1 //! The arena, a fast but limited type of allocator.
3 //! Arenas are a type of allocator that destroy the objects within, all at
4 //! once, once the arena itself is destroyed. They do not support deallocation
5 //! of individual objects while the arena itself is still alive. The benefit
6 //! of an arena is very fast allocation; just a pointer bump.
8 //! This crate implements several kinds of arena.
11 html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/",
12 test(no_crate_inject, attr(deny(warnings)))
14 #![feature(array_value_iter_slice)]
15 #![feature(dropck_eyepatch)]
16 #![feature(new_uninit)]
17 #![feature(maybe_uninit_slice)]
18 #![feature(array_value_iter)]
19 #![cfg_attr(bootstrap, feature(min_const_generics))]
20 #![feature(min_specialization)]
21 #![cfg_attr(test, feature(test))]
23 use smallvec::SmallVec;
25 use std::alloc::Layout;
26 use std::cell::{Cell, RefCell};
28 use std::marker::{PhantomData, Send};
29 use std::mem::{self, MaybeUninit};
35 pub fn cold_path<F: FnOnce() -> R, R>(f: F) -> R {
39 /// An arena that can hold objects of only one type.
40 pub struct TypedArena<T> {
41 /// A pointer to the next object to be allocated.
44 /// A pointer to the end of the allocated area. When this pointer is
45 /// reached, a new chunk is allocated.
48 /// A vector of arena chunks.
49 chunks: RefCell<Vec<TypedArenaChunk<T>>>,
51 /// Marker indicating that dropping the arena causes its owned
52 /// instances of `T` to be dropped.
56 struct TypedArenaChunk<T> {
57 /// The raw storage for the arena chunk.
58 storage: Box<[MaybeUninit<T>]>,
59 /// The number of valid entries in the chunk.
63 impl<T> TypedArenaChunk<T> {
65 unsafe fn new(capacity: usize) -> TypedArenaChunk<T> {
66 TypedArenaChunk { storage: Box::new_uninit_slice(capacity), entries: 0 }
69 /// Destroys this arena chunk.
71 unsafe fn destroy(&mut self, len: usize) {
72 // The branch on needs_drop() is an -O1 performance optimization.
73 // Without the branch, dropping TypedArena<u8> takes linear time.
74 if mem::needs_drop::<T>() {
75 ptr::drop_in_place(MaybeUninit::slice_assume_init_mut(&mut self.storage[..len]));
79 // Returns a pointer to the first allocated object.
81 fn start(&mut self) -> *mut T {
82 MaybeUninit::slice_as_mut_ptr(&mut self.storage)
85 // Returns a pointer to the end of the allocated space.
87 fn end(&mut self) -> *mut T {
89 if mem::size_of::<T>() == 0 {
90 // A pointer as large as possible for zero-sized elements.
93 self.start().add(self.storage.len())
99 // The arenas start with PAGE-sized chunks, and then each new chunk is twice as
100 // big as its predecessor, up until we reach HUGE_PAGE-sized chunks, whereupon
101 // we stop growing. This scales well, from arenas that are barely used up to
102 // arenas that are used for 100s of MiBs. Note also that the chosen sizes match
103 // the usual sizes of pages and huge pages on Linux.
104 const PAGE: usize = 4096;
105 const HUGE_PAGE: usize = 2 * 1024 * 1024;
107 impl<T> Default for TypedArena<T> {
108 /// Creates a new `TypedArena`.
109 fn default() -> TypedArena<T> {
111 // We set both `ptr` and `end` to 0 so that the first call to
112 // alloc() will trigger a grow().
