bump-scope

A fast bump allocator that supports allocation scopes / checkpoints. Aka an arena for values of arbitrary types.

What is bump allocation?

A bump allocator owns a big chunk of memory. It has a pointer that starts at one end of that chunk. When an allocation is made that pointer gets aligned and bumped towards the other end of the chunk by the allocation's size. When its chunk is full, it allocates another chunk with twice the size.

This makes allocations very fast. The drawback is that you can't reclaim memory like you do with a more general allocator. Memory for the most recent allocation can be reclaimed. You can also use scopes, checkpoints and reset to reclaim memory.

A bump allocator is great for phase-oriented allocations where you allocate objects in a loop and free them at the end of every iteration.

use bump_scope::Bump;
let mut bump: Bump = Bump::new();

loop {
    // use bump ...
    bump.reset();
}

The fact that the bump allocator allocates ever larger chunks and reset only keeps around the largest one means that after a few iterations, every bump allocation will be done on the same chunk and no more chunks need to be allocated.

The introduction of scopes makes this bump allocator also great for temporary allocations and stack-like usage.

Comparison to bumpalo

Bumpalo is a popular crate for bump allocation. This crate was inspired by bumpalo and Always Bump Downwards.

Unlike bumpalo, this crate...

• Supports scopes and checkpoints.
• Drop is always called for allocated values unless explicitly leaked or forgotten.
  • alloc* methods return a BumpBox<T> which owns and drops T. Types that don't need dropping can be turned into references with into_ref and into_mut.
• You can efficiently allocate items from any Iterator with alloc_iter_mut(_rev).
• Every method that panics on allocation failure has a fallible try_* counterpart.
• Bump's base allocator is generic.
• Bump and BumpScope have the same repr as NonNull<u8>. (vs 3x pointer sized)
• Won't try to allocate a smaller chunk if allocation failed.
• No built-in allocation limit. You can provide an allocator that enforces an allocation limit (see tests/limit_memory_usage.rs).
• Allocations are a bit more optimized. (see crates/inspect-asm/out/x86-64 and benchmarks)
• You can choose the bump direction. Bumps upwards by default.
• You can choose the minimum alignment.

Scopes and Checkpoints

You can create scopes to make allocations that live only for a part of its parent scope. Creating and exiting scopes is virtually free. Allocating within a scope has no overhead.

You can create a new scope either with a scoped closure or with a scope_guard:

use bump_scope::Bump;

let mut bump: Bump = Bump::new();

// you can use a closure
bump.scoped(|mut bump| {
    let hello = bump.alloc_str("hello");
    assert_eq!(bump.stats().allocated(), 5);
    
    bump.scoped(|bump| {
        let world = bump.alloc_str("world");

        println!("{hello} and {world} are both live");
        assert_eq!(bump.stats().allocated(), 10);
    });
    
    println!("{hello} is still live");
    assert_eq!(bump.stats().allocated(), 5);
});

assert_eq!(bump.stats().allocated(), 0);

// or you can use scope guards
{
    let mut guard = bump.scope_guard();
    let mut bump = guard.scope();

    let hello = bump.alloc_str("hello");
    assert_eq!(bump.stats().allocated(), 5);
    
    {
        let mut guard = bump.scope_guard();
        let bump = guard.scope();

        let world = bump.alloc_str("world");

        println!("{hello} and {world} are both live");
        assert_eq!(bump.stats().allocated(), 10);
    }
    
    println!("{hello} is still live");
    assert_eq!(bump.stats().allocated(), 5);
}

assert_eq!(bump.stats().allocated(), 0);

You can also use the unsafe checkpoint api to reset the bump pointer to a previous location.

let checkpoint = bump.checkpoint();

{
    let hello = bump.alloc_str("hello");
    assert_eq!(bump.stats().allocated(), 5);
}

unsafe { bump.reset_to(checkpoint); }
assert_eq!(bump.stats().allocated(), 0);

Allocator API

Bump and BumpScope implement allocator_api2::alloc::Allocator. With this you can bump allocate allocator_api2::boxed::Box, allocator_api2::vec::Vec and collections from other crates that support it like hashbrown::HashMap.

