[llvm-dev] RFC: Updating to CMake 3.15.0

[James Y Knight via llvm-dev]

On Sun, Nov 3, 2019 at 7:20 PM Neil Nelson via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Glad we are getting some reasonable justifications for CMake 3.15.0. There
> are two points we may be missing.
>
> Is our plan to just keep updating to the most recent CMake version or will
> our objectives be reached with this version? If we gain what we need with
> this version then after a while we will have the Ubuntu LTS aligned.
>
> For my current purposes, LLVM can get wild without much worry here. I just
> clone a VM and run what is useful and then delete it. A bit like use once
> and throw away. This is not for production purposes but for getting a
> better view of software structure that LLVM can provide.
>
> When I was managing IT and the servers for a small company that needed to
> be live 24/7 we used the latest Ubuntu LTS version because we needed
> rock-solid performance as best we could get. The software in the LTS
> version is tested and used by a large user base with necessary updates and
> so we expected to have high reliability.
>
> Those saying how easy it was to obtain the latest CMake have apparently
> not been faced with the absolute need for solid up-time. Installing
> hack-me-now, buggy, bleeding-edge software was severely discouraged. But
> in the LLVM environment, I can see where that is not a strong necessity.
>
> But at the same time, does LLVM strive to be the backbone of 24/7
> software? Or is it more of a cool thing with some interesting code analysis
> properties? I suspect its nature is a bit wild, bleeding edge. And that's
> OK.
>
I'm not really sure what your worry is here -- if you're downloading and
building a development version of LLVM onto this particular production
machine, why is it problematic to also download a newer release version of
CMake to build it with?

Maybe you're worried that you'd have to install the new cmake into
/usr/local/bin? If that's the worry, I'll note that you are not required to
install cmake into /usr/local/bin/ in order to use it to build LLVM, and
I'd recommend not doing so. You can actually invoke the cmake binary
directly from its build/download directory -- no "make install" required at
all. Used that way, it doesn't affect anything else on your system -- if
you like, the new cmake can be used ONLY for building LLVM, and affect
literally nothing else other than a tiny amount of disk-space.

Another important point here is that the version of cmake you use to build
llvm doesn't impact users of the llvm libraries or binaries that were built
with the new cmake. (I'd contrast that with llvm requiring a newer version
of a shared library, which _does_ have runtime impact, and thus is of
potentially larger concern.)
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