6. Working with the build system
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## 6. Working with the build system

This rest of this guide is intended for duffers like me, who aren't really interested in Makefiles and systems configurations, but who need a mental model of the interlocking pieces so that they can make them work, extend them consistently when adding new software, and lay hands on them gently when they don't work.

### 6.1. History

First, a historical note. The GHC build system used to be called "fptools": a generic build system used to build multiple projects (GHC, Happy, GreenCard, H/Direct, etc.). It had a concept of the generic project-independent parts, and project-specific parts that resided in a project subdirectory.

Nowadays, most of these other projects are using Cabal, or have faded away, and GHC is the only regular user of the fptools build system. We decided therefore to simplify the situation for developers, and specialise the build system for GHC. This resulted in a simpler organisation of the source tree and the build system, which hopefully makes the whole thing easier to understand.

You might find old comments that refer to "projects" or "fptools" in the documentation and/or source; please let us know if you do.

### 6.2. Build trees

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.

We often want to build multiple versions of our software for different architectures, or with different options (e.g. profiling). It's very desirable to share a single copy of the source code among all these builds.

So for every source tree we have zero or more build trees. Each build tree is initially an exact copy of the source tree, except that each file is a symbolic link to the source file, rather than being a copy of the source file. There are “standard” Unix utilities that make such copies, so standard that they go by different names: lndir, mkshadowdir are two (If you don't have either, the source distribution includes sources for the X11 lndir—check out `utils/lndir`). See Section 6.4, “The story so far” for a typical invocation.

The build tree does not need to be anywhere near the source tree in the file system. Indeed, one advantage of separating the build tree from the source is that the build tree can be placed in a non-backed-up partition, saving your systems support people from backing up untold megabytes of easily-regenerated, and rapidly-changing, gubbins. The golden rule is that (with a single exception—Section 6.3, “Getting the build you want”) absolutely everything in the build tree is either a symbolic link to the source tree, or else is mechanically generated. It should be perfectly OK for your build tree to vanish overnight; an hour or two compiling and you're on the road again.

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 a build tree!

Remember, that the source files in the build tree are symbolic links to the files in the source tree. (The build tree soon accumulates lots of built files like `Foo.o`, as well.) You can delete a source file from the build tree without affecting the source tree (though it's an odd thing to do). On the other hand, if you edit a source file from the build tree, you'll edit the source-tree file directly. (You can set up Emacs so that if you edit a source file from the build tree, Emacs will silently create an edited copy of the source file in the build tree, leaving the source file unchanged; but the danger is that you think you've edited the source file whereas actually all you've done is edit the build-tree copy. More commonly you do want to edit the source file.)

Like the source tree, the top level of your build tree must be (a linked copy of) the root directory of the GHC source tree.. Inside Makefiles, the root of your build tree is called `$(GHC_TOP)`. In the rest of this document path names are relative to `$(GHC_TOP)` unless otherwise stated. For example, the file `mk/target.mk` is actually `$(GHC_TOP)/mk/target.mk`.

### 6.3. Getting the build you want

When you build GHC you will be compiling code on a particular host platform, to run on a particular target platform (usually the same as the host platform). The difficulty is that there are minor differences between different platforms; minor, but enough that the code needs to be a bit different for each. There are some big differences too: for a different architecture we need to build GHC with a different native-code generator.

There are also knobs you can turn to control how the software is built. For example, you might want to build GHC optimised (so that it runs fast) or unoptimised (so that you can compile it fast after you've modified it. Or, you might want to compile it with debugging on (so that extra consistency-checking code gets included) or off. And so on.

All of this stuff is called the configuration of your build. You set the configuration using a three-step process.

Step 1: get ready for configuration.

  NOTE: if you're starting from a source distribution, rather than darcs sources, you can skip this step.

