Vertex shaders with derivative functions in dead code fail to compile on WebGL

[johanhelsing]

Description

If a wgsl shader contains a function with fwidth, dpdx or dpdy in it and that shader file is used as both a vertex and a fragment shader (just with different entry points), the shader fails in validation on WebGL even if the function is not used from the vertex shader.

i.e.

[[stage(vertex)]]
fn vertex(vertex: Vertex) -> VertexOutput {
    var out: VertexOutput;
    out.clip_position = view.view_proj * mesh.model * vec4<f32>(vertex.position, 1.0);
    out.color = vertex.color;
    return out;
}
struct FragmentInput {
    [[location(0)]] color: vec4<f32>;
};
// compiler errors when vertex shader compiles
fn fwidth_wrapper(color: vec3<f32>) -> vec3<f32> {
    return fwidth(color.rgb);
}
[[stage(fragment)]]
fn fragment(in: FragmentInput) -> [[location(0)]] vec4<f32> {
    // return vec4<f32>(fwidth(in.color.rgb), in.color.a); // <-- this works
    return vec4<f32>(fwidth_wrapper(in.color.rgb), in.color.a);
}

Repro steps

0. Install wasm toolchain
1. Check out https://github.com/johanhelsing/bevy/tree/wasm-fwidth-repro
2. cargo install wasm-server-runner
3. CARGO_TARGET_WASM32_UNKNOWN_UNKNOWN_RUNNER=wasm-server-runner cargo run --target wasm32-unknown-unknown --example mesh2d_manual
4. Open browser link and observe console log output

Expected behavior

I'd expect to see a pretty dark star (with some color on it) in the middle of the browser window.

Observed behavior

No star,

panicked at 'wgpu error: Validation Error

Caused by:
    In Device::create_render_pipeline
    Internal error in VERTEX shader: ERROR: 0:44: 'fwidth' : no matching overloaded function found
ERROR: 0:803: 'fwidth' : no matching overloaded function found

Extra materials

For what it's worth, it works fine on native (cargo run --example mesh2d_manual)

I don't understand the stack that well, but I assume this could be worked around by at some level trying to figure out whether fwidth and friends is called from a function that is unreachable by the entry point and in that case skip it.

Platform

• WebGL, Firefox 97.0.1 Windows
• wgpu v0.12.0

[kvark]

@JCapucho aren't we filtering out the functions unused by the entry point?

[JCapucho]

Hmm, weird seems like we at some point removed it and are just printing everything

[emilk]

I just hit this bug. We usually put the vertex shader and fragment shader in the same .wgsl file. Splitting them into two separate files solved the problem for us, as expected.

[james-j-obrien]

I have also hit this issue while trying to run https://github.com/james-j-obrien/bevy_vector_shapes on web.

[teoxoy]

This has been fixed by gfx-rs/naga#2531.