Not many people who work in C and C++ seem to know this:
apt-get install kcachegrind valgrind valgrind --tool=callgrind programname kcachegrind callgrind.out.nnnnn
You can see in what functions the program spent its time, for every function in the program how much time it spent in every of the called functions and all other sort of cool analysis. And it's graphical, interactive, useful, cool.
It helps to compile without libtool (otherwise you're valgrinding a shellscript
unless you do something like
valgrind --tool=callgrind .libs/lt-programname.
It also helps to compile with debugging symbols.