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

[Juergen Ributzka]

I also misunderstood your original transition proposal in this point. I agree with Jim that we should keep the current C-API where it is and have a separate location for the bindings. I envision that we will need the current C-API and the new stable C-API to overlap for at least one release cycle to allow a smooth transition without breaking all the clients out there. Some clients only read the release notes, so this process will take some time.

I added an initial patch for discussion. There is one point I am not sure about yet. You mentioned that existing API implementations might become NOOPS. I agree that this is a viable and sometimes necessary path, but what are the limitations for this? Which APIs can be safely converted to NOOPs?

—Juergen

 

> On Jul 20, 2015, at 3:26 PM, Eric Christopher <echristo at gmail.com> wrote:
> 
> On Mon, Jul 20, 2015 at 3:20 PM Jim Grosbach <grosbach at apple.com <mailto:grosbach at apple.com>> wrote:
>> On Jul 20, 2015, at 2:24 PM, Jim Grosbach <grosbach at apple.com <mailto:grosbach at apple.com>> wrote:
>> 
>> 
>>> On Jul 20, 2015, at 1:45 PM, Eric Christopher <echristo at gmail.com <mailto:echristo at gmail.com>> wrote:
>>> 
>>> 
>>> 
>>> On Mon, Jul 20, 2015 at 1:37 PM Juergen Ributzka <juergen at apple.com <mailto:juergen at apple.com>> wrote:
>>> Wow, this went of topic very quickly ;-)
>>> 
>>> 
>>> It did. I am sorry about that :)
>>>  
>>> As you know I am very interested in an stable API (C and/or C++) for LLVM, but maybe we should discuss this in a separate thread. Designing a good stable API from scratch will take some time and until that point I want to document our current “tribal knowledge”. I will post a patch as you suggested.
>>> 
>>> 
>>> Thanks. Given the direction it's taken and the positive feedback I've gotten on the existing C API we might want to document it as "in flux" :)
>>> 
>>> That said, here's how I see this happening if we were to go the direction I'm proposing:
>>> 
>>> I think we should bite the bullet and say that the growth we've had in the C API is past what we were originally promising and just call the existing C API the "bindings" api. From there we can move the LTO headers to a new directory (insert bikeshed) and say that the existing code in llvm-c isn't stable.  We can then migrate the things we want to be stable into a new directory with real stability guarantees. I'm seeing an API (possibly versioned) with a much lower surface area here similar, possibly, to the LTO stuff or libclang.
>> 
>> I’d prefer to do it the other way around. Make the new directory be for the bindings stuff. That way anything that works with what’s there as-is continues to do so. Any clients that need to move to the bindings API as we then work through deprecating things and otherwise cleaning up the stable API will be the ones that by definition want to opt-in to a less stable API and are thus more amenable to change. Changes for clients that are using the current API as a stable API will continue to work and will get the advance notice and transition planning from the deprecation process. If we make the stable API be in a new directory, that’s a build-time breaker right there for every client using the stable API. Let’s avoid that.
> 
> To clarify (I hope), what I care about here is that there is a clean migration path from the current structure to whatever new structure (and accordant API) we have. IMO, that’s easiest if we leave the current headers where they are and deprecate like crazy, but there are other ways (leaving behind copies w/ the entire file marked deprecated, e.g.). So long as there’s a migration path that doesn’t involve a hard break where clients are just screwed from one revision to the next, we should be fine.
> 
> 
> Totally in agreement here. I'm actually trying to minimize disruption for the most people. My original thought was more of a "tell people what's going on and do a slow degradation". I.e. leave the current C API where it is, keep expanding it for the bindings folks and let people willing to take a chance on the code not breaking just continue on while the stable API folk add one and migrate users to it (the process of which I'm sure will help define what a decent API should be :)
> 
> I'm not too invested in how we get from a to b, it was mostly a "let's try to minimize total code changes for everyone".
>  
> -eric

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