diff options
Diffstat (limited to 'library/std/src/sys/pal/unix/time.rs')
| -rw-r--r-- | library/std/src/sys/pal/unix/time.rs | 27 |
1 files changed, 12 insertions, 15 deletions
diff --git a/library/std/src/sys/pal/unix/time.rs b/library/std/src/sys/pal/unix/time.rs index bd7f74fea6a..c207f41cad4 100644 --- a/library/std/src/sys/pal/unix/time.rs +++ b/library/std/src/sys/pal/unix/time.rs @@ -134,28 +134,25 @@ impl Timespec { } pub fn sub_timespec(&self, other: &Timespec) -> Result<Duration, Duration> { + // When a >= b, the difference fits in u64. + fn sub_ge_to_unsigned(a: i64, b: i64) -> u64 { + debug_assert!(a >= b); + a.wrapping_sub(b).cast_unsigned() + } + if self >= other { - // NOTE(eddyb) two aspects of this `if`-`else` are required for LLVM - // to optimize it into a branchless form (see also #75545): - // - // 1. `self.tv_sec - other.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.tv_sec - 1 - other.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.tv_nsec.as_inner() >= other.tv_nsec.as_inner() { ( - (self.tv_sec - other.tv_sec) as u64, + sub_ge_to_unsigned(self.tv_sec, other.tv_sec), self.tv_nsec.as_inner() - other.tv_nsec.as_inner(), ) } else { + // Following sequence of assertions explain why `self.tv_sec - 1` does not underflow. + debug_assert!(self.tv_nsec < other.tv_nsec); + debug_assert!(self.tv_sec > other.tv_sec); + debug_assert!(self.tv_sec > i64::MIN); ( - (self.tv_sec - other.tv_sec - 1) as u64, + sub_ge_to_unsigned(self.tv_sec - 1, other.tv_sec), self.tv_nsec.as_inner() + (NSEC_PER_SEC as u32) - other.tv_nsec.as_inner(), ) }; |