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Location assignment for arrayed outputs #500
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- changed the title
[-](my bug report)[/-][+]Location assignment for tesselation, geometry and mesh shaders[/+]on Dec 28, 2025 - changed the title
[-]Location assignment for tesselation, geometry and mesh shaders[/-][+]Location assignment for arrayed outputs[/+]on Dec 28, 2025 - added a commit that references this issue
on Dec 28, 2025 Compiletest that has wrong location assignments 0, 9, 18, 21 instead of 0, 1, 2, 3:
// build-pass // compile-flags: -Ctarget-feature=+MeshShadingEXT,+ext:SPV_EXT_mesh_shader // compile-flags: -C llvm-args=--disassemble-globals // normalize-stderr-test "OpSource .*\n" -> "" // normalize-stderr-test "OpLine .*\n" -> "" // normalize-stderr-test "%\d+ = OpString .*\n" -> "" // normalize-stderr-test "; .*\n" -> "" // normalize-stderr-test "OpCapability VulkanMemoryModel\n" -> "" // normalize-stderr-test "OpMemoryModel Logical Vulkan" -> "OpMemoryModel Logical Simple" // ignore-spv1.0 // ignore-spv1.1 // ignore-spv1.2 // ignore-spv1.3 // ignore-vulkan1.0 // ignore-vulkan1.1 use spirv_std::arch::set_mesh_outputs_ext; use spirv_std::glam::{UVec3, Vec4}; use spirv_std::spirv; #[spirv(mesh_ext( threads(1), output_vertices = 9, output_primitives_ext = 3, output_triangles_ext ))] pub fn main( #[spirv(position)] positions: &mut [Vec4; 9], #[spirv(primitive_triangle_indices_ext)] indices: &mut [UVec3; 3], // location 0 out_per_vertex: &mut [u32; 9], // location 1 out_per_vertex2: &mut [f32; 9], // location 2 #[spirv(per_primitive_ext)] out_per_primitive: &mut [u32; 3], // location 3 #[spirv(per_primitive_ext)] out_per_primitive2: &mut [f32; 3], ) { unsafe { set_mesh_outputs_ext(9, 3); } for i in 0..3 { positions[i * 3 + 0] = Vec4::new(-0.5, 0.5, 0.0, 1.0); positions[i * 3 + 1] = Vec4::new(0.5, 0.5, 0.0, 1.0); positions[i * 3 + 2] = Vec4::new(0.0, -0.5, 0.0, 1.0); } for i in 0..9 { out_per_vertex[i] = i as u32; out_per_vertex2[i] = i as f32; } for i in 0..3 { indices[i] = UVec3::new(0, 1, 2) + UVec3::splat(i as u32); out_per_primitive[i] = 42; out_per_primitive2[i] = 69.; } }
PR #482 broke location assignment for tesselation, geometry and mesh shaders.
Specifically, it broke the following rules in 16.1.3. Interface Matching, which all have a quite similar pattern:
TLDR: Their output must be a spirv array (sized or unsized aka. rust slice). This "outer layer" must be ignored in terms of location assignment and only it's element type considered for location size, as all of these shaders expand from one element to many elements, and each element in the array is being sent to their respective output element.
Workaround: manual location assignment with
#[spirv(location = 123)]