Standardize #line handling in dxcompiler

Hi HLSL Team!

Hope everyone's doing well, it's been a while! On behalf of Nvidia, I'd like to revisit how the compiler handles #line directives in debug info, originally raised in https://github.com/microsoft/DirectXShaderCompiler/issues/4615. It's still causing problems for our shader tooling.

#line N "file" is meant to remap source locations back to the original authored file and line after preprocessing has flattened #include's, which is exactly what shader build systems like Unreal do. Clang, GCC and MSVC all honour it in their line tables, as does DXC's own SPIR-V backend. The normal DXIL path is the exception: it keeps the un-preprocessed source and line numbers, so downstream tools have to re-parse the embedded source to reconstruct the intended mapping.

The shader profiler and Aftermath could work around this because they only need a line table. A full source-level debugger can't, since it relies on the debug info being the source of truth, and source references are spread across more of the DWARF than just the line table. Reconstructing all of that downstream isn't really viable in the debugger core, which is shared with the CUDA debugger and works on standard ELF/DWARF. As we bring that same source-level debugging experience to DirectX, standard #line handling stops being something we can work around.

I understand dxcompiler is in maintenance mode and change in default behaviour is unlikely, so here are two proposals:

1. Near term: an opt-in flag for standard #line handling in the DXIL debug metadata, so it doesn't affect anyone relying on today's behaviour. That alone would unblock us and let developers make use of our tools. I'd be happy to draft a PR for this.

2. Longer term: since the HLSL/Clang modernization effort is aiming for standards alignment, it would be good to make standard #line semantics the default there from the start.

Thanks,
Adam

Start by tracing how the normal DXIL path emits debug metadata and compare its handling of #line directives with DXC's SPIR-V backend. Define the opt-in behavior so standard source mappings are available without changing the current default. Done means DXIL debug metadata preserves the authored file and line mappings needed by source-level debuggers.