david bishop

Go to:
http://www.vhdl.org/vhdl-200x/vhdl-200x-ft/packages/
you will need
math_utility_pkg.vhdl
fixed_pkg_c.vhdl

Then you can say:

use ieee_proposed.fixed_pkg.all;
....
signal xfixed : sfixed (3 downto -3);
signal xpower : integer;

xfixed <= to_sfixed (1.5, 3, -3);

xpower <= find_msb (abs(xfixed));

All synthesizable, and it should give you what you want.