[llvm-dev] A libc in LLVM

JF Bastien via llvm-dev
Wed, 26 Jun 2019 17:26:43 -0000

> 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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