[gcc r12-6826] libstdc++: Fix aliasing violation in std::shared_ptr [PR104019]

[Jonathan Wakely]

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_release):
            Set members to zero without type punning.

Diff:
---
 libstdc++-v3/include/bits/shared_ptr_base.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/shared_ptr_base.h b/libstdc++-v3/include/bits/shared_ptr_base.h
index 5b8f84b65be..b2f955b41f7 100644
--- a/libstdc++-v3/include/bits/shared_ptr_base.h
+++ b/libstdc++-v3/include/bits/shared_ptr_base.h
@@ -340,7 +340,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	      // we are releasing the last strong reference. No other
 	      // threads can observe the effects of this _M_release()
 	      // call (e.g. calling use_count()) without a data race.
-	      *(long long*)(&_M_use_count) = 0;
+	      _M_weak_count = _M_use_count = 0;
 	      _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_use_count);
 	      _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_weak_count);
 	      _M_dispose();