fixed-bump

A bump allocator (like bumpalo) that internally uses fixed-size chunks of memory.

Other bump allocators, like bumpalo, are optimized for throughput: they allocate chunks of memory with exponentially increasing sizes, which results in amortized constant-time allocations.

fixed-bump is optimized for latency: it internally allocates chunks of memory with a fixed, configurable size, and individual value allocations are performed in non-amortized constant time. However, a trade-off with using this crate is that it may not be able to allocate certain types or memory layouts if the specified chunk size or alignment is too small. See Bump::allocate for the conditions under which allocation may fail.

This crate depends only on core and alloc, so it can be used in no_std environments that support alloc.

Example

# #![cfg_attr(feature = "allocator_api", feature(allocator_api))]
use fixed_bump::Bump;
struct Item(u64);

// Use chunks large and aligned enough to hold 128 `Item`s.
let bump = Bump::<[Item; 128]>::new();
let item1: &mut Item = bump.alloc_value(Item(1));
let item2: &mut Item = bump.alloc_value(Item(2));
item1.0 += item2.0;

assert_eq!(item1.0, 3);
assert_eq!(item2.0, 2);

// Can also allocate different types:
let array: &mut [u8; 8] = bump.alloc_value([0, 1, 2, 3, 4, 5, 6, 7]);
assert_eq!(array.iter().sum::<u8>(), 28);

// Can also use `&Bump` as an `Allocator` (requires "allocator_api"):
// To avoid resizing, we create these `Vec`s with the maximum capacity
// we want them ever to have. Resizing would waste memory, since bump
// allocators don't reclaim or reuse memory until the entire allocator
// is dropped.
let mut vec1: Vec<u32, _> = Vec::with_capacity_in(8, &bump);
let mut vec2: Vec<u32, _> = Vec::with_capacity_in(4, &bump);
for i in 0..4 {
    vec1.push(i * 2);
    vec1.push(i * 2 + 1);
    vec2.push(i * 2);
}

assert_eq!(vec1, [0, 1, 2, 3, 4, 5, 6, 7]);
assert_eq!(vec2, [0, 2, 4, 6]);

Dropping

Bump can either return raw memory (see Bump::allocate) or allocate a value of a specific type and return a reference (see Bump::alloc_value and Bump::try_alloc_value). In the latter case where references are returned, note that destructors will not be automatically run. If this is an issue, you can do one of the following:

• Drop those values manually with ptr::drop_in_place.
• Enable the allocator_api feature, which lets you use bump allocators with various data structures like Box and Vec. Note that this requires Rust nightly.

Note that, as with other bump allocators, the memory used by an allocated object will not be reclaimed or reused until the entire bump allocator is dropped.

Crate features

If the crate feature allocator_api is enabled, the unstable Allocator trait will be implemented for T, &T, and crate::Rc<T>, where T is Bump or DynamicBump. This lets you use those types as allocators for various data structures like Box and Vec. Note that this feature requires Rust nightly. Alternatively, if the feature allocator-fallback is enabled, this crate will use the allocator API provided by allocator-fallback instead of the standard library’s.