113 ptr: Cell::new(ptr::null_mut()),
114 end: Cell::new(ptr::null_mut()),
115 chunks: RefCell::new(vec![]),
122 fn alloc_from_iter(self, arena: &TypedArena<T>) -> &mut [T];
125 impl<I, T> IterExt<T> for I
127 I: IntoIterator<Item = T>,
130 default fn alloc_from_iter(self, arena: &TypedArena<T>) -> &mut [T] {
131 let vec: SmallVec<[_; 8]> = self.into_iter().collect();
132 vec.alloc_from_iter(arena)
136 impl<T, const N: usize> IterExt<T> for std::array::IntoIter<T, N> {
138 fn alloc_from_iter(self, arena: &TypedArena<T>) -> &mut [T] {
139 let len = self.len();
143 // Move the content to the arena by copying and then forgetting it
145 let start_ptr = arena.alloc_raw_slice(len);
146 self.as_slice().as_ptr().copy_to_nonoverlapping(start_ptr, len);
148 slice::from_raw_parts_mut(start_ptr, len)
153 impl<T> IterExt<T> for Vec<T> {
155 fn alloc_from_iter(mut self, arena: &TypedArena<T>) -> &mut [T] {
156 let len = self.len();
160 // Move the content to the arena by copying and then forgetting it
162 let start_ptr = arena.alloc_raw_slice(len);
163 self.as_ptr().copy_to_nonoverlapping(start_ptr, len);
165 slice::from_raw_parts_mut(start_ptr, len)
170 impl<A: smallvec::Array> IterExt<A::Item> for SmallVec<A> {
172 fn alloc_from_iter(mut self, arena: &TypedArena<A::Item>) -> &mut [A::Item] {
173 let len = self.len();
177 // Move the content to the arena by copying and then forgetting it
179 let start_ptr = arena.alloc_raw_slice(len);
180 self.as_ptr().copy_to_nonoverlapping(start_ptr, len);
182 slice::from_raw_parts_mut(start_ptr, len)
187 impl<T> TypedArena<T> {
188 /// Allocates an object in the `TypedArena`, returning a reference to it.
190 pub fn alloc(&self, object: T) -> &mut T {
191 if self.ptr == self.end {
196 if mem::size_of::<T>() == 0 {
197 self.ptr.set((self.ptr.get() as *mut u8).wrapping_offset(1) as *mut T);
198 let ptr = mem::align_of::<T>() as *mut T;
199 // Don't drop the object. This `write` is equivalent to `forget`.
200 ptr::write(ptr, object);
203 let ptr = self.ptr.get();
204 // Advance the pointer.
205 self.ptr.set(self.ptr.get().offset(1));
206 // Write into uninitialized memory.
207 ptr::write(ptr, object);
214 fn can_allocate(&self, additional: usize) -> bool {
215 let available_bytes = self.end.get() as usize - self.ptr.get() as usize;
216 let additional_bytes = additional.checked_mul(mem::size_of::<T>()).unwrap();
217 available_bytes >= additional_bytes
220 /// Ensures there's enough space in the current chunk to fit `len` objects.
222 fn ensure_capacity(&self, additional: usize) {
223 if !self.can_allocate(additional) {
224 self.grow(additional);
225 debug_assert!(self.can_allocate(additional));
230 unsafe fn alloc_raw_slice(&self, len: usize) -> *mut T {
231 assert!(mem::size_of::<T>() != 0);
234 self.ensure_capacity(len);
236 let start_ptr = self.ptr.get();
237 self.ptr.set(start_ptr.add(len));
241 /// Allocates a slice of objects that are copied into the `TypedArena`, returning a mutable
242 /// reference to it. Will panic if passed a zero-sized types.
246 /// - Zero-sized types
247 /// - Zero-length slices
249 pub fn alloc_slice(&self, slice: &[T]) -> &mut [T]
254 let len = slice.len();
255 let start_ptr = self.alloc_raw_slice(len);
256 slice.as_ptr().copy_to_nonoverlapping(start_ptr, len);
257 slice::from_raw_parts_mut(start_ptr, len)
262 pub fn alloc_from_iter<I: IntoIterator<Item = T>>(&self, iter: I) -> &mut [T] {
263 assert!(mem::size_of::<T>() != 0);
264 iter.alloc_from_iter(self)
270 fn grow(&self, additional: usize) {
272 // We need the element size to convert chunk sizes (ranging from
273 // PAGE to HUGE_PAGE bytes) to element counts.
274 let elem_size = cmp::max(1, mem::size_of::<T>());
275 let mut chunks = self.chunks.borrow_mut();
277 if let Some(last_chunk) = chunks.last_mut() {
278 // If a type is `!needs_drop`, we don't need to keep track of how many elements
279 // the chunk stores - the field will be ignored anyway.
280 if mem::needs_drop::<T>() {
281 let used_bytes = self.ptr.get() as usize - last_chunk.start() as usize;
282 last_chunk.entries = used_bytes / mem::size_of::<T>();
285 // If the previous chunk's len is less than HUGE_PAGE
286 // bytes, then this chunk will be least double the previous
288 new_cap = last_chunk.storage.len().min(HUGE_PAGE / elem_size / 2);
291 new_cap = PAGE / elem_size;
293 // Also ensure that this chunk can fit `additional`.
