1 // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! The arena, a fast but limited type of allocator.
13 //! Arenas are a type of allocator that destroy the objects within, all at
14 //! once, once the arena itself is destroyed. They do not support deallocation
15 //! of individual objects while the arena itself is still alive. The benefit
16 //! of an arena is very fast allocation; just a pointer bump.
18 //! This crate has two arenas implemented: `TypedArena`, which is a simpler
19 //! arena but can only hold objects of a single type, and `Arena`, which is a
20 //! more complex, slower arena which can hold objects of any type.
22 #![crate_name = "arena"]
23 #![unstable(feature = "rustc_private", issue = "27812")]
24 #![crate_type = "rlib"]
25 #![crate_type = "dylib"]
26 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
27 html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
28 html_root_url = "https://doc.rust-lang.org/nightly/",
29 test(no_crate_inject, attr(deny(warnings))))]
32 #![feature(core_intrinsics)]
33 #![feature(drop_in_place)]
37 #![feature(staged_api)]
38 #![feature(dropck_parametricity)]
39 #![cfg_attr(test, feature(test))]
45 use std::cell::{Cell, RefCell};
48 use std::marker::{PhantomData, Send};
54 use alloc::raw_vec::RawVec;
58 /// Index of the first unused byte.
60 /// Indicates whether objects with destructors are stored in this chunk.
65 fn new(size: usize, is_copy: bool) -> Chunk {
67 data: RawVec::with_capacity(size),
69 is_copy: Cell::new(is_copy),
73 fn capacity(&self) -> usize {
77 unsafe fn as_ptr(&self) -> *const u8 {
81 // Walk down a chunk, running the destructors for any objects stored
83 unsafe fn destroy(&self) {
85 let buf = self.as_ptr();
86 let fill = self.fill.get();
89 let tydesc_data = buf.offset(idx as isize) as *const usize;
90 let (tydesc, is_done) = un_bitpack_tydesc_ptr(*tydesc_data);
91 let (size, align) = ((*tydesc).size, (*tydesc).align);
93 let after_tydesc = idx + mem::size_of::<*const TyDesc>();
95 let start = round_up(after_tydesc, align);
98 ((*tydesc).drop_glue)(buf.offset(start as isize) as *const i8);
101 // Find where the next tydesc lives
102 idx = round_up(start + size, mem::align_of::<*const TyDesc>());
107 /// A slower reflection-based arena that can allocate objects of any type.
109 /// This arena uses `RawVec<u8>` as a backing store to allocate objects from.
110 /// For each allocated object, the arena stores a pointer to the type descriptor
111 /// followed by the object (potentially with alignment padding after each
112 /// element). When the arena is destroyed, it iterates through all of its
113 /// chunks, and uses the tydesc information to trace through the objects,
114 /// calling the destructors on them. One subtle point that needs to be
115 /// addressed is how to handle panics while running the user provided
116 /// initializer function. It is important to not run the destructor on
117 /// uninitialized objects, but how to detect them is somewhat subtle. Since
118 /// `alloc()` can be invoked recursively, it is not sufficient to simply exclude
119 /// the most recent object. To solve this without requiring extra space, we
120 /// use the low order bit of the tydesc pointer to encode whether the object
121 /// it describes has been fully initialized.
123 /// As an optimization, objects with destructors are stored in different chunks
124 /// than objects without destructors. This reduces overhead when initializing
125 /// plain-old-data (`Copy` types) and means we don't need to waste time running
126 /// their destructors.
127 #[unstable(feature = "rustc_private",
128 reason = "Private to rustc", issue = "0")]
129 #[rustc_deprecated(since = "1.6.0-dev", reason =
130 "The reflection-based arena is superseded by the any-arena crate")]
131 pub struct Arena<'longer_than_self> {
132 // The heads are separated out from the list as a unbenchmarked
133 // microoptimization, to avoid needing to case on the list to access a head.
