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Subject: [Bug libstdc++/104019] Testsuite
 17_intro/headers/c++2020/stdc++_multiple_inclusion.cc failures
Date: Sun, 23 Jan 2022 22:49:39 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104019

--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:2d8a9ad4a922e3248d0f6c60a6331be6f47dc435

commit r12-6826-g2d8a9ad4a922e3248d0f6c60a6331be6f47dc435
Author: Jonathan Wakely <jwakely@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_releas=
e):
            Set members to zero without type punning.=