This thing is written in ML, not in Haskell, and the GHC docs themselves claim that Cmm (the GHC thingy) is "is rather like C--. The syntax is almost C-- (a few constructs are missing), and it is augmented with some macros that are expanded by GHC's code generator". So "GHC's Cmm/C--" sounds rather like an independent reimplementation of a superset of a subset of "this C--".