicating whether the new value was written and containing
/// the previous value. On success this value is guaranteed to be equal to `current`.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::atomic::AtomicCell;
///
/// let a = AtomicCell::new(1);
///
/// assert_eq!(a.compare_exchange(2, 3), Err(1));
/// assert_eq!(a.load(), 1);
///
/// assert_eq!(a.compare_exchange(1, 2), Ok(1));
/// assert_eq!(a.load(), 2);
/// ```
pub fn compare_exchange(&self, current: T, new: T) -> Result<T, T> {
unsafe { atomic_compare_exchange_weak(self.as_ptr(), current, new) }
}
/// Fetches the value, and applies a function to it that returns an optional
/// new value. Returns a `Result` of `Ok(previous_value)` if the function returned `Some(_)`, else
/// `Err(previous_value)`.
///
/// Note: This may call the function multiple times if the value has been changed from other threads in
/// the meantime, as long as the function returns `Some(_)`, but the function will have been applied
/// only once to the stored value.
///
/// # Examples
///
/// ```rust
/// use crossbeam_utils::atomic::AtomicCell;
///
/// let a = AtomicCell::new(7);
/// assert_eq!(a.fetch_update(|_| None), Err(7));
/// assert_eq!(a.fetch_update(|a| Some(a + 1)), Ok(7));
/// assert_eq!(a.fetch_update(|a| Some(a + 1)), Ok(8));
/// assert_eq!(a.load(), 9);
/// ```
#[inline]
pub fn fetch_update<F>(&self, mut f: F) -> Result<T, T>
where
F: FnMut(T) -> Option<T>,
{
let mut prev = self.load();
while let Some(next) = f(prev) {
match self.compare_exchange(prev, next) {
x @ Ok(_) => return x,
Err(next_prev) => prev = next_prev,
}
}
Err(prev)
}
}
// `MaybeUninit` prevents `T` from being dropped, so we need to implement `Drop`
// for `AtomicCell` to avoid leaks of non-`Copy` types.
impl<T> Drop for AtomicCell<T> {
fn drop(&mut self) {
if mem::needs_drop::<T>() {
// SAFETY:
// - the mutable reference guarantees that no other threads are concurrently accessing the atomic data
// - the raw pointer passed in is valid because we got it from a reference
// - `MaybeUninit` prevents double dropping `T`
unsafe {
self.as_ptr().drop_in_place();
}
}
}
}
macro_rules! impl_arithmetic {
($t:ty, fallback, $example:tt) => {
impl AtomicCell<$t> {
/// Increments the current value by `val` and returns the previous value.
///
/// The addition wraps on overflow.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::atomic::AtomicCell;
///
#[doc = $example]
///
/// assert_eq!(a.fetch_add(3), 7);
/// assert_eq!(a.load(), 10);
/// ```
#[inline]
pub fn fetch_add(&self, val: $t) -> $t {
let _guard = lock(self.as_ptr() as usize).write();
let value = unsafe { &mut *(self.as_ptr()) };
let old = *value;
*value = value.wrapping_add(val);
old
}
/// Decrements the current value by `val` and returns the previous value.