A bump allocator can grow, shrink and deallocate the most recent allocation. When bumping upwards it can even do so in place. Growing other allocations will require a new allocation and the old memory block becomes wasted space. Shrinking or deallocating other allocations does nothing which means wasted space.

A bump allocator does not require deallocate or shrink to free memory. After all, memory will be reclaimed when exiting a scope or calling reset. You can wrap a bump allocator in a type that makes deallocate and shrink a no-op using without_dealloc and without_shrink.

use bump_scope::Bump;
use allocator_api2::boxed::Box;
let bump: Bump = Bump::new();

let boxed = Box::new_in(5, &bump);
assert_eq!(bump.stats().allocated(), 4);
drop(boxed);
assert_eq!(bump.stats().allocated(), 0);

let boxed = Box::new_in(5, bump.without_dealloc());
assert_eq!(bump.stats().allocated(), 4);
drop(boxed);
assert_eq!(bump.stats().allocated(), 4);

Feature Flags

This crate supports no_std, unless the std feature is enabled.

• std (default):

  Adds implementations of std::io traits for BumpBox and {Fixed, Mut}BumpVec. Activates alloc feature.

• alloc (default):

  Adds implementations interacting with String and Cow<str>.

• serde:

  Adds Serialize implementations for BumpBox, strings and vectors. Adds DeserializeSeed implementations for strings and vectors.

• nightly-allocator-api (requires nightly):

  Enables allocator-api2's nightly feature which makes it reexport the nightly allocator api instead of its own implementation. With this you can bump allocate collections from the standard library.

• nightly-coerce-unsized (requires nightly):

  Makes BumpBox<T> implement CoerceUnsized. With this BumpBox<[i32;3]> coerces to BumpBox<[i32]>, BumpBox<dyn Debug> and so on.

• nightly-const-refs-to-static (requires nightly):

  Makes Bump::unallocated a const fn.

Bumping upwards or downwards?

Bump direction is controlled by the generic parameter const UP: bool. By default, UP is true, so the allocator bumps upwards.

• Bumping upwards...
  • has the advantage that the most recent allocation can be grown and shrunk in place.
  • makes alloc_iter(_mut) and alloc_fmt(_mut) faster.
• Bumping downwards...
  • uses slightly fewer instructions per allocation.
  • makes alloc_iter_mut_rev faster.

Minimum alignment?

The minimum alignment is controlled by the generic parameter const MIN_ALIGN: usize. By default, MIN_ALIGN is 1.

Changing the minimum alignment to e.g. 4 makes it so allocations with the alignment of 4 don't need to align the bump pointer anymore. This will penalize allocations of a smaller alignment as their size now needs to be rounded up the next multiple of 4.

This amounts to about 1 or 2 instructions per allocation.

GUARANTEED_ALLOCATED parameter?

When GUARANTEED_ALLOCATED is true, the bump allocator is in a guaranteed allocated state. That means that it must have already allocated a chunk from its backing allocator.

When GUARANTEED_ALLOCATED is false, the bump allocator may or may not have allocated chunks. You can create a bump allocator with no allocated chunks with Bump::unallocated.

You need a guaranteed allocated Bump(Scope) to create scopes via scoped and scope_guard. You can convert a maybe unallocated Bump(Scope) into a guaranteed allocated one with into_guaranteed_allocated or as_guaranteed_allocated(_mut).

The point of this is so Bumps can be created without allocating memory and even const constructed when the feature nightly-const-refs-to-static is enabled. At the same time Bump's that have already allocated a chunk don't suffer runtime checks for entering scopes and creating checkpoints.

License

Licensed under either of:

• MIT license (LICENSE-MIT or https://opensource.org/licenses/MIT)
• Apache License, Version 2.0, (LICENSE-APACHE or https://www.apache.org/licenses/LICENSE-2.0)

at your option.

Your contributions

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.