Testing clang 3.7.0 OpenMP support

Soon after llvm 3.7.0 reached the Gentoo tree, Jeremi Piotrowski opened two bugs to fix OpenMP support in our ebuilds. sys-devel/llvm-3.7.0-r1 (a revision bump that also now installs LLVM utilities) now has a post-install message with a short recap, but here is the blog version.

As detailed in upstream release notes, OpenMP in clang is disabled by default (and I kept this default in the Gentoo package), and the runtime is a separate package. So:

  • emerge sys-libs/libomp
  • add “-fopenmp=libomp” in your CFLAGS
  • check with a sample program like:
#include <stdio.h>
#include <omp.h>

int main() {
#pragma omp parallel
    printf("Hello from thread %d, nthreads %d\n", omp_get_thread_num(), omp_get_num_threads());
  • You should get multiple lines on STDOUT (default threads count is one per CPU):
% clang -fopenmp=libomp test_omp.c -o test_omp
% OMP_NUM_THREADS=6 ./test_omp
Hello from thread 4, nthreads 6
Hello from thread 5, nthreads 6
Hello from thread 1, nthreads 6
Hello from thread 3, nthreads 6
Hello from thread 0, nthreads 6
Hello from thread 2, nthreads 6

Some additional reading:

LLVM 3.7.0 released, Gentoo package changes

As announced here, the new LLVM release is now available. If you are interested after reading the release notes (or the clang ones), I just added the corresponding packages in Gentoo, although still masked for some additional testing. mesa-11.0 release candidates compile fine with it though.
2015/09/04 edit: llvm 3.7.0 is now unmasked!

On the Gentoo side, this version bump has some nice changes:

  • We now use the cmake/ninja build system: huge thanks to fellow developer mgorny for his hard work on this rewrite. This is the recommended build system upstream now, and some features will only be available with it
  • LLDB, the debugger  is now available via USE=lldb. This is not a separate package as the build system for llvm and subprojects is quite monolithic. clang is in the same situation: sys-devel/clang package currently does almost nothing, the magic is done in llvm[clang])
  • These new ebuilds include the usual series of fixes, including some recent ones by gienah

On the TODO list, some tests are still failing on 32bit multilib (bug #558604), ocaml doc compilation/installation has a strange bug (see the additional call on docs/ocaml_doc target in the ebuild). And let’s not forget adding other tools, like polly. Another recent suggestion: add llvm’s OpenMP library needed for OpenMP with clang.

Also, if people were still using the dragonegg GCC plugin, note that it is not released anymore upstream. As 3.6.x versions had compilation problems, this package received its last rites.