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Diffstat (limited to 'third_party/rust/crossbeam-utils-0.6.6/src/atomic/atomic_cell.rs')
| -rw-r--r-- | third_party/rust/crossbeam-utils-0.6.6/src/atomic/atomic_cell.rs | 968 |
1 files changed, 968 insertions, 0 deletions
diff --git a/third_party/rust/crossbeam-utils-0.6.6/src/atomic/atomic_cell.rs b/third_party/rust/crossbeam-utils-0.6.6/src/atomic/atomic_cell.rs new file mode 100644 index 0000000000..f3118ecacf --- /dev/null +++ b/third_party/rust/crossbeam-utils-0.6.6/src/atomic/atomic_cell.rs @@ -0,0 +1,968 @@ +use core::cell::UnsafeCell; +use core::fmt; +use core::mem; +use core::ptr; +use core::sync::atomic::{self, AtomicBool, AtomicUsize, Ordering}; + +#[cfg(feature = "std")] +use std::panic::{RefUnwindSafe, UnwindSafe}; + +use Backoff; + +/// 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`]: https://doc.rust-lang.org/std/cell/struct.Cell.html +/// [`AtomicCell::<T>::is_lock_free()`]: struct.AtomicCell.html#method.is_lock_free +/// [`Acquire`]: https://doc.rust-lang.org/std/sync/atomic/enum.Ordering.html#variant.Acquire +/// [`Release`]: https://doc.rust-lang.org/std/sync/atomic/enum.Ordering.html#variant.Release +// TODO(@jeehoonkang): when the minimum supported Rust version is bumped to 1.28+, apply the +// attribute `#[repr(transparent)]`. +pub struct AtomicCell<T: ?Sized> { + /// 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. + value: UnsafeCell<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 fn new(val: T) -> AtomicCell<T> { + AtomicCell { + value: UnsafeCell::new(val), + } + } + + /// Unwraps the atomic cell and returns its inner value. + /// + /// # Examples + /// + /// ``` + /// use crossbeam_utils::atomic::AtomicCell; + /// + /// let mut a = AtomicCell::new(7); + /// let v = a.into_inner(); + /// + /// assert_eq!(v, 7); + /// ``` + pub fn into_inner(self) -> T { + self.value.into_inner() + } + + /// 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 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.value.get(), 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.value.get(), val) } + } +} + +impl<T: ?Sized> AtomicCell<T> { + /// Returns a raw pointer to the underlying data in this atomic cell. + /// + /// # Examples + /// + /// ``` + /// use crossbeam_utils::atomic::AtomicCell; + /// + /// let mut a = AtomicCell::new(5); + /// + /// let ptr = a.as_ptr(); + /// ``` + #[inline] + pub fn as_ptr(&self) -> *mut T { + self.value.get() + } + + /// Returns a mutable reference to the inner value. + /// + /// # Examples + /// + /// ``` + /// use crossbeam_utils::atomic::AtomicCell; + /// + /// let mut a = AtomicCell::new(7); + /// *a.get_mut() += 1; + /// + /// assert_eq!(a.load(), 8); + /// ``` + #[doc(hidden)] + #[deprecated(note = "this method is unsound and will be removed in the next release")] + pub fn get_mut(&mut self) -> &mut T { + unsafe { &mut *self.value.get() } + } +} + +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. + /// + /// # 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.value.get()) } + } +} + +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 + /// + /// ``` + /// 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); + /// ``` + 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.value.get(), current, new) } + } +} + +macro_rules! impl_arithmetic { + ($t: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::AtomicUsize>() { + let a = unsafe { &*(self.value.get() as *const atomic::AtomicUsize) }; + a.fetch_add(val as usize, Ordering::AcqRel) as $t + } else { + let _guard = lock(self.value.get() as usize).write(); + let value = unsafe { &mut *(self.value.get()) }; + 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::AtomicUsize>() { + let a = unsafe { &*(self.value.get() as *const atomic::AtomicUsize) }; + a.fetch_sub(val as usize, Ordering::AcqRel) as $t + } else { + let _guard = lock(self.value.get() as usize).write(); + let value = unsafe { &mut *(self.value.get()) }; + 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::AtomicUsize>() { + let a = unsafe { &*(self.value.get() as *const atomic::AtomicUsize) }; + a.fetch_and(val as usize, Ordering::AcqRel) as $t + } else { + let _guard = lock(self.value.get() as usize).write(); + let value = unsafe { &mut *(self.value.get()) }; + let old = *value; + *value &= 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::AtomicUsize>() { + let a = unsafe { &*(self.value.get() as *const atomic::AtomicUsize) }; + a.fetch_or(val as usize, Ordering::AcqRel) as $t + } else { + let _guard = lock(self.value.get() as usize).write(); + let value = unsafe { &mut *(self.value.get()) }; + 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::AtomicUsize>() { + let a = unsafe { &*(self.value.get() as *const atomic::AtomicUsize) }; + a.fetch_xor(val as usize, Ordering::AcqRel) as $t + } else { + let _guard = lock(self.value.get() as usize).write(); + let value = unsafe { &mut *(self.value.get()) }; + let old = *value; + *value ^= val; + old + } + } + } + }; + ($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 { + let a = unsafe { &*(self.value.get() as *const $atomic) }; + a.fetch_add(val, Ordering::AcqRel) + } + + /// 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 a = unsafe { &*(self.value.get() as *const $atomic) }; + a.fetch_sub(val, Ordering::AcqRel) + } + + /// 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 a = unsafe { &*(self.value.get() as *const $atomic) }; + a.fetch_and(val, Ordering::AcqRel) + } + + /// 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 a = unsafe { &*(self.value.get() as *const $atomic) }; + a.fetch_or(val, Ordering::AcqRel) + } + + /// 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 a = unsafe { &*(self.value.get() as *const $atomic) }; + a.fetch_xor(val, Ordering::AcqRel) + } + } + }; + ($t:ty, $size:tt, $atomic:ty, $example:tt) => { + #[cfg(target_has_atomic = $size)] + impl_arithmetic!