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std/sys/unix/time: make it easier for LLVM to optimize Instant subtraction.

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Eduard-Mihai Burtescu 2020-08-15 02:41:41 +03:00
parent 8e21bd0633
commit a7ad899f9d
1 changed files with 20 additions and 8 deletions
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28
library/std/src/sys/unix/time.rs
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@ -20,17 +20,29 @@ impl Timespec {
fn sub_timespec(&self, other: &Timespec) -> Result<Duration, Duration> {
if self >= other {
Ok(if self.t.tv_nsec >= other.t.tv_nsec {
Duration::new(
(self.t.tv_sec - other.t.tv_sec) as u64,
(self.t.tv_nsec - other.t.tv_nsec) as u32,
)
// NOTE(eddyb) two aspects of this `if`-`else` are required for LLVM
// to optimize it into a branchless form (see also #75545):
//
// 1. `self.t.tv_sec - other.t.tv_sec` shows up as a common expression
// in both branches, i.e. the `else` must have its `- 1`
// subtraction after the common one, not interleaved with it
// (it used to be `self.t.tv_sec - 1 - other.t.tv_sec`)
//
// 2. the `Duration::new` call (or any other additional complexity)
// is outside of the `if`-`else`, not duplicated in both branches
//
// Ideally this code could be rearranged such that it more
// directly expresses the lower-cost behavior we want from it.
let (secs, nsec) = if self.t.tv_nsec >= other.t.tv_nsec {
((self.t.tv_sec - other.t.tv_sec) as u64, (self.t.tv_nsec - other.t.tv_nsec) as u32)
} else {
Duration::new(
(self.t.tv_sec - 1 - other.t.tv_sec) as u64,
(
(self.t.tv_sec - other.t.tv_sec - 1) as u64,
self.t.tv_nsec as u32 + (NSEC_PER_SEC as u32) - other.t.tv_nsec as u32,
)
})
};
Ok(Duration::new(secs, nsec))
} else {
match other.sub_timespec(self) {
Ok(d) => Err(d),