|Version 51 (modified by 7 years ago) (diff),|
Using the Build System
- Source trees and build trees
- Booting, configuring, cleaning
- Steps to prepare for building
- Building things
- Developing in a GHC build tree
- Installing extra packages
NOTE: this page now describes the current GHC build system. For documentation of the "old" build system, see Attic/Building/Old/Using.
Using the Build System
This part of the guide is about using the build system, that is, how to interact with the build system when you're developing some part of GHC, its libraries or tools. The section aims to be comprehensive; for a quick start, read Building/Hacking first.
Source trees and build trees
Sometimes we want to separate the build tree from the source tree. There are a few advantages to doing this:
- You can make multiple different builds from the same sources, perhaps for testing different build settings, or for building on different platforms.
- You might want to put the source tree on a remote, backed-up, filesystem, but keep your build tree on a local fast unbacked-up drive (this is a configuration we use regularly at GHC HQ). It doesn't matter if you lose the build tree: it can easily be regenerated.
- It's easy to blow away a build tree and start again, without
modifying your source tree.
make maintainer-cleanis usually good for this too, but it can miss files that it doesn't know about, or files that are remnants from older versions of GHC.
- It helps to avoid mistakes whereby you edit a file that happens
to be automatically generated, instead of the original source
file (e.g. editing
config.mk.in). If you only edit files in the source tree, then this can't happen.
However, if you just want to build the software once on a single platform, then your source tree can also be your build tree, and you can skip the rest of this section.
Windows users: so far as we know, symbolic links do not work right on MSYS at least, so we never use separate source and build trees on Windows.
A build tree is just an exact copy of the source tree, except that
every file in it is a symbolic link to the appropriate file in the
source tree. There are "standard" Unix utilities that make such
copies, so standard that they go by different names:
mkshadowdir are two (If you don't have either, the GHC source
tree contains sources for the X11
lndir check out
You need to be a bit careful, though, that any new files you create (if you do any development work) are in the source tree, not the build tree!
Booting, configuring, cleaning
GHC uses the
autoconf tools in the standard Unixy way, described in more detail in subsequent subsections on this page:
--------------- | | | Source code | <--------------------- | | | --------------- maintainer-clean | | perl boot (runs autoconf) | | | v | ---------------------------- | | Source tar-ball |>--------| | ./configure | | | libraries/*/GNUmakefile | | | libraries/*/ghc.mk |<-- | ---------------------------- | | | | | ./configure distclean | | | | v | | ------------------ | | | Configured |>------------ | | mk/config.mk | | | | |>------------------- ------------------ | | | | | make | | | | | v | | ------------------ | | | Built |>------------ | | | | | |>------------------- ------------------
Steps to prepare for building
We gave a quick getting-started introduction to building GHC in Building/QuickStart, the following sections describe each step in more detail.
In the build system, the path to the top of your build tree is
referred to as
$(TOP), and we will use that convention in the
Generate configure scripts
NOTE: if you're starting from a source distribution that you downloaded from the GHC web site, rather than darcs sources, you can skip this step. Source distributions come with configure scripts pre-generated for you.
Change directory to
$(TOP) and issue the command
$ sh boot
$ is the prompt. You don't type that bit.) This runs
autoreconf on the various
configure.ac scripts in the GHC build
configure scripts and other miscellaneous files.
You have to re-do this step if (and only if) you ever change one of
the files that
autoconf uses to generate the
Run the configure script
configure script, thus:
$ ./configure <args>
configure's mission is to scurry round your computer working out
what architecture it has, what operating system, whether it has the
vfork system call, where
tar is kept, whether
available, where various obscure
#include files are, whether
it's a leap year, and what the systems manager had for lunch. It
communicates these snippets of information in two ways:
- It translates various files such as
mk/config.mk, substituting for things between "
@" brackets. So, "
@HaveGcc@" will be replaced by "
YES" or "
NO" depending on what
mk/config.mkis included by every Makefile (directly or indirectly), so the configuration information is thereby communicated to all Makefiles.
- It translates
mk/config.h. The latter is
#included by various C source files, which can thereby make use of configuration information.
configure takes some optional arguments. Use
to get a list of the available arguments. Here are some of
the ones you might need:
Specifies the "build platform" (see platform names).
This is usually only necessary on Windows, where you should set it
- Set the "host platform" (see platform names). Usually not necessary.
- Set the "target platform" (see platform names). Usually not necessary.
Specifies the path to an installed GHC which you would like to use.
This compiler will be used for compiling GHC-specific code (eg. GHC
itself). This option cannot be specified using
build.mk(see later), because
configureneeds to auto-detect the version of GHC you're using. The default is to look for a compiler named
ghcin your path.
Specifies the path to the installed GCC. This compiler will be used
to compile all C files, except any generated by the installed
Haskell compiler, which will have its own idea of which C compiler
(if any) to use. The default is to use
gcc. On Windows, this should be set to the gcc that comes with MinGW, which by default is
ldprogram to use. Normally only necessary on Windows, where you should set it to the
ldthat comes with MinGW, which is usually
Tells the build system where you would like GHC to be eventually
installed. You don't have to install GHC in order to use it: it is
entirely possible to work on GHC and test it without ever modifying
anything outside the build tree. However, if you do with to install
GHC, then the
--prefixdirectory is the root of the install tree. Typciallly on Unix systems the default for
/usr/local. See also Building/Installing for more details.
