Constant PRIMS 

pub const PRIMS: &[PrimDesc];

THE SINGLE SOURCE OF TRUTH. host_cpu is always true (the host runs everything — the universal fallback). dram is true only for the ops UNMODIFIED ComputeDRAM realizes: RowClone (copy) + bulk bitwise AND/OR (the sense-amp majority) — the conservative tier the placement pass routes to. dram_ambit additionally covers NOT + XOR (the Ambit boolean basis) on minor-modification DRAM; the codegen_dram target uses this tier. Reductions (Popcount) are control-shaped → host only, on neither tier.