<div dir="ltr">Hello,<div><br></div><div>I have followed the advice about taking over the review above, and have gotten to a place where I'm working on getting the existing code to compile cleanly. A few functions were not implemented, so I have forwarded them to the serial backend for now. Just to get compilation working.</div><div><br></div><div>I have a few questions:</div><div><br></div><div>1. I notice that neither the TBB backend, nor the existing OpenMP backend code evaluates the execution policy to understand what to do. I may have misunderstood Louis Dionne, but it appears like the "sequential" mode is not handled at all if the user requests it. That seems wrong, so I must be missing something. I also notice that the execution modes are not enums, they are objects. However, when I try to overload on them in order to specialize for sequential, I get a compile error saying that the types are not fully defined. What is the design expectation for handling the different execution policies?<br></div><div><br></div><div>2. The existing code refers to a type: </div><div><br></div><pre style="color:rgb(8,8,8);font-family:"JetBrains Mono",monospace;font-size:11.3pt"><span style="color:rgb(0,51,179)">using </span>_CombinerType = __pstl::__internal::_Combiner<_Value, _Reduction>;<br></pre><div><span style="color:rgb(8,8,8);font-family:"JetBrains Mono",monospace;font-size:11.3pt">_CombinerType </span><span style="font-family:"JetBrains Mono",monospace;font-size:11.3pt;color:rgb(0,0,0)">__result</span><span style="color:rgb(8,8,8);font-family:"JetBrains Mono",monospace;font-size:11.3pt">{__identity, &__reduction};</span></div><div><br></div><div>However, this type does not exist in __pstl::__internal, at least so far as I can tell. Also, the D70530 code dump does not contain a definition of that object. Has this migrated? Should I provide my own implementation of it? </div><div><br></div><div>3. I have tried to implement a very, very simple function:</div><div><br></div><div><pre style="color:rgb(8,8,8);font-family:"JetBrains Mono",monospace;font-size:11.3pt"><span style="color:rgb(0,51,179)">template </span><<span style="color:rgb(0,51,179)">class </span><span style="color:rgb(55,31,128)">_ExecutionPolicy</span>, <span style="color:rgb(0,51,179)">typename </span><span style="color:rgb(55,31,128)">_F1</span>, <span style="color:rgb(0,51,179)">typename </span><span style="color:rgb(55,31,128)">_F2</span>><br><span style="color:rgb(0,51,179)">void </span><span style="color:rgb(0,98,122)">__parallel_invoke</span>(<span style="color:rgb(55,31,128)">_ExecutionPolicy </span>&&, <span style="color:rgb(55,31,128)">_F1 </span>&&__f1, <span style="color:rgb(55,31,128)">_F2 </span>&&__f2) {<br> <span style="color:rgb(0,51,179)">if </span>(omp_in_parallel()) {<br> _PSTL_PRAGMA(omp sections) {<br> _PSTL_PRAGMA(omp section)<br> <span style="color:rgb(0,128,128)">std</span>::forward<<span style="color:rgb(55,31,128)">_F1</span>>(__f1)();<br> _PSTL_PRAGMA(omp section)<br> <span style="color:rgb(0,128,128)">std</span>::forward<<span style="color:rgb(55,31,128)">_F2</span>>(__f2)();<br> }<br> } <span style="color:rgb(0,51,179)">else </span>{<br> _PSTL_PRAGMA(omp parallel)<br> _PSTL_PRAGMA(omp sections) {<br> _PSTL_PRAGMA(omp section)<br> <span style="color:rgb(0,128,128)">std</span>::forward<<span style="color:rgb(55,31,128)">_F1</span>>(__f1)();<br> _PSTL_PRAGMA(omp section)<br> <span style="color:rgb(0,128,128)">std</span>::forward<<span style="color:rgb(55,31,128)">_F2</span>>(__f2)();<br> }<br> }<br>}</pre>Does this look sane? I have just started reading through the OpenMP documentation. This looks like it could be correct, but there is also the "omp task" directive, and it's not clear which of these is superior in this case. Also, this seems awfully repetitive. Is this just OpenMP?</div><div><br></div><div>Thanks!</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Wed, Sep 16, 2020 at 9:28 AM Kukanov, Alexey <<a href="mailto:Alexey.Kukanov@intel.com">Alexey.Kukanov@intel.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
