blob: a69e0613ec965e617f12c097db863c99c01bb025 [file] [log] [blame]
use alloc::boxed::Box;
use core::borrow::{Borrow, BorrowMut};
use core::cmp;
use core::fmt;
use core::marker::PhantomData;
use core::mem;
use core::ops::{Deref, DerefMut};
use core::ptr;
use core::sync::atomic::Ordering;
use core::sync::atomic::{AtomicUsize, ATOMIC_USIZE_INIT};
use crossbeam_utils::atomic::AtomicConsume;
use guard::Guard;
/// Given ordering for the success case in a compare-exchange operation, returns the strongest
/// appropriate ordering for the failure case.
#[inline]
fn strongest_failure_ordering(ord: Ordering) -> Ordering {
use self::Ordering::*;
match ord {
Relaxed | Release => Relaxed,
Acquire | AcqRel => Acquire,
_ => SeqCst,
}
}
/// The error returned on failed compare-and-set operation.
pub struct CompareAndSetError<'g, T: 'g, P: Pointer<T>> {
/// The value in the atomic pointer at the time of the failed operation.
pub current: Shared<'g, T>,
/// The new value, which the operation failed to store.
pub new: P,
}
impl<'g, T: 'g, P: Pointer<T> + fmt::Debug> fmt::Debug for CompareAndSetError<'g, T, P> {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
f.debug_struct("CompareAndSetError")
.field("current", &self.current)
.field("new", &self.new)
.finish()
}
}
/// Memory orderings for compare-and-set operations.
///
/// A compare-and-set operation can have different memory orderings depending on whether it
/// succeeds or fails. This trait generalizes different ways of specifying memory orderings.
///
/// The two ways of specifying orderings for compare-and-set are:
///
/// 1. Just one `Ordering` for the success case. In case of failure, the strongest appropriate
/// ordering is chosen.
/// 2. A pair of `Ordering`s. The first one is for the success case, while the second one is
/// for the failure case.
pub trait CompareAndSetOrdering {
/// The ordering of the operation when it succeeds.
fn success(&self) -> Ordering;
/// The ordering of the operation when it fails.
///
/// The failure ordering can't be `Release` or `AcqRel` and must be equivalent or weaker than
/// the success ordering.
fn failure(&self) -> Ordering;
}
impl CompareAndSetOrdering for Ordering {
#[inline]
fn success(&self) -> Ordering {
*self
}
#[inline]
fn failure(&self) -> Ordering {
strongest_failure_ordering(*self)
}
}
impl CompareAndSetOrdering for (Ordering, Ordering) {
#[inline]
fn success(&self) -> Ordering {
self.0
}
#[inline]
fn failure(&self) -> Ordering {
self.1
}
}
/// Panics if the pointer is not properly unaligned.
#[inline]
fn ensure_aligned<T>(raw: *const T) {
assert_eq!(raw as usize & low_bits::<T>(), 0, "unaligned pointer");
}
/// Returns a bitmask containing the unused least significant bits of an aligned pointer to `T`.
#[inline]
fn low_bits<T>() -> usize {
(1 << mem::align_of::<T>().trailing_zeros()) - 1
}
/// Given a tagged pointer `data`, returns the same pointer, but tagged with `tag`.
///
/// `tag` is truncated to fit into the unused bits of the pointer to `T`.
#[inline]
fn data_with_tag<T>(data: usize, tag: usize) -> usize {
(data & !low_bits::<T>()) | (tag & low_bits::<T>())
}
/// Decomposes a tagged pointer `data` into the pointer and the tag.
#[inline]
fn decompose_data<T>(data: usize) -> (*mut T, usize) {
let raw = (data & !low_bits::<T>()) as *mut T;
let tag = data & low_bits::<T>();
(raw, tag)
}
/// An atomic pointer that can be safely shared between threads.
///
/// The pointer must be properly aligned. Since it is aligned, a tag can be stored into the unused
/// least significant bits of the address. More precisely, a tag should be less than `(1 <<
/// mem::align_of::<T>().trailing_zeros())`.
///
/// Any method that loads the pointer must be passed a reference to a [`Guard`].
