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Auto merge of #87667 - the8472:document-in-place-iter, r=yaahc
add module-level documentation for vec's in-place iteration As requested in the last libs team meeting and during previous reviews. Feel free to point out any gaps you encounter, after all non-obvious things may with hindsight seem obvious to me. r? `@yaahc` CC `@steffahn`
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| //! Inplace iterate-and-collect specialization for `Vec` | ||
| //! | ||
| //! Note: This documents Vec internals, some of the following sections explain implementation | ||
| //! details and are best read together with the source of this module. | ||
| //! | ||
| //! The specialization in this module applies to iterators in the shape of | ||
| //! `source.adapter().adapter().adapter().collect::<Vec<U>>()` | ||
| //! where `source` is an owning iterator obtained from [`Vec<T>`], [`Box<[T]>`][box] (by conversion to `Vec`) | ||
| //! or [`BinaryHeap<T>`], the adapters each consume one or more items per step | ||
| //! (represented by [`InPlaceIterable`]), provide transitive access to `source` (via [`SourceIter`]) | ||
| //! and thus the underlying allocation. And finally the layouts of `T` and `U` must | ||
| //! have the same size and alignment, this is currently ensured via const eval instead of trait bounds | ||
| //! in the specialized [`SpecFromIter`] implementation. | ||
| //! | ||
| //! [`BinaryHeap<T>`]: crate::collections::BinaryHeap | ||
| //! [box]: crate::boxed::Box | ||
| //! | ||
| //! By extension some other collections which use `collect::<Vec<_>>()` internally in their | ||
| //! `FromIterator` implementation benefit from this too. | ||
| //! | ||
| //! Access to the underlying source goes through a further layer of indirection via the private | ||
| //! trait [`AsVecIntoIter`] to hide the implementation detail that other collections may use | ||
| //! `vec::IntoIter` internally. | ||
| //! | ||
| //! In-place iteration depends on the interaction of several unsafe traits, implementation | ||
| //! details of multiple parts in the iterator pipeline and often requires holistic reasoning | ||
| //! across multiple structs since iterators are executed cooperatively rather than having | ||
| //! a central evaluator/visitor struct executing all iterator components. | ||
| //! | ||
| //! # Reading from and writing to the same allocation | ||
| //! | ||
| //! By its nature collecting in place means that the reader and writer side of the iterator | ||
| //! use the same allocation. Since `try_fold()` (used in [`SpecInPlaceCollect`]) takes a | ||
| //! reference to the iterator for the duration of the iteration that means we can't interleave | ||
| //! the step of reading a value and getting a reference to write to. Instead raw pointers must be | ||
| //! used on the reader and writer side. | ||
| //! | ||
| //! That writes never clobber a yet-to-be-read item is ensured by the [`InPlaceIterable`] requirements. | ||
| //! | ||
| //! # Layout constraints | ||
| //! | ||
| //! [`Allocator`] requires that `allocate()` and `deallocate()` have matching alignment and size. | ||
| //! Additionally this specialization doesn't make sense for ZSTs as there is no reallocation to | ||
| //! avoid and it would make pointer arithmetic more difficult. | ||
| //! | ||
| //! [`Allocator`]: core::alloc::Allocator | ||
| //! | ||
| //! # Drop- and panic-safety | ||
| //! | ||
| //! Iteration can panic, requiring dropping the already written parts but also the remainder of | ||
| //! the source. Iteration can also leave some source items unconsumed which must be dropped. | ||
| //! All those drops in turn can panic which then must either leak the allocation or abort to avoid | ||
| //! double-drops. | ||
| //! | ||
| //! This is handled by the [`InPlaceDrop`] guard for sink items (`U`) and by | ||
