Catégorie: "Debian"

Rebuild of Debian using Clang 3.5.0

September 11th, 2014

Clang 3.5.0 has just been released. A new rebuild has been done highlight the progress to get Debian built with clang.

tl;dr: Great progress. We decreased from 9.5% to 5.7% of failures. Full results are available on

At time of the rebuild with 3.4.2, we had 2040 packages failing to build with clang. With 3.5.0, this dropped to 1261 packages.


With Arthur Marble and Alexander Ovchinnikov, both GSoC students, we worked on various ways to decrease the number of errors.

Upstream fixes

First, the most obvious way, we fixed programming bugs/mistakes in upstream sources. Basically, we took categories of failure and fixed issues one after the other. We started with simple bugs like 'Wrong main declaration', 'non-void function should return a value' or 'Void function should not return a value'.

They are trivial to fix. We continued with harder fixes like ' Undefined reference' or 'Variable length array for a non POD (plain old data) element'.

So, besides these one, we worked on:

In total, we reported 295 bugs with patches. 85 of them have been fixed (meaning that the Debian maintainer uploaded a new version with the fix).

In parallel, I think that the switch by FreeBSD and Mac OS X to Clang also helped to fix various issues by upstreams.

Hacking in clang

As a parallel approach, we started to implement a suggestion from Linus Torvalds and a few others. Instead of trying to fix all upstream, where we can, we tried to update clang to improve the gcc compatibility.

gcc has many flags to disable or enable optimizations. Some of them are legacy, others have no sense in clang, etc. Instead of failing in clang with an error, we create a new category of warnings (showing optimization flag '%0' is not supported) and moved all relevant flags into it. Some examples, r212805, r213365, r214906 or r214907

We also updated clang to silent some useless arguments like -finput-charset=UTF-8 (r212110), clang being UTF-8 compliant.

Finally, we worked on the forwarding of linker flags. Clang and gcc have a very different behavior: when gcc does not know an argument, it is going to forward the argument to the linker. Clang, in this case, is going to reject the argument and fail with an error. In clang, we have to explicitly declare which arguments are going to be transfer to the linker. Of course, the correct way to pass arguments to the linker is to use -Xlinker or -Wl but the Debian rebuild proved that these shortcuts are used. Two of these arguments are now forwarded:

  • -z keyword - r213198
  • -u Force symbol to be entered in the output file as an undefined symbol - r211756. This one fixed most of the haskell build failures. It fixed the most common issue that we had (701 occurrences but this does not mean that all these packages build fine now, some haskell-based package are failing later in the process)

New errors

Just like in other releases, new warnings are added in clang. With (bad) usage of -Werror by upstream software, this causes new build failures:

I also took the opportunity to add some further categorizations in the list of errors. Some examples:

Next steps

The Debile project being close to ready with Clément Schreiner's GSoC, we will now have an automatic and transparent way to rebuild packages using clang.


As stated, we can see a huge drop in term of number of failures over time:

Hopefully, Clang getting better and better, more and more projects adopting it as the default compiler or as a base for plugin/extension developments, this percentage will continue to decrease.
Having some kind of release goal with clang for Jessie+1 can now be considered as potentially reachable.

Want to help?

There are several things which can be done to help:

  • Point me common error patterns in the Not categorized list of errors to create new categories
  • Report and fix packages
  • As an upstream, integrate clang as part of your continuous integration system
  • Hack on cqa-scanlogs, the error detection tool to detect error patterns (example: Undetected error). This tool is used also for the regular rebuilds of the archive.

  • Improve website


Thanks to David Suarez for the rebuilds of the archive, Arthur Marble and Alexander Ovchinnikov for their GSoC works and Nicolas Sévelin-Radiguet for the few fixes.

clang 3.4, 3.5 and 3.6 are now coinstallable in Debian

Août 11th, 2014

Clang is finally co installable on Debian. 3.4, 3.5 and the current trunk (snapshot) can be installed together.

So, just like gcc, the different version can be called with clang-3.4, clang-3.5 or clang-3.6.