294 new_cap = cmp::max(additional, new_cap);
296 let mut chunk = TypedArenaChunk::<T>::new(new_cap);
297 self.ptr.set(chunk.start());
298 self.end.set(chunk.end());
303 /// Clears the arena. Deallocates all but the longest chunk which may be reused.
304 pub fn clear(&mut self) {
306 // Clear the last chunk, which is partially filled.
307 let mut chunks_borrow = self.chunks.borrow_mut();
308 if let Some(mut last_chunk) = chunks_borrow.last_mut() {
309 self.clear_last_chunk(&mut last_chunk);
310 let len = chunks_borrow.len();
311 // If `T` is ZST, code below has no effect.
312 for mut chunk in chunks_borrow.drain(..len - 1) {
313 chunk.destroy(chunk.entries);
319 // Drops the contents of the last chunk. The last chunk is partially empty, unlike all other
321 fn clear_last_chunk(&self, last_chunk: &mut TypedArenaChunk<T>) {
322 // Determine how much was filled.
323 let start = last_chunk.start() as usize;
324 // We obtain the value of the pointer to the first uninitialized element.
325 let end = self.ptr.get() as usize;
326 // We then calculate the number of elements to be dropped in the last chunk,
327 // which is the filled area's length.
328 let diff = if mem::size_of::<T>() == 0 {
329 // `T` is ZST. It can't have a drop flag, so the value here doesn't matter. We get
330 // the number of zero-sized values in the last and only chunk, just out of caution.
331 // Recall that `end` was incremented for each allocated value.
334 (end - start) / mem::size_of::<T>()
336 // Pass that to the `destroy` method.
338 last_chunk.destroy(diff);
341 self.ptr.set(last_chunk.start());
345 unsafe impl<#[may_dangle] T> Drop for TypedArena<T> {
348 // Determine how much was filled.
349 let mut chunks_borrow = self.chunks.borrow_mut();
350 if let Some(mut last_chunk) = chunks_borrow.pop() {
351 // Drop the contents of the last chunk.
352 self.clear_last_chunk(&mut last_chunk);
353 // The last chunk will be dropped. Destroy all other chunks.
354 for chunk in chunks_borrow.iter_mut() {
355 chunk.destroy(chunk.entries);
358 // Box handles deallocation of `last_chunk` and `self.chunks`.
363 unsafe impl<T: Send> Send for TypedArena<T> {}
365 pub struct DroplessArena {
366 /// A pointer to the start of the free space.
367 start: Cell<*mut u8>,
369 /// A pointer to the end of free space.
371 /// The allocation proceeds from the end of the chunk towards the start.
372 /// When this pointer crosses the start pointer, a new chunk is allocated.
375 /// A vector of arena chunks.
376 chunks: RefCell<Vec<TypedArenaChunk<u8>>>,
379 unsafe impl Send for DroplessArena {}
381 impl Default for DroplessArena {
383 fn default() -> DroplessArena {
385 start: Cell::new(ptr::null_mut()),
386 end: Cell::new(ptr::null_mut()),
387 chunks: Default::default(),
395 fn grow(&self, additional: usize) {
397 let mut chunks = self.chunks.borrow_mut();
399 if let Some(last_chunk) = chunks.last_mut() {
400 // There is no need to update `last_chunk.entries` because that
401 // field isn't used by `DroplessArena`.
403 // If the previous chunk's len is less than HUGE_PAGE
404 // bytes, then this chunk will be least double the previous
406 new_cap = last_chunk.storage.len().min(HUGE_PAGE / 2);
411 // Also ensure that this chunk can fit `additional`.
412 new_cap = cmp::max(additional, new_cap);
414 let mut chunk = TypedArenaChunk::<u8>::new(new_cap);
415 self.start.set(chunk.start());
416 self.end.set(chunk.end());
421 /// Allocates a byte slice with specified layout from the current memory
422 /// chunk. Returns `None` if there is no free space left to satisfy the
425 fn alloc_raw_without_grow(&self, layout: Layout) -> Option<*mut u8> {
426 let start = self.start.get() as usize;
427 let end = self.end.get() as usize;
429 let align = layout.align();
430 let bytes = layout.size();
432 let new_end = end.checked_sub(bytes)? & !(align - 1);
433 if start <= new_end {
434 let new_end = new_end as *mut u8;
435 self.end.set(new_end);
443 pub fn alloc_raw(&self, layout: Layout) -> *mut u8 {
444 assert!(layout.size() != 0);
446 if let Some(a) = self.alloc_raw_without_grow(layout) {
449 // No free space left. Allocate a new chunk to satisfy the request.
