What's the difference between the option -fopenmp=libomp and -fopenmp?

hi, all
On some small case, I tried to compile with option -fopenmp=libomp and -fopenmp, and find they don’t have difference with their assemble.
While on some real application, I find the have difference when I compile a fortran project (I compare their dump file, and find the difference is not releated to the share libraries’ path). So I’m a little confused about the difference between the two options, does any guy known that ?

  • the left side is compiled with option -fopenmp, and it seems have 2 extra function calls (_mp_bcs_nest/_mp_ecs_nest)in my local fortran project convergence for example.