Function drreg_reserve_dead_register 

pub unsafe extern "C" fn drreg_reserve_dead_register(
    drcontext: *mut c_void,
    ilist: *mut instrlist_t,
    where_: *mut instr_t,
    reg_allowed: *mut drvector_t,
    reg: *mut reg_id_t,
) -> drreg_status_t

Identical to drreg_reserve_register() except returns failure if no register is available that does not require a spill.

@return whether successful or an error code on failure.