[libcxx-dev] An extension of libcxx
Olivier Giroux via libcxx-dev
libcxx-dev at lists.llvm.org
Tue Dec 11 10:01:10 PST 2018
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-lock-free cases, achieving the desired goal of being dependency-free without having to upset SG1.
We’re good with the rest. What you wrote closely resembles what we proposed to our people.
From: <jfbastien at apple.com> on behalf of JF Bastien <jfbastien at apple.com>
Date: Tuesday, December 11, 2018 at 9:40 AM
To: Olivier Giroux <OGiroux at nvidia.com>, Louis Dionne <ldionne at apple.com>
Cc: "libcxx-dev at lists.llvm.org" <libcxx-dev at lists.llvm.org>
Subject: Re: [libcxx-dev] An extension of libcxx
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:
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:
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:
<cfloat> : float.h
<limits> : stddef.h
As a result, I think the following are low-hanging fruit that do not require any runtime support AFAICT:
Other things we might be able to throw in with minimal effort:
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):
most if not all of <functional>
most of <algorithm>
lock-free parts of <atomic>
On Dec 10, 2018, at 9:23 PM, Olivier Giroux via libcxx-dev <libcxx-dev at lists.llvm.org<mailto:libcxx-dev at lists.llvm.org>> wrote:
In the discussion of 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) 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:
1. 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).
1. “Select” here means prioritizing headers in Freestanding now, or soon, basically the header-only facilities.
2. 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.
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