[SPIR-V] Fix usage of indices in subfunctions

tools/clang/lib/SPIRV/DeclResultIdMapper.cpp

@@ -860,7 +860,7 @@ bool DeclResultIdMapper::createStageOutputVar(const DeclaratorDecl *decl,
         QualType arrayType = astContext.getConstantArrayType(
             type, llvm::APInt(32, arraySize), clang::ArrayType::Normal, 0);
 
-        stageVarInstructions[cast<DeclaratorDecl>(decl)] =
+        msOutIndicesBuiltin =
             getBuiltinVar(builtinID, arrayType, decl->getLocation());
       } else {
         // For NV_mesh_shader, the built type is PrimitiveIndicesNV
@@ -871,7 +871,7 @@ bool DeclResultIdMapper::createStageOutputVar(const DeclaratorDecl *decl,
             astContext.UnsignedIntTy, llvm::APInt(32, arraySize),
             clang::ArrayType::Normal, 0);
 
-        stageVarInstructions[cast<DeclaratorDecl>(decl)] =
+        msOutIndicesBuiltin =
             getBuiltinVar(builtinID, arrayType, decl->getLocation());
       }
 

tools/clang/lib/SPIRV/DeclResultIdMapper.h

@@ -559,6 +559,11 @@ class DeclResultIdMapper {
     return value;
   }
 
+  SpirvVariable *getMSOutIndicesBuiltin() {
+    assert(msOutIndicesBuiltin && "Variable usage before decl parsing.");
+    return msOutIndicesBuiltin;
+  }
+
   /// Decorate with spirv intrinsic attributes with lamda function variable
   /// check
   void decorateWithIntrinsicAttrs(
@@ -1014,6 +1019,25 @@ class DeclResultIdMapper {
   /// creating that stage variable, so that we don't need to query them again
   /// for reading and writing.
   llvm::DenseMap<const ValueDecl *, SpirvVariable *> stageVarInstructions;
+
+  /// Special case for the Indices builtin:
+  /// - this builtin has a different layout in HLSL & SPIR-V, meaning it
+  /// requires
+  ///   the same kind of handling as classic stageVarInstructions:
+  ///   -> load into a HLSL compatible tmp
+  ///   -> write back into the SPIR-V compatible layout.
+  /// - but the builtin is shared across invocations (not only lanes).
+  ///   -> we must only write/read from the indices requested by the user.
+  /// - the variable can be passed to other functions as a out param
+  ///   -> we cannot copy-in/copy-out because shared across invocations.
+  ///   -> we cannot pass a simple pointer: layout differences between
+  ///   HLSL/SPIR-V.
+  ///
+  /// All this means we must keep track of the builtin, and each assignment to
+  /// this will have to handle the layout differences. The easiest solution is
+  /// to keep this builtin global to the module if present.
+  SpirvVariable *msOutIndicesBuiltin = nullptr;
+
   /// Vector of all defined resource variables.
   llvm::SmallVector<ResourceVar, 8> resourceVars;
   /// Mapping from {RW|Append|Consume}StructuredBuffers to their

tools/clang/lib/SPIRV/SpirvEmitter.cpp

@@ -8128,7 +8128,7 @@ void SpirvEmitter::assignToMSOutIndices(
   if (indices.size() > 1) {
     vecComponent = indices.back();
   }
-  auto *var = declIdMapper.getStageVarInstruction(decl);
+  auto *var = declIdMapper.getMSOutIndicesBuiltin();
   const auto *varTypeDecl = astContext.getAsConstantArrayType(decl->getType());
   QualType varType = varTypeDecl->getElementType();
   uint32_t numVertices = 1;

