-*- Outline -*- Read this file if you are a Debian Developer or would like to become one, or if you would like to create your own binary packages of GCC. * Overview From the GCC sources, Debian currently builds 3 source packages and almost 100 binary packages, using a single set of build scripts. The 3 source packages are: gcc-x.y: C, C++, Fortran, Objective-C and Objective-C++, plus many common libraries like libssp and libgcc. gnat-x.y: Ada. The way we do this is quite peculiar, so listen up :) When we build from the gcc-x.y source package, we produce, among many others, a gcc-x.y-source binary package that contains the pristine upstream tarball and some Debian-specific patches. Any user can then install this package on their Debian system, and will have the full souces in /usr/src/gcc-x.y/gcc-.tar.bz2, along with the Makefile snippets that unpack and patch them. The intended use for this package is twofold: (a) allow users to build their own cross-compilers, and (b) build the other packages like gnat-x.y. - gcc-x.y requires only a C compiler to build and produces C, C++, Fortran, Go and Objective-C compilers and libraries. It also produces the binary package gcc-x.y-source containing all the sources and patches in a tarball. - gnat-x.y build-depends on gcc-x.y-source and an Ada compiler. It does not even have an .orig.tar.bz2 package; it is a Debian native package. The benefits of this split are many: - bootstrapping a subset of languages is much faster than bootstrapping all languages and libraries (which can take a full week on slow architectures like mips or arm) - the language maintainers don't have to wait for each other - for new ports, the absence of a port of, say, gnat-x.y does not block the porting of gcc-x.y. gcc-x.y-source is also intended for interested users to build cross-compiler packages. Debian cannot provide all possible cross-compiler packages (i.e. all possible host, target, language and library combinations), so instead tries to facilitate building them. * The build sequence As for all other Debian packages, you build GCC by calling debian/rules. The first thing debian/rules does it to look at the top-most entry in debian/changelog: this tells it which source package it is building. For example, if the first entry in debian/changelog reads: gnat-6 (6.2.0-1) unstable; urgency=low * Upload as gnat-6. -- Ludovic Brenta Tue, 26 Jun 2007 00:26:42 +0200 then, debian/rules will build only the gnat binary packages. The second step is to build debian/control from debian/control.m4 and a complex set of rules specified in debian/rules.conf. The resulting control file contains only the binary packages to be built. The third step is to select which patches to apply (this is done in debian/rules.defs), and then to apply the selected patches (see debian/rules.patch). The result of this step is a generated debian/patches/series file for use by quilt. The fourth step is to unpack the GCC source tarball. This tarball is either in the build directory (when building gcc-x.y), or in /usr/src/gcc-x.y/gcc-x.y.z.tar.xz (when building the other source packages). The fifth step is to apply all patches to the unpacked sources with quilt. The sixth step is to create a "build" directory, cd into it, call ../src/configure, and bootstrap the compiler and libraries selected. This is in debian/rules2. The seventh step is to call "make install" in the build directory: this installs the compiler and libraries into debian/tmp (i.e. debian/tmp/usr/bin/gcc, etc.) The eighth step is to run the GCC test suite. This actually takes at least as much time as bootstrapping, and you can disable it by setting WITHOUT_CHECK to "yes" in the environment. The ninth step is to build the binary packages, i.e. the .debs. This is done by a set of language- and architecture-dependent Makefile snippets in the debian/rules.d/ directory, which move files from the debian/tmp tree to the debian/ trees. * Making your own packages In this example, we will build our own gnat-x.y package. 1) Install gcc-x.y-source, which contains the real sources: # aptitude install gcc-x.y-source 2) Create a build directory: $ mkdir gnat-x.y-x.y.z; cd gnat-x.y-x.y.z 3) Checkout from Subversion: $ svn checkout svn://svn.debian.org/gcccvs/branches/sid/gcc-x.y/debian 4) Edit the debian/changelog file, adding a new entry at the top that starts with "gnat-x.y". 5) Generate the debian/control file, adjusted for gnat: $ debian/rules control 8) Build: $ dpkg-buildpackage * Hints You need a powerful machine to build GCC. The larger, the better. The build scripts take advantage of as many CPU threads as are available in your box (for example: 2 threads on a dual-core amd64; 4 threads on a dual-core POWER5; 32 threads on an 8-core UltraSPARC T1, etc.). If you have 2 GB or more of physical RAM, you can achieve maximum performance by building in a tmpfs, like this: 1) as root, create the new tmpfs: # mount -t tmpfs -o size=1280m none /home/lbrenta/src/debian/ram By default, the tmpfs will be limited to half your physical RAM. The beauty of it is that it only consumes as much physical RAM as necessary to hold the files in it; deleting files frees up RAM. 2) As your regular user, create the working directory in the tmpfs $ cp --archive ~/src/debian/gcc-x.y-x.y.z ~/src/debian/ram 3) Build in there. On my dual-core, 2 GHz amd64, it takes 34 minutes to build gnat, and the tmpfs takes 992 MiB of physical RAM but exceeds 1 GiB during the build. Note that the build process uses a lot of temporary files. Your $TEMP directory should therefore also be in a ram disk. You can achieve that either by mounting it as tmpfs, or by setting TEMP to point to ~/src/debian/ram. Also note that each thread in your processor(s) will run a compiler in it and use up RAM. Therefore your physical memory should be: Physical_RAM >= 1.2 + 0.4 * Threads (in GiB) (this is an estimate; your mileage may vary). If you have less physical RAM than recommended, reduce the number of threads allocated to the build process, or do not use a tmpfs to build. * Patching GCC Debian applies a large number of patches to GCC as part of the build process. It uses quilt but the necessary debian/patches/series is not part of the packaging scripts; instead, "debian/rules patch" generates this file by looking at debian/control (which is itself generated!), debian/changelog and other files. Then it applies all the patches. At this point, you can use quilt as usual: $ cd ~/src/debian/gcc-x.y $ export QUILT_PATCHES=$PWD/debian/patches $ quilt series If you add new patches, remember to add them to the version control system too. -- Ludovic Brenta, 2012-04-02.