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 implements `TypedArena`, a simple arena that can only hold
19 //! objects of a single type.
21 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
22 html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
23 html_root_url = "https://doc.rust-lang.org/nightly/",
24 test(no_crate_inject, attr(deny(warnings))))]
27 #![feature(core_intrinsics)]
28 #![feature(dropck_eyepatch)]
29 #![cfg_attr(test, feature(test))]
34 extern crate rustc_data_structures;
36 use rustc_data_structures::sync::MTLock;
38 use std::cell::{Cell, RefCell};
41 use std::marker::{PhantomData, Send};
46 use alloc::raw_vec::RawVec;
48 /// An arena that can hold objects of only one type.
49 pub struct TypedArena<T> {
50 /// A pointer to the next object to be allocated.
53 /// A pointer to the end of the allocated area. When this pointer is
54 /// reached, a new chunk is allocated.
57 /// A vector of arena chunks.
58 chunks: RefCell<Vec<TypedArenaChunk<T>>>,
60 /// Marker indicating that dropping the arena causes its owned
61 /// instances of `T` to be dropped.
65 struct TypedArenaChunk<T> {
66 /// The raw storage for the arena chunk.
70 impl<T> TypedArenaChunk<T> {
72 unsafe fn new(capacity: usize) -> TypedArenaChunk<T> {
74 storage: RawVec::with_capacity(capacity),
78 /// Destroys this arena chunk.
80 unsafe fn destroy(&mut self, len: usize) {
81 // The branch on needs_drop() is an -O1 performance optimization.
82 // Without the branch, dropping TypedArena<u8> takes linear time.
83 if mem::needs_drop::<T>() {
84 let mut start = self.start();
85 // Destroy all allocated objects.
87 ptr::drop_in_place(start);
88 start = start.offset(1);
93 // Returns a pointer to the first allocated object.
95 fn start(&self) -> *mut T {
99 // Returns a pointer to the end of the allocated space.
101 fn end(&self) -> *mut T {
103 if mem::size_of::<T>() == 0 {
104 // A pointer as large as possible for zero-sized elements.
107 self.start().offset(self.storage.cap() as isize)
113 const PAGE: usize = 4096;
115 impl<T> TypedArena<T> {
116 /// Creates a new `TypedArena`.
118 pub fn new() -> TypedArena<T> {
120 // We set both `ptr` and `end` to 0 so that the first call to
121 // alloc() will trigger a grow().
122 ptr: Cell::new(0 as *mut T),
123 end: Cell::new(0 as *mut T),
124 chunks: RefCell::new(vec![]),
129 /// Allocates an object in the `TypedArena`, returning a reference to it.
131 pub fn alloc(&self, object: T) -> &mut T {
132 if self.ptr == self.end {
137 if mem::size_of::<T>() == 0 {
139 .set(intrinsics::arith_offset(self.ptr.get() as *mut u8, 1)
141 let ptr = mem::align_of::<T>() as *mut T;
142 // Don't drop the object. This `write` is equivalent to `forget`.
143 ptr::write(ptr, object);
146 let ptr = self.ptr.get();
147 // Advance the pointer.
148 self.ptr.set(self.ptr.get().offset(1));
149 // Write into uninitialized memory.
150 ptr::write(ptr, object);
156 /// Allocates a slice of objects that are copied into the `TypedArena`, returning a mutable
157 /// reference to it. Will panic if passed a zero-sized types.
