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#[cfg(not(crossbeam_no_atomic))]
use core::sync::atomic::Ordering;
/// Trait which allows reading from primitive atomic types with "consume" ordering.
pub trait AtomicConsume {
/// Type returned by `load_consume`.
type Val;
/// Loads a value from the atomic 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.
///
/// This is currently only implemented on ARM and AArch64, where a fence
/// can be avoided. On other architectures this will fall back to a simple
/// `load(Ordering::Acquire)`.
fn load_consume(&self) -> Self::Val;
}
#[cfg(not(crossbeam_no_atomic))]
// Miri and Loom don't support "consume" ordering and ThreadSanitizer doesn't treat
// load(Relaxed) + compiler_fence(Acquire) as "consume" load.
// LLVM generates machine code equivalent to fence(Acquire) in compiler_fence(Acquire)
// can be actually avoided here only on ARM and AArch64. See also
#[cfg(all(
any(target_arch = "arm", target_arch = "aarch64"),
not(any(miri, crossbeam_loom, crossbeam_sanitize_thread)),
))]
macro_rules! impl_consume {
() => {
#[inline]
fn load_consume(&self) -> Self::Val {
use crate::primitive::sync::atomic::compiler_fence;
let result = self.load(Ordering::Relaxed);
compiler_fence(Ordering::Acquire);
result
}
};
}
#[cfg(not(crossbeam_no_atomic))]
#[cfg(not(all(
any(target_arch = "arm", target_arch = "aarch64"),
not(any(miri, crossbeam_loom, crossbeam_sanitize_thread)),
)))]
macro_rules! impl_consume {
() => {
#[inline]
fn load_consume(&self) -> Self::Val {
self.load(Ordering::Acquire)
}
};
}
macro_rules! impl_atomic {
($atomic:ident, $val:ty) => {
#[cfg(not(crossbeam_no_atomic))]
impl AtomicConsume for core::sync::atomic::$atomic {
type Val = $val;
impl_consume!();
}
#[cfg(crossbeam_loom)]
impl AtomicConsume for loom::sync::atomic::$atomic {
type Val = $val;
impl_consume!();
}
};
}
impl_atomic!(AtomicBool, bool);
impl_atomic!(AtomicUsize, usize);
impl_atomic!(AtomicIsize, isize);
impl_atomic!(AtomicU8, u8);
impl_atomic!(AtomicI8, i8);
impl_atomic!(AtomicU16, u16);
impl_atomic!(AtomicI16, i16);
#[cfg(any(target_has_atomic = "32", not(target_pointer_width = "16")))]
impl_atomic!(AtomicU32, u32);
#[cfg(any(target_has_atomic = "32", not(target_pointer_width = "16")))]
impl_atomic!(AtomicI32, i32);
#[cfg(any(
target_has_atomic = "64",
not(any(target_pointer_width = "16", target_pointer_width = "32")),
))]
impl_atomic!(AtomicU64, u64);
#[cfg(any(
target_has_atomic = "64",
not(any(target_pointer_width = "16", target_pointer_width = "32")),
))]
impl_atomic!(AtomicI64, i64);
#[cfg(not(crossbeam_no_atomic))]
impl<T> AtomicConsume for core::sync::atomic::AtomicPtr<T> {
type Val = *mut T;
impl_consume!();
}
#[cfg(crossbeam_loom)]
impl<T> AtomicConsume for loom::sync::atomic::AtomicPtr<T> {
type Val = *mut T;
impl_consume!();
}