src/libarena/lib.rs

   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.
   4 //
   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.
  10
  11 //! The arena, a fast but limited type of allocator.
  12 //!
  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.
  17 //!
  18 //! This crate implements `TypedArena`, a simple arena that can only hold
  19 //! objects of a single type.
  20
  21 #![crate_name = "arena"]
  22 #![crate_type = "rlib"]
  23 #![crate_type = "dylib"]
  24 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
  25        html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
  26        html_root_url = "https://doc.rust-lang.org/nightly/",
  27        test(no_crate_inject, attr(deny(warnings))))]
  28 #![deny(warnings)]
  29
  30 #![feature(alloc)]
  31 #![feature(core_intrinsics)]
  32 #![feature(dropck_eyepatch)]
  33 #![feature(generic_param_attrs)]
  34 #![feature(needs_drop)]
  35 #![cfg_attr(test, feature(test))]
  36
  37 #![allow(deprecated)]
  38
  39 extern crate alloc;
  40
  41 use std::cell::{Cell, RefCell};
  42 use std::cmp;
  43 use std::intrinsics;
  44 use std::marker::{PhantomData, Send};
  45 use std::mem;
  46 use std::ptr;
  47 use std::slice;
  48
  49 use alloc::raw_vec::RawVec;
  50
  51 /// An arena that can hold objects of only one type.
  52 pub struct TypedArena<T> {
  53     /// A pointer to the next object to be allocated.
  54     ptr: Cell<*mut T>,
  55
  56     /// A pointer to the end of the allocated area. When this pointer is
  57     /// reached, a new chunk is allocated.
  58     end: Cell<*mut T>,
  59
  60     /// A vector of arena chunks.
  61     chunks: RefCell<Vec<TypedArenaChunk<T>>>,
  62
  63     /// Marker indicating that dropping the arena causes its owned
  64     /// instances of `T` to be dropped.
  65     _own: PhantomData<T>,
  66 }
  67
  68 struct TypedArenaChunk<T> {
  69     /// The raw storage for the arena chunk.
  70     storage: RawVec<T>,
  71 }
  72
  73 impl<T> TypedArenaChunk<T> {
  74     #[inline]
  75     unsafe fn new(capacity: usize) -> TypedArenaChunk<T> {
  76         TypedArenaChunk { storage: RawVec::with_capacity(capacity) }
  77     }
  78
  79     /// Destroys this arena chunk.
  80     #[inline]
  81     unsafe fn destroy(&mut self, len: usize) {
  82         // The branch on needs_drop() is an -O1 performance optimization.
  83         // Without the branch, dropping TypedArena<u8> takes linear time.
  84         if mem::needs_drop::<T>() {
  85             let mut start = self.start();
  86             // Destroy all allocated objects.
  87             for _ in 0..len {
  88                 ptr::drop_in_place(start);
  89                 start = start.offset(1);
  90             }
  91         }
  92     }
  93
  94     // Returns a pointer to the first allocated object.
  95     #[inline]
  96     fn start(&self) -> *mut T {
  97         self.storage.ptr()
  98     }
  99
 100     // Returns a pointer to the end of the allocated space.
 101     #[inline]
 102     fn end(&self) -> *mut T {
 103         unsafe {
 104             if mem::size_of::<T>() == 0 {
 105                 // A pointer as large as possible for zero-sized elements.
 106                 !0 as *mut T
 107             } else {
 108                 self.start().offset(self.storage.cap() as isize)
 109             }
 110         }
 111     }
 112 }
 113
 114 const PAGE: usize = 4096;
 115
 116 impl<T> TypedArena<T> {
 117     /// Creates a new `TypedArena`.
 118     #[inline]
 119     pub fn new() -> TypedArena<T> {
 120         TypedArena {
 121             // We set both `ptr` and `end` to 0 so that the first call to
 122             // alloc() will trigger a grow().
 123             ptr: Cell::new(0 as *mut T),
 124             end: Cell::new(0 as *mut T),
 125             chunks: RefCell::new(vec![]),
 126             _own: PhantomData,
 127         }
 128     }
 129
 130     /// Allocates an object in the `TypedArena`, returning a reference to it.
