[libcxx-dev] [llvm-dev] RFC: A top level monorepo CMake file

[Josh Stone via libcxx-dev]

On 6/18/20 11:27 AM, Steven Wu via llvm-dev wrote:
> I like the proposal but I would like to go even further. If we are going
> to create a top level CMake file, we should just go ahead and eliminate
> all the standalone build configuration. The standalone build should just
> be `cmake <monorepo-root> -DLLVM_ENABLE_PROJECTS=standalone-project
> ...`. That means less build configuration to maintain which is always good.

This would break the release of standalone source tarballs, no?

> That also means that we can refactor some of the cmake modules to put
> into the top level directories so sub-projects can just reuse them
> without duping the code worrying they are not available in the
> standalone build today.
> 
> Steven
> 
>> On Jun 18, 2020, at 10:57 AM, Louis Dionne via llvm-dev
>> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>
>> Hi folks,
>>
>> Building any LLVM project currently requires invoking CMake inside
>> <monorepo-root>/llvm, while setting the projects to enable in the
>> LLVM_ENABLE_PROJECTS variable. This has the downside that CMake
>> processing for the LLVM subproject happens even when one doesn't
>> really need or want it. It's also not great from a build hygiene
>> perspective, as LLVM globally sets some flags and subprojects pick
>> them up, when they don't really mean to. For example, see this
>> workaround:
>> https://github.com/llvm/llvm-project/blob/master/libcxx/CMakeLists.txt#L503-L507,
>> where we need to account for some flags that might have been set
>> globally by LLVM.
>>
>> I'm not sure about other projects, however this is quite problematic
>> for projects part of the C++ runtime (libc++/libc++abi/libunwind).
>> Indeed, we often try to build those projects targetting not widely
>> supported platforms, where the overall LLVM build doesn't work. For
>> example, trying to use the LLVM_ENABLE_PROJECTS approach for building
>> libc++ for Apple's DriverKit environment doesn't work, since it has a
>> few unusual things that the LLVM build chokes on. However, building
>> libc++ standalone works just fine because it has far fewer
>> requirements. It's also not just an issue of working vs not working:
>> because of global flag pollution, building libc++ standalone and as
>> part of the rest of LLVM can result in slightly different flags being
>> used, which could cause important and hard-to-diagnose issues.
>>
>> Hence, I think we should introduce a way to build LLVM projects (or at
>> least the runtimes) without going through
>> <monorepo-root>/llvm/CMakeLists.txt. What I suggest is to have a
>> top-level <monorepo-root>/CMakeLists.txt whose sole job is to include
>> subprojects. We could also place basic LLVM-wide things like the check
>> for the minimum CMake version there. More specifically, I would like
>> to be able to do:
>>
>> Â  Â  $ cd <monorepo-root>
>> Â  Â  $ mkdir build
>> Â  Â  $ (cd build && cmake <monorepo-root>
>> -DLLVM_ENABLE_PROJECTS="<projects-to-enable>")
>>
>> Pretty much the only difference with today is that you'd use `cmake
>> <monorepo-root>` instead of `cmake <monorepo-root>/llvm`.
>>
>> Like I said, this is a problem for the runtime projects, but I'm not
>> sure about other projects. For the runtime projects, another option
>> would be to only allow standalone builds. However, the runtime
>> projects are often built in lockstep, and so running three CMake
>> commands when one would suffice is both annoying and also an easy way
>> to screw things up. Furthermore, the current standalone builds add
>> complexity to the projects, because they require the ability to point
>> to arbitrary headers/libraries from the other projects, when we really
>> always want to point to the just-built ones.
>>
>> Relationship with Petr Hosek's "Runtimes" build
>> ---------------------------------------------------------------
>> What I'm proposing isn't a replacement for itl. The "Runtimes" build
>> can be seen as a driver that sets up the individual
>> libc++/libc++abi/libunwind builds with the just-built toolchain, and
>> for the provided targets. That's really great, however it is built *on
>> top of* the basic libc++/libc++abi/libunwind builds. So basically,
>> after my proposal, the "Runtimes" build could simply build all
>> elements from the runtime with a single CMake invocation, as opposed
>> to multiple invocations.
>>
>> Thoughts?
>> Louis
>>
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> 
> 
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