Thanks Jonathan! On Sun, Jan 23, 2022 at 5:51 PM Jonathan Wakely <jwak...@redhat.com> wrote:
> 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 > > > >
