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# Objective This gets Bevy building on Wasm when the `atomics` flag is enabled. This does not yet multithread Bevy itself, but it allows Bevy users to use a crate like `wasm_thread` to spawn their own threads and manually parallelize work. This is a first step towards resolving #4078 . Also fixes #9304. This provides a foothold so that Bevy contributors can begin to think about multithreaded Wasm's constraints and Bevy can work towards changes to get the engine itself multithreaded. Some flags need to be set on the Rust compiler when compiling for Wasm multithreading. Here's what my build script looks like, with the correct flags set, to test out Bevy examples on web: ```bash set -e RUSTFLAGS='-C target-feature=+atomics,+bulk-memory,+mutable-globals' \ cargo build --example breakout --target wasm32-unknown-unknown -Z build-std=std,panic_abort --release wasm-bindgen --out-name wasm_example \ --out-dir examples/wasm/target \ --target web target/wasm32-unknown-unknown/release/examples/breakout.wasm devserver --header Cross-Origin-Opener-Policy='same-origin' --header Cross-Origin-Embedder-Policy='require-corp' --path examples/wasm ``` A few notes: 1. `cpal` crashes immediately when the `atomics` flag is set. That is patched in https://github.com/RustAudio/cpal/pull/837, but not yet in the latest crates.io release. That can be temporarily worked around by patching Cpal like so: ```toml [patch.crates-io] cpal = { git = "https://github.com/RustAudio/cpal" } ``` 2. When testing out `wasm_thread` you need to enable the `es_modules` feature. ## Solution The largest obstacle to compiling Bevy with `atomics` on web is that `wgpu` types are _not_ Send and Sync. Longer term Bevy will need an approach to handle that, but in the near term Bevy is already configured to be single-threaded on web. Therefor it is enough to wrap `wgpu` types in a `send_wrapper::SendWrapper` that _is_ Send / Sync, but panics if accessed off the `wgpu` thread. --- ## Changelog - `wgpu` types that are not `Send` are wrapped in `send_wrapper::SendWrapper` on Wasm + 'atomics' - CommandBuffers are not generated in parallel on Wasm + 'atomics' ## Questions - Bevy should probably add CI checks to make sure this doesn't regress. Should that go in this PR or a separate PR? **Edit:** Added checks to build Wasm with atomics --------- Co-authored-by: François <mockersf@gmail.com> Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com> Co-authored-by: daxpedda <daxpedda@gmail.com> Co-authored-by: François <francois.mockers@vleue.com> |
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