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//! 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 implements several kinds of arena.
#![doc(
html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/",
test(no_crate_inject, attr(deny(warnings)))
)]
#![feature(core_intrinsics)]
#![feature(dropck_eyepatch)]
#![feature(new_uninit)]
#![feature(maybe_uninit_slice)]
#![feature(decl_macro)]
#![feature(pointer_byte_offsets)]
#![feature(rustc_attrs)]
#![cfg_attr(test, feature(test))]
#![feature(strict_provenance)]
#![deny(unsafe_op_in_unsafe_fn)]
#![deny(rustc::untranslatable_diagnostic)]
#![deny(rustc::diagnostic_outside_of_impl)]
#![allow(internal_features)]
#![allow(clippy::mut_from_ref)] // Arena allocators are one of the places where this pattern is fine.
use smallvec::SmallVec;
use std::alloc::Layout;
use std::cell::{Cell, RefCell};
use std::marker::PhantomData;
use std::mem::{self, MaybeUninit};
use std::ptr::{self, NonNull};
use std::slice;
use std::{cmp, intrinsics};
/// This calls the passed function while ensuring it won't be inlined into the caller.
#[inline(never)]
#[cold]
fn outline<F: FnOnce() -> R, R>(f: F) -> R {
f()
}
struct ArenaChunk<T = u8> {
/// The raw storage for the arena chunk.
storage: NonNull<[MaybeUninit<T>]>,
/// The number of valid entries in the chunk.
entries: usize,
}
unsafe impl<#[may_dangle] T> Drop for ArenaChunk<T> {
fn drop(&mut self) {
unsafe { drop(Box::from_raw(self.storage.as_mut())) }
}
}
impl<T> ArenaChunk<T> {
#[inline]
unsafe fn new(capacity: usize) -> ArenaChunk<T> {
ArenaChunk {
storage: NonNull::from(Box::leak(Box::new_uninit_slice(capacity))),
entries: 0,
}
}
/// Destroys this arena chunk.
///
/// # Safety
///
/// The caller must ensure that `len` elements of this chunk have been initialized.
#[inline]
unsafe fn destroy(&mut self, len: usize) {
// The branch on needs_drop() is an -O1 performance optimization.
// Without the branch, dropping TypedArena<T> takes linear time.
if mem::needs_drop::<T>() {
// SAFETY: The caller must ensure that `len` elements of this chunk have
// been initialized.
unsafe {
let slice = self.storage.as_mut();
ptr::drop_in_place(MaybeUninit::slice_assume_init_mut(&mut slice[..len]));
}
}
}
// Returns a pointer to the first allocated object.
#[inline]
fn start(&mut self) -> *mut T {
self.storage.as_ptr() as *mut T
}
// Returns a pointer to the end of the allocated space.
#[inline]
fn end(&mut self) -> *mut T {
unsafe {
if mem::size_of::<T>() == 0 {
// A pointer as large as possible for zero-sized elements.
ptr::invalid_mut(!0)
} else {
self.start().add(self.storage.len())
}
}
}
}
// The arenas start with PAGE-sized chunks, and then each new chunk is twice as
// big as its predecessor, up until we reach HUGE_PAGE-sized chunks, whereupon
// we stop growing. This scales well, from arenas that are barely used up to
// arenas that are used for 100s of MiBs. Note also that the chosen sizes match
// the usual sizes of pages and huge pages on Linux.
const PAGE: usize = 4096;
const HUGE_PAGE: usize = 2 * 1024 * 1024;
/// An arena that can hold objects of only one type.
pub struct TypedArena<T> {
/// A pointer to the next object to be allocated.
ptr: Cell<*mut T>,
/// A pointer to the end of the allocated area. When this pointer is
/// reached, a new chunk is allocated.
end: Cell<*mut T>,
/// A vector of arena chunks.
chunks: RefCell<Vec<ArenaChunk<T>>>,
/// Marker indicating that dropping the arena causes its owned
/// instances of `T` to be dropped.
