[PATCH] libstdc++: Skip atomic instructions in _Sp_counted_base::_M_release when both counts are 1

[Jonathan Wakely]

On Wed, 8 Dec 2021 at 19:21, Jonathan Wakely wrote:
>
> On Wed, 8 Dec 2021 at 19:17, Rainer Orth wrote:
> >
> > Hi Jonathan,
> >
> > > I've pushed this change to trunk now (it was posted and reviewed in
> > > stage 1, I just didn't get around to pushing it until now).
> > >
> > > The final version of the patch is attached to this mail.
> >
> > unfortunately, it breaks Solaris/SPARC bootstrap:
> >
> > In file included from /var/gcc/regression/master/11.4-gcc-gas/build/sparc-sun-solaris2.11/sparcv9/libstdc++-v3/include/bits/shared_ptr.h:53,
> >                  from /var/gcc/regression/master/11.4-gcc-gas/build/sparc-sun-solaris2.11/sparcv9/libstdc++-v3/include/memory:77,
> >                  from /vol/gcc/src/hg/master/local/libstdc++-v3/include/precompiled/stdc++.h:82:
> > /var/gcc/regression/master/11.4-gcc-gas/build/sparc-sun-solaris2.11/sparcv9/libstdc++-v3/include/bits/shared_ptr_base.h: In member function 'void std::_Sp_counted_base<_Lp>::_M_release() [with __gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic]':
> > /var/gcc/regression/master/11.4-gcc-gas/build/sparc-sun-solaris2.11/sparcv9/libstdc++-v3/include/bits/shared_ptr_base.h:329:26: error: right operand of shift expression '(1 << 64)' is greater than or equal to the precision 64 of the left operand [-fpermissive]
> >   329 |             = 1LL + (1LL << (__CHAR_BIT__ * sizeof(_Atomic_word)));
> >       |                     ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > make[9]: *** [Makefile:1875: sparc-sun-solaris2.11/bits/stdc++.h.gch/O2ggnu++0x.gch] Error 1
> >
> > For 64-bit SPARC, _Atomic_word is long.
>
> Ah yes, so we need to disable this optimization. Patch coming up ...

Gah, I remembered to check that:

      constexpr bool __double_word
        = sizeof(long long) == 2 * sizeof(_Atomic_word);
      // The ref-count members follow the vptr, so are aligned to
      // alignof(void*).
      constexpr bool __aligned = __alignof(long long) <= alignof(void*);
      if _GLIBCXX17_CONSTEXPR (__lock_free && __double_word && __aligned)


But for C++11 and C++14 that is a normal runtime condition not
if-constexpr, so the undefined shift still gets compiled, even though
it can never be reached at runtime.