134 head: RefCell<Chunk>,
135 copy_head: RefCell<Chunk>,
136 chunks: RefCell<Vec<Chunk>>,
137 _marker: PhantomData<*mut &'longer_than_self ()>,
141 /// Allocates a new Arena with 32 bytes preallocated.
142 pub fn new() -> Arena<'a> {
143 Arena::new_with_size(32)
146 /// Allocates a new Arena with `initial_size` bytes preallocated.
147 pub fn new_with_size(initial_size: usize) -> Arena<'a> {
149 head: RefCell::new(Chunk::new(initial_size, false)),
150 copy_head: RefCell::new(Chunk::new(initial_size, true)),
151 chunks: RefCell::new(Vec::new()),
152 _marker: PhantomData,
157 impl<'longer_than_self> Drop for Arena<'longer_than_self> {
160 self.head.borrow().destroy();
161 for chunk in self.chunks.borrow().iter() {
162 if !chunk.is_copy.get() {
171 fn round_up(base: usize, align: usize) -> usize {
172 (base.checked_add(align - 1)).unwrap() & !(align - 1)
175 // We encode whether the object a tydesc describes has been
176 // initialized in the arena in the low bit of the tydesc pointer. This
177 // is necessary in order to properly do cleanup if a panic occurs
178 // during an initializer.
180 fn bitpack_tydesc_ptr(p: *const TyDesc, is_done: bool) -> usize {
181 p as usize | (is_done as usize)
184 fn un_bitpack_tydesc_ptr(p: usize) -> (*const TyDesc, bool) {
185 ((p & !1) as *const TyDesc, p & 1 == 1)
188 // HACK(eddyb) TyDesc replacement using a trait object vtable.
189 // This could be replaced in the future with a custom DST layout,
190 // or `&'static (drop_glue, size, align)` created by a `const fn`.
192 // * rvalue promotion (issue #1056)
193 // * mem::{size_of, align_of} must be const fns
195 drop_glue: fn(*const i8),
204 impl<T: ?Sized> AllTypes for T {}
206 unsafe fn get_tydesc<T>() -> *const TyDesc {
207 use std::raw::TraitObject;
209 let ptr = &*(heap::EMPTY as *const T);
211 // Can use any trait that is implemented for all types.
212 let obj = mem::transmute::<&AllTypes, TraitObject>(ptr);
213 obj.vtable as *const TyDesc
216 impl<'longer_than_self> Arena<'longer_than_self> {
217 // Grows a given chunk and returns `false`, or replaces it with a bigger
218 // chunk and returns `true`.
219 // This method is shared by both parts of the arena.
221 fn alloc_grow(&self, head: &mut Chunk, used_cap: usize, n_bytes: usize) -> bool {
222 if head.data.reserve_in_place(used_cap, n_bytes) {
223 // In-place reallocation succeeded.
226 // Allocate a new chunk.
227 let new_min_chunk_size = cmp::max(n_bytes, head.capacity());
228 let new_chunk = Chunk::new((new_min_chunk_size + 1).next_power_of_two(), false);
229 let old_chunk = mem::replace(head, new_chunk);
230 if old_chunk.fill.get() != 0 {
231 self.chunks.borrow_mut().push(old_chunk);
237 // Functions for the copyable part of the arena.
240 fn alloc_copy_inner(&self, n_bytes: usize, align: usize) -> *const u8 {
241 let mut copy_head = self.copy_head.borrow_mut();
242 let fill = copy_head.fill.get();
243 let mut start = round_up(fill, align);
244 let mut end = start + n_bytes;
246 if end > copy_head.capacity() {
247 if self.alloc_grow(&mut *copy_head, fill, end - fill) {
248 // Continuing with a newly allocated chunk
251 copy_head.is_copy.set(true);
255 copy_head.fill.set(end);
257 unsafe { copy_head.as_ptr().offset(start as isize) }
261 fn alloc_copy<T, F>(&self, op: F) -> &mut T
262 where F: FnOnce() -> T
265 let ptr = self.alloc_copy_inner(mem::size_of::<T>(), mem::align_of::<T>());
266 let ptr = ptr as *mut T;
267 ptr::write(&mut (*ptr), op());
272 // Functions for the non-copyable part of the arena.
