rustgpu input SSBO disappearing?

Question

I am encountering a strange error in rustgpu. I had a shader that was compiling through a custom wrapper around rustgpu, but I recently switched to cargo gpu.

In that shader I have an array input that I use to initialize a voxel oct tree on the gpu, this is what it looks like:

#[spirv(fragment)]
pub fn main_fs(
    #[spirv(uniform, descriptor_set = 1, binding = 1)] window_data: &WindowData,
    #[spirv(uniform, descriptor_set = 1, binding = 2)] voctree_meta: &VoctreeMetadata,
    #[spirv(storage_buffer, descriptor_set = 1, binding = 4)] counter: &mut [u32; 1],
    #[spirv(storage_buffer, descriptor_set = 1, binding = 5)] nodes: &mut RuntimeArray<
        Node<SimpleNodeData>,
    >,
    #[spirv(frag_coord)] screen_pos: Vec4,
    out_color: &mut Vec4,
)
{
    let window_dimensions = window_data.dimensions.xy();
    let view_matrix = window_data.view_matrix;
    let screen_position = screen_pos.xy();

    let display_width = window_dimensions.x;
    let display_height = window_dimensions.y;
    let aspect_ratio = display_width / display_height;

    let x = screen_position.x * 2.0 / display_width - 1.0;
    let y = screen_position.y * 2.0 / display_height - 1.0;

    let screen_ray = generate_ray(&Vec2::new(x, y), aspect_ratio, 45.);
    let ray = (view_matrix.inverse() * screen_ray.extend(0.)).xyz();
    let cam_pos = (view_matrix.inverse() * Vec4::new(0., 0., 0., 1.)).xyz();

    let node_count = unsafe { counter.index_unchecked_mut(0_usize) };
    let voctree =
        GpuVoctree::<SimpleNodeData>::new(voctree_meta.clone(), node_count, nodes);

    let (cam_pos, ray) = create_ray_from_camera_data(
        &screen_pos.xy(),
        &window_data.dimensions.xy(),
        &window_data.view_matrix,
    );

    let (was_hit, hit_node, iter_count) = trace_svo(cam_pos, ray, &voctree);

    // let (was_hit, color) = trace_voxel_grid(cam_pos, ray, &volume_data, &volume);
    // *out_color = color.xyz().extend(1.0);

    if was_hit
    {
        *out_color = voctree.node_at(hit_node as usize).data.color;
    }
    else
    {
        *out_color = Vec3::splat(0.).extend(1.);
    }
}

I compile this shader via cargo gpu and then I am parsing its metadata through spirq. Which I have at the latest version.

According to spirq, there is no variable named counter in the emitted spirv code.

i.e. I iterate over the spriv code like this:

 let entry_list: Vec<_> = spirv_data
        .iter()
        .map(|spirv_data| {
            ReflectConfig::new()
                .spv(spirv_data.clone())
                .ref_all_rscs(true)
                .combine_img_samplers(true)
                .gen_unique_names(true)
                .reflect()
                .unwrap()
        })
        .collect();


    for entry_point in entry_list.iter().flatten()
    {
        for var in &entry_point.vars
        {
            match var
            {
                Variable::Input { name, location, ty } =>
                {
                   println!("{:?}", name);
                }
                Variable::Descriptor {
                    desc_bind,
                    desc_ty,
                    ty,
                    nbind,
                    name,
                    ..
                } =>
                {
                    println!("{:?}", name);
                   
                }
                Variable::Output { ty, name, .. } =>
                {
                    println!("{:?}", name);
                }
                _ => panic!(),
            }
        }
    }

Which outputs:

Some("window_data")
Some("voctree_meta")
Some("nodes")
Some("out_color")

I don't understand why cargo gpu would elide the variable when the old script I was using did not.

Makogan · Accepted Answer · 2025-04-07 00:21:59Z

1

The issue is that the variable is elided because the compiler considers it unused, adding:

let node_count = unsafe { counter.index_unchecked_mut(0_usize) };
*out_color = Vec3::splat(*node_count as f32).extend(1.);

to the fragment shader makes the variable be present.

So likely a different version of rustgpu with different optimization logic is used, explaining the difference in behaviour,

answered Apr 7 at 0:21

Makogan

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