--- /dev/null
+//! The implementation is based on Dmitry Vyukov's bounded MPMC queue.
+//!
+//! Source:
+//! - http://www.1024cores.net/home/lock-free-algorithms/queues/bounded-mpmc-queue
+//!
+//! Copyright & License:
+//! - Copyright (c) 2010-2011 Dmitry Vyukov
+//! - Simplified BSD License and Apache License, Version 2.0
+//! - http://www.1024cores.net/home/code-license
+
+use alloc::vec::Vec;
+use core::cell::UnsafeCell;
+use core::fmt;
+use core::marker::PhantomData;
+use core::mem;
+use core::ptr;
+use core::sync::atomic::{self, AtomicUsize, Ordering};
+
+use crossbeam_utils::{Backoff, CachePadded};
+
+use maybe_uninit::MaybeUninit;
+
+use err::{PopError, PushError};
+
+/// A slot in a queue.
+struct Slot<T> {
+ /// The current stamp.
+ ///
+ /// If the stamp equals the tail, this node will be next written to. If it equals head + 1,
+ /// this node will be next read from.
+ stamp: AtomicUsize,
+
+ /// The value in this slot.
+ value: UnsafeCell<MaybeUninit<T>>,
+}
+
+/// A bounded multi-producer multi-consumer queue.
+///
+/// This queue allocates a fixed-capacity buffer on construction, which is used to store pushed
+/// elements. The queue cannot hold more elements than the buffer allows. Attempting to push an
+/// element into a full queue will fail. Having a buffer allocated upfront makes this queue a bit
+/// faster than [`SegQueue`].
+///
+/// [`SegQueue`]: struct.SegQueue.html
+///
+/// # Examples
+///
+/// ```
+/// use crossbeam_queue::{ArrayQueue, PushError};
+///
+/// let q = ArrayQueue::new(2);
+///
+/// assert_eq!(q.push('a'), Ok(()));
+/// assert_eq!(q.push('b'), Ok(()));
+/// assert_eq!(q.push('c'), Err(PushError('c')));
+/// assert_eq!(q.pop(), Ok('a'));
+/// ```
+pub struct ArrayQueue<T> {
+ /// The head of the queue.
+ ///
+ /// This value is a "stamp" consisting of an index into the buffer and a lap, but packed into a
+ /// single `usize`. The lower bits represent the index, while the upper bits represent the lap.
+ ///
+ /// Elements are popped from the head of the queue.
+ head: CachePadded<AtomicUsize>,
+
+ /// The tail of the queue.
+ ///
+ /// This value is a "stamp" consisting of an index into the buffer and a lap, but packed into a
+ /// single `usize`. The lower bits represent the index, while the upper bits represent the lap.
+ ///
+ /// Elements are pushed into the tail of the queue.
+ tail: CachePadded<AtomicUsize>,
+
+ /// The buffer holding slots.
+ buffer: *mut Slot<T>,
+
+ /// The queue capacity.
+ cap: usize,
+
+ /// A stamp with the value of `{ lap: 1, index: 0 }`.
+ one_lap: usize,
+
+ /// Indicates that dropping an `ArrayQueue<T>` may drop elements of type `T`.
+ _marker: PhantomData<T>,
+}
+
+unsafe impl<T: Send> Sync for ArrayQueue<T> {}
+unsafe impl<T: Send> Send for ArrayQueue<T> {}
+
+impl<T> ArrayQueue<T> {
+ /// Creates a new bounded queue with the given capacity.
+ ///
+ /// # Panics
+ ///
+ /// Panics if the capacity is zero.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use crossbeam_queue::ArrayQueue;
+ ///
+ /// let q = ArrayQueue::<i32>::new(100);
+ /// ```
+ pub fn new(cap: usize) -> ArrayQueue<T> {
+ assert!(cap > 0, "capacity must be non-zero");
+
+ // Head is initialized to `{ lap: 0, index: 0 }`.
+ // Tail is initialized to `{ lap: 0, index: 0 }`.
+ let head = 0;
+ let tail = 0;
+
+ // Allocate a buffer of `cap` slots.
+ let buffer = {
+ let mut v = Vec::<Slot<T>>::with_capacity(cap);
+ let ptr = v.as_mut_ptr();
+ mem::forget(v);
+ ptr
+ };
+
+ // Initialize stamps in the slots.
+ for i in 0..cap {
+ unsafe {
+ // Set the stamp to `{ lap: 0, index: i }`.
+ let slot = buffer.add(i);
+ ptr::write(&mut (*slot).stamp, AtomicUsize::new(i));
+ }
+ }
+
+ // One lap is the smallest power of two greater than `cap`.
