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问题:

I have problems compiling OpenMP code using clang (both 3.6 and 3.8 ToT).

I followed this blog post http://blog.llvm.org/2015/05/openmp-support_22.html , but the problem is that the compiled program is executed on a one thread only. I'm using ubuntu 15.04 x64, I have both libgomp and libiopmp installed and I compile my code with the following command:

clang test.c -o test -fopenmp -L/usr/lib/gcc/x86_64-linux-gnu/5.1.1

When I use gcc instead, everything works fine: gcc test.c -o test -fopenmp

I also tried running export LD_LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/5.1.1:$LD_LIBRARY_PATH but it didn't help. `

Any suggestions?

回答1:

Update

Building the latest trunk of LLVM/Clang (clang-3.8), installing libiomp5, and specifying the location of the gomp omp header files worked. Note that the Ubuntu package for libiomp5 isn't quite correct, so you will need to add a symlink in /usr/lib from /usr/lib/libiomp5.so to /usr/lib/libiomp5.so.5.

./clang++ -I/usr/lib/gcc/x86_64-linux-gnu/4.9/include -fopenmp=libiomp5 -o test test.cpp

I'm using g++-5.1 and clang++-3.6 on Linux Mint 17.2 (essentially Ubuntu trusty) and I see the same results with the following code.

#include  #include  int main() {     #pragma omp parallel num_threads(4)     {         #pragma omp critical         std::cout

Running this under ltrace reveals the issue:

g++

$ g++ -fopenmp -o test test.cpp $ ./test tid = 0 tid = 3 tid = 2 tid = 1 $ ltrace ./test __libc_start_main(0x400af6, 1, 0x7ffc937b8198, 0x400bc0  _ZNSt8ios_base4InitC1Ev(0x6021b1, 0xffff, 0x7ffc937b81a8, 5)   = 0 __cxa_atexit(0x4009f0, 0x6021b1, 0x602090, 0x7ffc937b7f70)     = 0 GOMP_parallel(0x400b6d, 0, 4, 0  GOMP_critical_start(0, 128, 0, 0)                              = 0 tid = 3 tid = 2 omp_get_thread_num(0x7f9fe13894a8, 1, 0, 0x493e0)              = 0 _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc(0x6020a0, 0x400c44, 0, 0x493e0) = 0x6020a0 _ZNSolsEi(0x6020a0, 0, 0x7f9fe1a03988, 0x203d2064)             = 0x6020a0 _ZNSolsEPFRSoS_E(0x6020a0, 0x400920, 0x7f9fe1a03988, 0  _ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_(0x6020a0, 0x400920, 0x7f9fe1a03988, 0) = 0x6020a0 <... _znsolsepfrsos_e="" resumed=""> )                               = 0x6020a0 GOMP_critical_end(0x7f9fe0d2d400, 0x7f9fe0d2e9e0, 0, -1)       = 0 tid = 1 tid = 0 <... gomp_parallel="" resumed=""> )                                  = 0 _ZNSt8ios_base4InitD1Ev(0x6021b1, 0, 224, 0x7f9fe0d2df50)      = 0x7f9fe1a08940 +++ exited (status 0) +++

clang

$ clang++ -fopenmp -o test test.cpp $ ./test tid = 0 $ ltrace ./test __libc_start_main(0x4009a0, 1, 0x7ffde4782538, 0x400a00  _ZNSt8ios_base4InitC1Ev(0x6013f4, 0x7ffde4782538, 0x7ffde4782548, 5) = 0 __cxa_atexit(0x400830, 0x6013f4, 0x6012c8, 0x7ffde4782310)     = 0 _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc(0x6012e0, 0x400a84, 0x7ffde4782548, 6) = 0x6012e0 omp_get_thread_num(0x7f3e4698c006, 0x7f3e4698c000, 0x7f3e46764988, 1024) = 0 _ZNSolsEi(0x6012e0, 0, 0x7f3e46764988, 1024)                   = 0x6012e0 _ZNSolsEPFRSoS_E(0x6012e0, 0x4007a0, 0x7f3e46764988, 0  _ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_(0x6012e0, 0x4007a0, 0x7f3e46764988, 0) = 0x6012e0 tid = 0 <... _znsolsepfrsos_e="" resumed=""> )                               = 0x6012e0 _ZNSt8ios_base4InitD1Ev(0x6013f4, 0, 224, 0x7f3e45886f50)      = 0x7f3e46769940 +++ exited (status 0) +++

You can immediately see the problem: clang++ never calls GOMP_parallel, so you always get one thread. This is crazy behavior on the part of clang. Have you tried building and using the "special" OpenMP version of clang?

回答2:

Some additional comments:

1) You need to use -fopenmp=libomp to enable OpenMP in clang. -fopenmp just links libgomp but ignores all the pragmas. Weird, I know -- and will be changed in the trunk soon.

2) 3.7 is the first version that supports OpenMP. 3.6 doesn't.

3) clang is only able to work with libomp. Don't put libgomp (headers or the library) in the way of libomp! clang uses Intel API, not supported by libgomp. -fopenmp=libomp should link correct library.

Yours,