Crate bump_scope

Expand description

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, this allocator 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 (but ignores the title).

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 allocate a slice from any Iterator with alloc_iter.
Every method that panics on allocation failure has a fallible try_* counterpart.
Bump’s base allocator is generic.
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 bump-scope-inspect-asm/out/x86-64 and benchmarks)
You can choose the bump direction. Bumps upwards by default.
You can choose the minimum alignment. 1 by default.

§Allocator Methods

The bump allocator provides many methods to conveniently allocate values, strings and slices. Have a look at the documentation of Bump for a method overview.

§Scopes and Checkpoints

You can create scopes to make allocations that live only for a part of its parent scope. Entering 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 mut bump: Bump = Bump::new();
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);

§Collections

bump-scope provides bump allocated variants of Vec and String called BumpVec and BumpString. They are also available in the following variants:

Fixed* for fixed capacity collections
Mut* for collections optimized for a mutable bump allocator

§Api changes

The collections are designed to have a similar api to their std counterparts but they do make some breaking changes:

split_off — splits the collection in place without allocation; the parameter is a range instead of a single index
retain — takes a closure with a &mut T parameter like Vec::retain_mut

§New features

append — allows appending all kinds of owned slice types like [T; N], Box<[T]>, Vec<T>, Drain<T> etc
map — maps the elements, potentially reusing the existing allocation
map_in_place — maps the elements without allocation
conversions between the regular collections, their Fixed* variants and BumpBox<[T]> / BumpBox<str>

§Parallel Allocation

Bump is !Sync which means it can’t be shared between threads.

To bump allocate in parallel you can use a BumpPool.

§Allocator API

Bump and BumpScope implement allocator_api2’s Allocator trait. They can be used to allocate collections.

A bump allocator can grow, shrink and deallocate the most recent allocation. When bumping upwards it can even do so in place. Growing allocations other than the most recent one will require a new allocation and the old memory block becomes wasted space. Shrinking or deallocating allocations other than the most recent one 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, calling reset or dropping the Bump. You can wrap a bump allocator in a type that makes deallocate and shrink a no-op using WithoutDealloc and WithoutShrink.

use bump_scope::{ Bump, WithoutDealloc };
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, WithoutDealloc(&bump));
assert_eq!(bump.stats().allocated(), 4);
drop(boxed);
assert_eq!(bump.stats().allocated(), 4);

§Feature Flags

std (enabled by default) — Adds BumpPool and implementations of std::io traits.
alloc (enabled by default) — Adds Global as the default base allocator and some interactions with alloc collections.
panic-on-alloc (enabled by default) — Adds functions and traits that will panic when the allocation fails. Without this feature, allocation failures cannot cause panics, and only try_-prefixed allocation methods will be available.
serde — Adds Serialize implementations for BumpBox, strings and vectors, and DeserializeSeed for strings and vectors.
zerocopy — Adds alloc_zeroed(_slice), init_zeroed, resize_zeroed and extend_zeroed.

§Nightly features

nightly-allocator-api — 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 — Makes BumpBox<T> implement CoerceUnsized. With this BumpBox<[i32;3]> coerces to BumpBox<[i32]>, BumpBox<dyn Debug> and so on. You can unsize a BumpBox in stable without this feature using unsize_bump_box.
nightly-exact-size-is-empty — Implements is_empty manually for some iterators.
nightly-trusted-len — Implements TrustedLen for some iterators.
nightly-fn-traits — Implements Fn* traits for BumpBox<T>. Makes BumpBox<T: FnOnce + ?Sized> callable. Requires alloc crate.

§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. This benefits collections as well as alloc_iter(_mut) and alloc_fmt(_mut) with the exception of MutBumpVecRev and alloc_iter_mut_rev. MutBumpVecRev can be grown and shrunk in place iff bumping downwards.

Bumping downwards shaves off a few non-branch instructions per allocation.

§Minimum alignment?

