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// Copyright 2018-2024 the Deno authors. All rights reserved. MIT license.
use std::alloc::Layout;
use std::cell::Cell;
use std::mem::ManuallyDrop;
use std::ptr::NonNull;
use bit_set::BitSet;
use bit_vec::BitVec;
use super::alloc;
use super::alloc_layout;
/// In debug mode we use a signature to ensure that raw pointers are pointing to the correct
/// shape of arena object.
#[cfg(debug_assertions)]
const SIGNATURE: usize = 0x1234567812345678;
/// A very-`unsafe`, arena for raw pointers that falls back to raw allocation when full. This
/// should be used with great care, and ideally you should only be using the higher-level arenas
/// built on top of this.
///
/// # Safety
///
/// Items placed into the RawArena are dropped, but there is no check to ensure that an allocated
/// item is valid before dropping it. Use `recycle_without_drop` to return an item to the arena
/// without dropping it.
///
/// # Example
///
/// ```rust
/// # use deno_core::arena::RawArena;
/// // Create a RawArena with a capacity of 10 elements
/// let arena = RawArena::<usize>::with_capacity(10);
///
/// // Allocate elements in the arena
/// unsafe {
/// let mut elements = Vec::new();
/// for i in 0..10 {
/// let mut element_ptr = arena.allocate();
/// *element_ptr.as_mut() = i * 2;
/// elements.push(element_ptr);
/// }
///
/// // Recycle elements back into the arena
/// for &element_ptr in elements.iter() {
/// arena.recycle(element_ptr);
/// }
/// }
/// ```
pub struct RawArena<T> {
#[cfg(debug_assertions)]
signature: usize,
alloc: NonNull<RawArenaEntry<T>>,
past_alloc_end: NonNull<RawArenaEntry<T>>,
max: Cell<NonNull<RawArenaEntry<T>>>,
next: Cell<NonNull<RawArenaEntry<T>>>,
allocated: Cell<usize>,
capacity: usize,
}
/// The [`RawArena`] is [`Send`], but not [`Sync`].
unsafe impl<T> Send for RawArena<T> {}
static_assertions::assert_impl_one!(RawArena<()>: Send);
static_assertions::assert_not_impl_any!(RawArena<()>: Sync);
union RawArenaEntry<T> {
/// If this is a vacant entry, points to the next entry.
next: NonNull<RawArenaEntry<T>>,
/// If this is a valid entry, contains the raw data.
value: ManuallyDrop<T>,
}
impl<T> RawArenaEntry<T> {
#[inline(always)]
unsafe fn next(
entry: NonNull<RawArenaEntry<T>>,
) -> NonNull<RawArenaEntry<T>> {
(*(entry.as_ptr())).next
}
#[inline(always)]
unsafe fn drop(entry: NonNull<RawArenaEntry<T>>) {
std::ptr::drop_in_place(
std::ptr::addr_of_mut!((*entry.as_ptr()).value) as *mut T
);
}
}
impl<T> RawArena<T> {
/// Returns the constant overhead per allocation to assist with making allocations
/// page-aligned.
pub const fn overhead() -> usize {
Self::allocation_size() - std::mem::size_of::<T>()
}
/// Returns the size of each allocation.
pub const fn allocation_size() -> usize {
std::mem::size_of::<RawArenaEntry<T>>()
}
/// Allocate an arena, completely initialized. This memory is not zeroed, and
/// we use the high-water mark to keep track of what we've initialized so far.
///
/// This is safe, because dropping the [`RawArena`] without doing anything to
/// it is safe.
pub fn with_capacity(capacity: usize) -> Self {
let alloc = alloc_layout(Self::layout(capacity));
Self {
#[cfg(debug_assertions)]
signature: SIGNATURE,
alloc,
past_alloc_end: unsafe {
NonNull::new_unchecked(alloc.as_ptr().add(capacity))
},
max: alloc.into(),
next: Cell::new(alloc),
allocated: Default::default(),
capacity,
}
}
// TODO(mmastrac): const when https://github.com/rust-lang/rust/issues/67521 is fixed
fn layout(capacity: usize) -> Layout {
match Layout::array::<RawArenaEntry<T>>(capacity) {
Ok(l) => l,
_ => panic!("Zero-sized objects are not supported"),
}
}
/// Helper method to transmute internal pointers.
