[Patch, libstdc++] Fix data races in basic_string implementation

[Jonathan Wakely]

On 01/09/15 16:56 +0200, Dmitry Vyukov wrote:
>I don't understand how a new gcc may not support __atomic builtins on
>ints. How it is even possible? That's a portable API provided by
>recent gcc's...

The built-in function is always defined, but it might expand to a call
to an external function in libatomic, and it would be a regression for
code using std::string to start requiring libatomic (although maybe it
would be necessary if it's the only way to make the code correct).

I don't know if there are any targets that define __GTHREADS and also
don't support __atomic_load(int*, ...) without libatomic. If such
targets exist then adding a new configure check that only depends on
__atomic_load(int*, ...) would mean we keep supporting those targets.

Another option would be to simply do:

         bool
         _M_is_shared() const _GLIBCXX_NOEXCEPT
#if defined(__GTHREADS)
+        { return __atomic_load(&this->_M_refcount, __ATOMIC_ACQUIRE) > 0; }
+#else
         { return this->_M_refcount > 0; }
+#endif

and see if anyone complains!