The question about using OpenMP taskloop feature in gcc

Xi Ruoyao ryxi@stu.xidian.edu.cn
Tue Nov 21 08:56:00 GMT 2017


On 2017-11-21 16:50 +0800, Xi Ruoyao wrote:
> On 2017-11-21 15:49 +0800, Nan Xiao wrote:
> > 
> > #include <omp.h>
> > #include <stdio.h>
> > 
> > int main(void) {
> >     #pragma omp parallel for
> >     for (auto i = 0; i < 10; i++) {
> >           int sum = 0;
> >           #pragma omp taskloop shared(sum)
> >           for (auto j = 0; j < 1000000; j++) {
> >                  sum += j;
> >            }
> >            printf("%d\n", sum);
> >      }
> >      return 0;
> > }
> 
> There are two bugs in your code.  First, signed overflow is an undefined
> behaviour and may generate arbitary result.  Second, the access to shared
> variable sum is racing, the result may vary with scheduling.

Fix:

    #pragma omp parallel for
    for (auto i = 0; i < 10; i++) {
          long long sum = 0;
          #pragma omp taskloop shared(sum)
          for (auto j = 0; j < 1000000; j++) {
                 __atomic_add_fetch(&sum, j, __ATOMIC_RELAXED);
           }
           printf("%lld\n", sum);
    }

This would generate "lock addq" instruction for "sum", instead of loading
it into a register.
-- 
Xi Ruoyao <ryxi@stu.xidian.edu.cn>
School of Aerospace Science and Technology, Xidian University



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