This brings back memories of when I wrote a lot of DSP code. The architecture I was on had a decent C compiler, but it wouldn't correctly take advantage of some of the special instructions. By rewriting a viterbi decoder in assembly to take advantage of their special subtract-compare-branch instructions, I recall getting a huge performance increase. But the compiler did a decent job at optimizing for multiply-accumulate instructions especially if you gave it the right hints with pragmas.