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use core::sync::atomic::{compiler_fence, AtomicBool, AtomicU32, Ordering};
#[derive(Clone, Copy)]
pub(crate) struct Token(());
/// Creates a token and passes it to the closure.
///
/// This is a no-op replacement for `CriticalSection::with` because we don't need any locking.
pub(crate) fn locked<R>(f: impl FnOnce(Token) -> R) -> R {
f(Token(()))
}
// Must be kept in sync with the layout of `State`!
pub(crate) const STATE_SPAWNED: u32 = 1 << 0;
pub(crate) const STATE_RUN_QUEUED: u32 = 1 << 8;
#[repr(C, align(4))]
pub(crate) struct State {
/// Task is spawned (has a future)
spawned: AtomicBool,
/// Task is in the executor run queue
run_queued: AtomicBool,
pad: AtomicBool,
pad2: AtomicBool,
}
impl State {
pub const fn new() -> State {
Self {
spawned: AtomicBool::new(false),
run_queued: AtomicBool::new(false),
pad: AtomicBool::new(false),
pad2: AtomicBool::new(false),
}
}
fn as_u32(&self) -> &AtomicU32 {
unsafe { &*(self as *const _ as *const AtomicU32) }
}
/// If task is idle, mark it as spawned + run_queued and return true.
#[inline(always)]
pub fn spawn(&self) -> bool {
compiler_fence(Ordering::Release);
let r = self
.as_u32()
.compare_exchange(
0,
STATE_SPAWNED | STATE_RUN_QUEUED,
Ordering::Relaxed,
Ordering::Relaxed,
)
.is_ok();
compiler_fence(Ordering::Acquire);
r
}
/// Unmark the task as spawned.
#[inline(always)]
pub fn despawn(&self) {
compiler_fence(Ordering::Release);
self.spawned.store(false, Ordering::Relaxed);
}
/// Mark the task as run-queued if it's spawned and isn't already run-queued. Run the given
/// function if the task was successfully marked.
#[inline(always)]
pub fn run_enqueue(&self, f: impl FnOnce(Token)) {
let old = self.run_queued.swap(true, Ordering::AcqRel);
if !old {
locked(f);
}
}
/// Unmark the task as run-queued. Return whether the task is spawned.
#[inline(always)]
pub fn run_dequeue(&self) -> bool {
compiler_fence(Ordering::Release);
let r = self.spawned.load(Ordering::Relaxed);
self.run_queued.store(false, Ordering::Relaxed);
r
}
}
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