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<p class="MsoNormal"><span style="font-size:11.0pt">Hello libc++-dev,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">In the discussion of <a href="https://reviews.llvm.org/D55517">
https://reviews.llvm.org/D55517</a>, I mentioned that we are attempting a vendor variant of libcxx that uses _VSTD differently. Eric pointed out that I should have started here, so we could talk about design goals. He’s right, I’m sorry.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Not one to bury the lede, I’d like to talk about a CUDA C++ standard library.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">The ultimate goal of something like that should be that most things in C++, if not bolted too-tightly onto the operating system, should be able to be passed and used between CPU and GPU. There’s no fundamental
reason why we don’t have a big chunk of C++ working like this, <i>today</i>, if we’re talking about contemporary HPC-friendly GPUs. The reason we don’t have much is that it’s a huge pile of work and everyone has managed to avoid doing it so far.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">One exploration vehicle was shown at CppCon in September (by me, see: YouTube, and
<a href="https://github.com/ogiroux/freestanding">https://github.com/ogiroux/freestanding</a>) and then we made but failed to present a more detailed poster at the LLVM dev meeting in October. And now we’re here.
</span><span style="font-size:11.0pt;font-family:"Apple Color Emoji"">😊</span><span style="font-size:11.0pt"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">After making a few exploration vehicles (2 overall, 4 for <atomic>), we now think we’ll create version 1 this way:<o:p></o:p></span></p>
<ol style="margin-top:0in" start="1" type="1">
<li class="MsoListParagraph" style="margin-left:0in;mso-list:l0 level1 lfo1"><span style="font-size:11.0pt">Wrap select libcxx <*> headers with <cuda/*> to introduce symbols in cuda::* instead of std::*, and…<o:p></o:p></span>
<ol style="margin-top:0in" start="1" type="a">
<li class="MsoListParagraph" style="margin-left:0in;mso-list:l0 level2 lfo1"><span style="font-size:11.0pt">These facilities are always heterogeneous, NORTTI, and NOEXCEPTIONS.<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0in;mso-list:l0 level2 lfo1"><span style="font-size:11.0pt">Enable users to include them
<i>on top of</i> their host library (that being CPU only).<o:p></o:p></span></li></ol>
</li><li class="MsoListParagraph" style="margin-left:0in;mso-list:l0 level1 lfo1"><span style="font-size:11.0pt">“Select” here means prioritizing headers in Freestanding now, or soon, basically the header-only facilities.<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0in;mso-list:l0 level1 lfo1"><span style="font-size:11.0pt">Subsequently help maintain the intersection of libcxx and Freestanding.<o:p></o:p></span></li></ol>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">In terms of libcxx design, we think that we could layer on this surface:<o:p></o:p></span></p>
<ul style="margin-top:0in" type="disc">
<li class="MsoListParagraph" style="margin-left:0in;mso-list:l1 level1 lfo2"><span style="font-size:11.0pt">A freestanding mode, say LIBCXX_FREESTANDING, with a design goal of placing low-OS-coupling variants of code for facilities under this mode, and some
agreement that Freestanding libraries have different ABI goals than their closest Hosted relative.<o:p></o:p></span>
<ul style="margin-top:0in" type="circle">
<li class="MsoListParagraph" style="margin-left:0in;mso-list:l1 level2 lfo2"><span style="font-size:11.0pt">For example, in <atomic>, it would be preferable for Freestanding implementations (and users) if the
<i>lock-in-atomic</i> strategy was used for non-lock-free atomics (instead of the
<i>sharded-lock-table</i> strategy tucked inside __cxa_atomic_*) because that then frees the program from dependencies on libatomic.<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0in;mso-list:l1 level2 lfo2"><span style="font-size:11.0pt">It is my intention to contribute the code for this 3<sup>rd</sup> strategy, and other maintenance to <atomic>, some of which I’ve already made in my
branch.<o:p></o:p></span></li></ul>
</li><li class="MsoListParagraph" style="margin-left:0in;mso-list:l1 level1 lfo2"><span style="font-size:11.0pt">An extension point that allows std::* symbols to be put into another namespace, both for ABI and to co-exist.<o:p></o:p></span>
<ul style="margin-top:0in" type="circle">
<li class="MsoListParagraph" style="margin-left:0in;mso-list:l1 level2 lfo2"><span style="font-size:11.0pt">This is in tension with Eric’s proposed change.<o:p></o:p></span></li></ul>
</li><li class="MsoListParagraph" style="margin-left:0in;mso-list:l1 level1 lfo2"><span style="font-size:11.0pt">An extension point that allows us to tune visibility control, e.g. add __device__ linkage to local-linkage symbols in those headers included in the subset
(Freestanding minimum, or the implementation-defined choice).<o:p></o:p></span>
<ul style="margin-top:0in" type="circle">
<li class="MsoListParagraph" style="margin-left:0in;mso-list:l1 level2 lfo2"><span style="font-size:11.0pt">This was at one point in tension with changes Louis was making, but I think we’re Ok right now.<o:p></o:p></span></li></ul>
</li><li class="MsoListParagraph" style="margin-left:0in;mso-list:l1 level1 lfo2"><span style="font-size:11.0pt">And, generally speaking, good header inclusion hygiene that tries to minimize what’s pulled into a facility’s header.<o:p></o:p></span></li></ul>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">That should isolate most of the ugly stuff in our code; version 1 will indeed be fairly ugly, no doubt about that. But then, hopefully, this all ends with libcxx gaining a new implementer!<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Thanks for reading, I’ll try to answer your questions as best I can.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Sincerely,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Olivier<br>
<br>
<o:p></o:p></span></p>
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