How to implement a FIR filter with the ALTSHIFT_TAPS megafunction

The idea is to save registers when you have a long filter plenty of ram to spare. Losing few ram locations is not an issue then and use of ram is highly recommended for speed as well.

As regard shift and what you call "empty locations" you don't insert zeros. there are no empty locations but only the data is shifted through.

In the case of FIR you need to tap each and every location. So if you set shift at tap distance of 3 and run at 3 x Fs then each tap will appear every three fast clocks i.e every Fs clock.

edit:

I should add that you enter your data into the shift at data rate but you shift it in at 3 times faster so you don't need to worry about inserting zeros.