From 625b18027d4d8d9b2092cf3706b29a5812f76357 Mon Sep 17 00:00:00 2001
From: Karl Meakin <karl.meakin@arm.com>
Date: Fri, 25 Jul 2025 19:51:21 +0100
Subject: [PATCH] Optimize `SliceIndex::get` impl for `RangeInclusive`

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`.
---
 library/core/src/slice/index.rs            | 38 ++++++++++++----------
 tests/codegen-llvm/slice-range-indexing.rs | 26 ++++++---------
 2 files changed, 31 insertions(+), 33 deletions(-)

diff --git a/library/core/src/slice/index.rs b/library/core/src/slice/index.rs
index 31d9931e474a..3d769010ea1c 100644
--- a/library/core/src/slice/index.rs
+++ b/library/core/src/slice/index.rs
@@ -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)
 }
diff --git a/tests/codegen-llvm/slice-range-indexing.rs b/tests/codegen-llvm/slice-range-indexing.rs
index 5c47cab6234e..c4b3492dc4dc 100644
--- a/tests/codegen-llvm/slice-range-indexing.rs
+++ b/tests/codegen-llvm/slice-range-indexing.rs
@@ -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