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 --git a/libstdc++-v3/include/bits/shared_ptr_base.h b/libstdc++-v3/include/bits/shared_ptr_base.h
index 5b8f84b65be4d9396ca162421d8ee0761c315311..b2f955b41f786a5874c211e867bd11ebd10b63d7 100644 (file)
--- 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();