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Rollup merge of #103594 - maniwani:fix-issue-91417, r=thomcc 

Fix non-associativity of `Instant` math on `aarch64-apple-darwin` targets

This is a duplicate of #94100 (since the original author is unresponsive), which resolves #91417.

On `aarch64-apple-darwin` targets, the internal resolution of `Instant` is lower than that of `Duration`, so math between them becomes non-associative with small-enough durations.

This PR makes this target use the standard Unix implementation (where `Instant` has 1ns resolution), but with `CLOCK_UPTIME_RAW` so it still returns the same values as `mach_absolute_time`[^1].

(Edit: I need someone to confirm that this still works, I do not have access to an M1 device.)

[^1]: https://www.manpagez.com/man/3/clock_gettime/
This commit is contained in:
Matthias Krüger 2022-11-18 14:13:36 +01:00 committed by GitHub
parent 3efbf30220 f4f515973e
commit 6419151f8b
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2 changed files with 24 additions and 9 deletions
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25
library/std/src/sys/unix/time.rs
View File

@ -149,7 +149,11 @@ impl From<libc::timespec> for Timespec {
}
}
#[cfg(any(target_os = "macos", target_os = "ios", target_os = "watchos"))]
#[cfg(any(
all(target_os = "macos", any(not(target_arch = "aarch64"), miri)),
target_os = "ios",
target_os = "watchos"
))]
mod inner {
use crate::sync::atomic::{AtomicU64, Ordering};
use crate::sys::cvt;
@ -265,7 +269,11 @@ mod inner {
}
}
#[cfg(not(any(target_os = "macos", target_os = "ios", target_os = "watchos")))]
#[cfg(not(any(
all(target_os = "macos", any(not(target_arch = "aarch64"), miri)),
target_os = "ios",
target_os = "watchos"
)))]
mod inner {
use crate::fmt;
use crate::mem::MaybeUninit;
@ -281,7 +289,11 @@ mod inner {
impl Instant {
pub fn now() -> Instant {
Instant { t: Timespec::now(libc::CLOCK_MONOTONIC) }
#[cfg(target_os = "macos")]
const clock_id: libc::clockid_t = libc::CLOCK_UPTIME_RAW;
#[cfg(not(target_os = "macos"))]
const clock_id: libc::clockid_t = libc::CLOCK_MONOTONIC;
Instant { t: Timespec::now(clock_id) }
}
pub fn checked_sub_instant(&self, other: &Instant) -> Option<Duration> {
@ -312,13 +324,8 @@ mod inner {
}
}
#[cfg(not(any(target_os = "dragonfly", target_os = "espidf", target_os = "horizon")))]
pub type clock_t = libc::c_int;
#[cfg(any(target_os = "dragonfly", target_os = "espidf", target_os = "horizon"))]
pub type clock_t = libc::c_ulong;
impl Timespec {
pub fn now(clock: clock_t) -> Timespec {
pub fn now(clock: libc::clockid_t) -> Timespec {
// Try to use 64-bit time in preparation for Y2038.
#[cfg(all(target_os = "linux", target_env = "gnu", target_pointer_width = "32"))]
{

8
library/std/src/time/tests.rs
View File

@ -88,6 +88,14 @@ fn instant_math_is_associative() {
// Changing the order of instant math shouldn't change the results,
// especially when the expression reduces to X + identity.
assert_eq!((now + offset) - now, (now - now) + offset);
// On any platform, `Instant` should have the same resolution as `Duration` (e.g. 1 nanosecond)
// or better. Otherwise, math will be non-associative (see #91417).
let now = Instant::now();
let provided_offset = Duration::from_nanos(1);
let later = now + provided_offset;
let measured_offset = later - now;
assert_eq!(measured_offset, provided_offset);
}
#[test]