[flang-dev] RFC: new Flang driver - next steps

[Andrzej Warzynski via flang-dev]

Hello all,

We have been busy prototyping the new Flang driver [1] and investigating 
how to best re-use the goodness already available in Clang [2]. Earlier 
today we've sent a refined design of the Flang driver to cfe-dev [3]. 
That RFC focuses on what changes are required in Clang in order for us 
to be able to share that infrastructure without Flang depending on 
Clang. In this email we would like to discuss the next steps for the new 
Flang driver from the perspective of Flang.

If the Clang community finds our design feasible and is happy with the 
refactoring that we proposed, we will start working on that immediately. 
We are also keen to get early feedback from the Flang community and 
would like to start upstreaming our prototype driver [1] into Flang at 
the same time. This would inform the refactoring of Clang and guarantee 
that the integration goes smoothly. We also feel that this would allow 
us to parallelize the implementation a bit better.

The main drawback of this approach is that we need to introduce a 
dependency on Clang in the interim. We propose the following _temporary_ 
workaround for this:
* the new Flang driver will be guarded with a CMake flag (i.e. it will 
only be build conditionally)
* the same CMake flag will guard any Clang dependencies (i.e. by default 
Flang will not depend on Clang)
* we will set-up a dedicated build-bot for the new Flang driver to avoid 
and to catch early any breakage

Once the refactoring in Clang is complete, this workaround can be 
removed. The former boils down to making libclangDriver independent of 
Clang and moving it out of Clang (please see [3] for more details) and 
looks much less involved than we originally thought.

Your feedback is much appreciated.

Last, but not least, we'll need some help reviewing the patches as they 
come back to master. If anyone is interested in helping here please 
shout out!

Andrzej Warzynski
On behalf of the Arm Fortran Team

[1] https://github.com/banach-space/llvm-project
[2] http://lists.llvm.org/pipermail/llvm-dev/2020-June/141994.html
[3] http://lists.llvm.org/pipermail/cfe-dev/2020-July/066393.html