I thought I'd CC'd Maged on this patch, but apparently not. I've pushed it to trunk now.
On Fri, 21 Jan 2022 at 13:50, Jonathan Wakely wrote: > > 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 >