275 fn alloc_noncopy_inner(&self, n_bytes: usize, align: usize) -> (*const u8, *const u8) {
276 let mut head = self.head.borrow_mut();
277 let fill = head.fill.get();
279 let mut tydesc_start = fill;
280 let after_tydesc = fill + mem::size_of::<*const TyDesc>();
281 let mut start = round_up(after_tydesc, align);
282 let mut end = round_up(start + n_bytes, mem::align_of::<*const TyDesc>());
284 if end > head.capacity() {
285 if self.alloc_grow(&mut *head, tydesc_start, end - tydesc_start) {
286 // Continuing with a newly allocated chunk
288 start = round_up(mem::size_of::<*const TyDesc>(), align);
289 end = round_up(start + n_bytes, mem::align_of::<*const TyDesc>());
296 let buf = head.as_ptr();
297 (buf.offset(tydesc_start as isize),
298 buf.offset(start as isize))
303 fn alloc_noncopy<T, F>(&self, op: F) -> &mut T
304 where F: FnOnce() -> T
307 let tydesc = get_tydesc::<T>();
308 let (ty_ptr, ptr) = self.alloc_noncopy_inner(mem::size_of::<T>(), mem::align_of::<T>());
309 let ty_ptr = ty_ptr as *mut usize;
310 let ptr = ptr as *mut T;
311 // Write in our tydesc along with a bit indicating that it
312 // has *not* been initialized yet.
313 *ty_ptr = bitpack_tydesc_ptr(tydesc, false);
314 // Actually initialize it
315 ptr::write(&mut (*ptr), op());
316 // Now that we are done, update the tydesc to indicate that
317 // the object is there.
318 *ty_ptr = bitpack_tydesc_ptr(tydesc, true);
324 /// Allocates a new item in the arena, using `op` to initialize the value,
325 /// and returns a reference to it.
327 pub fn alloc<T: 'longer_than_self, F>(&self, op: F) -> &mut T
328 where F: FnOnce() -> T
331 if intrinsics::needs_drop::<T>() {
332 self.alloc_noncopy(op)
339 /// Allocates a slice of bytes of requested length. The bytes are not guaranteed to be zero
340 /// if the arena has previously been cleared.
344 /// Panics if the requested length is too large and causes overflow.
345 pub fn alloc_bytes(&self, len: usize) -> &mut [u8] {
347 // Check for overflow.
348 self.copy_head.borrow().fill.get().checked_add(len).expect("length overflow");
349 let ptr = self.alloc_copy_inner(len, 1);
350 intrinsics::assume(!ptr.is_null());
351 slice::from_raw_parts_mut(ptr as *mut _, len)
355 /// Clears the arena. Deallocates all but the longest chunk which may be reused.
356 pub fn clear(&mut self) {
358 self.head.borrow().destroy();
359 self.head.borrow().fill.set(0);
360 self.copy_head.borrow().fill.set(0);
361 for chunk in self.chunks.borrow().iter() {
362 if !chunk.is_copy.get() {
366 self.chunks.borrow_mut().clear();
371 /// A faster arena that can hold objects of only one type.
372 pub struct TypedArena<T> {
373 /// A pointer to the next object to be allocated.
376 /// A pointer to the end of the allocated area. When this pointer is
377 /// reached, a new chunk is allocated.
380 /// A vector arena segments.
381 chunks: RefCell<Vec<TypedArenaChunk<T>>>,
383 /// Marker indicating that dropping the arena causes its owned
384 /// instances of `T` to be dropped.
385 _own: PhantomData<T>,
388 struct TypedArenaChunk<T> {
389 /// Pointer to the next arena segment.
393 impl<T> TypedArenaChunk<T> {
395 unsafe fn new(capacity: usize) -> TypedArenaChunk<T> {
396 TypedArenaChunk { storage: RawVec::with_capacity(capacity) }
399 /// Destroys this arena chunk.
