The proposal describes only allowing subgroup_matrix types in Function or Private address spaces. Private is a little problematic because of the subgroup uniformity analysis. All mutable global variables are considered non-uniform. So any subgroup_matrix type in private address space would be non-uniform and effectively require disabling the analysis to use.

Suggest disallowing private for now, but consider supporting global variables in uniformity analysis via a desugaring as a separate language feature. If added we could then also allow private subgroup matrices.