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

Fri Jun 19 07:26:55 PDT 2020

> On Jun 18, 2020, at 17:26, Petr Hosek <phosek at chromium.org> wrote:
> 
> +1 for this change.
> 
> On Thu, Jun 18, 2020 at 10:57 AM Louis Dionne <ldionne at apple.com <mailto:ldionne at apple.com>> 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 <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:
> 
>     $ cd <monorepo-root>
>     $ mkdir build
>     $ (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.
> 
> I think there may be a misunderstanding of how the "runtimes" build work. It already uses an equivalent of:
> 
> cmake -DCMAKE_CXX_COMPILER=<path to just built clang> -DCMAKE_CXX_COMPILER=<path to just built clang++> <options>  -DLLVM_ENABLE_PROJECTS="libcxx;libcxxabi;libunwind" <llvm-project-root>
> 
> The reason why it doesn't do exactly that is because LLVM's root CMakeLists.txt does too many things, and doesn't do some of the things we need.
> 
> Instead we use a trick where llvm/runtimes/CMakeLists.txt re-invokes itself for different targets. When invoked as a the root file it drives the build for all runtimes, resembling the CMake invocation above, but it also exposes a "build API" to the parent build, so as the user of the "runtimes" build, you use the parent build and it drives the child builds through this API.
> 
> When using runtimes build, you have to make one CMake invocation to build tools, and then one CMake invocation per-target to build runtimes (but *not* one CMake per project). I don't think there's a way to get down to a single CMake invocation unless CMake gains support for "scoped toolchains" (today there's only one global host toolchain), which is something that GN has and why in GN this is possible.
> 
> I don't think that having a top-level CMake file changes anything for the "runtimes" build. We could consider merging llvm/runtimes/CMakeLists.txt into the top-level CMake file, but I don't see any immediate gains aside from clearer file structure.

I agree with everything here. I didn't know the runtimes build used a single CMake invocation per target, though, thanks for correcting me on this. But the point I was trying to make, which you've made even clearer, is that there's basically no relationship between my proposed change and the runtimes build.

Louis

> 
> I think a bigger win, and not just for the runtimes build, would be to have a global CMake modules directory that would be shared by all subprojects avoiding the duplication we currently have, and allow sharing cached variables between runtimes which should significantly reduce the number of CMake checks we have to run. For example, today every runtime does the same set of checks to ensure your system has libc, libm, pthreads, etc. We really should only ever have to run those once per CMake invocation.
> 
> Thoughts?
> Louis

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