[PATCH] D98063: [flang][fir] Add the pre-code gen rewrite pass and codegen ops.

Sat Mar 20 09:30:17 PDT 2021

kiranchandramohan added a comment.

The OpenMP for Flang team has consistently been asking for upstreaming the fir-dev branch. I made an initial attempt to upstream a portion in May last year (https://reviews.llvm.org/D79731) which was discarded since it
did not have any community support. The current situation presents a few difficulties for the OpenMP team.

1. The OpenMP dialect is in the llvm-project repo, while FIR codegen is developed in the fir-dev repo. So we have to first make a patch to llvm-project/mlir and then wait for it to be merged into the fir-dev repo (which can take from one to two weeks) and then make the relevant changes in the fir-dev repo. If FIR codegen was also upstream then this delay and committing to multiple repositories can be avoided.
2. Since the bridge code (parse-tree to FIR) and codegen is not available in llvm-project/flang, any commits that we make to fir-dev cannot be upstreamed. So all our changes are also increasing the diff between fir-dev and upstream llvm-project/flang. Left uncontrolled this might become an untameable monster and we might never be able to fully upstream fir-dev.
3. Since the OpenMP code which works with FIR is in fir-dev we cannot often show the context to MLIR core team. On at least one occasion this has become an issue while seeking help. If the code is upstream this will facilitate better discussions with the MLIR core team.

Given the issues mentioned above, we favour a faster upstreaming process so that the entire community can work on a single code base.

The MLIR core team has been very helpful with the OpenMP dialect work and we have benefitted from their advise and review comments. I also fondly recall that Mehdi has stepped in to support Flang/F18 <https://reviews.llvm.org/F18> (https://lists.llvm.org/pipermail/llvm-dev/2020-January/138219.html) when there was an opinion to consider other candidates. Mehdi has been very gracious with his time and has provided several reviews for FIR upstreaming. It will be unwise to not consider his review comments. We should try to address the DCE and registration comments. If addressing a comment requires substantial work then may be we can considered for later with a suitable mechanism to track this publicly.

I hope we can soon find a way out of this impasse and make progress.

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D98063/new/

https://reviews.llvm.org/D98063