[flang-dev] RFC:

[Mehdi AMINI via flang-dev]

Hi,

I have strong concerns about having a "code drop" for the FIR components
and anything that has a dependency on MLIR.
I'd like that any such component is upstreamed individually (each passes in
a single review for example) and reviewed on Phabricator appropriately.

Thanks,

-- 
Mehdi


On Thu, Dec 10, 2020 at 3:38 PM McCormick, Pat via flang-dev <
flang-dev at lists.llvm.org> wrote:

> This RFC aims to address challenges and concerns (e.g., disjoint
> development activities) in providing a functional code base for Flang via
> the main branch of the llvm-project.
>
> *Background*
>
> When Flang upstreamed to llvm-project, only the frontend (preprocessor,
> parser, and semantic analysis) went into the main repo. This retained the
> commit history for the project and post hoc reviews of the upstreamed code
> were expected.  The work on this front-end has continued in llvm-project
> using the smaller, incremental commits that is normal LLVM practice. The
> middle-end  (lowering from parse trees to the MLIR dialect for the Fortran
> intermediate representation, FIR, that was not initially upstreamed) has
> been actively developed in a "fir-dev" fork:
>
>   https://github.com/flang-compiler/f18-llvm-project/tree/fir-dev
>
> This codebase is heavily dependent upon MLIR for both Fortran-IR (FIR) and
> plans for an OpenMP dialect, and obviously on LLVM for the final lowering.
> Active work across all these components, and efforts to get a working
> Fortran 77 front-end implemented on a timely schedule, has expanded the
> "fir-dev”  capabilities and has significantly extended the codebase as it
> currently exists in the main branch.
>
> To date, the community has struggled to reach an agreement on how to best
> merge the critical functionality in "fir-dev" into the main branch.
> Unfortunately, the end targets of smaller commits and timely delivery of a
> working Fortran front end in the main llvm-project branch have been at
> odds. Furthermore, as time has passed this bifurcation has become
> detrimental to both Flang and established community practices. Therefore,
> we would like to propose the following approach to address and reduce
> the risks associated with the growing divergence across the code base.
>
>
> *Proposed Merge Strategy  *
> The proposed strategy aims to provide a timely push of "fir-dev"
> capabilities into Flang's main branch. Although not ideal, we feel it
> addresses both schedule risks and the overarching goal is to bring the
> community back together in working on a unified codebase. The proposal is
> to upstream groups of functionality with a revised history. One can imagine
> staging a sequence of multiple commits in git as a chain (say a handful of
> commits at most). This entire chain can be pushed in one go, creating a
> "history" or by splitting up the group into a few individual commits.
>
> One potential grouping of functionality could be:
>
>    1. FIR dialect and code generation,
>    2. Optimization passes, and
>    3. Lowering to FIR.
>
>
> We welcome other suggestions for these groupings.
>
> Unlike previous proposals, we would like to encourage timeliness by
> avoiding any initial source code modifications beyond organizing the
> merging of code into these functional groups.  We think there are two
> potential approaches to upstreaming this code in the master branch.  The
> first would handle each group independently as a single or few smaller
> commits.  It may be difficult to completely test each of these individual
> commits until the overall upstream process is completed, however we will,
> at the very least, make sure the code still builds appropriately.
>
> Alternatively, we could eliminate the three functional groups and roll up
> all the "fir-dev" changes into a single commit.  Within the broader LLVM
> effort, there have been cases where very large, cross-cutting changes have
>  been committed in one shot to avoid breaking functionality.  The middle
> stages of Flang's functionality would meet a similar goal.
>
> The primary advantage of the 2nd (“everything at once”) approach is that
> it would provide a more timely working (F77)  implementation of Flang and
> avoid a continued risk of a much longer duration and continued bifurcation
> of efforts across repositories.
>
> We encourage the community's thoughts on these two paths, any alternative
> approaches, etc.
>
> A series of important steps need to happen as part of the incorporation of
> fir-dev into the master repo:
>
>
>    1. We need to make certain that all build-bot functionality is sound
>    and doesn’t negatively impact community-wide, LLVM regression checks.  Our
>    proposed approach would be to complete all this testing (by hand-running
>    buildbots) before finally landing the fir-dev merged code upstream.
>    2. Make certain those who have contributed to fir-dev have their
>    contributions maintained in the git history. This potentially will require
>    that we temporarily disable LLVM’s commit mailer to avoid an onslaught of
>    email traffic across the community. We need to understand how best to
>    achieve this.  One alternative would be to capture and start a contributors
>    list as part of the project if there is no clean way to retain history.
>    3. It would be valuable to identify a small number of people from
>    across the community (2-4 seems reasonable) to help coordinate and oversee
>    this process -- it is likely too much for a single person.
>    4. As a final step of the proposed process, the "fir-dev" repository
>    will be archived and all future development activities will use the
>    llvm-project main repository.
>
>
> We believe with this process, and the fir-dev code successfully merged in,
> the first functional Fortran 77 front end, and middle stage(s), can be
> completed and provide the community with a starting point for refactoring,
> adding new capabilities, and exploring additional opportunities for
> contributing to Flang and leveraging the broader LLVM code base.
>
> We look forward to your feedback, suggestions, additions, etc.
>
> Thanks,
>
> —Pat McCormick, Los Alamos National Laboratory
> —Steve Scalpone, NVIDIA
>
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