[llvm-dev] Upgrading LLVM's minimum required CMake version

Thu Apr 9 13:19:51 PDT 2020

I would add my voice to Chris's: building out of the box on standard
distros is a valuable feature. I don't have time to really participate in
this discussion, but I'd discourage us from adding any more steps at all to
the LLVM getting started document.

When I started working on compilers, it was important that I could build
LLVM on the system and hardware that I had. That was a meaningful
differentiating advantage over GCC, which sent me off on a side quest to
check out two unfamiliar arbitrary precision math libraries, and asked me
if I wanted to do a two-stage bootstrap. Forget that. From a modern OS,
building LLVM should be as simple as:
- Install a standard C++ toolchain
- Clone source
- Paste a standard configuration command
- make

If we get too far away from that, we've lost something.

On Wed, Apr 8, 2020 at 2:49 PM James Y Knight via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> I am strongly in favor of increasing the minimum CMake version.
>
> I think we should NOT be waiting for the next LLVM release to do so, but
> should do so as soon as practical (e.g. maybe a month from now). A warning
> message emitted in CMake spam is not likely to help users very much, IMO.
> An entry in the release notes saying "This version of LLVM now requires
> CMake X.Y.Z." along with a link to the completely-trivial instructions on
> how to get, build, and use a new version for building LLVM (WITHOUT having
> to install it!), should both suffice -- and is more likely to be useful.
>
> I do NOT think we should have a policy tying ourselves to versions
> supported by certain LTS releases. Users of such LTSes can download a newer
> cmake. We should, of course, strive to not be unnecessarily annoying --
> even though the amount of work to download a new version is small, it's not
> zero. We should _consider_ the versions that developers are likely to have
> pre-installed on their machines, and drop support for those versions only
> when the new features are judged compelling enough to offset the cost
> (small-pain-per-developer times number-of-deveopers-affected).
>
> I agree with others who say we should only upgrade when would be truly
> valuable -- not automatically just because a new version exists. We should
> avoid changing the minimum-requirement too often.
>
> FInally, I think we should put the decision of when such an upgrade is
> judged valuable into the hands of those who are spending the most time
> working on the build system. We do not need to redo this discussion every
> time.
>
>
> On Wed, Apr 8, 2020 at 4:36 PM Chris Tetreault via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> We should decide who we consider the major distros to be. In my mind this
>> is Ubuntu, Debian, Fedora, and CentOS/RHEL. We should also consider Visual
>> Studio releases and whatever OSX and the major BSD’s have. (I honestly have
>> no idea so I’ll refrain from speculating)
>>
>>
>>
>> For all of these, we should try very hard to support the most recent LTS.
>> For the previous LTS, it would be nice if we could support it, but we
>> shouldn’t require it. Old LTSs tend to have really out of date packages,
>> especially in times like now with Ubuntu where we’re really close to the
>> current LTS becoming the old LTS.
>>
>>
>>
>> That said, if CMake version X has a killer feature that we need, we
>> should consider upgrading even if it doesn’t fit this criteria. Similarly,
>> we should not just upgrade because the minimum bound of CMake versions
>> supported by this set of OSs increased. The point is to upgrade only when
>> there’s a compelling reason, and this set of OSs is just a heuristic of
>> “most people probably already have this CMake version.”
>>
>>
>>
>> *From:* Shoaib Meenai <smeenai at fb.com>
>> *Sent:* Wednesday, April 8, 2020 12:58 PM
>> *To:* Chris Tetreault <ctetreau at quicinc.com>; Eric Christopher <
>> echristo at gmail.com>
>> *Cc:* llvm-dev at lists.llvm.org
>> *Subject:* [EXT] Re: [llvm-dev] Upgrading LLVM's minimum required CMake
>> version
>>
>>
>>
>> Yeah, I don’t anticipate Windows posing problems. Also, it’s pretty
>> common in Windows to just install software yourself, and CMake ships
>> prebuilt binaries and an installer, so it’s pretty easy to get set up with
>> it.
>>
>>
>>
>> Chris, I’m gonna reiterate a question of mine from an earlier email,
>> since you may have thoughts on it:
>>
>>
>>
>> * If we want to limit ourselves to CMake versions supported by LTS
>> releases of distros, which distros should we consider, and how far back
>> should we go (i.e. is it just the latest LTS or the last two LTS versions)?
>>
>>
>>
>> *From: *llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Chris
>> Tetreault via llvm-dev <llvm-dev at lists.llvm.org>
>> *Reply-To: *Chris Tetreault <ctetreau at quicinc.com>
>> *Date: *Wednesday, April 8, 2020 at 12:51 PM
>> *To: *Eric Christopher <echristo at gmail.com>
>> *Cc: *"llvm-dev at lists.llvm.org" <llvm-dev at lists.llvm.org>
>> *Subject: *Re: [llvm-dev] Upgrading LLVM's minimum required CMake version
>>
>>
>>
>> Visual studio 2019 ships with CMake 3.15.5, which is pretty darn new IMO.