450 // On failure the grow will panic or abort.
451 self.grow(layout.size());
456 pub fn alloc<T>(&self, object: T) -> &mut T {
457 assert!(!mem::needs_drop::<T>());
459 let mem = self.alloc_raw(Layout::for_value::<T>(&object)) as *mut T;
462 // Write into uninitialized memory.
463 ptr::write(mem, object);
468 /// Allocates a slice of objects that are copied into the `DroplessArena`, returning a mutable
469 /// reference to it. Will panic if passed a zero-sized type.
473 /// - Zero-sized types
474 /// - Zero-length slices
476 pub fn alloc_slice<T>(&self, slice: &[T]) -> &mut [T]
480 assert!(!mem::needs_drop::<T>());
481 assert!(mem::size_of::<T>() != 0);
482 assert!(!slice.is_empty());
484 let mem = self.alloc_raw(Layout::for_value::<[T]>(slice)) as *mut T;
487 mem.copy_from_nonoverlapping(slice.as_ptr(), slice.len());
488 slice::from_raw_parts_mut(mem, slice.len())
493 unsafe fn write_from_iter<T, I: Iterator<Item = T>>(
500 // Use a manual loop since LLVM manages to optimize it better for
503 let value = iter.next();
504 if i >= len || value.is_none() {
505 // We only return as many items as the iterator gave us, even
506 // though it was supposed to give us `len`
507 return slice::from_raw_parts_mut(mem, i);
509 ptr::write(mem.add(i), value.unwrap());
515 pub fn alloc_from_iter<T, I: IntoIterator<Item = T>>(&self, iter: I) -> &mut [T] {
516 let iter = iter.into_iter();
517 assert!(mem::size_of::<T>() != 0);
518 assert!(!mem::needs_drop::<T>());
520 let size_hint = iter.size_hint();
523 (min, Some(max)) if min == max => {
524 // We know the exact number of elements the iterator will produce here
531 let mem = self.alloc_raw(Layout::array::<T>(len).unwrap()) as *mut T;
532 unsafe { self.write_from_iter(iter, len, mem) }
535 cold_path(move || -> &mut [T] {
536 let mut vec: SmallVec<[_; 8]> = iter.collect();
540 // Move the content to the arena by copying it and then forgetting
541 // the content of the SmallVec
545 self.alloc_raw(Layout::for_value::<[T]>(vec.as_slice())) as *mut T;
546 vec.as_ptr().copy_to_nonoverlapping(start_ptr, len);
548 slice::from_raw_parts_mut(start_ptr, len)
556 /// Calls the destructor for an object when dropped.
558 drop_fn: unsafe fn(*mut u8),
562 unsafe fn drop_for_type<T>(to_drop: *mut u8) {
563 std::ptr::drop_in_place(to_drop as *mut T)
566 impl Drop for DropType {
568 unsafe { (self.drop_fn)(self.obj) }
572 /// An arena which can be used to allocate any type.
573 /// Allocating in this arena is unsafe since the type system
574 /// doesn't know which types it contains. In order to
575 /// allocate safely, you must store a PhantomData<T>
576 /// alongside this arena for each type T you allocate.
578 pub struct DropArena {
579 /// A list of destructors to run when the arena drops.
580 /// Ordered so `destructors` gets dropped before the arena
581 /// since its destructor can reference memory in the arena.
582 destructors: RefCell<Vec<DropType>>,
583 arena: DroplessArena,
588 pub unsafe fn alloc<T>(&self, object: T) -> &mut T {
589 let mem = self.arena.alloc_raw(Layout::new::<T>()) as *mut T;
590 // Write into uninitialized memory.
