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/",
12 test(no_crate_inject, attr(deny(warnings)))
14 #![feature(core_intrinsics)]
15 #![feature(dropck_eyepatch)]
16 #![feature(raw_vec_internals)]
17 #![cfg_attr(test, feature(test))]
22 use rustc_data_structures::cold_path;
23 use smallvec::SmallVec;
25 use std::alloc::Layout;
26 use std::cell::{Cell, RefCell};
29 use std::marker::{PhantomData, Send};
34 use alloc::raw_vec::RawVec;
36 /// An arena that can hold objects of only one type.
37 pub struct TypedArena<T> {
38 /// A pointer to the next object to be allocated.
41 /// A pointer to the end of the allocated area. When this pointer is
42 /// reached, a new chunk is allocated.
45 /// A vector of arena chunks.
46 chunks: RefCell<Vec<TypedArenaChunk<T>>>,
48 /// Marker indicating that dropping the arena causes its owned
49 /// instances of `T` to be dropped.
53 struct TypedArenaChunk<T> {
54 /// The raw storage for the arena chunk.
56 /// The number of valid entries in the chunk.
60 impl<T> TypedArenaChunk<T> {
62 unsafe fn new(capacity: usize) -> TypedArenaChunk<T> {
63 TypedArenaChunk { storage: RawVec::with_capacity(capacity), entries: 0 }
66 /// Destroys this arena chunk.
68 unsafe fn destroy(&mut self, len: usize) {
69 // The branch on needs_drop() is an -O1 performance optimization.
70 // Without the branch, dropping TypedArena<u8> takes linear time.
71 if mem::needs_drop::<T>() {
72 let mut start = self.start();
73 // Destroy all allocated objects.
75 ptr::drop_in_place(start);
76 start = start.offset(1);
81 // Returns a pointer to the first allocated object.
83 fn start(&self) -> *mut T {
87 // Returns a pointer to the end of the allocated space.
89 fn end(&self) -> *mut T {
91 if mem::size_of::<T>() == 0 {
92 // A pointer as large as possible for zero-sized elements.
95 self.start().add(self.storage.capacity())
101 // The arenas start with PAGE-sized chunks, and then each new chunk is twice as
102 // big as its predecessor, up until we reach HUGE_PAGE-sized chunks, whereupon
103 // we stop growing. This scales well, from arenas that are barely used up to
104 // arenas that are used for 100s of MiBs. Note also that the chosen sizes match
105 // the usual sizes of pages and huge pages on Linux.
106 const PAGE: usize = 4096;
107 const HUGE_PAGE: usize = 2 * 1024 * 1024;
109 impl<T> Default for TypedArena<T> {
110 /// Creates a new `TypedArena`.
111 fn default() -> TypedArena<T> {
113 // We set both `ptr` and `end` to 0 so that the first call to
114 // alloc() will trigger a grow().
115 ptr: Cell::new(ptr::null_mut()),
116 end: Cell::new(ptr::null_mut()),
117 chunks: RefCell::new(vec![]),
123 impl<T> TypedArena<T> {
124 /// Allocates an object in the `TypedArena`, returning a reference to it.
126 pub fn alloc(&self, object: T) -> &mut T {
127 if self.ptr == self.end {
132 if mem::size_of::<T>() == 0 {
133 self.ptr.set(intrinsics::arith_offset(self.ptr.get() as *mut u8, 1) as *mut T);
134 let ptr = mem::align_of::<T>() as *mut T;
135 // Don't drop the object. This `write` is equivalent to `forget`.
136 ptr::write(ptr, object);
139 let ptr = self.ptr.get();
140 // Advance the pointer.
141 self.ptr.set(self.ptr.get().offset(1));
142 // Write into uninitialized memory.
143 ptr::write(ptr, object);
150 fn can_allocate(&self, additional: usize) -> bool {
151 let available_bytes = self.end.get() as usize - self.ptr.get() as usize;
152 let additional_bytes = additional.checked_mul(mem::size_of::<T>()).unwrap();
153 available_bytes >= additional_bytes
156 /// Ensures there's enough space in the current chunk to fit `len` objects.
