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<p class="MsoNormal">Thanks!<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The only part of what you propose at the bottom that I’ll take exception to is the proposed initial exclusion and later subsetting of <atomic>. It’s in the Freestanding subset now, probably in the Freestanding subset in the future also,
we just polled this in SG1 ~weeks ago and we don’t want to subset it. Rather, we can eliminate the library dependency by having the lock-byte strategy in libcxx itself for non-<i>lock-free</i> cases, achieving the desired goal of being dependency-free without
having to upset SG1.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">We’re good with the rest. What you wrote closely resembles what we proposed to our people.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Olivier<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><b><span style="font-size:12.0pt;color:black">From: </span></b><span style="font-size:12.0pt;color:black"><jfbastien@apple.com> on behalf of JF Bastien <jfbastien@apple.com><br>
<b>Date: </b>Tuesday, December 11, 2018 at 9:40 AM<br>
<b>To: </b>Olivier Giroux <OGiroux@nvidia.com>, Louis Dionne <ldionne@apple.com><br>
<b>Cc: </b>"libcxx-dev@lists.llvm.org" <libcxx-dev@lists.llvm.org><br>
<b>Subject: </b>Re: [libcxx-dev] An extension of libcxx<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I’m very excited to have NVIDIA collaborate on libc++. It’s worth supporting your weirdo macro hack as a transitional tool.
<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I’m especially interested in working on freestanding in clang / libc++, bringing the good parts of it from the current C++ standard, and working with you and other on the Committee to make C++23 freestanding actually nice (Ben Craig has
been working on <a href="http://wg21.link/P0829R3">wg21.link/P0829R3</a>). I hope that we can experiment on what’s “nice” in clang / libc++ in the next few months.<o:p></o:p></p>
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<p class="MsoNormal">One design constraint around freestanding: I want to make sure that clang can keep supporting other STL implementations.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I’d like to understand if we can have a different ABI for freestanding, given that it’s not supported in libc++ today. This might be an opportunity to fix some mistakes.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On “freestanding” macro, clang does the following today:<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Courier"> if (LangOpts.Freestanding)</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Courier"> Builder.defineMacro("__STDC_HOSTED__", "0");</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Courier"> else</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Courier"> Builder.defineMacro("__STDC_HOSTED__");</span><o:p></o:p></p>
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<p class="MsoNormal">Otherwise, clang’s lib/Headers do some stuff with HOSTED as well, which might interfere with freestanding.<o:p></o:p></p>
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<p class="MsoNormal"><br>
Good header hygiene indeed seems necessary, especially for <algorithm>. Louis mentioned that he was interested in looking into this.<o:p></o:p></p>
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<p class="MsoNormal">Louis did a survey and found the following:<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Freestanding in the current C++20 draft requires the following headers:<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Courier"> <ciso646></span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Courier"> <cstddef></span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Courier"> <limits> </span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Courier"> <climits></span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Courier"> <cstdint></span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Courier"> <exception></span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Courier"> <initializer_list></span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Courier"> <cstdarg></span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Courier"> <type_traits></span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Courier"> <atomic></span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Of those headers, I think the following are easy to provide with minimal changes to libc++ and without having to ship a libc++ shared object (or compiler-rt), and they use the following parts of the C Standard Library:<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Courier"> <ciso646>: nothing</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Courier"> <cstddef>: stddef.h</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Courier"> <cfloat> : float.h</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Courier"> <limits> : stddef.h</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Courier"> <cstdlib>: stdlib.h</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Courier"> <initializer_list>: stddef.h</span><o:p></o:p></p>
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<p class="MsoNormal">As a result, I think the following are low-hanging fruit that do not require any runtime support AFAICT:<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Courier"> <ciso646></span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Courier"> <cstddef></span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Courier"> <limits></span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Courier"> <climits></span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Courier"> <cstdint></span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Courier"> <initializer_list></span><o:p></o:p></p>
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<p class="MsoNormal">Other things we might be able to throw in with minimal effort:<o:p></o:p></p>
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<p class="MsoNormal">Other things that we SHOULD be able to have, but that would require refactoring in libc++ (and most of them are not part of the current freestanding):<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Courier"> most if not all of <functional></span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Courier"> most of <algorithm></span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Courier"> lock-free parts of <atomic></span><o:p></o:p></p>
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<o:p></o:p></p>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<div>
