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Try to guess a smarter initial capacity in Vec::from_iter #53086

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Try to guess a smarter initial capacity in Vec::from_iter #53086

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d545902
Try to guess a smarter initial capacity in Vec::from_iter
scottmcm Aug 5, 2018
aa080f4
Be much less clever about things
scottmcm Aug 8, 2018
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24 changes: 13 additions & 11 deletions src/liballoc/vec.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1857,18 +1857,20 @@ impl<T, I> SpecExtend<T, I> for Vec<T>
// empty, but the loop in extend_desugared() is not going to see the
// vector being full in the few subsequent loop iterations.
// So we get better branch prediction.
let mut vector = match iterator.next() {
None => return Vec::new(),
Some(element) => {
let (lower, _) = iterator.size_hint();
let mut vector = Vec::with_capacity(lower.saturating_add(1));
unsafe {
ptr::write(vector.get_unchecked_mut(0), element);
vector.set_len(1);
}
vector
}
let element =
if let Some(x) = iterator.next() { x }
else { return Vec::new() };
let (lower, upper) = iterator.size_hint();
let guess = if let Some(upper) = upper {
lower.saturating_mul(2).min(upper)
} else {
lower
};
let mut vector = Vec::with_capacity(guess.saturating_add(1));
unsafe {
ptr::write(vector.get_unchecked_mut(0), element);
vector.set_len(1);
}
<Vec<T> as SpecExtend<T, I>>::spec_extend(&mut vector, iterator);
vector
}
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