 131     #[inline]
 132     pub fn alloc(&self, object: T) -> &mut T {
 133         if self.ptr == self.end {
 134             self.grow(1)
 135         }
 136
 137         unsafe {
 138             if mem::size_of::<T>() == 0 {
 139                 self.ptr.set(intrinsics::arith_offset(self.ptr.get() as *mut u8, 1) as *mut T);
 140                 let ptr = mem::align_of::<T>() as *mut T;
 141                 // Don't drop the object. This `write` is equivalent to `forget`.
 142                 ptr::write(ptr, object);
 143                 &mut *ptr
 144             } else {
 145                 let ptr = self.ptr.get();
 146                 // Advance the pointer.
 147                 self.ptr.set(self.ptr.get().offset(1));
 148                 // Write into uninitialized memory.
 149                 ptr::write(ptr, object);
 150                 &mut *ptr
 151             }
 152         }
 153     }
 154
 155     /// Allocates a slice of objects that are copy into the `TypedArena`, returning a mutable
 156     /// reference to it. Will panic if passed a zero-sized types.
 157     ///
 158     /// Panics:
 159     ///  - Zero-sized types
 160     ///  - Zero-length slices
 161     #[inline]
 162     pub fn alloc_slice(&self, slice: &[T]) -> &mut [T]
 163         where T: Copy {
 164         assert!(mem::size_of::<T>() != 0);
 165         assert!(slice.len() != 0);
 166
 167         let available_capacity_bytes = self.end.get() as usize - self.ptr.get() as usize;
 168         let at_least_bytes = slice.len() * mem::size_of::<T>();
 169         if available_capacity_bytes < at_least_bytes {
 170             self.grow(slice.len());
 171         }
 172
 173         unsafe {
 174             let start_ptr = self.ptr.get();
 175             let arena_slice = slice::from_raw_parts_mut(start_ptr, slice.len());
 176             self.ptr.set(start_ptr.offset(arena_slice.len() as isize));
 177             arena_slice.copy_from_slice(slice);
 178             arena_slice
 179         }
 180     }
 181
 182     /// Grows the arena.
 183     #[inline(never)]
 184     #[cold]
 185     fn grow(&self, n: usize) {
 186         unsafe {
 187             let mut chunks = self.chunks.borrow_mut();
 188             let (chunk, mut new_capacity);
 189             if let Some(last_chunk) = chunks.last_mut() {
 190                 let used_bytes = self.ptr.get() as usize - last_chunk.start() as usize;
 191                 let currently_used_cap = used_bytes / mem::size_of::<T>();
 192                 if last_chunk.storage.reserve_in_place(currently_used_cap, n) {
 193                     self.end.set(last_chunk.end());
 194                     return;
 195                 } else {
 196                     new_capacity = last_chunk.storage.cap();
 197                     loop {
 198                         new_capacity = new_capacity.checked_mul(2).unwrap();
 199                         if new_capacity >= currently_used_cap + n {
 200                             break;
 201                         }
 202                     }
 203                 }
 204             } else {
 205                 let elem_size = cmp::max(1, mem::size_of::<T>());
 206                 new_capacity = cmp::max(n, PAGE / elem_size);
 207             }
 208             chunk = TypedArenaChunk::<T>::new(new_capacity);
 209             self.ptr.set(chunk.start());
 210             self.end.set(chunk.end());
 211             chunks.push(chunk);
 212         }
 213     }
 214
 215     /// Clears the arena. Deallocates all but the longest chunk which may be reused.
 216     pub fn clear(&mut self) {
 217         unsafe {
 218             // Clear the last chunk, which is partially filled.
 219             let mut chunks_borrow = self.chunks.borrow_mut();
 220             if let Some(mut last_chunk) = chunks_borrow.pop() {
 221                 self.clear_last_chunk(&mut last_chunk);
 222                 // If `T` is ZST, code below has no effect.
 223                 for mut chunk in chunks_borrow.drain(..) {
 224                     let cap = chunk.storage.cap();
 225                     chunk.destroy(cap);
 226                 }
 227                 chunks_borrow.push(last_chunk);
 228             }
 229         }
 230     }
 231
 232     // Drops the contents of the last chunk. The last chunk is partially empty, unlike all other
 233     // chunks.
 234     fn clear_last_chunk(&self, last_chunk: &mut TypedArenaChunk<T>) {
 235         // Determine how much was filled.
 236         let start = last_chunk.start() as usize;
 237         // We obtain the value of the pointer to the first uninitialized element.
 238         let end = self.ptr.get() as usize;
 239         // We then calculate the number of elements to be dropped in the last chunk,
 240         // which is the filled area's length.
