Function plan_wpe 

pub fn plan_wpe(
    pdk: &'static PdkTarget,
    profile: &WpeProfile,
    off_chip: &[ArrayRole],
    logic_cells: u64,
) -> Result<WpeFloorplan, WpeUnplaceable>

Plan every array of profile onto pdk, sending the roles in off_chip behind the memory edge, and check the result against the vehicle’s user area alongside logic_cells of engine.

An array whose role is off-chip costs no die area and contributes to off_chip_bits. An array that is read combinationally is never given a macro, because a macro read is registered and the placement would be unbuildable — the planner refuses to produce a floorplan it knows is a lie.