The way that the blob obtains the subgroup id on compute shaders is by
just and'ing gl_LocalInvocationIndex with 63, since it advertizes a
subgroupSize of 64. In order to support VK_EXT_subgroup_size_control and
expose a subgroupSize of 128, we'll have to do something a little more
flexible. Sometimes we have to fall back to a subgroup size of 64 due to
various constraints, and in that case we have to fake a subgroup size of
128 while actually using 64 under the hood, by just pretending that the
upper 64 invocations are all disabled. However when computing the
subgroup id we need to use the "real" subgroup size. For this purpose we
plumb through a driver param which exposes the real subgroup size. If
the user forces a particular subgroup size then we lower
load_subgroup_size in nir_lower_subgroups, otherwise we let it through,
and we assume when translating to ir3 that load_subgroup_size means
"give me the *actual* subgroup size that you decided in RA" and give you
the driver param.
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6752>
68b8b9e9e1