rust/src/libarena/lib.rs
2015-06-11 13:56:07 +01:00

671 lines
20 KiB
Rust

// Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.
//! The arena, a fast but limited type of allocator.
//!
//! Arenas are a type of allocator that destroy the objects within, all at
//! once, once the arena itself is destroyed. They do not support deallocation
//! of individual objects while the arena itself is still alive. The benefit
//! of an arena is very fast allocation; just a pointer bump.
//!
//! This crate has two arenas implemented: `TypedArena`, which is a simpler
//! arena but can only hold objects of a single type, and `Arena`, which is a
//! more complex, slower arena which can hold objects of any type.
// Do not remove on snapshot creation. Needed for bootstrap. (Issue #22364)
#![cfg_attr(stage0, feature(custom_attribute))]
#![crate_name = "arena"]
#![unstable(feature = "rustc_private")]
#![staged_api]
#![crate_type = "rlib"]
#![crate_type = "dylib"]
#![doc(html_logo_url = "http://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
html_root_url = "http://doc.rust-lang.org/nightly/")]
#![feature(alloc)]
#![feature(box_syntax)]
#![feature(core)]
#![feature(staged_api)]
#![feature(unboxed_closures)]
#![cfg_attr(test, feature(test))]
extern crate alloc;
use std::cell::{Cell, RefCell};
use std::cmp;
use std::intrinsics;
use std::marker;
use std::mem;
use std::ptr;
use std::rc::Rc;
use std::rt::heap::{allocate, deallocate};
// The way arena uses arrays is really deeply awful. The arrays are
// allocated, and have capacities reserved, but the fill for the array
// will always stay at 0.
#[derive(Clone, PartialEq)]
struct Chunk {
data: Rc<RefCell<Vec<u8>>>,
fill: Cell<usize>,
is_copy: Cell<bool>,
}
impl Chunk {
fn capacity(&self) -> usize {
self.data.borrow().capacity()
}
unsafe fn as_ptr(&self) -> *const u8 {
self.data.borrow().as_ptr()
}
}
/// A slower reflection-based arena that can allocate objects of any type.
///
/// This arena uses `Vec<u8>` as a backing store to allocate objects from. For
/// each allocated object, the arena stores a pointer to the type descriptor
/// followed by the object (potentially with alignment padding after each
/// element). When the arena is destroyed, it iterates through all of its
/// chunks, and uses the tydesc information to trace through the objects,
/// calling the destructors on them. One subtle point that needs to be
/// addressed is how to handle panics while running the user provided
/// initializer function. It is important to not run the destructor on
/// uninitialized objects, but how to detect them is somewhat subtle. Since
/// `alloc()` can be invoked recursively, it is not sufficient to simply exclude
/// the most recent object. To solve this without requiring extra space, we
/// use the low order bit of the tydesc pointer to encode whether the object
/// it describes has been fully initialized.
///
/// As an optimization, objects with destructors are stored in different chunks
/// than objects without destructors. This reduces overhead when initializing
/// plain-old-data (`Copy` types) and means we don't need to waste time running
/// their destructors.
pub struct Arena<'longer_than_self> {
// The head is separated out from the list as a unbenchmarked
// microoptimization, to avoid needing to case on the list to access the
// head.
head: RefCell<Chunk>,
copy_head: RefCell<Chunk>,
chunks: RefCell<Vec<Chunk>>,
_marker: marker::PhantomData<*mut &'longer_than_self()>,
}
impl<'a> Arena<'a> {
/// Allocates a new Arena with 32 bytes preallocated.
pub fn new() -> Arena<'a> {
Arena::new_with_size(32)
}
/// Allocates a new Arena with `initial_size` bytes preallocated.
