Custom FP sqrt function

I assume you have connected the floating point custom instruction hardware (FPH2) to the custom instruction port of the Nios?

The sqrtf function is not directly inferred. You will need to explicitly include the headers into your C code.

# include "altera_nios_custom_instr_floating_point_2.h"

Then you can use sqrtf function normally. You should always include the headers when using the FPH2 module. Also you can check from objdump to re-confirm on the custom instructions being called.