///
/// # Safety
///
/// For internal use.
#[inline(always)]
unsafe fn entry_to_data(entry: NonNull<RawArenaEntry<T>>) -> NonNull<T> {
// Transmute the union
entry.cast()
}
/// Helper method to transmute internal pointers.
///
/// # Safety
///
/// For internal use.
#[inline(always)]
unsafe fn data_to_entry(data: NonNull<T>) -> NonNull<RawArenaEntry<T>> {
// Transmute the union
data.cast()
}
/// Gets the next free entry, allocating if necessary. This is `O(1)` if we have free space in
/// the arena, `O(?)` if we need to allocate from the allocator (where `?` is defined by the
/// system allocator).
///
/// # Safety
///
/// As the memory area is considered uninitialized and you must be careful to fully and validly
/// initialize the underlying data, this method is marked as unsafe.
///
/// This pointer will be invalidated when we drop the `RawArena`, so the allocator API is `unsafe`
/// as there are no lifetimes here.
///
/// **IMPORTANT:** Ensure all allocated entries are fully initialized before dropping `RawArena`,
/// or use `recycle_without_drop` to manually handle recycling, as dropping the arena does not
/// perform any validation or cleanup on the allocated items. Dropping `RawArena` will automatically
/// trigger the drop of all items allocated within.
pub unsafe fn allocate(&self) -> NonNull<T> {
#[cfg(debug_assertions)]
debug_assert_eq!(self.signature, SIGNATURE);
let next = self.next.get();
let max = self.max.get();
// Check to see if we have gone past our high-water mark, and we need to extend it. The high-water
// mark allows us to leave the allocation uninitialized, and assume that the remaining part of the
// next-free list is a trivial linked-list where each node points to the next one.
if max == next {
// Are we out of room?
if max == self.past_alloc_end {
// We filled the RawArena, so start allocating
return Self::entry_to_data(alloc());
}
// Nope, we can extend by one
let next = NonNull::new_unchecked(self.max.get().as_ptr().add(1));
self.next.set(next);
self.max.set(next);
} else {
// We haven't passed the high-water mark, so walk the internal next-free list
// for our next allocation
self.next.set(RawArenaEntry::next(next));
}
// Update accounting
self.allocated.set(self.allocated.get() + 1);
Self::entry_to_data(next)
}
/// Gets the next free entry, returning null if full. This is `O(1)`.
///
/// # Safety
///
/// As the memory area is considered uninitialized and you must be careful to fully and validly
/// initialize the underlying data, this method is marked as unsafe.
///
/// This pointer will be invalidated when we drop the `RawArena`, so the allocator API is `unsafe`
/// as there are no lifetimes here.
///
/// **IMPORTANT:** Ensure all allocated entries are fully initialized before dropping `RawArena`,
/// or use `recycle_without_drop` to manually handle recycling, as dropping the arena does not
/// perform any validation or cleanup on the allocated items. Dropping `RawArena` will automatically
/// trigger the drop of all items allocated within.
pub unsafe fn allocate_if_space(&self) -> Option<NonNull<T>> {
#[cfg(debug_assertions)]
debug_assert_eq!(self.signature, SIGNATURE);
let next = self.next.get();
let max = self.max.get();
// Check to see if we have gone past our high-water mark, and we need to extend it. The high-water
// mark allows us to leave the allocation uninitialized, and assume that the remaining part of the
// next-free list is a trivial linked-list where each node points to the next one.
if max == next {
// Are we out of room?