  Change directory to `$(GHC_TOP)` and issue the command

      $ autoreconf

  (with no arguments). This GNU program (recursively) converts `$(GHC_TOP)/configure.ac` and `$(GHC_TOP)/aclocal.m4` to a shell script called `$(GHC_TOP)/configure`. If autoreconf bleats that it can't write the file `configure`, then delete the latter and try again. Note that you must use autoreconf, and not the old autoconf! If you erroneously use the latter, you'll get a message like "No rule to make target 'mk/config.h.in'".

  Some parts of the source tree, particularly libraries, have their own configure script. autoreconf takes care of that, too, so all you have to do is calling autoreconf in the top-level directory `$(GHC_TOP)`.

  These steps are completely platform-independent; they just mean that the human-written files (`configure.ac` and `aclocal.m4`) can be short, although the resulting files (the configure shell scripts and the C header template `mk/config.h.in`) are long.

Step 2: system configuration.

  Runs the newly-created 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 gcc is 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 `mk/config.mk.in` to `mk/config.mk`, substituting for things between “`@`” brackets. So, “`@HaveGcc@`” will be replaced by “`YES`” or “`NO`” depending on what configure finds. `mk/config.mk` is included by every Makefile (directly or indirectly), so the configuration information is thereby communicated to all Makefiles.

  - It translates `mk/config.h.in` to `mk/config.h`. The latter is `#include`d by various C programs, which can thereby make use of configuration information.

  configure takes some optional arguments. Use `./configure --help` to get a list of the available arguments. Here are some of the ones you might need:

  `--with-ghc=`path``

    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 configure needs to auto-detect the version of GHC you're using. The default is to look for a compiler named `ghc` in your path.

  `--with-hc=`path``

    Specifies the path to any installed Haskell compiler. This compiler will be used for compiling generic Haskell code. The default is to use `ghc`. (NOTE: I'm not sure it actually works to specify a compiler other than GHC here; unless you really know what you're doing I suggest not using this option at all.)

  `--with-gcc=`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`.

Step 3: build configuration.

  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.) So `mk/build.mk` never exists in the source tree—you create one in each build tree from the template. We'll discuss what to put in it shortly.

And that's it for configuration. Simple, eh?

What do you put in your build-specific configuration file `mk/build.mk`? For almost all purposes all you will do is put make variable definitions that override those in `mk/config.mk.in`. The whole point of `mk/config.mk.in`—and its derived counterpart `mk/config.mk`—is to define the build configuration. It is heavily commented, as you will see if you look at it. So generally, what you do is look at `mk/config.mk.in`, and add definitions in `mk/build.mk` that override any of the `config.mk` definitions that you want to change. (The override occurs because the main boilerplate file, `mk/boilerplate.mk`, includes `build.mk` after `config.mk`.)

For your convenience, there's a file called `build.mk.sample` that can serve as a starting point for your `build.mk`.

For example, `config.mk.in` contains the definition:

    GhcHcOpts=-Rghc-timing

The accompanying comment explains that this is the list of flags passed to GHC when building GHC itself. For doing development, it is wise to add `-DDEBUG`, to enable debugging code. So you would add the following to `build.mk`:

    GhcHcOpts += -DDEBUG

GNU make allows existing definitions to have new text appended using the “`+=`” operator, which is quite a convenient feature.

Haskell compilations by default have `-O` turned on, by virtue of this setting from `config.mk`:

    SRC_HC_OPTS += -H16m -O

`SRC_HC_OPTS` means "options for HC from the source tree", where HC stands for Haskell Compiler. `SRC_HC_OPTS` are added to every Haskell compilation. To turn off optimisation, you could add this to `build.mk`:

    SRC_HC_OPTS = -H16m -O0

Or you could just add `-O0` to `GhcHcOpts` to turn off optimisation for the compiler. See Section 5, “Quick start for GHC developers” for some more suggestions.

