The GAP build system

This file is meant to give an overview of how the GAP build system works. It is targeted at people who need to work on the build system itself (to extend it, fixes bugs in it, etc.). It should not be necessary to read this if all you want to do is compile GAP and work on the GAP library and kernel.

Note that this really is just an overview; for details, please refer to the comments inside the various parts of the build system.

Prerequisites

In order to work on the build system, you need at least the following:

• GNU autoconf (we recommend 2.69 or later)
• GNU make

Note that we extensively use features provided by GNU make, so in general another version of make, such as BSD make, is not suitable.

Quick start: building GAP with no frills

If you are working with a fresh clone of the GAP repository, you need to run the autogen.sh script first, which will generate the configure script. Afterwards, or if you are using a release version of GAP, you can follow the standard procedure:

./configure make

== Overview of the files constituting the GAP build system

• autogen.sh: sets up the build system; typically the first thing to run in a fresh clone of the GAP repository. It runs autoreconf which in turn runs several tools include aclocal, autoheader, autoconf, libtoolize

• configure: generated by autogen.sh from configure.ac.

• configure.ac: the GNU autoconf source of our configure script.

• GNUmakefile, GNUmakefile.in: The former file is generated from the latter by configure. It is the primary Makefile (GNU make prefers it over Makefile). It only contains variables and vpath settings, and includes Makefile.rules for the actual build rules.

• Makefile: This is a placeholder file, and serves two purposes:

  1. If the user runs make before configure, it prints a warning.
  2. If configure did run, but make is not GNU make, it produces a corresponding error message.

• Makefile.rules: This is the core of the build system. If you want to add or remove a kernel C source file, you need to add or remove its name here and only here.

• bin/: This directory is created for compatibility mode (see below).

• cnf/: All files in this directory are part of the build system.

• extern/: External libraries we bundle with GAP (such as GMP) are put in here.

• gen/: Generated code (such as config.h and gap_version.c) is put into this directory.

• obj/: All *.o files are placed into this directory.

• .deps/ directories contain *.d files generated by the build system, and which are used to track dependencies, e.g. of C source files on header files.

• .libs/ directories are created by libtool. Please refer to the libtool documentation for details.

Out-of-tree builds

The old GAP build system had a concept of "configs" and "CONFIGNAME", which allowed you to build GAP in different configurations from a single set of sources. This is gone in the current build system. However, a similar goal can be achieved by using so-called "out-of-tree builds".

In the following and also in the files that make up the build system, "srcdir" refers to the directory containing the GAP files, i.e. it contains this README, the src and lib directories and more.

To create a new out-of-tree build, create a new directory anywhere in your filesystem. A typical setup places the out-of-tree dirs into subdirectories of a "build" directory inside the srcdir. So you might have directories

srcdir/build/default srcdir/build/default32 srcdir/build/hpcgap srcdir/build/hpcgap32 ...

We will refer to this directory from now on as the "builddir".

To initialize the out-of-tree build, change into the builddir and execute the configure script from the srcdir, like this:

cd $builddir $srcdir/configure

You can pass any additional options you like to configure, e.g. ABI=32 or --enable-hpcgap.

Once the configure script has completed, you can run make as usual, and all the object files and the gap executable will be placed inside builddir. Your srcdir will remain untouched.

Compatibility mode

Compatibility mode emulates the build environment of the old GAP build system in order to allow packages with kernel extensions to be used with the new build system unmodified. As such, it mainly exists to ease the transition between new and old build system, and the plan is to remove it once all packages have been adapted to the new build system. However, that is still far off.

Compatibility mode does the following things:

• create sysinfo.gap file in the build dir
• create a symlink sysinfo.gap-default$ABI pointing at sysinfo.gap
• create a bin/$GAPARCH/config.h symlink pointing at gen/config.h
• create a bin/$GAPARCH/gac symlink pointing at gac
• create a bin/$GAPARCH/src symlink pointing at either $srcdir/src or (for HPC-GAP build) at $srcdir/hpcgap/src
• for out-of-tree builds, it creates a ${builddir}/src/compiled.h file
• ...

For now, using compatibility mode is required if one wants to build the kernel extension for most packages which have one. In the future, we will provide an alternative way to do this, and also will extend gac to cleanly supported building kernel extensions.

Dependency tracking

The build system tracks depdencies between files, such as between C source and header files, via *.d files in .deps directories below obj/ and gen/. These files are mostly generated by the compiler; for this, the compiler needs to support the relevant flags (gcc, clang, icc all do so).

For a detailed explanation of a very similar scheme, see here: http://make.mad-scientist.net/papers/advanced-auto-dependency-generation/

HPC-GAP integration

One of the main features of the new build system is that it optionally allows to build HPC-GAP instead of plain GAP. HPC-GAP is an experimental fork of GAP which implements concurrent programming, multi-threading, etc..

The HPC-GAP kernel and library were forked from the GAP kernel and library and developed semi-independently for several years, with occasional merges between the two. In order to recombine the two, we developed the following plan: We merged the HPC-GAP fork into a subdirectory hpcgap of the GAP repository. Then, all files inside hpcgap which were identical to their counterparts in the GAP repository were deleted (e.g. hpcgap/src/ariths.c was deleted as it was identical to src/ariths.c).

The new build system was then modified to optionally allow building HPC-GAP (when the --enable-hpcgap flag is passed to the configure script). For this work, and the resulting HPC-GAP binary to work, several techniques are employed:

1. For the kernel C source files, whenever both src/FOO.c and hpcgap/src/FOO.c exist, the latter is compiled, the former ignored.

2. The include paths for C headers are ordered so that files in hpcgap/src are found before those in src

3. However, the C preprocessor directive #include "FOO.h" always searches first in the current directory. This would lead to incorrect compilation, as src/FOO.h might get picked up even if there is a hpcgap/src/FOO.h header. To prevent this, all include directives were changed to the form #include <src/FOO.h>, which does not search in the current directory first. The src/ prefix was added so that internal headers can be quickly distinguished from external headers (e.g. of the standard C library). We expect that this change will be undone again in the future, once the GAP and HPC-GAP kernels have been merged.

4. In order to get the library to work right, HPC-GAP mode employs multiple GAP root paths. Specifically, the GAP kernel function SySetGapRootPath was modified so that for every root directory FOO that gets added, we first add FOO/hpcgap to the list of root directories.

5. Ward (a tool for scanning our C sources and inserting guard statements into it) is integrated as follows: Normally, src/FOO.c gets compiled to obj/FOO.o. With ward in the mix, src/FOO.c or (if it exists) hpcgap/src/FOO.c is first turned into gen/FOO.c by ward, and then the compiler turns the latter into obj/FOO.o. For each conversion, dependencies are tracked via .../.deps/FOO.d files. In particular, if hpcgap/src/FOO.c exists and is deleted, then the dependency rules in gen/.deps/FOO.d will ensure that gen/FOO.c is regenerated from src/FOO.c.

Open tasks

There are many things that still need to be done in the new build system. For a (possibly incomplete) list, please refer to the GAP issue tracker under the label build system. For now also refer to https://github.com/fingolfin/gap/issues