I think that your solution is better in that it's simpler, but I'm not sure what the compiler will do in this case. Theoretically, in[i+j] could point to temp_

I think that your solution is better in that it's simpler, but I'm not sure what the compiler will do in this case. Theoretically, in[i+j] could point to temp_out, but it could only be through very tricky means, since temp_out is on the stack, and the caller presumably doesn't know where it will be allocated. An FFT wouldn't be faster in this case since the filter only has five taps. Generally there has to be a decent sized kernel for the FFT to be worth it -- the FFT is O(N log N), and this algorithm is only 5*N operations.