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VecDeque::resize
should re-use the buffer in the passed-in element
#104435
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Original file line number | Diff line number | Diff line change |
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@@ -0,0 +1,195 @@ | ||
use crate::iter::{FusedIterator, TrustedLen}; | ||
use crate::mem::ManuallyDrop; | ||
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/// Creates a new iterator that repeats a single element a given number of times. | ||
/// | ||
/// The `repeat_n()` function repeats a single value exactly `n` times. | ||
/// | ||
/// This is very similar to using [`repeat()`] with [`Iterator::take()`], | ||
/// but there are two differences: | ||
/// - `repeat_n()` can return the original value, rather than always cloning. | ||
/// - `repeat_n()` produces an [`ExactSizeIterator`]. | ||
/// | ||
/// [`repeat()`]: crate::iter::repeat | ||
/// | ||
/// # Examples | ||
/// | ||
/// Basic usage: | ||
/// | ||
/// ``` | ||
/// #![feature(iter_repeat_n)] | ||
/// use std::iter; | ||
/// | ||
/// // four of the the number four: | ||
/// let mut four_fours = iter::repeat_n(4, 4); | ||
/// | ||
/// assert_eq!(Some(4), four_fours.next()); | ||
/// assert_eq!(Some(4), four_fours.next()); | ||
/// assert_eq!(Some(4), four_fours.next()); | ||
/// assert_eq!(Some(4), four_fours.next()); | ||
/// | ||
/// // no more fours | ||
/// assert_eq!(None, four_fours.next()); | ||
/// ``` | ||
/// | ||
/// For non-`Copy` types, | ||
/// | ||
/// ``` | ||
/// #![feature(iter_repeat_n)] | ||
/// use std::iter; | ||
/// | ||
/// let v: Vec<i32> = Vec::with_capacity(123); | ||
/// let mut it = iter::repeat_n(v, 5); | ||
/// | ||
/// for i in 0..4 { | ||
/// // It starts by cloning things | ||
/// let cloned = it.next().unwrap(); | ||
/// assert_eq!(cloned.len(), 0); | ||
/// assert_eq!(cloned.capacity(), 0); | ||
/// } | ||
/// | ||
/// // ... but the last item is the original one | ||
/// let last = it.next().unwrap(); | ||
/// assert_eq!(last.len(), 0); | ||
/// assert_eq!(last.capacity(), 123); | ||
/// | ||
/// // ... and now we're done | ||
/// assert_eq!(None, it.next()); | ||
/// ``` | ||
#[inline] | ||
#[unstable(feature = "iter_repeat_n", issue = "104434")] | ||
#[doc(hidden)] // waiting on ACP#120 to decide whether to expose publicly | ||
pub fn repeat_n<T: Clone>(element: T, count: usize) -> RepeatN<T> { | ||
let mut element = ManuallyDrop::new(element); | ||
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if count == 0 { | ||
// SAFETY: we definitely haven't dropped it yet, since we only just got | ||
// passed it in, and because the count is zero the instance we're about | ||
// to create won't drop it, so to avoid leaking we need to now. | ||
unsafe { ManuallyDrop::drop(&mut element) }; | ||
} | ||
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RepeatN { element, count } | ||
} | ||
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/// An iterator that repeats an element an exact number of times. | ||
/// | ||
/// This `struct` is created by the [`repeat_n()`] function. | ||
/// See its documentation for more. | ||
#[derive(Clone, Debug)] | ||
#[unstable(feature = "iter_repeat_n", issue = "104434")] | ||
#[doc(hidden)] // waiting on ACP#120 to decide whether to expose publicly | ||
pub struct RepeatN<A> { | ||
count: usize, | ||
// Invariant: has been dropped iff count == 0. | ||
element: ManuallyDrop<A>, | ||
} | ||
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impl<A> RepeatN<A> { | ||
/// If we haven't already dropped the element, return it in an option. | ||
/// | ||
/// Clears the count so it won't be dropped again later. | ||
#[inline] | ||
fn take_element(&mut self) -> Option<A> { | ||
if self.count > 0 { | ||
self.count = 0; | ||
// SAFETY: We just set count to zero so it won't be dropped again, | ||
// and it used to be non-zero so it hasn't already been dropped. | ||
unsafe { Some(ManuallyDrop::take(&mut self.element)) } | ||
} else { | ||
None | ||
} | ||
} | ||
} | ||
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#[unstable(feature = "iter_repeat_n", issue = "104434")] | ||
impl<A> Drop for RepeatN<A> { | ||
fn drop(&mut self) { | ||
self.take_element(); | ||
} | ||
} | ||
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#[unstable(feature = "iter_repeat_n", issue = "104434")] | ||
impl<A: Clone> Iterator for RepeatN<A> { | ||
type Item = A; | ||
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#[inline] | ||
fn next(&mut self) -> Option<A> { | ||
if self.count == 0 { | ||
return None; | ||
} | ||
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self.count -= 1; | ||
Some(if self.count == 0 { | ||
// SAFETY: the check above ensured that the count used to be non-zero, | ||
// so element hasn't been dropped yet, and we just lowered the count to | ||
// zero so it won't be dropped later, and thus it's okay to take it here. | ||
unsafe { ManuallyDrop::take(&mut self.element) } | ||
