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Optimize SliceIndex::get impl for RangeInclusive

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The checks for `self.end() == usize::MAX` and `self.end() + 1 > slice.len()`
can be replaced with `self.end() >= slice.len()`, since
`self.end() < slice.len()` implies both
`self.end() <= slice.len()` and
`self.end() < usize::MAX`.
This commit is contained in:
Karl Meakin 2025-07-25 19:51:21 +01:00
parent d74b276d1d
commit 625b18027d
2 changed files with 31 additions and 33 deletions
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38
library/core/src/slice/index.rs
View file

@ -663,7 +663,6 @@ unsafe impl<T> const SliceIndex<[T]> for ops::RangeFull {
}
/// The methods `index` and `index_mut` panic if:
/// - the end of the range is `usize::MAX` or
/// - the start of the range is greater than the end of the range or
/// - the end of the range is out of bounds.
#[stable(feature = "inclusive_range", since = "1.26.0")]
@ -673,12 +672,12 @@ unsafe impl<T> const SliceIndex<[T]> for ops::RangeInclusive<usize> {
#[inline]
fn get(self, slice: &[T]) -> Option<&[T]> {
if *self.end() == usize::MAX { None } else { self.into_slice_range().get(slice) }
if *self.end() >= slice.len() { None } else { self.into_slice_range().get(slice) }
}
#[inline]
fn get_mut(self, slice: &mut [T]) -> Option<&mut [T]> {
if *self.end() == usize::MAX { None } else { self.into_slice_range().get_mut(slice) }
if *self.end() >= slice.len() { None } else { self.into_slice_range().get_mut(slice) }
}
#[inline]
@ -950,8 +949,7 @@ where
R: ops::RangeBounds<usize>,
{
let len = bounds.end;
let r = into_range(len, (range.start_bound().copied(), range.end_bound().copied()))?;
if r.start > r.end || r.end > len { None } else { Some(r) }
into_range(len, (range.start_bound().copied(), range.end_bound().copied()))
}
/// Converts a pair of `ops::Bound`s into `ops::Range` without performing any
@ -982,21 +980,27 @@ pub(crate) const fn into_range(
len: usize,
(start, end): (ops::Bound<usize>, ops::Bound<usize>),
) -> Option<ops::Range<usize>> {
use ops::Bound;
let start = match start {
Bound::Included(start) => start,
Bound::Excluded(start) => start.checked_add(1)?,
Bound::Unbounded => 0,
};
let end = match end {
Bound::Included(end) => end.checked_add(1)?,
Bound::Excluded(end) => end,
Bound::Unbounded => len,
ops::Bound::Included(end) if end >= len => return None,
// Cannot overflow because `end < len` implies `end < usize::MAX`.
ops::Bound::Included(end) => end + 1,
ops::Bound::Excluded(end) if end > len => return None,
ops::Bound::Excluded(end) => end,
ops::Bound::Unbounded => len,
};
// Don't bother with checking `start < end` and `end <= len`
// since these checks are handled by `Range` impls
let start = match start {
ops::Bound::Excluded(start) if start >= end => return None,
// Cannot overflow because `start < end` implies `start < usize::MAX`.
ops::Bound::Excluded(start) => start + 1,
ops::Bound::Included(start) if start > end => return None,
ops::Bound::Included(start) => start,
ops::Bound::Unbounded => 0,
};
Some(start..end)
}

26
tests/codegen-llvm/slice-range-indexing.rs
View file

@ -33,17 +33,14 @@ macro_rules! tests {
// CHECK: ret
tests!(Range<usize>, get_range, index_range);
// 3 comparisons required:
// end != usize::MAX && end + 1 <= len && start <= end + 1
// CHECK-LABEL: @get_range_inclusive
// CHECK-COUNT-3: %{{.+}} = icmp
// CHECK-NOT: %{{.+}} = icmp
// CHECK: ret
// 2 comparisons required:
// end < len && start <= end + 1
// CHECK-LABEL: @get_range_inclusive
// CHECK-COUNT-2: %{{.+}} = icmp
// CHECK-NOT: %{{.+}} = icmp
// CHECK: ret
// CHECK-LABEL: @index_range_inclusive
// CHECK-COUNT-2: %{{.+}} = icmp
// CHECK-NOT: %{{.+}} = icmp
@ -64,17 +61,14 @@ tests!(RangeInclusive<usize>, get_range_inclusive, index_range_inclusive);
// CHECK: ret
tests!(RangeTo<usize>, get_range_to, index_range_to);
// 2 comparisons required:
// end != usize::MAX && end + 1 <= len
// CHECK-LABEL: @get_range_to_inclusive
// CHECK-COUNT-2: %{{.+}} = icmp
// CHECK-NOT: %{{.+}} = icmp
// CHECK: ret
// 1 comparison required:
// end < len
// CHECK-LABEL: @get_range_to_inclusive
// CHECK-COUNT-1: %{{.+}} = icmp
// CHECK-NOT: %{{.+}} = icmp
// CHECK: ret
// CHECK-LABEL: @index_range_to_inclusive
// CHECK-COUNT-1: %{{.+}} = icmp
// CHECK-NOT: %{{.+}} = icmp