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<p class="MsoNormal"><span lang="EN-US" style="font-family:Cambria,serif;color:rgb(31,73,125)">Hi Cristopher,<u></u><u></u></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:Cambria,serif;color:rgb(31,73,125)"><u></u> <u></u></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:Cambria,serif;color:rgb(31,73,125)">One good way to contribute, I think, is to develop an OpenMP-based parallel backend. LLVM already supports OpenMP, so it resolves the dependency
problem Louis mentioned. While it’s arguably not the best default engine in the long term, there is certainly some demand for it. The GCC community is also interested in it. Moreover, Mikhail and the team at Intel in collaboration with Thomas (CC’d) from GCC
already developed a basic prototype: <a href="https://reviews.llvm.org/D70530" target="_blank">https://reviews.llvm.org/D70530</a>, but further work is postponed. If you are interested to continue, you are more than welcome, and we will help with guidance and feedback.<u></u><u></u></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:Cambria,serif;color:rgb(31,73,125)"><u></u> <u></u></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:Cambria,serif;color:rgb(31,73,125)">Regards,<u></u><u></u></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:Cambria,serif;color:rgb(31,73,125)">- Alexey<u></u><u></u></span></p>
<p class="MsoNormal"><span lang="EN-US"><u></u> <u></u></span></p>
<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> libcxx-dev <<a href="mailto:libcxx-dev-bounces@lists.llvm.org" target="_blank">libcxx-dev-bounces@lists.llvm.org</a>>
<b>On Behalf Of </b>Christopher Nelson via libcxx-dev<br>
<b>Sent:</b> Wednesday, September 16, 2020 2:43 PM<br>
<b>To:</b> Louis Dionne <<a href="mailto:ldionne@apple.com" target="_blank">ldionne@apple.com</a>><br>
<b>Cc:</b> Dvorskiy, Mikhail <<a href="mailto:mikhail.dvorskiy@intel.com" target="_blank">mikhail.dvorskiy@intel.com</a>>; <br>
<b>Subject:</b> Re: [libcxx-dev] Parallel STL<u></u><u></u></span></p>
<p class="MsoNormal"><span lang="EN-US"><u></u> <u></u></span></p>
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<p class="MsoNormal"><span lang="EN-US">Fantastic. I will study the serial backend and see what I can do!<u></u><u></u></span></p>
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<p class="MsoNormal"><span lang="EN-US"><u></u> <u></u></span></p>
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<p class="MsoNormal"><span lang="EN-US">On Tue, Sep 15, 2020 at 5:27 PM Louis Dionne <</span><a href="mailto:ldionne@apple.com" target="_blank"><span lang="EN-US">ldionne@apple.com</span></a><span lang="EN-US">> wrote:<u></u><u></u></span></p>
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<p class="MsoNormal"><span lang="EN-US">+ Mikhail, who wrote most of the PSTL<u></u><u></u></span></p>
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<p class="MsoNormal"><span lang="EN-US">On Sep 15, 2020, at 15:40, Christopher Nelson <</span><a href="mailto:nadiasvertex@gmail.com" target="_blank"><span lang="EN-US">nadiasvertex@gmail.com</span></a><span lang="EN-US">> wrote:<u></u><u></u></span></p>
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<p class="MsoNormal"><span lang="EN-US">Okay, that makes sense. I can see how you might want to use Grand Central Dispatch on macOS, and the Windows system thread pool on Windows. I'm not really sure what that means for Linux, though. Other than maybe pthreads,
which is not great.<u></u><u></u></span></p>
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<p class="MsoNormal"><span lang="EN-US">Is there any documentation on what is needed to create a backend? Or are there perhaps already plans in motion? I don't want to step on any toes, but I would love to have a usable pstl on macOS and Linux for the next
LLVM release.<u></u><u></u></span></p>
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<p class="MsoNormal"><span lang="EN-US">We use libc++ on Linux as well as macOS. Depending on what's involved, I might be able to contribute a backend for those two platforms.<u></u><u></u></span></p>