Next, you say how this build of GHC is to differ from the standard
defaults by creating a new file
mk/build.mk in the build tree.
This file is the one and only file you edit in the build tree,
precisely because it says how this build differs from the source.
(Just in case your build tree does die, you might want to keep a
private directory of
build.mk files, and use a symbolic link in each
build tree to point to the appropriate one.)
mk/build.mk is purely for overriding settings that are found in
mk/config.mk. You should never edit
mk/config.mk directly, since
it is automatically generated from
Neither should you edit
mk/config.mk.in; just provide your settings
We provide a sample file,
mk/build.mk.sample, which you can copy to
mk/build.mk and edit. It provides a number of pre-defined
configurations, such as a "fast build" or a "development build". Take
a look in the file itself for details.
To understand more about what you can put in
mk/build.mk, read on.
Common build.mk options
The following are some common variables that you might want to set in
mk/build.mk. For other variables that you can override,
take a look in mk/config.mk.in.
Options passed to GHC for all Haskell compilations.
Options added when compiling GHC (all stages)
Options added when compiling the stage 1 GHC.
Options added when compiling the stage 2 GHC.
Options added when compiling the stage 3 GHC.
Options added when compiling the libraries.
YESto enable profiling for GHC itself (stage 2). You'll also need to add
GhcLibWays += pto make this work.
-debugwhen building GHC (stage 2).
Ways in which to build the libraries. Must contain at least
v(the vanilla way). Also contains
pby default (profiling). For other ways, see
When set to
YES, static library object files are split into smaller pieces. This means that less of the library code needs to be linked into the final application, which makes smaller binaries. It takes longer to build libraries this way, though.
When set to
YES, these build the docbook documentation (e.g. the users guide) as HTML, PS and PDF respectively.
By default this is set to
integer-gmp, which means Integer is implemented on top of the C GMP library. If you set it to
integer-simplethan a simple, BSD-licensed Haskell implementation will be used instead.
How to make GHC build quickly
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 are the
build.mk settings that we use to build fast:
# My build settings for hacking on stage 2 SRC_HC_OPTS = -H32m -O -fasm -Rghc-timing GhcStage1HcOpts = -O -fasm GhcStage2HcOpts = -O0 -DDEBUG -Wall GhcLibHcOpts = -O -fasm -XGenerics GhcLibWays = v 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-timingprints out a line of timing info about each compilation. It's handy to keep an eye on.
-Wallturns on all the warnings; GHC is meant to be warning-clean with
GhcStage1HcOpts = -O -fasm
- Build stage 1 optimised: we're going to be rebuilding stage 2 a lot, so we want the compiler that does the building to be fast.
GhcStage2HcOpts = -O0 -DDEBUG -Wall
We turn off optimisation here, assuming you'll be modifying and
testing stage 2. 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 much less stuff is recompiled
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 -fasm -XGenerics
You almost certainly want optimisation on when building
libraries, otherwise the code you build with this compiler
goes really slowly.
-fgenericsadd 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 = v
Normally the profiled libraries are built. Setting
GhcLibWaysto just "v" 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.
At this point you have made yourself a fully-configured build tree, so you are ready to start building real things.
The first thing you need to know is that you must use GNU
make. On some systems (eg. FreeBSD) this is called
gmake, whereas on others it is the standard
In this document we will always refer to it as
gmake if your system requires it. If you use a
make you will get all sorts of error messages (but no
damage) because the GHC
Makefiles use GNU
To just build the whole thing,
cd to the top of your build tree and
make. This will prepare the tree and build the various parts
in the correct order, resulting in a complete build of GHC that can
even be used directly from the tree (as
without being installed first.
Order of the build
Here is a high level view of what happens when you build GHC:
- First, we build a few packages that GHC itself depends on, such as
filepath, using your installed GHC. These packages are under
libraries, and each is built in a subdirectory
dist-boot; for example the bootstrap build of Cabal will be in
- Then we build package
ghc, still using the installed GHC. The
ghcpackage is a perfectly ordinary package. The source code for the package is all the files in
- Now, still using the installed GHC, we build the
ghcexecutable, from source code in
ghc/. This source code is just a small Haskell program that depends on package
The resulting executable is called the "stage 1" compiler (see stages). You can run the stage 1 compiler by invoking
inplace/bin/ghc-stage1. The stage 1 build of GHC happens in
- The stage 1 compiler is now used to build all the packages in the
librariessubdirectory, and the runtime system in
- Finally, the stage 1 compiler is used to build GHC itself again,
this time against the libraries we just built. This GHC is called
stage 2, and can be invokes as
There's an optional final stage, using the stage 2 compiler to build a stage 3 compiler, but this isn't strictly necessary, and is only used to check that the stage 2 compiler is working properly.
