Function dr_insert_write_tls_field 

pub unsafe extern "C" fn dr_insert_write_tls_field(
    drcontext: *mut c_void,
    ilist: *mut instrlist_t,
    where_: *mut instr_t,
    reg: reg_id_t,
)

Inserts into \p ilist prior to \p where meta-instruction(s) to write the general-purpose full-size register \p reg to the user-controlled drcontext field for this thread. Writes to the same field as dr_set_tls_field().