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.

Jason

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