To make use of the cost centre profiling system all modules must be compiled and linked with the -prof option. Any _scc_ constructs you've put in your source will spring to life.
Without a -prof option, your _scc_s are ignored; so you can compiled _scc_-laden code without changing it.
There are a few other profiling-related compilation options. Use them in addition to -prof. These do not have to be used consistently for all modules in a program.
GHC will automatically add _scc_ constructs for all top-level, exported functions.
All top-level functions, exported or not, will be automatically _scc_'d.
The costs of all CAFs in a module are usually attributed to one “big” CAF cost-centre. With this option, all CAFs get their own cost-centre. An “if all else fails” option…
Ignore any _scc_ constructs, so a module which already has _scc_s can be compiled for profiling with the annotations ignored.