 241         let diff = if mem::size_of::<T>() == 0 {
 242             // `T` is ZST. It can't have a drop flag, so the value here doesn't matter. We get
 243             // the number of zero-sized values in the last and only chunk, just out of caution.
 244             // Recall that `end` was incremented for each allocated value.
 245             end - start
 246         } else {
 247             (end - start) / mem::size_of::<T>()
 248         };
 249         // Pass that to the `destroy` method.
 250         unsafe {
 251             last_chunk.destroy(diff);
 252         }
 253         // Reset the chunk.
 254         self.ptr.set(last_chunk.start());
 255     }
 256 }
 257
 258 unsafe impl<#[may_dangle] T> Drop for TypedArena<T> {
 259     fn drop(&mut self) {
 260         unsafe {
 261             // Determine how much was filled.
 262             let mut chunks_borrow = self.chunks.borrow_mut();
 263             if let Some(mut last_chunk) = chunks_borrow.pop() {
 264                 // Drop the contents of the last chunk.
 265                 self.clear_last_chunk(&mut last_chunk);
 266                 // The last chunk will be dropped. Destroy all other chunks.
 267                 for chunk in chunks_borrow.iter_mut() {
 268                     let cap = chunk.storage.cap();
 269                     chunk.destroy(cap);
 270                 }
 271             }
 272             // RawVec handles deallocation of `last_chunk` and `self.chunks`.
 273         }
 274     }
 275 }
 276
 277 unsafe impl<T: Send> Send for TypedArena<T> {}
 278
 279 pub struct DroplessArena {
 280     /// A pointer to the next object to be allocated.
 281     ptr: Cell<*mut u8>,
 282
 283     /// A pointer to the end of the allocated area. When this pointer is
 284     /// reached, a new chunk is allocated.
 285     end: Cell<*mut u8>,
 286
 287     /// A vector of arena chunks.
 288     chunks: RefCell<Vec<TypedArenaChunk<u8>>>,
 289 }
 290
 291 impl DroplessArena {
 292     pub fn new() -> DroplessArena {
 293         DroplessArena {
 294             ptr: Cell::new(0 as *mut u8),
 295             end: Cell::new(0 as *mut u8),
 296             chunks: RefCell::new(vec![]),
 297         }
 298     }
 299
 300     pub fn in_arena<T: ?Sized>(&self, ptr: *const T) -> bool {
 301         let ptr = ptr as *const u8 as *mut u8;
 302         for chunk in &*self.chunks.borrow() {
 303             if chunk.start() <= ptr && ptr < chunk.end() {
 304                 return true;
 305             }
 306         }
 307
 308         false
 309     }
 310
 311     fn align_for<T>(&self) {
 312         let align = mem::align_of::<T>();
 313         let final_address = ((self.ptr.get() as usize) + align - 1) & !(align - 1);
 314         self.ptr.set(final_address as *mut u8);
 315         assert!(self.ptr <= self.end);
 316     }
 317
 318     #[inline(never)]
 319     #[cold]
 320     fn grow<T>(&self, n: usize) {
 321         let needed_bytes = n * mem::size_of::<T>();
 322         unsafe {
 323             let mut chunks = self.chunks.borrow_mut();
 324             let (chunk, mut new_capacity);
 325             if let Some(last_chunk) = chunks.last_mut() {
 326                 let used_bytes = self.ptr.get() as usize - last_chunk.start() as usize;
 327                 if last_chunk.storage.reserve_in_place(used_bytes, needed_bytes) {
 328                     self.end.set(last_chunk.end());
 329                     return;
 330                 } else {
 331                     new_capacity = last_chunk.storage.cap();
 332                     loop {
 333                         new_capacity = new_capacity.checked_mul(2).unwrap();
 334                         if new_capacity >= used_bytes + needed_bytes {
 335                             break;
 336                         }
 337                     }
 338                 }
 339             } else {
 340                 new_capacity = cmp::max(needed_bytes, PAGE);
 341             }
 342             chunk = TypedArenaChunk::<u8>::new(new_capacity);
 343             self.ptr.set(chunk.start());
 344             self.end.set(chunk.end());
 345             chunks.push(chunk);
 346         }
 347     }
 348
 349     #[inline]
 350     pub fn alloc<T>(&self, object: T) -> &mut T {
 351         unsafe {
 352             assert!(!mem::needs_drop::<T>());
 353             assert!(mem::size_of::<T>() != 0);
 354
 355             self.align_for::<T>();
 356             let future_end = intrinsics::arith_offset(self.ptr.get(), mem::size_of::<T>() as isize);
 357             if (future_end as *mut u8) >= self.end.get() {
 358                 self.grow::<T>(1)
 359             }
 360
 361             let ptr = self.ptr.get();
 362             // Set the pointer past ourselves
 363             self.ptr.set(intrinsics::arith_offset(
 364                     self.ptr.get(), mem::size_of::<T>() as isize
 365             ) as *mut u8);
 366             // Write into uninitialized memory.
