Is a function without loop parallelizable? - c

Question

considering the code below, can we consider it parallel even if there are no loops?
#include <omp.h>
int main(void) {
#pragma omp parallel
{
int a = 1;
a = 0;
}
return 0;
}

Answer 1

Direct Answer:
Yes, here, the section of your code,
int a = 1;
a = 0;
Runs in parallel, P times, where P is the number of cores on your machine.
For example on a four core machine, the following code (with the relevant imports),
int main(void) {
#pragma omp parallel
{
printf("Thread number %d", omp_get_thread_num());
}
return 0;
}
would output:
Thread number 0
Thread number 1
Thread number 2
Thread number 3
Note that when running in parallel, there is no guarantee on the order of the output, so the output could just as likely be something like:
Thread number 1
Thread number 2
Thread number 0
Thread number 3
Additionally, if you wanted to specify the number of threads used in the parallel region, instead of #pragma omp parallel you could write, #pragma omp parallel num_threads(4).
Further Explanation:
If you are still confused, it may be helpful to better understand the difference between parallel for loops and parallel code regions.
#pragma omp parallel tells the compiler that the following code block may be executed in parallel. It guarantees that all code within the parallel region will have finished execution before continuing to subsequent code.
In the following (toy) example, the programmer is guaranteed that after the parallel region, the array will have all entries set to zero.
int *arr = malloc(sizeof(int) * 128);
const int P = omp_get_max_threads();
#pragma omp parallel num_threads(P)
{
int local_start = omp_get_thread_num();
int local_end = local_start + (100 / P);
for (int i = local_start; i < local_end; ++i) {
arr[i] = 0;
}
}
// any code from here onward is guaranteed that arr contains all zeros!
Ignoring differences in scheduling, this task could equivalently be accomplished using a parallel for loop as follows:
int *arr = malloc(sizeof(int) * 128);
const int P = omp_get_max_threads();
#pragma omp parallel num_threads(P) for
for (int i = 0; i < 128; ++i) {
arr[i] = 0;
}
// any code from here onward is guaranteed that arr contains all zeros!
Essentially, #pragma omp parallel enables you to describe regions of code that can execute in parallel - this can be much more flexible than a parallel for loop. In contrast, #pragma omp parallel for should generally be used to parallelize loops with independent iterations.
I can further elaborate on the differences in performance, if you would like.