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<p class="MsoNormal"><span lang="EN-US"><u></u> <u></u></span></p>
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<p class="MsoNormal"><span lang="EN-US">You're not stepping on any toes, far from that. If we have backends with satisfactory performance and we're confident about ABI stability, I don't see a reason why we wouldn't ship the PSTL as soon as we have those. One
big issue to shipping it so far has been that the only backends are serial (not great to ship that), and the other one relies on an external dependency (TBB).<u></u><u></u></span></p>
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<p class="MsoNormal"><span lang="EN-US"><u></u> <u></u></span></p>
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<p class="MsoNormal"><span lang="EN-US">Mikhail might be able to provide documentation. We should check it into the PSTL repository. I meant to write such documentation when I wrote the serial backend, but never got around to writing something that was enough
to check in. You can see the minimal API needed to implement a backend here: pstl/include/pstl/internal/parallel_backend_serial.h. It's the serial backend, which tries to be as trivial as possible.<u></u><u></u></span></p>
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<p class="MsoNormal"><span lang="EN-US">Are you familiar with libc++ contribution? If so, contributing to PSTL works basically the same -- just send a Phabricator review and I'll review it. We can also chat on Slack in the Cpplang workspace and I can give some
guidance -- look for "ldionne".<u></u><u></u></span></p>
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<p class="MsoNormal"><span lang="EN-US">Cheers,<u></u><u></u></span></p>
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<p class="MsoNormal"><span lang="EN-US">Louis<u></u><u></u></span></p>
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<p class="MsoNormal"><span lang="EN-US">On Tue, Sep 15, 2020 at 2:50 PM Louis Dionne <</span><a href="mailto:ldionne@apple.com" target="_blank"><span lang="EN-US">ldionne@apple.com</span></a><span lang="EN-US">> wrote:<u></u><u></u></span></p>
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<p class="MsoNormal" style="margin-bottom:12pt"><span lang="EN-US">Hi,<br>
<br>
Long story short, the PSTL is pretty much ready to be shipped with LLVM. I did the integration between it and libc++, and it all worked last time I checked. I think the next step would be to change whatever LLVM scripts are used to create releases to also install
the PSTL, which is the part I haven't had time to look into yet.<br>
<br>
That being said, the PSTL will then default to using the Serial backend, which isn't very useful. We could decide to ship a different backend if we wanted, however I think what makes sense is to use a backend specific to the platform we're running on instead
of adding a dependency to LLVM.<br>
<br>
Louis<br>
<br>
> On Sep 8, 2020, at 08:25, Christopher Nelson via libcxx-dev <</span><a href="mailto:libcxx-dev@lists.llvm.org" target="_blank"><span lang="EN-US">libcxx-dev@lists.llvm.org</span></a><span lang="EN-US">> wrote:<br>
> <br>
> Hello friends,<br>
> <br>
> I have spent some time looking at the mailing archives and git logs for the parallel STL. I'm not clear what state it is in, since the oneAPI/tbb seems to be production ready and comes with the parallel STL. Also, it appears the GCC has shipped a PSTL based
on the same code that clang is using.<br>
> <br>
> I was wondering if someone could clarify for me what state the PSTL is in, and if there is some work needed to help get it over the finish line I may be able to help. I'm very interested in using it in our production software, so I'm a motivated helper. :-)<br>
> <br>
> Thank you for your time,<br>
> -={C}=-<br>
> _______________________________________________<br>
> libcxx-dev mailing list<br>
> </span><a href="mailto:libcxx-dev@lists.llvm.org" target="_blank"><span lang="EN-US">libcxx-dev@lists.llvm.org</span></a><span lang="EN-US"><br>
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