lol_alloc

A laughably simple wasm global_allocator.

Like wee_alloc, but smaller since I used skinnier letters in the name.

lol_alloc is a experimental wasm global_allocator.

I wrote lol_alloc to learn about allocators (I hadn't written one before) and because wee_alloc seems unmaintained and has a leak. After looking at wee_alloc's implementation (which I failed to understand or fix), I wanted to find out how hard it really is to make a wasm global_allocator, and it seemed like providing one could be useful to the rust wasm community.

Soundness

Use of Pointers

Soundness of the pointer manipulation in this library is currently unclear. Since wasm32::memory_grow does not return a pointer there is no "original pointer" so the Strict Provenance rules can not be followed. Attempting to determine if this library's use of pointes at least meets the requirements for being dereferenceable when it dereferences them is similarly challenging as that is defined as:

dereferenceable: the memory range of the given size starting at the pointer must all be within the bounds of a single allocated object.

The definition of "allocated object" is not clear here. If the growable wasm heap counts as a single allocated object, then all these allocators are likely ok (in this aspect at least). However if each call to wasm32::memory_grow is considered to create a new allocated object, then the free list coalescing in FreeListAllocator in unsound and could result in undefined behavior.

Thread Safety

LeakingAllocator and FreeListAllocator are not thread-safe and use unsound implementations of Sync to allow their use as the global allocator.

Multithreading is possible in wasm these days: applications have multiple threads should not use these allocators.

Making thread safe versions of them is possible, but none of the allocators in this library are intended for high performance: if for some reason you have multithreaded wasm, and don't care about performance, it should be possible to modify these allocators to meet your needs, but consider just using Rust's built in allocator instead (it is thread safe, and fast).

Status

Not production ready.

Current a few allocators are provided with minimal testing. If you use it, please report any bugs. If it actually works for you, also let me know (you can post an issue with your report).

Currently only FailAllocator and LeakingPageAllocator are thread-safe safe.

Sizes of allocators include overhead from example:

• FailAllocator: 195 bytes: errors on allocations. Operations are O(1),
• LeakingPageAllocator: 230 bytes: Allocates pages for each allocation. Operations are O(1).
• LeakingAllocator: 356 bytes: Bump pointer allocator, growing the heap as needed and does not reuse/free memory. Operations are O(1).
• FreeListAllocator: 656 bytes: Free list based allocator. Operations (both allocation and freeing) are O(length of free list), but it does coalesce adjacent free list nodes.

LeakingAllocator should have no allocating space overhead other than for alignment.

FreeListAllocator rounds allocations up to at least 2 words in size, but otherwise should use all the space. Even gaps from high alignment allocations end up in its free list for use by smaller allocations.

Builtin rust allocator is 5053 bytes in this same test. If you can afford the extra code size, use it: its a much better allocator.

Supports only wasm32: other targets may build, but the allocators will not work on them (except: FailAllocator, it errors on all platforms just fine).

Usage

You can replace the global_allocator in wasm32 with FreeListAllocator builds using:

#[cfg(target_arch = "wasm32")]
use lol_alloc::FreeListAllocator;

#[cfg(target_arch = "wasm32")]
#[global_allocator]
static ALLOCATOR: FreeListAllocator = FreeListAllocator::new();

Testing

There are some normal rust unit tests (run with cargo run test), which use a test implementation of MemoryGrower.

There are also some wasm-pack tests (run with wasm-pack test --node lol_alloc)

Size testing:

wasm-pack build --release example && ls -l example/pkg/lol_alloc_example_bg.wasm