401 unsafe fn destroy(&mut self, len: usize) {
402 // The branch on needs_drop() is an -O1 performance optimization.
403 // Without the branch, dropping TypedArena<u8> takes linear time.
404 if intrinsics::needs_drop::<T>() {
405 let mut start = self.start();
406 // Destroy all allocated objects.
408 ptr::drop_in_place(start);
409 start = start.offset(1);
414 // Returns a pointer to the first allocated object.
416 fn start(&self) -> *mut T {
420 // Returns a pointer to the end of the allocated space.
422 fn end(&self) -> *mut T {
424 if mem::size_of::<T>() == 0 {
425 // A pointer as large as possible for zero-sized elements.
428 self.start().offset(self.storage.cap() as isize)
434 const PAGE: usize = 4096;
436 impl<T> TypedArena<T> {
437 /// Creates a new `TypedArena` with preallocated space for many objects.
439 pub fn new() -> TypedArena<T> {
440 // Reserve at least one page.
441 let elem_size = cmp::max(1, mem::size_of::<T>());
442 TypedArena::with_capacity(PAGE / elem_size)
445 /// Creates a new `TypedArena` with preallocated space for the given number of
448 pub fn with_capacity(capacity: usize) -> TypedArena<T> {
450 let chunk = TypedArenaChunk::<T>::new(cmp::max(1, capacity));
452 ptr: Cell::new(chunk.start()),
453 end: Cell::new(chunk.end()),
454 chunks: RefCell::new(vec![chunk]),
460 /// Allocates an object in the `TypedArena`, returning a reference to it.
462 pub fn alloc(&self, object: T) -> &mut T {
463 if self.ptr == self.end {
468 if mem::size_of::<T>() == 0 {
469 self.ptr.set(intrinsics::arith_offset(self.ptr.get() as *mut u8, 1) as *mut T);
470 let ptr = heap::EMPTY as *mut T;
471 // Don't drop the object. This `write` is equivalent to `forget`.
472 ptr::write(ptr, object);
475 let ptr = self.ptr.get();
476 // Advance the pointer.
477 self.ptr.set(self.ptr.get().offset(1));
478 // Write into uninitialized memory.
479 ptr::write(ptr, object);
490 let mut chunks = self.chunks.borrow_mut();
491 let prev_capacity = chunks.last().unwrap().storage.cap();
492 let new_capacity = prev_capacity.checked_mul(2).unwrap();
493 if chunks.last_mut().unwrap().storage.double_in_place() {
494 self.end.set(chunks.last().unwrap().end());
496 let chunk = TypedArenaChunk::<T>::new(new_capacity);
497 self.ptr.set(chunk.start());
498 self.end.set(chunk.end());
503 /// Clears the arena. Deallocates all but the longest chunk which may be reused.
504 pub fn clear(&mut self) {
506 // Clear the last chunk, which is partially filled.
507 let mut chunks_borrow = self.chunks.borrow_mut();
508 let last_idx = chunks_borrow.len() - 1;
509 self.clear_last_chunk(&mut chunks_borrow[last_idx]);
510 // If `T` is ZST, code below has no effect.
511 for mut chunk in chunks_borrow.drain(..last_idx) {
512 let cap = chunk.storage.cap();
518 // Drops the contents of the last chunk. The last chunk is partially empty, unlike all other
520 fn clear_last_chunk(&self, last_chunk: &mut TypedArenaChunk<T>) {
521 // Determine how much was filled.
522 let start = last_chunk.start() as usize;
523 // We obtain the value of the pointer to the first uninitialized element.
524 let end = self.ptr.get() as usize;
525 // We then calculate the number of elements to be dropped in the last chunk,
526 // which is the filled area's length.
527 let diff = if mem::size_of::<T>() == 0 {
528 // `T` is ZST. It can't have a drop flag, so the value here doesn't matter. We get
529 // the number of zero-sized values in the last and only chunk, just out of caution.
