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.
In addition to the `-prof' option your system might be setup to enable you to compile and link with the `-prof-details' option instead. This enables additional detailed counts to be reported with the `-P' RTS option.