///
/// The subtraction wraps on overflow.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::atomic::AtomicCell;
///
#[doc = $example]
///
/// assert_eq!(a.fetch_sub(3), 7);
/// assert_eq!(a.load(), 4);
/// ```
#[inline]
pub fn fetch_sub(&self, val: $t) -> $t {
let _guard = lock(self.as_ptr() as usize).write();
let value = unsafe { &mut *(self.as_ptr()) };
let old = *value;
*value = value.wrapping_sub(val);
old
}
/// Applies bitwise "and" to the current value and returns the previous value.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::atomic::AtomicCell;
///
#[doc = $example]
///
/// assert_eq!(a.fetch_and(3), 7);
/// assert_eq!(a.load(), 3);
/// ```
#[inline]
pub fn fetch_and(&self, val: $t) -> $t {
let _guard = lock(self.as_ptr() as usize).write();
let value = unsafe { &mut *(self.as_ptr()) };
let old = *value;
*value &= val;
old
}
/// Applies bitwise "nand" to the current value and returns the previous value.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::atomic::AtomicCell;
///
#[doc = $example]
///
/// assert_eq!(a.fetch_nand(3), 7);
/// assert_eq!(a.load(), !(7 & 3));
/// ```
#[inline]
pub fn fetch_nand(&self, val: $t) -> $t {
let _guard = lock(self.as_ptr() as usize).write();
let value = unsafe { &mut *(self.as_ptr()) };
let old = *value;
*value = !(old & val);
old
}
/// Applies bitwise "or" to the current value and returns the previous value.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::atomic::AtomicCell;
///
#[doc = $example]
///
/// assert_eq!(a.fetch_or(16), 7);
/// assert_eq!(a.load(), 23);
/// ```
#[inline]
pub fn fetch_or(&self, val: $t) -> $t {
let _guard = lock(self.as_ptr() as usize).write();
let value = unsafe { &mut *(self.as_ptr()) };
let old = *value;
*value |= val;
old
}
/// Applies bitwise "xor" to the current value and returns the previous value.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::atomic::AtomicCell;
///
#[doc = $example]
///
/// assert_eq!(a.fetch_xor(2), 7);
/// assert_eq!(a.load(), 5);
/// ```
#[inline]
pub fn fetch_xor(&self, val: $t) -> $t {
let _guard = lock(self.as_ptr() as usize).write();
let value = unsafe { &mut *(self.as_ptr()) };
let old = *value;
*value ^= val;
old
}
/// Compares and sets the maximum of the current value and `val`,
/// and returns the previous value.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::atomic::AtomicCell;
///
#[doc = $example]
///
/// assert_eq!(a.fetch_max(2), 7);
/// assert_eq!(a.load(), 7);
/// ```
#[inline]
pub fn fetch_max(&self, val: $t) -> $t {
let _guard = lock(self.as_ptr() as usize).write();
let value = unsafe { &mut *(self.as_ptr()) };
let old = *value;
*value = cmp::max(old, val);
old
}
/// Compares and sets the minimum of the current value and `val`,
/// and returns the previous value.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::atomic::AtomicCell;
///
#[doc = $example]
///
/// assert_eq!(a.fetch_min(2), 7);
/// assert_eq!(a.load(), 2);
/// ```
#[inline]
pub fn fetch_min(&self, val: $t) -> $t {
let _guard = lock(self.as_ptr() as usize).write();
let value = unsafe { &mut *(self.as_ptr()) };
let old = *value;
*value = cmp::min(old, val);
old
}
}
};
($t:ty, $atomic:ty, $example:tt) => {
impl AtomicCell<$t> {
/// Increments the current value by `val` and returns the previous value.
///
/// The addition wraps on overflow.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::atomic::AtomicCell;
///
#[doc = $example]
///
/// assert_eq!(a.fetch_add(3), 7);
/// assert_eq!(a.load(), 10);
/// ```
#[inline]
pub fn fetch_add(&self, val: $t) -> $t {
if can_transmute::<$t, $atomic>() {
let a = unsafe { &*(self.as_ptr() as *const $atomic) };
a.fetch_add(val, Ordering::AcqRel)
} else {
let _guard = lock(self.as_ptr() as usize).write();
let value = unsafe { &mut *(self.as_ptr()) };
let old = *value;
*value = value.wrapping_add(val);
old
}
}
/// Decrements the current value by `val` and returns the previous value.
