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Drop array::from_fn workaround #3

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Merged
merged 3 commits into from
Oct 2, 2023
Merged

Drop array::from_fn workaround #3

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faef954
Drop array::from_fn workaround
HadrienG2 Oct 2, 2023
c4a7496
Oops
HadrienG2 Oct 2, 2023
54eac24
Wrong bet
HadrienG2 Oct 2, 2023
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73 changes: 2 additions & 71 deletions src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -146,7 +146,6 @@ use core::{
cell::RefCell,
hint::unreachable_unchecked,
iter::{ExactSizeIterator, Iterator},
mem::MaybeUninit,
ops::{Add, Mul},
};
use num_traits::{One, Zero};
Expand Down Expand Up @@ -321,7 +320,7 @@ pub trait IteratorILP: Iterator + Sized + TrustedLowerBound {
for _ in 0..stream_len {
// Fetch one Option<Item> per stream
let item_opts: [Option<Self::Item>; STREAMS] =
array_from_fn(|_| unsafe { next_unchecked(&mut iter) });
core::array::from_fn(|_| unsafe { next_unchecked(&mut iter) });

// Check if the item of interest was found
if let Some(item) = item_opts.into_iter().flatten().next() {
Expand Down Expand Up @@ -424,7 +423,7 @@ pub trait IteratorILP: Iterator + Sized + TrustedLowerBound {
assert_ne!(STREAMS, 0, "Need at least one stream to make progress");

// Set up accumulators
let mut accumulators = array_from_fn::<STREAMS, _>(|_| Some(neutral()));
let mut accumulators: [Option<Acc>; STREAMS] = core::array::from_fn(|_| Some(neutral()));
let mut accumulate = |accumulator: &mut Option<Acc>, item| {
if let Some(prev_acc) = accumulator.take() {
*accumulator = Some(accumulate(prev_acc, item));
Expand Down Expand Up @@ -545,74 +544,6 @@ unsafe fn next_unchecked<Item>(iter: &mut impl Iterator<Item = Item>) -> Item {
}
}

/// Build an array using a mapping from index to value
///
/// This may look suspiciously like `std::array::from_fn()`, because it is
/// actually a clone of that function. Unfortunately, at the time of
/// writing, the real thing does not optimize well because the compiler fails to
/// inline some steps, so we need to clone it for performance...
#[inline]
fn array_from_fn<const SIZE: usize, T>(mut idx_to_elem: impl FnMut(usize) -> T) -> [T; SIZE] {
let mut array = PartialArray::new();
for idx in 0..SIZE {
array.push(idx_to_elem(idx));
}
array.collect()
}

/// Partially initialized array
struct PartialArray<T, const N: usize> {
inner: MaybeUninit<[T; N]>,
num_initialized: usize,
}
//
impl<T, const N: usize> PartialArray<T, N> {
/// Prepare to iteratively initialize an array
#[inline]
fn new() -> Self {
Self {
inner: MaybeUninit::uninit(),
num_initialized: 0,
}
}

/// Initialize the next array element
#[inline]
fn push(&mut self, value: T) {
assert!(self.num_initialized < N);
unsafe {
let ptr = self
.inner
.as_mut_ptr()
.cast::<T>()
.add(self.num_initialized);
ptr.write(value);
self.num_initialized += 1;
}
}

/// Assume the array is fully initialized and collect its value
#[inline]
fn collect(self) -> [T; N] {
assert_eq!(self.num_initialized, N);
unsafe {
let result = self.inner.assume_init_read();
core::mem::forget(self);
result
}
}
}
//
impl<T, const N: usize> Drop for PartialArray<T, N> {
/// Drop already initialized elements on panic
fn drop(&mut self) {
let ptr = self.inner.as_mut_ptr().cast::<T>();
for idx in 0..self.num_initialized {
unsafe { ptr.add(idx).drop_in_place() };
}
}
}

/// An iterator that reports an accurate lower bound using [`size_hint()`]
///
/// # Safety
Expand Down