Add decoration PerPrimitiveEXT when a mesh output variable is decorated with PrimitiveID (#3895)

Fixes bug 3872

2 files changed, 58 insertions, 2 deletions

diff --git a/source/slang/slang-emit-spirv.cpp b/source/slang/slang-emit-spirv.cpp
index 942e089db..9f116a1e2 100644
--- a/source/slang/slang-emit-spirv.cpp
+++ b/source/slang/slang-emit-spirv.cpp
@@ -3827,6 +3827,7 @@ struct SPIRVEmitContext
                     // non-tessellation shader stages, and if so include a declaration of
                     // Geometry capability.
                     bool needGeometryCapability = true;
+                    bool needPerPrimitiveEXTDecoration = false;
                     if (entryPoints)
                     {
                         for (auto entryPoint : *entryPoints)
@@ -3835,9 +3836,13 @@ struct SPIRVEmitContext
                             {
                                 switch (entryPointDecor->getProfile().getStage())
                                 {
+                                case Stage::Mesh:
+                                    // For outputs decorated with PrimitiveID it must be decorated
+                                    // with PerPrimitiveEXT to comply with Vulkan validation rules
+                                    if (m_capabilities.contains(SpvCapabilityMeshShadingEXT))
+                                        needPerPrimitiveEXTDecoration = true;
                                 case Stage::Geometry:
                                 case Stage::Intersection:
-                                case Stage::Mesh:
                                 case Stage::Amplification:
                                 case Stage::AnyHit:
                                 case Stage::ClosestHit:
@@ -3851,7 +3856,14 @@ struct SPIRVEmitContext
                     }
                     if (needGeometryCapability)
                         requireSPIRVCapability(SpvCapabilityGeometry);
-                    return getBuiltinGlobalVar(inst->getFullType(), SpvBuiltInPrimitiveId);
+                    auto varInst = getBuiltinGlobalVar(inst->getFullType(), SpvBuiltInPrimitiveId);
+                    if (needPerPrimitiveEXTDecoration)
+                        emitOpDecorate(
+                            getSection(SpvLogicalSectionID::Annotations),
+                            nullptr,
+                            varInst,
+                            SpvDecorationPerPrimitiveEXT);
+                    return varInst;
                 }
                 else if (semanticName == "sv_rendertargetarrayindex")
                 {
diff --git a/tests/pipeline/rasterization/mesh/mesh-PerPrimitiveEXT.slang b/tests/pipeline/rasterization/mesh/mesh-PerPrimitiveEXT.slang
new file mode 100644
index 000000000..bf46cce63
--- /dev/null
+++ b/tests/pipeline/rasterization/mesh/mesh-PerPrimitiveEXT.slang
@@ -0,0 +1,44 @@
+// Test that a mesh shader generates PerPrimitiveNV decoration for OutputPrimitives
+//TEST:SIMPLE(filecheck=CHECK):-target spirv -entry main -stage mesh -emit-spirv-directly
+
+// CHECK: OpDecorate %gl_PrimitiveID BuiltIn PrimitiveId
+// CHECK: OpDecorate %gl_PrimitiveID PerPrimitiveNV
+
+const static let color: float3[] = {
+  float3(1.0, 0.0, 0.0),
+  float3(0.0, 1.0, 0.0),
+  float3(0.0, 0.0, 1.0),
+};
+
+const static let position: float3[] = {
+  float3( 0.5,  0.5, 0.0),
+  float3( 0.0, -0.5, 0.0),
+  float3(-0.5,  0.5, 0.0),
+};
+
+struct SVertexOutput {
+  float4 Position: SV_Position;
+  float3 Color;
+};
+
+struct SPrimitiveOutput {
+  uint PrimitiveId: SV_PrimitiveID;
+};
+
+[shader("mesh")]
+[numthreads(1, 1, 1)]
+[outputtopology("triangle")]
+func main(
+  OutputVertices<SVertexOutput, 3> outputVertices,
+  OutputPrimitives<SPrimitiveOutput, 1> outputPrimitives,
+  OutputIndices<uint3, 1> outputIndices,
+) -> void {
+  SetMeshOutputCounts(3, 1);
+  outputVertices[0] = { float4(position[0], 1.0), color[0] };
+  outputVertices[1] = { float4(position[1], 1.0), color[1] };
+  outputVertices[2] = { float4(position[2], 1.0), color[2] };
+  outputIndices[0] = uint3(0, 1, 2);
+
+  outputPrimitives[0] = { 0 };
+}
+