[llvm-dev] RFC: Updating to CMake 3.15.0

[Tom Stellard via llvm-dev]

On 10/29/2019 10:10 AM, Chris Bieneman via llvm-dev wrote:
> At the LLVM Developer Meeting on October 23rd we held a CMake roundtable. One of the items we discussed was adopting a more regular timeline for CMake upgrades. During the roundtable there was overwhelming support for upgrading CMake, and support for treating CMake differently than how we treat upgrading host compilers.
> 
> Historically we've taken into account recent versions of Visual Studio, Xcode and Linux distribution long term support packages when deciding what CMake version to require. While this does work, it has held us to old versions of CMake for long periods of time.
> 
> During the roundtable we discussed that the amount of effort involved in updating to a modern CMake is more or less the same regardless of what version we choose. We believe this to be true because most OS package managers have older versions, like the Ubuntu 18.04 LTS which has CMake 3.10.2 which was released January 18, 2018. Additionally CMake.org provides binary and source downloads, and building CMake from source has lower system requirements than LLVM and is very simple.
> 
> With all of this in mind at the roundtable we decided to propose raising the minimum required CMake version for all LLVM sub-projects to 3.15.0. Assuming there are no objections to this proposal, I propose a cut-over date of December 2, 2019.
> 

Will users of the CMake export files generated by LLVM also need to use 3.15.0 or
is it possibly to have the export files require an older version?

The 10.0 branchpoint will be in early January, I think it would be better
to wait until after the branch.  Mainly because that doesn't give us a
lot of time to actually take advantage of the new features  in 3.15.0 before the
branch, so I don't see much benefit in switching over at that date.

I think we should also decide on a long-term plan for CMake requirements.
If the conclusion is that users should be able to handle using the latest
(3.15.0) version of CMake, maybe we should just plan to always up-date to the
latest version in trunk after each release branch is created.

-Tom

> Thanks,
> -Chris
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