when I use the IP tool bench to make a nco dds, the third step generate will stuck in "generating megacore function top-level", and there's no .qip files generated.
Which version of Quartus are you using?
Quartus II 13.1
If the other files are generated, just add all the files nco_library and <ip_name>.vhd to your project instead of the QIP file.
For more complete information about compiler optimizations, see our Optimization Notice.