} else { | ||
A::clone(&mut self.element) | ||
}) | ||
} | ||
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#[inline] | ||
fn size_hint(&self) -> (usize, Option<usize>) { | ||
let len = self.len(); | ||
(len, Some(len)) | ||
} | ||
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#[inline] | ||
fn advance_by(&mut self, skip: usize) -> Result<(), usize> { | ||
let len = self.count; | ||
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if skip >= len { | ||
self.take_element(); | ||
} | ||
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if skip > len { | ||
Err(len) | ||
} else { | ||
self.count = len - skip; | ||
Ok(()) | ||
} | ||
} | ||
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#[inline] | ||
fn last(mut self) -> Option<A> { | ||
self.take_element() | ||
} | ||
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#[inline] | ||
fn count(self) -> usize { | ||
self.len() | ||
} | ||
} | ||
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#[unstable(feature = "iter_repeat_n", issue = "104434")] | ||
impl<A: Clone> ExactSizeIterator for RepeatN<A> { | ||
fn len(&self) -> usize { | ||
self.count | ||
} | ||
} | ||
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#[unstable(feature = "iter_repeat_n", issue = "104434")] | ||
impl<A: Clone> DoubleEndedIterator for RepeatN<A> { | ||
#[inline] | ||
fn next_back(&mut self) -> Option<A> { | ||
self.next() | ||
} | ||
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#[inline] | ||
fn advance_back_by(&mut self, n: usize) -> Result<(), usize> { | ||
self.advance_by(n) | ||
} | ||
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#[inline] | ||
fn nth_back(&mut self, n: usize) -> Option<A> { | ||
self.nth(n) | ||
} | ||
} | ||
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#[unstable(feature = "iter_repeat_n", issue = "104434")] | ||
impl<A: Clone> FusedIterator for RepeatN<A> {} | ||
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#[unstable(feature = "trusted_len", issue = "37572")] | ||
unsafe impl<A: Clone> TrustedLen for RepeatN<A> {} |
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Original file line number | Diff line number | Diff line change |
---|---|---|
@@ -0,0 +1,56 @@ | ||
// compile-flags: -O | ||
// only-x86_64 | ||
// ignore-debug: the debug assertions get in the way | ||
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#![crate_type = "lib"] | ||
#![feature(iter_repeat_n)] | ||
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#[derive(Clone)] | ||
pub struct NotCopy(u16); | ||
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impl Drop for NotCopy { | ||
fn drop(&mut self) {} | ||
} | ||
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// For a type where `Drop::drop` doesn't do anything observable and a clone is the | ||
// same as a move, make sure that the extra case for the last item disappears. | ||
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#[no_mangle] | ||
// CHECK-LABEL: @iter_repeat_n_next | ||
pub fn iter_repeat_n_next(it: &mut std::iter::RepeatN<NotCopy>) -> Option<NotCopy> { | ||
// CHECK-NEXT: start: | ||
// CHECK-NOT: br | ||
// CHECK: %[[COUNT:.+]] = load i64 | ||
// CHECK-NEXT: %[[COUNT_ZERO:.+]] = icmp eq i64 %[[COUNT]], 0 | ||
// CHECK-NEXT: br i1 %[[COUNT_ZERO]], label %[[EMPTY:.+]], label %[[NOT_EMPTY:.+]] | ||
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// CHECK: [[NOT_EMPTY]]: | ||
// CHECK-NEXT: %[[DEC:.+]] = add i64 %[[COUNT]], -1 | ||
// CHECK-NEXT: store i64 %[[DEC]] | ||
// CHECK-NOT: br | ||
// CHECK: %[[VAL:.+]] = load i16 | ||
// CHECK-NEXT: br label %[[EMPTY]] | ||
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// CHECK: [[EMPTY]]: | ||
// CHECK-NOT: br | ||
// CHECK: phi i16 [ undef, %start ], [ %[[VAL]], %[[NOT_EMPTY]] ] | ||
// CHECK-NOT: br | ||
// CHECK: ret | ||
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it.next() | ||
} | ||
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// And as a result, using the iterator can optimize without special cases for | ||
// the last iteration, like `memset`ing all the items in one call. | ||
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#[no_mangle] | ||
// CHECK-LABEL: @vec_extend_via_iter_repeat_n | ||
pub fn vec_extend_via_iter_repeat_n() -> Vec<u8> { | ||
// CHECK: %[[ADDR:.+]] = tail call dereferenceable_or_null(1234) ptr @__rust_alloc(i64 1234, i64 1) | ||
// CHECK: tail call void @llvm.memset.p0.i64(ptr noundef nonnull align 1 dereferenceable(1234) %[[ADDR]], i8 42, i64 1234, | ||
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let n = 1234_usize; | ||
let mut v = Vec::with_capacity(n); | ||
v.extend(std::iter::repeat_n(42_u8, n)); | ||
v | ||
} |
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This helps show that we can probably use the iterator rather than special custom extend code like
rust/library/alloc/src/vec/mod.rs
Lines 2525 to 2529 in 101e182
but
Vec
is really important so I don't want to do that in the same PR as this fix.