On Wed, 1 Dec 2021 at 18:16, Florian Weimer wrote: > > * 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?
Hmm, yes. > (I doubt the conditional lack of a compiler barrier leads to > optimization improvements elsewhere.) Probably not. I'll revert the change to _M_is_leaked() and just keep it for _M_is_shared(). Thanks for pointing that out.
