On Fri, 15 Nov 2024 at 15:20, Jonathan Wakely <jwak...@redhat.com> wrote:
>
> On Fri, 15 Nov 2024 at 15:12, Jason Merrill <ja...@redhat.com> wrote:
> >
> > On 11/15/24 9:20 AM, Jonathan Wakely wrote:
> > > On 15/11/24 08:53 -0500, Jason Merrill wrote:
> > >> On 11/15/24 6:04 AM, Jonathan Wakely wrote:
> > >>> On 14/11/24 23:09 -0500, Jason Merrill wrote:
> >
> > >>>> Since bits/stdc++.h also intends to include the whole standard
> > >>>> library, I
> > >>>> include it rather than duplicate it. But stdc++.h comments out
> > >>>> <execution>,
> > >>>> so I include it separately. Alternatively, we could uncomment it in
> > >>>> stdc++.h.
> > >>>
> > >>> It's not included by default because it pulls in a dependency on
> > >>> libtbb.so even if you don't use the parallel algos:
> > >>>
> > >>> $ g++ -include algorithm -include execution -x c++ - <<< 'int main()
> > >>> { }'
> > >>> /usr/bin/ld: /tmp/ccA76qrz.o: in function
> > >>> `tbb::detail::d1::execution_slot(tbb::detail::d1::execution_data
> > >>> const&)':
> > >>> <stdin>:
> > >>> (.text._ZN3tbb6detail2d114execution_slotERKNS1_14execution_dataE[_ZN3tbb6detail2d114execution_slotERKNS1_14execution_dataE]+0x14):
> > >>> undefined reference to
> > >>> `tbb::detail::r1::execution_slot(tbb::detail::d1::execution_data
> > >>> const*)'
> > >>> /usr/bin/ld: /tmp/ccA76qrz.o: in function
> > >>> `tbb::detail::d1::current_thread_index()':
> > >>> <stdin>:
> > >>> (.text._ZN3tbb6detail2d120current_thread_indexEv[_ZN3tbb6detail2d120current_thread_indexEv]+0xe):
> > >>> undefined reference to
> > >>> `tbb::detail::r1::execution_slot(tbb::detail::d1::execution_data
> > >>> const*)'
> > >>> collect2: error: ld returned 1 exit status
> > >>>
> > >>> The way we arranged the PSTL inclusion is that you only get a
> > >>> dependeny on TBB if you include a header containing a parallel algo
> > >>> (<algorithm>, <numeric>) *and* include <execution>. Since including
> > >>> <execution> is formally required by the standard in order to pass one
> > >>> of std::execution::par, std::execution::seq etc. to a parallel algo,
> > >>> we switch on whether <execution> has been included or not to detect
> > >>> whether the user actually wants to use parallel algos.
> > >>>
> > >>> Is this not a problem for 'import std;'?
> > >>
> > >> Hmm, I haven't been seeing that in my testing, std.o only contains the
> > >> reference to ios_base_library_init.
> > >>
> > >> But then, I don't get that error with your testcase above, either.
> > >> With trunk, the compiler output for that only contains main for me.
> > >
> > > Then I assume you don't have tbb-devel installed. If the tbb headers
> > > aren't present, then <execution> doesn't include them.
> > >
> > > Could you do 'dnf install tbb-devel' and try again?
> >
> > Indeed, that's what the difference was. With tbb-devel installed,
> > compiling std.cc fails with an error about TBB code exposing
> > task_ptr_or_nullptr in the anonymous namespace.
> >
> > tbb-devel also isn't installed on tor/torsion, and I don't seem to have
> > sudo on them anymore.
> >
> > > Maybe we should figure out how to fix it for #include <execution>
> > > anyway. When we eventually support senders/receivers in <execution>
> > > for C++26, users are going to want to include that for reasons other
> > > than using the parallel algos and their execution policies.
> >
> > Is there a way I can suppress pstl with a macro? If not, I guess I'll
> > just leave <execution> out for now.
>
> Defining _GLIBCXX_USE_TBB_PAR_BACKEND to 0 will work.
That only suppresses the TBB backend, so you can still use the
execution policies in <execution> and call parallel algos ... they
just don't run in parallel :-)
All the declarations required by the standard will be present.
