Function subgroup_f_add 

pub fn subgroup_f_add(val: f32) -> f32

Subgroup (wave/warp) floating-point add reduction.

Returns the sum of val across all invocations in the subgroup.

On SPIR-V: calls spirv_std::arch::subgroup_f_add. On CUDA: uses warp-level __shfl_xor_sync butterfly reduction. On CPU: returns val unchanged (subgroup size = 1).