| //! [`vec::IntoIter::forget_allocation_drop_remaining()`] for remaining source items (`T`). | ||
| //! | ||
| //! [`vec::IntoIter::forget_allocation_drop_remaining()`]: super::IntoIter::forget_allocation_drop_remaining() | ||
| //! | ||
| //! # O(1) collect | ||
| //! | ||
| //! The main iteration itself is further specialized when the iterator implements | ||
| //! [`TrustedRandomAccessNoCoerce`] to let the optimizer see that it is a counted loop with a single | ||
| //! [induction variable]. This can turn some iterators into a noop, i.e. it reduces them from O(n) to | ||
| //! O(1). This particular optimization is quite fickle and doesn't always work, see [#79308] | ||
| //! | ||
| //! [#79308]: https://github.com/rust-lang/rust/issues/79308 | ||
| //! [induction variable]: https://en.wikipedia.org/wiki/Induction_variable | ||
| //! | ||
| //! Since unchecked accesses through that trait do not advance the read pointer of `IntoIter` | ||
| //! this would interact unsoundly with the requirements about dropping the tail described above. | ||
| //! But since the normal `Drop` implementation of `IntoIter` would suffer from the same problem it | ||
| //! is only correct for `TrustedRandomAccessNoCoerce` to be implemented when the items don't | ||
| //! have a destructor. Thus that implicit requirement also makes the specialization safe to use for | ||
| //! in-place collection. | ||
| //! Note that this safety concern is about the correctness of `impl Drop for IntoIter`, | ||
| //! not the guarantees of `InPlaceIterable`. | ||
| //! | ||
| //! # Adapter implementations | ||
| //! | ||
| //! The invariants for adapters are documented in [`SourceIter`] and [`InPlaceIterable`], but | ||
| //! getting them right can be rather subtle for multiple, sometimes non-local reasons. | ||
| //! For example `InPlaceIterable` would be valid to implement for [`Peekable`], except | ||
| //! that it is stateful, cloneable and `IntoIter`'s clone implementation shortens the underlying | ||
| //! allocation which means if the iterator has been peeked and then gets cloned there no longer is | ||
| //! enough room, thus breaking an invariant ([#85322]). | ||
| //! | ||
| //! [#85322]: https://github.com/rust-lang/rust/issues/85322 | ||
| //! [`Peekable`]: core::iter::Peekable | ||
| //! | ||
| //! | ||
| //! # Examples | ||
| //! | ||
| //! Some cases that are optimized by this specialization, more can be found in the `Vec` | ||
| //! benchmarks: | ||
| //! | ||
| //! ```rust | ||
| //! # #[allow(dead_code)] | ||
| //! /// Converts a usize vec into an isize one. | ||
| //! pub fn cast(vec: Vec<usize>) -> Vec<isize> { | ||
| //! // Does not allocate, free or panic. On optlevel>=2 it does not loop. | ||
| //! // Of course this particular case could and should be written with `into_raw_parts` and | ||
| //! // `from_raw_parts` instead. | ||
| //! vec.into_iter().map(|u| u as isize).collect() | ||
| //! } | ||
| //! ``` | ||
| //! | ||
| //! ```rust | ||
| //! # #[allow(dead_code)] | ||
| //! /// Drops remaining items in `src` and if the layouts of `T` and `U` match it | ||
| //! /// returns an empty Vec backed by the original allocation. Otherwise it returns a new | ||
| //! /// empty vec. | ||
| //! pub fn recycle_allocation<T, U>(src: Vec<T>) -> Vec<U> { | ||
| //! src.into_iter().filter_map(|_| None).collect() | ||
| //! } | ||
| //! ``` | ||
| //! | ||
| //! ```rust | ||
| //! let vec = vec![13usize; 1024]; | ||
| //! let _ = vec.into_iter() | ||
| //! .enumerate() | ||
| //! .filter_map(|(idx, val)| if idx % 2 == 0 { Some(val+idx) } else {None}) | ||
| //! .collect::<Vec<_>>(); | ||
| //! | ||
| //! // is equivalent to the following, but doesn't require bounds checks | ||
| //! | ||
| //! let mut vec = vec![13usize; 1024]; | ||
| //! let mut write_idx = 0; | ||
