[WIP RFC] libstdc++: add module std

[Jason Merrill]

On 11/15/24 10:21 AM, Jonathan Wakely wrote:
> On Fri, 15 Nov 2024 at 15:20, Jonathan Wakely <jwakely@redhat.com> wrote:
>>
>> On Fri, 15 Nov 2024 at 15:12, Jason Merrill <jason@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.

Hmm, I'm not sure whether that is preferable to leaving them out 
entirely.  But here's a v3 that does that, along with your other 
suggestions:

-------------- next part --------------
A non-text attachment was scrubbed...
Name: 0001-libstdc-add-module-std-PR106852.patch
Type: text/x-patch
Size: 117382 bytes
Desc: not available
URL: <https://gcc.gnu.org/pipermail/libstdc++/attachments/20241115/c59d9167/attachment-0001.bin>