[Haskell-cafe] separate compilation for library profiling

Why does GHC start a separate compilation when I build with cabal --enable-library-profiling? At least parsing and type-checking should be the same as in the non-profiling compilation. Why aren't these steps shared between profiling and non-profiling compilation? I remember that in the past GHC also started a separate compilation for shared object generation. However, starting with GHC-7.8 dyn-files are just generated along with static compilation.