158 fn ensure_capacity(&self, additional: usize) {
159 if !self.can_allocate(additional) {
160 self.grow(additional);
161 debug_assert!(self.can_allocate(additional));
166 unsafe fn alloc_raw_slice(&self, len: usize) -> *mut T {
167 assert!(mem::size_of::<T>() != 0);
170 self.ensure_capacity(len);
172 let start_ptr = self.ptr.get();
173 self.ptr.set(start_ptr.add(len));
177 /// Allocates a slice of objects that are copied into the `TypedArena`, returning a mutable
178 /// reference to it. Will panic if passed a zero-sized types.
182 /// - Zero-sized types
183 /// - Zero-length slices
185 pub fn alloc_slice(&self, slice: &[T]) -> &mut [T]
190 let len = slice.len();
191 let start_ptr = self.alloc_raw_slice(len);
192 slice.as_ptr().copy_to_nonoverlapping(start_ptr, len);
193 slice::from_raw_parts_mut(start_ptr, len)
198 pub fn alloc_from_iter<I: IntoIterator<Item = T>>(&self, iter: I) -> &mut [T] {
199 assert!(mem::size_of::<T>() != 0);
200 let mut vec: SmallVec<[_; 8]> = iter.into_iter().collect();
204 // Move the content to the arena by copying it and then forgetting
205 // the content of the SmallVec
208 let start_ptr = self.alloc_raw_slice(len);
209 vec.as_ptr().copy_to_nonoverlapping(start_ptr, len);
211 slice::from_raw_parts_mut(start_ptr, len)
218 fn grow(&self, additional: usize) {
220 // We need the element size to convert chunk sizes (ranging from
221 // PAGE to HUGE_PAGE bytes) to element counts.
222 let elem_size = cmp::max(1, mem::size_of::<T>());
223 let mut chunks = self.chunks.borrow_mut();
225 if let Some(last_chunk) = chunks.last_mut() {
226 let used_bytes = self.ptr.get() as usize - last_chunk.start() as usize;
227 last_chunk.entries = used_bytes / mem::size_of::<T>();
229 // If the previous chunk's capacity is less than HUGE_PAGE
230 // bytes, then this chunk will be least double the previous
232 new_cap = last_chunk.storage.capacity();
233 if new_cap < HUGE_PAGE / elem_size {
234 new_cap = new_cap.checked_mul(2).unwrap();
237 new_cap = PAGE / elem_size;
239 // Also ensure that this chunk can fit `additional`.
240 new_cap = cmp::max(additional, new_cap);
242 let chunk = TypedArenaChunk::<T>::new(new_cap);
243 self.ptr.set(chunk.start());
244 self.end.set(chunk.end());
249 /// Clears the arena. Deallocates all but the longest chunk which may be reused.
250 pub fn clear(&mut self) {
252 // Clear the last chunk, which is partially filled.
253 let mut chunks_borrow = self.chunks.borrow_mut();
254 if let Some(mut last_chunk) = chunks_borrow.last_mut() {
255 self.clear_last_chunk(&mut last_chunk);
256 let len = chunks_borrow.len();
257 // If `T` is ZST, code below has no effect.
258 for mut chunk in chunks_borrow.drain(..len - 1) {
259 chunk.destroy(chunk.entries);
265 // Drops the contents of the last chunk. The last chunk is partially empty, unlike all other
267 fn clear_last_chunk(&self, last_chunk: &mut TypedArenaChunk<T>) {
268 // Determine how much was filled.
269 let start = last_chunk.start() as usize;
270 // We obtain the value of the pointer to the first uninitialized element.
271 let end = self.ptr.get() as usize;
272 // We then calculate the number of elements to be dropped in the last chunk,
273 // which is the filled area's length.
274 let diff = if mem::size_of::<T>() == 0 {
275 // `T` is ZST. It can't have a drop flag, so the value here doesn't matter. We get
276 // the number of zero-sized values in the last and only chunk, just out of caution.
277 // Recall that `end` was incremented for each allocated value.
