gecko-dev/third_party/rust/bumpalo

bumpalo

A fast bump allocation arena for Rust.

Bump Allocation

Bump allocation is a fast, but limited approach to allocation. We have a chunk of memory, and we maintain a pointer within that memory. Whenever we allocate an object, we do a quick check that we have enough capacity left in our chunk to allocate the object and then update the pointer by the object's size. That's it!

The disadvantage of bump allocation is that there is no general way to deallocate individual objects or reclaim the memory region for a no-longer-in-use object.

These trade offs make bump allocation well-suited for phase-oriented allocations. That is, a group of objects that will all be allocated during the same program phase, used, and then can all be deallocated together as a group.

Deallocation en Masse, but no Drop

To deallocate all the objects in the arena at once, we can simply reset the bump pointer back to the start of the arena's memory chunk. This makes mass deallocation extremely fast, but allocated objects' Drop implementations are not invoked.

However: bumpalo::boxed::Box<T> can be used to wrap T values allocated in the Bump arena, and calls T's Drop implementation when the Box<T> wrapper goes out of scope. This is similar to how std::boxed::Box works, except without deallocating its backing memory.

What happens when the memory chunk is full?

This implementation will allocate a new memory chunk from the global allocator and then start bump allocating into this new memory chunk.

Example

use bumpalo::Bump;
use std::u64;

struct Doggo {
    cuteness: u64,
    age: u8,
    scritches_required: bool,
}

// Create a new arena to bump allocate into.
let bump = Bump::new();

// Allocate values into the arena.
let scooter = bump.alloc(Doggo {
    cuteness: u64::max_value(),
    age: 8,
    scritches_required: true,
});

// Exclusive, mutable references to the just-allocated value are returned.
assert!(scooter.scritches_required);
scooter.age += 1;

Collections

When the "collections" cargo feature is enabled, a fork of some of the std library's collections are available in the collections module. These collection types are modified to allocate their space inside bumpalo::Bump arenas.

#[cfg(feature = "collections")]
{
    use bumpalo::{Bump, collections::Vec};

    // Create a new bump arena.
    let bump = Bump::new();

    // Create a vector of integers whose storage is backed by the bump arena. The
    // vector cannot outlive its backing arena, and this property is enforced with
    // Rust's lifetime rules.
    let mut v = Vec::new_in(&bump);

    // Push a bunch of integers onto `v`!
    for i in 0..100 {
        v.push(i);
    }
}

Eventually all std collection types will be parameterized by an allocator and we can remove this collections module and use the std versions.

For unstable, nightly-only support for custom allocators in std, see the allocator_api section below.

bumpalo::boxed::Box

When the "boxed" cargo feature is enabled, a fork of std::boxed::Box is available in the boxed module. This Box type is modified to allocate its space inside bumpalo::Bump arenas.

A Box<T> runs T's drop implementation when the Box<T> is dropped. You can use this to work around the fact that Bump does not drop values allocated in its space itself.

#[cfg(feature = "boxed")]
{
    use bumpalo::{Bump, boxed::Box};
    use std::sync::atomic::{AtomicUsize, Ordering};

    static NUM_DROPPED: AtomicUsize = AtomicUsize::new(0);

    struct CountDrops;

    impl Drop for CountDrops {
        fn drop(&mut self) {
            NUM_DROPPED.fetch_add(1, Ordering::SeqCst);
        }
    }

    // Create a new bump arena.
    let bump = Bump::new();

    // Create a `CountDrops` inside the bump arena.
    let mut c = Box::new_in(CountDrops, &bump);

    // No `CountDrops` have been dropped yet.
    assert_eq!(NUM_DROPPED.load(Ordering::SeqCst), 0);

    // Drop our `Box<CountDrops>`.
    drop(c);

    // Its `Drop` implementation was run, and so `NUM_DROPS` has been incremented.
    assert_eq!(NUM_DROPPED.load(Ordering::SeqCst), 1);
}

#![no_std] Support

Bumpalo is a no_std crate. It depends only on the alloc and core crates.

Thread support

The Bump is !Sync, which makes it hard to use in certain situations around threads ‒ for example in rayon.

The bumpalo-herd crate provides a pool of Bump allocators for use in such situations.

Nightly Rust allocator_api Support

The unstable, nightly-only Rust allocator_api feature defines an Allocator trait and exposes custom allocators for std types. Bumpalo has a matching allocator_api cargo feature to enable implementing Allocator and using Bump with std collections. Note that, as feature(allocator_api) is unstable and only in nightly Rust, Bumpalo's matching allocator_api cargo feature should be considered unstable, and will not follow the semver conventions that the rest of the crate does.

First, enable the allocator_api feature in your Cargo.toml:

[dependencies]
bumpalo = { version = "3.9", features = ["allocator_api"] }

Next, enable the allocator_api nightly Rust feature in your src/lib.rs or src/main.rs:

#![feature(allocator_api)]

Finally, use std collections with Bump, so that their internal heap allocations are made within the given bump arena:

use bumpalo::Bump;

// Create a new bump arena.
let bump = Bump::new();

// Create a `Vec` whose elements are allocated within the bump arena.
let mut v = Vec::new_in(&bump);
v.push(0);
v.push(1);
v.push(2);

Minimum Supported Rust Version (MSRV)

This crate is guaranteed to compile on stable Rust 1.54 and up. It might compile with older versions but that may change in any new patch release.

We reserve the right to increment the MSRV on minor releases, however we will strive to only do it deliberately and for good reasons.