How to implement the Greatest Common Divisor in Xilinx ? [VHDL]

Question

I'm trying to implement a simple GCD algorithm with VHDL language but i get this error "ERROR:HDLCompiler:981: Non-static loop limit exceeded".

So, How the while statement can work without the error above? and if it's not adaptable with xilinx 14.7 with what statement i can replace it?

signal a,b,GCD: integer;

a<=100; --example of a
b<=50; --example of b
while NOT (a*b=0) loop
if a>b then
a<=a-b;
else
b<=b-a;
end if;
end loop;
if a=0 then --result
GCD<=b; 
else GCD<=a; 
end if;

Thank you,