Permalink
Fetching contributors…
Cannot retrieve contributors at this time
225 lines (182 sloc) 10.1 KB

wee_alloc

Travis CI Build Status AppVeyor Build status

wee_alloc: The Wasm-Enabled, Elfin Allocator.

  • Elfin, i.e. small: Generates less than a kilobyte of uncompressed WebAssembly code. Doesn't pull in the heavy panicking or formatting infrastructure. wee_alloc won't bloat your .wasm download size on the Web.

  • WebAssembly enabled: Designed for the wasm32-unknown-unknown target and #![no_std].

wee_alloc is focused on targeting WebAssembly, producing a small .wasm code size, and having a simple, correct implementation. It is geared towards code that makes a handful of initial dynamically sized allocations, and then performs its heavy lifting without any further allocations. This scenario requires some allocator to exist, but we are more than happy to trade allocation performance for small code size. In contrast, wee_alloc would be a poor choice for a scenario where allocation is a performance bottleneck.

Although WebAssembly is the primary target, wee_alloc also has an mmap based implementation for unix systems and a VirtualAlloc implementation for Windows. This enables testing wee_alloc, and code using wee_alloc, without a browser or WebAssembly engine.

⚠ Custom allocators currently require Nightly Rust. ⚠

Using wee_alloc as the Global Allocator

To get the smallest .wasm sizes, you want to use #![no_std] with a custom panicking hook that avoids using any of the core::fmt infrastructure. Nevertheless, wee_alloc is also usable with std.

With #![no_std]

// We aren't using the standard library.
#![no_std]

// Required to use the `alloc` crate and its types, the `abort` intrinsic, and a
// custom panic handler.
#![feature(alloc, core_intrinsics, panic_implementation, lang_items, alloc_error_handler)]

extern crate alloc;
extern crate wee_alloc;

// Use `wee_alloc` as the global allocator.
#[global_allocator]
static ALLOC: wee_alloc::WeeAlloc = wee_alloc::WeeAlloc::INIT;

// Need to provide a tiny `panic` implementation for `#![no_std]`.
// This translates into an `unreachable` instruction that will
// raise a `trap` the WebAssembly execution if we panic at runtime.
#[panic_implementation]
#[no_mangle]
pub fn panic(_info: &::core::panic::PanicInfo) -> ! {
    unsafe {
        ::core::intrinsics::abort();
    }
}

// Need to provide an allocation error handler which just aborts
// the execution with trap.
#[alloc_error_handler]
#[no_mangle]
pub extern "C" fn oom(_: ::core::alloc::Layout) -> ! {
    unsafe {
        ::core::intrinsics::abort();
    }
}

// And now you can use `alloc` types!
use alloc::arc::Arc;
use alloc::boxed::Box;
use alloc::vec::Vec;
// etc...

With std

// Required to replace the global allocator.
#![feature(global_allocator)]

extern crate wee_alloc;

// Use `wee_alloc` as the global allocator.
#[global_allocator]
static ALLOC: wee_alloc::WeeAlloc = wee_alloc::WeeAlloc::INIT;

cargo Features

  • size_classes: On by default. Use size classes for smaller allocations to provide amortized O(1) allocation for them. Increases uncompressed .wasm code size by about 450 bytes (up to a total of ~1.2K).

  • extra_assertions: Enable various extra, expensive integrity assertions and defensive mechanisms, such as poisoning freed memory. This incurs a large runtime overhead. It is useful when debugging a use-after-free or wee_alloc itself.

  • static_array_backend: Force the use of an OS-independent backing implementation with a global maximum size fixed at compile time. Suitable for deploying to non-WASM/Unix/Windows #![no_std] environments, such as on embedded devices with esoteric or effectively absent operating systems. The size defaults to 32 MiB (33554432 bytes), and may be controlled at build-time by supplying an optional environment variable to cargo, WEE_ALLOC_STATIC_ARRAY_BACKEND_BYTES

Implementation Notes and Constraints

  • wee_alloc imposes two words of overhead on each allocation for maintaining its internal free lists.

  • Deallocation is an O(1) operation.

  • wee_alloc will never return freed pages to the WebAssembly engine / operating system. Currently, WebAssembly can only grow its heap, and can never shrink it. All allocated pages are indefinitely kept in wee_alloc's internal free lists for potential future allocations, even when running on unix targets.

  • wee_alloc uses a simple, first-fit free list implementation. This means that allocation is an O(n) operation.

    Using the size_classes feature enables extra free lists dedicated to small allocations (less than or equal to 256 words). The size classes' free lists are populated by allocating large blocks from the main free list, providing amortized O(1) allocation time. Allocating from the size classes' free lists uses the same first-fit routines that allocating from the main free list does, which avoids introducing more code bloat than necessary.

Finally, here is a diagram giving an overview of wee_alloc's implementation:

+------------------------------------------------------------------------------+
| WebAssembly Engine / Operating System                                        |
+------------------------------------------------------------------------------+
                   |
                   |
                   | 64KiB Pages
                   |
                   V
+------------------------------------------------------------------------------+
| Main Free List                                                               |
|                                                                              |
|          +------+     +------+     +------+     +------+                     |
| Head --> | Cell | --> | Cell | --> | Cell | --> | Cell | --> ...             |
|          +------+     +------+     +------+     +------+                     |
|                                                                              |
+------------------------------------------------------------------------------+
                   |                                    |            ^
                   |                                    |            |
                   | Large Blocks                       |            |
                   |                                    |            |
                   V                                    |            |
+---------------------------------------------+         |            |
| Size Classes                                |         |            |
|                                             |         |            |
|             +------+     +------+           |         |            |
| Head(1) --> | Cell | --> | Cell | --> ...   |         |            |
|             +------+     +------+           |         |            |
|                                             |         |            |
|             +------+     +------+           |         |            |
| Head(2) --> | Cell | --> | Cell | --> ...   |         |            |
|             +------+     +------+           |         |            |
|                                             |         |            |
| ...                                         |         |            |
|                                             |         |            |
|               +------+     +------+         |         |            |
| Head(256) --> | Cell | --> | Cell | --> ... |         |            |
|               +------+     +------+         |         |            |
|                                             |         |            |
+---------------------------------------------+         |            |
                      |            ^                    |            |
                      |            |                    |            |
          Small       |      Small |        Large       |      Large |
          Allocations |      Frees |        Allocations |      Frees |
                      |            |                    |            |
                      |            |                    |            |
                      |            |                    |            |
                      |            |                    |            |
                      |            |                    |            |
                      V            |                    V            |
+------------------------------------------------------------------------------+
| User Application                                                             |
+------------------------------------------------------------------------------+

License

Licensed under the Mozilla Public License 2.0.

TL;DR?

Permissions of this weak copyleft license are conditioned on making available source code of licensed files and modifications of those files under the same license (or in certain cases, one of the GNU licenses). Copyright and license notices must be preserved. Contributors provide an express grant of patent rights. However, a larger work using the licensed work may be distributed under different terms and without source code for files added in the larger work.

Contribution

See CONTRIBUTING.md for hacking!