if max == self.past_alloc_end {
// We filled the RawArena, so return None
return None;
}
// Nope, we can extend by one
let next = NonNull::new_unchecked(self.max.get().as_ptr().add(1));
self.next.set(next);
self.max.set(next);
} else {
// We haven't passed the high-water mark, so walk the internal next-free list
// for our next allocation
self.next.set((*(next.as_ptr())).next);
}
// Update accounting
self.allocated.set(self.allocated.get() + 1);
Some(Self::entry_to_data(next))
}
/// Returns the remaining capacity of this [`RawArena`] that can be provided without allocation.
pub fn remaining(&self) -> usize {
self.capacity - self.allocated.get()
}
/// Returns the remaining capacity of this [`RawArena`] that can be provided without allocation.
pub fn allocated(&self) -> usize {
self.allocated.get()
}
/// Clear all internally-allocated entries, resetting the arena state to its original state. Any
/// non-vacant entries are dropped.
///
/// This operation must walk the vacant list and is worst-case `O(n)`, where `n` is the largest
/// size of this arena since the last clear operation.
///
/// # Safety
///
/// Does not clear system-allocator entries. Pointers previously [`allocate`](Self::allocate)d may still be in use.
pub unsafe fn clear_allocated(&self) {
#[cfg(debug_assertions)]
debug_assert_eq!(self.signature, SIGNATURE);
// We need to drop the allocated pointers, but we don't know which ones they are. We only
// know the vacant slots.
if self.allocated.get() > 0 {
unsafe {
// How many entries are we possibly using?
let max = self.max.get();
// Compute the vacant set by walking the `next` pointers
let count = max.as_ptr().offset_from(self.alloc.as_ptr()) as usize;
let mut vacant = BitVec::with_capacity(count);
vacant.grow(count, false);
let mut next = self.next.get();
while next != max {
let i = next.as_ptr().offset_from(self.alloc.as_ptr()) as usize;
vacant.set(i, true);
next = RawArenaEntry::next(next);
}
vacant.negate();
// Iterate over the inverse of the vacant set and free those items
for alloc in BitSet::from_bit_vec(vacant).into_iter() {
let entry = self.alloc.as_ptr().add(alloc);
std::ptr::drop_in_place(
std::ptr::addr_of_mut!((*entry).value) as *mut T
);
}
}
}
self.max.set(self.alloc);
self.next.set(self.alloc);
self.allocated.set(0);
}
/// Recycle a used item, returning it to the next-free list. Drops the associated item
/// in place before recycling.
///
/// # Safety
///
/// We assume this pointer is either internal to the arena (in which case we return it
/// to the arena), or allocated via [`std::alloc::alloc`] in [`allocate`](Self::allocate).
pub unsafe fn recycle(&self, data: NonNull<T>) -> bool {
#[cfg(debug_assertions)]
debug_assert_eq!(self.signature, SIGNATURE);
let mut entry = Self::data_to_entry(data);
let mut emptied = false;
RawArenaEntry::drop(entry);
if entry >= self.alloc && entry < self.past_alloc_end {
let next = self.next.get();
let count = self.allocated.get() - 1;
emptied = count == 0;
self.allocated.set(count);
entry.as_mut().next = next;
self.next.set(entry);
} else {
std::alloc::dealloc(
entry.as_ptr() as _,
Layout::new::<RawArenaEntry<T>>(),
);
}
emptied
}
/// Recycle a used item, returning it to the next-free list.
///
/// # Safety
///
/// We assume this pointer is either internal to the arena (in which case we return it
/// to the arena), or allocated via [`std::alloc::alloc`] in [`allocate`](Self::allocate).
pub unsafe fn recycle_without_drop(&self, data: NonNull<T>) -> bool {
#[cfg(debug_assertions)]
debug_assert_eq!(self.signature, SIGNATURE);
let mut entry = Self::data_to_entry(data);
let mut emptied = false;
if entry >= self.alloc && entry < self.past_alloc_end {
let next = self.next.get();
let count = self.allocated.get() - 1;
emptied = count == 0;
self.allocated.set(count);
entry.as_mut().next = next;
self.next.set(entry);
} else {
std::alloc::dealloc(
entry.as_ptr() as _,
Layout::new::<RawArenaEntry<T>>(),
);
}
emptied
}
}
impl<T> Drop for RawArena<T> {
/// Drop the arena. All pointers are invalidated at this point, except for those
/// allocated outside outside of the arena.