280 (end - start) / mem::size_of::<T>()
282 // Pass that to the `destroy` method.
284 last_chunk.destroy(diff);
287 self.ptr.set(last_chunk.start());
291 unsafe impl<#[may_dangle] T> Drop for TypedArena<T> {
294 // Determine how much was filled.
295 let mut chunks_borrow = self.chunks.borrow_mut();
296 if let Some(mut last_chunk) = chunks_borrow.pop() {
297 // Drop the contents of the last chunk.
298 self.clear_last_chunk(&mut last_chunk);
299 // The last chunk will be dropped. Destroy all other chunks.
300 for chunk in chunks_borrow.iter_mut() {
301 chunk.destroy(chunk.entries);
304 // RawVec handles deallocation of `last_chunk` and `self.chunks`.
309 unsafe impl<T: Send> Send for TypedArena<T> {}
311 pub struct DroplessArena {
312 /// A pointer to the next object to be allocated.
315 /// A pointer to the end of the allocated area. When this pointer is
316 /// reached, a new chunk is allocated.
319 /// A vector of arena chunks.
320 chunks: RefCell<Vec<TypedArenaChunk<u8>>>,
323 unsafe impl Send for DroplessArena {}
325 impl Default for DroplessArena {
327 fn default() -> DroplessArena {
329 ptr: Cell::new(ptr::null_mut()),
330 end: Cell::new(ptr::null_mut()),
331 chunks: Default::default(),
339 fn grow(&self, additional: usize) {
341 let mut chunks = self.chunks.borrow_mut();
343 if let Some(last_chunk) = chunks.last_mut() {
344 // There is no need to update `last_chunk.entries` because that
345 // field isn't used by `DroplessArena`.
347 // If the previous chunk's capacity is less than HUGE_PAGE
348 // bytes, then this chunk will be least double the previous
350 new_cap = last_chunk.storage.capacity();
351 if new_cap < HUGE_PAGE {
352 new_cap = new_cap.checked_mul(2).unwrap();
357 // Also ensure that this chunk can fit `additional`.
358 new_cap = cmp::max(additional, new_cap);
360 let chunk = TypedArenaChunk::<u8>::new(new_cap);
361 self.ptr.set(chunk.start());
362 self.end.set(chunk.end());
367 /// Allocates a byte slice with specified layout from the current memory
368 /// chunk. Returns `None` if there is no free space left to satisfy the
371 fn alloc_raw_without_grow(&self, layout: Layout) -> Option<*mut u8> {
372 let ptr = self.ptr.get() as usize;
373 let end = self.end.get() as usize;
374 let align = layout.align();
375 let bytes = layout.size();
376 // The allocation request fits into the current chunk iff:
378 // let aligned = align_to(ptr, align);
379 // ptr <= aligned && aligned + bytes <= end
381 // Except that we work with fixed width integers and need to be careful
382 // about potential overflow in the calcuation. If the overflow does
383 // happen, then we definitely don't have enough free and need to grow
385 let aligned = ptr.checked_add(align - 1)? & !(align - 1);
386 let new_ptr = aligned.checked_add(bytes)?;
388 self.ptr.set(new_ptr as *mut u8);
389 Some(aligned as *mut u8)
396 pub fn alloc_raw(&self, layout: Layout) -> *mut u8 {
397 assert!(layout.size() != 0);
399 if let Some(a) = self.alloc_raw_without_grow(layout) {
402 // No free space left. Allocate a new chunk to satisfy the request.
403 // On failure the grow will panic or abort.
404 self.grow(layout.size());
409 pub fn alloc<T>(&self, object: T) -> &mut T {
410 assert!(!mem::needs_drop::<T>());
412 let mem = self.alloc_raw(Layout::for_value::<T>(&object)) as *mut T;
415 // Write into uninitialized memory.
416 ptr::write(mem, object);
421 /// Allocates a slice of objects that are copied into the `DroplessArena`, returning a mutable
422 /// reference to it. Will panic if passed a zero-sized type.
