On 08/04/16 12:43, Nathan Sidwell wrote:
How about:
gcov_t expected;
atomic_load (&counter[0], val, ...);
gcov_t delta = val == value ? 1 : -1;
atomic_add (&counter[1], delta); <-- or atomic_add_fetch
if (delta < 0) {
/* can we set counter[0]? */
atomic_load (&counter[1], &expected, ...);
if (expected < 0) {
atomic_store (&counter[0], value, ...);
atomic_add (&counter[1], 2, ...);
}
}
atomic_add (&counter[2], 1, ...);
we could do better by using compare_exchange storing value, and detect the race
I mentioned:
gcov_t expected, val;
atomic_load (&counter[0], &val, ...);
gcov_t delta = val == value ? 1 : -1;
atomic_add (&counter[1], delta);
if (delta < 0) {
retry:
/* can we set counter[0]? */
atomic_load (&counter[1], &expected, ...);
if (expected < 0) {
bool stored = atomic_compare_exchange (&counter[0], &val, &value, ...);
if (!stored && val != value)
goto retry;
atomic_add (&counter[1], 2, ...);
}
}
atomic_add (&counter[2], 1, ...);
This corrects the off-by one issue.
nathan
