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Add an overflow check in the Iter::next() impl for Range<_> to help with vectorization. #43595

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Aug 9, 2017
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Add an overflow check in the Iter::next() impl for Range<_> to help with vectorization. #43595

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13 changes: 10 additions & 3 deletions src/libcore/iter/range.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -214,9 +214,16 @@ impl<A: Step> Iterator for ops::Range<A> {
#[inline]
fn next(&mut self) -> Option<A> {
if self.start < self.end {
let mut n = self.start.add_one();
mem::swap(&mut n, &mut self.start);
Some(n)
// We check for overflow here, even though it can't actually
// happen. Adding this check does however help llvm vectorize loops
// for some ranges that don't get vectorized otherwise,
// and this won't actually result in an extra check in an optimized build.
if let Some(mut n) = self.start.add_usize(1) {
mem::swap(&mut n, &mut self.start);
Some(n)
} else {
None
}
} else {
None
}
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