Tested powerpc64le-linux. Does anybody see a problem with this change?
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:
* include/bits/shared_ptr_base.h (_Sp_counted_base<>::_M_release):
Set members to zero without type punning.
---
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();
--
2.31.1
