<p>Anyone know this compiler feature? It seems GCC support that. How does it work? What is the potential gain? In which case it's good? Inner loops?</p>

<p>(this question is specific, not about optimization in general, thanks)</p>

The fun thing about optimization is that speed gains are found in the unlikeliest of places.
It's also the reason you need a profiler, rather than guessing where the speed problems are.
I recommend starting with a profiler (
```
gperf
```
if you're using GCC) and just start poking around the results of running your application through some normal operations.