Struct static_alloc::unsync::Bump

#[repr(C)]
pub struct Bump<T> { /* private fields */ }

A bump allocator whose storage capacity and alignment is given by T.

This type dereferences to the generic MemBump that implements the allocation behavior. Note that MemBump is an unsized type. In contrast this type is sized so it is possible to construct an instance on the stack or leak one from another bump allocator such as a global one.

Usage

For on-stack usage this works the same as Bump. Note that it is not possible to use as a global allocator though.

One interesting use case for this struct is as scratch space for subroutines. This ensures good locality and cache usage. It can also allows such subroutines to use a dynamic amount of space without the need to actually allocate. Contrary to other methods where the caller provides some preallocated memory it will also not ‘leak’ private data types. This could be used in handling web requests.

use static_alloc::unsync::Bump;

let mut stack_buffer: Bump<[usize; 64]> = Bump::uninit();
subroutine_one(&stack_buffer);
stack_buffer.reset();
subroutine_two(&stack_buffer);

Note that you need not use the stack for the Bump itself. Indeed, you could allocate a large contiguous instance from the global (synchronized) allocator and then do subsequent allocations from the Bump you’ve obtained. This avoids potential contention on a lock of the global allocator, especially in case you must do many small allocations. If you’re writing an allocator yourself you might use this technique as an internal optimization.

use static_alloc::unsync::{Bump, MemBump};
let mut local_page: Box<Bump<[u64; 64]>> = Box::new(Bump::uninit());

for request in iterate_recv() {
    local_page.reset();
    handle_request(&local_page, request);
}

Implementations

impl<T> Bump<T>

pub fn uninit() -> Self

Create an allocator with uninitialized memory.

All allocations coming from the allocator will need to be initialized manually.

pub fn zeroed() -> Self

Create an allocator with zeroed memory.

The caller can rely on all allocations to be zeroed.

Methods from Deref<Target = MemBump>

pub fn capacity(&self) -> usize

Returns capacity of this MemBump. This is how many bytes can be allocated within this node.

pub fn data_ptr(&self) -> NonNull<u8>

Get a raw pointer to the data.

Note that any use of the pointer must be done with extreme care as it may invalidate existing references into the allocated region. Furthermore, bytes may not be initialized. The length of the valid region is MemBump::capacity.

Prefer MemBump::get_unchecked for reconstructing a prior allocation.

pub fn alloc(&self, layout: Layout) -> Option<NonNull<u8>>

Allocate a region of memory.

This is a safe alternative to GlobalAlloc::alloc.

Panics

This function will panic if the requested layout has a size of 0. For the use in a GlobalAlloc this is explicitely forbidden to request and would allow any behaviour but we instead strictly check it.

FIXME(breaking): this could well be a Result<_, Failure>.

pub fn alloc_at( &self, layout: Layout, level: Level ) -> Result<NonNull<u8>, Failure>

Try to allocate some layout with a precise base location.

The base location is the currently consumed byte count, without correction for the alignment of the allocation. This will succeed if it can be allocate exactly at the expected location.

Panics

This function may panic if the provided level is from a different slab.

pub fn get<V>(&self) -> Option<Allocation<'_, V>>

Get an allocation for a specific type.

It is not yet initialized but provides an interface for that initialization.

Usage

use core::cell::{Ref, RefCell};

let slab: Bump<[Ref<'static, usize>; 1]> = Bump::uninit();
let data = RefCell::new(0xff);

// We can place a `Ref` here but we did not yet.
let alloc = slab.get::<Ref<usize>>().unwrap();
let cell_ref = unsafe {
    alloc.leak(data.borrow())
};

assert_eq!(**cell_ref, 0xff);

FIXME(breaking): this could well be a Result<_, Failure>.

pub fn get_at<V>(&self, level: Level) -> Result<Allocation<'_, V>, Failure>

Get an allocation for a specific type at a specific level.

See get for usage. This can be used to ensure that data is contiguous in concurrent access to the allocator.

pub unsafe fn get_unchecked<V>(&self, level: Level) -> Allocation<'_, V>

Reacquire an allocation that has been performed previously.

This call won’t invalidate any other allocations.

Safety

The caller must guarantee that no other pointers to this prior allocation are alive, or can be created. This is guaranteed if the allocation was performed previously, has since been discarded, and reset can not be called (for example, the caller holds a shared reference).

Usage

// Create an initial allocation.
let level = alloc.level();
let allocation = alloc.get_at::<usize>(level)?;
let address = allocation.ptr.as_ptr() as usize;
// pretend to lose the owning pointer of the allocation.
let _ = { allocation };

// Restore our access.
let renewed = unsafe { alloc.get_unchecked::<usize>(level) };
assert_eq!(address, renewed.ptr.as_ptr() as usize);

Critically, you can rely on other allocations to stay valid.

let level = alloc.level();
alloc.get_at::<usize>(level)?;

let other_val = alloc.bump_box()?;
let other_val = LeakBox::write(other_val, 0usize);

let renew = unsafe { alloc.get_unchecked::<usize>(level) };
assert_eq!(*other_val, 0); // Not UB!

pub fn bump_box<'bump, T: 'bump>( &'bump self ) -> Result<LeakBox<'bump, MaybeUninit<T>>, Failure>

Allocate space for one T without initializing it.

Note that the returned MaybeUninit can be unwrapped from LeakBox. Or you can store an arbitrary value and ensure it is safely dropped before the borrow ends.

Usage

use core::cell::RefCell;
use static_alloc::leaked::LeakBox;

let slab: Bump<[usize; 4]> = Bump::uninit();
let data = RefCell::new(0xff);

let slot = slab.bump_box().unwrap();
let cell_box = LeakBox::write(slot, data.borrow());

assert_eq!(**cell_box, 0xff);
drop(cell_box);

assert!(data.try_borrow_mut().is_ok());

FIXME(breaking): should return evidence of the level (observed, and post). Something similar to Allocation but containing a LeakBox<T> instead? Introduce that to the sync Bump allocator as well.

FIXME(breaking): align with sync Bump::get (probably rename get to bump_box).

pub fn bump_array<'bump, T: 'bump>( &'bump self, n: usize ) -> Result<LeakBox<'bump, [MaybeUninit<T>]>, Failure>

Allocate space for a slice of Ts without initializing any.

Retrieve individual MaybeUninit elements and wrap them as a LeakBox to store values. Or use the slice as backing memory for one of the containers from without-alloc. Or manually initialize them.

Usage

Quicksort, implemented recursively, requires a maximum of log n stack frames in the worst case when implemented optimally. Since each frame is quite large this is wasteful. We can use a properly sized buffer instead and implement an iterative solution. (Left as an exercise to the reader, or see the examples for without-alloc where we use such a dynamic allocation with an inline vector as our stack).

pub fn level(&self) -> Level

Get the number of already allocated bytes.

pub fn reset(&mut self)

Reset the bump allocator.

This requires a unique reference to the allocator hence no allocation can be alive at this point. It will reset the internal count of used bytes to zero.

Trait Implementations

impl<T> Deref for Bump<T>

type Target = MemBump

The resulting type after dereferencing.

fn deref(&self) -> &MemBump

Dereferences the value.

impl<T> DerefMut for Bump<T>

fn deref_mut(&mut self) -> &mut MemBump

Mutably dereferences the value.