Bug 278946 – [WGSL] Default initialization of [[position]] can lead to division by zero depending on the hardware implementation

• Status: NEW
• Component: WebGPU

[Mike Wyrzykowski]

Since position performs an effective division https://www.w3.org/TR/WGSL/#position-builtin-value

its default initializer should be 0,0,0,1 instead of 0,0,0,0. Otherwise this result is undefined, as v.position is not written to. Specifically, on Apple Silicon it renders the pixel but on AMD / Intel, the render is skipped because (x,y,z)/w = (0,0,0)/0 = inf:

struct VertexOutput {
  @builtin(position) position : vec4f,
  @location(0) @interpolate(flat) something: u32,
}

@vertex
fn v(@location(0) fromVertexBuffer: u32) -> VertexOutput {
  var v = VertexOutput();
  v.something = fromVertexBuffer;
  return v;
}

@fragment
fn f(@location(0) @interpolate(flat) something: u32) -> @location(0) u32 {
  return something;
}

[Mike Wyrzykowski]

Related: https://github.com/gpuweb/gpuweb/issues/4845

Instead of changing the initializer, we may want to insert a clamp at the end, like:

position.w = select(position.w, 1., position.w == 0)

[Radar WebKit Bug Importer]

<rdar://problem/135039804>