how to use the floating point hardware in nios II custom instructions

i check the system.h

and i find this:

/*

* Custom instruction macros

*

*/

# define ALT_CI_NIOS_CUSTOM_INSTR_FLOATING_POINT_0(n,A,B) __builtin_custom_inii(ALT_CI_NIOS_CUSTOM_INSTR_FLOATING_POINT_0_N+(n&ALT_CI_NIOS_CUSTOM_INSTR_FLOATING_POINT_0_N_MASK),(A),(B))

# define ALT_CI_NIOS_CUSTOM_INSTR_FLOATING_POINT_0_N 0xfc

# define ALT_CI_NIOS_CUSTOM_INSTR_FLOATING_POINT_0_N_MASK ((1<<2)-1)

i also tried

c=ALT_CI_NIOS_CUSTOM_INSTR_FLOATING_POINT_0(3,a,b);

c=ALT_CI_NIOS_CUSTOM_INSTR_FLOATING_POINT_0(1,a,b);

c=ALT_CI_NIOS_CUSTOM_INSTR_FLOATING_POINT_0(2,b,a);

c=ALT_CI_NIOS_CUSTOM_INSTR_FLOATING_POINT_0(0,b,a)

but the result is 0.0