请大神帮忙看看vhdl循环寄存功能这样怎么无法实现？

Question

LIBRARY IEEE;
USE IEEE.STD_LOGIC_1164.ALL;
ENTITY xyw10218104 IS
  PORT(clk_1:IN STD_LOGIC;
  d0,d1,d2,d3,d4,d5,d6,d7:buffer STD_LOGIC_VECTOR(3 DOWNTO 0));
END xyw10218104;
ARCHITECTURE one OF xyw10218104 IS
begin
  process(clk_1)
  begin
    d7<="1000";
	d6<="0111";
	d5<="0110";
	d4<="0101";
	d3<="0100";
	d2<="0011";
	d1<="0010";
	d0<="0001";
  if(clk_1'event and clk_1='1') then
    d7<=d0;
	d6<=d1;
	d5<=d2;
	d4<=d3;
	d3<=d4;
	d2<=d5;
	d1<=d6;
	d0<=d7;
  END  IF;
  END PROCESS;
END one;

出现这样的错误不明白是什么原因 应该怎么修改才能正确赋初值呢？