[Feature Request] WaveSize attribute support for Mesh and Amplification shaders

[Devaniti]

Is your feature request related to a problem? Please describe.
Despite the fact that Mesh and Amplification shaders are very similar to compute shaders, they are not allowed to have WaveSize attribute. Compiling given shader as as_6_6 fails:

[WaveSize(32)]
[numthreads(32, 1, 1)]
void main(uint DTiD : SV_DispatchThreadID, uint GTiD : SV_GroupThreadID) 
{
}

with error: attribute WaveSize only valid for CS.
If GPU advertises support for multiple different wave sizes, you can not force it to select wave size your shader is compatible with.

Describe the solution you'd like
WaveSize should be valid on Mesh and Amplification shaders
Reasoning for supporting WaveSize on AS/MS is same as for CS described in HLSL Wave Size doc

Describe alternatives you've considered
In case when GPU supports multiple wave sizes, your only option is to author shaders that are compatible with any of supported wave size. The drawbacks of such approach are:

• To test such shader you would still need WaveSize attribute to confirm that it runs as expected on different wave sizes.
• You can't set up group size to match wave size, meaning you are forced to use groupshared memory to share data between waves inside the threadgroup, which can potentially be slower.
• It's easier to author shaders that only support single wave size when prototyping.

[llvm-beanz]

Moving to hlsl-specs since this is a language change that requires a shader model change.

[Nielsbishere]

Okay so I guess this is not supported currently.