[llvm-dev] Flang landing in the monorepo

[James Y Knight via llvm-dev]

I think it would probably make the most sense to land this as a single
merge-commit (from the 2700-commit rewritten history you've created) onto
llvm-project master, rather than as 2700 individual toplevel commits to
master. (Which means: disable the merge-commit prohibition in the github
configuration, temporarily, push this commit, and then enable it again).


On Tue, Dec 17, 2019 at 5:10 PM Tom Stellard via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On 12/17/2019 01:30 PM, Peter Waller via llvm-dev wrote:
> > Hi All,
> >
> > The flang project (a Fortran compiler) is getting ready to join the
> > monorepo. We intend to preserve the existing history by rewriting the
> > existing commits as a linear series of commits on top of llvm-project.
> >
> > I understand the flang community would like to do this before the LLVM
> > 10 branch in due in mid January, so please speak up soon if you see
> > anything needing fixing in what I write below.
> >
> > I've taken into account the discussion raised during the MLIR landing
> > discussion found at
> > http://lists.llvm.org/pipermail/llvm-dev/2019-November/136813.html. As
> > with MLIR, we rewrite the commits so that flang's work all appears to
> > happen in the flang directory, starting with llvm-project master as it
> > appears today. The topology of the f18 history was fairly interesting,
> > which is why I ended up writing a new program to rewrite it rather than
> > using an existing one.
> >
> > === Key links
> >
> > * Resulting tree of the rewrite:
> >
> https://github.com/peterwaller-arm/f18/tree/rewritten-history-v2-llvm-project-merge
> >
> > * Rewritten history, with flang commits applied on top of llvm-project
> > master:
> >
> https://github.com/peterwaller-arm/f18/commits/rewritten-history-v2-llvm-project-merge
> >
> > * The history rewriting program is published here:
> >    https://github.com/flang-compiler/f18/pull/854
> >
> > * Latest mailing list discussion of rewrite on flang-dev:
> > http://lists.llvm.org/pipermail/flang-dev/2019-December/000122.html
> >
> > === Additional considerations
> >
> > * Existing references to pull request and issue numbers are rewritten so
> > that they point at the originals as, e.g. flang-compiler/f18#123. This
> > prevents those patches from generating bogus references to Issues/PRs of
> > llvm-project if/when those appear in the llvm-project repository.
> >
> > * Developers using the llvm-project repo, when they pull after this
> > push, will see 2,700ish commits appear on the tip. These will follow on
> > as normal commits from wherever master is at the time of the push. The
> > fetch takes 40s and I see my ".git" directory grow by approximately
> > 90MiB when I simulate this.
> >
>
> I think we may want to disable the commit emailer before all these
> new commits are pushed.  Once you are ready to push, can you pick a
> specific time and date for the push and then coordinate with Mike (cc'd)
> and myself, so we can avoid spamming the mail server.
>
> Thanks,
> Tom
>
> > * Rewriting and validating the rewritten f18 history is sufficiently
> > fast that I don't think it will be necessary to pause commits to LLVM.
> > The script runs in a few seconds. Before this is done though, I think
> > new commits should no longer be accepted on the original repository.
> >
> > * You can simulate the experience of the fresh merge with `git remote
> > add peterwaller-arm https://github.com/peterwaller-arm/f18 && time git
> > fetch peterwaller-arm rewritten-history-v2-llvm-project-merge`, and then
> > look at the peterwaller-arm/rewritten-history-v2-llvm-project-merge
> > branch with git log.
> >
> > * Remember that you can restrict the "git log" output to what you are
> > interested in by specifying a directory, e.g. `git log clang/`.
> >
> > That's all for now. Season's greetings!
> >
> > - Peter
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
>
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