Rust wasm32-unknown-unknown Math functions not linking

Question

I'm trying the new wasm32-unknown-unknown target for Rust, and I'm experiencing issues invoking math functions (e.g. sin, cos, exp, atan2).

Cargo.toml:

[package]
name = "wasm_math"
version = "0.1.0"
authors = ["..."]

[lib]
path = "src/lib.rs"
crate-type = ["cdylib"]

[dependencies]

src/lib.rs:

#[no_mangle]
pub extern "C" fn invoke_sin(x: f64) -> f64 {
    x.sin()
}

index.html:

<!doctype html>
<html lang="en">
  <head><meta charset="utf-8"><title>Wasm Math</title></head>
  <body></body>
  <script>
    const imports = { env: { } };
    fetch("target/wasm32-unknown-unknown/release/wasm_math.wasm").then(response =>
      response.arrayBuffer()
    ).then(bytes =>
      WebAssembly.instantiate(bytes, imports)
    ).then(results => {
      alert(results.instance.exports.invoke_sin(1.0));
    });
  </script>
</html>

I build the project with the command

cargo build --release --target wasm32-unknown-unknown

When I open the html file in firefox, I get the following error:

LinkError: import object field 'sin' is not a Function

Is there something wrong with my setup? Or is this a shortcoming in Rust/WebAssembly/Firefox? I could manually add the sin function to the imports.env object in the javascript, but that seems very hacky and I would have to do that for every math function that I use. Is there a better way?

I'm using the nightly Rust toolchain (nightly-x86_64-unknown-linux-gnu rustc 1.24.0-nightly (cddc4a62d 2017-12-26)) and Firefox 57.0.1 (64-bit).

Kyle Kyle · Accepted Answer · 2017-12-27T21:35:32

According to the WASM FAQ sin isn't included.

•WebAssembly doesn’t include its own math functions like sin , cos , exp , pow , and so on. WebAssembly’s strategy for such functions is to allow them to be implemented as library routines in WebAssembly itself (note that x86’s sin and cos instructions are slow and imprecise and are generally avoided these days anyway). Users wishing to use faster and less precise math functions on WebAssembly can simply select a math library implementation which does so.

Rust appears to rely on LLVM to provide sin (f64 impl here) which it does not do for WASM. I would think LLVM should provide it as part of their llvm.sin.f64 intrinsic, but it appears that they don't guarantee implementation for every target per https://llvm.org/docs/LangRef.html#llvm-sin-intrinsic (emphasis mine):

This is an overloaded intrinsic. You can use llvm.sin on any floating-point or vector of floating-point type. Not all targets support all types however.

Maybe, given this, Rust should consider implementing sin itself.

Rust wasm32-unknown-unknown Math functions not linking

2 Answers