Use openMP only when an argument is passed to the program

Question

Is there a good way to use OpenMP to parallelize a for-loop, only if an -omp argument is passed to the program?

This seems not possible, since #pragma omp parallel for is a preprocessor directive and thus evaluated even before compile time and of course it is only certain if the argument is passed to the program at runtime.

At the moment I am using a very ugly solution to achieve this, which leads to an enormous duplication of code.

if(ompDefined) {
#pragma omp parallel for
  for(...)
    ...
}
else {
  for(...)
    ...
}

Answer 1

I think what you are looking for can be solved using a CPU dispatcher technique.

For benchmarking OpenMP code vs. non-OpenMP code you can create different object files from the same source code like this

//foo.c
#ifdef _OPENMP
double foo_omp() {
#else
double foo() {
#endif
  double sum = 0;
  #pragma omp parallel for reduction(+:sum)
  for(int i=0; i<1000000000; i++) sum += i%10;
  return sum;
}

Compile like this

gcc -O3 -c foo.c
gcc -O3 -fopenmp -c foo.c -o foo_omp.o

This creates two object files foo.o and foo_omp.o. Then you can call one of these functions like this

//bar.c
#include <stdio.h>

double foo();
double foo_omp();
double (*fp)();

int main(int argc, char *argv[]) {
  if(argc>1) {
    fp = foo_omp;
  }
  else {
    fp = foo;
  }
  double sum = fp();
  printf("sum %e\n", sum);
}

Compile and link like this

gcc -O3 -fopenmp bar.c foo.o foo_omp.o

Then I time the code like this

time ./a.out -omp
time ./a.out

and the first case takes about 0.4 s and the second case about 1.2 s on my system with 4 cores/8 hardware threads.

---

Here is a solution which only needs a single source file

#include <stdio.h>

typedef double foo_type();

foo_type foo, foo_omp, *fp;

#ifdef _OPENMP
#define FUNCNAME foo_omp
#else
#define FUNCNAME foo
#endif

double FUNCNAME () {
  double sum = 0;
  #pragma omp parallel for reduction(+:sum)
  for(int i=0; i<1000000000; i++) sum += i%10;
  return sum;
}

#ifdef _OPENMP
int main(int argc, char *argv[]) {
  if(argc>1) {
    fp = foo_omp;
  }
  else {
    fp = foo;
  }
  double sum = fp();
  printf("sum %e\n", sum);
}
#endif

Compile like this

gcc -O3 -c foo.c
gcc -O3 -fopenmp foo.c foo.o