///
/// The subtraction wraps on overflow.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::atomic::AtomicCell;
///
#[doc = $example]
///
/// assert_eq!(a.fetch_sub(3), 7);
/// assert_eq!(a.load(), 4);
/// ```
#[inline]
pub fn fetch_sub(&self, val: $t) -> $t {
if can_transmute::<$t, $atomic>() {
let a = unsafe { &*(self.as_ptr() as *const $atomic) };
a.fetch_sub(val, Ordering::AcqRel)
} else {
let _guard = lock(self.as_ptr() as usize).write();
let value = unsafe { &mut *(self.as_ptr()) };
let old = *value;
*value = value.wrapping_sub(val);
old
}
}
/// Applies bitwise "and" to the current value and returns the previous value.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::atomic::AtomicCell;
///
#[doc = $example]
///
/// assert_eq!(a.fetch_and(3), 7);
/// assert_eq!(a.load(), 3);
/// ```
#[inline]
pub fn fetch_and(&self, val: $t) -> $t {
if can_transmute::<$t, $atomic>() {
let a = unsafe { &*(self.as_ptr() as *const $atomic) };
a.fetch_and(val, Ordering::AcqRel)
} else {
let _guard = lock(self.as_ptr() as usize).write();
let value = unsafe { &mut *(self.as_ptr()) };
let old = *value;
*value &= val;
old
}
}
/// Applies bitwise "nand" to the current value and returns the previous value.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::atomic::AtomicCell;
///
#[doc = $example]
///
/// assert_eq!(a.fetch_nand(3), 7);
/// assert_eq!(a.load(), !(7 & 3));
/// ```
#[inline]
pub fn fetch_nand(&self, val: $t) -> $t {
if can_transmute::<$t, $atomic>() {
let a = unsafe { &*(self.as_ptr() as *const $atomic) };
a.fetch_nand(val, Ordering::AcqRel)
} else {
let _guard = lock(self.as_ptr() as usize).write();
let value = unsafe { &mut *(self.as_ptr()) };
let old = *value;
*value = !(old & val);
old
}
}
/// Applies bitwise "or" to the current value and returns the previous value.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::atomic::AtomicCell;
///
#[doc = $example]
///
/// assert_eq!(a.fetch_or(16), 7);
/// assert_eq!(a.load(), 23);
/// ```
#[inline]
pub fn fetch_or(&self, val: $t) -> $t {
if can_transmute::<$t, $atomic>() {
let a = unsafe { &*(self.as_ptr() as *const $atomic) };
a.fetch_or(val, Ordering::AcqRel)
} else {
let _guard = lock(self.as_ptr() as usize).write();
let value = unsafe { &mut *(self.as_ptr()) };
let old = *value;
*value |= val;
old
}
}
/// Applies bitwise "xor" to the current value and returns the previous value.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::atomic::AtomicCell;
///
#[doc = $example]
///
/// assert_eq!(a.fetch_xor(2), 7);
/// assert_eq!(a.load(), 5);
/// ```
#[inline]
pub fn fetch_xor(&self, val: $t) -> $t {
if can_transmute::<$t, $atomic>() {
let a = unsafe { &*(self.as_ptr() as *const $atomic) };
a.fetch_xor(val, Ordering::AcqRel)
} else {
let _guard = lock(self.as_ptr() as usize).write();
let value = unsafe { &mut *(self.as_ptr()) };
let old = *value;
*value ^= val;
old
}
}
/// Compares and sets the maximum of the current value and `val`,
/// and returns the previous value.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::atomic::AtomicCell;
///
#[doc = $example]
///
/// assert_eq!(a.fetch_max(9), 7);
/// assert_eq!(a.load(), 9);
/// ```
#[inline]
pub fn fetch_max(&self, val: $t) -> $t {
if can_transmute::<$t, $atomic>() {
// TODO: Atomic*::fetch_max requires Rust 1.45.
self.fetch_update(|old| Some(cmp::max(old, val))).unwrap()
} else {
let _guard = lock(self.as_ptr() as usize).write();
let value = unsafe { &mut *(self.as_ptr()) };
let old = *value;
*value = cmp::max(old, val);
old
}
}
/// Compares and sets the minimum of the current value and `val`,
/// and returns the previous value.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::atomic::AtomicCell;
///
#[doc = $example]
///
/// assert_eq!(a.fetch_min(2), 7);
/// assert_eq!(a.load(), 2);
/// ```
#[inline]
pub fn fetch_min(&self, val: $t) -> $t {
if can_transmute::<$t, $atomic>() {
// TODO: Atomic*::fetch_min requires Rust 1.45.
