[llvm-dev] A libc in LLVM

JF Bastien via llvm-dev llvm-dev at lists.llvm.org
Wed Jun 26 10:26:43 PDT 2019

> On Jun 24, 2019, at 3:23 PM, Siva Chandra via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> Hello LLVM Developers,
> Within Google, we have a growing range of needs that existing libc implementations don't quite address. This is pushing us to start working on a new libc implementation.
> Informal conversations with others within the LLVM community has told us that a libc in LLVM is actually a broader need, and we are increasingly consolidating our toolchains around LLVM. Hence, we wanted to see if the LLVM project would be interested in us developing this upstream as part of the project. 
> To be very clear: we don't expect our needs to exactly match everyone else's -- part of our impetus is to simplify things wherever we can, and that may not quite match what others want in a libc. That said, we do believe that the effort will still be directly beneficial and usable for the broader LLVM community, and may serve as a starting point for others in the community to flesh out an increasingly complete set of libc functionality.
> We are still in the early stages, but we do have some high-level goals and guiding principles of the initial scope we are interested in pursuing:
> The project should mesh with the "as a library" philosophy of the LLVM project: even though "the C Standard Library" is nominally "a library," most implementations are, in practice, quite monolithic.
> The libc should support static non-PIE and static-PIE linking. This means, providing the CRT (the C runtime) and a PIE loader for static non-PIE and static-PIE linked executables.
> If there is a specification, we should follow it. The scope that we need includes most of the C Standard Library; POSIX additions; and some necessary, system-specific extensions. This does not mean we should (or can) follow the entire specification -- there will be some parts which simply aren't worth implementing, and some parts which cannot be safely used in modern coding practice.

I’d love to hear what you have in mind with point 3 above, and see it expanded. libc++ implements C++11 and subsequent standards, and that makes me wonder:

Which standards would this libc implement?
Would you implement upcoming C standards, and how would you manage “experimental” features (API changes, ABI changes, etc)?
What parts of the standard wouldn’t you follow, why, how would the LLVM community determine this?
Which parts aren’t worth implementing?
Which parts cannot be safely used in modern coding practice? How would you remedy what’s perceived as “the bad parts”?
I’d love it if the C Standards Committee, WG14, got renewed involvement through this project. Is that an explicit goal? Who will join WG14 in this effort?
What part of C do you see this project help improve over time?

> Vendor extensions must be considered very carefully, and only admitted when necessary. Similar to Clang and libc++, it does seem inevitable that we will need to provide some level of compatibility with other vendors' extensions.
> The project should be an exemplar of developing with LLVM tooling. Two examples are fuzz testing from the start, and sanitizer-supported testing.

How do you intend to test this C library? Fuzzing and all that is nice, but just straight conformance testing is what I’d like to hear about.

> There are also few areas which we do not intend to invest in at this point:
> Implement dynamic loading and linking support.
> Support for more architectures (we'll start with just x86-64 for simplicity).
> For these areas, the community is of course free to contribute. Our hope is that, preserving the "as a library" design philosophy will make such extensions easy, and allow retaining the simplicity when these features aren't needed.
> We intend to build the new libc in a gradual manner. To begin with,  the new libc will be a layer sitting between the application and the system libc. Eventually, when the implementation is sufficiently complete, it will be able to replace the system libc at least for some use cases and contexts.
> So, what do you think about incorporating this new libc under the LLVM project?
> Thank you,
> Siva Chandra and the rest of the Google LLVM contributors

Somewhat off-topic… this last line is unfortunate. It would be great to not sign as "the rest of the Google LLVM contributors” when subsequent responses show that many Google LLVM contributors aren’t co-signing this proposal (even if they’re interested!). Scope and purpose within your organization would have been more helpful, here it sounds like all of Google is in agreement… which never happen 🙂

 - JF (and not the rest of the Apple LLVM contributors 😉)

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