[llvm-dev] RFC: Adding a staging branch (temporarily) to facilitate upstreaming

[Nikita Popov via llvm-dev]

On Tue, Jun 30, 2020 at 7:15 AM Chandler Carruth via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On Mon, Jun 29, 2020 at 9:43 PM Mehdi AMINI via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hey Duncan,
>>
>> On Mon, Jun 29, 2020 at 8:28 PM Duncan Exon Smith via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> To facilitate collaboration on an upstreaming effort (see "More context"
>>> below), we'd like to *push a branch* (with history) called "
>>> *staging/apple*" to github.com/llvm/llvm-project to serve as an
>>> official contribution to the LLVM project. This enables motivated parties
>>> to work with us to craft incremental patches for review on Phabricator.
>>> This branch would live during the effort and then be deleted after. It
>>> would not be merged.
>>>
>>> *Does this seem fine?* If you have a strong objection, please share
>>> your concern.
>>>
>>> For reference, I ran some experiments:
>>>
>>>    - A `--bare` clone (just the Git database) I have of
>>>    github.com/llvm/llvm-project was around ~1GB. Fetching this branch
>>>    from github.com/apple/llvm-project increased it to ~1.2GB. Running
>>>    `git gc --aggressive` brought it down to ~850MB.
>>>    - The worktree of the "master" branch is ~1GB. Adding the Git
>>>    database gives ~2GB, ~2.2GB, and ~1.9GB.
>>>    - The diff of the proposed staging/apple branch is 3.1MB at `-U0`,
>>>    4.1MB at `-U3`, and 32MB at `-U999999` (Phabricator settings).
>>>
>>>
>>>
>>> *More context*
>>>
>>> We're making a major push over the next few months to upstream
>>> changes that have accumulated over time in the branch called "apple/master"
>>> at github.com/apple/llvm-project. It has always been a non-goal for us
>>> to have changes, but over the years we've accumulated a non-trivial diff
>>> vs. github.com/llvm/llvm-project. This includes (but is not limited
>>> to) tweaks/features related to tuple hashing, modules hashing, source
>>> attributes, API notes, pointer authentication, indexing-while-building, and
>>> local refactoring.
>>>
>>> Our goal is to eliminate this difference. Besides paying off some debt,
>>> this upstreaming effort unblocks the Swift compiler (
>>> github.com/apple/swift) from building directly against an upstream
>>> checkout of LLVM. That's why non-Apple contributors are motivated to help
>>> craft incremental patches.
>>>
>>>
>>> *Alternatives considered*
>>>
>>> As an alternative, we could post a GitHub pull request and close it
>>> without merging. From our perspective this would serve the same purpose.
>>> However, pull requests are contentious in LLVM.
>>>
>>> Another alternative is to post a bulk Phabricator review and then
>>> "abandon" it. However, this has the disadvantage of not contributing the
>>> history (~30k commits).
>>>
>>
>> Seeing the alternatives, if a closed pull-request would fit your
>> use-case, then I'm not sure why you need the branch to actually live in the
>> monorepo instead of in any fork (github.com/apple/llvm-project or
>> another)? It seems publicly accessible the same way?
>>
>
> As I understand it, a key need is to explicitly contribute this to the
> LLVM project to make it unambiguous that it has been contributed and is
> completely available for folks not at Apple to iterate on the code and turn
> it into code-reviewable chunks.
>
> So whatever happens needs to be quite explicit in its nature as a
> contribution. IMO, a branch of the repository definitely qualifies.
>
> IMO, a pull request isn't as clear given that they haven't been used for
> contributions before. This is not a time to be innovative IMO. A branch as
> a staging location has been used many times over the history of the project
> though and seems nicely unambiguous in that regard.
>

If this is just a legal matter, can't it be solved as such? I would expect
just releasing a statement that you contribute the diff between
apple/llvm-project at XXX and llvm/llvm-project at YYY under the LLVM license
should be sufficient.

I'm really not seeing a technical reason to include this branch in the
monorepo, and make it larger for everyone. This seems like exactly the case
where the GitHub fork model works well.

Regard,
Nikita
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