pub fn new_with_size(initial_size: usize) -> Arena<'a> {
Arena {
head: RefCell::new(chunk(initial_size, false)),
copy_head: RefCell::new(chunk(initial_size, true)),
chunks: RefCell::new(Vec::new()),
_marker: marker::PhantomData,
}
}
}
fn chunk(size: usize, is_copy: bool) -> Chunk {
Chunk {
data: Rc::new(RefCell::new(Vec::with_capacity(size))),
fill: Cell::new(0),
is_copy: Cell::new(is_copy),
}
}
impl<'longer_than_self> Drop for Arena<'longer_than_self> {
fn drop(&mut self) {
unsafe {
destroy_chunk(&*self.head.borrow());
for chunk in self.chunks.borrow().iter() {
if !chunk.is_copy.get() {
destroy_chunk(chunk);
}
}
}
}
}
#[inline]
fn round_up(base: usize, align: usize) -> usize {
(base.checked_add(align - 1)).unwrap() & !(align - 1)
}
// Walk down a chunk, running the destructors for any objects stored
// in it.
unsafe fn destroy_chunk(chunk: &Chunk) {
let mut idx = 0;
let buf = chunk.as_ptr();
let fill = chunk.fill.get();
while idx < fill {
let tydesc_data: *const usize = mem::transmute(buf.offset(idx as isize));
let (tydesc, is_done) = un_bitpack_tydesc_ptr(*tydesc_data);
let (size, align) = ((*tydesc).size, (*tydesc).align);
let after_tydesc = idx + mem::size_of::<*const TyDesc>();
let start = round_up(after_tydesc, align);
//debug!("freeing object: idx = {}, size = {}, align = {}, done = {}",
// start, size, align, is_done);
if is_done {
((*tydesc).drop_glue)(buf.offset(start as isize) as *const i8);
}
// Find where the next tydesc lives
idx = round_up(start + size, mem::align_of::<*const TyDesc>());
}
}
// We encode whether the object a tydesc describes has been
// initialized in the arena in the low bit of the tydesc pointer. This
// is necessary in order to properly do cleanup if a panic occurs
// during an initializer.
#[inline]
fn bitpack_tydesc_ptr(p: *const TyDesc, is_done: bool) -> usize {
p as usize | (is_done as usize)
}
#[inline]
fn un_bitpack_tydesc_ptr(p: usize) -> (*const TyDesc, bool) {
((p & !1) as *const TyDesc, p & 1 == 1)
}
// HACK(eddyb) TyDesc replacement using a trait object vtable.
// This could be replaced in the future with a custom DST layout,
// or `&'static (drop_glue, size, align)` created by a `const fn`.
struct TyDesc {
drop_glue: fn(*const i8),
size: usize,
align: usize
}
trait AllTypes { fn dummy(&self) { } }
impl<T:?Sized> AllTypes for T { }
unsafe fn get_tydesc<T>() -> *const TyDesc {
use std::raw::TraitObject;
let ptr = &*(1 as *const T);
// Can use any trait that is implemented for all types.
let obj = mem::transmute::<&AllTypes, TraitObject>(ptr);
obj.vtable as *const TyDesc
}
impl<'longer_than_self> Arena<'longer_than_self> {
fn chunk_size(&self) -> usize {
self.copy_head.borrow().capacity()
}
// Functions for the POD part of the arena
fn alloc_copy_grow(&self, n_bytes: usize, align: usize) -> *const u8 {
// Allocate a new chunk.
let new_min_chunk_size = cmp::max(n_bytes, self.chunk_size());
self.chunks.borrow_mut().push(self.copy_head.borrow().clone());
*self.copy_head.borrow_mut() =
chunk((new_min_chunk_size + 1).next_power_of_two(), true);
return self.alloc_copy_inner(n_bytes, align);
}
#[inline]
fn alloc_copy_inner(&self, n_bytes: usize, align: usize) -> *const u8 {
let start = round_up(self.copy_head.borrow().fill.get(), align);
let end = start + n_bytes;
if end > self.chunk_size() {
return self.alloc_copy_grow(n_bytes, align);
}
let copy_head = self.copy_head.borrow();
copy_head.fill.set(end);
unsafe {
copy_head.as_ptr().offset(start as isize)
}
}
#[inline]
fn alloc_copy<T, F>(&self, op: F) -> &mut T where F: FnOnce() -> T {
unsafe {
let ptr = self.alloc_copy_inner(mem::size_of::<T>(),
mem::min_align_of::<T>());
let ptr = ptr as *mut T;
ptr::write(&mut (*ptr), op());
return &mut *ptr;
}
}
// Functions for the non-POD part of the arena
fn alloc_noncopy_grow(&self, n_bytes: usize,
align: usize) -> (*const u8, *const u8) {
// Allocate a new chunk.
