need a help with strange error" left bound of the range must be a constant "

here is more of my code

Reg_P:process (CLK)

begin

if (CLK'event and CLK='1') then

if RESET='1' then

p<= (others => '0');

else

case CS is

-----------------------------------

when S1 =>

p <= X_in;

-----------------------------------

when S7 =>

if (mul_result = '1') then

p <= mul_c;

end if;

-----------------------------------

when S8 =>

p <= t;

-----------------------------------

when S10 =>

p <= p(m_size-2 downto 0) & p(m_size-1);

-----------------------------------

when others =>

null;

-----------------------------------

end case;

end if;

end if;

end process;

-----------------------------------------------

Reg_Q:process (CLK, r_by_2)

begin

if (CLK'event and CLK='1') then

if RESET='1' then

q<= (others => '0');

else

case CS is

-----------------------------------

when S2 =>

q<=p;

-----------------------------------

when S4 =>

if(r_by_2 = 1) then

q <= q (m_size-r_by_2-1 downto 0) & q(m_size-1);

else

q(m_size-1 downto r_by_2) <= q (m_size - r_by_2 - 1 downto 0);

q(r_by_2-1 downto 0) <= q(m_size-1 downto m_size - r_by_2);

end if;

-----------------------------------

when others =>

null;

-----------------------------------

end case;

end if;

end if;

end process;