OpenMP named critical section: if a program variable is used, is it evaluated or is it used as string without being evaluated?

Question

I have the following piece of code which I want to add omp for to:

for ( int i = 0 ; i < N ; i++ ) {
    double max2 = 0.; 
    (calculate max2);

    for ( int j = 0 ; j < 3 ; j++) {
        int m = group[i].member[j];
        if ( members[m].norm < max2 ) {
            members[m].norm = max2; 
        }
    }
}

The access to members needs to be protected but I would like to use a named critical section so as to avoid the speed penalty of effectively serializing that loop. What I want to do is this:

#pragma omp parallel for
for ( int i = 0 ; i < N ; i++ ) {
    double max2 = 0.; 
    (calculate max2);

    for ( int j = 0 ; j < 3 ; j++) {
        int m = group[i].member[j];
#pragma omp critical (m)
        if ( members[m].norm < max2 ) {
            members[m].norm = max2; 
        }
    }
}

so that only threads actually writing to the same m value wait for each other. My question is: is "m" used as a string for the critical name or is it actually evaluated and its value used as the name of the critical section? It does compile but I do not know if it does what I think it does. I would greatly appreciate if anyone can clarify what omp does here (eg how omp implements these).

Thank you * very much.

score 2 · Answer 1 · answered Jun 19 '20 at 16:28

The name of a critical must be a compile-time constant. But it's not a string or something, it's just a name that you put in parenthesis, e.g.

#pragma omp critical (foo)

For your pattern you will have to use locks, similar to this:

#pragma omp parallel for
for ( int i = 0 ; i < N ; i++ ) {
    double max2 = 0.; 
    (calculate max2);

    for ( int j = 0 ; j < 3 ; j++) {
        omp_lock_t * lck = group[i].member[j].my_lock;
        omp_set_lock(*lck);
        if ( members[m].norm < max2 ) {
            members[m].norm = max2; 
        }
        omp_unset_lock(*lck);
    }
}

Please note, locks in OpenMP have to be initialized. Please see https://www.openmp.org/spec-html/5.0/openmpse31.html#x191-9120003.3 for more information.

OpenMP named critical section: if a program variable is used, is it evaluated or is it used as string without being evaluated?

1 Answers1