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| -rw-r--r-- | src/doc/rustc/src/platform-support/wasm32-unknown-unknown.md | 33 |
1 files changed, 33 insertions, 0 deletions
diff --git a/src/doc/rustc/src/platform-support/wasm32-unknown-unknown.md b/src/doc/rustc/src/platform-support/wasm32-unknown-unknown.md index 85915cf90a9..2bd50f6e6db 100644 --- a/src/doc/rustc/src/platform-support/wasm32-unknown-unknown.md +++ b/src/doc/rustc/src/platform-support/wasm32-unknown-unknown.md @@ -155,3 +155,36 @@ You'll need to consult your WebAssembly engine's documentation to learn more about the supported WebAssembly features the engine has. [reference]: https://doc.rust-lang.org/reference/attributes/codegen.html#wasm32-or-wasm64 + +Note that it is still possible for Rust crates and libraries to enable +WebAssembly features on a per-function level. This means that the build +command above may not be sufficent to disable all WebAssembly features. If the +final binary still has SIMD instructions, for example, the function in question +will need to be found and the crate in question will likely contain something +like: + +```rust +#[target_feature(enable = "simd128")] +fn foo() { + // ... +} +``` + +In this situation there is no compiler flag to disable emission of SIMD +instructions and the crate must instead be modified to not include this function +at compile time either by default or through a Cargo feature. For crate authors +it's recommended to avoid `#[target_feature(enable = "...")]` except where +necessary and instead use: + +```rust +#[cfg(target_feature = "simd128")] +fn foo() { + // ... +} +``` + +That is to say instead of enabling target features it's recommended to +conditionally compile code instead. This is notably different to the way native +platforms such as x86\_64 work, and this is due to the fact that WebAssembly +binaries must only contain code the engine understands. Native binaries work so +long as the CPU doesn't execute unknown code dynamically at runtime. |