426 /// - Zero-sized types
427 /// - Zero-length slices
429 pub fn alloc_slice<T>(&self, slice: &[T]) -> &mut [T]
433 assert!(!mem::needs_drop::<T>());
434 assert!(mem::size_of::<T>() != 0);
435 assert!(!slice.is_empty());
437 let mem = self.alloc_raw(Layout::for_value::<[T]>(slice)) as *mut T;
440 mem.copy_from_nonoverlapping(slice.as_ptr(), slice.len());
441 slice::from_raw_parts_mut(mem, slice.len())
446 unsafe fn write_from_iter<T, I: Iterator<Item = T>>(
453 // Use a manual loop since LLVM manages to optimize it better for
456 let value = iter.next();
457 if i >= len || value.is_none() {
458 // We only return as many items as the iterator gave us, even
459 // though it was supposed to give us `len`
460 return slice::from_raw_parts_mut(mem, i);
462 ptr::write(mem.add(i), value.unwrap());
468 pub fn alloc_from_iter<T, I: IntoIterator<Item = T>>(&self, iter: I) -> &mut [T] {
469 let iter = iter.into_iter();
470 assert!(mem::size_of::<T>() != 0);
471 assert!(!mem::needs_drop::<T>());
473 let size_hint = iter.size_hint();
476 (min, Some(max)) if min == max => {
477 // We know the exact number of elements the iterator will produce here
484 let mem = self.alloc_raw(Layout::array::<T>(len).unwrap()) as *mut T;
485 unsafe { self.write_from_iter(iter, len, mem) }
488 cold_path(move || -> &mut [T] {
489 let mut vec: SmallVec<[_; 8]> = iter.collect();
493 // Move the content to the arena by copying it and then forgetting
494 // the content of the SmallVec
498 self.alloc_raw(Layout::for_value::<[T]>(vec.as_slice())) as *mut T;
499 vec.as_ptr().copy_to_nonoverlapping(start_ptr, len);
501 slice::from_raw_parts_mut(start_ptr, len)
509 /// Calls the destructor for an object when dropped.
511 drop_fn: unsafe fn(*mut u8),
515 unsafe fn drop_for_type<T>(to_drop: *mut u8) {
516 std::ptr::drop_in_place(to_drop as *mut T)
519 impl Drop for DropType {
521 unsafe { (self.drop_fn)(self.obj) }
525 /// An arena which can be used to allocate any type.
526 /// Allocating in this arena is unsafe since the type system
527 /// doesn't know which types it contains. In order to
528 /// allocate safely, you must store a PhantomData<T>
529 /// alongside this arena for each type T you allocate.
531 pub struct DropArena {
532 /// A list of destructors to run when the arena drops.
533 /// Ordered so `destructors` gets dropped before the arena
534 /// since its destructor can reference memory in the arena.
535 destructors: RefCell<Vec<DropType>>,
536 arena: DroplessArena,
541 pub unsafe fn alloc<T>(&self, object: T) -> &mut T {
542 let mem = self.arena.alloc_raw(Layout::new::<T>()) as *mut T;
543 // Write into uninitialized memory.
