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Merged
merged 5 commits into from
Dec 22, 2020
Merged

Added [T; N]::zip() #79451

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3b8617b
Added [T; N]::zip()
usbalbin Nov 26, 2020
2f35fb1
Remove redundant tests
usbalbin Nov 27, 2020
be2c8f2
Update zip for better codegen, see discussion
usbalbin Dec 16, 2020
baa5e47
Update doc comment
usbalbin Dec 16, 2020
8b37259
Added reference to tracking issue
usbalbin Dec 16, 2020
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31 changes: 31 additions & 0 deletions library/core/src/array/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -463,6 +463,37 @@ impl<T, const N: usize> [T; N] {
unsafe { crate::mem::transmute_copy::<_, [U; N]>(&dst) }
}

/// 'Zips up' two arrays into a single array of pairs.
///
/// `zip()` returns a new array where every element is a tuple where the
/// first element comes from the first array, and the second element comes
/// from the second array. In other words, it zips two arrays together,
/// into a single one.
///
/// # Examples
///
/// ```
/// #![feature(array_zip)]
/// let x = [1, 2, 3];
/// let y = [4, 5, 6];
/// let z = x.zip(y);
/// assert_eq!(z, [(1, 4), (2, 5), (3, 6)]);
/// ```
#[unstable(feature = "array_zip", issue = "80094")]
pub fn zip<U>(self, rhs: [U; N]) -> [(T, U); N] {
use crate::mem::MaybeUninit;

let mut dst = MaybeUninit::uninit_array::<N>();
for (i, (lhs, rhs)) in IntoIter::new(self).zip(IntoIter::new(rhs)).enumerate() {
dst[i].write((lhs, rhs));
}
// FIXME: Convert to crate::mem::transmute once it works with generics.
// unsafe { crate::mem::transmute::<[MaybeUninit<U>; N], [U; N]>(dst) }
// SAFETY: At this point we've properly initialized the whole array
// and we just need to cast it to the correct type.
unsafe { crate::mem::transmute_copy::<_, [(T, U); N]>(&dst) }
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@cynecx cynecx Nov 27, 2020 •
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This implementation (x86-64 codegen with -O) uses more stack space and includes branches/a loop compared to this one (which is branch-free and utilizes less stack, at least for smaller sizes):

let mut dst = MaybeUninit::<[(T, U); N]>::uninit();
let ptr = dst.as_mut_ptr() as *mut (T, U);
for (idx, (lhs, rhs)) in IntoIter::new(lhs).zip(IntoIter::new(rhs)).enumerate() {
    unsafe { ptr.add(idx).write((lhs, rhs)) }
}
unsafe { dst.assume_init() }

https://godbolt.org/z/T4aT78

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@cynecx cynecx Nov 27, 2020 •
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However, that codegen is still not optimal (the memcpy is basically redundant and N/RVO could definitely help here).

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@cynecx cynecx Nov 27, 2020 •
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Are [MaybeUninit<(T, U)>; N] and [(T, U); N] layout compatible/equivalent? Otherwise that transmute_copy would be UB.

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@usbalbin usbalbin Nov 27, 2020

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I am quite new to working with unsafe but the doc states

[...] MaybeUninit will always guarantee that it has the same size, alignment, and ABI as T [...]

and there are examples with [Vec<u32>; N]) and [u8; N]. However I am not sure if there is anything special with [(T, U); N] that would make it not work.

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@usbalbin usbalbin Nov 27, 2020

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Looking at map() from #75212, which I based my function on, uses unsafe { crate::mem::transmute_copy::<_, [U; N]>(&dst) } for any U. That would make the same for [(T, U); N] (not the same U) fine, right?

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@usbalbin usbalbin Dec 12, 2020

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Sorry for the delay. I added a zippy2 to @cynecx's godbolt link here. However I am not too familiar with judging asm quality :)

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@JulianKnodt JulianKnodt Dec 14, 2020 •
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ah ty for the update! The asm just helps me see what differences there are between the two versions, mainly looking for bounds checks which shouldn't be necessary here. The version you have has a loop in the LBB1 block, whereas cynecx's has no loop and is unrolled as he mentions in his top comment. If you increase the size of the arrays, you'll start to see more differences between the two.

The one thing is, if you change

for ((lhs, rhs), dst) in IntoIter::new(lhs).zip(IntoIter::new(rhs)).zip(&mut dst) {
  dst.write((lhs, rhs));
}

into

for (i, (lhs, rhs)) in IntoIter::new(lhs).zip(IntoIter::new(rhs)).enumerate() {
  dst[i].write((lhs, rhs));
}

both versions output the same asm. So the main difference between the two is here if you choose to zip or enumerate, rather than whether you use a pointer or a MaybeUninit

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@usbalbin usbalbin Dec 15, 2020

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Then would that be the preferred implementation (zip3 on godbolt)? Seems to me like best of both worlds, no extra unsafe yet the same asm?

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@JulianKnodt JulianKnodt Dec 15, 2020 •
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To me that seems good, but I think that's ultimately up to you & the reviewer! I was just curious how each impl compared to the others.

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@usbalbin usbalbin Dec 16, 2020

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Do you all think zip3 is the way to go? Or do you have any other suggestions? :)

}

/// Returns a slice containing the entire array. Equivalent to `&s[..]`.
#[unstable(feature = "array_methods", issue = "76118")]
pub fn as_slice(&self) -> &[T] {
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