 367             ptr::write(ptr as *mut T, object);
 368             &mut *(ptr as *mut T)
 369         }
 370     }
 371
 372     /// Allocates a slice of objects that are copied into the `DroplessArena`, returning a mutable
 373     /// reference to it. Will panic if passed a zero-sized type.
 374     ///
 375     /// Panics:
 376     ///  - Zero-sized types
 377     ///  - Zero-length slices
 378     #[inline]
 379     pub fn alloc_slice<T>(&self, slice: &[T]) -> &mut [T]
 380         where T: Copy {
 381         assert!(!mem::needs_drop::<T>());
 382         assert!(mem::size_of::<T>() != 0);
 383         assert!(slice.len() != 0);
 384         self.align_for::<T>();
 385
 386         let future_end = unsafe {
 387             intrinsics::arith_offset(self.ptr.get(), (slice.len() * mem::size_of::<T>()) as isize)
 388         };
 389         if (future_end as *mut u8) >= self.end.get() {
 390             self.grow::<T>(slice.len());
 391         }
 392
 393         unsafe {
 394             let arena_slice = slice::from_raw_parts_mut(self.ptr.get() as *mut T, slice.len());
 395             self.ptr.set(intrinsics::arith_offset(
 396                     self.ptr.get(), (slice.len() * mem::size_of::<T>()) as isize
 397             ) as *mut u8);
 398             arena_slice.copy_from_slice(slice);
 399             arena_slice
 400         }
 401     }
 402 }
 403
 404 #[cfg(test)]
 405 mod tests {
 406     extern crate test;
 407     use self::test::Bencher;
 408     use super::TypedArena;
 409     use std::cell::Cell;
 410
 411     #[allow(dead_code)]
 412     #[derive(Debug, Eq, PartialEq)]
 413     struct Point {
 414         x: i32,
 415         y: i32,
 416         z: i32,
 417     }
 418
 419     #[test]
 420     pub fn test_unused() {
 421         let arena: TypedArena<Point> = TypedArena::new();
 422         assert!(arena.chunks.borrow().is_empty());
 423     }
 424
 425     #[test]
 426     fn test_arena_alloc_nested() {
 427         struct Inner {
 428             value: u8,
 429         }
 430         struct Outer<'a> {
 431             inner: &'a Inner,
 432         }
 433         enum EI<'e> {
 434             I(Inner),
 435             O(Outer<'e>),
 436         }
 437
 438         struct Wrap<'a>(TypedArena<EI<'a>>);
 439
 440         impl<'a> Wrap<'a> {
 441             fn alloc_inner<F: Fn() -> Inner>(&self, f: F) -> &Inner {
 442                 let r: &EI = self.0.alloc(EI::I(f()));
 443                 if let &EI::I(ref i) = r {
 444                     i
 445                 } else {
 446                     panic!("mismatch");
 447                 }
 448             }
 449             fn alloc_outer<F: Fn() -> Outer<'a>>(&self, f: F) -> &Outer {
 450                 let r: &EI = self.0.alloc(EI::O(f()));
 451                 if let &EI::O(ref o) = r {
 452                     o
 453                 } else {
 454                     panic!("mismatch");
 455                 }
 456             }
 457         }
 458
 459         let arena = Wrap(TypedArena::new());
 460
 461         let result =
 462             arena.alloc_outer(|| Outer { inner: arena.alloc_inner(|| Inner { value: 10 }) });
 463
 464         assert_eq!(result.inner.value, 10);
 465     }
 466
 467     #[test]
 468     pub fn test_copy() {
 469         let arena = TypedArena::new();
 470         for _ in 0..100000 {
 471             arena.alloc(Point { x: 1, y: 2, z: 3 });
 472         }
 473     }
 474
 475     #[bench]
 476     pub fn bench_copy(b: &mut Bencher) {
 477         let arena = TypedArena::new();
 478         b.iter(|| arena.alloc(Point { x: 1, y: 2, z: 3 }))
 479     }
 480
 481     #[bench]
 482     pub fn bench_copy_nonarena(b: &mut Bencher) {
 483         b.iter(|| {
 484             let _: Box<_> = Box::new(Point { x: 1, y: 2, z: 3 });
