[llvm-dev] Flang landing in the monorepo

[Eric Christopher via llvm-dev]

Hi Peter,

I have a few concerns and questions so far:

 - Supported C++ compilers: It looks like nothing too recent has been used
to test? And the flang page still has version 7 listed as the "latest llvm".
 - "It will be closely aligned with LLVM best practices and written in the
style of LLVM and clang":
    - The directory structure is quite different from clang's directory
structure.
    - IR generation still appears to be text string based?
    - I didn't see a single reference to ADT or any of llvm's libraries on
a cursory look through the f18 directory (grep -ri ADT and grep -ri llvm)
    - It looks like flang is a C based project and f18 is C++? Looking at
the flang directory itself for IR generation is quite confusing and while
named C++ in some places is actually just C with few abstractions?

I haven't done a full reading of the code, this is just a starting point
for discussion and review.

Mostly it seems I have a lot of questions and so getting an updated idea of
what you expect to merge, what the sources are, and more would be a first
start I think. If some of my concerns hold through discussion I can't see
this being merged as-is.

Thanks!

-eric

On Tue, Dec 17, 2019 at 1:31 PM Peter Waller via llvm-dev <
llvm-dev at lists.llvm.org> 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.
>
> * 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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