///
/// [`Guard`]: struct.Guard.html
pub struct Atomic<T> {
data: AtomicUsize,
_marker: PhantomData<*mut T>,
}
unsafe impl<T: Send + Sync> Send for Atomic<T> {}
unsafe impl<T: Send + Sync> Sync for Atomic<T> {}
impl<T> Atomic<T> {
/// Returns a new atomic pointer pointing to the tagged pointer `data`.
fn from_usize(data: usize) -> Self {
Self {
data: AtomicUsize::new(data),
_marker: PhantomData,
}
}
/// Returns a new null atomic pointer.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::Atomic;
///
/// let a = Atomic::<i32>::null();
/// ```
#[cfg(not(feature = "nightly"))]
pub fn null() -> Atomic<T> {
Self {
data: ATOMIC_USIZE_INIT,
_marker: PhantomData,
}
}
/// Returns a new null atomic pointer.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::Atomic;
///
/// let a = Atomic::<i32>::null();
/// ```
#[cfg(feature = "nightly")]
pub const fn null() -> Atomic<T> {
Self {
data: ATOMIC_USIZE_INIT,
_marker: PhantomData,
}
}
/// Allocates `value` on the heap and returns a new atomic pointer pointing to it.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::Atomic;
///
/// let a = Atomic::new(1234);
/// ```
pub fn new(value: T) -> Atomic<T> {
Self::from(Owned::new(value))
}
/// Loads a `Shared` from the atomic pointer.
///
/// This method takes an [`Ordering`] argument which describes the memory ordering of this
/// operation.
///
/// [`Ordering`]: https://doc.rust-lang.org/std/sync/atomic/enum.Ordering.html
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::new(1234);
/// let guard = &epoch::pin();
/// let p = a.load(SeqCst, guard);
/// ```
pub fn load<'g>(&self, ord: Ordering, _: &'g Guard) -> Shared<'g, T> {
unsafe { Shared::from_usize(self.data.load(ord)) }
}
/// Loads a `Shared` from the atomic pointer using a "consume" memory ordering.
///
/// This is similar to the "acquire" ordering, except that an ordering is
/// only guaranteed with operations that "depend on" the result of the load.
/// However consume loads are usually much faster than acquire loads on
/// architectures with a weak memory model since they don't require memory
/// fence instructions.
///
/// The exact definition of "depend on" is a bit vague, but it works as you
/// would expect in practice since a lot of software, especially the Linux
/// kernel, rely on this behavior.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic};
///
/// let a = Atomic::new(1234);
/// let guard = &epoch::pin();
/// let p = a.load_consume(guard);
/// ```
pub fn load_consume<'g>(&self, _: &'g Guard) -> Shared<'g, T> {
unsafe { Shared::from_usize(self.data.load_consume()) }
}
/// Stores a `Shared` or `Owned` pointer into the atomic pointer.
///
/// This method takes an [`Ordering`] argument which describes the memory ordering of this
/// operation.
///
/// [`Ordering`]: https://doc.rust-lang.org/std/sync/atomic/enum.Ordering.html
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic, Owned, Shared};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::new(1234);
/// a.store(Shared::null(), SeqCst);
/// a.store(Owned::new(1234), SeqCst);
/// ```
pub fn store<'g, P: Pointer<T>>(&self, new: P, ord: Ordering) {
self.data.store(new.into_usize(), ord);
}
/// Stores a `Shared` or `Owned` pointer into the atomic pointer, returning the previous
/// `Shared`.
///
/// This method takes an [`Ordering`] argument which describes the memory ordering of this
/// operation.
///
/// [`Ordering`]: https://doc.rust-lang.org/std/sync/atomic/enum.Ordering.html
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic, Owned, Shared};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::new(1234);
/// let guard = &epoch::pin();
/// let p = a.swap(Shared::null(), SeqCst, guard);
/// ```
pub fn swap<'g, P: Pointer<T>>(&self, new: P, ord: Ordering, _: &'g Guard) -> Shared<'g, T> {
unsafe { Shared::from_usize(self.data.swap(new.into_usize(), ord)) }
}
/// Stores the pointer `new` (either `Shared` or `Owned`) into the atomic pointer if the current
/// value is the same as `current`. The tag is also taken into account, so two pointers to the
/// same object, but with different tags, will not be considered equal.
///
/// The return value is a result indicating whether the new pointer was written. On success the
/// pointer that was written is returned. On failure the actual current value and `new` are
/// returned.
///
/// This method takes a [`CompareAndSetOrdering`] argument which describes the memory
/// ordering of this operation.
