The logic is now in mk/compiler-ghc.mk rather than being duplicated in ghc/Makefile and compiler/Makefile.
From http://hackage.haskell.org/trac/ghc/wiki/Building/Architecture/Idiom/PhaseOrdering Phase 0: Includes: package-data.mk files for things built by the bootstrapping compiler. Builds: the dependency files for hsc2hs and genprimopcode. We need to do this now, as hsc2hs needs to be buildable in phase 1's includes (so that we can make the hpc library's .hs source files, which in turn is necessary for making its dependency files), and genprimopcode needs to be buildable in phase 1's includes (so that we can make the primop-*.hs-incl files, which are sources for the stage1 compiler library, and thus necessary for making its dependency files). Phase 1: Includes: dependency files for things built by the bootstrapping compiler. Builds: package-data.mk files for everything else. Note that this requires configuring the packages, which means telling cabal which ghc to use, and thus the stage1 compiler gets built during this phase. Phase "": Includes: dependency files for everything else. Builds: Everything else.