///
/// The allocation APIs are unsafe because we don't track lifetimes here.
fn drop(&mut self) {
unsafe { self.clear_allocated() };
#[cfg(debug_assertions)]
{
debug_assert_eq!(self.signature, SIGNATURE);
self.signature = 0;
}
unsafe {
std::alloc::dealloc(self.alloc.as_ptr() as _, Self::layout(self.capacity))
}
}
}
#[cfg(test)]
mod tests {
use super::*;
#[must_use = "If you don't use this, it'll leak!"]
unsafe fn allocate(arena: &RawArena<usize>, i: usize) -> NonNull<usize> {
let mut new = arena.allocate();
*new.as_mut() = i;
new
}
#[test]
fn test_add_remove_many() {
let arena = RawArena::<usize>::with_capacity(1024);
unsafe {
for i in 0..2000 {
let v = allocate(&arena, i);
assert_eq!(arena.remaining(), 1023);
assert_eq!(*v.as_ref(), i);
arena.recycle(v);
assert_eq!(arena.remaining(), 1024);
}
}
}
#[test]
fn test_add_clear_many() {
let arena = RawArena::<usize>::with_capacity(1024);
unsafe {
for i in 0..2000 {
_ = allocate(&arena, i);
assert_eq!(arena.remaining(), 1023);
arena.clear_allocated();
assert_eq!(arena.remaining(), 1024);
}
}
}
#[test]
fn test_add_remove_many_separate() {
let arena = RawArena::<usize>::with_capacity(1024);
unsafe {
let mut nodes = vec![];
// This will spill over into memory allocations
for i in 0..2000 {
nodes.push(allocate(&arena, i));
}
assert_eq!(arena.remaining(), 0);
for i in (0..2000).rev() {
let node = nodes.pop().unwrap();
assert_eq!(*node.as_ref(), i);
arena.recycle(node);
}
assert_eq!(arena.remaining(), 1024);
}
}
#[test]
fn test_droppable() {
// Make sure we correctly drop all the items in this arena if they are droppable
let arena = RawArena::<_>::with_capacity(16);
unsafe {
let mut nodes = vec![];
// This will spill over into memory allocations
for i in 0..20 {
let node = arena.allocate();
std::ptr::write(
node.as_ptr(),
Box::new(std::future::ready(format!("iteration {i}"))),
);
nodes.push(node);
}
assert_eq!(arena.remaining(), 0);
for node in nodes {
arena.recycle(node);
}
assert_eq!(arena.remaining(), 16);
}
}
#[test]
fn test_no_drop() {
let arena = RawArena::<String>::with_capacity(16);
unsafe {
arena.recycle_without_drop(arena.allocate());
arena.clear_allocated();
}
}
#[test]
fn test_drops() {
let arena = RawArena::<_>::with_capacity(16);
unsafe {
for i in 0..2 {
let ptr = arena.allocate();
std::ptr::write(ptr.as_ptr(), format!("iteration {i}"));
}
// Leave a space in the internal allocations
let ptr = arena.allocate();
std::ptr::write(ptr.as_ptr(), "deleted".to_owned());
arena.recycle(ptr);
arena.clear_allocated();
}
}
#[test]
fn test_drops_full() {
#[allow(dead_code)]
struct Droppable(String);
let arena = RawArena::<_>::with_capacity(16);
unsafe {
for i in 0..2 {
let ptr = arena.allocate();
std::ptr::write(ptr.as_ptr(), Droppable(format!("iteration {i}")));
}
arena.clear_allocated();
}
}
}