544 ptr::write(mem, object);
545 let result = &mut *mem;
546 // Record the destructor after doing the allocation as that may panic
547 // and would cause `object`'s destuctor to run twice if it was recorded before
550 .push(DropType { drop_fn: drop_for_type::<T>, obj: result as *mut T as *mut u8 });
555 pub unsafe fn alloc_from_iter<T, I: IntoIterator<Item = T>>(&self, iter: I) -> &mut [T] {
556 let mut vec: SmallVec<[_; 8]> = iter.into_iter().collect();
562 let start_ptr = self.arena.alloc_raw(Layout::array::<T>(len).unwrap()) as *mut T;
564 let mut destructors = self.destructors.borrow_mut();
565 // Reserve space for the destructors so we can't panic while adding them
566 destructors.reserve(len);
568 // Move the content to the arena by copying it and then forgetting
569 // the content of the SmallVec
570 vec.as_ptr().copy_to_nonoverlapping(start_ptr, len);
571 mem::forget(vec.drain(..));
573 // Record the destructors after doing the allocation as that may panic
574 // and would cause `object`'s destuctor to run twice if it was recorded before
576 destructors.push(DropType {
577 drop_fn: drop_for_type::<T>,
578 obj: start_ptr.offset(i as isize) as *mut u8,
582 slice::from_raw_parts_mut(start_ptr, len)
587 macro_rules! arena_for_type {
589 $crate::TypedArena<$ty>
591 ([few $(, $attrs:ident)*][$ty:ty]) => {
592 ::std::marker::PhantomData<$ty>
594 ([$ignore:ident $(, $attrs:ident)*]$args:tt) => {
595 $crate::arena_for_type!([$($attrs),*]$args)
600 macro_rules! which_arena_for_type {
601 ([][$arena:expr]) => {
602 ::std::option::Option::Some($arena)
604 ([few$(, $attrs:ident)*][$arena:expr]) => {
605 ::std::option::Option::None
607 ([$ignore:ident$(, $attrs:ident)*]$args:tt) => {
608 $crate::which_arena_for_type!([$($attrs),*]$args)
613 macro_rules! declare_arena {
614 ([], [$($a:tt $name:ident: $ty:ty,)*], $tcx:lifetime) => {
616 pub struct Arena<$tcx> {
617 pub dropless: $crate::DroplessArena,
618 drop: $crate::DropArena,
619 $($name: $crate::arena_for_type!($a[$ty]),)*
622 pub trait ArenaAllocatable<'tcx, T = Self>: Sized {
623 fn allocate_on<'a>(self, arena: &'a Arena<'tcx>) -> &'a mut Self;
624 fn allocate_from_iter<'a>(
625 arena: &'a Arena<'tcx>,
626 iter: impl ::std::iter::IntoIterator<Item = Self>,
630 impl<'tcx, T: Copy> ArenaAllocatable<'tcx, ()> for T {
632 fn allocate_on<'a>(self, arena: &'a Arena<'tcx>) -> &'a mut Self {
633 arena.dropless.alloc(self)
636 fn allocate_from_iter<'a>(
637 arena: &'a Arena<'tcx>,
638 iter: impl ::std::iter::IntoIterator<Item = Self>,
639 ) -> &'a mut [Self] {
640 arena.dropless.alloc_from_iter(iter)
645 impl<$tcx> ArenaAllocatable<$tcx, $ty> for $ty {
647 fn allocate_on<'a>(self, arena: &'a Arena<$tcx>) -> &'a mut Self {
648 if !::std::mem::needs_drop::<Self>() {
649 return arena.dropless.alloc(self);
651 match $crate::which_arena_for_type!($a[&arena.$name]) {
652 ::std::option::Option::<&$crate::TypedArena<Self>>::Some(ty_arena) => {
655 ::std::option::Option::None => unsafe { arena.drop.alloc(self) },
660 fn allocate_from_iter<'a>(
661 arena: &'a Arena<$tcx>,
662 iter: impl ::std::iter::IntoIterator<Item = Self>,
663 ) -> &'a mut [Self] {
664 if !::std::mem::needs_drop::<Self>() {
665 return arena.dropless.alloc_from_iter(iter);
667 match $crate::which_arena_for_type!($a[&arena.$name]) {
668 ::std::option::Option::<&$crate::TypedArena<Self>>::Some(ty_arena) => {
669 ty_arena.alloc_from_iter(iter)
671 ::std::option::Option::None => unsafe { arena.drop.alloc_from_iter(iter) },
677 impl<'tcx> Arena<'tcx> {
679 pub fn alloc<T: ArenaAllocatable<'tcx, U>, U>(&self, value: T) -> &mut T {
680 value.allocate_on(self)
684 pub fn alloc_slice<T: ::std::marker::Copy>(&self, value: &[T]) -> &mut [T] {
685 if value.is_empty() {
688 self.dropless.alloc_slice(value)
691 pub fn alloc_from_iter<'a, T: ArenaAllocatable<'tcx, U>, U>(
693 iter: impl ::std::iter::IntoIterator<Item = T>,
695 T::allocate_from_iter(self, iter)