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// Copyright 2016 Amanieu d'Antras
//
// Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or
// copied, modified, or distributed except according to those terms.
use crate::thread_parker;
use core::hint::spin_loop;
// Wastes some CPU time for the given number of iterations,
// using a hint to indicate to the CPU that we are spinning.
#[inline]
fn cpu_relax(iterations: u32) {
for _ in 0..iterations {
spin_loop()
}
}
/// A counter used to perform exponential backoff in spin loops.
#[derive(Default)]
pub struct SpinWait {
counter: u32,
}
impl SpinWait {
/// Creates a new `SpinWait`.
#[inline]
pub fn new() -> Self {
Self::default()
}
/// Resets a `SpinWait` to its initial state.
#[inline]
pub fn reset(&mut self) {
self.counter = 0;
}
/// Spins until the sleep threshold has been reached.
///
/// This function returns whether the sleep threshold has been reached, at
/// which point further spinning has diminishing returns and the thread
/// should be parked instead.
///
/// The spin strategy will initially use a CPU-bound loop but will fall back
/// to yielding the CPU to the OS after a few iterations.
#[inline]
pub fn spin(&mut self) -> bool {
if self.counter >= 10 {
return false;
}
self.counter += 1;
if self.counter <= 3 {
cpu_relax(1 << self.counter);
} else {
thread_parker::thread_yield();
}
true
}
/// Spins without yielding the thread to the OS.
///
/// Instead, the backoff is simply capped at a maximum value. This can be
/// used to improve throughput in `compare_exchange` loops that have high
/// contention.
#[inline]
pub fn spin_no_yield(&mut self) {
self.counter += 1;
if self.counter > 10 {
self.counter = 10;
}
cpu_relax(1 << self.counter);
}
}