///
/// [`CompareAndSetOrdering`]: trait.CompareAndSetOrdering.html
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic, Owned, Shared};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::new(1234);
///
/// let guard = &epoch::pin();
/// let mut curr = a.load(SeqCst, guard);
/// let res1 = a.compare_and_set(curr, Shared::null(), SeqCst, guard);
/// let res2 = a.compare_and_set(curr, Owned::new(5678), SeqCst, guard);
/// ```
pub fn compare_and_set<'g, O, P>(
&self,
current: Shared<T>,
new: P,
ord: O,
_: &'g Guard,
) -> Result<Shared<'g, T>, CompareAndSetError<'g, T, P>>
where
O: CompareAndSetOrdering,
P: Pointer<T>,
{
let new = new.into_usize();
self.data
.compare_exchange(current.into_usize(), new, ord.success(), ord.failure())
.map(|_| unsafe { Shared::from_usize(new) })
.map_err(|current| unsafe {
CompareAndSetError {
current: Shared::from_usize(current),
new: P::from_usize(new),
}
})
}
/// Stores the pointer `new` (either `Shared` or `Owned`) into the atomic pointer if the current
/// value is the same as `current`. The tag is also taken into account, so two pointers to the
/// same object, but with different tags, will not be considered equal.
///
/// Unlike [`compare_and_set`], this method is allowed to spuriously fail even when comparison
/// succeeds, which can result in more efficient code on some platforms. The return value is a
/// result indicating whether the new pointer was written. On success the pointer that was
/// written is returned. On failure the actual current value and `new` are returned.
///
/// This method takes a [`CompareAndSetOrdering`] argument which describes the memory
/// ordering of this operation.
///
/// [`compare_and_set`]: struct.Atomic.html#method.compare_and_set
/// [`CompareAndSetOrdering`]: trait.CompareAndSetOrdering.html
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic, Owned, Shared};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::new(1234);
/// let guard = &epoch::pin();
///
/// let mut new = Owned::new(5678);
/// let mut ptr = a.load(SeqCst, guard);
/// loop {
/// match a.compare_and_set_weak(ptr, new, SeqCst, guard) {
/// Ok(p) => {
/// ptr = p;
/// break;
/// }
/// Err(err) => {
/// ptr = err.current;
/// new = err.new;
/// }
/// }
/// }
///
/// let mut curr = a.load(SeqCst, guard);
/// loop {
/// match a.compare_and_set_weak(curr, Shared::null(), SeqCst, guard) {
/// Ok(_) => break,
/// Err(err) => curr = err.current,
/// }
/// }
/// ```
pub fn compare_and_set_weak<'g, O, P>(
&self,
current: Shared<T>,
new: P,
ord: O,
_: &'g Guard,
) -> Result<Shared<'g, T>, CompareAndSetError<'g, T, P>>
where
O: CompareAndSetOrdering,
P: Pointer<T>,
{
let new = new.into_usize();
self.data
.compare_exchange_weak(current.into_usize(), new, ord.success(), ord.failure())
.map(|_| unsafe { Shared::from_usize(new) })
.map_err(|current| unsafe {
CompareAndSetError {
current: Shared::from_usize(current),
new: P::from_usize(new),
}
})
}
/// Bitwise "and" with the current tag.
///
/// Performs a bitwise "and" operation on the current tag and the argument `val`, and sets the
/// new tag to the result. Returns the previous pointer.
///
/// This method takes an [`Ordering`] argument which describes the memory ordering of this
/// operation.
///
/// [`Ordering`]: https://doc.rust-lang.org/std/sync/atomic/enum.Ordering.html
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic, Shared};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::<i32>::from(Shared::null().with_tag(3));
/// let guard = &epoch::pin();
/// assert_eq!(a.fetch_and(2, SeqCst, guard).tag(), 3);
/// assert_eq!(a.load(SeqCst, guard).tag(), 2);
/// ```
pub fn fetch_and<'g>(&self, val: usize, ord: Ordering, _: &'g Guard) -> Shared<'g, T> {
unsafe { Shared::from_usize(self.data.fetch_and(val | !low_bits::<T>(), ord)) }
}
/// Bitwise "or" with the current tag.
///
/// Performs a bitwise "or" operation on the current tag and the argument `val`, and sets the
/// new tag to the result. Returns the previous pointer.