let new_min_chunk_size = cmp::max(n_bytes, self.chunk_size());
self.chunks.borrow_mut().push(self.head.borrow().clone());
*self.head.borrow_mut() =
chunk((new_min_chunk_size + 1).next_power_of_two(), false);
return self.alloc_noncopy_inner(n_bytes, align);
}
#[inline]
fn alloc_noncopy_inner(&self, n_bytes: usize,
align: usize) -> (*const u8, *const u8) {
// Be careful to not maintain any `head` borrows active, because
// `alloc_noncopy_grow` borrows it mutably.
let (start, end, tydesc_start, head_capacity) = {
let head = self.head.borrow();
let fill = head.fill.get();
let tydesc_start = fill;
let after_tydesc = fill + mem::size_of::<*const TyDesc>();
let start = round_up(after_tydesc, align);
let end = start + n_bytes;
(start, end, tydesc_start, head.capacity())
};
if end > head_capacity {
return self.alloc_noncopy_grow(n_bytes, align);
}
let head = self.head.borrow();
head.fill.set(round_up(end, mem::align_of::<*const TyDesc>()));
unsafe {
let buf = head.as_ptr();
return (buf.offset(tydesc_start as isize), buf.offset(start as isize));
}
}
#[inline]
fn alloc_noncopy<T, F>(&self, op: F) -> &mut T where F: FnOnce() -> T {
unsafe {
let tydesc = get_tydesc::<T>();
let (ty_ptr, ptr) =
self.alloc_noncopy_inner(mem::size_of::<T>(),
mem::min_align_of::<T>());
let ty_ptr = ty_ptr as *mut usize;
let ptr = ptr as *mut T;
// Write in our tydesc along with a bit indicating that it
// has *not* been initialized yet.
*ty_ptr = mem::transmute(tydesc);
// Actually initialize it
ptr::write(&mut(*ptr), op());
// Now that we are done, update the tydesc to indicate that
// the object is there.
*ty_ptr = bitpack_tydesc_ptr(tydesc, true);
return &mut *ptr;
}
}
/// Allocates a new item in the arena, using `op` to initialize the value,
/// and returns a reference to it.
#[inline]
pub fn alloc<T:'longer_than_self, F>(&self, op: F) -> &mut T where F: FnOnce() -> T {
unsafe {
if intrinsics::needs_drop::<T>() {
self.alloc_noncopy(op)
} else {
self.alloc_copy(op)
}
}
}
}
#[test]
fn test_arena_destructors() {
let arena = Arena::new();
for i in 0..10 {
// Arena allocate something with drop glue to make sure it
// doesn't leak.
arena.alloc(|| Rc::new(i));
// Allocate something with funny size and alignment, to keep
// things interesting.
arena.alloc(|| [0u8, 1u8, 2u8]);
}
}
#[test]
#[should_panic]
fn test_arena_destructors_fail() {
let arena = Arena::new();
// Put some stuff in the arena.
for i in 0..10 {
// Arena allocate something with drop glue to make sure it
// doesn't leak.
arena.alloc(|| { Rc::new(i) });
// Allocate something with funny size and alignment, to keep
// things interesting.
arena.alloc(|| { [0u8, 1, 2] });
}
// Now, panic while allocating
arena.alloc::<Rc<i32>, _>(|| {
panic!();
});
}
/// A faster arena that can hold objects of only one type.
pub struct TypedArena<T> {
/// A pointer to the next object to be allocated.
ptr: Cell<*const T>,
/// A pointer to the end of the allocated area. When this pointer is
/// reached, a new chunk is allocated.
end: Cell<*const T>,
/// A pointer to the first arena segment.
first: RefCell<*mut TypedArenaChunk<T>>,
/// Marker indicating that dropping the arena causes its owned
/// instances of `T` to be dropped.