_own: PhantomData<T>,
}
impl<T> Default for TypedArena<T> {
/// Creates a new `TypedArena`.
fn default() -> TypedArena<T> {
TypedArena {
// We set both `ptr` and `end` to 0 so that the first call to
// alloc() will trigger a grow().
ptr: Cell::new(ptr::null_mut()),
end: Cell::new(ptr::null_mut()),
chunks: Default::default(),
_own: PhantomData,
}
}
}
impl<T> TypedArena<T> {
/// 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(1)
}
unsafe {
if mem::size_of::<T>() == 0 {
self.ptr.set(self.ptr.get().wrapping_byte_add(1));
let ptr = ptr::NonNull::<T>::dangling().as_ptr();
// Don't drop the object. This `write` is equivalent to `forget`.
ptr::write(ptr, object);
&mut *ptr
} else {
let ptr = self.ptr.get();
// Advance the pointer.
self.ptr.set(self.ptr.get().add(1));
// Write into uninitialized memory.
ptr::write(ptr, object);
&mut *ptr
}
}
}
#[inline]
fn can_allocate(&self, additional: usize) -> bool {
// FIXME: this should *likely* use `offset_from`, but more
// investigation is needed (including running tests in miri).
let available_bytes = self.end.get().addr() - self.ptr.get().addr();
let additional_bytes = additional.checked_mul(mem::size_of::<T>()).unwrap();
available_bytes >= additional_bytes
}
#[inline]
fn alloc_raw_slice(&self, len: usize) -> *mut T {
assert!(mem::size_of::<T>() != 0);
assert!(len != 0);
// Ensure the current chunk can fit `len` objects.
if !self.can_allocate(len) {
self.grow(len);
debug_assert!(self.can_allocate(len));
}
let start_ptr = self.ptr.get();
// SAFETY: `can_allocate`/`grow` ensures that there is enough space for
// `len` elements.
unsafe { self.ptr.set(start_ptr.add(len)) };
start_ptr
}
#[inline]
pub fn alloc_from_iter<I: IntoIterator<Item = T>>(&self, iter: I) -> &mut [T] {
// This implementation is entirely separate to
// `DroplessIterator::alloc_from_iter`, even though conceptually they
// are the same.
//
// `DroplessIterator` (in the fast case) writes elements from the
// iterator one at a time into the allocated memory. That's easy
// because the elements don't implement `Drop`. But for `TypedArena`
// they do implement `Drop`, which means that if the iterator panics we
// could end up with some allocated-but-uninitialized elements, which
// will then cause UB in `TypedArena::drop`.
//
// Instead we use an approach where any iterator panic will occur
// before the memory is allocated. This function is much less hot than
// `DroplessArena::alloc_from_iter`, so it doesn't need to be
// hyper-optimized.
assert!(mem::size_of::<T>() != 0);
let mut vec: SmallVec<[_; 8]> = iter.into_iter().collect();
if vec.is_empty() {
return &mut [];
}
// Move the content to the arena by copying and then forgetting it.
let len = vec.len();
let start_ptr = self.alloc_raw_slice(len);
unsafe {
vec.as_ptr().copy_to_nonoverlapping(start_ptr, len);
vec.set_len(0);
slice::from_raw_parts_mut(start_ptr, len)
}
}
/// Grows the arena.
#[inline(never)]
#[cold]
fn grow(&self, additional: usize) {
unsafe {
// We need the element size to convert chunk sizes (ranging from
// PAGE to HUGE_PAGE bytes) to element counts.
let elem_size = cmp::max(1, mem::size_of::<T>());
let mut chunks = self.chunks.borrow_mut();
let mut new_cap;
if let Some(last_chunk) = chunks.last_mut() {
// If a type is `!needs_drop`, we don't need to keep track of how many elements
// the chunk stores - the field will be ignored anyway.
if mem::needs_drop::<T>() {
// FIXME: this should *likely* use `offset_from`, but more
// investigation is needed (including running tests in miri).
let used_bytes = self.ptr.get().addr() - last_chunk.start().addr();
last_chunk.entries = used_bytes / mem::size_of::<T>();
}
// If the previous chunk's len is less than HUGE_PAGE
// bytes, then this chunk will be least double the previous
// chunk's size.
new_cap = last_chunk.storage.len().min(HUGE_PAGE / elem_size / 2);
new_cap *= 2;
} else {
new_cap = PAGE / elem_size;
}
// Also ensure that this chunk can fit `additional`.
new_cap = cmp::max(additional, new_cap);
let mut chunk = ArenaChunk::<T>::new(new_cap);
self.ptr.set(chunk.start());
self.end.set(chunk.end());
chunks.push(chunk);
}
}
// Drops the contents of the last chunk. The last chunk is partially empty, unlike all other
// chunks.
fn clear_last_chunk(&self, last_chunk: &mut ArenaChunk<T>) {
// Determine how much was filled.
let start = last_chunk.start().addr();
// We obtain the value of the pointer to the first uninitialized element.
let end = self.ptr.get().addr();
// We then calculate the number of elements to be dropped in the last chunk,
// which is the filled area's length.
let diff = if mem::size_of::<T>() == 0 {
// `T` is ZST. It can't have a drop flag, so the value here doesn't matter. We get
// the number of zero-sized values in the last and only chunk, just out of caution.