/usr/bin/clang, /usr/bin/clang++, /usr/bin/scan-build and /usr/bin/scan-view are now handled through the llvm-defaults package.

llvm-defaults is also now managing clang-check, clang-tblgen, c-index-test, clang-apply-replacements, clang-tidy, pp-trace and clang-query.

Changes are also available on
The next step will be to manage also llvm-defaults on to simplify the transition for people using these packages.

So, with:

# /etc/apt/sources.list
deb llvm-toolchain main
deb llvm-toolchain-3.4 main
deb llvm-toolchain-3.5 main
$ apt-get install clang-3.4 clang-3.5 clang-3.6

$ clang-3.4 --version
Debian clang version 3.4.2 (branches/release_34) (based on LLVM 3.4.2)
Target: x86_64-pc-linux-gnu
Thread model: posix

$ clang-3.5 --version
Debian clang version 3.5.0-+rc2-1~exp1 (tags/RELEASE_350/rc2) (based on LLVM 3.5.0)
Target: x86_64-pc-linux-gnu
Thread model: posix

$ clang-3.6 --version
Debian clang version 3.6.0-svn214990-1~exp1 (trunk) (based on LLVM 3.6.0)
Target: x86_64-pc-linux-gnu
Thread model: posix

Debian Twitter accounts are back

August 7th, 2014

After some downtime due to the changes, the Debian Twitter accounts are now back.

New Twitter feed ideas are welcome.

Rebuild of Debian using Clang 3.4

Mars 21st, 2014

Using the AWS donation, David Suarez and myself have been able to rebuild the whole archive with Clang 3.4.
The rebuild has been done January 10th but, with my new job, I did not find the time to publish the result.
Releases after releases, the results are getting better and better.

Currently, 2193 packages in the archive are failing to build from source.
That is roughly the same number of build failures as with the precedent rebuild with Clang 3.3.
However, this is good news for two reasons:
* the number of new packages in Debian increased (18854 at time of the 3.3 release, 21204 for the 3.4)
* clang 3.4 has more checks and error detections.

I also started to update clang to make it closer to gcc. For example, I transformed wrong usage of -O (> 6) error to be treated as regular warning.

However, a critical bug has emerged during this release. When using -D_FORTIFY_SOURCE=2 (which is the case of many Debian packages), the binaries produced freeze (infinity loop in the ASM). This has been reported upstream as bug 16821 and concerns about 150 packages (including Firefox, gcc, LLVM, etc). Hopefully, this will be fixed in 3.5 (this is unlikely for 3.4.1).

About the new build failures, now, Clang triggers an error (or warning + -Werror) on:

* Wrong usage of the default argument (should be done in the definition)
16 occurrences

* Usage of C++11 feature without the appropriate argument
7 occurrences

* Unused static const declaration
5 occurrences

* Recursive template instantiation exceeded
4 occurrences

* Defitinion of a builtin function
3 occurrences

* Read-only variable is not assignable
2 occurrences

By the way, I proposed a Google Summer of Code Project to work faster on a support of Debian built by Clang. As requirements, I asked students to fix some bugs, they already did a great job.

Some updates on

Février 22nd, 2014

I made some changes on for the last 2 months.

  • Added trusty, Ubuntu 14.04, as a new supported distribution (on the request of Michael Larabel, Phoronix)

  • Support both the stable and development version. Currently, that means that the release_34 branch and the trunk are built. So, for example, clang-3.4 and clang-3.5 can be installed.
    release_34 are only built when a new commit is submitted in this branch. trunk is built twice a day.

  • Add a new package llvm-{3.4,3.5}-tools which contains various tools to build software/packages on top of llvm. Contributed by Martin Nowack in the context of Klee.

  • Since a C++ 11 compiler is now mandatory, I had to force the usage of a backported gcc/g++ 4.8 (thanks Doko).
    This is the case for Ubuntu Precise (12.04), Quantal (12.10) and raring (13.04).
    The thing is that it triggers a dependency on the libstdc++ 4.8 causing the PPA to be mandatory.
    deb $DISTRIBUTION main

    For now, because of the lack of backport of gcc 4.8, I am not providing support for Debian stable (wheezy).