_own: marker::PhantomData<T>,
}
struct TypedArenaChunk<T> {
marker: marker::PhantomData<T>,
/// Pointer to the next arena segment.
next: *mut TypedArenaChunk<T>,
/// The number of elements that this chunk can hold.
capacity: usize,
// Objects follow here, suitably aligned.
}
fn calculate_size<T>(capacity: usize) -> usize {
let mut size = mem::size_of::<TypedArenaChunk<T>>();
size = round_up(size, mem::min_align_of::<T>());
let elem_size = mem::size_of::<T>();
let elems_size = elem_size.checked_mul(capacity).unwrap();
size = size.checked_add(elems_size).unwrap();
size
}
impl<T> TypedArenaChunk<T> {
#[inline]
unsafe fn new(next: *mut TypedArenaChunk<T>, capacity: usize)
-> *mut TypedArenaChunk<T> {
let size = calculate_size::<T>(capacity);
let chunk = allocate(size, mem::min_align_of::<TypedArenaChunk<T>>())
as *mut TypedArenaChunk<T>;
if chunk.is_null() { alloc::oom() }
(*chunk).next = next;
(*chunk).capacity = capacity;
chunk
}
/// Destroys this arena chunk. If the type descriptor is supplied, the
/// drop glue is called; otherwise, drop glue is not called.
#[inline]
unsafe fn destroy(&mut self, len: usize) {
// Destroy all the allocated objects.
if intrinsics::needs_drop::<T>() {
let mut start = self.start();
for _ in 0..len {
ptr::read(start as *const T); // run the destructor on the pointer
start = start.offset(mem::size_of::<T>() as isize)
}
}
// Destroy the next chunk.
let next = self.next;
let size = calculate_size::<T>(self.capacity);
let self_ptr: *mut TypedArenaChunk<T> = self;
deallocate(self_ptr as *mut u8, size,
mem::min_align_of::<TypedArenaChunk<T>>());
if !next.is_null() {
let capacity = (*next).capacity;
(*next).destroy(capacity);
}
}
// Returns a pointer to the first allocated object.
#[inline]
fn start(&self) -> *const u8 {
let this: *const TypedArenaChunk<T> = self;
unsafe {
mem::transmute(round_up(this.offset(1) as usize,
mem::min_align_of::<T>()))
}
}
// Returns a pointer to the end of the allocated space.
#[inline]
fn end(&self) -> *const u8 {
unsafe {
let size = mem::size_of::<T>().checked_mul(self.capacity).unwrap();
self.start().offset(size as isize)
}
}
}
impl<T> TypedArena<T> {
/// Creates a new `TypedArena` with preallocated space for eight objects.
#[inline]
pub fn new() -> TypedArena<T> {
TypedArena::with_capacity(8)
}
/// Creates a new `TypedArena` with preallocated space for the given number of
/// objects.
#[inline]
pub fn with_capacity(capacity: usize) -> TypedArena<T> {
unsafe {
let chunk = TypedArenaChunk::<T>::new(ptr::null_mut(), capacity);
TypedArena {
ptr: Cell::new((*chunk).start() as *const T),
end: Cell::new((*chunk).end() as *const T),
first: RefCell::new(chunk),
_own: marker::PhantomData,
}
}
}
/// Allocates an object in the `TypedArena`, returning a reference to it.
#[inline]
pub fn alloc(&self, object: T) -> &mut T {
if self.ptr == self.end {
self.grow()
}
let ptr: &mut T = unsafe {
let ptr: &mut T = mem::transmute(self.ptr.clone());
ptr::write(ptr, object);
self.ptr.set(self.ptr.get().offset(1));
ptr
};
ptr
}
/// Grows the arena.
