[Openmp-dev] [RFC] Requiring C++14 for the OpenMP subproject

[Johannes Doerfert via Openmp-dev]

More than a year ago [0,1] LLVM moved to C++14 as the minimal required
C++ versions.

Today we like to finally follow with the OpenMP subproject [2].

This RFC allows provides visibility for the proposed change and allows
people to voice their opinion. Please reply if you are in favor or not
with as much reasoning as possible (or needed).

I'll mention a few reasons why the change is justified and good for us
below, the LLVM discussion has probably a similar section:

  - The OpenMP runtime is tied to the LLVM project, it should require a
    very good reason not to follow the coding rules, standards, best
    practices, minimal versions, ... of the LLVM project.
  - It is 2020, LLVM requires C++14 for a year, it seems people are able
    to find an C++14 enabled compiler. In the *worst case* it requires
    people that *only* want to build the *newest* OpenMP runtime to
    bootstrap, or download, an older Clang release.
  - We are not compatible with LLVM-core, thus we cannot reuse, in any
    way, core code that uses C++14 features. The reuse discussion came
    up multiple times recently and there is good reason we will actually
    go down that road soon.
  - We limit the code we can write which limits productivity and even
    shows in code reviews [3]. It starts at features like `make_unique`
    [3] but there are other features we will be able to use in the
    runtime over time.

Now one can say that HPC systems, or other old systems for that matter,
have old compilers but want to use the new runtimes. We do explicitly
support this use case now and in the future, with the caveat that the
new runtimes require a moderately modern compiler to be build from
scratch. This might be an inconvenience but I don't see it as a deal
breaker for users. On the other hand, I expect modernization to help
us in the long run with development and maintenance as it helped other
parts of the project.

Thanks for reading,
  Johannes


[0] https://reviews.llvm.org/D66195
[1] http://lists.llvm.org/pipermail/llvm-dev/2019-January/129452.html
[2] https://reviews.llvm.org/D74258
[3] https://reviews.llvm.org/D74145
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