[libcxx-dev] An extension of libcxx

[JF Bastien via libcxx-dev]

I’m very excited to have NVIDIA collaborate on libc++. It’s worth supporting your weirdo macro hack as a transitional tool.

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 wg21.link/P0829R3 <http://wg21.link/P0829R3>). I hope that we can experiment on what’s “nice” in clang / libc++ in the next few months.
One design constraint around freestanding: I want to make sure that clang can keep supporting other STL implementations.

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.

On “freestanding” macro, clang does the following today:
  if (LangOpts.Freestanding)
    Builder.defineMacro("__STDC_HOSTED__", "0");
  else
    Builder.defineMacro("__STDC_HOSTED__");
Otherwise, clang’s lib/Headers do some stuff with HOSTED as well, which might interfere with freestanding.

Good header hygiene indeed seems necessary, especially for <algorithm>. Louis mentioned that he was interested in looking into this.
Louis did a survey and found the following:

Freestanding in the current C++20 draft requires the following headers:

    <ciso646>
    <cstddef>
    <cfloat> 
    <limits> 
    <climits>
    <cstdint>
    <cstdlib>
    <new>
    <typeinfo>
    <exception>
    <initializer_list>
    <cstdarg>
    <type_traits>
    <atomic>

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:

    <ciso646>: nothing
    <cstddef>: stddef.h
    <cfloat> : float.h
    <limits> : stddef.h
    <climits>: limits.h
    <cstdint>: stdint.h
    <cstdlib>: stdlib.h
    <initializer_list>: stddef.h
    <cstdarg>: stdarg.h
    <type_traits>: stddef.h

As a result, I think the following are low-hanging fruit that do not require any runtime support AFAICT:

    <ciso646>
    <cstddef>
    <cfloat>
    <limits>
    <climits>
    <cstdint>
    <initializer_list>
    <type_traits>

Other things we might be able to throw in with minimal effort:

    <bit>
    <ratio>

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):

    <tuple>
    <pair>
    most if not all of <functional>
    most of <algorithm>
    <span>
    <array>
    <string_view>
    lock-free parts of <atomic>



> On Dec 10, 2018, at 9:23 PM, Olivier Giroux via libcxx-dev <libcxx-dev at lists.llvm.org> wrote:
> 
> Hello libc++-dev,
>  
> In the discussion of https://reviews.llvm.org/D55517 <https://reviews.llvm.org/D55517>, 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.
>  
> Not one to bury the lede, I’d like to talk about a CUDA C++ standard library.
>  
> 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, today, 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.
>  
> One exploration vehicle was shown at CppCon in September (by me, see: YouTube, and https://github.com/ogiroux/freestanding <https://github.com/ogiroux/freestanding>) and then we made but failed to present a more detailed poster at the LLVM dev meeting in October. And now we’re here. 😊
>  
> After making a few exploration vehicles (2 overall, 4 for <atomic>), we now think we’ll create version 1 this way:
> Wrap select libcxx <*> headers with <cuda/*> to introduce symbols in cuda::* instead of std::*, and…
> These facilities are always heterogeneous, NORTTI, and NOEXCEPTIONS.
> Enable users to include them on top of their host library (that being CPU only).
> “Select” here means prioritizing headers in Freestanding now, or soon, basically the header-only facilities.
> Subsequently help maintain the intersection of libcxx and Freestanding.
>  
> In terms of libcxx design, we think that we could layer on this surface:
> 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.
> For example, in <atomic>, it would be preferable for Freestanding implementations (and users) if the lock-in-atomic strategy was used for non-lock-free atomics (instead of the sharded-lock-table strategy tucked inside __cxa_atomic_*) because that then frees the program from dependencies on libatomic.
> It is my intention to contribute the code for this 3rd strategy, and other maintenance to <atomic>, some of which I’ve already made in my branch.
> An extension point that allows std::* symbols to be put into another namespace, both for ABI and to co-exist.
> This is in tension with Eric’s proposed change.
> 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).
> This was at one point in tension with changes Louis was making, but I think we’re Ok right now.
> And, generally speaking, good header inclusion hygiene that tries to minimize what’s pulled into a facility’s header.
>  
> 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!
>  
> Thanks for reading, I’ll try to answer your questions as best I can.
>  
> Sincerely,
>  
> Olivier
> 
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