When reading `config.mk.in`, remember that anything between “@...@” signs is going to be substituted by configure later. You can override the resulting definition if you want, but you need to be a bit surer what you are doing. For example, there's a line that says:

    TAR = @TarCmd@

This defines the Make variables `TAR` to the pathname for a tar that configure finds somewhere. If you have your own pet tar you want to use instead, that's fine. Just add this line to `mk/build.mk`:

    TAR = mytar

You do not have to have a `mk/build.mk` file at all; if you don't, you'll get all the default settings from `mk/config.mk.in`.

You can also use `build.mk` to override anything that configure got wrong. One place where this happens often is with the definition of `GHC_TOP_ABS`: this variable is supposed to be the canonical path to the top of your source tree, but if your system uses an automounter then the correct directory is hard to find automatically. If you find that configure has got it wrong, just put the correct definition in `build.mk`.

### 6.4. The story so far

Let's summarise the steps you need to carry to get yourself a fully-configured build tree from scratch.

1. Get your source tree from somewhere (darcs repository or source distribution). Say you call the root directory `myghc` (it does not have to be called `ghc`).

2. (Optional) Use lndir or mkshadowdir to create a build tree.

       $ cd myghc
       $ mkshadowdir . /scratch/joe-bloggs/myghc-x86

   (N.B. mkshadowdir's first argument is taken relative to its second.) You probably want to give the build tree a name that suggests its main defining characteristic (in your mind at least), in case you later add others.

3. Change directory to the build tree. Everything is going to happen there now.

       $ cd /scratch/joe-bloggs/myghc-x86

4. Prepare for system configuration:

       $ autoreconf

   (You can skip this step if you are starting from a source distribution, and you already have `configure` and `mk/config.h.in`.)

5. Do system configuration:

       $ ./configure

   Don't forget to check whether you need to add any arguments to `configure`; for example, a common requirement is to specify which GHC to use with `--with-ghc=`ghc``.

6. Create the file `mk/build.mk`, adding definitions for your desired configuration options.

You can make subsequent changes to `mk/build.mk` as often as you like. You do not have to run any further configuration programs to make these changes take effect. In theory you should, however, say make clean; make, because configuration option changes could affect anything—but in practice you are likely to know what's affected.

### 6.5. Making things

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 make command. In this document we will always refer to it as make; please substitute with gmake if your system requires it. If you use a the wrong make you will get all sorts of error messages (but no damage) because the GHC Makefiles use GNU make's facilities extensively.

To just build the whole thing, cd to the top of your build tree and type 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, without being installed first.

### 6.6. Bootstrapping GHC

GHC requires a 2-stage bootstrap in order to provide full functionality, including GHCi. By a 2-stage bootstrap, we mean that the compiler is built once using the installed GHC, and then again using the compiler built in the first stage. You can also build a stage 3 compiler, but this normally isn't necessary except to verify that the stage 2 compiler is working properly.

Note that when doing a bootstrap, the stage 1 compiler must be built, followed by the runtime system and libraries, and then the stage 2 compiler. The correct ordering is implemented by the top-level `Makefile`, so if you want everything to work automatically it's best to start make from the top of the tree. The top-level `Makefile` is set up to do a 2-stage bootstrap by default (when you say make). Some other targets it supports are:

stage1

  Build everything as normal, including the stage 1 compiler.

stage2

  Build the stage 2 compiler only.

stage3

  Build the stage 3 compiler only.

bootstrap, bootstrap2,

  Build stage 1 followed by stage 2.

bootstrap3

  Build stages 1, 2 and 3.

install

  Install everything, including the compiler built in stage 2. To override the stage, say `make install stage=`n`` where `n` is the stage to install.

`binary-dist`

  make a binary distribution. This is the target we use to build the binary distributions of GHC.

`dist`

  make a source distribution. Note that this target does “make distclean” as part of its work; don't use it if you want to keep what you've built.

The top-level `Makefile` also arranges to do the appropriate `make boot` steps (see below) before actually building anything.

The `stage1`, `stage2` and `stage3` targets also work in the `compiler` directory, but don't forget that each stage requires its own `make boot` step: for example, you must do

    $ make boot stage=2

before `make stage2` in `compiler`.

