https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67843
--- Comment #11 from Jonathan Wakely <redi at gcc dot gnu.org> --- Author: redi Date: Tue Nov 27 23:25:56 2018 New Revision: 266533 URL: https://gcc.gnu.org/viewcvs?rev=266533&root=gcc&view=rev Log: PR libstdc++/67843 set shared_ptr lock policy at build-time This resolves a longstanding issue where the lock policy for shared_ptr reference counting depends on compilation options when the header is included, so that different -march options can cause ABI changes. For example, objects compiled with -march=armv7 will use atomics to synchronize reference counts, and objects compiled with -march=armv5t will use a mutex. That means the shared_ptr control block will have a different layout in different objects, causing ODR violations and undefined behaviour. This was the root cause of PR libstdc++/42734 as well as PR libstdc++/67843. The solution is to decide on the lock policy at build time, when libstdc++ is configured. The configure script checks for the availability of the necessary atomic built-ins for the target and fixes that choice permanently. Different -march flags used to compile user code will not cause changes to the lock policy. This results in an ABI change for certain compilations, but only where there was already an ABI incompatibility between the libstdc++.so library and objects built with an incompatible -march option. In general, this means a more stable ABI that isn't silently altered when -march flags make addition atomic ops available. To force a target to use "atomic" or "mutex" the new configure option --with-libstdcxx-lock-policy can be used. In order to turn ODR violations into linker errors, the uses of shared_ptr in filesystem directory iterators have been replaced with __shared_ptr, and explicit instantiations are declared. This ensures that object files using those types cannot link to libstdc++ libs unless they use the same lock policy. PR libstdc++/67843 * acinclude.m4 (GLIBCXX_ENABLE_LOCK_POLICY): Add new macro that defines _GLIBCXX_HAVE_ATOMIC_LOCK_POLICY. * config.h.in: Regenerate. * configure: Regenerate. * configure.ac: Use GLIBCXX_ENABLE_LOCK_POLICY. * doc/xml/manual/configure.xml: Document new configure option. * include/bits/fs_dir.h (directory_iterator): Use __shared_ptr instead of shared_ptr. (recursive_directory_iterator): Likewise. (__shared_ptr<_Dir>): Add explicit instantiation declaration. (__shared_ptr<recursive_directory_iterator::_Dir_stack>): Likewise. * include/bits/shared_ptr_base.h (__allocate_shared, __make_shared): Add default template argument for _Lock_policy template parameter. * include/ext/concurrence.h (__default_lock_policy): Check macro _GLIBCXX_HAVE_ATOMIC_LOCK_POLICY instead of checking if the current target supports the builtins for compare-and-swap. * src/filesystem/std-dir.cc (__shared_ptr<_Dir>): Add explicit instantiation definition. (__shared_ptr<recursive_directory_iterator::_Dir_stack>): Likewise. (directory_iterator, recursive_directory_iterator): Use __make_shared instead of make_shared. Modified: trunk/libstdc++-v3/ChangeLog trunk/libstdc++-v3/acinclude.m4 trunk/libstdc++-v3/config.h.in trunk/libstdc++-v3/configure trunk/libstdc++-v3/configure.ac trunk/libstdc++-v3/doc/xml/manual/configure.xml trunk/libstdc++-v3/include/bits/fs_dir.h trunk/libstdc++-v3/include/bits/shared_ptr_base.h trunk/libstdc++-v3/include/ext/concurrence.h trunk/libstdc++-v3/src/filesystem/std-dir.cc
