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.
