[llvm-dev] Flang landing in the monorepo

Peter Waller via llvm-dev llvm-dev at lists.llvm.org
Wed Dec 18 01:44:20 PST 2019

Hi Eric,

Apologies, I failed to disambiguate clearly, because there are multiple projects named flang. I was referring to the "new" flang, whose repository is currently found at https://github.com/flang-compiler/f18. It will land in the monorepo under a directory called "/flang/".

f18 has been approved to join, for reference see "[llvm-dev] f18 is accepted as part of LLVM project!", Chriss Lattner, April 10 2019: http://lists.llvm.org/pipermail/llvm-dev/2019-April/131703.html.

I would like to emphasize that it will not be turned on by default in the llvm build.


- Peter

On 18/12/2019 01:19, Eric Christopher wrote:
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.



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

* Rewritten history, with flang commits applied on top of llvm-project

* The history rewriting program is published here:

* Latest mailing list discussion of rewrite on flang-dev:

=== 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

LLVM Developers mailing list
llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20191218/e4bf6470/attachment.html>

More information about the llvm-dev mailing list