This is an emacs major mode for editing GHC's flavor of [C--][wiki]. At this point there is no up-to-date specification describing GHC C--'s syntax. For this it's best to use the [source][].
[wiki]: https://en.wikipedia.org/wiki/C--) [source]: https://github.com/ghc/ghc/blob/master/compiler/cmm/PprCmm.hs