///
/// This method takes an [`Ordering`] argument which describes the memory ordering of this
/// operation.
///
/// [`Ordering`]: https://doc.rust-lang.org/std/sync/atomic/enum.Ordering.html
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic, Shared};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::<i32>::from(Shared::null().with_tag(1));
/// let guard = &epoch::pin();
/// assert_eq!(a.fetch_or(2, SeqCst, guard).tag(), 1);
/// assert_eq!(a.load(SeqCst, guard).tag(), 3);
/// ```
pub fn fetch_or<'g>(&self, val: usize, ord: Ordering, _: &'g Guard) -> Shared<'g, T> {
unsafe { Shared::from_usize(self.data.fetch_or(val & low_bits::<T>(), ord)) }
}
/// Bitwise "xor" with the current tag.
///
/// Performs a bitwise "xor" operation on the current tag and the argument `val`, and sets the
/// new tag to the result. Returns the previous pointer.
///
/// This method takes an [`Ordering`] argument which describes the memory ordering of this
/// operation.
///
/// [`Ordering`]: https://doc.rust-lang.org/std/sync/atomic/enum.Ordering.html
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic, Shared};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::<i32>::from(Shared::null().with_tag(1));
/// let guard = &epoch::pin();
/// assert_eq!(a.fetch_xor(3, SeqCst, guard).tag(), 1);
/// assert_eq!(a.load(SeqCst, guard).tag(), 2);
/// ```
pub fn fetch_xor<'g>(&self, val: usize, ord: Ordering, _: &'g Guard) -> Shared<'g, T> {
unsafe { Shared::from_usize(self.data.fetch_xor(val & low_bits::<T>(), ord)) }
}
}
impl<T> fmt::Debug for Atomic<T> {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
let data = self.data.load(Ordering::SeqCst);
let (raw, tag) = decompose_data::<T>(data);
f.debug_struct("Atomic")
.field("raw", &raw)
.field("tag", &tag)
.finish()
}
}
impl<T> fmt::Pointer for Atomic<T> {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
let data = self.data.load(Ordering::SeqCst);
let (raw, _) = decompose_data::<T>(data);
fmt::Pointer::fmt(&raw, f)
}
}
impl<T> Clone for Atomic<T> {
/// Returns a copy of the atomic value.
///
/// Note that a `Relaxed` load is used here. If you need synchronization, use it with other
/// atomics or fences.
fn clone(&self) -> Self {
let data = self.data.load(Ordering::Relaxed);
Atomic::from_usize(data)
}
}
impl<T> Default for Atomic<T> {
fn default() -> Self {
Atomic::null()
}
}
impl<T> From<Owned<T>> for Atomic<T> {
/// Returns a new atomic pointer pointing to `owned`.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{Atomic, Owned};
///
/// let a = Atomic::<i32>::from(Owned::new(1234));
/// ```
fn from(owned: Owned<T>) -> Self {
let data = owned.data;
mem::forget(owned);
Self::from_usize(data)
}
}
impl<T> From<Box<T>> for Atomic<T> {
fn from(b: Box<T>) -> Self {
Self::from(Owned::from(b))
}
}
impl<T> From<T> for Atomic<T> {
fn from(t: T) -> Self {
Self::new(t)
}
}
impl<'g, T> From<Shared<'g, T>> for Atomic<T> {
/// Returns a new atomic pointer pointing to `ptr`.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{Atomic, Shared};
///
/// let a = Atomic::<i32>::from(Shared::<i32>::null());
/// ```
fn from(ptr: Shared<'g, T>) -> Self {
Self::from_usize(ptr.data)
}
}
impl<T> From<*const T> for Atomic<T> {
/// Returns a new atomic pointer pointing to `raw`.
///
/// # Examples
///
/// ```
/// use std::ptr;
/// use crossbeam_epoch::Atomic;
///
/// let a = Atomic::<i32>::from(ptr::null::<i32>());
/// ```
fn from(raw: *const T) -> Self {
Self::from_usize(raw as usize)
}
}
/// A trait for either `Owned` or `Shared` pointers.
pub trait Pointer<T> {
/// Returns the machine representation of the pointer.
fn into_usize(self) -> usize;
/// Returns a new pointer pointing to the tagged pointer `data`.
unsafe fn from_usize(data: usize) -> Self;
}
/// An owned heap-allocated object.