591 ptr::write(mem, object);
592 let result = &mut *mem;
593 // Record the destructor after doing the allocation as that may panic
594 // and would cause `object`'s destructor to run twice if it was recorded before
597 .push(DropType { drop_fn: drop_for_type::<T>, obj: result as *mut T as *mut u8 });
602 pub unsafe fn alloc_from_iter<T, I: IntoIterator<Item = T>>(&self, iter: I) -> &mut [T] {
603 let mut vec: SmallVec<[_; 8]> = iter.into_iter().collect();
609 let start_ptr = self.arena.alloc_raw(Layout::array::<T>(len).unwrap()) as *mut T;
611 let mut destructors = self.destructors.borrow_mut();
612 // Reserve space for the destructors so we can't panic while adding them
613 destructors.reserve(len);
615 // Move the content to the arena by copying it and then forgetting
616 // the content of the SmallVec
617 vec.as_ptr().copy_to_nonoverlapping(start_ptr, len);
618 mem::forget(vec.drain(..));
620 // Record the destructors after doing the allocation as that may panic
621 // and would cause `object`'s destructor to run twice if it was recorded before
624 .push(DropType { drop_fn: drop_for_type::<T>, obj: start_ptr.add(i) as *mut u8 });
627 slice::from_raw_parts_mut(start_ptr, len)
632 macro_rules! arena_for_type {
634 $crate::TypedArena<$ty>
636 ([few $(, $attrs:ident)*][$ty:ty]) => {
637 ::std::marker::PhantomData<$ty>
639 ([$ignore:ident $(, $attrs:ident)*]$args:tt) => {
640 $crate::arena_for_type!([$($attrs),*]$args)
645 macro_rules! which_arena_for_type {
646 ([][$arena:expr]) => {
647 ::std::option::Option::Some($arena)
649 ([few$(, $attrs:ident)*][$arena:expr]) => {
650 ::std::option::Option::None
652 ([$ignore:ident$(, $attrs:ident)*]$args:tt) => {
653 $crate::which_arena_for_type!([$($attrs),*]$args)
658 macro_rules! declare_arena {
659 ([], [$($a:tt $name:ident: $ty:ty,)*], $tcx:lifetime) => {
661 pub struct Arena<$tcx> {
662 pub dropless: $crate::DroplessArena,
663 drop: $crate::DropArena,
664 $($name: $crate::arena_for_type!($a[$ty]),)*
667 pub trait ArenaAllocatable<'tcx, T = Self>: Sized {
668 fn allocate_on<'a>(self, arena: &'a Arena<'tcx>) -> &'a mut Self;
669 fn allocate_from_iter<'a>(
670 arena: &'a Arena<'tcx>,
671 iter: impl ::std::iter::IntoIterator<Item = Self>,
675 impl<'tcx, T: Copy> ArenaAllocatable<'tcx, ()> for T {
677 fn allocate_on<'a>(self, arena: &'a Arena<'tcx>) -> &'a mut Self {
678 arena.dropless.alloc(self)
681 fn allocate_from_iter<'a>(
682 arena: &'a Arena<'tcx>,
683 iter: impl ::std::iter::IntoIterator<Item = Self>,
684 ) -> &'a mut [Self] {
685 arena.dropless.alloc_from_iter(iter)
690 impl<$tcx> ArenaAllocatable<$tcx, $ty> for $ty {
692 fn allocate_on<'a>(self, arena: &'a Arena<$tcx>) -> &'a mut Self {
693 if !::std::mem::needs_drop::<Self>() {
694 return arena.dropless.alloc(self);
696 match $crate::which_arena_for_type!($a[&arena.$name]) {
697 ::std::option::Option::<&$crate::TypedArena<Self>>::Some(ty_arena) => {
700 ::std::option::Option::None => unsafe { arena.drop.alloc(self) },
705 fn allocate_from_iter<'a>(
706 arena: &'a Arena<$tcx>,
707 iter: impl ::std::iter::IntoIterator<Item = Self>,
708 ) -> &'a mut [Self] {
709 if !::std::mem::needs_drop::<Self>() {
710 return arena.dropless.alloc_from_iter(iter);
712 match $crate::which_arena_for_type!($a[&arena.$name]) {
713 ::std::option::Option::<&$crate::TypedArena<Self>>::Some(ty_arena) => {
714 ty_arena.alloc_from_iter(iter)
716 ::std::option::Option::None => unsafe { arena.drop.alloc_from_iter(iter) },
722 impl<'tcx> Arena<'tcx> {
724 pub fn alloc<T: ArenaAllocatable<'tcx, U>, U>(&self, value: T) -> &mut T {
725 value.allocate_on(self)
729 pub fn alloc_slice<T: ::std::marker::Copy>(&self, value: &[T]) -> &mut [T] {
730 if value.is_empty() {
733 self.dropless.alloc_slice(value)
736 pub fn alloc_from_iter<'a, T: ArenaAllocatable<'tcx, U>, U>(
738 iter: impl ::std::iter::IntoIterator<Item = T>,
740 T::allocate_from_iter(self, iter)