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_allocwon't bloat your.wasmdownload size on the Web. -
WebAssembly enabled: Designed for the
wasm32-unknown-unknowntarget 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. This enables testing wee_alloc, and code
using wee_alloc, without a browser or WebAssembly engine.
⚠ Custom allocators currently require Nightly Rust. ⚠
- Using
wee_allocas the Global Allocator cargoFeatures- Implementation Notes and Constraints
- License
- Contribution
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.
// We aren't using the standard library.
#![no_std]
// Required to replace the global allocator.
#![feature(global_allocator)]
// Required to use the `alloc` crate and its types, the `abort` intrinsic, and a
// custom panic handler.
#![feature(alloc, core_intrinsics, lang_items)]
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_fmt` lang-item implementation for `#![no_std]`.
// This implementation will translate panics into traps in the resulting
// WebAssembly.
#[lang = "panic_fmt"]
extern "C" fn panic_fmt(
_args: ::core::fmt::Arguments,
_file: &'static str,
_line: u32
) -> ! {
use core::intrinsics;
unsafe {
intrinsics::abort();
}
}
// And now you can use `alloc` types!
use alloc::arc::Arc;
use alloc::boxed::Box;
use alloc::vec::Vec;
// etc...// 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;-
size_classes: On by default. Use size classes for smaller allocations to provide amortized O(1) allocation for them. Increases uncompressed
.wasmcode 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_allocitself.
-
wee_allocimposes two words of overhead on each allocation for maintaining its internal free lists. -
The maximum alignment supported is word alignment.
-
Deallocation is an O(1) operation.
-
wee_allocwill 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 inwee_alloc's internal free lists for potential future allocations, even when running on unix targets. -
wee_allocuses a simple, first-fit free list implementation. This means that allocation is an O(n) operation.Using the
size_classesfeature 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 |
+------------------------------------------------------------------------------+
Licensed under the Mozilla Public License 2.0.
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See CONTRIBUTING.md for hacking!