///
/// This type is very similar to `Box<T>`.
///
/// The pointer must be properly aligned. Since it is aligned, a tag can be stored into the unused
/// least significant bits of the address.
pub struct Owned<T> {
data: usize,
_marker: PhantomData<Box<T>>,
}
impl<T> Pointer<T> for Owned<T> {
#[inline]
fn into_usize(self) -> usize {
let data = self.data;
mem::forget(self);
data
}
/// Returns a new pointer pointing to the tagged pointer `data`.
///
/// # Panics
///
/// Panics if the data is zero in debug mode.
#[inline]
unsafe fn from_usize(data: usize) -> Self {
debug_assert!(data != 0, "converting zero into `Owned`");
Owned {
data: data,
_marker: PhantomData,
}
}
}
impl<T> Owned<T> {
/// Allocates `value` on the heap and returns a new owned pointer pointing to it.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::Owned;
///
/// let o = Owned::new(1234);
/// ```
pub fn new(value: T) -> Owned<T> {
Self::from(Box::new(value))
}
/// Returns a new owned pointer pointing to `raw`.
///
/// This function is unsafe because improper use may lead to memory problems. Argument `raw`
/// must be a valid pointer. Also, a double-free may occur if the function is called twice on
/// the same raw pointer.
///
/// # Panics
///
/// Panics if `raw` is not properly aligned.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::Owned;
///
/// let o = unsafe { Owned::from_raw(Box::into_raw(Box::new(1234))) };
/// ```
pub unsafe fn from_raw(raw: *mut T) -> Owned<T> {
ensure_aligned(raw);
Self::from_usize(raw as usize)
}
/// Converts the owned pointer into a [`Shared`].
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Owned};
///
/// let o = Owned::new(1234);
/// let guard = &epoch::pin();
/// let p = o.into_shared(guard);
/// ```
///
/// [`Shared`]: struct.Shared.html
pub fn into_shared<'g>(self, _: &'g Guard) -> Shared<'g, T> {
unsafe { Shared::from_usize(self.into_usize()) }
}
/// Converts the owned pointer into a `Box`.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Owned};
///
/// let o = Owned::new(1234);
/// let b: Box<i32> = o.into_box();
/// assert_eq!(*b, 1234);
/// ```
pub fn into_box(self) -> Box<T> {
let (raw, _) = decompose_data::<T>(self.data);
mem::forget(self);
unsafe { Box::from_raw(raw) }
}
/// Returns the tag stored within the pointer.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::Owned;
///
/// assert_eq!(Owned::new(1234).tag(), 0);
/// ```
pub fn tag(&self) -> usize {
let (_, tag) = decompose_data::<T>(self.data);
tag
}
/// Returns the same pointer, but tagged with `tag`. `tag` is truncated to be fit into the
/// unused bits of the pointer to `T`.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::Owned;
///
/// let o = Owned::new(0u64);
/// assert_eq!(o.tag(), 0);
/// let o = o.with_tag(2);
/// assert_eq!(o.tag(), 2);
/// ```
pub fn with_tag(self, tag: usize) -> Owned<T> {
let data = self.into_usize();
unsafe { Self::from_usize(data_with_tag::<T>(data, tag)) }
}
}
impl<T> Drop for Owned<T> {
fn drop(&mut self) {
let (raw, _) = decompose_data::<T>(self.data);
unsafe {
drop(Box::from_raw(raw));
}
}
}
impl<T> fmt::Debug for Owned<T> {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
let (raw, tag) = decompose_data::<T>(self.data);
f.debug_struct("Owned")
.field("raw", &raw)
.field("tag", &tag)
.finish()
}
}
impl<T: Clone> Clone for Owned<T> {
fn clone(&self) -> Self {
Owned::new((**self).clone()).with_tag(self.tag())
}
}
impl<T> Deref for Owned<T> {
type Target = T;
fn deref(&self) -> &T {
let (raw, _) = decompose_data::<T>(self.data);
unsafe { &*raw }
}
}
impl<T> DerefMut for Owned<T> {
fn deref_mut(&mut self) -> &mut T {
let (raw, _) = decompose_data::<T>(self.data);
unsafe { &mut *raw }
}
}
impl<T> From<T> for Owned<T> {
fn from(t: T) -> Self {
Owned::new(t)
}
}
impl<T> From<Box<T>> for Owned<T> {
/// Returns a new owned pointer pointing to `b`.