self.fetch_update(|old| Some(cmp::min(old, val))).unwrap()
} else {
let _guard = lock(self.as_ptr() as usize).write();
let value = unsafe { &mut *(self.as_ptr()) };
let old = *value;
*value = cmp::min(old, val);
old
}
}
}
};
}
impl_arithmetic!(u8, atomic::AtomicU8, "let a = AtomicCell::new(7u8);");
impl_arithmetic!(i8, atomic::AtomicI8, "let a = AtomicCell::new(7i8);");
impl_arithmetic!(u16, atomic::AtomicU16, "let a = AtomicCell::new(7u16);");
impl_arithmetic!(i16, atomic::AtomicI16, "let a = AtomicCell::new(7i16);");
impl_arithmetic!(u32, atomic::AtomicU32, "let a = AtomicCell::new(7u32);");
impl_arithmetic!(i32, atomic::AtomicI32, "let a = AtomicCell::new(7i32);");
#[cfg(not(crossbeam_no_atomic_64))]
impl_arithmetic!(u64, atomic::AtomicU64, "let a = AtomicCell::new(7u64);");
#[cfg(not(crossbeam_no_atomic_64))]
impl_arithmetic!(i64, atomic::AtomicI64, "let a = AtomicCell::new(7i64);");
#[cfg(crossbeam_no_atomic_64)]
impl_arithmetic!(u64, fallback, "let a = AtomicCell::new(7u64);");
#[cfg(crossbeam_no_atomic_64)]
impl_arithmetic!(i64, fallback, "let a = AtomicCell::new(7i64);");
// TODO: AtomicU128 is unstable
// impl_arithmetic!(u128, atomic::AtomicU128, "let a = AtomicCell::new(7u128);");
// impl_arithmetic!(i128, atomic::AtomicI128, "let a = AtomicCell::new(7i128);");
impl_arithmetic!(u128, fallback, "let a = AtomicCell::new(7u128);");
impl_arithmetic!(i128, fallback, "let a = AtomicCell::new(7i128);");
impl_arithmetic!(
usize,
atomic::AtomicUsize,
"let a = AtomicCell::new(7usize);"
);
impl_arithmetic!(
isize,
atomic::AtomicIsize,
"let a = AtomicCell::new(7isize);"
);
impl AtomicCell<bool> {
/// Applies logical "and" to the current value and returns the previous value.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::atomic::AtomicCell;
///
/// let a = AtomicCell::new(true);
///
/// assert_eq!(a.fetch_and(true), true);
/// assert_eq!(a.load(), true);
///
/// assert_eq!(a.fetch_and(false), true);
/// assert_eq!(a.load(), false);
/// ```
#[inline]
pub fn fetch_and(&self, val: bool) -> bool {
let a = unsafe { &*(self.as_ptr() as *const AtomicBool) };
a.fetch_and(val, Ordering::AcqRel)
}
/// Applies logical "nand" to the current value and returns the previous value.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::atomic::AtomicCell;
///
/// let a = AtomicCell::new(true);
///
/// assert_eq!(a.fetch_nand(false), true);
/// assert_eq!(a.load(), true);
///
/// assert_eq!(a.fetch_nand(true), true);
/// assert_eq!(a.load(), false);
///
/// assert_eq!(a.fetch_nand(false), false);
/// assert_eq!(a.load(), true);
/// ```
#[inline]
pub fn fetch_nand(&self, val: bool) -> bool {
let a = unsafe { &*(self.as_ptr() as *const AtomicBool) };
a.fetch_nand(val, Ordering::AcqRel)
}
/// Applies logical "or" to the current value and returns the previous value.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::atomic::AtomicCell;
///
/// let a = AtomicCell::new(false);
///
/// assert_eq!(a.fetch_or(false), false);
/// assert_eq!(a.load(), false);
///
/// assert_eq!(a.fetch_or(true), false);
/// assert_eq!(a.load(), true);
/// ```
#[inline]
pub fn fetch_or(&self, val: bool) -> bool {
let a = unsafe { &*(self.as_ptr() as *const AtomicBool) };
a.fetch_or(val, Ordering::AcqRel)
}
/// Applies logical "xor" to the current value and returns the previous value.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::atomic::AtomicCell;
///
/// let a = AtomicCell::new(true);
///
/// assert_eq!(a.fetch_xor(false), true);
/// assert_eq!(a.load(), true);
///
/// assert_eq!(a.fetch_xor(true), true);
/// assert_eq!(a.load(), false);
/// ```
#[inline]
pub fn fetch_xor(&self, val: bool) -> bool {
let a = unsafe { &*(self.as_ptr() as *const AtomicBool) };
a.fetch_xor(val, Ordering::AcqRel)
}
}
impl<T: Default> Default for AtomicCell<T> {
fn default() -> AtomicCell<T> {
AtomicCell::new(T::default())
}
}
impl<T> From<T> for AtomicCell<T> {
#[inline]
fn from(val: T) -> AtomicCell<T> {
AtomicCell::new(val)
}
}
impl<T: Copy + fmt::Debug> fmt::Debug for AtomicCell<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("AtomicCell")
.field("value", &self.load())
.finish()
}
}
/// Returns `true` if values of type `A` can be transmuted into values of type `B`.
const fn can_transmute<A, B>() -> bool {
// Sizes must be equal, but alignment of `A` must be greater or equal than that of `B`.