| //! for idx in 0..vec.len() { | ||
| //! if idx % 2 == 0 { | ||
| //! vec[write_idx] = vec[idx] + idx; | ||
| //! write_idx += 1; | ||
| //! } | ||
| //! } | ||
| //! vec.truncate(write_idx); | ||
| //! ``` | ||
| use core::iter::{InPlaceIterable, SourceIter, TrustedRandomAccessNoCoerce}; | ||
| use core::mem::{self, ManuallyDrop}; | ||
| use core::ptr::{self}; | ||
|
|
||
| use super::{InPlaceDrop, SpecFromIter, SpecFromIterNested, Vec}; | ||
|
|
||
| /// Specialization marker for collecting an iterator pipeline into a Vec while reusing the | ||
| /// source allocation, i.e. executing the pipeline in place. | ||
| #[rustc_unsafe_specialization_marker] | ||
| pub(super) trait InPlaceIterableMarker {} | ||
|
|
||
| impl<T> InPlaceIterableMarker for T where T: InPlaceIterable {} | ||
|
|
||
| impl<T, I> SpecFromIter<T, I> for Vec<T> | ||
| where | ||
| I: Iterator<Item = T> + SourceIter<Source: AsVecIntoIter> + InPlaceIterableMarker, | ||
| { | ||
| default fn from_iter(mut iterator: I) -> Self { | ||
| // See "Layout constraints" section in the module documentation. We rely on const | ||
| // optimization here since these conditions currently cannot be expressed as trait bounds | ||
| if mem::size_of::<T>() == 0 | ||
| || mem::size_of::<T>() | ||
| != mem::size_of::<<<I as SourceIter>::Source as AsVecIntoIter>::Item>() | ||
| || mem::align_of::<T>() | ||
| != mem::align_of::<<<I as SourceIter>::Source as AsVecIntoIter>::Item>() | ||
| { | ||
| // fallback to more generic implementations | ||
| return SpecFromIterNested::from_iter(iterator); | ||
| } | ||
|
|
||
| let (src_buf, src_ptr, dst_buf, dst_end, cap) = unsafe { | ||
| let inner = iterator.as_inner().as_into_iter(); | ||
| ( | ||
| inner.buf.as_ptr(), | ||
| inner.ptr, | ||
| inner.buf.as_ptr() as *mut T, | ||
| inner.end as *const T, | ||
| inner.cap, | ||
| ) | ||
| }; | ||
|
|
||
| let len = SpecInPlaceCollect::collect_in_place(&mut iterator, dst_buf, dst_end); | ||
|
|
||
| let src = unsafe { iterator.as_inner().as_into_iter() }; | ||
| // check if SourceIter contract was upheld | ||
| // caveat: if they weren't we might not even make it to this point | ||
| debug_assert_eq!(src_buf, src.buf.as_ptr()); | ||
| // check InPlaceIterable contract. This is only possible if the iterator advanced the | ||
| // source pointer at all. If it uses unchecked access via TrustedRandomAccess | ||
| // then the source pointer will stay in its initial position and we can't use it as reference | ||
| if src.ptr != src_ptr { | ||
| debug_assert!( | ||
| unsafe { dst_buf.add(len) as *const _ } <= src.ptr, | ||
| "InPlaceIterable contract violation, write pointer advanced beyond read pointer" | ||
| ); | ||
| } | ||
|
|
||
| // Drop any remaining values at the tail of the source but prevent drop of the allocation | ||
| // itself once IntoIter goes out of scope. | ||
| // If the drop panics then we also leak any elements collected into dst_buf. | ||
| // | ||
| // Note: This access to the source wouldn't be allowed by the TrustedRandomIteratorNoCoerce | ||
| // contract (used by SpecInPlaceCollect below). But see the "O(1) collect" section in the | ||
| // module documenttation why this is ok anyway. | ||
| src.forget_allocation_drop_remaining(); | ||
|
|
||
| let vec = unsafe { Vec::from_raw_parts(dst_buf, len, cap) }; | ||
|
|
||
| vec | ||
| } | ||
| } | ||
|
|
||
| fn write_in_place_with_drop<T>( | ||
| src_end: *const T, | ||
| ) -> impl FnMut(InPlaceDrop<T>, T) -> Result<InPlaceDrop<T>, !> { | ||
| move |mut sink, item| { | ||
| unsafe { | ||
| // the InPlaceIterable contract cannot be verified precisely here since | ||
| // try_fold has an exclusive reference to the source pointer | ||
| // all we can do is check if it's still in range | ||
| debug_assert!(sink.dst as *const _ <= src_end, "InPlaceIterable contract violation"); | ||