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

For example changing the minimum alignment to 4 makes it so allocations with the alignment of 4 don’t need to align the bump pointer anymore. This will penalize allocations whose sizes are not a multiple of 4 as their size now needs to be rounded up the next multiple of 4.

The overhead of aligning and rounding up is 1 (UP = false) or 2 (UP = true) non-branch instructions on x86-64.

§`GUARANTEED_ALLOCATED` parameter?

If GUARANTEED_ALLOCATED is true then the bump allocator is guaranteed to have at least one allocated chunk. This is usually the case unless it was created with Bump::unallocated.

You need a guaranteed allocated Bump(Scope) to create scopes via scoped and scope_guard. You can make a Bump(Scope) guaranteed allocated using guaranteed_allocated(_ref/_mut).

The point of this is so Bumps can be created without allocating memory and even const constructed since rust version 1.83. At the same time Bumps that have already allocated a chunk don’t suffer runtime checks for entering scopes and creating checkpoints.

Re-exports§

pub use allocator_api2;

Modules§

bump_vec: Contains BumpVec and associated types.
mut_bump_vec: Contains MutBumpVec and associated types.
owned_slice: Contains types associated with owned slices.
owned_str: Contains types associated with owned strings.
stats: Types that provide statistics about the memory usage of the bump allocator.

Macros§

bump_format: This is like format! but allocates inside a bump allocator, returning a BumpString.
bump_vec: This is like vec! but allocates inside a bump allocator, returning a BumpVec.
mut_bump_format: This is like format! but allocates inside a mutable bump allocator, returning a MutBumpString.
mut_bump_vec: This is like vec! but allocates inside a bump allocator, returning a MutBumpVec.
mut_bump_vec_rev: This is like vec! but allocates inside a bump allocator, returning a MutBumpVecRev.
unsize_bump_box: Allows unsizing to be performed on T of BumpBox<T>.

Structs§

Bump: The bump allocator.
BumpBox: A pointer type that uniquely owns a bump allocation of type T. This type is returned whenever a bump allocation is made.
BumpPoolstd: A pool of bump allocators.
BumpPoolGuardstd: This is a wrapper around Bump that mutably derefs to a BumpScope and returns its Bump back to the BumpPool on drop.
BumpScope: A bump allocation scope.
BumpScopeGuard: Returned from BumpScope::scope_guard.
BumpScopeGuardRoot: Returned from Bump::scope_guard.
BumpString: A bump allocated String.
BumpVec: A bump allocated Vec.
Checkpoint: This is returned from checkpoint and used for reset_to.
FixedBumpString: A type like BumpString but with a fixed capacity.
FixedBumpVec: A type like BumpVec but with a fixed capacity.
FromUtf8Error: A possible error value when converting a string from a UTF-8 byte vector.
FromUtf16Error: A possible error value when converting a string from a UTF-16 byte slice.
MinimumAlignment: Specifies the current minimum alignment of a bump allocator.
MutBumpString: A type like BumpString, optimized for a mutable bump allocator.
MutBumpVec: A type like BumpVec, optimized for a mutable bump allocator.
MutBumpVecRev: A type like MutBumpVec but new elements are pushed to the front.
Stats: Provides statistics about the memory usage of the bump allocator.
WithoutDealloc: Wraps an bump allocator and does nothing on deallocate.
WithoutShrink: Wraps an bump allocator and does nothing on shrink.

Traits§

BaseAllocator: Trait that any allocator used as a base allocator of a bump allocator needs to implement.
BumpAllocator: A bump allocator.
BumpAllocatorScope: An allocator that makes allocations with a lifetime of 'a.
MutBumpAllocator: A BumpAllocator who has exclusive access to allocation.
MutBumpAllocatorScope: Shorthand for MutBumpAllocator + BumpAllocatorScope<’a>
NoDrop: This trait marks types that don’t need dropping.
SupportedMinimumAlignment: Statically guarantees that a minimum alignment is marked as supported.

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