[SPIR-V] Emit `OpacityMicromapIdKHR` execution mode for OMM-enabled `RayQuery`

Description

SPIR-V's equivalent for DXIL's RAYQUERY_FLAG_ALLOW_OPACITY_MICROMAPS on inline ray traces seems to be an execution mode with OpacityMicromapIdKHR [^1] referencing an OpSpecConstantTrue (default-true spec constant). Is this something DXC can emit for us? It seems to be cumbersome to use https://github.com/microsoft/DirectXShaderCompiler/wiki/Inline-SPIR%E2%80%90V to emit a vk::ext_execution_mode_id(OpacityMicromapIdKHR, true) via a custom wrapper while still keeping the original constant working for DXIL.

[^1]: Using https://github.com/KhronosGroup/Vulkan-Docs/issues/2810 to clarify that this exclusively applies to inline ray tracing, while ray tracing pipelines use the VK_PIPELINE_CREATE_RAY_TRACING_OPACITY_MICROMAP_BIT_KHR host flag. Ray tracing pipeline shaders which perform inline ray queries with OMMs do need the execution mode though.

And there's a further limitation linked there:

If using a lib_6_x target profile with multiple entry points, it will only be applied to one of them.

We have multiple entrypoints for RTPSO shaders that all use ray queries, so a possible workaround would need to apply to all of them.

And note that any shader containing OpExtension SPV_KHR_opacity_micromap in current DXC does not get properly deadstripped until the SPIRV-Tools submodule is updated to include https://github.com/KhronosGroup/SPIRV-Tools/pull/6833.

Attributes on constants

As an aside I'd been quite happy to learn that extension and capability attributes can be applied on constants like so:

[[vk::ext_extension("SPV_KHR_opacity_micromap")]]
[[vk::ext_capability(RayTracingOpacityMicromapExecutionModeKHR)]]
static const uint OpacityMicromapIdKHR = 6031;

But these are emitted in the final SPIR-V even if the constant itself is clearly never referenced: https://godbolt.org/z/xc3aPrvad

Is that intended?

Actual Behavior

No OpExecutionModeId OpacityMicromapIdKHR %true is emitted for RayQuery shaders with RAYQUERY_FLAG_ALLOW_OPACITY_MICROMAPS.

Likely complications live in the fact that this execution mode can be a spec constant, likely mimicking the host-settable behaviour for RTPSOs via VK_PIPELINE_CREATE_RAY_TRACING_OPACITY_MICROMAP_BIT_KHR. Unlike DXIL, SPIR-V users might want the ability to actually configure the spec constant instead of hardcoding it based on the presence of RAYQUERY_FLAG_ALLOW_OPACITY_MICROMAPS?

Environment

• DXC version: https://github.com/microsoft/DirectXShaderCompiler/releases/tag/v1.10.2605.37
• Host Operating System: any

The issue does not name implementation files or tests. Start by tracing DXC's SPIR-V lowering for RayQuery and RAYQUERY_FLAG_ALLOW_OPACITY_MICROMAPS, then inspect generated SPIR-V for OpExecutionModeId OpacityMicromapIdKHR and handling of multiple entry points and unused extension attributes. Done means the intended behavior is implemented and covered for the described cases.