https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99341
--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> --- The new implementation added these new symbols to libstdc++.so: _ZNSt9once_flag11_M_activateEv _ZNSt9once_flag9_M_finishEb I think I'd like to keep them, but have the new implementation disabled by default (except for the ABI-changing gnu-versioned-namespace build). The new implementation is superior (it fixes PR 66146, and performs better in some cases due to inlining an initial check to see if executions should be passive) so I'd like to retain it for users who explicitly request it. By default we need to be compatible with the old version based on pthread_once. At some point (maybe when there are further ABI changes queued) maybe we'll do another painful transition like the cxx11-abi one and switch to the new std::once_flag by default. Maybe the new std::once_flag should be moved to an inline namespace, and then the symbols above defined as aliases for the equivalents using the inline namespace in the mangled name (the aliases are needed if we want to support binaries that are already using them, e.g. packages in the upcoming Fedora 34 which were compiled with gcc-11.0.1 already).
