openmp critical

Question

Following this question, for the code below (from MS OpenMP docs example)

// omp_critical.cpp
// compile with: /openmp
#include <omp.h>
#include <stdio.h>
#include <stdlib.h>

#define SIZE 10

int main()
{
    int i;
    int max;
    int a[SIZE];

    for (i = 0; i < SIZE; i++)
    {
        a[i] = rand();
        printf_s("%d\n", a[i]);
    }

    max = a[0];
    #pragma omp parallel for num_threads(4)
    for (i = 1; i < SIZE; i++)
    {
        if (a[i] > max)
        {
            #pragma omp critical
            {
                // compare a[i] and max again because max
                // could have been changed by another thread after
                // the comparison outside the critical section
                if (a[i] > max)
                    max = a[i];
            }
        }
    }

    printf_s("max = %d\n", max);
}

Can I remove the outside if test and do

max = a[0];
#pragma omp parallel for num_threads(4)
for (i = 1; i < SIZE; i++)
{
    #pragma omp critical
    {
        // compare a[i] and max again because max
        // could have been changed by another thread after
        // the comparison outside the critical section
        if (a[i] > max)
            max = a[i];
    }
}

Answer 1

You can, but this effectively results in sequential execution. The threads are constantly waiting to enter the critical section such that only one thread executes the loop body at a time. Hence, you get the same performance (maybe even worse due to synchronization overhead) than a plain serial loop.

The example from the MS docs only synchronizes if a new maximum value has been encountered. This allows to process all lower values up to this point in parallel.

As suggested in the comments, use a reduction construct.