In make mode, GHC keeps a lot of in-memory data structures about modules it has built and interface files it has read. These data structures are long-lived and immutable (at least, per module/mutually recursive module group/package), so they may be candidates for storing in compact regions to reduce the work done by the GC.

Potential problem: if we rely heavily on lazy evaluation in these structures for the purpose of avoiding work that will never be needed, then we can't store them in a compact region (since the contents of a compact region must be fully evaluated).