[MarijnS95]

Re-created #2831 to actually be reviewed, this time without accidental insta-merge.

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The HLSL mapping paired HitObject::GetInstanceID with OpHitObjectGetInstanceIdEXT and HitObject::GetInstanceIndex with OpHitObjectGetInstanceCustomIndexEXT by name, which is backwards. HLSL's InstanceID() is the application specified 24-bit value (InstanceCustomIndexKHR, gl_InstanceCustomIndexEXT) while InstanceIndex() is the index of the instance in the TLAS (the InstanceId BuiltIn, gl_InstanceID), which is what OpHitObjectGetInstanceIdEXT returns per GLSL_EXT_shader_invocation_reorder.

[dgkoch]

I checked the new mapping against independent sources and it holds up:

• DXR functional spec (InstanceIndex, InstanceID): InstanceIndex() is "the autogenerated index of the current instance in the top-level structure", and InstanceID() is "the user-provided InstanceID on the bottom-level acceleration structure instance".
• Vulkan spec, InstanceId and InstanceCustomIndexKHR BuiltIns: InstanceId is the index of the instance that intersects the ray, and InstanceCustomIndexKHR is VkAccelerationStructureInstanceKHR::instanceCustomIndex (lower 24 bits).
• GLSL_EXT_ray_tracing: gl_InstanceID maps to InstanceId, and gl_InstanceCustomIndexEXT maps to InstanceCustomIndexKHR.
• GLSL_EXT_shader_invocation_reorder: hitObjectGetInstanceIdEXT returns "the index of the instance", and hitObjectGetInstanceCustomIndexEXT returns "the custom index".
• DXC's SPIR-V backend already lowers InstanceIndex() to BuiltIn::InstanceId and InstanceID() to InstanceCustomIndexNV/KHR. The ray query intrinsics split the same way (CandidateInstanceIndex to ...InstanceIdKHR, CandidateInstanceID to ...InstanceCustomIndexKHR).

So GetInstanceIndex maps to OpHitObjectGetInstanceIdEXT and GetInstanceID maps to OpHitObjectGetInstanceCustomIndexEXT, as this PR now has it. The old text was swapped.

I also built the proposal with asciidoctor with the patch applied. There are no warnings, and both {spirvextensions} links and the refpage link render as anchors. LGTM.

▎ GenAI disclosure: Analysis assisted by Claude (claude-sonnet-5-5, Anthropic, commercial SaaS — Claude Code CLI).

[dgkoch]

LGTM.

Signed off by the Ray TSG 10/22