161 /// - Zero-sized types
162 /// - Zero-length slices
164 pub fn alloc_slice(&self, slice: &[T]) -> &mut [T]
168 assert!(mem::size_of::<T>() != 0);
169 assert!(slice.len() != 0);
171 let available_capacity_bytes = self.end.get() as usize - self.ptr.get() as usize;
172 let at_least_bytes = slice.len() * mem::size_of::<T>();
173 if available_capacity_bytes < at_least_bytes {
174 self.grow(slice.len());
178 let start_ptr = self.ptr.get();
179 let arena_slice = slice::from_raw_parts_mut(start_ptr, slice.len());
180 self.ptr.set(start_ptr.offset(arena_slice.len() as isize));
181 arena_slice.copy_from_slice(slice);
189 fn grow(&self, n: usize) {
191 let mut chunks = self.chunks.borrow_mut();
192 let (chunk, mut new_capacity);
193 if let Some(last_chunk) = chunks.last_mut() {
194 let used_bytes = self.ptr.get() as usize - last_chunk.start() as usize;
195 let currently_used_cap = used_bytes / mem::size_of::<T>();
196 if last_chunk.storage.reserve_in_place(currently_used_cap, n) {
197 self.end.set(last_chunk.end());
200 new_capacity = last_chunk.storage.cap();
202 new_capacity = new_capacity.checked_mul(2).unwrap();
203 if new_capacity >= currently_used_cap + n {
209 let elem_size = cmp::max(1, mem::size_of::<T>());
210 new_capacity = cmp::max(n, PAGE / elem_size);
212 chunk = TypedArenaChunk::<T>::new(new_capacity);
213 self.ptr.set(chunk.start());
214 self.end.set(chunk.end());
219 /// Clears the arena. Deallocates all but the longest chunk which may be reused.
220 pub fn clear(&mut self) {
222 // Clear the last chunk, which is partially filled.
223 let mut chunks_borrow = self.chunks.borrow_mut();
224 if let Some(mut last_chunk) = chunks_borrow.pop() {
225 self.clear_last_chunk(&mut last_chunk);
226 // If `T` is ZST, code below has no effect.
227 for mut chunk in chunks_borrow.drain(..) {
228 let cap = chunk.storage.cap();
231 chunks_borrow.push(last_chunk);
236 // Drops the contents of the last chunk. The last chunk is partially empty, unlike all other
238 fn clear_last_chunk(&self, last_chunk: &mut TypedArenaChunk<T>) {
239 // Determine how much was filled.
240 let start = last_chunk.start() as usize;
241 // We obtain the value of the pointer to the first uninitialized element.
242 let end = self.ptr.get() as usize;
243 // We then calculate the number of elements to be dropped in the last chunk,
244 // which is the filled area's length.
245 let diff = if mem::size_of::<T>() == 0 {
246 // `T` is ZST. It can't have a drop flag, so the value here doesn't matter. We get
247 // the number of zero-sized values in the last and only chunk, just out of caution.
248 // Recall that `end` was incremented for each allocated value.
251 (end - start) / mem::size_of::<T>()
253 // Pass that to the `destroy` method.
255 last_chunk.destroy(diff);
258 self.ptr.set(last_chunk.start());
262 unsafe impl<#[may_dangle] T> Drop for TypedArena<T> {
265 // Determine how much was filled.
266 let mut chunks_borrow = self.chunks.borrow_mut();
267 if let Some(mut last_chunk) = chunks_borrow.pop() {
268 // Drop the contents of the last chunk.
269 self.clear_last_chunk(&mut last_chunk);
270 // The last chunk will be dropped. Destroy all other chunks.
271 for chunk in chunks_borrow.iter_mut() {
272 let cap = chunk.storage.cap();
276 // RawVec handles deallocation of `last_chunk` and `self.chunks`.
281 unsafe impl<T: Send> Send for TypedArena<T> {}
283 pub struct DroplessArena {
284 /// A pointer to the next object to be allocated.
287 /// A pointer to the end of the allocated area. When this pointer is
288 /// reached, a new chunk is allocated.
291 /// A vector of arena chunks.