<p class="MsoNormal">On Dec 10, 2018, at 9:23 PM, Olivier Giroux via libcxx-dev <<a href="mailto:libcxx-dev@lists.llvm.org">libcxx-dev@lists.llvm.org</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Hello libc++-dev,<span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal"> <span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal">In the discussion of<span class="apple-converted-space"> </span><a href="https://reviews.llvm.org/D55517"><span style="color:#954F72">https://reviews.llvm.org/D55517</span></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.<span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal"> <span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal">Not one to bury the lede, I’d like to talk about a CUDA C++ standard library.<span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal"> <span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal">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,<span class="apple-converted-space"> </span><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.<span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal"> <span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal">One exploration vehicle was shown at CppCon in September (by me, see: YouTube, and<span class="apple-converted-space"> </span><a href="https://github.com/ogiroux/freestanding"><span style="color:#954F72">https://github.com/ogiroux/freestanding</span></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 class="apple-converted-space"> </span><span style="font-family:"Apple Color Emoji"">😊</span><span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal"> <span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal">After making a few exploration vehicles (2 overall, 4 for <atomic>), we now think we’ll create version 1 this way:<span style="font-size:12.0pt"><o:p></o:p></span></p>
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<ol style="margin-top:0in" start="1" type="1">
<li class="MsoListParagraph" style="margin-top:0in;margin-bottom:0in;margin-bottom:.0001pt;mso-list:l1 level1 lfo1">
Wrap select libcxx <*> headers with <cuda/*> to introduce symbols in cuda::* instead of std::*, and…<span style="font-size:12.0pt">
<o:p></o:p></span></li></ol>
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<ol style="margin-top:0in" start="1" type="a">
<li class="MsoListParagraph" style="margin-top:0in;margin-bottom:0in;margin-bottom:.0001pt;mso-list:l1 level2 lfo1">
These facilities are always heterogeneous, NORTTI, and NOEXCEPTIONS.<span style="font-size:12.0pt"><o:p></o:p></span></li><li class="MsoListParagraph" style="margin-top:0in;margin-bottom:0in;margin-bottom:.0001pt;mso-list:l1 level2 lfo1">
Enable users to include them<span class="apple-converted-space"> </span><i>on top of</i><span class="apple-converted-space"> </span>their host library (that being CPU only).<span style="font-size:12.0pt"><o:p></o:p></span></li></ol>
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<ol style="margin-top:0in" start="2" type="1">
<li class="MsoListParagraph" style="margin-top:0in;margin-bottom:0in;margin-bottom:.0001pt;mso-list:l1 level1 lfo1">
“Select” here means prioritizing headers in Freestanding now, or soon, basically the header-only facilities.<span style="font-size:12.0pt"><o:p></o:p></span></li><li class="MsoListParagraph" style="margin-top:0in;margin-bottom:0in;margin-bottom:.0001pt;mso-list:l1 level1 lfo1">
Subsequently help maintain the intersection of libcxx and Freestanding.<span style="font-size:12.0pt"><o:p></o:p></span></li></ol>
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<p class="MsoNormal"> <span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal">In terms of libcxx design, we think that we could layer on this surface:<span style="font-size:12.0pt"><o:p></o:p></span></p>
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<li class="MsoListParagraph" style="margin-top:0in;margin-bottom:0in;margin-bottom:.0001pt;mso-list:l0 level1 lfo2">
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.<span style="font-size:12.0pt">
<o:p></o:p></span></li></ul>
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<li class="MsoListParagraph" style="margin-top:0in;margin-bottom:0in;margin-bottom:.0001pt;mso-list:l0 level2 lfo2">
For example, in <atomic>, it would be preferable for Freestanding implementations (and users) if the<span class="apple-converted-space"> </span><i>lock-in-atomic</i><span class="apple-converted-space"> </span>strategy was used for non-lock-free atomics (instead
of the<span class="apple-converted-space"> </span><i>sharded-lock-table</i><span class="apple-converted-space"> </span>strategy tucked inside __cxa_atomic_*) because that then frees the program from dependencies on libatomic.<span style="font-size:12.0pt"><o:p></o:p></span></li><li class="MsoListParagraph" style="margin-top:0in;margin-bottom:0in;margin-bottom:.0001pt;mso-list:l0 level2 lfo2">
It is my intention to contribute the code for this 3<sup>rd</sup><span class="apple-converted-space"> </span>strategy, and other maintenance to <atomic>, some of which I’ve already made in my branch.<span style="font-size:12.0pt"><o:p></o:p></span></li></ul>
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<ul style="margin-top:0in" type="disc">
<li class="MsoListParagraph" style="margin-top:0in;margin-bottom:0in;margin-bottom:.0001pt;mso-list:l0 level1 lfo2">
An extension point that allows std::* symbols to be put into another namespace, both for ABI and to co-exist.<span style="font-size:12.0pt">
<o:p></o:p></span></li></ul>
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<li class="MsoListParagraph" style="margin-top:0in;margin-bottom:0in;margin-bottom:.0001pt;mso-list:l0 level2 lfo2">
This is in tension with Eric’s proposed change.<span style="font-size:12.0pt"><o:p></o:p></span></li></ul>
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<li class="MsoListParagraph" style="margin-top:0in;margin-bottom:0in;margin-bottom:.0001pt;mso-list:l0 level1 lfo2">
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).<span style="font-size:12.0pt">
<o:p></o:p></span></li></ul>
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<li class="MsoListParagraph" style="margin-top:0in;margin-bottom:0in;margin-bottom:.0001pt;mso-list:l0 level2 lfo2">
This was at one point in tension with changes Louis was making, but I think we’re Ok right now.<span style="font-size:12.0pt"><o:p></o:p></span></li></ul>
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<li class="MsoListParagraph" style="margin-top:0in;margin-bottom:0in;margin-bottom:.0001pt;mso-list:l0 level1 lfo2">
And, generally speaking, good header inclusion hygiene that tries to minimize what’s pulled into a facility’s header.<span style="font-size:12.0pt"><o:p></o:p></span></li></ul>
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<p class="MsoNormal">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!<span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal"> <span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal">Thanks for reading, I’ll try to answer your questions as best I can.<span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal"> <span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal">Sincerely,<span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal"> <span style="font-size:12.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal">Olivier<br>
<br>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">_______________________________________________<br>
libcxx-dev mailing list<br>
</span><a href="mailto:libcxx-dev@lists.llvm.org"><span style="font-size:9.0pt;font-family:Helvetica;color:#954F72">libcxx-dev@lists.llvm.org</span></a><span style="font-size:9.0pt;font-family:Helvetica"><br>
</span><a href="http://lists.llvm.org/cgi-bin/mailman/listinfo/libcxx-dev"><span style="font-size:9.0pt;font-family:Helvetica;color:#954F72">http://lists.llvm.org/cgi-bin/mailman/listinfo/libcxx-dev</span></a><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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