Wave Intrinsics in HDRP

Hi, I was told that Unity doesn't support wave intrinsics in Compute Shader, Due to the reason
"WaveInstrisics require DXC compiler and SM 6.0. Unity currently only support FXC path for DX11 and DX12."
https://forum.unity.com/threads/wave-intrinsics-support-for-compute-shader.824916/

But after reading some shader code in current HDRP package, I found there are plenty of places where wave/cross-lane operations were used. for example:

• MotionBlur.compute
  
  Code (CSharp):

  1. [numthreads(16, 16, 1)]
  2. void MotionBlurCS(uint3 dispatchThreadId : SV_DispatchThreadID)
  3. {
  4.    // .......
  5. #if defined(PLATFORM_SUPPORTS_WAVE_INTRINSICS)
  6.     earlyOut = WaveActiveAllTrue(earlyOut);
  7.     fastPath = WaveActiveAllTrue(fastPath);
  8. #endif
  9.   // .......
  10. }
• DealUtilities.compute

Code (CSharp):

1. #if SCALARIZE_LIGHT_LOOP
2.        if (!fastPath)
3.        {
4.            // If we are not in fast path, v_lightIdx is not scalar, so we need to query the Min value across the wave.
5.            s_decalIdx = WaveActiveMin(v_decalIdx);
6.            // If WaveActiveMin returns 0xffffffff it means that all lanes are actually dead, so we can safely ignore the loop and move forward.
7.            // This could happen as an helper lane could reach this point, hence having a valid v_lightIdx, but their values will be ignored by the WaveActiveMin
8.            if (s_decalIdx == -1)
9.            {
10.                break;
11.            }
12.        }
13.        // Note that the WaveReadLaneFirst should not be needed, but the compiler might insist in putting the result in VGPR.
14.        // However, we are certain at this point that the index is scalar.
15.        s_decalIdx = WaveReadLaneFirst(s_decalIdx);
16. #endif // SCALARIZE_LIGHT_LOOP
17.  

​

And I found that the macro PLATFORM_SUPPORTS_WAVE_INTRINSICS basically controls wether you can use wave intrinsics in code, I wonder if I could make this work in my own code? or it's just a useless macro for now, even in official HDRP package? I don't have much experience on shader compilation stuff, so hopefully you guys could give me some help, thanks!

 

Thanks for reply! I wonder when Unity will be able to supprt these new optimization possibilities for compute shader on PC platform?

 

Is there somewhere we can follow this? I'm getting an infinite loop in the fxc based compiler that doesn't happen with dxc

 

https://github.com/Unity-Technologies/ScriptableRenderPipeline/pull/6304

 

For any one who struggles to implement SM6.0 features in Unity,
I suggest to update your Unity to the newest version with DX12 backend as follows:
https://forum.unity.com/threads/shader-model-6-0-or-vendor-intrinsics.977700/

However, I noticed that the performance of compute shader in DX12 backend is slower than DX11, due to their poorly designed fence and barrier strategies... and even worse, RenderDoc has poor support (performance counter invalid for indirect dispatches, and no kernel name to diaplay for you) .
So I suggest to re-think about it before switch to SM6.0 with your project.

 

I really hope that there is no D11OnD12 sorta poor implementation in place