+ let one_lap = (cap + 1).next_power_of_two();
+
+ ArrayQueue {
+ buffer,
+ cap,
+ one_lap,
+ head: CachePadded::new(AtomicUsize::new(head)),
+ tail: CachePadded::new(AtomicUsize::new(tail)),
+ _marker: PhantomData,
+ }
+ }
+
+ /// Attempts to push an element into the queue.
+ ///
+ /// If the queue is full, the element is returned back as an error.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use crossbeam_queue::{ArrayQueue, PushError};
+ ///
+ /// let q = ArrayQueue::new(1);
+ ///
+ /// assert_eq!(q.push(10), Ok(()));
+ /// assert_eq!(q.push(20), Err(PushError(20)));
+ /// ```
+ pub fn push(&self, value: T) -> Result<(), PushError<T>> {
+ let backoff = Backoff::new();
+ let mut tail = self.tail.load(Ordering::Relaxed);
+
+ loop {
+ // Deconstruct the tail.
+ let index = tail & (self.one_lap - 1);
+ let lap = tail & !(self.one_lap - 1);
+
+ // Inspect the corresponding slot.
+ let slot = unsafe { &*self.buffer.add(index) };
+ let stamp = slot.stamp.load(Ordering::Acquire);
+
+ // If the tail and the stamp match, we may attempt to push.
+ if tail == stamp {
+ let new_tail = if index + 1 < self.cap {
+ // Same lap, incremented index.
+ // Set to `{ lap: lap, index: index + 1 }`.
+ tail + 1
+ } else {
+ // One lap forward, index wraps around to zero.
+ // Set to `{ lap: lap.wrapping_add(1), index: 0 }`.
+ lap.wrapping_add(self.one_lap)
+ };
+
+ // Try moving the tail.
+ match self.tail.compare_exchange_weak(
+ tail,
+ new_tail,
+ Ordering::SeqCst,
+ Ordering::Relaxed,
+ ) {
+ Ok(_) => {
+ // Write the value into the slot and update the stamp.
+ unsafe {
+ slot.value.get().write(MaybeUninit::new(value));
+ }
+ slot.stamp.store(tail + 1, Ordering::Release);
+ return Ok(());
+ }
+ Err(t) => {
+ tail = t;
+ backoff.spin();
+ }
+ }
+ } else if stamp.wrapping_add(self.one_lap) == tail + 1 {
+ atomic::fence(Ordering::SeqCst);
+ let head = self.head.load(Ordering::Relaxed);
+
+ // If the head lags one lap behind the tail as well...
+ if head.wrapping_add(self.one_lap) == tail {
+ // ...then the queue is full.
+ return Err(PushError(value));
+ }
+
+ backoff.spin();
+ tail = self.tail.load(Ordering::Relaxed);
+ } else {
+ // Snooze because we need to wait for the stamp to get updated.
+ backoff.snooze();
+ tail = self.tail.load(Ordering::Relaxed);
+ }
+ }
+ }
+
+ /// Attempts to pop an element from the queue.
+ ///
+ /// If the queue is empty, an error is returned.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use crossbeam_queue::{ArrayQueue, PopError};
+ ///
+ /// let q = ArrayQueue::new(1);
+ /// assert_eq!(q.push(10), Ok(()));
+ ///
+ /// assert_eq!(q.pop(), Ok(10));
+ /// assert_eq!(q.pop(), Err(PopError));
+ /// ```
+ pub fn pop(&self) -> Result<T, PopError> {
+ let backoff = Backoff::new();
+ let mut head = self.head.load(Ordering::Relaxed);
+
+ loop {
+ // Deconstruct the head.
+ let index = head & (self.one_lap - 1);
+ let lap = head & !(self.one_lap - 1);
+
+ // Inspect the corresponding slot.
+ let slot = unsafe { &*self.buffer.add(index) };
+ let stamp = slot.stamp.load(Ordering::Acquire);
+
+ // If the the stamp is ahead of the head by 1, we may attempt to pop.
+ if head + 1 == stamp {
+ let new = if index + 1 < self.cap {
+ // Same lap, incremented index.
+ // Set to `{ lap: lap, index: index + 1 }`.
+ head + 1
+ } else {
+ // One lap forward, index wraps around to zero.
+ // Set to `{ lap: lap.wrapping_add(1), index: 0 }`.
+ lap.wrapping_add(self.one_lap)
+ };
+
+ // Try moving the head.
+ match self.head.compare_exchange_weak(
+ head,
+ new,
+ Ordering::SeqCst,
+ Ordering::Relaxed,
+ ) {
+ Ok(_) => {
+ // Read the value from the slot and update the stamp.
