The current documentation says this:
unify (CCACHE_UNIFY) [boolean]
... Also note that enabling the unifier implies turning off the direct mode.
It seems like these could be used together. For instance, if the direct hash lookup misses, run the preprocessor and unifier, and lookup that hash.
Alternatively, if ccache already behaves this way, please update the documentation.
Moved to https://github.com/ccache/ccache/issues/99.