diff options
Diffstat (limited to 'rust/alloc/slice.rs')
| -rw-r--r-- | rust/alloc/slice.rs | 445 |
1 files changed, 54 insertions, 391 deletions
diff --git a/rust/alloc/slice.rs b/rust/alloc/slice.rs index e444e97fa145..245e01590df7 100644 --- a/rust/alloc/slice.rs +++ b/rust/alloc/slice.rs @@ -1,84 +1,14 @@ // SPDX-License-Identifier: Apache-2.0 OR MIT -//! A dynamically-sized view into a contiguous sequence, `[T]`. +//! Utilities for the slice primitive type. //! //! *[See also the slice primitive type](slice).* //! -//! Slices are a view into a block of memory represented as a pointer and a -//! length. +//! Most of the structs in this module are iterator types which can only be created +//! using a certain function. For example, `slice.iter()` yields an [`Iter`]. //! -//! ``` -//! // slicing a Vec -//! let vec = vec![1, 2, 3]; -//! let int_slice = &vec[..]; -//! // coercing an array to a slice -//! let str_slice: &[&str] = &["one", "two", "three"]; -//! ``` -//! -//! Slices are either mutable or shared. The shared slice type is `&[T]`, -//! while the mutable slice type is `&mut [T]`, where `T` represents the element -//! type. For example, you can mutate the block of memory that a mutable slice -//! points to: -//! -//! ``` -//! let x = &mut [1, 2, 3]; -//! x[1] = 7; -//! assert_eq!(x, &[1, 7, 3]); -//! ``` -//! -//! Here are some of the things this module contains: -//! -//! ## Structs -//! -//! There are several structs that are useful for slices, such as [`Iter`], which -//! represents iteration over a slice. -//! -//! ## Trait Implementations -//! -//! There are several implementations of common traits for slices. Some examples -//! include: -//! -//! * [`Clone`] -//! * [`Eq`], [`Ord`] - for slices whose element type are [`Eq`] or [`Ord`]. -//! * [`Hash`] - for slices whose element type is [`Hash`]. -//! -//! ## Iteration -//! -//! The slices implement `IntoIterator`. The iterator yields references to the -//! slice elements. -//! -//! ``` -//! let numbers = &[0, 1, 2]; -//! for n in numbers { -//! println!("{n} is a number!"); -//! } -//! ``` -//! -//! The mutable slice yields mutable references to the elements: -//! -//! ``` -//! let mut scores = [7, 8, 9]; -//! for score in &mut scores[..] { -//! *score += 1; -//! } -//! ``` -//! -//! This iterator yields mutable references to the slice's elements, so while -//! the element type of the slice is `i32`, the element type of the iterator is -//! `&mut i32`. -//! -//! * [`.iter`] and [`.iter_mut`] are the explicit methods to return the default -//! iterators. -//! * Further methods that return iterators are [`.split`], [`.splitn`], -//! [`.chunks`], [`.windows`] and more. -//! -//! [`Hash`]: core::hash::Hash -//! [`.iter`]: slice::iter -//! [`.iter_mut`]: slice::iter_mut -//! [`.split`]: slice::split -//! [`.splitn`]: slice::splitn -//! [`.chunks`]: slice::chunks -//! [`.windows`]: slice::windows +//! A few functions are provided to create a slice from a value reference +//! or from a raw pointer. #![stable(feature = "rust1", since = "1.0.0")] // Many of the usings in this module are only used in the test configuration. // It's cleaner to just turn off the unused_imports warning than to fix them. @@ -88,20 +18,23 @@ use core::borrow::{Borrow, BorrowMut}; #[cfg(not(no_global_oom_handling))] use core::cmp::Ordering::{self, Less}; #[cfg(not(no_global_oom_handling))] -use core::mem; -#[cfg(not(no_global_oom_handling))] -use core::mem::size_of; +use core::mem::{self, SizedTypeProperties}; #[cfg(not(no_global_oom_handling))] use core::ptr; +#[cfg(not(no_global_oom_handling))] +use core::slice::sort; use crate::alloc::Allocator; #[cfg(not(no_global_oom_handling))] -use crate::alloc::Global; +use crate::alloc::{self, Global}; #[cfg(not(no_global_oom_handling))] use