530 // Recall that `end` was incremented for each allocated value.
533 (end - start) / mem::size_of::<T>()
535 // Pass that to the `destroy` method.
537 last_chunk.destroy(diff);
540 self.ptr.set(last_chunk.start());
544 impl<T> Drop for TypedArena<T> {
545 #[unsafe_destructor_blind_to_params]
548 // Determine how much was filled.
549 let mut chunks_borrow = self.chunks.borrow_mut();
550 let mut last_chunk = chunks_borrow.pop().unwrap();
551 // Drop the contents of the last chunk.
552 self.clear_last_chunk(&mut last_chunk);
553 // The last chunk will be dropped. Destroy all other chunks.
554 for chunk in chunks_borrow.iter_mut() {
555 let cap = chunk.storage.cap();
558 // RawVec handles deallocation of `last_chunk` and `self.chunks`.
563 unsafe impl<T: Send> Send for TypedArena<T> {}
568 use self::test::Bencher;
569 use super::{Arena, TypedArena};
574 #[derive(Debug, Eq, PartialEq)]
582 fn test_arena_alloc_nested() {
594 struct Wrap<'a>(TypedArena<EI<'a>>);
597 fn alloc_inner<F: Fn() -> Inner>(&self, f: F) -> &Inner {
598 let r: &EI = self.0.alloc(EI::I(f()));
599 if let &EI::I(ref i) = r {
605 fn alloc_outer<F: Fn() -> Outer<'a>>(&self, f: F) -> &Outer {
606 let r: &EI = self.0.alloc(EI::O(f()));
607 if let &EI::O(ref o) = r {
615 let arena = Wrap(TypedArena::new());
617 let result = arena.alloc_outer(|| {
618 Outer { inner: arena.alloc_inner(|| Inner { value: 10 }) }
621 assert_eq!(result.inner.value, 10);
626 let arena = TypedArena::new();
628 arena.alloc(Point { x: 1, y: 2, z: 3 });
633 pub fn bench_copy(b: &mut Bencher) {
634 let arena = TypedArena::new();
635 b.iter(|| arena.alloc(Point { x: 1, y: 2, z: 3 }))
639 pub fn bench_copy_nonarena(b: &mut Bencher) {
641 let _: Box<_> = Box::new(Point { x: 1, y: 2, z: 3 });
646 pub fn bench_copy_old_arena(b: &mut Bencher) {
647 let arena = Arena::new();
648 b.iter(|| arena.alloc(|| Point { x: 1, y: 2, z: 3 }))
658 pub fn test_noncopy() {
659 let arena = TypedArena::new();
661 arena.alloc(Noncopy {
662 string: "hello world".to_string(),
663 array: vec![1, 2, 3, 4, 5],
669 pub fn test_typed_arena_zero_sized() {
670 let arena = TypedArena::new();
677 pub fn test_arena_zero_sized() {
678 let arena = Arena::new();
679 let mut points = vec![];
684 let point = arena.alloc(|| Point { x: 1, y: 2, z: 3 });
687 for point in &points {
688 assert_eq!(**point, Point { x: 1, y: 2, z: 3 });
693 pub fn test_typed_arena_clear() {
694 let mut arena = TypedArena::new();
698 arena.alloc(Point { x: 1, y: 2, z: 3 });
704 pub fn test_arena_clear() {
705 let mut arena = Arena::new();
709 arena.alloc(|| Point { x: 1, y: 2, z: 3 });
712 string: "hello world".to_string(),
721 pub fn test_arena_alloc_bytes() {
722 let arena = Arena::new();
724 arena.alloc(|| Point { x: 1, y: 2, z: 3 });
725 for byte in arena.alloc_bytes(i % 42).iter_mut() {
732 fn test_arena_destructors() {
733 let arena = Arena::new();
735 // Arena allocate something with drop glue to make sure it
737 arena.alloc(|| Rc::new(i));
738 // Allocate something with funny size and alignment, to keep
739 // things interesting.
