OpenMP: #pragma omp parallel (nested)

omp_parallel_nested.txt

#include <stdio.h>
#include <stdlib.h>
#include <omp.h>

int main(void)
{
	#pragma omp parallel num_threads(4)
	{
		printf("%d: Hello outer parallel region!\n", omp_get_thread_num());

		#pragma omp parallel
		{
			printf("%d: Hello inner parallel region!\n", omp_get_thread_num());
		}
	}

	return 0;
}

==============================================================================
After OpenMP lowering and expansion:
==============================================================================

/* libgomp/parallel.c:
 * - GOMP_parallel_start
 * - GOMP_parallel_end
 */

void main_omp_fn_1(void *omp_data)
{
	printf("%d: Hello inner parallel region!\n", omp_get_thread_num());
}

void main_omp_fn_0(void *omp_data)
{
	printf("%d: Hello outer parallel region!\n", omp_get_thread_num());

	__builtin_GOMP_parallel_start(main_omp_fn_1, NULL, 0);

	main_omp_fn_1(NULL);
	
	__builtin_GOMP_parallel_end();
}

int main(void)
{
	// Spawns a team of threads
	__builtin_GOMP_parallel_start(main_omp_fn_0, NULL, 4);

	main_omp_fn_0(NULL);
	
	// Terminates current team of threads 
	__builtin_GOMP_parallel_end();

	return 0;
}