+ let msg = unsafe { slot.value.get().read().assume_init() };
+ slot.stamp
+ .store(head.wrapping_add(self.one_lap), Ordering::Release);
+ return Ok(msg);
+ }
+ Err(h) => {
+ head = h;
+ backoff.spin();
+ }
+ }
+ } else if stamp == head {
+ atomic::fence(Ordering::SeqCst);
+ let tail = self.tail.load(Ordering::Relaxed);
+
+ // If the tail equals the head, that means the channel is empty.
+ if tail == head {
+ return Err(PopError);
+ }
+
+ backoff.spin();
+ head = self.head.load(Ordering::Relaxed);
+ } else {
+ // Snooze because we need to wait for the stamp to get updated.
+ backoff.snooze();
+ head = self.head.load(Ordering::Relaxed);
+ }
+ }
+ }
+
+ /// Returns the capacity of the queue.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use crossbeam_queue::{ArrayQueue, PopError};
+ ///
+ /// let q = ArrayQueue::<i32>::new(100);
+ ///
+ /// assert_eq!(q.capacity(), 100);
+ /// ```
+ pub fn capacity(&self) -> usize {
+ self.cap
+ }
+
+ /// Returns `true` if the queue is empty.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use crossbeam_queue::{ArrayQueue, PopError};
+ ///
+ /// let q = ArrayQueue::new(100);
+ ///
+ /// assert!(q.is_empty());
+ /// q.push(1).unwrap();
+ /// assert!(!q.is_empty());
+ /// ```
+ pub fn is_empty(&self) -> bool {
+ let head = self.head.load(Ordering::SeqCst);
+ let tail = self.tail.load(Ordering::SeqCst);
+
+ // Is the tail lagging one lap behind head?
+ // Is the tail equal to the head?
+ //
+ // Note: If the head changes just before we load the tail, that means there was a moment
+ // when the channel was not empty, so it is safe to just return `false`.
+ tail == head
+ }
+
+ /// Returns `true` if the queue is full.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use crossbeam_queue::{ArrayQueue, PopError};
+ ///
+ /// let q = ArrayQueue::new(1);
+ ///
+ /// assert!(!q.is_full());
+ /// q.push(1).unwrap();
+ /// assert!(q.is_full());
+ /// ```
+ pub fn is_full(&self) -> bool {
+ let tail = self.tail.load(Ordering::SeqCst);
+ let head = self.head.load(Ordering::SeqCst);
+
+ // Is the head lagging one lap behind tail?
+ //
+ // Note: If the tail changes just before we load the head, that means there was a moment
+ // when the queue was not full, so it is safe to just return `false`.
+ head.wrapping_add(self.one_lap) == tail
+ }
+
+ /// Returns the number of elements in the queue.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use crossbeam_queue::{ArrayQueue, PopError};
+ ///
+ /// let q = ArrayQueue::new(100);
+ /// assert_eq!(q.len(), 0);
+ ///
+ /// q.push(10).unwrap();
+ /// assert_eq!(q.len(), 1);
+ ///
+ /// q.push(20).unwrap();
+ /// assert_eq!(q.len(), 2);
+ /// ```
+ pub fn len(&self) -> usize {
+ loop {
+ // Load the tail, then load the head.
+ let tail = self.tail.load(Ordering::SeqCst);
+ let head = self.head.load(Ordering::SeqCst);
+
+ // If the tail didn't change, we've got consistent values to work with.
+ if self.tail.load(Ordering::SeqCst) == tail {
+ let hix = head & (self.one_lap - 1);
+ let tix = tail & (self.one_lap - 1);
+
+ return if hix < tix {
+ tix - hix
+ } else if hix > tix {
+ self.cap - hix + tix
+ } else if tail == head {
+ 0
+ } else {
+ self.cap
+ };
+ }
+ }
+ }
+}
+
+impl<T> Drop for ArrayQueue<T> {
+ fn drop(&mut self) {
+ // Get the index of the head.
+ let hix = self.head.load(Ordering::Relaxed) & (self.one_lap - 1);
+
+ // Loop over all slots that hold a message and drop them.
+ for i in 0..self.len() {
+ // Compute the index of the next slot holding a message.
+ let index = if hix + i < self.cap {
+ hix + i
+ } else {
+ hix + i - self.cap
+ };
+
+ unsafe {
+ let p = {
+ let slot = &mut *self.buffer.add(index);
+ let value = &mut *slot.value.get();
+ value.as_mut_ptr()
+ };
+ p.drop_in_place();
+ }
+ }
+
+ // Finally, deallocate the buffer, but don't run any destructors.
+ unsafe {
+ Vec::from_raw_parts(self.buffer, 0, self.cap);
+ }
+ }
+}
+
+impl<T> fmt::Debug for ArrayQueue<T> {
+ fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+ f.pad("ArrayQueue { .. }")
+ }
+}
projects / rustc.git / blobdiff