[LLVMdev] [RFC] Developer Policy for LLVM C API

Hayden Livingston halivingston at gmail.com
Sun Jul 19 20:37:11 PDT 2015


While I understand the people committing to the primarily C++ codebase
of LLVM find it as additional burden, far more number of people enjoy
the benefits of an official LLVM C API support than are vocal here.

While Clang maybe the first-class LLVM citizen for the foreseeable
future, I can tell you LLVM is used in many more situations (I'm
talking about Rust, Go, Julia, DLang, etc.) than this alias realized.
I alone am using it interesting projects I hope to release sometime
this year and all of these projects rely on the comfort of the
"official" C API.

I sympathize with the community that is not using them but having to
still maintain and fix bugs. To those of you (on this thread and
others) who are wanting to remove the C API from the project, please
tell the community what can be done to improve the situation. Do we
need bots? People writing tests?

I would like to put my support behind discouraging the idea of pushing
the bindings to a side project.

On Sun, Jul 19, 2015 at 7:24 PM, Eric Christopher <echristo at gmail.com> wrote:
> Hi Chris,
>
> On Sun, Jul 19, 2015 at 9:16 AM Chris Lattner <clattner at apple.com> wrote:
>>
>> On Jul 18, 2015, at 11:27 AM, Hal Finkel <hfinkel at anl.gov> wrote:
>> >> I am strongly in favor of moving the bindings, C or otherwise, to
>> >> another project.
>> >
>> > I agree. From my viewpoint we have two primary problems with the C API:
>> >
>> > 1. Many of the LLVM contributors don't use it, and thus, don't have a
>> > great understanding of how it can be most-usefully updated/improved, and
>> > what functionality needs to be exposed. We have most, but not all,
>> > transformations; many, but not all, IR features, etc.
>> >
>> > 2. We don't have a good set of tests for it, nor do we have a good set
>> > of tutorials/documentation for it. Our tutorials, specifically, are in C++,
>> > not in C. We could break the C API and we'd likely remain unaware for quite
>> > awhile.
>> >
>> > Putting it in a separate project will force those who have a stake in
>> > its existence to take the responsibility of moving it forward. Separate
>> > project, or not, however, we should have a better developer policy regarding
>> > the C API (and the other bindings) that fall under the LLVM umbrella.
>> > Specifically, we should outline what features need to be exposed, and we
>> > should actively maintain (and test for) full coverage of those features.
>>
>> I don’t understand the motivation for this.  Is there tension between
>> clients of the C API that would warrant having different
>> approaches/implementations?  Moving it out to a subproject to get better
>> testing seems a bit silly.
>>
>> Personally, unless there is a strong and compelling reason to do so, I’d
>> prefer to keep a single set of C bindings to encourage standardization and
>> avoid fragmenting the community.
>>
>
>  So, I made this proposal for what I think is a pretty good reason. There's
> an "unofficial" as Juergen said, policy that the C API is the stable API.
> There's nothing wrong with a stable C API, but that's what I'm proposing
> should move out of tree to where those that are most concerned with it can
> develop it and ensure that it remains stable for whatever guarantees they
> want.
>
> Some background here:
>
> Right now we definitely have this dichotomy between a "bindings" C API and a
> "stable" C API. The unofficial policy as I mentioned above is that there's
> one C API and that's the stable API. Over the last 3-5 years or so the
> "stable" C API has started growing in ways that encompass just about every
> class and API in llvm. We've occasionally denied bindings level API support
> because we knew the code in that area was going to change - but just imagine
> we'd let a few of them in, we wouldn't have been able to do the IR/Metadata
> split at all. As it is we technically broke the C API, just not in a way
> that any external user cared about.
>
> Back to the proposal:
>
> What I'm proposing is that we make the C API that exists in tree a bindings
> API that has the same stability guarantees as the C++ API. Honestly it'll
> probably much more stable, but at least then we won't have to worry or
> revert work because the C API was "too close to the machine" or rather the
> C++ code.  This means that someone that wants a stable C API can go off and
> develop one (tests and all) and we can possibly look at bringing it back
> into tree at some point in the future. For example, if someone comes up with
> a good "libjit" api then we can look at how the API design works and make
> sure it's general enough that it's not going to cause undue solidification
> of the existing APIs.
>
> Caveat: I'm not talking about the existing libclang or liblto libraries.
> Those seem to work and have a small enough API surface that they seem
> reasonable to support and we can move to a new API if they seem to be
> hindering development in the future.
>
> This help explain where I'm coming from here?
>
> Thanks!
>
> -eric
>
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