[llvm-dev] A libc in LLVM

Tue Jun 25 17:18:02 PDT 2019

On 6/25/19 6:29 PM, Siva Chandra wrote:

On Mon, Jun 24, 2019 at 3:45 PM Finkel, Hal J. <hfinkel at anl.gov<mailto:hfinkel at anl.gov>> wrote:
On 6/24/19 5:23 PM, Siva Chandra via llvm-dev 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,

+1 - This has also been my experience: Many people over many years have expressed a desire to have a libc has part of the LLVM project. It is currently a large gap in our LLVM toolchain offering. Moreover, from the standpoint of my organization, an LLVM libc could provide benefits on both production platforms and research/experimental hardware.

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:

  1.  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.

  2.  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.

  3.  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.

  4.  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.

  5.  The project should be an exemplar of developing with LLVM tooling. Two examples are fuzz testing from the start, and sanitizer-supported testing.

Great.

There are also few areas which we do not intend to invest in at this point:

  1.  Implement dynamic loading and linking support.

It will be useful to have a design document that describes the kind of system and capabilities that you're targeting, and then we can discuss how the libc might have a modular design that can be adapted for other use cases. I mention modularity because, for example, we have accelerator hardware and various kind of low-variability/embedded environments where many, but not all, POSIX/libc capabilities make sense.

I am of the opinion that modularity should be as fine-grained as possible. For example, one should be able to pick and package individual functions into a libc as suitable for their platform.
That said, I am open to other ideas you might have about modularity. I am also open to getting convinced that function level granularity is an overkill.

This sounds like a good starting position to me. We can adjust over time.

Thanks again,

Hal

  1.  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?

This is something that I'd like to see.

 -Hal

Thank you,

Siva Chandra and the rest of the Google LLVM contributors

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--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory
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