 485         })
 486     }
 487
 488     #[allow(dead_code)]
 489     struct Noncopy {
 490         string: String,
 491         array: Vec<i32>,
 492     }
 493
 494     #[test]
 495     pub fn test_noncopy() {
 496         let arena = TypedArena::new();
 497         for _ in 0..100000 {
 498             arena.alloc(Noncopy {
 499                 string: "hello world".to_string(),
 500                 array: vec![1, 2, 3, 4, 5],
 501             });
 502         }
 503     }
 504
 505     #[test]
 506     pub fn test_typed_arena_zero_sized() {
 507         let arena = TypedArena::new();
 508         for _ in 0..100000 {
 509             arena.alloc(());
 510         }
 511     }
 512
 513     #[test]
 514     pub fn test_typed_arena_clear() {
 515         let mut arena = TypedArena::new();
 516         for _ in 0..10 {
 517             arena.clear();
 518             for _ in 0..10000 {
 519                 arena.alloc(Point { x: 1, y: 2, z: 3 });
 520             }
 521         }
 522     }
 523
 524     // Drop tests
 525
 526     struct DropCounter<'a> {
 527         count: &'a Cell<u32>,
 528     }
 529
 530     impl<'a> Drop for DropCounter<'a> {
 531         fn drop(&mut self) {
 532             self.count.set(self.count.get() + 1);
 533         }
 534     }
 535
 536     #[test]
 537     fn test_typed_arena_drop_count() {
 538         let counter = Cell::new(0);
 539         {
 540             let arena: TypedArena<DropCounter> = TypedArena::new();
 541             for _ in 0..100 {
 542                 // Allocate something with drop glue to make sure it doesn't leak.
 543                 arena.alloc(DropCounter { count: &counter });
 544             }
 545         };
 546         assert_eq!(counter.get(), 100);
 547     }
 548
 549     #[test]
 550     fn test_typed_arena_drop_on_clear() {
 551         let counter = Cell::new(0);
 552         let mut arena: TypedArena<DropCounter> = TypedArena::new();
 553         for i in 0..10 {
 554             for _ in 0..100 {
 555                 // Allocate something with drop glue to make sure it doesn't leak.
 556                 arena.alloc(DropCounter { count: &counter });
 557             }
 558             arena.clear();
 559             assert_eq!(counter.get(), i * 100 + 100);
 560         }
 561     }
 562
 563     thread_local! {
 564         static DROP_COUNTER: Cell<u32> = Cell::new(0)
 565     }
 566
 567     struct SmallDroppable;
 568
 569     impl Drop for SmallDroppable {
 570         fn drop(&mut self) {
 571             DROP_COUNTER.with(|c| c.set(c.get() + 1));
 572         }
 573     }
 574
 575     #[test]
 576     fn test_typed_arena_drop_small_count() {
 577         DROP_COUNTER.with(|c| c.set(0));
 578         {
 579             let arena: TypedArena<SmallDroppable> = TypedArena::new();
 580             for _ in 0..100 {
 581                 // Allocate something with drop glue to make sure it doesn't leak.
 582                 arena.alloc(SmallDroppable);
 583             }
 584             // dropping
 585         };
 586         assert_eq!(DROP_COUNTER.with(|c| c.get()), 100);
 587     }
 588
 589     #[bench]
 590     pub fn bench_noncopy(b: &mut Bencher) {
 591         let arena = TypedArena::new();
 592         b.iter(|| {
 593             arena.alloc(Noncopy {
 594                 string: "hello world".to_string(),
 595                 array: vec![1, 2, 3, 4, 5],
 596             })
 597         })
 598     }
 599
 600     #[bench]
 601     pub fn bench_noncopy_nonarena(b: &mut Bencher) {
 602         b.iter(|| {
 603             let _: Box<_> = Box::new(Noncopy {
 604                 string: "hello world".to_string(),
 605                 array: vec![1, 2, 3, 4, 5],
 606             });
 607         })
 608     }
 609 }