(mem::size_of::<A>() == mem::size_of::<B>()) & (mem::align_of::<A>() >= mem::align_of::<B>())
}
/// Returns a reference to the global lock associated with the `AtomicCell` at address `addr`.
///
/// This function is used to protect atomic data which doesn't fit into any of the primitive atomic
/// types in `std::sync::atomic`. Operations on such atomics must therefore use a global lock.
///
/// However, there is not only one global lock but an array of many locks, and one of them is
/// picked based on the given address. Having many locks reduces contention and improves
/// scalability.
#[inline]
#[must_use]
fn lock(addr: usize) -> &'static SeqLock {
// The number of locks is a prime number because we want to make sure `addr % LEN` gets
// dispersed across all locks.
//
// Note that addresses are always aligned to some power of 2, depending on type `T` in
// `AtomicCell<T>`. If `LEN` was an even number, then `addr % LEN` would be an even number,
// too, which means only half of the locks would get utilized!
//
// It is also possible for addresses to accidentally get aligned to a number that is not a
// power of 2. Consider this example:
//
// ```
// #[repr(C)]
// struct Foo {
// a: AtomicCell<u8>,
// b: u8,
// c: u8,
// }
// ```
//
// Now, if we have a slice of type `&[Foo]`, it is possible that field `a` in all items gets
// stored at addresses that are multiples of 3. It'd be too bad if `LEN` was divisible by 3.
// In order to protect from such cases, we simply choose a large prime number for `LEN`.
const LEN: usize = 97;
#[allow(clippy::declare_interior_mutable_const)]
const L: SeqLock = SeqLock::new();
static LOCKS: [SeqLock; LEN] = [L; LEN];
// If the modulus is a constant number, the compiler will use crazy math to transform this into
// a sequence of cheap arithmetic operations rather than using the slow modulo instruction.
&LOCKS[addr % LEN]
}
/// An atomic `()`.
///
/// All operations are noops.
struct AtomicUnit;
impl AtomicUnit {
#[inline]
fn load(&self, _order: Ordering) {}
#[inline]
fn store(&self, _val: (), _order: Ordering) {}
#[inline]
fn swap(&self, _val: (), _order: Ordering) {}
#[inline]
fn compare_exchange_weak(
&self,
_current: (),
_new: (),
_success: Ordering,
_failure: Ordering,
) -> Result<(), ()> {
Ok(())
}
}
macro_rules! atomic {
// If values of type `$t` can be transmuted into values of the primitive atomic type `$atomic`,
// declares variable `$a` of type `$atomic` and executes `$atomic_op`, breaking out of the loop.
(@check, $t:ty, $atomic:ty, $a:ident, $atomic_op:expr) => {
if can_transmute::<$t, $atomic>() {
let $a: &$atomic;
break $atomic_op;
}
};
// If values of type `$t` can be transmuted into values of a primitive atomic type, declares
// variable `$a` of that type and executes `$atomic_op`. Otherwise, just executes
// `$fallback_op`.
($t:ty, $a:ident, $atomic_op:expr, $fallback_op:expr) => {
loop {
atomic!(@check, $t, AtomicUnit, $a, $atomic_op);
atomic!(@check, $t, atomic::AtomicU8, $a, $atomic_op);
atomic!(@check, $t, atomic::AtomicU16, $a, $atomic_op);
atomic!(@check, $t, atomic::AtomicU32, $a, $atomic_op);
#[cfg(not(crossbeam_no_atomic_64))]
atomic!(@check, $t, atomic::AtomicU64, $a, $atomic_op);
// TODO: AtomicU128 is unstable
// atomic!(@check, $t, atomic::AtomicU128, $a, $atomic_op);
break $fallback_op;
}
};
}
/// Returns `true` if operations on `AtomicCell<T>` are lock-free.
const fn atomic_is_lock_free<T>() -> bool {
// HACK(taiki-e): This is equivalent to `atomic! { T, _a, true, false }`, but can be used in const fn even in our MSRV (Rust 1.38).
let is_lock_free = can_transmute::<T, AtomicUnit>()
| can_transmute::<T, atomic::AtomicU8>()
| can_transmute::<T, atomic::AtomicU16>()
| can_transmute::<T, atomic::AtomicU32>();
#[cfg(not(crossbeam_no_atomic_64))]
let is_lock_free = is_lock_free | can_transmute::<T, atomic::AtomicU64>();
// TODO: AtomicU128 is unstable
// let is_lock_free = is_lock_free | can_transmute::<T, atomic::AtomicU128>();
is_lock_free
}
/// Atomically reads data from `src`.