// Recall that `end` was incremented for each allocated value.
end - start
} else {
// FIXME: this should *likely* use `offset_from`, but more
// investigation is needed (including running tests in miri).
(end - start) / mem::size_of::<T>()
};
// Pass that to the `destroy` method.
unsafe {
last_chunk.destroy(diff);
}
// Reset the chunk.
self.ptr.set(last_chunk.start());
}
}
unsafe impl<#[may_dangle] T> Drop for TypedArena<T> {
fn drop(&mut self) {
unsafe {
// Determine how much was filled.
let mut chunks_borrow = self.chunks.borrow_mut();
if let Some(mut last_chunk) = chunks_borrow.pop() {
// Drop the contents of the last chunk.
self.clear_last_chunk(&mut last_chunk);
// The last chunk will be dropped. Destroy all other chunks.
for chunk in chunks_borrow.iter_mut() {
chunk.destroy(chunk.entries);
}
}
// Box handles deallocation of `last_chunk` and `self.chunks`.
}
}
}
unsafe impl<T: Send> Send for TypedArena<T> {}
#[inline(always)]
fn align_down(val: usize, align: usize) -> usize {
debug_assert!(align.is_power_of_two());
val & !(align - 1)
}
#[inline(always)]
fn align_up(val: usize, align: usize) -> usize {
debug_assert!(align.is_power_of_two());
(val + align - 1) & !(align - 1)
}
// Pointer alignment is common in compiler types, so keep `DroplessArena` aligned to them
// to optimize away alignment code.
const DROPLESS_ALIGNMENT: usize = mem::align_of::<usize>();
/// An arena that can hold objects of multiple different types that impl `Copy`
/// and/or satisfy `!mem::needs_drop`.
pub struct DroplessArena {
/// A pointer to the start of the free space.
start: Cell<*mut u8>,
/// A pointer to the end of free space.
///
/// The allocation proceeds downwards from the end of the chunk towards the
/// start. (This is slightly simpler and faster than allocating upwards,
/// see <https://fitzgeraldnick.com/2019/11/01/always-bump-downwards.html>.)
/// When this pointer crosses the start pointer, a new chunk is allocated.
///
/// This is kept aligned to DROPLESS_ALIGNMENT.
end: Cell<*mut u8>,
/// A vector of arena chunks.
chunks: RefCell<Vec<ArenaChunk>>,
}
unsafe impl Send for DroplessArena {}
impl Default for DroplessArena {
#[inline]
fn default() -> DroplessArena {
DroplessArena {
// We set both `start` and `end` to 0 so that the first call to
// alloc() will trigger a grow().
start: Cell::new(ptr::null_mut()),
end: Cell::new(ptr::null_mut()),
chunks: Default::default(),
}
}
}
impl DroplessArena {
#[inline(never)]
#[cold]
fn grow(&self, layout: Layout) {
// Add some padding so we can align `self.end` while
// still fitting in a `layout` allocation.
let additional = layout.size() + cmp::max(DROPLESS_ALIGNMENT, layout.align()) - 1;
unsafe {
let mut chunks = self.chunks.borrow_mut();
let mut new_cap;
if let Some(last_chunk) = chunks.last_mut() {
// There is no need to update `last_chunk.entries` because that
// field isn't used by `DroplessArena`.
