https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104019
--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jonathan Wakely <r...@gcc.gnu.org>: https://gcc.gnu.org/g:2d8a9ad4a922e3248d0f6c60a6331be6f47dc435 commit r12-6826-g2d8a9ad4a922e3248d0f6c60a6331be6f47dc435 Author: Jonathan Wakely <jwak...@redhat.com> Date: Fri Jan 21 12:08:20 2022 +0000 libstdc++: Fix aliasing violation in std::shared_ptr [PR104019] The non-atomic store that sets both reference counts to zero uses a type-punned pointer, which has undefined behaviour. We could use memset to write 8 bytes, but we don't actually need it to be a single store anyway. No other thread can observe the values, that's why it's safe to use non-atomic stores in the first place. So we can just set each count to zero. With -fstore-merging (which is enabled by default at -O2) GCC produces the same code for this as for memset or the type punned store. Clang does that store merging even at -O1. libstdc++-v3/ChangeLog: PR libstdc++/104019 * include/bits/shared_ptr_base.h (_Sp_counted_base<>::_M_release): Set members to zero without type punning.