### 6.7. Standard Targets

In any directory you should be able to make the following:

`boot`

  does the one-off preparation required to get ready for the real work. Notably, it does make depend in all directories that contain programs. It also builds the necessary tools for compilation to proceed.

  Invoking the `boot` target explicitly is not normally necessary. From the top-level directory, invoking `make` causes `make boot` to be invoked in various subdirectories first, in the right order. Unless you really know what you are doing, it is best to always say `make` from the top level first.

  If you're working in a subdirectory somewhere and need to update the dependencies, `make boot` is a good way to do it.

`all`

  makes all the final target(s) for this Makefile. Depending on which directory you are in a “final target” may be an executable program, a library archive, a shell script, or a Postscript file. Typing make alone is generally the same as typing make all.

`install`

  installs the things built by `all` (except for the documentation). Where does it install them? That is specified by `mk/config.mk.in`; you can override it in `mk/build.mk`, or by running configure with command-line arguments like `--bindir=/home/simonpj/bin`; see `./configure --help` for the full details.

`install-docs`

  installs the documentation. Otherwise behaves just like `install`.

`uninstall`

  reverses the effect of `install` (WARNING: probably doesn't work).

`clean`

  Delete all files from the current directory that are normally created by building the program. Don't delete the files that record the configuration, or files generated by make boot. Also preserve files that could be made by building, but normally aren't because the distribution comes with them.

`distclean`

  Delete all files from the current directory that are created by configuring or building the program. If you have unpacked the source and built the program without creating any other files, `make distclean` should leave only the files that were in the distribution.

`mostlyclean`

  Like `clean`, but may refrain from deleting a few files that people normally don't want to recompile.

`maintainer-clean`

  Delete everything from the current directory that can be reconstructed with this Makefile. This typically includes everything deleted by `distclean`, plus more: C source files produced by Bison, tags tables, Info files, and so on.

  One exception, however: `make maintainer-clean` should not delete `configure` even if `configure` can be remade using a rule in the `Makefile`. More generally, `make maintainer-clean` should not delete anything that needs to exist in order to run `configure` and then begin to build the program.

  After a `maintainer-clean`, a `configure` will be necessary before building again.

All of these standard targets automatically recurse into sub-directories. Certain other standard targets do not:

`depend`

  make a `.depend` file in each directory that needs it. This `.depend` file contains mechanically-generated dependency information; for example, suppose a directory contains a Haskell source module `Foo.lhs` which imports another module `Baz`. Then the generated `.depend` file will contain the dependency:

      Foo.o : Baz.hi

  which says that the object file `Foo.o` depends on the interface file `Baz.hi` generated by compiling module `Baz`. The `.depend` file is automatically included by every Makefile.

Some `Makefile`s have targets other than these. You can discover them by looking in the `Makefile` itself.

### 6.8. Using GHC from the build tree

If you want to build GHC and just use it direct from the build tree without doing `make install` first, you can run the in-place driver script. To run the stage 1 compiler, use `compiler/stage1/ghc-inplace`, stage 2 is `compiler/stage2/ghc-inplace`, and so on.

Do NOT use `compiler/stage1/ghc`, or `compiler/stage1/ghc-6.xx`, as these are the scripts intended for installation, and contain hard-wired paths to the installed libraries, rather than the libraries in the build tree.

### 6.9. Fast Making

Sometimes the dependencies get in the way: if you've made a small change to one file, and you're absolutely sure that it won't affect anything else, but you know that make is going to rebuild everything anyway, the following hack may be useful:

    $ make FAST=YES

This tells the make system to ignore dependencies and just build what you tell it to. In other words, it's equivalent to temporarily removing the `.depend` file in the current directory (where mkdependHS and friends store their dependency information).

A bit of history: GHC used to come with a fastmake script that did the above job, but GNU make provides the features we need to do it without resorting to a script. Also, we've found that fastmaking is less useful since the advent of GHC's recompilation checker (see the User's Guide section on "Separate Compilation").

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