///
/// This operation uses the `Acquire` ordering. If possible, an atomic instructions is used, and a
/// global lock otherwise.
unsafe fn atomic_load<T>(src: *mut T) -> T
where
T: Copy,
{
atomic! {
T, a,
{
a = &*(src as *const _ as *const _);
mem::transmute_copy(&a.load(Ordering::Acquire))
},
{
let lock = lock(src as usize);
// Try doing an optimistic read first.
if let Some(stamp) = lock.optimistic_read() {
// We need a volatile read here because other threads might concurrently modify the
// value. In theory, data races are *always* UB, even if we use volatile reads and
// discard the data when a data race is detected. The proper solution would be to
// do atomic reads and atomic writes, but we can't atomically read and write all
// kinds of data since `AtomicU8` is not available on stable Rust yet.
// Load as `MaybeUninit` because we may load a value that is not valid as `T`.
let val = ptr::read_volatile(src.cast::<MaybeUninit<T>>());
if lock.validate_read(stamp) {
return val.assume_init();
}
}
// Grab a regular write lock so that writers don't starve this load.
let guard = lock.write();
let val = ptr::read(src);
// The value hasn't been changed. Drop the guard without incrementing the stamp.
guard.abort();
val
}
}
}
/// Atomically writes `val` to `dst`.
///
/// This operation uses the `Release` ordering. If possible, an atomic instructions is used, and a
/// global lock otherwise.
unsafe fn atomic_store<T>(dst: *mut T, val: T) {
atomic! {
T, a,
{
a = &*(dst as *const _ as *const _);
a.store(mem::transmute_copy(&val), Ordering::Release);
mem::forget(val);
},
{
let _guard = lock(dst as usize).write();
ptr::write(dst, val);
}
}
}
/// Atomically swaps data at `dst` with `val`.
///
/// This operation uses the `AcqRel` ordering. If possible, an atomic instructions is used, and a
/// global lock otherwise.
unsafe fn atomic_swap<T>(dst: *mut T, val: T) -> T {
atomic! {
T, a,
{
a = &*(dst as *const _ as *const _);
let res = mem::transmute_copy(&a.swap(mem::transmute_copy(&val), Ordering::AcqRel));
mem::forget(val);
res
},
{
let _guard = lock(dst as usize).write();
ptr::replace(dst, val)
}
}
}
/// Atomically compares data at `dst` to `current` and, if equal byte-for-byte, exchanges data at
/// `dst` with `new`.
///
/// Returns the old value on success, or the current value at `dst` on failure.
///
/// This operation uses the `AcqRel` ordering. If possible, an atomic instructions is used, and a
/// global lock otherwise.
#[allow(clippy::let_unit_value)]
unsafe fn atomic_compare_exchange_weak<T>(dst: *mut T, mut current: T, new: T) -> Result<T, T>
where
T: Copy + Eq,
{
atomic! {
T, a,
{
a = &*(dst as *const _ as *const _);
let mut current_raw = mem::transmute_copy(¤t);
let new_raw = mem::transmute_copy(&new);
loop {
match a.compare_exchange_weak(
current_raw,
new_raw,
Ordering::AcqRel,
Ordering::Acquire,
) {
Ok(_) => break Ok(current),
Err(previous_raw) => {
let previous = mem::transmute_copy(&previous_raw);
if !T::eq(&previous, ¤t) {
break Err(previous);
}
// The compare-exchange operation has failed and didn't store `new`. The
// failure is either spurious, or `previous` was semantically equal to
// `current` but not byte-equal. Let's retry with `previous` as the new
// `current`.
current = previous;
current_raw = previous_raw;
}
}
}
},
{
let guard = lock(dst as usize).write();
if T::eq(&*dst, ¤t) {
Ok(ptr::replace(dst, new))
} else {
let val = ptr::read(dst);
// The value hasn't been changed. Drop the guard without incrementing the stamp.
guard.abort();
Err(val)
}
}
}
}