crate::borrow::ToOwned; use crate::boxed::Box; use crate::vec::Vec; +#[cfg(test)] +mod tests; + #[unstable(feature = "slice_range", issue = "76393")] pub use core::slice::range; #[unstable(feature = "array_chunks", issue = "74985")] @@ -116,6 +49,8 @@ pub use core::slice::EscapeAscii; pub use core::slice::SliceIndex; #[stable(feature = "from_ref", since = "1.28.0")] pub use core::slice::{from_mut, from_ref}; +#[unstable(feature = "slice_from_ptr_range", issue = "89792")] +pub use core::slice::{from_mut_ptr_range, from_ptr_range}; #[stable(feature = "rust1", since = "1.0.0")] pub use core::slice::{from_raw_parts, from_raw_parts_mut}; #[stable(feature = "rust1", since = "1.0.0")] @@ -275,7 +210,7 @@ impl<T> [T] { where T: Ord, { - merge_sort(self, |a, b| a.lt(b)); + stable_sort(self, T::lt); } /// Sorts the slice with a comparator function. @@ -331,7 +266,7 @@ impl<T> [T] { where F: FnMut(&T, &T) -> Ordering, { - merge_sort(self, |a, b| compare(a, b) == Less); + stable_sort(self, |a, b| compare(a, b) == Less); } /// Sorts the slice with a key extraction function. @@ -374,7 +309,7 @@ impl<T> [T] { F: FnMut(&T) -> K, K: Ord, { - merge_sort(self, |a, b| f(a).lt(&f(b))); + stable_sort(self, |a, b| f(a).lt(&f(b))); } /// Sorts the slice with a key extraction function. @@ -530,7 +465,7 @@ impl<T> [T] { hack::into_vec(self) } - /// Creates a vector by repeating a slice `n` times. + /// Creates a vector by copying a slice `n` times. /// /// # Panics /// @@ -725,7 +660,7 @@ impl [u8] { /// /// ```error /// error[E0207]: the type parameter `T` is not constrained by the impl trait, self type, or predica -/// --> src/liballoc/slice.rs:608:6 +/// --> library/alloc/src/slice.rs:608:6 /// | /// 608 | impl<T: Clone, V: Borrow<[T]>> Concat for [V] { /// | ^ unconstrained type parameter @@ -836,14 +771,14 @@ impl<T: Clone, V: Borrow<[T]>> Join<&[T]> for [V] { //////////////////////////////////////////////////////////////////////////////// #[stable(feature = "rust1", since = "1.0.0")] -impl<T> Borrow<[T]> for Vec<T> { +impl<T, A: Allocator> Borrow<[T]> for Vec<T, A> { fn borrow(&self) -> &[T] { &self[..] } } #[stable(feature = "rust1", since = "1.0.0")] -impl<T> BorrowMut<[T]> for Vec<T> { +impl<T, A: Allocator> BorrowMut<[T]> for Vec<T, A> { fn borrow_mut(&mut self) -> &mut [T] { &mut self[..] } @@ -881,324 +816,52 @@ impl<T: Clone> ToOwned for [T] { // Sorting //////////////////////////////////////////////////////////////////////////////// -/// Inserts `v[0]` into pre-sorted sequence `v[1..]` so that whole `v[..]` becomes sorted. -/// -/// This is the integral subroutine of insertion sort. +#[inline] #[cfg(not(no_global_oom_handling))] -fn insert_head<T, F>(v: &mut [T], is_less: &mut F) +fn stable_sort<T, F>(v: &mut [T], mut is_less: F) where F: FnMut(&T, &T) -> bool, { - if v.len() >= 2 && is_less(&v[1], &v[0]) { - unsafe { - // There are three ways to implement insertion here: - // - // 1. Swap adjacent elements until the first one gets to its final destination. - // However, this way we copy data around more than is necessary. If elements are big - // structures (costly to copy), this method will be slow. - // - // 2. Iterate until the right place for the first element is found. Then shift the - // elements succeeding it to make room for it and finally place it into the - // remaining hole. This is a good method. - // - // 3. Copy the first element into a temporary variable. Iterate until the right place - // for it is found. As we go along, copy every traversed element into the slot - // preceding it. Finally, copy data from the temporary variable into the remaining - // hole. This method is very good. Benchmarks demonstrated slightly better - // performance than with the 2nd method. - // - // All methods were benchmarked, and the 3rd showed best