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Optimize slice.{r}position result bounds check #47333

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Jan 17, 2018
Merged

Optimize slice.{r}position result bounds check #47333

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37 changes: 37 additions & 0 deletions src/libcore/slice/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1215,6 +1215,43 @@ macro_rules! iterator {
}
accum
}

#[inline]
#[rustc_inherit_overflow_checks]
fn position<P>(&mut self, mut predicate: P) -> Option<usize> where
Self: Sized,
P: FnMut(Self::Item) -> bool,
{
// The addition might panic on overflow
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I think this needs either Add::add instead of + or #[rustc_inherit_overflow_checks].

Also, it's a shame there's no way to "call base" here. Would it be worth making a pub(crate) helper here to avoid duplicating the logic? Or using specialization to do the assume for all Self: TrustedLen?

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I don't think the duplication is a concern as these are really small. Having a helper would probably end up adding more lines than saving.

Good points about the overflow and specialization though. I have to test if the assume survives the abstraction/inlining.

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I couldn't make it work with TrustedLen 😞 https://godbolt.org/g/bFA3tk

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Thanks for giving it a try! Good to see this get in 🙂

let n = self.len();
self.try_fold(0, move |i, x| {
if predicate(x) { Err(i) }
else { Ok(i + 1) }
}).err()
.map(|i| {
unsafe { assume(i < n) };
i
})
}

#[inline]
fn rposition<P>(&mut self, mut predicate: P) -> Option<usize> where
P: FnMut(Self::Item) -> bool,
Self: Sized + ExactSizeIterator + DoubleEndedIterator
{
// No need for an overflow check here, because `ExactSizeIterator`
// implies that the number of elements fits into a `usize`.
let n = self.len();
self.try_rfold(n, move |i, x| {
let i = i - 1;
if predicate(x) { Err(i) }
else { Ok(i) }
}).err()
.map(|i| {
unsafe { assume(i < n) };
i
})
}
}

#[stable(feature = "rust1", since = "1.0.0")]
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19 changes: 19 additions & 0 deletions src/libcore/tests/slice.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -10,6 +10,25 @@

use core::result::Result::{Ok, Err};


#[test]
fn test_position() {
let b = [1, 2, 3, 5, 5];
assert!(b.iter().position(|&v| v == 9) == None);
assert!(b.iter().position(|&v| v == 5) == Some(3));
assert!(b.iter().position(|&v| v == 3) == Some(2));
assert!(b.iter().position(|&v| v == 0) == None);
}

#[test]
fn test_rposition() {
let b = [1, 2, 3, 5, 5];
assert!(b.iter().rposition(|&v| v == 9) == None);
assert!(b.iter().rposition(|&v| v == 5) == Some(4));
assert!(b.iter().rposition(|&v| v == 3) == Some(2));
assert!(b.iter().rposition(|&v| v == 0) == None);
}

#[test]
fn test_binary_search() {
let b: [i32; 0] = [];
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