#[inline(never)]
fn grow(&self) {
unsafe {
let chunk = *self.first.borrow_mut();
let new_capacity = (*chunk).capacity.checked_mul(2).unwrap();
let chunk = TypedArenaChunk::<T>::new(chunk, new_capacity);
self.ptr.set((*chunk).start() as *const T);
self.end.set((*chunk).end() as *const T);
*self.first.borrow_mut() = chunk
}
}
}
impl<T> Drop for TypedArena<T> {
fn drop(&mut self) {
unsafe {
// Determine how much was filled.
let start = self.first.borrow().as_ref().unwrap().start() as usize;
let end = self.ptr.get() as usize;
let diff = (end - start) / mem::size_of::<T>();
// Pass that to the `destroy` method.
(**self.first.borrow_mut()).destroy(diff)
}
}
}
#[cfg(test)]
mod tests {
extern crate test;
use self::test::Bencher;
use super::{Arena, TypedArena};
#[allow(dead_code)]
struct Point {
x: i32,
y: i32,
z: i32,
}
#[test]
fn test_arena_alloc_nested() {
struct Inner { value: u8 }
struct Outer<'a> { inner: &'a Inner }
enum EI<'e> { I(Inner), O(Outer<'e>) }
struct Wrap<'a>(TypedArena<EI<'a>>);
impl<'a> Wrap<'a> {
fn alloc_inner<F:Fn() -> Inner>(&self, f: F) -> &Inner {
let r: &EI = self.0.alloc(EI::I(f()));
if let &EI::I(ref i) = r {
i
} else {
panic!("mismatch");
}
}
fn alloc_outer<F:Fn() -> Outer<'a>>(&self, f: F) -> &Outer {
let r: &EI = self.0.alloc(EI::O(f()));
if let &EI::O(ref o) = r {
o
} else {
panic!("mismatch");
}
}
}
let arena = Wrap(TypedArena::new());
let result = arena.alloc_outer(|| Outer {
inner: arena.alloc_inner(|| Inner { value: 10 }) });
assert_eq!(result.inner.value, 10);
}
#[test]
pub fn test_copy() {
let arena = TypedArena::new();
for _ in 0..100000 {
arena.alloc(Point {
x: 1,
y: 2,
z: 3,
});
}
}
#[bench]
pub fn bench_copy(b: &mut Bencher) {
let arena = TypedArena::new();
b.iter(|| {
arena.alloc(Point {
x: 1,
y: 2,
z: 3,
})
})
}
#[bench]
pub fn bench_copy_nonarena(b: &mut Bencher) {
b.iter(|| {
let _: Box<_> = box Point {
x: 1,
y: 2,
z: 3,
};
})
}
#[bench]
pub fn bench_copy_old_arena(b: &mut Bencher) {
let arena = Arena::new();
b.iter(|| {
arena.alloc(|| {
Point {
x: 1,
y: 2,
z: 3,
}
})
})
}
#[allow(dead_code)]
struct Noncopy {
string: String,
array: Vec<i32>,
}
#[test]
pub fn test_noncopy() {
let arena = TypedArena::new();
for _ in 0..100000 {
arena.alloc(Noncopy {
string: "hello world".to_string(),
array: vec!( 1, 2, 3, 4, 5 ),
});
}
}
#[bench]
pub fn bench_noncopy(b: &mut Bencher) {
let arena = TypedArena::new();
b.iter(|| {
arena.alloc(Noncopy {
string: "hello world".to_string(),
array: vec!( 1, 2, 3, 4, 5 ),
})
})
}
#[bench]
pub fn bench_noncopy_nonarena(b: &mut Bencher) {
b.iter(|| {
let _: Box<_> = box Noncopy {
string: "hello world".to_string(),
array: vec!( 1, 2, 3, 4, 5 ),
};
})
}
#[bench]
pub fn bench_noncopy_old_arena(b: &mut Bencher) {
let arena = Arena::new();
b.iter(|| {
arena.alloc(|| Noncopy {
string: "hello world".to_string(),
array: vec!( 1, 2, 3, 4, 5 ),
})
})
}
}