results. So we chose that one. - let tmp = mem::ManuallyDrop::new(ptr::read(&v[0])); - - // Intermediate state of the insertion process is always tracked by `hole`, which - // serves two purposes: - // 1. Protects integrity of `v` from panics in `is_less`. - // 2. Fills the remaining hole in `v` in the end. - // - // Panic safety: - // - // If `is_less` panics at any point during the process, `hole` will get dropped and - // fill the hole in `v` with `tmp`, thus ensuring that `v` still holds every object it - // initially held exactly once. - let mut hole = InsertionHole { src: &*tmp, dest: &mut v[1] }; - ptr::copy_nonoverlapping(&v[1], &mut v[0], 1); - - for i in 2..v.len() { - if !is_less(&v[i], &*tmp) { - break; - } - ptr::copy_nonoverlapping(&v[i], &mut v[i - 1], 1); - hole.dest = &mut v[i]; - } - // `hole` gets dropped and thus copies `tmp` into the remaining hole in `v`. - } - } - - // When dropped, copies from `src` into `dest`. - struct InsertionHole<T> { - src: *const T, - dest: *mut T, - } - - impl<T> Drop for InsertionHole<T> { - fn drop(&mut self) { - unsafe { - ptr::copy_nonoverlapping(self.src, self.dest, 1); - } - } + if T::IS_ZST { + // Sorting has no meaningful behavior on zero-sized types. Do nothing. + return; } -} - -/// Merges non-decreasing runs `v[..mid]` and `v[mid..]` using `buf` as temporary storage, and -/// stores the result into `v[..]`. -/// -/// # Safety -/// -/// The two slices must be non-empty and `mid` must be in bounds. Buffer `buf` must be long enough -/// to hold a copy of the shorter slice. Also, `T` must not be a zero-sized type. -#[cfg(not(no_global_oom_handling))] -unsafe fn merge<T, F>(v: &mut [T], mid: usize, buf: *mut T, is_less: &mut F) -where - F: FnMut(&T, &T) -> bool, -{ - let len = v.len(); - let v = v.as_mut_ptr(); - let (v_mid, v_end) = unsafe { (v.add(mid), v.add(len)) }; - // The merge process first copies the shorter run into `buf`. Then it traces the newly copied - // run and the longer run forwards (or backwards), comparing their next unconsumed elements and - // copying the lesser (or greater) one into `v`. - // - // As soon as the shorter run is fully consumed, the process is done. If the longer run gets - // consumed first, then we must copy whatever is left of the shorter run into the remaining - // hole in `v`. - // - // Intermediate state of the process is always tracked by `hole`, which serves two purposes: - // 1. Protects integrity of `v` from panics in `is_less`. - // 2. Fills the remaining hole in `v` if the longer run gets consumed first. - // - // Panic safety: - // - // If `is_less` panics at any point during the process, `hole` will get dropped and fill the - // hole in `v` with the unconsumed range in `buf`, thus ensuring that `v` still holds every - // object it initially held exactly once. - let mut hole; + let elem_alloc_fn = |len: usize| -> *mut T { + // SAFETY: Creating the layout is safe as long as merge_sort never calls this with len > + // v.len(). Alloc in general will only be used as 'shadow-region' to store temporary swap + // elements. + unsafe { alloc::alloc(alloc::Layout::array::<T>(len).unwrap_unchecked()) as *mut T } + }; - if mid <= len - mid { - // The left run is shorter. + let elem_dealloc_fn = |buf_ptr: *mut T, len: usize| { + // SAFETY: Creating the layout is safe as long as merge_sort never calls this with len > + // v.len(). The caller must ensure that buf_ptr was created by elem_alloc_fn with the same + // len. unsafe { - ptr::copy_nonoverlapping(v, buf, mid); - hole = MergeHole { start: buf, end: buf.add(mid), dest: v }; + alloc::dealloc(buf_ptr as *mut u8, alloc::Layout::array::<T>(len).unwrap_unchecked()); } + }; - // Initially, these pointers point to the beginnings of their arrays. - let left = &mut hole.start; - let mut right = v_mid; - let out = &mut hole.dest; - - while *left < hole.end && right < v_end { - // Consume the lesser side. - // If equal, prefer the left run to maintain stability. - unsafe { - let to_copy = if is_less(&*right, &**left) { - get_and_increment(&mut right) - } else { - get_and_increment(left) - }; - ptr::copy_nonoverlapping(to_copy, get_and_increment(out), 1); - } - } - } else { - // The right run is shorter. + let run_alloc_fn = |len: usize| -> *mut sort::TimSortRun { + // SAFETY: Creating the layout is safe as long as merge_sort never calls this with an + // obscene length or 0. unsafe { - ptr::copy_nonoverlapping(v_mid, buf, len - mid); - hole = MergeHole { start: buf, end: buf.add(len - mid), dest: v_mid }; + alloc::alloc(alloc::Layout::array::<sort::TimSortRun>(len).unwrap_unchecked()) + as *mut sort::TimSortRun } + }; - // Initially, these pointers point past the ends of their arrays. - let left = &mut hole.dest; - let right = &mut hole.end; - let mut out = v_end; - - while v < *left && buf < *right { - // Consume the greater side. - // If equal, prefer the right run to maintain stability. - unsafe { - let to_copy = if is_less(&*right.offset(-1), &*left.offset(-1)) { - decrement_and_get(left) - } else { - decrement_and_get(right) - }; - ptr::copy_nonoverlapping(to_copy, decrement_and_get(&mut out), 1); - } - } - } - // Finally, `hole` gets dropped. If the shorter run was not fully consumed, whatever remains of - // it will now be copied into the hole in `v`. - - unsafe fn get_and_increment<T>(ptr: &mut *mut T) -> *mut T { - let old = *ptr; - *ptr = unsafe { ptr.offset(1) }; - old - } - - unsafe fn decrement_and_get<T>(ptr: &mut *mut T) -> *mut T { - *ptr = unsafe { ptr.offset(-1) }; - *ptr - } - - // When dropped, copies the range `start..end` into `dest..`. - struct MergeHole<T> { - start: *mut T, - end: *mut T, - dest: *mut T, - } - - impl<T> Drop for MergeHole<T> { - fn drop(&mut self) { - // `T` is not a zero-sized type, and these are pointers into a slice's elements. - unsafe { - let len = self.end.sub_ptr(self.start); - ptr::copy_nonoverlapping(self.start, self.dest, len); - } - } - } -} - -/// This merge sort borrows some (but not all) ideas from TimSort, which is described in detail -/// [here](https://github.com/python/cpython/blob/main/Objects/listsort.txt). -/// -/// The algorithm identifies strictly descending and non-descending subsequences, which are called -/// natural runs. There is a stack of pending runs yet to be merged. Each newly found run is pushed -/// onto the stack, and then some pairs of adjacent runs are merged until these two invariants are -/// satisfied: -/// -/// 1. for every `i` in `1..runs.len()`: `runs[i - 1].len > runs[i].len` -/// 2. for every `i` in `2..runs.len()`: `runs[i - 2].len > runs[i - 1].len + runs[i].len` -/// -/// The invariants ensure that the total running time is *O*(*n* \* log(*n*)) worst-case. -#[cfg(not(no_global_oom_handling))] -fn merge_sort<T, F>(v: &mut [T], mut is_less: F) -where - F: FnMut(&T, &T) -> bool, -{ - // Slices of up to this length get sorted using insertion sort. - const MAX_INSERTION: usize = 20; - // Very short runs are extended using insertion sort to span at least this many elements. - const MIN_RUN: usize = 10; - - // Sorting has no meaningful behavior on zero-sized types. - if size_of::<T>() == 0 { - return; - } - - let len = v.len(); - - // Short arrays get sorted in-place via insertion sort to avoid allocations. - if len <= MAX_INSERTION { - if len >= 2 { - for i in (0..len - 1).rev() { - insert_head(&mut v[i..], &mut is_less); - } - } - return; - } - - // Allocate a buffer to use as scratch memory. We keep the length 0 so we can keep in it - // shallow copies of the contents of `v` without risking the dtors running on copies if - // `is_less` panics. When merging two sorted runs, this buffer holds a copy of the shorter run, - // which will always have length at most `len / 2`. - let mut buf = Vec::with_capacity(len / 2); - - // In order to identify natural runs in `v`, we traverse it backwards. That might seem like a - // strange decision, but consider the fact that merges more often go in the opposite direction - // (forwards). According to benchmarks, merging forwards is slightly faster than merging - // backwards. To conclude, identifying runs by traversing backwards improves performance. - let mut runs = vec![]; - let mut end = len; - while end > 0 { - // Find the next natural run, and reverse it if it's strictly descending. - let mut start = end - 1; - if start > 0 { - start -= 1; - unsafe { - if is_less(v.get_unchecked(start + 1), v.get_unchecked(start)) { - while start > 0 && is_less(v.get_unchecked(start), v.get_unchecked(start - 1)) { - start -= 1; - } - v[start..end].reverse(); - } else { - while start > 0 && !is_less(v.get_unchecked(start), v.get_unchecked(start - 1)) - { - start -= 1; - } - } - } - } - - // Insert some more elements into the run if it's too short. Insertion sort is faster than - // merge sort on short sequences, so this significantly improves performance. - while start > 0 && end - start < MIN_RUN { - start -= 1; - insert_head(&mut v[start..end], &mut is_less); - } - - // Push this run onto the stack. - runs.push(Run { start, len: end - start }); - end = start; - - // Merge some pairs of adjacent runs to satisfy the invariants. - while let Some(r) = collapse(&runs) { - let left = runs[r + 1]; - let right = runs[r]; - unsafe { - merge( - &mut v[left.start..right.start + right.len], - left.len, - buf.as_mut_ptr(), - &mut is_less, - ); - } - runs[r] = Run { start: left.start, len: left.len + right.len }; - runs.remove(r + 1); - } - } - - // Finally, exactly one run must remain in the stack. - debug_assert!(runs.len() == 1 && runs[0].start == 0 && runs[0].len == len); - - // Examines the stack of runs and identifies the next pair of runs to merge. More specifically, - // if `Some(r)` is returned, that means `runs[r]` and `runs[r + 1]` must be merged next. If the - // algorithm should continue building a new run instead, `None` is returned. - // - // TimSort is infamous for its buggy implementations, as described here: - // http://envisage-project.eu/timsort-specification-and-verification/ - // - // The gist of the story is: we must enforce the invariants on the top four runs on the stack. - // Enforcing them on just top three is not sufficient to ensure that the invariants will still - // hold for *all* runs in the stack. - // - // This function correctly checks invariants for the top four runs. Additionally, if the top - // run starts at index 0, it will always demand a merge operation until the stack is fully - // collapsed, in order to complete the sort. - #[inline] - fn collapse(runs: &[Run]) -> Option<usize> { - let n = runs.len(); - if n >= 2 - && (runs[n - 1].start == 0 - || runs[n - 2].len <= runs[n - 1].len - || (n >= 3 && runs[n - 3].len <= runs[n - 2].len + runs[n - 1].len) - || (n >= 4 && runs[n - 4].len <= runs[n - 3].len + runs[n - 2].len)) - { - if n >= 3 && runs[n - 3].len < runs[n - 1].len { Some(n - 3) } else { Some(n - 2) } - } else { - None + let run_dealloc_fn = |buf_ptr: *mut sort::TimSortRun, len: usize| { + // SAFETY: The caller must ensure that buf_ptr was created by elem_alloc_fn with the same + // len. + unsafe { + alloc::dealloc( + buf_ptr as *mut u8, + alloc::Layout::array::<sort::TimSortRun>(len).unwrap_unchecked(), + ); } - } + }; - #[derive(Clone, Copy)] - struct Run { - start: usize, - len: usize, - } + sort::merge_sort(v, &mut is_less, elem_alloc_fn, elem_dealloc_fn, run_alloc_fn, run_dealloc_fn); } |