// If the previous chunk's len is less than HUGE_PAGE
// bytes, then this chunk will be least double the previous
// chunk's size.
new_cap = last_chunk.storage.len().min(HUGE_PAGE / 2);
new_cap *= 2;
} else {
new_cap = PAGE;
}
// Also ensure that this chunk can fit `additional`.
new_cap = cmp::max(additional, new_cap);
let mut chunk = ArenaChunk::new(align_up(new_cap, PAGE));
self.start.set(chunk.start());
// Align the end to DROPLESS_ALIGNMENT.
let end = align_down(chunk.end().addr(), DROPLESS_ALIGNMENT);
// Make sure we don't go past `start`. This should not happen since the allocation
// should be at least DROPLESS_ALIGNMENT - 1 bytes.
debug_assert!(chunk.start().addr() <= end);
self.end.set(chunk.end().with_addr(end));
chunks.push(chunk);
}
}
#[inline]
pub fn alloc_raw(&self, layout: Layout) -> *mut u8 {
assert!(layout.size() != 0);
// This loop executes once or twice: if allocation fails the first
// time, the `grow` ensures it will succeed the second time.
loop {
let start = self.start.get().addr();
let old_end = self.end.get();
let end = old_end.addr();
// Align allocated bytes so that `self.end` stays aligned to
// DROPLESS_ALIGNMENT.
let bytes = align_up(layout.size(), DROPLESS_ALIGNMENT);
// Tell LLVM that `end` is aligned to DROPLESS_ALIGNMENT.
unsafe { intrinsics::assume(end == align_down(end, DROPLESS_ALIGNMENT)) };
if let Some(sub) = end.checked_sub(bytes) {
let new_end = align_down(sub, layout.align());
if start <= new_end {
let new_end = old_end.with_addr(new_end);
// `new_end` is aligned to DROPLESS_ALIGNMENT as `align_down`
// preserves alignment as both `end` and `bytes` are already
// aligned to DROPLESS_ALIGNMENT.
self.end.set(new_end);
return new_end;
}
}
// No free space left. Allocate a new chunk to satisfy the request.
// On failure the grow will panic or abort.
self.grow(layout);
}
}
#[inline]
pub fn alloc<T>(&self, object: T) -> &mut T {
assert!(!mem::needs_drop::<T>());
assert!(mem::size_of::<T>() != 0);
let mem = self.alloc_raw(Layout::new::<T>()) as *mut T;
unsafe {
// Write into uninitialized memory.
ptr::write(mem, object);
&mut *mem
}
}
/// Allocates a slice of objects that are copied into the `DroplessArena`, returning a mutable
/// reference to it. Will panic if passed a zero-sized type.
///
/// Panics:
///
/// - Zero-sized types
/// - Zero-length slices
#[inline]
pub fn alloc_slice<T>(&self, slice: &[T]) -> &mut [T]
where
T: Copy,
{
assert!(!mem::needs_drop::<T>());
assert!(mem::size_of::<T>() != 0);
assert!(!slice.is_empty());
let mem = self.alloc_raw(Layout::for_value::<[T]>(slice)) as *mut T;
unsafe {
mem.copy_from_nonoverlapping(slice.as_ptr(), slice.len());
slice::from_raw_parts_mut(mem, slice.len())
}
}
/// # Safety
///
/// The caller must ensure that `mem` is valid for writes up to
/// `size_of::<T>() * len`.
#[inline]
unsafe fn write_from_iter<T, I: Iterator<Item = T>>(
&self,
mut iter: I,
len: usize,
mem: *mut T,
) -> &mut [T] {
let mut i = 0;
// Use a manual loop since LLVM manages to optimize it better for
// slice iterators
loop {
// SAFETY: The caller must ensure that `mem` is valid for writes up to
// `size_of::<T>() * len`.
unsafe {
match iter.next() {
Some(value) if i < len => mem.add(i).write(value),
Some(_) | None => {
// We only return as many items as the iterator gave us, even
// though it was supposed to give us `len`
return slice::from_raw_parts_mut(mem, i);
}
}
}
i += 1;
}
}
#[inline]
pub fn alloc_from_iter<T, I: IntoIterator<Item = T>>(&self, iter: I) -> &mut [T] {
let iter = iter.into_iter();
assert!(mem::size_of::<T>() != 0);
assert!(!mem::needs_drop::<T>());
let size_hint = iter.size_hint();
match size_hint {
(min, Some(max)) if min == max => {
// We know the exact number of elements the iterator will produce here
let len = min;
if len == 0 {
return &mut [];
}
let mem = self.alloc_raw(Layout::array::<T>(len).unwrap()) as *mut T;
unsafe { self.write_from_iter(iter, len, mem) }
}
(_, _) => {
outline(move || -> &mut [T] {
let mut vec: SmallVec<[_; 8]> = iter.collect();
if vec.is_empty() {
return &mut [];
}
// Move the content to the arena by copying it and then forgetting
// the content of the SmallVec
unsafe {
let len = vec.len();
let start_ptr =
self.alloc_raw(Layout::for_value::<[T]>(vec.as_slice())) as *mut T;
vec.as_ptr().copy_to_nonoverlapping(start_ptr, len);
vec.set_len(0);
slice::from_raw_parts_mut(start_ptr, len)
}
})
}
}
}
}
/// Declare an `Arena` containing one dropless arena and many typed arenas (the
/// types of the typed arenas are specified by the arguments).