292 chunks: RefCell<Vec<TypedArenaChunk<u8>>>,
295 unsafe impl Send for DroplessArena {}
298 pub fn new() -> DroplessArena {
300 ptr: Cell::new(0 as *mut u8),
301 end: Cell::new(0 as *mut u8),
302 chunks: RefCell::new(vec![]),
306 pub fn in_arena<T: ?Sized>(&self, ptr: *const T) -> bool {
307 let ptr = ptr as *const u8 as *mut u8;
308 for chunk in &*self.chunks.borrow() {
309 if chunk.start() <= ptr && ptr < chunk.end() {
317 fn align(&self, align: usize) {
318 let final_address = ((self.ptr.get() as usize) + align - 1) & !(align - 1);
319 self.ptr.set(final_address as *mut u8);
320 assert!(self.ptr <= self.end);
325 fn grow(&self, needed_bytes: usize) {
327 let mut chunks = self.chunks.borrow_mut();
328 let (chunk, mut new_capacity);
329 if let Some(last_chunk) = chunks.last_mut() {
330 let used_bytes = self.ptr.get() as usize - last_chunk.start() as usize;
333 .reserve_in_place(used_bytes, needed_bytes)
335 self.end.set(last_chunk.end());
338 new_capacity = last_chunk.storage.cap();
340 new_capacity = new_capacity.checked_mul(2).unwrap();
341 if new_capacity >= used_bytes + needed_bytes {
347 new_capacity = cmp::max(needed_bytes, PAGE);
349 chunk = TypedArenaChunk::<u8>::new(new_capacity);
350 self.ptr.set(chunk.start());
351 self.end.set(chunk.end());
357 pub fn alloc_raw(&self, bytes: usize, align: usize) -> &mut [u8] {
363 let future_end = intrinsics::arith_offset(self.ptr.get(), bytes as isize);
364 if (future_end as *mut u8) >= self.end.get() {
368 let ptr = self.ptr.get();
369 // Set the pointer past ourselves
371 intrinsics::arith_offset(self.ptr.get(), bytes as isize) as *mut u8,
373 slice::from_raw_parts_mut(ptr, bytes)
378 pub fn alloc<T>(&self, object: T) -> &mut T {
379 assert!(!mem::needs_drop::<T>());
381 let mem = self.alloc_raw(
383 mem::align_of::<T>()) as *mut _ as *mut T;
386 // Write into uninitialized memory.
387 ptr::write(mem, object);
392 /// Allocates a slice of objects that are copied into the `DroplessArena`, returning a mutable
393 /// reference to it. Will panic if passed a zero-sized type.
397 /// - Zero-sized types
398 /// - Zero-length slices
400 pub fn alloc_slice<T>(&self, slice: &[T]) -> &mut [T]
404 assert!(!mem::needs_drop::<T>());
405 assert!(mem::size_of::<T>() != 0);
406 assert!(slice.len() != 0);
408 let mem = self.alloc_raw(
409 slice.len() * mem::size_of::<T>(),
410 mem::align_of::<T>()) as *mut _ as *mut T;
413 let arena_slice = slice::from_raw_parts_mut(mem, slice.len());
414 arena_slice.copy_from_slice(slice);
420 pub struct SyncTypedArena<T> {
421 lock: MTLock<TypedArena<T>>,
424 impl<T> SyncTypedArena<T> {
426 pub fn new() -> SyncTypedArena<T> {
428 lock: MTLock::new(TypedArena::new())
433 pub fn alloc(&self, object: T) -> &mut T {
434 // Extend the lifetime of the result since it's limited to the lock guard
435 unsafe { &mut *(self.lock.lock().alloc(object) as *mut T) }
439 pub fn alloc_slice(&self, slice: &[T]) -> &mut [T]
443 // Extend the lifetime of the result since it's limited to the lock guard
444 unsafe { &mut *(self.lock.lock().alloc_slice(slice) as *mut [T]) }
448 pub fn clear(&mut self) {
449 self.lock.get_mut().clear();
453 pub struct SyncDroplessArena {
454 lock: MTLock<DroplessArena>,
457 impl SyncDroplessArena {
459 pub fn new() -> SyncDroplessArena {
461 lock: MTLock::new(DroplessArena::new())
466 pub fn in_arena<T: ?Sized>(&self, ptr: *const T) -> bool {
467 self.lock.lock().in_arena(ptr)
471 pub fn alloc_raw(&self, bytes: usize, align: usize) -> &mut [u8] {