///
/// # Panics
///
/// Panics if the pointer (the `Box`) is not properly aligned.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::Owned;
///
/// let o = unsafe { Owned::from_raw(Box::into_raw(Box::new(1234))) };
/// ```
fn from(b: Box<T>) -> Self {
unsafe { Self::from_raw(Box::into_raw(b)) }
}
}
impl<T> Borrow<T> for Owned<T> {
fn borrow(&self) -> &T {
&**self
}
}
impl<T> BorrowMut<T> for Owned<T> {
fn borrow_mut(&mut self) -> &mut T {
&mut **self
}
}
impl<T> AsRef<T> for Owned<T> {
fn as_ref(&self) -> &T {
&**self
}
}
impl<T> AsMut<T> for Owned<T> {
fn as_mut(&mut self) -> &mut T {
&mut **self
}
}
/// A pointer to an object protected by the epoch GC.
///
/// The pointer is valid for use only during the lifetime `'g`.
///
/// The pointer must be properly aligned. Since it is aligned, a tag can be stored into the unused
/// least significant bits of the address.
pub struct Shared<'g, T: 'g> {
data: usize,
_marker: PhantomData<(&'g (), *const T)>,
}
impl<'g, T> Clone for Shared<'g, T> {
fn clone(&self) -> Self {
Shared {
data: self.data,
_marker: PhantomData,
}
}
}
impl<'g, T> Copy for Shared<'g, T> {}
impl<'g, T> Pointer<T> for Shared<'g, T> {
#[inline]
fn into_usize(self) -> usize {
self.data
}
#[inline]
unsafe fn from_usize(data: usize) -> Self {
Shared {
data: data,
_marker: PhantomData,
}
}
}
impl<'g, T> Shared<'g, T> {
/// Returns a new null pointer.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::Shared;
///
/// let p = Shared::<i32>::null();
/// assert!(p.is_null());
/// ```
pub fn null() -> Shared<'g, T> {
Shared {
data: 0,
_marker: PhantomData,
}
}
/// Returns `true` if the pointer is null.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic, Owned};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::null();
/// let guard = &epoch::pin();
/// assert!(a.load(SeqCst, guard).is_null());
/// a.store(Owned::new(1234), SeqCst);
/// assert!(!a.load(SeqCst, guard).is_null());
/// ```
pub fn is_null(&self) -> bool {
self.as_raw().is_null()
}
/// Converts the pointer to a raw pointer (without the tag).
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic, Owned};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let o = Owned::new(1234);
/// let raw = &*o as *const _;
/// let a = Atomic::from(o);
///
/// let guard = &epoch::pin();
/// let p = a.load(SeqCst, guard);
/// assert_eq!(p.as_raw(), raw);
/// ```
pub fn as_raw(&self) -> *const T {
let (raw, _) = decompose_data::<T>(self.data);
raw
}
/// Dereferences the pointer.
///
/// Returns a reference to the pointee that is valid during the lifetime `'g`.
///
/// # Safety
///
/// Dereferencing a pointer is unsafe because it could be pointing to invalid memory.
///
/// Another concern is the possiblity of data races due to lack of proper synchronization.
/// For example, consider the following scenario:
///
/// 1. A thread creates a new object: `a.store(Owned::new(10), Relaxed)`
/// 2. Another thread reads it: `*a.load(Relaxed, guard).as_ref().unwrap()`
///
/// The problem is that relaxed orderings don't synchronize initialization of the object with
/// the read from the second thread. This is a data race. A possible solution would be to use
/// `Release` and `Acquire` orderings.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::new(1234);
/// let guard = &epoch::pin();
/// let p = a.load(SeqCst, guard);
/// unsafe {
/// assert_eq!(p.deref(), &1234);
/// }
/// ```
pub unsafe fn deref(&self) -> &'g T {
&*self.as_raw()
}
/// Converts the pointer to a reference.
///
/// Returns `None` if the pointer is null, or else a reference to the object wrapped in `Some`.
///
/// # Safety
///
/// Dereferencing a pointer is unsafe because it could be pointing to invalid memory.
///
/// Another concern is the possiblity of data races due to lack of proper synchronization.
