[WIP RFC] libstdc++: add module std

[Jason Merrill]

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