[SPIR-V] Support EvaluateAttribute* intrinsics

[jaebaek]

Currently, SPIR-V backend does not support EvaluateAttribute* intrinsics.

According to the discussion in KhronosGroup/glslang#2584, OpInterpolate* needs a pointer operand rather than OpLoad. Based on the offline discussion with @greg-lunarg we need a spirv-opt pass to replace OpLoad operand of OpInterpolate* to the pointer loaded by the OpLoad.

When a spirv-opt pass is prepared, we can simply let DXC emit OpInterpolate* for EvaluateAttribute*. Since it will just follow typical code generation routine, it will use OpLoad for its operand. We can rely on the spirv-opt pass to make it correct (as a part of the legalization pass).

[jaebaek]

KhronosGroup/SPIRV-Tools#4220 is the spirv-opt PR.

[jaebaek]

We have two use three instructions InterpolateAtCentroid, InterpolateAtSample, and InterpolateAtOffset of GLSL.std.450 extended instruction set for corresponding EvaluateAttribute*.

[gjaegy]

We are also relying on EvaluateAttributeAtSample() HLSL instruction in our code. This is now the only thing we haven't been able to port to Vulkan. Not dramatic, but our tree shadows looked better in D3D11, what is a bit frustrating.

Is there any update on that issue ?

Thanks a lot !

[pdoane]

I'm also interested in this. I tried using the SPIR-V intrinsics but not sure how to get the interpolant into the Input storage class. Here is what I tried:

[[vk::ext_capability(/* InterpolationFunction */ 52)]]
[[vk::ext_instruction(/* InterpolateAtSample */ 77, "GLSL.std.450")]]
float2 InterpolateAtSample([[vk::ext_reference]] float2 x, uint);

which outputs:

fatal error: generated SPIR-V is invalid: GLSL.std.450 InterpolateAtSample: expected Interpolant storage class to be Input
  %76 = OpExtInst %v2float %1 InterpolateAtSample %75 %70