LLVM Project News and Details from the Trenches

Tuesday, September 25, 2018

Integration of libc++ and OpenMP packages into llvm-toolchain

A bit more than a year ago, we gave an update about recent changes in apt.llvm.org. Since then, we noticed an important increase of the usage of the service. Just last month, we saw more than 16.5TB of data being transferred from our CDN.
Thanks to the Google Summer of Code 2018, and after number of requests, we decided to focus our energy to bring new great projects from the LLVM ecosystems into apt.llvm.org.

Starting from version 7, libc++, libc++abi and OpenMP packages are available into the llvm-toolchain packages. This means that, just like clang, lldb or lldb, libc++, libc++abi and OpenMP packages are also built, tested and shipped on https://apt.llvm.org/.

The integration focuses to preserve the current usage of these libraries. The newly merged packages have adopted the llvm-toolchain versioning:

libc++ packages
  • libc++1-7
  • libc++-7-dev
libc++abi packages
  • libc++abi1-7
  • libc++abi-7-dev
OpenMP packages
  • libomp5-7
  • libomp-7-dev
  • libomp-7-doc
This packages are built twice a day for trunk. For version 7, only when new changes happen in the SVN branches.
Integration of libc++* packages

Both libc++ and libc++abi packages are built at same time using the clang built during the process. The existing libc++ and libc++abi packages present in Debian and Ubuntu repositories will not be affected (they will be removed at some point). Newly integrated libcxx* packages are not co-installable with them.

Symlinks have been provided from the original locations to keep the library usage same.

Example:  /usr/lib/x86_64-linux-gnu/libc++.so.1.0 -> /usr/lib/llvm-7/lib/libc++.so.1.0

The usage of the libc++ remains super easy:
Usage:
$ clang++-7 -std=c++11 -stdlib=libc++ foo.cpp
$ ldd ./a.out|grep libc++
  libc++.so.1 => /usr/lib/x86_64-linux-gnu/libc++.so.1 (0x00007f62a1a90000)
  libc++abi.so.1 => /usr/lib/x86_64-linux-gnu/libc++abi.so.1 (0x00007f62a1a59000)

In order to test new developments in libc++, we are also building the experimental features.
For example, the following command will work out of the box:

$ clang++-7 -std=c++17 -stdlib=libc++ foo.cpp -lc++experimental -lc++fs

Integration of OpenMP packages

While OpenMP packages have been present in the Debian and Ubuntu archives for a while, only a single version of the package was available.

For now, the newly integrated packages creates a symlink from /usr/lib/libomp.so.5 to /usr/lib/llvm-7/lib/libomp.so.5 keeping the current usage same and making them non co-installable.

It can be used with clang through -fopenmp flag:
$ clang -fopenmp foo.c

The dependency packages providing the default libc++* and OpenMP package are also integrated in llvm-defaults. This means that the following command will install all these new packages at the current version:
$ apt-get install libc++-dev libc++abi-dev libomp-dev

LLVM 7 => 8 transition

In parallel of the libc++ and OpenMP work, https://apt.llvm.org/ has been updated to reflect the branching of 7 from the trunk branches.
Therefore, we have currently on the platform:

Stable
6.0
Qualification
7
Development
8


Please note that, from version 7, the packages and libraries are called 7 (and not 7.0).
For the rational and implementation, see https://reviews.llvm.org/D41869 & https://reviews.llvm.org/D41808.

Stable packages of LLVM toolchain are already officially available in Debian Buster and in Ubuntu Cosmic.

Cosmic support

In order to make sure that the LLVM toolchain does not have too many regressions with this new version, we also support the next Ubuntu version, 18.10, aka Cosmic.

A Note on coinstallability

We tried to make them coinstallable, in the resulting packages we had no control over the libraries used during the runtime. This could lead to many unforeseen issues. Keeping these in mind we settled to keep them conflicting with other versions.

Future work
  • Code coverage build fails for newly integrated packages
  • Move to a 2 phases build to generate clang binary using clang

Sources of the project are available on the gitlab instance of Debian: https://salsa.debian.org/pkg-llvm-team/llvm-toolchain/tree/7


Reshabh Sharma & Sylvestre Ledru