Unexpected Error Log in Unity Vulkan

[patricknasralla]

Since the addition of the Vulkan wave size hack, I've noticed that the following error appears in the Unity 6 console when running the prefix sum shaders:

Local keyword VULKAN doesn't exist in the compute shader.

This is on Unity 6000.0.34, in Windows under the Vulkan Graphics API. Having looked at my buffers in RenderDoc, there does not seem to be any issues with the prefix summation. I assume that the non-vulkan fallback is working as intended.

[b0nes164]

1. Just because it is passing on your hardware, does not mean it is correct for all hardware, and I would be highly cautious if the keyword isn't being picked up.

2. No idea why the keyword is not being picked up. Likely something was changed about multicompile functionality, but from reading the docs, not seeing anything obvious.

[patricknasralla]

No, I couldn't see anything in the docs either. Out of interest, the Vulkan keyword was only added recently. Can I ask what was happening in the Vulkan case that caused the wave group to become divergent? Also, what is the purpose of the double underscore in the

#pragma multi_compile __ VULKAN

pragma?

Sorry if this is obvious, my hlsl is okay but not quite at the level of this stuff!

[b0nes164]

All this does: #pragma multi_compile __ VULKAN is to tell Unity to compile two different versions of the shader, one with the keyword enabled, one without.

Wave divergence is a whole other issue, which is beyond the scope of this comment. If you want to read more:
https://llvm.org/docs/ConvergentOperations.html
https://llvm.org/docs/ConvergenceAndUniformity.html

OK, so we have two shader variants, why?
The older version of Unity used an older version of Vulkan that would not always return the correct wave size when WaveGetLaneCount() was called in the shader. So we have to dynamically acquire the wave size, but only on Vulkan. D3D12 is fine.

[patricknasralla]

Thanks for the explanation and links! Much appreciated!