* Jonathan Wakely via Libstdc: > diff --git a/libstdc++-v3/include/bits/cow_string.h > b/libstdc++-v3/include/bits/cow_string.h > index ced395b80b8..4fae1d02981 100644 > --- a/libstdc++-v3/include/bits/cow_string.h > +++ b/libstdc++-v3/include/bits/cow_string.h > @@ -105,7 +105,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION > * destroy the empty-string _Rep object. > * > * All but the last paragraph is considered pretty conventional > - * for a C++ string implementation. > + * for a Copy-On-Write C++ string implementation. > */ > // 21.3 Template class basic_string > template<typename _CharT, typename _Traits, typename _Alloc> > @@ -207,10 +207,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION > // so we need to use an atomic load. However, _M_is_leaked > // predicate does not change concurrently (i.e. the string is either > // leaked or not), so a relaxed load is enough. > - return __atomic_load_n(&this->_M_refcount, __ATOMIC_RELAXED) < 0; > -#else > - return this->_M_refcount < 0; > + if (!__gnu_cxx::__is_single_threaded()) > + return __atomic_load_n(&this->_M_refcount, __ATOMIC_RELAXED) < 0; > #endif > + return this->_M_refcount < 0; > }
Relaxed MO loads of word-size values on all current architectures only have a compiler barrier, so I think the optimization makes things worse? (I doubt the conditional lack of a compiler barrier leads to optimization improvements elsewhere.) Thanks, Florian
