[DirectX] Design an approach for handling DebugInfo when writing DXIL

[bogner]

We currently fall apart in DXILBitcodeWriter when trying to emit DXIL with DebugInfo. Some of this is relatively easy to deal with, but a lot of problems stem from the fact that the DI representation in metadata has changed a lot since DXIL was codified, and the naive "downgrade" approach kind of falls apart. We need to decide how we're going to handle this.

Presumably, the way forward is based on one of the following approaches:

• Handle all DI metadata in DXILBitcodeWriter. This is probably very difficult.
• Run a pass to simplify DI metadata so that DXILBitcodeWriter only has to deal with a subset. This might be more tenable but risks losing information.
• Try to "side channel" the debug info somehow

AC:

• Determine which of the above approaches we want to take
  • research what is the best way forward for debug info
  • spike on the level of effort to implement our desired solution
• Create a spec in hlsl-specs that outlines how this is handled
• Possibly create an artifact within LLVM describing or linking to how HLSL is handling this

[llvmbot]

@llvm/issue-subscribers-debuginfo

[bogner]

On further reflection I suspect a lot of this will just kind of work out. A lot of the constructs we struggle with are things that we aren't lowering to the DXIL representations yet (like globals associated with buffers), so once all of that's in place it's likely that we just won't have that many things to deal with here.

[bogner]

As evidenced by #85884 we will need to do something about debug intrinsics, which are being eliminated:

• https://discourse.llvm.org/t/rfc-instruction-api-changes-needed-to-eliminate-debug-intrinsics-from-ir/68939/12
• https://llvm.org/docs/RemoveDIsDebugInfo.html

Presumably we'll need to reintroduce these when emitting DXIL

[jmorse]

Hola -- this is a fly-by comment, but we've designed the DPValue / DbgRecord / whatever-it-is-now information to have a one-to-one correspondence with debug intrinsics, so converting things back to intrinsic form should be straight forwards (there's a createDebugIntrinsic method that does most of it). Perhaps things will change more in the future, but for now it's pretty trivial, there's a convertFromNewDbgValues method hanging around on Module / Function / BasicBlock that does it for you.