Avoid superfluous UB checks in IndexRange

`IndexRange::len` is justified as an overall invariant, and `take_prefix` and `take_suffix` are justified by local branch conditions. A few more UB-checked calls remain in cases that are only supported locally by `debug_assert!`, which won't do anything in distributed builds, so those UB checks may still be useful. We generally expect core's `#![rustc_preserve_ub_checks]` to optimize away in user's release builds, but the mere presence of that extra code can sometimes inhibit optimization, as seen in #131563.

Author: cuviper

library/core/src/ops/index_range.rs

@@ -45,7 +45,8 @@ impl IndexRange {
  #[inline]
  pub const fn len(&self) -> usize {
  // SAFETY: By invariant, this cannot wrap
- unsafe { self.end.unchecked_sub(self.start) }
+ // Using the intrinsic avoids a superfluous UB check.
+ unsafe { crate::intrinsics::unchecked_sub(self.end, self.start) }
  }
 
  /// # Safety
@@ -82,7 +83,8 @@ impl IndexRange {
  let mid = if n <= self.len() {
  // SAFETY: We just checked that this will be between start and end,
  // and thus the addition cannot overflow.
- unsafe { self.start.unchecked_add(n) }
+ // Using the intrinsic avoids a superfluous UB check.
+ unsafe { crate::intrinsics::unchecked_add(self.start, n) }
  } else {
  self.end
  };
@@ -100,8 +102,9 @@ impl IndexRange {
  pub fn take_suffix(&mut self, n: usize) -> Self {
  let mid = if n <= self.len() {
  // SAFETY: We just checked that this will be between start and end,
- // and thus the addition cannot overflow.
- unsafe { self.end.unchecked_sub(n) }
+ // and thus the subtraction cannot overflow.
+ // Using the intrinsic avoids a superfluous UB check.
+ unsafe { crate::intrinsics::unchecked_sub(self.end, n) }
  } else {
  self.start
  };

tests/codegen/issues/issue-101082.rs

@@ -1,4 +1,9 @@
 //@ compile-flags: -O
+//@ revisions: host x86-64-v3
+
+// This particular CPU regressed in #131563
+//@[x86-64-v3] only-x86_64
+//@[x86-64-v3] compile-flags: -Ctarget-cpu=x86-64-v3
 
 #![crate_type = "lib"]