SPV Output: Why RayQueryTerminate Is Ignored?

Introduction

This article delves into a critical issue encountered in the gfx-rs ecosystem, specifically within the wgpu project. The focus is on the rayQueryTerminate function and its unexpected behavior in the SPV (Standard Portable Intermediate Representation) output. This problem was initially flagged in the gfx-rs discussion forum under the category of wgpu, highlighting a discrepancy in how rayQueryTerminate is handled across different backends. Understanding the root cause and implications of this issue is crucial for developers working with ray tracing and compute shaders in wgpu.

Description of the Issue

The core of the problem lies in the fact that the SPV output currently ignores RayQueryFunction::Terminate. In other words, when a shader program containing rayQueryTerminate is compiled to SPV, the terminate call is effectively omitted. This is problematic because rayQueryTerminate is intended to prematurely end a ray query, which is a fundamental operation in ray tracing algorithms. On other platforms, such as Metal, this function is correctly translated to .abort, which serves the intended purpose. Although there is limited documentation, spirv-cross translates directly from glsl's rayQueryTerminateEXT (which is the same as spirv's). It also translates to .Abort on dx12, which has very similar documentation to spirv's OpRayQueryTerminateKHR, only it's allowed in more places. The discrepancy in behavior between SPV and other backends introduces inconsistencies and potential errors in applications that rely on ray tracing functionality.

The omission of rayQueryTerminate in SPV output can lead to several adverse effects. Firstly, it can cause ray queries to continue executing even when they should be terminated, leading to wasted computation and potential performance bottlenecks. Secondly, it can result in incorrect results if the application logic depends on the termination of ray queries under certain conditions. For example, if a ray query is intended to find the closest intersection with a scene, prematurely terminating the query can prevent it from finding the correct result.

Reproducing the Issue

To demonstrate this issue, one can examine the Naga snapshots, which are generated as part of the wgpu testing process. Specifically, comparing the HLSL (High-Level Shading Language) output with the SPV assembly output reveals the discrepancy. In the HLSL output (wgsl-ray-query.hlsl), the rayQueryTerminate function is correctly translated, whereas in the corresponding SPV assembly (wgsl-ray-query.spvasm), the function call is missing or represented by an empty block. This difference in output clearly illustrates that the SPV backend is not properly handling rayQueryTerminate.

By inspecting the Naga snapshots, developers can gain a concrete understanding of how rayQueryTerminate is being mishandled in SPV output. This can aid in debugging and identifying potential workarounds for the issue. Additionally, the Naga snapshots serve as a valuable resource for contributors who are working on fixing the problem in the wgpu codebase.

The specific lines of code that highlight the issue are the HLSL output at line 161 (wgsl-ray-query.hlsl) and the SPV assembly output at lines 536-537 (wgsl-ray-query.spvasm). These lines demonstrate the absence of a corresponding terminate call in the SPV output, confirming that the function is being ignored during SPV compilation.

Expected vs. Observed Behavior

The expected behavior is that the SPV output should include a mechanism to terminate ray queries, mirroring the functionality provided by other backends such as Metal and DirectX 12. This would ensure consistency across different platforms and prevent unexpected behavior in ray tracing applications. The observed behavior, however, is that the SPV output omits the rayQueryTerminate call, effectively disabling the ability to prematurely end ray queries. This discrepancy can lead to incorrect results, performance issues, and other problems in applications that rely on ray tracing.

The ideal solution would be for the SPV backend to generate code that accurately reflects the intended behavior of rayQueryTerminate. This could involve emitting specific SPV instructions that terminate the ray query or using alternative mechanisms to achieve the same effect. By ensuring that the SPV output correctly handles rayQueryTerminate, developers can have confidence that their ray tracing applications will behave consistently across different platforms.

Additional Resources

To further investigate this issue, it is helpful to examine how spirv-cross, another tool in the SPIR-V ecosystem, handles rayQueryTerminate. Spirv-cross's GLSL output (and potentially its GLSL input) provides insights into how the function can be translated in a different context. Additionally, examining spirv-cross's MSL (Metal Shading Language) output reveals how rayQueryTerminate is handled on Metal, which is a platform where the function is known to work correctly.

The spirv-cross code snippets provided in the original issue report (https://github.com/KhronosGroup/SPIRV-Cross/blob/8d838b4e2e1eb36deb292f89b8b5fc6ce84bf02c/shaders-msl/comp/ray-query.spv14.vk.ios.msl24..invalid.comp#L37 and https://github.com/KhronosGroup/SPIRV-Cross/blob/8d838b4e2e1eb36deb292f89b8b5fc6ce84bf02c/reference/shaders-msl/comp/ray-query.spv14.vk.ios.msl24..invalid.comp#L56) offer valuable clues about how rayQueryTerminate can be translated to different shading languages. By comparing these translations with the SPV output generated by wgpu, developers can identify the specific steps needed to fix the issue.

Platform Information

The issue described in this article is platform-agnostic, meaning it is not specific to any particular operating system or hardware configuration. The problem lies in the SPV backend of wgpu, which is responsible for generating SPV code regardless of the target platform. Therefore, the omission of rayQueryTerminate in SPV output will affect any application that uses ray tracing and relies on this function, regardless of the platform on which it is running.

This platform independence underscores the importance of addressing the issue in the wgpu codebase. By fixing the SPV backend, developers can ensure that ray tracing applications behave consistently across all platforms supported by wgpu.

Conclusion

In conclusion, the omission of rayQueryTerminate in the SPV output of wgpu is a significant issue that can lead to incorrect behavior and performance problems in ray tracing applications. The discrepancy between SPV and other backends highlights the need for a consistent approach to handling ray query termination across different platforms. By understanding the root cause of the problem and examining the behavior of other tools in the SPIR-V ecosystem, developers can work towards a solution that ensures the correct and efficient execution of ray tracing algorithms in wgpu.

For more information on SPIR-V and its role in graphics and compute, visit the Khronos Group's SPIR-V page. This resource provides comprehensive documentation and specifications for SPIR-V, which can be helpful in understanding the underlying technology and potential solutions for the rayQueryTerminate issue.