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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/ 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/ (mk/ is the place to look for
*all* the things you might want to tune).
A good 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.
GhcStage1HcOpts = -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.
GhcStage1HcOpts = -O
GhcStage2HcOpts = -O0 -DDEBUG
Take a look through mk/ for more settings you might want to
override in Remember: don't modify 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 GHC Developer's Wiki
The home for GHC Developers, with information on accessing the latest sources,
the bug tracker, and further documentation on the code.
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 if you have difficulties.
If you're working with the current CVS sources of GHC, then might be a more appropriate (developers hang
out here). See for
Happy Hacking! --The GHC Team
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