[cfe-dev] [llvm-dev] Fwd: Raising CMake minimum version to 3.4.3

[Renato Golin via cfe-dev]

On 27 April 2016 at 18:41, Chris Bieneman <beanz at apple.com> wrote:
> Renato, in your most recent email you comment about differentiating *stable* vs random versions of CMake built from source. I believe as a community our recommendation should be that people download CMake sources from https://CMake.org/download/ instead of git. Then they get the source of a known stable release. If people choose to pull random git hashes or tags, that is their decision, but since the minimum version I'm proposing is widely available I think it will be uncommon.

I'm guilty of cloning the git repo on my local tree, because it's
easier to do that than to find the link, download, untar, clean up,
etc. Then there's a bug, and you need to upgrade, "git pull" is much
simpler than repeating the whole process again. :)

Also, recent "stable", as Chuck has shown, is less stable than
previous stable. I'd like to err in the side of caution.


> I've been working to try and un-do the horrible Darwin approach we have because I think it has some serious problems, but an alternative solution would be to extend the Darwin approach to every other platform. If you do this you don't need ExternalProject, but you do need to do some really dirty hacks. If you want to understand those hacks, just look at CompilerRTDarwinUtils.cmake in compiler-rt. That is a bit of evil that I’m responsible for, and I’ll be paying down that debt for a long time to come.

Point taken, it is horrible. (sorry)


> Apart from this we also have another problem. The build system makes configuration decisions based on the compiler *at configuration time*. The problem is you really want to configure compiler-rt (and the other runtimes) *after* you've already built clang so the runtimes are configured and built with the clang you just built. The only way to do that is with some mechanism similar to ExternalProject.

<devil's advocate>We could come up with a set of rules that makes it
possible (not preferable) to do so at config time.</da>

Is that where the current complication comes from? What makes it so
horrible and ends up as a home brew version of ExternalProject?


> We could roll our own. At this point I'm going to put on my "non-constructive reality" hat. I'm not going to do that.

Absolutely not. Ten shots to the head before we start rolling thing
our own that are perfectly covered by a tool we already use.


> Replicating ExternalProject would be a lot of work...

Just to be clear, I didn't mean that. If you're saying that, to get
the desired functionality we'll either need ExternalProject or
something identical to it, then the choice is clear. I was asking if
there was another (good, but less good) solution to the problem.

Rolling our own is not even a solution, much less a good one.


> As a data point, the one place in LLVM where we don't use ExternalProject (and we should be) is the cross-compiling goop, and it is a gigantic mess (see: CrossCompile.cmake and every mention of LLVM_NATIVE_BUILD in TableGen.cmake). It is filled with bugs and in desperate need of disentangling. This is another one of my sins that I need to repent.

Oh, so ExternalProject would also fix cross compilation!? I'm sold! :)


> I feel I should also point out that ExternalProject isn't a silver bullet. It isn't going to solve all our problems, and since it is one of the more actively changing parts of CMake we may find ourselves re-visiting this conversation in a year or two talking about a newer version of CMake.

Right, this is an interesting point. I don't want to do this every
year, so let's be as reasonable as we can.

Is there anything in 3.5 that can really save us a lot of effort in
the current work?

I'm now using CMake 3.2.2 on all buildbots. If I have to move it up to
3.4.3, I'm fairly confident that there will be absolutely no problem
with it.

But as Chuck said, there were some crashes on 3.5.2 that needed 3.5.3,
and by using it on so many different platforms, we may uncover bugs,
and may have to roll back to 3.4 temporarily, and then forwards to
3.5, etc.

I really want to avoid that. But I also want to avoid doing this over
again next year.

Finally, Linaro cares more about Linux on ARM (v7+) than bare-metal,
but we can't just break the rest. I know there are people out there
that have infrastructure to test that kind of restricted environment,
not only on ARM, but also MIPS and old PPC32 stuff. I think they
should voice their concerns before we take any harsh decision.

cheers,
--renato