From what I remember someone tried to parallelize GHC but it turned out to me tricky in practice. At the moment very trying to parallelize Cabal which would allow us to build packages/modules in parallel using ghc -c and let Cabal handle dependency management (including preprocessing of .hsc files).
Right, that's probably the one I mentioned. And I think he was trying to parallelize ghc internally, so even compiling one file could parallelize. That would be cool and all, but seems like a lot of work compared to just parallelizing at the file level, as make would do. A parallel cabal build would be excellent, but AFAIK not much help for mixed language projects, though I admit I haven't tried cabal for that yet. I'm sure it could launch make to build the C, but can it track .h -> .hsc dependencies? Parallel cabal build would tempt me to give it a try.