[SM 6.10] Group Wave Index

[alan-baker]

A function or system value to know which wave within the threadgroup a particular thread is part of. This is equivalent to the SPIRV SubgroupId built-in and the simdgroup_index_in_threadgroup attribute in MSL.

[llvm-beanz]

I think we should track this separately from the linalg::Matrix proposal since it is more broadly useful, but we should get this on the roadmap sooner rather than later.

[alan-baker]

Also, if it is not guaranteed that all but one Wave will have WaveGetLaneCount threads it probably makes sense to an equivalent to num_subgroups too (i.e. number of waves in threadgroup).

[damyanp]

PR for proposal: #670