[llvm-dev] Flang landing in the monorepo

Eric Christopher via llvm-dev llvm-dev at lists.llvm.org
Wed Dec 18 13:49:40 PST 2019

Hi Peter,

Yes, I looked through those sources and a number of my questions around
which clang versions have been supported and directory structure. I think
the only difference is removing the direct questions about earlier flang,
but I still don't see code generation or uses of llvm libraries that would
conform to "written in the style of LLVM and clang". Can you perhaps point
me to where I'm missing these things?

Chris's earlier acceptance aside I don't see any evidence of code review as
part of that and so I'd expect we'd see more here.



On Wed, Dec 18, 2019 at 1:44 AM Peter Waller <Peter.Waller at arm.com> wrote:

> 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.
> Regards,
> - 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.
> 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
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
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