[llvm-dev] RFC: Updating to CMake 3.15.0

Chris Bieneman via llvm-dev llvm-dev at lists.llvm.org
Fri Nov 8 10:38:24 PST 2019

> On Nov 8, 2019, at 6:10 AM, Stefan Gränitz via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> Strong agree in general. A number of policy proposals were discussed
> already. The thread still reads like a split between supporters and
> opponents?
> In my opinion the update process on build bots was not discussed enough.
> If we choose a version and then see that it's gonna be a problem with
> many build bots, this will be a blocker and potential time sink for
> maintainers right? Should we do it first and then bump the CMake version?
> Here's a proposal that gives build bot maintainers enough time:
> (1) Once a new LLVM release version branches: start updating CMake to
> latest available binary releases on all build bots
> (2) Once the new LLVM version is released: bump minimum CMake
> requirement on master to the lowest common denominator of the (updated)
> build bots
> What do you think?

I think that the timeline here is correct. Post-branch we should have a window to update the bots (a couple weeks maybe to get everything squared). I don't think we should update to a lowest common denominator though. I think we should make a decision and tell the bots maintainers which version to update to. One of the points of this thread is to get our project away from being held hostage on old versions of CMake because package distributions don't update frequently enough.

> On 29/10/2019 18:10, 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.
>> Thanks,
>> -Chris
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