rtx structure

[Bharati Bhole]

I am not able to understand the structure of rtx_def fully its too complicated.
Could anybody please guide me. Its defined in rtl.h.
struct rtx_def GTY((chain_next ("RTX_NEXT (&%h)"),
                    chain_prev ("RTX_PREV (&%h)")))
{
   ENUM_BITFIELD(rtx_code) code: 16;
  ENUM_BITFIELD(machine_mode) mode : 8;
  unsigned int jump : 1;
  unsigned int call : 1;
  unsigned int unchanging : 1;
  unsigned int volatil : 1;
  unsigned int in_struct : 1;
   unsigned int used : 1;
  unsigned frame_related : 1;
  unsigned return_val : 1;
  union u {
    rtunion fld[1];
    HOST_WIDE_INT hwint[1];
  } GTY ((special ("rtx_def"), desc ("GET_CODE (&%0)"))) u;
};

Thanks,
Bharati.