tools/clang/test/CodeGenSPIRV/meshshading.nv.triangle.indices.out.hlsl

@@ -0,0 +1,65 @@
+// RUN: %dxc -T ms_6_5 -E outie -fcgl  %s -spirv | FileCheck %s
+// RUN: %dxc -T ms_6_5 -E innie -fcgl  %s -spirv | FileCheck %s
+
+// CHECK-DAG: [[v4_n05_05_0_1:%[0-9]+]] = OpConstantComposite %v4float %float_n0_5 %float_0_5 %float_0 %float_1
+// CHECK-DAG:  [[v4_05_05_0_1:%[0-9]+]] = OpConstantComposite %v4float %float_0_5 %float_0_5 %float_0 %float_1
+// CHECK-DAG:  [[v4_0_n05_0_1:%[0-9]+]] = OpConstantComposite %v4float %float_0 %float_n0_5 %float_0 %float_1
+// CHECK-DAG:  [[v3_1_0_0:%[0-9]+]] = OpConstantComposite %v3float %float_1 %float_0 %float_0
+// CHECK-DAG:  [[v3_0_1_0:%[0-9]+]] = OpConstantComposite %v3float %float_0 %float_1 %float_0
+// CHECK-DAG:  [[v3_0_0_1:%[0-9]+]] = OpConstantComposite %v3float %float_0 %float_0 %float_1
+// CHECK-DAG:  [[u3_0_1_2:%[0-9]+]] = OpConstantComposite %v3uint %uint_0 %uint_1 %uint_2
+
+// CHECK-DAG:  OpDecorate [[indices:%[0-9]+]] BuiltIn PrimitiveIndicesNV
+
+struct MeshOutput {
+  float4 position : SV_Position;
+  float3 color : COLOR0;
+};
+
+[outputtopology("triangle")]
+[numthreads(1, 1, 1)]
+void innie(out indices uint3 triangles[1], out vertices MeshOutput verts[3]) {
+    SetMeshOutputCounts(3, 2);
+
+    triangles[0] = uint3(0, 1, 2);
+// CHECK: [[off:%[0-9]+]] = OpIMul %uint %uint_0 %uint_3
+// CHECK: [[ptr:%[0-9]+]] = OpAccessChain %_ptr_Output_uint [[indices]] [[off]]
+// CHECK: [[tmp:%[0-9]+]] = OpCompositeExtract %uint [[u3_0_1_2]] 0
+// CHECK:                   OpStore [[ptr]] [[tmp]]
+// CHECK: [[idx:%[0-9]+]] = OpIAdd %uint [[off]] %uint_1
+// CHECK: [[ptr:%[0-9]+]] = OpAccessChain %_ptr_Output_uint [[indices]] [[idx]]
+// CHECK: [[tmp:%[0-9]+]] = OpCompositeExtract %uint [[u3_0_1_2]] 1
+// CHECK:                   OpStore [[ptr]] [[tmp]]
+// CHECK: [[idx:%[0-9]+]] = OpIAdd %uint [[off]] %uint_2
+// CHECK: [[ptr:%[0-9]+]] = OpAccessChain %_ptr_Output_uint [[indices]] [[idx]]
+// CHECK: [[tmp:%[0-9]+]] = OpCompositeExtract %uint [[u3_0_1_2]] 2
+// CHECK:                   OpStore [[ptr]] [[tmp]]
+
+    verts[0].position = float4(-0.5, 0.5, 0.0, 1.0);
+// CHECK: [[ptr:%[0-9]+]] = OpAccessChain %_ptr_Output_v4float %gl_Position %int_0
+// CHECK:                   OpStore [[ptr]] [[v4_n05_05_0_1]]
+    verts[0].color = float3(1.0, 0.0, 0.0);
+// CHECK: [[ptr:%[0-9]+]] = OpAccessChain %_ptr_Output_v3float %out_var_COLOR0 %int_0
+// CHECK:                   OpStore [[ptr]] [[v3_1_0_0]]
+
+    verts[1].position = float4(0.5, 0.5, 0.0, 1.0);
+// CHECK: [[ptr:%[0-9]+]] = OpAccessChain %_ptr_Output_v4float %gl_Position %int_1
+// CHECK:                   OpStore [[ptr]] [[v4_05_05_0_1]]
+    verts[1].color = float3(0.0, 1.0, 0.0);
+// CHECK: [[ptr:%[0-9]+]] = OpAccessChain %_ptr_Output_v3float %out_var_COLOR0 %int_1
+// CHECK:                   OpStore [[ptr]] [[v3_0_1_0]]
+
+    verts[2].position = float4(0.0, -0.5, 0.0, 1.0);
+// CHECK: [[ptr:%[0-9]+]] = OpAccessChain %_ptr_Output_v4float %gl_Position %int_2
+// CHECK:                   OpStore [[ptr]] [[v4_0_n05_0_1]]
+    verts[2].color = float3(0.0, 0.0, 1.0);
+// CHECK: [[ptr:%[0-9]+]] = OpAccessChain %_ptr_Output_v3float %out_var_COLOR0 %int_2
+// CHECK:                   OpStore [[ptr]] [[v3_0_0_1]]
+
+}
+
+[outputtopology("triangle")]
+[numthreads(1, 1, 1)]
+void outie(out indices uint3 triangles[1], out vertices MeshOutput verts[3]) {
+	innie(triangles, verts);
+}