[flang-dev] RFC: LLVM 11 branch

[David Truby via flang-dev]

Hi all,

The LLVM 11 branch is happening soon so we should have a discussion
about what we want Flang to look like in that branch.


tldr; I see a couple of issues currently that we should discuss for the
release:

* Flang defaults to calling a proprietary compiler (pgfortran)
* Flang enables Werror by default
* Shared library builds don't work

Does anyone else see any other things we should consider that I haven't
listed here?

To elaborate on my issues list:

I think it's reasonable for us to leave flang in the release, rather
than for example removing it, and add info in the Release Notes about
the state of Flang (e.g. that it fully parses and semantically analyses
Fortran 2018 code but then calls out to another compiler to build it).

The biggest issue I see with this is that currently flang defaults to
calling out to a proprietary compiler that many people in an open
source community like LLVM are unlikely to have installed. I would
propose that we either add a CMake flag to select the default compiler
to call out to, or that we rely on the existing mechanism of defining
FC, but that either way we should default to gfortran if no other
compiler is specified. That way we would be calling out to a compiler
that almost anyone interested in Fortran is likely to have installed,
and if not it is at least easily available.

The other major issue I forsee is that we currently enable Werror by
default in flang. This is likely to break the release for people
building it with as-yet-unreleased compilers, with downstream
proprietary compilers we haven't tested with, or other unforseen
circumstances. I think there are too many potential issues for Werror
by default to go into a release version of LLVM.

Shared library builds also don't currently work due to a circular
dependency between two libraries. I think it's farily uncontroversial
to say that this should be fixed, and I am happy to start working on it
personally.


Does anyone have any thoughts on this? 

Thanks
David Truby