[SPIR-V] Struct within Mesh Shader Vetex Output crashes spirv generation

Hello 🙂

We've got a crash when building mesh shaders for spirv related to how our output vertex setup is constructed. I've got a repro here showing the issue: https://godbolt.org/z/9jYY3z1eE - I took the DX12 sample project and simply moved a few attributes out of the VertexOut and in to another struct and the spirv generation now crashes.

// Adding this struct causes DXC to crash with SPIR-V
struct VertexAttributes
{
    float3 PositionVS   : POSITION0;
    float3 Normal       : NORMAL0;
};

struct VertexOut
{
    float4 PositionHS   : SV_Position;
    VertexAttributes attributes;
    uint   MeshletIndex : COLOR0;
};

The original code looked like this, which does not crash:

struct VertexOut
{
    float4 PositionHS   : SV_Position;
    float3 PositionVS   : POSITION0;
    float3 Normal       : NORMAL0;
    uint   MeshletIndex : COLOR0;
};

I believe this is the callstack on the crash:

dxcompiler.dll!hlsl::NodeFlags::operator unsigned int(void)
dxcompiler.dll!clang::spirv::SpirvEmitter::assignToMSOutAttribute(class clang::DeclaratorDecl const *,class clang::spirv::SpirvInstruction *,class llvm::SmallVector<class clang::spirv::SpirvInstruction *,4> const &)
dxcompiler.dll!clang::spirv::SpirvEmitter::tryToAssignToMSOutAttrsOrIndices(class clang::Expr const *,class clang::spirv::SpirvInstruction *,class clang::spirv::SpirvInstruction *,bool)
dxcompiler.dll!clang::spirv::SpirvEmitter::processAssignment(class clang::Expr const *,class clang::spirv::SpirvInstruction *,bool,class clang::spirv::SpirvInstruction *,class clang::SourceRange)
dxcompiler.dll!clang::spirv::SpirvEmitter::doBinaryOperator(class clang::BinaryOperator const *)
dxcompiler.dll!clang::spirv::SpirvEmitter::doExpr(class clang::Expr const *,class clang::SourceRange)
dxcompiler.dll!clang::spirv::SpirvEmitter::doStmt(class clang::Stmt const *,class llvm::ArrayRef<class clang::Attr const *>)
dxcompiler.dll!clang::spirv::SpirvEmitter::doStmt(class clang::Stmt const *,class llvm::ArrayRef<class clang::Attr const *>)
dxcompiler.dll!clang::spirv::SpirvEmitter::doIfStmt(class clang::IfStmt const *,class llvm::ArrayRef<class clang::Attr const *>)
dxcompiler.dll!clang::spirv::SpirvEmitter::doStmt(class clang::Stmt const *,class llvm::ArrayRef<class clang::Attr const *>)
dxcompiler.dll!clang::spirv::SpirvEmitter::doStmt(class clang::Stmt const *,class llvm::ArrayRef<class clang::Attr const *>)
dxcompiler.dll!clang::spirv::SpirvEmitter::doFunctionDecl(class clang::FunctionDecl const *)
dxcompiler.dll!clang::spirv::SpirvEmitter::doDecl(class clang::Decl const *)
dxcompiler.dll!clang::spirv::SpirvEmitter::HandleTranslationUnit(class clang::ASTContext &)
dxcompiler.dll!clang::ParseAST(class clang::Sema &,bool,bool)
dxcompiler.dll!clang::ASTFrontendAction::ExecuteAction(void)
dxcompiler.dll!clang::FrontendAction::Execute(void)

As a workaround, we can attempt to try and move our additional vertex data out of the structure but it's a bit of a pain for us to do.

Any support would be greatly appreciated.

I'm running on PC on DXC version dxcompiler.dll: 1.9(5184-21d28f72)(1.9.2602.17) - 1.9.2602.17 (21d28f727)

Thanks!

Alex

Start with the SPIR-V emitter entry points named in the call stack, especially assignToMSOutAttribute and tryToAssignToMSOutAttrsOrIndices, then reproduce the crash using the Godbolt example. Done means the nested VertexAttributes mesh-shader output compiles to SPIR-V without crashing.