使用OpenMP与clang [英] Using OpenMP with clang

问题描述

我在使用clang（3.6和3.8 ToT）编译OpenMP代码时遇到问题。

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

我跟着这个博客文章 http://blog.llvm.org/2015/05/openmp-support_22.html ，但问题是，编译程序只在一个线程上执行。
我使用ubuntu 15.04 x64，我安装了libgomp和libiopmp，并用以下命令编译我的代码：

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

当我使用gcc代替，一切正常： gcc test.c -o test -fopenmp

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

我也试过运行 export LD_LIBRARY_PATH = / usr / lib / gcc / x86_64-linux-gnu / 5.1.1：$ LD_LIBRARY_PATH 帮帮我。
`

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. `

有任何建议吗？

推荐答案

更新

建筑LLVM / Clang的（铛-3.8）的最新行李箱，安装libiomp5，并指定gomp OMP头文件的位置工作。注意，用于libiomp5的Ubuntu包不是很正确，因此您需要在/ usr / lib中将符号链接从/usr/lib/libiomp5.so添加到/usr/lib/libiomp5.so.5.

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

---

我使用g ++ - 5.1和clang ++ - 3.6在Linux Mint 17.2（本质上是Ubuntu trusty）和我看到相同的结果与以下代码。

---

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 <iostream>
#include <omp.h>
int main() {
    #pragma omp parallel num_threads(4)
    {
        #pragma omp critical
        std::cout << "tid = " << omp_get_thread_num() << std::endl;
    }
}

在ltrace下运行此操作会显示问题：

Running this under ltrace reveals the issue:

$ g++ -fopenmp -o test test.cpp
$ ./test
tid = 0
tid = 3
tid = 2
tid = 1
$ ltrace ./test
__libc_start_main(0x400af6, 1, 0x7ffc937b8198, 0x400bc0 <unfinished ...>
_ZNSt8ios_base4InitC1Ev(0x6021b1, 0xffff, 0x7ffc937b81a8, 5)   = 0
__cxa_atexit(0x4009f0, 0x6021b1, 0x602090, 0x7ffc937b7f70)     = 0
GOMP_parallel(0x400b6d, 0, 4, 0 <unfinished ...>
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 <unfinished ...>
_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

$ clang++ -fopenmp -o test test.cpp
$ ./test
tid = 0
$ ltrace ./test
__libc_start_main(0x4009a0, 1, 0x7ffde4782538, 0x400a00 <unfinished ...>
_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 <unfinished ...>
_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) +++

你可以立即看到问题：clang ++从不调用GOMP_parallel，所以你总是得到一个线程。这是cl的一部分的疯狂行为。您是否尝试过构建和使用clang的特殊 OpenMP版本？

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?

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