($t, $atomic, $example); + }; +} + +cfg_if! { + if #[cfg(feature = "nightly")] { + impl_arithmetic!(u8, "8", atomic::AtomicU8, "let a = AtomicCell::new(7u8);"); + impl_arithmetic!(i8, "8", atomic::AtomicI8, "let a = AtomicCell::new(7i8);"); + impl_arithmetic!(u16, "16", atomic::AtomicU16, "let a = AtomicCell::new(7u16);"); + impl_arithmetic!(i16, "16", atomic::AtomicI16, "let a = AtomicCell::new(7i16);"); + impl_arithmetic!(u32, "32", atomic::AtomicU32, "let a = AtomicCell::new(7u32);"); + impl_arithmetic!(i32, "32", atomic::AtomicI32, "let a = AtomicCell::new(7i32);"); + impl_arithmetic!(u64, "64", atomic::AtomicU64, "let a = AtomicCell::new(7u64);"); + impl_arithmetic!(i64, "64", atomic::AtomicI64, "let a = AtomicCell::new(7i64);"); + impl_arithmetic!(u128, "let a = AtomicCell::new(7u128);"); + impl_arithmetic!(i128, "let a = AtomicCell::new(7i128);"); + } else { + impl_arithmetic!(u8, "let a = AtomicCell::new(7u8);"); + impl_arithmetic!(i8, "let a = AtomicCell::new(7i8);"); + impl_arithmetic!(u16, "let a = AtomicCell::new(7u16);"); + impl_arithmetic!(i16, "let a = AtomicCell::new(7i16);"); + impl_arithmetic!(u32, "let a = AtomicCell::new(7u32);"); + impl_arithmetic!(i32, "let a = AtomicCell::new(7i32);"); + impl_arithmetic!(u64, "let a = AtomicCell::new(7u64);"); + impl_arithmetic!(i64, "let a = AtomicCell::new(7i64);"); + impl_arithmetic!(u128, "let a = AtomicCell::new(7u128);"); + impl_arithmetic!(i128, "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.value.get() as *const AtomicBool) }; + a.fetch_and(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.value.get() 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.value.get() 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: 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`. +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>() +} + +/// A simple stamped lock. +struct Lock { + /// The current state of the lock. + /// + /// All bits except the least significant one hold the current stamp. When locked, the state + /// equals 1 and doesn't contain a valid stamp. + state: AtomicUsize, +} + +impl Lock { + /// If not locked, returns the current stamp. + /// + /// This method should be called before optimistic reads. + #[inline] + fn optimistic_read(&self) -> Option<usize> { + let state = self.state.load(Ordering::Acquire); + if state == 1 { + None + } else { + Some(state) + } + } + + /// Returns `true` if the current stamp is equal to `stamp`. + /// + /// This method should be called after optimistic reads to check whether they are valid. The + /// argument `stamp` should correspond to the one returned by method `optimistic_read`. + #[inline] + fn validate_read(&self, stamp: usize) -> bool { + atomic::fence(Ordering::Acquire); + self.state.load(Ordering::Relaxed) == stamp + } + + /// Grabs the lock for writing. + #[inline] + fn write(&'static self) -> WriteGuard { + let backoff = Backoff::new(); + loop { + let previous = self.state.swap(1, Ordering::Acquire); + + if previous != 1 { + atomic::fence(Ordering::Release); + + return WriteGuard { + lock: self, + state: previous, + }; + } + + backoff.snooze(); + } + } +} + +/// A RAII guard that releases the lock and increments the stamp when dropped. +struct WriteGuard { + /// The parent lock. + lock: &'static Lock, + + /// The stamp before locking. + state: usize, +} + +impl WriteGuard { + /// Releases the lock without incrementing the stamp. + #[inline] + fn abort(self) { + self.lock.state.store(self.state, Ordering::Release); + } +} + +impl Drop for WriteGuard { + #[inline] + fn drop(&mut self) { + // Release the lock and increment the stamp. + self.lock + .state + .store(self.state.wrapping_add(2), Ordering::Release); + } +} + +/// 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 Lock { + // 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; + + const L: Lock = Lock { + state: AtomicUsize::new(0), + }; + static LOCKS: [Lock; LEN] = [ + L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, + L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, + L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, + L, L, L, L, L, L, L, + ]; + + // 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::AtomicUsize, $a, $atomic_op); + + #[cfg(feature = "nightly")] + { + #[cfg(target_has_atomic = "8")] + atomic!(@check, $t, atomic::AtomicU8, $a, $atomic_op); + #[cfg(target_has_atomic = "16")] + atomic!(@check, $t, atomic::AtomicU16, $a, $atomic_op); + #[cfg(target_has_atomic = "32")] + atomic!(@check, $t, atomic::AtomicU32, $a, $atomic_op); + #[cfg(target_has_atomic = "64")] + atomic!(@check, $t, atomic::AtomicU64, $a, $atomic_op); + } + + break $fallback_op; + } + }; +} + +/// Returns `true` if operations on `AtomicCell<T>` are lock-free. +fn atomic_is_lock_free<T>() -> bool { + atomic! { T, _a, true, false } +} + +/// 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. + let val = ptr::read_volatile(src); + + if lock.validate_read(stamp) { + return val; + } + } + + // 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. +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) + } + } + } +} |