https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102698
--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> --- !$omp atomic capture v = x x = ior(x, expr) !$omp end atomic etc. is conceptually similar to !$omp atomic load v = x !$omp end atomic !$omp atomic update x = ior(x, expr) !$omp end atomic except that the capture form is atomic even together, the v is really the value of x right before the successful atomic ior, while in the latter case some other thread could modify x in between those 2 atomic constructs. And similarly, the forms that have v = x at the end are similar to two atomics in the other order. There are many cases where it is useful, e.g. for the fetch before operation you atomically or some flag but then you want to perform some code in the current thread only if the flag wasn't set before, etc. Similarly for the fetch after the operation. It really depends on what the code wants to achieve. What your testcase does is not useful though, you basically want to have 2 arrays the same content, you can achieve that by copying the atomically changed array to the other one at the end of parallel region, or by doing the atomics on both arrays separately.
