[LLVMdev] RFC: Staging area proposal for new backends

[Tom Stellard]

Hi,

I would like to try to keep the staging area discussion going.  There
seems to be a general consensus that a staging area for backends and also
new features would be acceptable for the LLVM project.  What actions
are required to make the staging area a reality?  Is more discussion
needed? Is anyone willing to volunteer to be the "Code Owner" for the
staging area, to help move the process forward?

Thanks,
Tom

On Fri, Jul 20, 2012 at 11:51:03AM -0400, Tom Stellard wrote:
> Hi,
> 
> I would like to follow up on the recent discussion on the mailing list
> about requirements for new backends[1] by submitting the following
> proposal for a staging area for new LLVM backends.  This proposal
> incorporates ideas from Owen, Chandler, and others who chimed in on
> the original thread, and I hope the LLVM developers will be able to
> come to a consensus on this proposal or a modified version, so the
> project is able to accept new backends.
> 
> The goals of the staging area will be:
>   1. Facilitate communication between the LLVM project and backend
>      developers
>   2. Ensure that new backends meet LLVM standards
>   3. Give the backend more exposure to users and prospective developers
> 
> ++ Staging Area:
> 
> Similar to the Linux kernel, the staging area for new backends will
> be in the main LLVM tree, with building of the backend being disabled
> by default.  There will also be a TODO file in the backend's root
> directory, that contains a list of improvements that are required to
> promote the backend out of the staging area.  The backend will be
> assigned a steward who's role will be to guide the backend through
> the staging process and help solicit feedback from other developers.
> 
> There are several advantages to having the staging area be in the main
> tree as opposed to a separate branch:
> 
> 1. It will be easier for LLVM developers to become familiar with the
>    new backend and identify areas for improvement.
> 
>   If the new backend is in the main tree, LLVM developers are more
>   likely to encounter it in their day to day development.  Imagine a
>   scenario where a developer makes a change to LLVM core that impacts
>   several backends.  The developer may grep the code looking for
>   backends that make use of the feature that they have added or
>   changed.  If the new backend is in tree and uses that feature,
>   the developer will see the code and might take a few moments to
>   read through.  While doing this the developer may notice an area for
>   improvement for the backend and can update the backends's TODO file.
>   The end result of this is that the LLVM developer has been able to
>   provide some feedback with a minimal time commitment on their part.
> 
>   If the backend were staged in a separate tree, this kind of
>   simple review would not be possible, and I would be concerned that
>   developers would be too busy to ever get around to checking out
>   the staging tree.
> 
> 2. It will allow the backend developers to always develop against TOT.
> 
>   Developing against TOT is the recommended development procedure for
>   anyone working on LLVM, and this is regularly reiterated on the
>   mailing list.  If the new backend is included in the main tree,
>   the backend developers will have no choice but to work against TOT.
> 
> 3. It will make it easier for end users and distributions to test and
>    also make it easier for new contributors.
> 
>   New backends will be more visible to the public if they are in the
>   main tree.  This will mean more users, an expanded testing base, and
>   more potential developers which will lead to a higher quality backend.
> 
> 
> ++ Promotion/Demotion from staging area:
> 
>   After a period of time, or when the tasks in the TODO file have been
>   completed, the backend developers or the steward can initiate the
>   review process.  The review process will be conducted by either the
>   steward, a committee, or some select developers, who will decide
>   (maybe by vote in the case of a committee) whether the backend
>   should be:
> 
>   - Promoted = Build of backend will be enabled by default.
>   - Extended = Backend remains in the staging area.
>   - Demoted  = Removed from the main tree
>     (I can't really think of any disadvantages to having a backend be
>     in the main tree as long as its not being built by default, so maybe
>     demotion would be reserved for cases of long term absence
>     of maintainership)
> 
>   The Promoted/Extended/Demoted decision will be made using the
>   following criteria (These won't necessarily all be absolutely
>   required, they merely serve as a way for a backend's progress to
>   be measured) :
> 
>   - Progress towards completion of TODO tasks
>   - Active maintainership
>   - Use of incremental development techniques
>   - Adherence to LLVM coding style
>   - Usage of modern LLVM features
>   - Quality and quantity of regression tests
>   - Availability of buildbots
>   - Size of user base
>   - Other criteria deemed important by LLVM developers
> 
>   - Contributions to core LLVM
>     In the previous mailing list discussions there were differing
>     opinions of how important contributing to the core LLVM code is
>     for having a backend accepted.  It seems like a good middle ground
>     would be that backends should be free of code that works around
>     bugs or deficiency in core LLVM and instead fix the problem in
>     shared code, and also should make an effort to push optimization
>     passes that may be useful to other targets into the shared parts
>     of the code.
> 
> 
> ++ What is needed from the LLVM developers:
> 
>   In order to make this staging program successful, the LLVM project
>   will need to appoint a "code owner" for the staging process, who
>   backend developers can contact when they are interested in getting
>   the backend included in the main tree.  An LLVM developer will also
>   be needed to act as a steward for the new backend and help guide
>   the backend developers through the process.
> 
> Looking forward to comments on this proposal.
> 
> Thanks,
> Tom Stellard
> 
> [1] http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20120716/146560.html
> 
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