>> From what I can tell, CMake versions are tied to visual studio releases. So
>> assuming we go with “what do recent LTS distros have” as our metric, I
>> think it’s reasonable to say “what do recent visual studio versions have”.
>> It probably makes sense to confirm with MS though before we assume that
>> this is the case.
>>
>>
>>
>> *From:* Eric Christopher <echristo at gmail.com>
>> *Sent:* Wednesday, April 8, 2020 12:41 PM
>> *To:* Chris Tetreault <ctetreau at quicinc.com>
>> *Cc:* Mehdi AMINI <joker.eph at gmail.com>; Louis Dionne <ldionne at apple.com>;
>> llvm-dev at lists.llvm.org
>> *Subject:* [EXT] Re: [llvm-dev] Upgrading LLVM's minimum required CMake
>> version
>>
>>
>>
>> Hi All,
>>
>>
>>
>> Throwing a couple of comments in:
>>
>>
>>
>> Chris's position here has a lot of good points and we want to make sure
>> we're not raising the barrier too high. I definitely want to be able to
>> push ahead with our versions of tools; being able to update quickly is one
>> of the hallmarks of the llvm project. That said, binary packages that can
>> be updated are a minimal first step IMO. I'd really like to not build
>> anything from source :) It seems like there are binaries available for
>> cmake for all of our current platforms, but the windows use case that he
>> brings is definitely a significant one. Can we perhaps reach out and find
>> out the likelihood of a reasonably soonish update there? Linux distros are
>> probably less of a problem - while we all can't use ppas we should be able
>> to do something, similarly with osx.
>>
>>
>>
>> Thoughts?
>>
>>
>>
>> -eric
>>
>>
>>
>> On Wed, Apr 8, 2020 at 9:53 AM Chris Tetreault via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>
>>
>> A line has to be drawn in the sand somewhere. How many “easy” things are
>> we going to require the user to do? Today it’s build a specific CMake from
>> source. What’s next?
>>
>>
>>
>> Not having to manually track down a bunch of dependencies before building
>> is a feature. Not having to have an internet connection at build time (if
>> we were to script the getting of the custom CMake) is a feature. Being able
>> to just call cmake instead of using some build_llvm.sh that (probably
>> poorly) wraps cmake and downloads the correct version is a feature. Being
>> able to use CMake that is distributed with visual studio so that invoking
>> cmake from the developer powershell just works without fiddling with PATHs
>> is a feature. Not having to install msys so that I can invoke
>> download_cmake.sh is a feature. Not having to have the correct version of
>> python (is it 2 or 3?) be on the path in order to invoke download_cmake.py
>> is a feature. Not having to remember to do --recurse-submodules on the llvm
>> repo if we include it as a git submodule is a feature. The list goes on.
>> Yeah, these are all little things, but a bunch of little things adds up to
>> a huge barrier.
>>
>>
>>
>> People use Linux distos because by and large they just have all the
>> dependencies that they need. I know I personally hate installing some open
>> source thing on my machines when they have some dependency that’s not in
>> the repos. Sure, it may be easy to build CMake from source. But now I have
>> two CMakes: one that is automatically updated when I do sudo apt-get
>> upgrade, and one that is just randomly in some folder that’s probably not
>> on the PATH. I personally would really appreciate it if we made an attempt
>> to reduce this sort of friction.
>>
>>
>>
>> Thanks,
>>
>>    Christopher Tetreault
>>
>>
>>
>>
>>
>> *From:* llvm-dev <llvm-dev-bounces at lists.llvm.org> *On Behalf Of *Mehdi
>> AMINI via llvm-dev
>> *Sent:* Wednesday, April 8, 2020 9:06 AM
>> *To:* Louis Dionne <ldionne at apple.com>
>> *Cc:* llvm-dev at lists.llvm.org
>> *Subject:* [EXT] Re: [llvm-dev] Upgrading LLVM's minimum required CMake
>> version
>>
>>
>>
>>
>>
>>
>>
>> On Wed, Apr 8, 2020 at 9:02 AM Louis Dionne <ldionne at apple.com> wrote:
>>
>>
>>
>>
>>
>> On Apr 7, 2020, at 22:16, Mehdi AMINI via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>
>>
>>
>>
>>
>>
>> On Tue, Apr 7, 2020 at 11:27 AM David Blaikie <dblaikie at gmail.com> wrote:
>>
>> I think it does make a difference how many things we ask new developers
>> to do to get up and running - because we've asked them to do one thing
>> doesn't mean it's low-cost to ask them to do another thing.
>>
>>
>>
>> In this case I see it rather that if we ask them to do one quite big
>> thing already, we should be OK with what seems like a trivial one.
>>
>>
>>
>> I strongly agree. I think Mehdi's point can be summarized as (Mehdi, feel
>> free to correct me):
>>
>>
>>
>>     It's incredibly trivial to install CMake, so if a user is *already*
>> required to install a non-default toolchain (which is not so trivial),
>> requiring them to install a non-default CMake is not increasing the barrier
>> by much.
>>
>>
>>
>> Thanks, this is my point indeed!
>>
>>
>>
>> I think it is even slightly stronger than what you wrote since you don't
>> even need to *install* CMake as it can be built and used directly from the
>> build directory: it is entirely non-intrusive on the system.
>>
>>
>>
>> --
>>
>> Mehdi
>>
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