<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">I’m very excited to have NVIDIA collaborate on libc++. It’s worth supporting your weirdo macro hack as a transitional tool.<div class=""><br class=""></div><div class="">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" class="">wg21.link/P0829R3</a>). I hope that we can experiment on what’s “nice” in clang / libc++ in the next few months.</div><div class=""><div class="">One design constraint around freestanding: I want to make sure that clang can keep supporting other STL implementations.</div><div class=""><br class=""></div><div class="">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.</div><div class=""><br class=""></div></div><div class="">On “freestanding” macro, clang does the following today:</div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Courier" class=""> if (LangOpts.Freestanding)</font></div><div class=""><font face="Courier" class=""> Builder.defineMacro("__STDC_HOSTED__", "0");</font></div><div class=""><font face="Courier" class=""> else</font></div><div class=""><font face="Courier" class=""> Builder.defineMacro("__STDC_HOSTED__</font><span style="font-family: Courier;" class="">"</span><span style="font-family: Courier;" class="">);</span></div></blockquote><div class="">Otherwise, clang’s lib/Headers do some stuff with HOSTED as well, which might interfere with freestanding.</div><div class=""><br class="">Good header hygiene indeed seems necessary, especially for <algorithm>. Louis mentioned that he was interested in looking into this.</div><div class="">Louis did a survey and found the following:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><div class=""><div class="">Freestanding in the current C++20 draft requires the following headers:</div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""><br class=""></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <ciso646></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <cstddef></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <cfloat> </font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <limits> </font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <climits></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <cstdint></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <cstdlib></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <new></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <typeinfo></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <exception></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <initializer_list></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <cstdarg></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <type_traits></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <atomic></font></div></div></div><div class=""><div class=""><div class=""><br class=""></div></div></div><div class=""><div class=""><div class="">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:</div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""><br class=""></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <ciso646>: nothing</font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <cstddef>: stddef.h</font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <cfloat> : float.h</font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <limits> : stddef.h</font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <climits>: limits.h</font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <cstdint>: stdint.h</font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <cstdlib>: stdlib.h</font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <initializer_list>: stddef.h</font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <cstdarg>: stdarg.h</font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <type_traits>: stddef.h</font></div></div></div><div class=""><div class=""><div class=""><br class=""></div></div></div><div class=""><div class=""><div class="">As a result, I think the following are low-hanging fruit that do not require any runtime support AFAICT:</div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""><br class=""></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <ciso646></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <cstddef></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <cfloat></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <limits></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <climits></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <cstdint></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <initializer_list></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <type_traits></font></div></div></div><div class=""><div class=""><div class=""><br class=""></div></div></div><div class=""><div class=""><div class="">Other things we might be able to throw in with minimal effort:</div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""><br class=""></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <bit></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <ratio></font></div></div></div><div class=""><div class=""><div class=""><br class=""></div></div></div><div class=""><div class=""><div class="">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):</div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""><br class=""></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <tuple></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <pair></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> most if not all of <functional></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> most of <algorithm></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <span></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <array></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> <string_view></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> lock-free parts of <atomic></font></div></div></div></blockquote><div class=""><div class=""><div class=""><br class=""></div></div></div><div class=""><br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On Dec 10, 2018, at 9:23 PM, Olivier Giroux via libcxx-dev <<a href="mailto:libcxx-dev@lists.llvm.org" class="">libcxx-dev@lists.llvm.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="WordSection1" style="page: WordSection1; caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;"><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 11pt;" class="">Hello libc++-dev,<o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 11pt;" class=""><o:p class=""> </o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 11pt;" class="">In the discussion of<span class="Apple-converted-space"> </span><a href="https://reviews.llvm.org/D55517" style="color: rgb(149, 79, 114); text-decoration: underline;" class="">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 class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 11pt;" class=""><o:p class=""> </o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 11pt;" class="">Not one to bury the lede, I’d like to talk about a CUDA C++ standard library.