What to do after
make has finished
make completes successfully, then it should have created
inplace/bin/ghc-stage2, which is a GHC binary you can run directly.
It supports all the usual features, including GHCi if you pass the
In fact, the
inplace directory looks a lot like an installed copy of
GHC (see Building/Installing): there is a
subdirectory containing various programs that can be run, including
ghc-pkg, Haddock and
You can now run the testsuite, see Building/RunningTests.
You can now install GHC, by typing
What to do if you get a build failure
GHC is a complex system, with many platform-dependent components. We try our best to make sure it builds out of the box as often as possible, but build failures are not uncommon. If you get some kind of failure, don't panic.
The chances are, someone else already encountered the same problem as you and has reported it on a mailing list or as a ticket. Search for the error message using your favourite search engine, and if that doesn't turn anything up then search directly on this Trac. Finally, if you don't find a solution:
Developing in a GHC build tree
This section describes how to make changes to the sources of GHC, or its libraries and tools, and use the build system to build and test those changes.
Bringing the whole tree up to date
The easy way to bring the tree up to date after making some changes is
simply to go to the top of the tree and say
make. The build system
has enough dependencies encoded into it that this should rebuild
everything that needs to be rebuilt, in the correct order. The
downside is that it will build everything right though to the stage2
compiler and including all the documentation, which might be overkill
if all you wanted to do was to test a small change to GHC.
Building a single sub-component
Each subdirectory of the source tree has a stub makefile, most of which follow this pattern:
dir = libraries/base TOP = ../.. include $(TOP)/mk/sub-makefile.mk
the main purpose of the stub makefile is to invoke
make at the
top-level of the tree: GHC's build system is
in effect there is really just one
Makefile, at the top level.
However, the stub makefile has another purpose: it passes a target to
Makefile, telling it to build just the components of
the system in this directory. For example, when you say
make in the
rts directory, this is actually equivalent to
$ make -C .. all_rts
-C .." tells make to invoke the
Makefile in the directory "
all_rts is the target that makes everything in the
make all_libraries/base at the top level would build
everything in the
libraries/base subdirectory. To understand how
all targets are defined, see
You can also clean a single component of the tree, just by saying
make clean in a subdirectory. Again this is equivalent to issuing a
command at the top of the tree of the form
Rebuilding the GHC binary after making changes
Suppose you want to make a small change to GHC itself and test it. Assuming that your tree is already up to date, the best way to do this is as follows:
$ cd ghc $ make stage=2
This will bring the stage 2 compiler up to date only. Setting
stage=2 has the effect of disabling all the
rules that build the stage 1 compiler, so the build system will ignore the fact that the stage 1 compiler is also out of date, and hence all the libraries are also potentially out of date. If you did
from the top-level, all of these dependencies would be taken into
account, and a lot of rebuilding would probably ensue. There's another target
that takes an even quicker shortcut:
$ cd ghc $ make 2
This is like
make stage=2, except that it omits the dependency-building phase (
make 2 is in fact just shorthand for
make stage=2 FAST=YES; see Fast Rebuilding below). If you have changed the imports in any modules, those new dependencies will not be taken into account by the build system, so you might get a build failure. On the other hand, this shortcut usually works and the few seconds it saves can make GHC development a much more interactive experience. There are also targets
to make the stage 1 and stage 3 compilers respectively. These targets work in both the
compiler subdirectories (Commentary/SourceTree).
Building a single file
It's possible to tell make to build a single file, from any subdirectory in the tree. For example, suppose I want to build just the module
Control.Monad in the
base package, I can do it like this:
$ make libraries/base/dist-install/build/Control/Monad.o
(you have to know that
dist-install is the distdir for a package, and object files are put in the subdirectory
build). It's also possible to do this from the
$ cd libraries/base $ make dist-install/build/Control/Monad.o
suppose you wanted to build this module with a few extra flags, perhaps because you want to see what GHC's optimiser is doing on this module:
$ rm dist-install/build/Control/Monad.o $ make dist-install/build/Control/Monad.o EXTRA_HC_OPTS=-dcore-lint
you could also cut-and-paste the command-line to add flags, but sometimes the
EXTRA_HC_OPTS method is more convenient.
Often when you're working in a particular part of the tree, the rest of the tree is up to date, and you just want to rebuild the component you're working on after making changes. To speed things up, we'd like to avoid having
make check that everything else in the tree is up-to-date before building the component we're working on. So the GHC build system provides a couple of ways to do this:
Only has an effect in a subdirectory. Setting
maketo omit rebuilding the
.dependfile (if any), and also omits some of the phases.
FAST=YESis allowed in conjunction with any other target; for example, it makes sense when rebuilding a single file, as in the previous section.
make all FAST=YES.
Another useful trick is
Does not build any GHC stages at all.
stage=0can be used in combination with other targets and settings.
Installing extra packages
The boot libraries are built as part of building GHC; they are built with the stage1 compiler, and imported when the stage2 compiler is compiled with stage1.
All other libraries are stand-alone Cabal packages, and the build system knows nothing about them. Nevertheless, it is common to want to install extra packages for the GHC in your build tree. Here are instructions for how to do so.