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

this is what my public.mk shows :

# Hardware Divider present.

# setting HARDWARE_DIVIDE is false

ALT_CFLAGS += -mno-hw-div

# Hardware Multiplier present.

# setting HARDWARE_MULTIPLY is true

ALT_CFLAGS += -mhw-mul

# Hardware Mulx present.

# setting HARDWARE_MULX is false

ALT_CFLAGS += -mno-hw-mulx

......

# Enable BSP generation to query if SOPC system floating point custom

# instruction with a divider is present. If true ignores export of 'ALT_CFLAGS

# += -mcustom-fpu-cfg=60-2' and 'ALT_LDFLAGS += -mcustom-fpu-cfg=60-2' to

# public.mk if the custom instruction is found in the system. none

# setting hal.make.ignore_system_derived.hardware_fp_cust_inst_divider_present is false

there are no

"# Hardware Floating Point Custom Instruction without Divider present.

ALT_CFLAGS += -mcustom-fpu-cfg=60-1

ALT_LDFLAGS += -mcustom-fpu-cfg=60-1"

in my project , the bsp editor=>settings=>advanced=>hal.make.ignore_system_derived

i found the "hardware_fp_cust_inst_no_divider_present" is checked,

i think maybe that is the reason, so i uncheck it, then regenerate the bsp

rebuild the project, change the system.h again(the soft ware generate the "__builtin_custom_inii" again, i have to change it to "__builtin_custom_fnff" manually)

then i find in objdump file:

c=a*b;

1082c0: e0fffd17 ldw r3,-12(fp)

1082c4: e0bffe17 ldw r2,-8(fp)

1082c8: 1885ff32 custom 252,r2,r3,r2

1082cc: e0bfff15 stw r2,-4(fp)

so problem solved!

so it seems like if i want to use the floating point hardware in nios II custom instructions, i have to add the ip in Qsys=>change the bsp setting=>change the system.h

why the user guide did tell me? "tt_floating_point_custom_instructions.pdf"