HLSL requires the number of samples for a MS texture as part of the type?

[ben-clayton]

According to https://docs.microsoft.com/en-us/windows/win32/direct3dhlsl/dx-graphics-hlsl-to-type:

Multisampled objects (Texture2DMS and Texture2DMSArray) require the texture size to be explicitly stated and expressed as the number of samples. Object2 [<Type, Samples>] Name;

Later it gives an example:

Texture2DMS <float4, 128> MyMSTex;

However, WGSL does not currently include the number of samples in the texture_multisampled_* types.

It seems that DXC actually treats the number of samples argument as optional, but it's I've not found any documentation to explain what omitting the number of samples means.

The DXIL output for Texture2DMS<float4> and Texture2DMS<float4, 8> is clearly different, so it seems this information is being preserved by DXC.

If the number of samples is required to function properly, we may need to change the spec to include this as part of the type, or we'll have to inject this information into the shader at pipeline creation time.

Microsoft folks - are you able to provide any information on whether this template argument is required, and what the behaviour is if omitted?

[dj2]

@pow2clk @RafaelCintron

[kvark]

It's definitely not required for correctness. We've been running MSAA-enabled workloads with SPIRV-Cross translation for a while, and SPIR-V doesn't carry the number of samples, so the generated HLSL does neither. The question is more of whether or not there is any (performance) advantage in specifying the sample count in there.

[magcius]

It's required for Shader Model 4.0 (D3D10), but not Shader Model 5.0+ (D3D11+).

[ben-clayton]

Thank you both. It certainly sounds like this is a non-issue then. Closing.