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use AtomicU64::fetch_update instead of handrolled RMW-loop

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This commit is contained in:
The8472 2021-09-17 18:42:16 +02:00
parent 2b512cc329
commit 57465d9c1b
2 changed files with 10 additions and 14 deletions
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1
library/std/src/lib.rs
View File

@ -234,6 +234,7 @@
#![feature(atomic_mut_ptr)]
#![feature(auto_traits)]
#![feature(bench_black_box)]
#![feature(bool_to_option)]
#![feature(box_syntax)]
#![feature(c_unwind)]
#![feature(c_variadic)]

23
library/std/src/time/monotonic.rs
View File

@ -37,20 +37,15 @@ pub(in crate::time) fn monotonize_impl(mono: &AtomicU64, raw: time::Instant) ->
// 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 mut old = mono.load(Relaxed);
loop {
if old == UNINITIALIZED || packed.wrapping_sub(old) < u64::MAX / 2 {
match mono.compare_exchange_weak(old, packed, Relaxed, Relaxed) {
Ok(_) => return raw,
Err(x) => {
old = x;
continue;
}
}
} else {
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 = old >> 32;
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.
@ -69,8 +64,8 @@ pub(in crate::time) fn monotonize_impl(mono: &AtomicU64, raw: time::Instant) ->
seconds_upper = seconds_upper.wrapping_add(0x1_0000_0000);
}
let secs = seconds_upper | seconds_lower;
let nanos = old as u32;
return ZERO.checked_add_duration(&Duration::new(secs, nanos)).unwrap();
let nanos = newer as u32;
ZERO.checked_add_duration(&Duration::new(secs, nanos)).unwrap()
}
}
}