[llvm-dev] Flang landing in the monorepo

[Tom Stellard via llvm-dev]

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
> 
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