We have a target that executes a non-power-of-2 number of threads in lock-step. It would be natural to make a subgroup have one member per thread, but we are not able to because GL_KHR_shader_subgroup states:

If the extension GL_KHR_shader_subgroup_basic is enabled, the variable
<gl_SubgroupSize> is the number of invocations within a subgroup, and its
value is always a power of 2.

Why is there a restriction that it must be a power of 2? Is there any real need for that?

(As far as I can see, neither SPIR-V nor vulkan have this restriction, it is only in GLSL, so we could in theory have shaders written in other languages running on vulkan with a non-power-of-2 subgroup size.)