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wgsl @location attribute not emitted for entry-point varyings #5933

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meggsOmatic opened this issue Dec 22, 2024 · 0 comments · May be fixed by #5937
Open

wgsl @location attribute not emitted for entry-point varyings #5933

meggsOmatic opened this issue Dec 22, 2024 · 0 comments · May be fixed by #5937

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@meggsOmatic
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I'm trying to slang-ify a very simple slang "hello triangle" example flor WebGPU, targeting WGSL. Here's the minimal slang shader:

struct VertexOutput {
    float4 clipPos : SV_POSITION;
    float3 color;
}

[shader("vertex")]
func vertexMain(float3 pos: POSITION, float3 color: COLOR0)->VertexOutput {
    VertexOutput out;
    out.clipPos = float4(pos, 1.0);
    out.color = color;
    return out;
}

I compile it on the 2024.17 release using this command:

.\bin\slangc.exe shaders.slang -target wgsl -entry vertexMain -o ..\src\vertex.wgsl

And then I get this invalid WGSL as the output:

struct VertexOutput_0
{
    @builtin(position) clipPos_0 : vec4<f32>,
    @location(0) color_0 : vec3<f32>,
};

@vertex
fn vertexMain( pos_0 : vec3<f32>,  color_1 : vec3<f32>) -> VertexOutput_0
{
    var out_0 : VertexOutput_0;
    out_0.clipPos_0 = vec4<f32>(pos_0, 1.0f);
    out_0.color_0 = color_1;
    return out_0;
}

This fails to compile in WGSL with the error:

      Entry point vertexMain at Vertex is invalid
        Argument 0 varying error
          Entry point arguments and return values must all have bindings

If I manually edit the WGSL to add @location attributes, it's fine:

fn vertexMain( @location(0) pos_0 : vec3<f32>,  @location(1) color_1 : vec3<f32>) -> VertexOutput_0

Note that if I instead use an input structure instead of entry parameters, the wgsl @location attributes are correctly emitted. The problem is specific to entry parameters. Here's some slang using an input struct:

struct VertexInput {
    float3 pos : POSITION;
    float3 color : COLOR0;
}

struct VertexOutput {
    float4 clipPos : SV_POSITION;
    float3 color;
}

[shader("vertex")]
func vertexMain(VertexInput v)->VertexOutput {
    VertexOutput out;
    out.clipPos = float4(v.pos, 1.0);
    out.color = v.color;
    return out;
}

And here's the valid WGSL it produces. Note the @location attributes correctly applied to the members of VertexInput_0:

struct VertexOutput_0
{
    @builtin(position) clipPos_0 : vec4<f32>,
    @location(0) color_0 : vec3<f32>,
};

struct VertexInput_0
{
    @location(0) pos_0 : vec3<f32>,
    @location(1) color_1 : vec3<f32>,
};

@vertex
fn vertexMain( v_0 : VertexInput_0) -> VertexOutput_0
{
    var out_0 : VertexOutput_0;
    out_0.clipPos_0 = vec4<f32>(v_0.pos_0, 1.0f);
    out_0.color_0 = v_0.color_1;
    return out_0;
}
@fairywreath fairywreath linked a pull request Dec 24, 2024 that will close this issue
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Fix WGSL varying input attributes fairywreath/slang
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@meggsOmatic