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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.
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@ -45,7 +45,8 @@ impl IndexRange {
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#[inline]
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pub const fn len(&self) -> usize {
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// SAFETY: By invariant, this cannot wrap
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unsafe { self.end.unchecked_sub(self.start) }
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// Using the intrinsic because a UB check here impedes LLVM optimization. (#131563)
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unsafe { crate::intrinsics::unchecked_sub(self.end, self.start) }
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}
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/// # Safety
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@ -82,7 +83,8 @@ impl IndexRange {
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let mid = if n <= self.len() {
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// SAFETY: We just checked that this will be between start and end,
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// and thus the addition cannot overflow.
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unsafe { self.start.unchecked_add(n) }
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// Using the intrinsic avoids a superfluous UB check.
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unsafe { crate::intrinsics::unchecked_add(self.start, n) }
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} else {
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self.end
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};
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@ -100,8 +102,9 @@ impl IndexRange {
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pub fn take_suffix(&mut self, n: usize) -> Self {
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let mid = if n <= self.len() {
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// SAFETY: We just checked that this will be between start and end,
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// and thus the addition cannot overflow.
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unsafe { self.end.unchecked_sub(n) }
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// and thus the subtraction cannot overflow.
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// Using the intrinsic avoids a superfluous UB check.
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unsafe { crate::intrinsics::unchecked_sub(self.end, n) }
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} else {
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self.start
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};
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@ -1,4 +1,9 @@
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//@ compile-flags: -O
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//@ revisions: host x86-64-v3
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// This particular CPU regressed in #131563
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//@[x86-64-v3] only-x86_64
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//@[x86-64-v3] compile-flags: -Ctarget-cpu=x86-64-v3
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#![crate_type = "lib"]
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