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use crate::sys::time;
#[inline]
pub(super) fn monotonize(raw: time::Instant) -> time::Instant {
inner::monotonize(raw)
}
#[cfg(any(all(target_has_atomic = "64", not(target_has_atomic = "128")), target_arch = "aarch64"))]
pub mod inner {
use crate::sync::atomic::AtomicU64;
use crate::sync::atomic::Ordering::*;
use crate::sys::time;
use crate::time::Duration;
pub(in crate::time) const ZERO: time::Instant = time::Instant::zero();
// bits 30 and 31 are never used since the nanoseconds part never exceeds 10^9
const UNINITIALIZED: u64 = 0b11 << 30;
static MONO: AtomicU64 = AtomicU64::new(UNINITIALIZED);
#[inline]
pub(super) fn monotonize(raw: time::Instant) -> time::Instant {
monotonize_impl(&MONO, raw)
}
#[inline]
pub(in crate::time) fn monotonize_impl(mono: &AtomicU64, raw: time::Instant) -> time::Instant {
let delta = raw.checked_sub_instant(&ZERO).unwrap();
let secs = delta.as_secs();
// occupies no more than 30 bits (10^9 seconds)
let nanos = delta.subsec_nanos() as u64;
// This wraps around every 136 years (2^32 seconds).
// To detect backsliding we use wrapping arithmetic and declare forward steps smaller
// than 2^31 seconds as expected and everything else as a backslide which will be
// monotonized.
// This could be a problem for programs that call instants at intervals greater
// than 68 years. Interstellar probes may want to ensure that actually_monotonic() is true.
let packed = (secs << 32) | nanos;
let updated = mono.fetch_update(Relaxed, Relaxed, |old| {
(old == UNINITIALIZED || packed.wrapping_sub(old) < u64::MAX / 2).then_some(packed)
});
match updated {
Ok(_) => raw,
Err(newer) => {
// Backslide occurred. We reconstruct monotonized time from the upper 32 bit of the
// passed in value and the 64bits loaded from the atomic
let seconds_lower = newer >> 32;
let mut seconds_upper = secs & 0xffff_ffff_0000_0000;
if secs & 0xffff_ffff > seconds_lower {
// Backslide caused the lower 32bit of the seconds part to wrap.
// This must be the case because the seconds part is larger even though
// we are in the backslide branch, i.e. the seconds count should be smaller or equal.
//
// We assume that backslides are smaller than 2^32 seconds
// which means we need to add 1 to the upper half to restore it.
//
// Example:
// most recent observed time: 0xA1_0000_0000_0000_0000u128
// bits stored in AtomicU64: 0x0000_0000_0000_0000u64
// backslide by 1s
// caller time is 0xA0_ffff_ffff_0000_0000u128
// -> we can fix up the upper half time by adding 1 << 32
seconds_upper = seconds_upper.wrapping_add(0x1_0000_0000);
}
let secs = seconds_upper | seconds_lower;
let nanos = newer as u32;
ZERO.checked_add_duration(&Duration::new(secs, nanos)).unwrap()
}
}
}
}
#[cfg(all(target_has_atomic = "128", not(target_arch = "aarch64")))]
pub mod inner {
use crate::sync::atomic::AtomicU128;
use crate::sync::atomic::Ordering::*;
use crate::sys::time;
use crate::time::Duration;
const ZERO: time::Instant = time::Instant::zero();
static MONO: AtomicU128 = AtomicU128::new(0);
#[inline]
pub(super) fn monotonize(raw: time::Instant) -> time::Instant {
let delta = raw.checked_sub_instant(&ZERO).unwrap();
// Split into seconds and nanos since Duration doesn't have a
// constructor that takes a u128
let secs = delta.as_secs() as u128;
let nanos = delta.subsec_nanos() as u128;
let timestamp: u128 = secs << 64 | nanos;
let timestamp = MONO.fetch_max(timestamp, Relaxed).max(timestamp);
let secs = (timestamp >> 64) as u64;
let nanos = timestamp as u32;
ZERO.checked_add_duration(&Duration::new(secs, nanos)).unwrap()
}
}
#[cfg(not(any(target_has_atomic = "64", target_has_atomic = "128")))]
pub mod inner {
use crate::cmp;
use crate::sys::time;
use crate::sys_common::mutex::StaticMutex;
#[inline]
pub(super) fn monotonize(os_now: time::Instant) -> time::Instant {
static LOCK: StaticMutex = StaticMutex::new();
static mut LAST_NOW: time::Instant = time::Instant::zero();
unsafe {
let _lock = LOCK.lock();
let now = cmp::max(LAST_NOW, os_now);
LAST_NOW = now;
now
}
}
}
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