[Bug libstdc++/70554] New: [6 Regression] _GLIBCXX_ATOMIC_BUILTINS changed value

["redi at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70554

            Bug ID: 70554
           Summary: [6 Regression] _GLIBCXX_ATOMIC_BUILTINS changed value
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

https://gcc.gnu.org/ml/gcc-patches/2016-03/msg01159.html observes that r232147
changed the value of _GLIBCXX_ATOMIC_BUILTINS in libstdc++, which affects how
atomic ref-counting is done. Previously most targets used atomics for
ref-counts, but now many targets will use the generic fallback which relies on
a mutex.

Code compiled with previous releases will still be using atomics, but code
compiled with gcc-6 will lock a mutex and then perform a non-atomic
increment/decrement, possibly concurrently with other threads that aren't using
the mutex.

This needs to be fixed for gcc-6.