Re: [PATCH 1/4] Cherry-pick fprofile-generate-atomic from google/gcc-4_9 branch

[Nathan Sidwell <nathan at acm dot org>]

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