Workgroup size from spec constant

When you use subgroups, you can often simplify code by assuming subgroup size == workgroup size for any subgroup intrinsics. But subgroup size is defined by the HW you are executing on, so what you'd need to do is use a specialization constant to set your workgroup size. You can already do this in glsl since vulkan first released:

The built-in vector gl_WorkGroupSize can be specialized using special layout local_size_{xyz}_id's applied to the "in" qualifier. For example:
layout(local_size_x_id = 18, local_size_z_id = 19) in;
This leaves gl_WorkGroupSize.y as a non-specialization constant, with gl_WorkGroupSize being a partially specialized vector. Its x and z components can be later specialized using the ID's 18 and 19.

https://github.com/KhronosGroup/GLSL/blob/3e0d9a3b3f54651ef53d533392a7401a3b19e16d/extensions/khr/GL_KHR_vulkan_glsl.txt#L192

Beginne mit der verknüpften GL_KHR_vulkan_glsl-Spezifikationspassage, die specialization constants für gl_WorkGroupSize beschreibt, und finde dann den Rust-GPU-Compilerpfad, der workgroup size und specialization constants behandelt. Als erledigt gilt die Aufgabe, wenn Annahmen zur workgroup size im Zusammenhang mit subgroups eine specialization constant verwenden können und eine Abdeckung für das unterstützte Verhalten hinzugefügt wurde.