472 // Extend the lifetime of the result since it's limited to the lock guard
473 unsafe { &mut *(self.lock.lock().alloc_raw(bytes, align) as *mut [u8]) }
477 pub fn alloc<T>(&self, object: T) -> &mut T {
478 // Extend the lifetime of the result since it's limited to the lock guard
479 unsafe { &mut *(self.lock.lock().alloc(object) as *mut T) }
483 pub fn alloc_slice<T>(&self, slice: &[T]) -> &mut [T]
487 // Extend the lifetime of the result since it's limited to the lock guard
488 unsafe { &mut *(self.lock.lock().alloc_slice(slice) as *mut [T]) }
495 use self::test::Bencher;
496 use super::TypedArena;
500 #[derive(Debug, Eq, PartialEq)]
508 pub fn test_unused() {
509 let arena: TypedArena<Point> = TypedArena::new();
510 assert!(arena.chunks.borrow().is_empty());
514 fn test_arena_alloc_nested() {
526 struct Wrap<'a>(TypedArena<EI<'a>>);
529 fn alloc_inner<F: Fn() -> Inner>(&self, f: F) -> &Inner {
530 let r: &EI = self.0.alloc(EI::I(f()));
531 if let &EI::I(ref i) = r {
537 fn alloc_outer<F: Fn() -> Outer<'a>>(&self, f: F) -> &Outer {
538 let r: &EI = self.0.alloc(EI::O(f()));
539 if let &EI::O(ref o) = r {
547 let arena = Wrap(TypedArena::new());
549 let result = arena.alloc_outer(|| Outer {
550 inner: arena.alloc_inner(|| Inner { value: 10 }),
553 assert_eq!(result.inner.value, 10);
558 let arena = TypedArena::new();
560 arena.alloc(Point { x: 1, y: 2, z: 3 });
565 pub fn bench_copy(b: &mut Bencher) {
566 let arena = TypedArena::new();
567 b.iter(|| arena.alloc(Point { x: 1, y: 2, z: 3 }))
571 pub fn bench_copy_nonarena(b: &mut Bencher) {
573 let _: Box<_> = Box::new(Point { x: 1, y: 2, z: 3 });
584 pub fn test_noncopy() {
585 let arena = TypedArena::new();
587 arena.alloc(Noncopy {
588 string: "hello world".to_string(),
589 array: vec![1, 2, 3, 4, 5],
595 pub fn test_typed_arena_zero_sized() {
596 let arena = TypedArena::new();
603 pub fn test_typed_arena_clear() {
604 let mut arena = TypedArena::new();
608 arena.alloc(Point { x: 1, y: 2, z: 3 });
615 struct DropCounter<'a> {
616 count: &'a Cell<u32>,
619 impl<'a> Drop for DropCounter<'a> {
621 self.count.set(self.count.get() + 1);
626 fn test_typed_arena_drop_count() {
627 let counter = Cell::new(0);
629 let arena: TypedArena<DropCounter> = TypedArena::new();
631 // Allocate something with drop glue to make sure it doesn't leak.
632 arena.alloc(DropCounter { count: &counter });
635 assert_eq!(counter.get(), 100);
639 fn test_typed_arena_drop_on_clear() {
640 let counter = Cell::new(0);
641 let mut arena: TypedArena<DropCounter> = TypedArena::new();
644 // Allocate something with drop glue to make sure it doesn't leak.
645 arena.alloc(DropCounter { count: &counter });
648 assert_eq!(counter.get(), i * 100 + 100);
653 static DROP_COUNTER: Cell<u32> = Cell::new(0)
656 struct SmallDroppable;
658 impl Drop for SmallDroppable {
660 DROP_COUNTER.with(|c| c.set(c.get() + 1));
665 fn test_typed_arena_drop_small_count() {
666 DROP_COUNTER.with(|c| c.set(0));
668 let arena: TypedArena<SmallDroppable> = TypedArena::new();
670 // Allocate something with drop glue to make sure it doesn't leak.
671 arena.alloc(SmallDroppable);
675 assert_eq!(DROP_COUNTER.with(|c| c.get()), 100);
679 pub fn bench_noncopy(b: &mut Bencher) {
680 let arena = TypedArena::new();
682 arena.alloc(Noncopy {
683 string: "hello world".to_string(),
684 array: vec![1, 2, 3, 4, 5],
690 pub fn bench_noncopy_nonarena(b: &mut Bencher) {
692 let _: Box<_> = Box::new(Noncopy {
693 string: "hello world".to_string(),
694 array: vec![1, 2, 3, 4, 5],