C - How to find all inner loops using grep?

I have a giant C project with numerous C files. I have to find all inner-loops. I am sure there is no any O(n³) block in the project, so only O(n²)-compexity blocks must be found (a loop in a loop).

Is it possible to find all inner loops using grep? If yes, what regexp may I use to find all occurrences of inner loops of all kinds like {for,for}, {while,for}, {for, while}, {do, while}, etc. ? If no, is there any simple unix-way method to do it (maybe multiple greps or a kind of awk)?