The libraries/ directory contains all the packages that GHC needs to build. It has one sub-directory for each package repository (e.g. base, haskell98, random). Usually each such repository builds just one package, but there is more than one in dph.
compiler/ghc.cabal.in: the Cabal file for GHC is generated from this. If you add a module to GHC's source code, you must add it in the ghc.cabal.in file too, else you'll get link errors.
The following directories appear only in the build tree:
compiler/stage1: generated files for the stage1 build of GHC. There are a handful of files (ghc_boot_platform.h etc), and a directory compiler/stage1/build/ that contains all the .o and .hi files for the compiler.
compiler/stage2: similarly stage2.
You can't run a binary from here: look in the inplace/ directory below for that.
The utils directory contains support utilities that GHC uses.
These utils may be built with the bootstrapping compiler, for use during the build, or with the stage1 or stage2 compiler, for installing. Some of them are built with both; we can't install the utils built with the bootstrapping compiler as they may use different versions of C libraries. The reason we use sometimes stage2 rather than stage1 is that some utils, e.g. haddock, need the GHC API package.
utils/ghc-cabal is a little program we use for building the libraries. It's similar to cabal-install, but without the dependencies on http etc.
utils/count_lines is a program that counts the number of source-code lines in GHC's code-base. It distinguishes comments from non-comments.
This contains some simple wrapper programs and scripts, for example the ghci wrapper that invokes the ghc binary with the --interactive flag. These wrappers tend to be executable programs on Windows and scripts on Unix systems.
ghc-tarballs/ (Windows only)
This contains some tarball files (binary packages) that GHC relies upon. Used for easier development / deployment on windows.
The testsuite/ and nofib/ directories contain apparatus for testing GHC.
The mk/ and rules.mk directories contains all the build system Makefile boilerplate; see GHC Build System Architecture. Some particular files are interesting:
mk/build.mk: contains Makefile settings that control your build. Details here. The file mk/build.mk.sample contains a starting point that you can copy to mk/build.mk if you want.
mk/are-validating.mk: this file records the fact that you are doing validation, by containing the single line Validating=YES. That in turn means the the build system gets its settings from mk/validate-settings.mk instead of from mk/build.mk. Remove the file to stop validating.
mk/validate.mk: just like build.mk, but applies when validating. Use this file to override the default settings for validation, which are in mk/validate-settings.mk.
Miscellaneous files for building distributions.
Stuff that appears only in a build tree
The inplace/ directory is where we "install" stage1 and stage2 compilers, and other utility programs, when they are built, to be used when building other things in the build tree. The layout is exactly the same as that of an installed GHC on the host platform.
inplace/bin/: executables, including
inplace/lib/: suppporting libraries for the executables.
In many directories, dist* subdirectories appear. These are where Cabal, and the build system makefiles, put all of the files generated while building. Some particularly interesting files are:
docs/users_guide/users_guide/index.html: the HTML for the user manual
libraries/LIB/dist-install/doc/html/LIB: contains the Haddock'd documentation for library LIB