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where available use 64- or 128bit atomics instead of a Mutex to monot…
…onize time
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use crate::sys::time; | ||
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#[inline] | ||
pub(super) fn monotonize(raw: time::Instant) -> time::Instant { | ||
inner::monotonize(raw) | ||
} | ||
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#[cfg(all(target_has_atomic = "64", not(target_has_atomic = "128")))] | ||
pub mod inner { | ||
use crate::sync::atomic::AtomicU64; | ||
use crate::sync::atomic::Ordering::*; | ||
use crate::sys::time; | ||
use crate::time::Duration; | ||
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const ZERO: time::Instant = time::Instant::zero(); | ||
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// bits 30 and 31 are never used since the seconds part never exceeds 10^9 | ||
const UNINITIALIZED: u64 = 0xff00_0000; | ||
static MONO: AtomicU64 = AtomicU64::new(UNINITIALIZED); | ||
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#[inline] | ||
pub(super) fn monotonize(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; | ||
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// 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 old = MONO.load(Relaxed); | ||
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if packed == UNINITIALIZED || packed.wrapping_sub(old) < u64::MAX / 2 { | ||
MONO.store(packed, Relaxed); | ||
raw | ||
} else { | ||
// Backslide occurred. We reconstruct monotonized time by assuming the clock will never | ||
// backslide more than 2`32 seconds which means we can reuse the upper 32bits from | ||
// the seconds. | ||
let secs = (secs & 0xffff_ffff << 32) | old >> 32; | ||
let nanos = old as u32; | ||
ZERO.checked_add_duration(&Duration::new(secs, nanos)).unwrap() | ||
} | ||
} | ||
} | ||
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#[cfg(target_has_atomic = "128")] | ||
pub mod inner { | ||
use crate::sync::atomic::AtomicU128; | ||
use crate::sync::atomic::Ordering::*; | ||
use crate::sys::time; | ||
use crate::time::Duration; | ||
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const ZERO: time::Instant = time::Instant::zero(); | ||
static MONO: AtomicU128 = AtomicU128::new(0); | ||
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#[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 an 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() | ||
} | ||
} | ||
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#[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; | ||
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#[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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