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# wee_alloc |
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1. [What is `wee_alloc`?](#what-is-wee_alloc) |
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2. [Enabling `wee_alloc`](#enabling-wee_alloc) |
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3. [Rust nightly](#rust-nightly) |
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## What is `wee_alloc`? |
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Reducing the size of compiled WebAssembly code is important, since it is often transmitted over the Internet or placed on embedded devices. |
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> `wee_alloc` is a tiny allocator designed for WebAssembly that has a (pre-compression) code-size footprint of only a single kilobyte. |
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[An analysis](http://fitzgeraldnick.com/2018/02/09/wee-alloc.html) suggests that over half of the bare minimum WebAssembly memory footprint is required by Rust's default memory allocator. Yet, WebAssembly code often does not require a sophisticated allocator, since it often just requests a couple of large initial allocations. |
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`wee_alloc` trades off size for speed. Although it has a tiny code-size footprint, it is relatively slow if additional allocations are needed. |
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For even more details, see the [`wee_alloc` repository](https://github.com/rustwasm/wee_alloc). |
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## Enabling `wee_alloc` |
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In `lib.rs`, we have the configuration for `wee_alloc` inside a `cfg_if!` macro: |
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```rust |
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cfg_if! { |
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if #[cfg(feature = "wee_alloc")] { |
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extern crate wee_alloc; |
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#[global_allocator] |
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static ALLOC: wee_alloc::WeeAlloc = wee_alloc::WeeAlloc::INIT; |
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} |
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} |
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``` |
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This code block is intended to initialize `wee_alloc` as the global memory allocator, but only if the `wee_alloc` feature is enabled in `Cargo.toml`. |
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To do so we need to append `"wee_alloc"` to the `default` vector in `Cargo.toml`. Then, the `cfg_if!` block is replaced with the contents of the `if` block, shown above. |
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```toml |
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[features] |
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default = ["console_error_panic_hook", "wee_alloc"] |
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``` |
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## Rust nightly |
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`wee_alloc` currently relies on features only available in Rust nightly. As such it requires you to use the nightly toolchain for compilation. If you have [Rustup](https://rustup.rs/) set up, you can install the nightly toolchain as follows: |
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``` |
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rustup toolchain add nightly |
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``` |
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To use `wasm-pack` with Rust nightly run: |
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``` |
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rustup run nightly wasm-pack build |
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``` |
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