/// For example, consider the following scenario:
///
/// 1. A thread creates a new object: `a.store(Owned::new(10), Relaxed)`
/// 2. Another thread reads it: `*a.load(Relaxed, guard).as_ref().unwrap()`
///
/// The problem is that relaxed orderings don't synchronize initialization of the object with
/// the read from the second thread. This is a data race. A possible solution would be to use
/// `Release` and `Acquire` orderings.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::new(1234);
/// let guard = &epoch::pin();
/// let p = a.load(SeqCst, guard);
/// unsafe {
/// assert_eq!(p.as_ref(), Some(&1234));
/// }
/// ```
pub unsafe fn as_ref(&self) -> Option<&'g T> {
self.as_raw().as_ref()
}
/// Takes ownership of the pointee.
///
/// # Panics
///
/// Panics if this pointer is null, but only in debug mode.
///
/// # Safety
///
/// This method may be called only if the pointer is valid and nobody else is holding a
/// reference to the same object.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::new(1234);
/// unsafe {
/// let guard = &epoch::unprotected();
/// let p = a.load(SeqCst, guard);
/// drop(p.into_owned());
/// }
/// ```
pub unsafe fn into_owned(self) -> Owned<T> {
debug_assert!(
self.as_raw() != ptr::null(),
"converting a null `Shared` into `Owned`"
);
Owned::from_usize(self.data)
}
/// Returns the tag stored within the pointer.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic, Owned};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::<u64>::from(Owned::new(0u64).with_tag(2));
/// let guard = &epoch::pin();
/// let p = a.load(SeqCst, guard);
/// assert_eq!(p.tag(), 2);
/// ```
pub fn tag(&self) -> usize {
let (_, tag) = decompose_data::<T>(self.data);
tag
}
/// Returns the same pointer, but tagged with `tag`. `tag` is truncated to be fit into the
/// unused bits of the pointer to `T`.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::{self as epoch, Atomic};
/// use std::sync::atomic::Ordering::SeqCst;
///
/// let a = Atomic::new(0u64);
/// let guard = &epoch::pin();
/// let p1 = a.load(SeqCst, guard);
/// let p2 = p1.with_tag(2);
///
/// assert_eq!(p1.tag(), 0);
/// assert_eq!(p2.tag(), 2);
/// assert_eq!(p1.as_raw(), p2.as_raw());
/// ```
pub fn with_tag(&self, tag: usize) -> Shared<'g, T> {
unsafe { Self::from_usize(data_with_tag::<T>(self.data, tag)) }
}
}
impl<'g, T> From<*const T> for Shared<'g, T> {
/// Returns a new pointer pointing to `raw`.
///
/// # Panics
///
/// Panics if `raw` is not properly aligned.
///
/// # Examples
///
/// ```
/// use crossbeam_epoch::Shared;
///
/// let p = unsafe { Shared::from(Box::into_raw(Box::new(1234)) as *const _) };
/// assert!(!p.is_null());
/// ```
fn from(raw: *const T) -> Self {
ensure_aligned(raw);
unsafe { Self::from_usize(raw as usize) }
}
}
impl<'g, T> PartialEq<Shared<'g, T>> for Shared<'g, T> {
fn eq(&self, other: &Self) -> bool {
self.data == other.data
}
}
impl<'g, T> Eq for Shared<'g, T> {}
impl<'g, T> PartialOrd<Shared<'g, T>> for Shared<'g, T> {
fn partial_cmp(&self, other: &Self) -> Option<cmp::Ordering> {
self.data.partial_cmp(&other.data)
}
}
impl<'g, T> Ord for Shared<'g, T> {
fn cmp(&self, other: &Self) -> cmp::Ordering {
self.data.cmp(&other.data)
}
}
impl<'g, T> fmt::Debug for Shared<'g, T> {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
let (raw, tag) = decompose_data::<T>(self.data);
f.debug_struct("Shared")
.field("raw", &raw)
.field("tag", &tag)
.finish()
}
}
impl<'g, T> fmt::Pointer for Shared<'g, T> {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
fmt::Pointer::fmt(&self.as_raw(), f)
}
}
impl<'g, T> Default for Shared<'g, T> {
fn default() -> Self {
Shared::null()
}
}
#[cfg(test)]
mod tests {
use super::Shared;
#[test]
fn valid_tag_i8() {
Shared::<i8>::null().with_tag(0);
}
#[test]
fn valid_tag_i64() {
Shared::<i64>::null().with_tag(7);
}
}