GCC使用OpenMP
OpenMP采用fork-join的执行模式。开始的时候只存在一个主线程，当需要进行并行计算的时候，派生出若干个分支线程来执行并行任务。当并行代码执行完成之后，分支线程会合，并把控制流程交给单独的主线程。
一个典型的fork-join执行模型的示意图如下：
OpenMP编程模型以线程为基础，通过编译制导指令制导并行化，有三种编程要素可以实现并行化控制，他们分别是编译directive、API函数集和环境变量。
具体可以参考这篇博客：
OpenMP基本概念_yu132563的专栏-CSDN博客_openmp OpenMP是一种用于共享内存并行系统的多线程程序设计方案，支持的编程语言包括C、C++和Fortran。OpenMP提供了对并行算法的高层抽象描述，特别适合在多核CPU机器上的并行程序设计。编译器根据程序中添加的pragma指令，自动将程序并行处理，使用OpenMP降低了并行编程的难度和复杂度。当编译器不支持OpenMP时，程序会退化成普通（串行）程序。程序中已有的OpenMP指令不会影响...https://blog.csdn.net/yu132563/article/details/82704993测试代码：
#include
int main(void) {
#pragma omp parallel
{
int i;
printf("Hello World\n");
for(i=0;i <6;i++)
printf("Iter:%d\n",i);
}
printf("GoodBye World\n");
} 编译：
gcc -o test openmp.c -fopenmp -Wl,-Map=openmp.map增加了-fopenmap选项,如果需要调试和静态链接，可以继续添加 -static 和 -g选项。
Map文件，从中可以看出，引用了额外的libgomp库链接.
czl@czl-VirtualBox:~/WorkSpace/cnn$ cat openmp.map
Archive member included to satisfy reference by file (symbol)
/usr/lib/x86_64-linux-gnu/libc_nonshared.a(elf-init.oS)
/usr/lib/gcc/x86_64-linux-gnu/7/../../../x86_64-linux-gnu/Scrt1.o (__libc_csu_init)
As-needed library included to satisfy reference by file (symbol)
libgomp.so.1 /tmp/ccFkqztK.o (GOMP_parallel@@GOMP_4.0)
libc.so.6 /tmp/ccFkqztK.o (puts@@GLIBC_2.2.5)
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.comment 0x0000000000000029 0x2a /usr/lib/gcc/x86_64-linux-gnu/7/crtoffloadbegin.o
.comment 0x0000000000000029 0x2a /tmp/ccFkqztK.o
.comment 0x0000000000000029 0x2a /usr/lib/gcc/x86_64-linux-gnu/7/crtendS.o
.comment 0x0000000000000029 0x2a /usr/lib/gcc/x86_64-linux-gnu/7/crtoffloadend.o
.debug
*(.debug)
.line
*(.line)
.debug_srcinfo
*(.debug_srcinfo)
.debug_sfnames
*(.debug_sfnames)
.debug_aranges
*(.debug_aranges)
.debug_pubnames
*(.debug_pubnames)
.debug_info
*(.debug_info .gnu.linkonce.wi.*)
.debug_abbrev
*(.debug_abbrev)
.debug_line
*(.debug_line .debug_line.* .debug_line_end)
.debug_frame
*(.debug_frame)
.debug_str
*(.debug_str)
.debug_loc
*(.debug_loc)
.debug_macinfo
*(.debug_macinfo)
.debug_weaknames
*(.debug_weaknames)
.debug_funcnames
*(.debug_funcnames)
.debug_typenames
*(.debug_typenames)
.debug_varnames
*(.debug_varnames)
.debug_pubtypes
*(.debug_pubtypes)
.debug_ranges
*(.debug_ranges)
.debug_macro
*(.debug_macro)
.debug_addr
*(.debug_addr)
.gnu.attributes
*(.gnu.attributes)
/DISCARD/
*(.note.GNU-stack)
*(.gnu_debuglink)
*(.gnu.lto_*)
OUTPUT(test elf64-x86-64)
czl@czl-VirtualBox:~/WorkSpace/cnn$
总结：
OpenMP虽好，但它只对多核的平台起到加速的作用，对于单核心来说，OpenMP不但不能性能，还会由于多出来的任务调度降低系统计算能力。