[llvm-dev] Bumping the CMake requirement for libc++ and libc++abi

[Louis Dionne via llvm-dev]

> On Mar 25, 2020, at 13:07, Nikita Popov <nikita.ppv at gmail.com> wrote:
> 
> On Wed, Mar 25, 2020 at 5:01 PM Tom Stellard via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> On 03/25/2020 06:20 AM, Louis Dionne wrote:
> > 
> > 
> >> On Mar 25, 2020, at 00:47, Tom Stellard <tstellar at redhat.com <mailto:tstellar at redhat.com>> wrote:
> >>
> >> On 03/24/2020 09:00 PM, Petr Hosek via llvm-dev wrote:
> >>> In October, there was a discussion about updating CMake to 3.15: http://lists.llvm.org/pipermail/llvm-dev/2019-October/136295.html <http://lists.llvm.org/pipermail/llvm-dev/2019-October/136295.html>. No decision was made, but maybe we should revisit that proposal? If we're going to require a newer version of CMake for some subprojects, I'd prefer to bump the minimum CMake version for all of LLVM.
> > 
> > My personal opinion is that there's a tendency to view all subprojects under the LLVM umbrella as a single, monolithic project. That leads to the desire to make decisions for the whole project, which is often difficult, as opposed to making the right decision for each subproject, which is often easier. This results on subprojects being blocked from doing the right thing for them, like we've seen happen for pre-commit CI. But that's a much larger (non-technical) discussion than the scope of a simple CMake version bump.
> > 
> > Let's try to bump CMake for all of LLVM and see how that goes.
> > 
> >> Yes, I agree we should bump the version for all of LLVM, but I don't
> >> think we should bump the version without a long-term cmake usage plan.
> >> e.g. something like: After every release branch, we bump the cmake version
> >> to whatever version of cmake is X months old.
> >>
> >> I think the concern that this was our one chance to bump the CMake version
> >> led to the choice of 3.15 as the next version, which would be too new for some Linux distros.
> >> I think if we had a planned upgrade path, it would be easier for those of us that
> >> want something really new to settle on a release that is a little bit older.
> > 
> > Ok, how about the following policy:
> > 
> >   After every release branch, we bump the CMake version to whatever version of CMake is 12 months old.
> > 
> > This is simple, straightforward, and it gives a full year of old CMakes being supported. If we did this right now, this would take us to CMake 3.14.0, released around March 14th, 2019 (https://github.com/Kitware/CMake/releases/tag/v3.14.0 <https://github.com/Kitware/CMake/releases/tag/v3.14.0>). I believe the expectation should be that recent CMakes are upgraded using some package manager or download from the site -- we can't really expect the CMake version to be the one provided by the system, because that is either non-existent or very old on most Linux distributions AFAICT. Fortunately, installing a new CMake is incredibly easy.
> > 
> > Is everybody OK with the above policy? What would be the preferred place to document it?
> > 
> 
> 12 months is fine with me.
> 
> I'm not sure the best place to document the policy.  Some suggestions are here:
> https://llvm.org/docs/CMake.html <https://llvm.org/docs/CMake.html> or in the Programmer's manual.
> 
> I think the relevant metric here needs to be "which version is available on current LTS distros", and not "how old is the release". The current cmake version I have on Ubuntu 18.04 is cmake 3.10.2, so I personally would be happy with a bump to that version, but not something newer. People using other distributions probably have other considerations. If someone wants to perform a cmake version bump, I believe the first step would be to gather what the cmake version availability across distros looks like right now, so an informed decision can be made.

I'll actually suggest that we might want to do something else entirely. If we depend on the version available in distros, then we are at the mercy of those distros not updating. It also means that we need to look at several distributions and try to satisfy everybody. That's a lot harder than requiring that people download a recent CMake from cmake.org.

CMake has the benefit of being incredibly easy to install, either from source or with the binaries provided on their website. Do you think it's an unreasonable requirement to ask folks to download CMake instead of using the one provided with their OS?

Louis

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