Why do programs load multiple threads even if they don't contain multi-threaded code?

Question

C++ / C#

Here on this site https://benchmarksgame-team.pages.debian.net/bench... it is shown that the program, for example, loads all 4 threads at 100%, but if you look at the code itself, there is not a single line about multithreading , how is this possible ?

Answer 1

Link to multi-threaded program

#pragma omp parallel for
   for (i = 0; i < n; i++) {
      __m128d sum = _mm_setzero_pd();

      int j;
      for (j = 0; j < n; j += 2) {
         __m128d b = _mm_set_pd(v[j],v[j+1]);
         __m128d a = _mm_set_pd(A(i,j), A(i,j+1));
         sum = _mm_add_pd(sum, _mm_div_pd(b, a));
      }
      out[i] = sum[0] + sum[1];
   }
}

The #pragma omp parallel for directive is a multi-threaded OpenMP library, if I'm not mistaken