[cfe-dev] Clang/LLVM OpenMP program not spawning threads

[Andrey Bokhanko]

Hi Jaime,

Adding -fopenmp=libomp should help.

Sorry for this confusion -- blog description was correct at the time
of writing. But then runtime library defaults changed... for some
reasons. We hope that the default runtime lib will be changed to
libomp again "really soon", so the blog description will become
correct again as wll.

Yours,
Andrey Bokhanko
==============
Software Engineer
Intel Compiler Team
Intel

On Tue, Jul 28, 2015 at 1:19 AM, Jaime Arteaga <jandres742 at gmail.com> wrote:
> According to http://blog.llvm.org/2015/05/openmp-support_22.html, OpenMP
> support in Clang is completed. However, I'm having difficulties trying a
> simple program.
>
> I've installed Clang/LLVM as explained in
> http://clang.llvm.org/get_started.html and the OpenMP runtime as explained
> in http://openmp.llvm.org/.
>
> The test program is:
>
> #include "omp.h"
> #include <stdio.h>
> #include <stdlib.h>
>
> int main(int argc, char** argv)
> {
>     #pragma omp parallel
>     {
>         printf("thread %d\n", omp_get_thread_num());
>     }
>     return 0;
> }
>
> The compilation line is:
>
> clang -lomp -I/.../openmp/runtime/exports/common/include
> -L/.../openmp/runtime/exports/lin_32e/lib ./test-openmp.c -o ./test-openmp
>
> Using which, I check I'm using the correct clang binary.
>
> With ldd, I check I'm linking to the correct OpenMP library:
>
> $ ldd ./test-openmp
> linux-vdso.so.1 =>  (0x00007ffdaf6d7000)
> libomp.so => /.../openmp/runtime/exports/lin_32e/lib/libomp.so
> (0x00007f7d47552000)
> libc.so.6 => /lib64/libc.so.6 (0x00007f7d47191000)
> libdl.so.2 => /lib64/libdl.so.2 (0x00007f7d46f8d000)
> libpthread.so.0 => /lib64/libpthread.so.0 (0x00007f7d46d71000)
> /lib64/ld-linux-x86-64.so.2 (0x00007f7d477fb000)
>
> But when running, it executes only one thread:
>
> $ OMP_NUM_THREADS=4 ./test-openmp
> thread 0
>
> The reason I'm linking with lomp is because if I link with fopenmp, the code
> is wrongly linked to the gcc omp library. However in that case, the result
> is the same:
>
> $ clang -fopenmp -I/.../openmp/runtime/exports/common/include
> -L/.../openmp/runtime/exports/lin_32e/lib ./test-openmp.c -o ./test-openmp
>
> $ ldd ./test-openmp
> linux-vdso.so.1 =>  (0x00007ffdf351f000)
> libgomp.so.1 => /lib64/libgomp.so.1 (0x00007fbc1c3e1000)
> librt.so.1 => /lib64/librt.so.1 (0x00007fbc1c1d9000)
> libpthread.so.0 => /lib64/libpthread.so.0 (0x00007fbc1bfbd000)
> libc.so.6 => /lib64/libc.so.6 (0x00007fbc1bbfc000)
> /lib64/ld-linux-x86-64.so.2 (0x00007fbc1c5f8000)
>
> $ OMP_NUM_THREADS=4 ./test-openmp
> thread 0
>
> When using gcc, it works as expected:
>
> $ gcc -fopenmp ./test-openmp.c -o ./test-openmp
>
> $ ldd ./test-openmp
> linux-vdso.so.1 =>  (0x00007ffc444e0000)
> libgomp.so.1 => /lib64/libgomp.so.1 (0x00007f7d425ce000)
> libpthread.so.0 => /lib64/libpthread.so.0 (0x00007f7d423b2000)
> libc.so.6 => /lib64/libc.so.6 (0x00007f7d41ff1000)
> /lib64/ld-linux-x86-64.so.2 (0x00007f7d427e5000)
>
> $ OMP_NUM_THREADS=4 ./test-openmp
> thread 0
> thread 2
> thread 3
> thread 1
>
> I've used in the past the implementation described in
> http://clang-omp.github.io/ and I know that one works (a different repo is
> presented there for Clang and LLVM, but the OpenMP repo is the same).
> However that page was (apparently) updated in 2014, and the blog
> inhttp://blog.llvm.org/2015/05/openmp-support_22.html is from May 2015,
> which makes you think that you can use the latest Clang/LLVM for OpenMP.
>
> So my question is, am I missing something or the blog from May 2015 is
> actually referring to Clang/LLVM implementation from in
> http://clang-omp.github.io and not to the latest one?
>
> Thanks
>
>
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