///
/// There are three cases of interest.
/// - Types that are `Copy`: these need not be specified in the arguments. They
/// will use the `DroplessArena`.
/// - Types that are `!Copy` and `!Drop`: these must be specified in the
/// arguments. An empty `TypedArena` will be created for each one, but the
/// `DroplessArena` will always be used and the `TypedArena` will stay empty.
/// This is odd but harmless, because an empty arena allocates no memory.
/// - Types that are `!Copy` and `Drop`: these must be specified in the
/// arguments. The `TypedArena` will be used for them.
///
#[rustc_macro_transparency = "semitransparent"]
pub macro declare_arena([$($a:tt $name:ident: $ty:ty,)*]) {
#[derive(Default)]
pub struct Arena<'tcx> {
pub dropless: $crate::DroplessArena,
$($name: $crate::TypedArena<$ty>,)*
}
pub trait ArenaAllocatable<'tcx, C = rustc_arena::IsNotCopy>: Sized {
#[allow(clippy::mut_from_ref)]
fn allocate_on<'a>(self, arena: &'a Arena<'tcx>) -> &'a mut Self;
#[allow(clippy::mut_from_ref)]
fn allocate_from_iter<'a>(
arena: &'a Arena<'tcx>,
iter: impl ::std::iter::IntoIterator<Item = Self>,
) -> &'a mut [Self];
}
// Any type that impls `Copy` can be arena-allocated in the `DroplessArena`.
impl<'tcx, T: Copy> ArenaAllocatable<'tcx, rustc_arena::IsCopy> for T {
#[inline]
#[allow(clippy::mut_from_ref)]
fn allocate_on<'a>(self, arena: &'a Arena<'tcx>) -> &'a mut Self {
arena.dropless.alloc(self)
}
#[inline]
#[allow(clippy::mut_from_ref)]
fn allocate_from_iter<'a>(
arena: &'a Arena<'tcx>,
iter: impl ::std::iter::IntoIterator<Item = Self>,
) -> &'a mut [Self] {
arena.dropless.alloc_from_iter(iter)
}
}
$(
impl<'tcx> ArenaAllocatable<'tcx, rustc_arena::IsNotCopy> for $ty {
#[inline]
fn allocate_on<'a>(self, arena: &'a Arena<'tcx>) -> &'a mut Self {
if !::std::mem::needs_drop::<Self>() {
arena.dropless.alloc(self)
} else {
arena.$name.alloc(self)
}
}
#[inline]
#[allow(clippy::mut_from_ref)]
fn allocate_from_iter<'a>(
arena: &'a Arena<'tcx>,
iter: impl ::std::iter::IntoIterator<Item = Self>,
) -> &'a mut [Self] {
if !::std::mem::needs_drop::<Self>() {
arena.dropless.alloc_from_iter(iter)
} else {
arena.$name.alloc_from_iter(iter)
}
}
}
)*
impl<'tcx> Arena<'tcx> {
#[inline]
#[allow(clippy::mut_from_ref)]
pub fn alloc<T: ArenaAllocatable<'tcx, C>, C>(&self, value: T) -> &mut T {
value.allocate_on(self)
}
// Any type that impls `Copy` can have slices be arena-allocated in the `DroplessArena`.
#[inline]
#[allow(clippy::mut_from_ref)]
pub fn alloc_slice<T: ::std::marker::Copy>(&self, value: &[T]) -> &mut [T] {
if value.is_empty() {
return &mut [];
}
self.dropless.alloc_slice(value)
}
#[allow(clippy::mut_from_ref)]
pub fn alloc_from_iter<T: ArenaAllocatable<'tcx, C>, C>(
&self,
iter: impl ::std::iter::IntoIterator<Item = T>,
) -> &mut [T] {
T::allocate_from_iter(self, iter)
}
}
}
// Marker types that let us give different behaviour for arenas allocating
// `Copy` types vs `!Copy` types.
pub struct IsCopy;
pub struct IsNotCopy;
#[cfg(test)]
mod tests;