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So I tried to use OpenMP with one of the latest version of clang, clang version 3.4.2 (tags/RELEASE_34/dot2-final). Followed the procedure to compile and add the PATHs of omp.h, then Compiling my hello.c using : 

clang -fopenmp hello.c

and then running it, still it can't use more than 1 threads:

Bash-4.1$ ./a.out Hello from thread 0, nthreads 1

P.S: I tried to manually export export OMP_NUM_THREADS=8 but that didn't solve anything as well. Any ideas?

UPDATE: This is the hello.c:

#include <omp.h> #include <stdio.h> int main() { #pragma omp parallel printf("Hello from thread %d, nthreads %d\n", omp_get_thread_num(), omp_get_num_threads());}
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  • Please show us the code for hello.c. Commented Nov 6, 2014 at 23:29
  • here it is @Mysticial Commented Nov 6, 2014 at 23:31
  • Why not try #pragma omp parallel num_threads(#noofthreads) Commented Nov 7, 2014 at 19:19
  • @TejasPatel, i already tried that but didn't work. the same Hello from thread 0, nthreads 1 apears Commented Nov 7, 2014 at 19:57

3 Answers 3

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Despite the fact that its kinda late regarding the time-stamp of my original question, but I would like to mention the answer here so at-least it saves people's time facing similar issue.

LLVM itself currently doesn't support Openmp right out-of-the-box. You can make it compile and run the omp tagged code with Intel Runtime Support. However, if you want to have a clean clang supporting OpenMP, there is a trunk of the project at OpenMP-Clang which you can clone and build. The current support is OpenMP 3.1 specification and they will reach to support OpenMP 4.0 specification soon:

$ git clone https://github.com/clang-omp/llvm_trunk llvm $ git clone https://github.com/clang-omp/compiler-rt_trunk llvm/projects/compiler-rt $ git clone https://github.com/clang-omp/clang_trunk llvm/tools/clang

Don't forget to build the Intel® OpenMP* Runtime Library after this as you need omp.h and /path/to/llvm/projects/openmp/runtime/lin_32e/lib/libomp.so

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0

Try setting number of threads using omp_set_num_thread() function. If it doesn't work, try setting up clang again.

#include <omp.h> #include <stdio.h> int main() { omp_set_num_threads(4); #pragma omp parallel { printf("Hello from thread %d, nthreads %d\n", omp_get_thread_num(), omp_get_num_threads()); } }

Also try calling mp_get_max_threads() in both parallel and serial region, and see what you get

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interesting thing is when I execute this : clang -g -emit-llvm -S -c -fopenmp hello.c -o hello.ll i am getting this warning: clang: warning: argument unused during compilation: '-fopenmp', do you know why ??
If you don't use -fopenmp, does compiler throw error about undefined reference to functions of openMP. If not the library might be included by default
no I have carried out the procedures here : http://openmp.llvm.org/ and install the library and exported the path. before that I had error but now not anymore.
so if you don't use -fopenmp, it compiles succesfully?
strange behaviour. sorry i have no further idea about it. maybe u should have better chance on openmp forum
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I was only seeing one thread too, with clang version 3.8.0 (trunk 252425). I then read some recent news at https://clang-omp.github.io:

November 27, 2015 - Further development of OpenMP support in clang/llvm compiler moved to www.llvm.org. This site is maintained for archival purposes only. Thank you to everyone who contributed all these years!

...and so I compiled LLVM/Clang from trunk; compiled the OpenMP runtime library using the excellent instructions here; and now it does work.

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