740 arena.alloc(|| [0u8, 1u8, 2u8]);
746 fn test_arena_destructors_fail() {
747 let arena = Arena::new();
748 // Put some stuff in the arena.
750 // Arena allocate something with drop glue to make sure it
752 arena.alloc(|| Rc::new(i));
753 // Allocate something with funny size and alignment, to keep
754 // things interesting.
755 arena.alloc(|| [0u8, 1, 2]);
757 // Now, panic while allocating
758 arena.alloc::<Rc<i32>, _>(|| {
765 struct DropCounter<'a> {
766 count: &'a Cell<u32>,
769 impl<'a> Drop for DropCounter<'a> {
771 self.count.set(self.count.get() + 1);
776 fn test_arena_drop_count() {
777 let counter = Cell::new(0);
779 let arena = Arena::new();
781 // Allocate something with drop glue to make sure it doesn't leak.
782 arena.alloc(|| DropCounter { count: &counter });
783 // Allocate something with funny size and alignment, to keep
784 // things interesting.
785 arena.alloc(|| [0u8, 1u8, 2u8]);
789 assert_eq!(counter.get(), 100);
793 fn test_arena_drop_on_clear() {
794 let counter = Cell::new(0);
796 let mut arena = Arena::new();
798 // Allocate something with drop glue to make sure it doesn't leak.
799 arena.alloc(|| DropCounter { count: &counter });
800 // Allocate something with funny size and alignment, to keep
801 // things interesting.
802 arena.alloc(|| [0u8, 1u8, 2u8]);
805 assert_eq!(counter.get(), i * 100 + 100);
810 fn test_typed_arena_drop_count() {
811 let counter = Cell::new(0);
813 let arena: TypedArena<DropCounter> = TypedArena::new();
815 // Allocate something with drop glue to make sure it doesn't leak.
816 arena.alloc(DropCounter { count: &counter });
819 assert_eq!(counter.get(), 100);
823 fn test_typed_arena_drop_on_clear() {
824 let counter = Cell::new(0);
825 let mut arena: TypedArena<DropCounter> = TypedArena::new();
828 // Allocate something with drop glue to make sure it doesn't leak.
829 arena.alloc(DropCounter { count: &counter });
832 assert_eq!(counter.get(), i * 100 + 100);
837 static DROP_COUNTER: Cell<u32> = Cell::new(0)
840 struct SmallDroppable;
842 impl Drop for SmallDroppable {
844 DROP_COUNTER.with(|c| c.set(c.get() + 1));
849 fn test_arena_drop_small_count() {
850 DROP_COUNTER.with(|c| c.set(0));
852 let arena = Arena::new();
855 // Allocate something with drop glue to make sure it doesn't leak.
856 arena.alloc(|| SmallDroppable);
858 // Allocate something with funny size and alignment, to keep
859 // things interesting.
860 arena.alloc(|| [0u8, 1u8, 2u8]);
864 assert_eq!(DROP_COUNTER.with(|c| c.get()), 100);
868 fn test_typed_arena_drop_small_count() {
869 DROP_COUNTER.with(|c| c.set(0));
871 let arena: TypedArena<SmallDroppable> = TypedArena::new();
873 // Allocate something with drop glue to make sure it doesn't leak.
874 arena.alloc(SmallDroppable);
878 assert_eq!(DROP_COUNTER.with(|c| c.get()), 100);
882 pub fn bench_noncopy(b: &mut Bencher) {
883 let arena = TypedArena::new();
885 arena.alloc(Noncopy {
886 string: "hello world".to_string(),
887 array: vec![1, 2, 3, 4, 5],
893 pub fn bench_noncopy_nonarena(b: &mut Bencher) {
895 let _: Box<_> = Box::new(Noncopy {
896 string: "hello world".to_string(),
897 array: vec![1, 2, 3, 4, 5],
903 pub fn bench_noncopy_old_arena(b: &mut Bencher) {
904 let arena = Arena::new();
908 string: "hello world".to_string(),
909 array: vec![1, 2, 3, 4, 5],