6. Quick start for GHC developers

This section is a copy of the file ghc/HACKING from the GHC source tree. It describes how to get started with setting up your build tree for developing GHC or its libraries, and how to start building.

Getting started with hacking on GHC

So you've decided to hack on GHC, congratulations!  We hope you have a
rewarding experience.  This file contains a few nuggets of information
that will help get you started right away, and point you in the
direction of more comprehensive documentation for later.

Setting up your build

The GHC build tree is set up so that, by default, it builds a compiler
ready for installing and using.  That means full optimisation, and the
build can take a *long* time.  If you unpack your source tree and
right away say "./configure; make", expect to have to wait a while.

For hacking, you want the build to be quick - quick to build in the
first place, and quick to rebuild after making changes.  Tuning your
build setup can make the difference between several hours to build
GHC, and less than an hour.  Here's how to do it.

mk/build.mk is a GNU makefile that contains all your build settings.
By default, this file doesn't exist, and all the parameters are set to
their defaults in mk/config.mk (mk/config.mk is the place to look for
*all* the things you might want to tune).

A good mk/build.mk to start hacking on GHC is:

SRC_HC_OPTS     = -H32m -O -fasm -Rghc-timing
GhcStage1HcOpts = -O0 -DDEBUG
GhcLibHcOpts    = -O -fgenerics
GhcLibWays      =
SplitObjs       = NO

What do these options do?

SRC_HC_OPTS = -H32m -O -fasm -Rghc-timing

  These options are added to the command line for all Haskell
  compilations.  We turn on -fasm, because that halves compilation
  time at the expense of a few percent performance.  -Rghc-timing
  prints out a line of timing info about each compilation.  It's handy
  to keep an eye on.

GhcStage1HcOpts = -O0 -DDEBUG

  The options for building the stage1 compiler (these come after
  SRC_HC_OPTS, so you can override settings from there).  We turn off
  optimisation here, assuming you'll be modifying and testing stage1.
  With optimisation off, rebuilding GHC after modifying it will be
  *much* quicker, not only because the individual compilations will be
  quicker, but also there will be fewer dependencies between modules,
  so less stuff needs to be rebuilt after each modification.

  Also we turn on -DDEBUG, because that enables assertions and
  debugging code in the compiler itself.  Turning on DEBUG makes
  the compiler about 30% slower.

GhcLibHcOpts = -O -fgenerics

  You almost certainly want optimisation *on* when building
  libraries, otherwise the code you build with this compiler
  goes really slowly.  -fgenerics add generics support to the
  libraries - you can turn this off if you like (it'll make the
  libraries a bit smaller), but you won't be able to use Generics in
  the code you build against these libraries.

GhcLibWays =

  Normally the profiled libs are built.  Setting GhcLibWays to
  empty disables this, so you only build the normal libs.

SplitObjs = NO

  Object splitting causes each module to be split into smaller
  pieces in the final library, to reduce executable sizes when
  linking against the library.  It can be quite time and
  memory-consuming, so turn it off when you're hacking.

Actually building the bits

To just build everything, from the top level:

  $ autoreconf
  $ ./configure
  $ make
  $ make install

Building individual parts of the tree

The first thing to understand is that the source tree is built in two
passes.  First 'make boot' builds dependencies and any other tools
required as part of the build itself.  For example,
ghc/utils/genprimopcode is built as part of 'make boot', because it is
required to preprocess ghc/compiler/prelude/primops.txt.pp.

After 'make boot', 'make' will build everything.

If you say 'make' from the very top-level, the build system will
arrange to do the appropriate 'make boot' steps for you.  If you just
want to build in a subdirectory (eg. ghc), you have to do 'make boot'
yourself.  You don't need to 'make boot' after every single change,
but you might want to do it to update dependencies, for example.

Refining the setup

If you will be hacking mostly on libraries, then you probably want to
build stage1 with optimisation, because you're only building it once
but using it many times.

  GhcState1HcOpts = -O

If you are working on GHCi or Template Haskell, then you will be
building and modifying the stage 2 compiler.  Hence, you want to build
stage 1 with, and stage 2 without, optimisation.

  GhcState1HcOpts = -O
  GhcState2HcOpts = -O0 -DDEBUG

Take a look through mk/config.mk for more settings you might want to
override in build.mk.  Remember: don't modify config.mk directly (it
gets overwritten when you run ./configure).

Full optimisation

To turn up everything to the max, for running performance tests for
example, try theses:

  SRC_HC_OPTS  = -H64m -O2 
  GhcLibHcOpts = -O2
  SplitObjs    = YES

You can even add some more aggresive options, such as
-fliberate-case-threshold50, -funfolding-use-threshold50.


A rough roadmap to the source tree:

 distrib        materials for building distributions

 docs           build system documentation

 ghc            The GHC Compiler
   rts          the runtime system and storage manager
   lib          libraries used in GHC and its tools
   utils	tools that come with GHC, and tools used in the build
   compiler     the compiler itself
   driver       various scripts, and package databases
   docs         compiler documentation
   includes     header files shipped with GHC

 glafp-utils    tools for the build system

 libraries	The hierarchical libraries

 nofib          A benchmark suite

 testsuite      The regression test suite


The Building Guide

   Full documentation on the build system.

The GHC Commentary

   Notes on the internals and architecture of GHC.  Much of this isn't
   up to date, but there is still lots of useful stuff in there.  Read
   in conjunction with the source code.

Mailing lists

   Ask on glasgow-haskell-users@haskell.org if you have difficulties.
   If you're working with the current CVS sources of GHC, then
   cvs-ghc@haskell.org might be a more appropriate (developers hang
   out here).  See http://www.haskell.org/mailman/listinfo for

Happy Hacking!  --The GHC Team