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

Pavel Labath via libcxx-dev libcxx-dev at lists.llvm.org
Wed Mar 25 09:33:14 PDT 2020

If we're going to be updating the cmake version requirements llvm-wide
(and in particular, if we're going to make a policy out of that), I
think that should be done in a new RFC thread with an appropriate
subject -- people may not notice this discussion because they don't
follow libc++ and ignore any discussions relating to it.

As for the policy itself, I can't say I am fan of automatically bumping
version requirements, just because. A similar idea was proposed when we
were bumping the compiler versions, and was eventually rejected in favor
of the process described at
The current process involves proposing a specific version and discussing
the trade-offs implied by it.

Now, bumping the cmake version is not as involved as bumping the
compiler, but maybe it would be good if the process for doing it was
similar? If for nothing else, then for consistency?


On 25/03/2020 17:04, Louis Dionne via llvm-dev wrote:
>> On Mar 25, 2020, at 12:00, Tom Stellard <tstellar at redhat.com
>> <mailto:tstellar at redhat.com>> 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.
>>>>> 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). 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 or in the Programmer's manual.
> Okay, so assuming nobody objects to this, the process would then be:
> - I put up a Phabricator review updating the documentation and the
> `cmake_minimum_required` fields for (all?) LLVM projects I can find.
> - I wait for <some amount of time> before committing the change for
> build bot owners to update their CMake
> - I commit the change -- at that point using a too-old CMake will error
> out and we can point any remaining failing bot to the policy
> Does that sound like a reasonable plan? What should be the <some amount
> of time>? I'd suggest something like two weeks to give folks ample time
> to update builders.
> Louis
>> -Tom
>>> Cheers,
>>> Louis
>>>>> On Tue, Mar 24, 2020 at 8:11 PM Louis Dionne via llvm-dev
>>>>> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>>>>> <mailto:llvm-dev at lists.llvm.org>> wrote:
>>>>>   Hi,
>>>>>   The minimum CMake version currently advertised for libc++ and
>>>>> libc++abi is currently 3.4.3. I think the oldest version of CMake
>>>>> actually being tested on any builder is 3.7.0, so advertising 3.4.3
>>>>> is somewhat of a lie (I'm pretty sure we're using features that
>>>>> require a more recent version already). However, we do need to bump
>>>>> it to 3.8.0 at least because CMake 3.7 doesn't know about C++17 in
>>>>> its compilation features, and we'll need that to build libc++
>>>>> properly going forward. This will mean for bot owners:
>>>>>   1. They need to upgrade CMake on the builders to at least 3.8.0
>>>>> (which is really easy), or
>>>>>   2. they can disable processing of libc++ and libc++abi's CMake
>>>>> files by making sure they do not appear in LLVM_ENABLE_PROJECTS
>>>>>   Any objections?
>>>>>   Cheers,
>>>>>   Louis
>>>>>   _______________________________________________
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