| ptr::write(sink.dst, item); | ||
| // Since this executes user code which can panic we have to bump the pointer | ||
| // after each step. | ||
| sink.dst = sink.dst.add(1); | ||
| } | ||
| Ok(sink) | ||
| } | ||
| } | ||
|
|
||
| /// Helper trait to hold specialized implementations of the in-place iterate-collect loop | ||
| trait SpecInPlaceCollect<T, I>: Iterator<Item = T> { | ||
| /// Collects an iterator (`self`) into the destination buffer (`dst`) and returns the number of items | ||
| /// collected. `end` is the last writable element of the allocation and used for bounds checks. | ||
| /// | ||
| /// This method is specialized and one of its implementations makes use of | ||
| /// `Iterator::__iterator_get_unchecked` calls with a `TrustedRandomAccessNoCoerce` bound | ||
| /// on `I` which means the caller of this method must take the safety conditions | ||
| /// of that trait into consideration. | ||
| fn collect_in_place(&mut self, dst: *mut T, end: *const T) -> usize; | ||
| } | ||
|
|
||
| impl<T, I> SpecInPlaceCollect<T, I> for I | ||
| where | ||
| I: Iterator<Item = T>, | ||
| { | ||
| #[inline] | ||
| default fn collect_in_place(&mut self, dst_buf: *mut T, end: *const T) -> usize { | ||
| // use try-fold since | ||
| // - it vectorizes better for some iterator adapters | ||
| // - unlike most internal iteration methods, it only takes a &mut self | ||
| // - it lets us thread the write pointer through its innards and get it back in the end | ||
| let sink = InPlaceDrop { inner: dst_buf, dst: dst_buf }; | ||
| let sink = | ||
| self.try_fold::<_, _, Result<_, !>>(sink, write_in_place_with_drop(end)).unwrap(); | ||
| // iteration succeeded, don't drop head | ||
| unsafe { ManuallyDrop::new(sink).dst.offset_from(dst_buf) as usize } | ||
| } | ||
| } | ||
|
|
||
| impl<T, I> SpecInPlaceCollect<T, I> for I | ||
| where | ||
| I: Iterator<Item = T> + TrustedRandomAccessNoCoerce, | ||
| { | ||
| #[inline] | ||
| fn collect_in_place(&mut self, dst_buf: *mut T, end: *const T) -> usize { | ||
| let len = self.size(); | ||
| let mut drop_guard = InPlaceDrop { inner: dst_buf, dst: dst_buf }; | ||
| for i in 0..len { | ||
| // Safety: InplaceIterable contract guarantees that for every element we read | ||
| // one slot in the underlying storage will have been freed up and we can immediately | ||
| // write back the result. | ||
| unsafe { | ||
| let dst = dst_buf.offset(i as isize); | ||
| debug_assert!(dst as *const _ <= end, "InPlaceIterable contract violation"); | ||
| ptr::write(dst, self.__iterator_get_unchecked(i)); | ||
| // Since this executes user code which can panic we have to bump the pointer | ||
| // after each step. | ||
| drop_guard.dst = dst.add(1); | ||
| } | ||
| } | ||
| mem::forget(drop_guard); | ||
| len | ||
| } | ||
| } | ||
|
|
||
| /// Internal helper trait for in-place iteration specialization. | ||
| /// | ||
| /// Currently this is only implemented by [`vec::IntoIter`] - returning a reference to itself - and | ||
| /// [`binary_heap::IntoIter`] which returns a reference to its inner representation. | ||
| /// | ||
| /// Since this is an internal trait it hides the implementation detail `binary_heap::IntoIter` | ||
| /// uses `vec::IntoIter` internally. | ||
| /// | ||
| /// [`vec::IntoIter`]: super::IntoIter | ||
| /// [`binary_heap::IntoIter`]: crate::collections::binary_heap::IntoIter | ||
| /// | ||
| /// # Safety | ||
| /// | ||
| /// In-place iteration relies on implementation details of `vec::IntoIter`, most importantly that | ||
| /// it does not create references to the whole allocation during iteration, only raw pointers | ||
| #[rustc_specialization_trait] | ||
| pub(crate) unsafe trait AsVecIntoIter { | ||
| type Item; | ||
| fn as_into_iter(&mut self) -> &mut super::IntoIter<Self::Item>; | ||
| } |
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