<o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 11pt;" class=""><o:p class=""> </o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 11pt;" class="">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 class="">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 class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 11pt;" class=""><o:p class=""> </o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 11pt;" class="">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" style="color: rgb(149, 79, 114); text-decoration: underline;" class="">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 class="Apple-converted-space"> </span></span><span style="font-size: 11pt; font-family: "Apple Color Emoji";" class="">😊</span><span style="font-size: 11pt;" class=""><o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 11pt;" class=""><o:p class=""> </o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 11pt;" class="">After making a few exploration vehicles (2 overall, 4 for <atomic>), we now think we’ll create version 1 this way:<o:p class=""></o:p></span></div><ol start="1" type="1" style="margin-bottom: 0in; margin-top: 0in;" class=""><li class="MsoListParagraph" style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;"><span style="font-size: 11pt;" class="">Wrap select libcxx <*> headers with <cuda/*> to introduce symbols in cuda::* instead of std::*, and…<o:p class=""></o:p></span><ol start="1" type="a" style="margin-bottom: 0in; margin-top: 0in;" class=""><li class="MsoListParagraph" style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;"><span style="font-size: 11pt;" class="">These facilities are always heterogeneous, NORTTI, and NOEXCEPTIONS.<o:p class=""></o:p></span></li><li class="MsoListParagraph" style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;"><span style="font-size: 11pt;" class="">Enable users to include them<span class="Apple-converted-space"> </span><i class="">on top of</i><span class="Apple-converted-space"> </span>their host library (that being CPU only).<o:p class=""></o:p></span></li></ol></li><li class="MsoListParagraph" style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;"><span style="font-size: 11pt;" class="">“Select” here means prioritizing headers in Freestanding now, or soon, basically the header-only facilities.<o:p class=""></o:p></span></li><li class="MsoListParagraph" style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;"><span style="font-size: 11pt;" class="">Subsequently help maintain the intersection of libcxx and Freestanding.<o:p class=""></o:p></span></li></ol><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 11pt;" class=""><o:p class=""> </o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 11pt;" class="">In terms of libcxx design, we think that we could layer on this surface:<o:p class=""></o:p></span></div><ul type="disc" style="margin-bottom: 0in; margin-top: 0in;" class=""><li class="MsoListParagraph" style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;"><span style="font-size: 11pt;" class="">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 class=""></o:p></span><ul type="circle" style="margin-bottom: 0in; margin-top: 0in;" class=""><li class="MsoListParagraph" style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;"><span style="font-size: 11pt;" class="">For example, in <atomic>, it would be preferable for Freestanding implementations (and users) if the<span class="Apple-converted-space"> </span><i class="">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 class="">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.<o:p class=""></o:p></span></li><li class="MsoListParagraph" style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;"><span style="font-size: 11pt;" class="">It is my intention to contribute the code for this 3<sup class="">rd</sup><span class="Apple-converted-space"> </span>strategy, and other maintenance to <atomic>, some of which I’ve already made in my branch.<o:p class=""></o:p></span></li></ul></li><li class="MsoListParagraph" style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;"><span style="font-size: 11pt;" class="">An extension point that allows std::* symbols to be put into another namespace, both for ABI and to co-exist.<o:p class=""></o:p></span><ul type="circle" style="margin-bottom: 0in; margin-top: 0in;" class=""><li class="MsoListParagraph" style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;"><span style="font-size: 11pt;" class="">This is in tension with Eric’s proposed change.<o:p class=""></o:p></span></li></ul></li><li class="MsoListParagraph" style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;"><span style="font-size: 11pt;" class="">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 class=""></o:p></span><ul type="circle" style="margin-bottom: 0in; margin-top: 0in;" class=""><li class="MsoListParagraph" style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;"><span style="font-size: 11pt;" class="">This was at one point in tension with changes Louis was making, but I think we’re Ok right now.<o:p class=""></o:p></span></li></ul></li><li class="MsoListParagraph" style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;"><span style="font-size: 11pt;" class="">And, generally speaking, good header inclusion hygiene that tries to minimize what’s pulled into a facility’s header.<o:p class=""></o:p></span></li></ul><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 11pt;" class=""><o:p class=""> </o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 11pt;" class="">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 class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 11pt;" class=""><o:p class=""> </o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 11pt;" class="">Thanks for reading, I’ll try to answer your questions as best I can.<o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 11pt;" class=""><o:p class=""> </o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 11pt;" class="">Sincerely,<o:p class=""></o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 11pt;" class=""><o:p class=""> </o:p></span></div><div style="margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 11pt;" class="">Olivier<br class=""><br class=""><o:p class=""></o:p></span></div></div><div style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class=""><hr class=""></div><div style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class="">This email message is for the sole use of the intended recipient(s) and may contain confidential information. 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